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SEPT.OCT.2010.IJPM cover_July_August IJPM cover 9/22/2010 11:28 AM Page 1
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September/October 2010
Focus Issue: PM Titanium
Newsmaker: Paul Beiss, FAPMI
46/5
Powder Metallurgy Titanium—Challenges and Opportunities Metal Powder Injection Molding of Titanium Titanium-Powder-Production Methods Cold Compaction and Sintering of Titanium and Its Alloys Mechanical Properties of Powder Metallurgy Titanium
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FRONT MATTER_ FRONT MATTER 9/22/2010 9:54 AM Page 1
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EDITORIAL REVIEW COMMITTEE P.W. Taubenblat, FAPMI, Chairman I.E. Anderson, FAPMI T. Ando S.G. Caldwell S.C. Deevi D. Dombrowski J.J. Dunkley Z. Fang B.L. Ferguson W. Frazier K. Kulkarni, FAPMI K.S. Kumar T.F. Murphy, FAPMI J.W. Newkirk P.D. Nurthen J.H. Perepezko P.K. Samal D.W. Smith, FAPMI R. Tandon T.A. Tomlin D.T. Whychell, Sr., FAPMI M. Wright, PMT A. Zavaliangos INTERNATIONAL LIAISON COMMITTEE D. Whittaker (UK) Chairman V. Arnhold (Germany) E.C. Barba (Mexico) P. Beiss, FAPMI (Germany) C. Blais (Canada) G.F. Bocchini (Italy) F. Chagnon (Canada) C-L Chu (Taiwan) O. Coube (Europe) H. Danninger, FAPMI (Austria) U. Engström (Sweden) O. Grinder (Sweden) S. Guo (China) F-L Han (China) K.S. Hwang (Taiwan) Y.D. Kim (Korea) G. L’Espérance, FAPMI (Canada) H. Miura (Japan) C.B. Molins (Spain) R.L. Orban (Romania) T.L. Pecanha (Brazil) F. Petzoldt (Germany) G.B. Schaffer (Australia) L. Sigl (Austria) Y. Takeda (Japan) G.S. Upadhyaya (India) Publisher C. James Trombino, CAE
[email protected] Editor-in-Chief Alan Lawley, FAPMI
[email protected] Managing Editor James P. Adams
[email protected] Contributing Editor Peter K. Johnson
[email protected] Advertising Manager Jessica S. Tamasi
[email protected] Copy Editor Donni Magid
[email protected] Production Assistant Dora Schember
[email protected] Graphics Debby Stab
[email protected] President of APMI International Nicholas T. Mares
[email protected] Executive Director/CEO, APMI International C. James Trombino, CAE
[email protected]
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46/5 September/October 2010
2 Editor’s Note 4 Newsmaker ...Paul Beiss, FAPMI 7 Consultants’ Corner James G. Marsden, FAPMI FOCUS: PM Titanium 9 Powder Metallurgy Titanium—Challenges and Opportunities Z.Z. Fang
11 Status of Metal Powder Injection Molding of Titanium Randall M. German
19 Review of Titanium-Powder-Production Methods C.G. McCracken, C. Motchenbacher and D.P. Barbis
29 Cold Compaction and Sintering of Titanium and Its Alloys for Near-Net-Shape or Preform Fabrication M. Qian
45 A Critical Review of Mechanical Properties of Powder Metallurgy Titanium H. Wang, Z.Z. Fang and P. Sun
58 61 62 63 64
DEPARTMENTS PM Industry News in Review Meetings and Conferences APMI Membership Application PM Bookshelf Advertisers’ Index Cover: Titanium powder produced via Armstrong process. Photo courtesy Colin G. McCracken, Reading Alloys Inc.
The International Journal of Powder Metallurgy (ISSN No. 0888-7462) is a professional publication serving the scientific and technological needs and interests of the powder metallurgist and the metal powder producing and consuming industries. Advertising carried in the Journal is selected so as to meet these needs and interests. Unrelated advertising cannot be accepted. Published bimonthly by APMI International, 105 College Road East, Princeton, N.J. 08540-6692 USA. Telephone (609) 4527700. Periodical postage paid at Princeton, New Jersey, and at additional mailing offices. Copyright © 2010 by APMI International. Subscription rates to non-members; USA, Canada and Mexico: $100.00 individuals, $230.00 institutions; overseas: additional $40.00 postage; single issues $55.00. Printed in USA. Postmaster send address changes to the International Journal of Powder Metallurgy, 105 College Road East, Princeton, New Jersey 08540 USA USPS#267-120 ADVERTISING INFORMATION Jessica Tamasi, APMI International 105 College Road East, Princeton, New Jersey 08540-6692 USA Tel: (609) 452-7700 • Fax: (609) 987-8523 • E-mail:
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FRONT MATTER_ FRONT MATTER 9/22/2010 9:54 AM Page 2
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EDITOR’S NOTE
E
ven a cursory review of the history of titanium powder metallurgy (PM) reveals a chronology permeated with advances and setbacks in the technology. Currently, non-military applications of titanium are limited, a reflection of its high cost, regardless of the process by which it is produced. Since the aerospace industry dominates the titanium market, any PM approach must compete with ingot metallurgy on a cost basis and address the sensitivity of fatigue properties to oxygen and porosity. Achieving low cost and high performance is the key challenge in the PM processing of titanium and its alloys. Where does the technology go from here and what are the future prospects for PM titanium? This “Focus Issue,” coordinated by Zak Fang, University of Utah, addresses these challenges in relation to titanium powder production, compaction and sintering, injection molding, and mechanical properties. Collectively, these reviews reflect a positive outlook for the future of PM titanium. Returning to the “Consultants’ Corner,” Jim Marsden address issues related to the sintering of ferrous materials. Specifically, he rationalizes the absence of a standard for “combined carbon,” and cites reasons for the discoloration of austenitic and martensitic stainless steels sintered in pure dissociated ammonia. “Newsmaker” Paul Beiss, FAPMI, recently retired from RWTH Aachen following a distinguished and productive career embracing teaching and research in powder metallurgy, with a focus on fatigue. Of particular note is his attention to real-world problem solving, a career-long mission that reflects close ties with the PM industry in Europe.
Alan Lawley Editor-in-Chief
In the previous issue of the Journal, I wrote briefly on differentiating between engineering and science, and hence between engineers and scientists. In a nutshell, “scientists seek to understand what is, whereas engineers seek to create what never was.” As such, it is engineering that has a direct influence on our overall standard of living. Given the paucity of media coverage, it is left to the engineering profession to educate the non-technical populace on the influence of engineering in their daily lives and on its many significant accomplishments. How is this being done and who speaks for the profession? For the “materials” discipline, primary advocates include the Minerals, Metals and Materials Society (TMS), ASM International, the American Ceramic Society (ACS), and the Federation of Materials Societies (FMS). And powder metallurgy and particulate materials is receiving increasing exposure on the beltway through the efforts of the Metal Powder Industries Federation (MPIF) in touting their “green” energy-efficient technology. Arguably, across the entire engineering discipline, the leading voice is the National Academy of Engineering (NAE). The goal of their proactive programs is to better prepare America to navigate our technology-dependent society in relation to technological literacy. Examples include: K-12 engineering education; the public understanding of engineering through public and media relations; diversity in the engineering workforce; engineering and health; and the interface between the nation’s economy and engineering innovation. Further insight into the programs and activities of the NAE is available at www.nae.edu.
2
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
FRONT MATTER_ FRONT MATTER 9/22/2010 9:54 AM Page 3
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Features and Benefits t Improved machining performance/tool life t Stain free t No detrimental effects on mechanical properties
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NEWSMAKER_ NEWSMAKER 9/22/2010 9:56 AM Page 4
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NEWSMAKER
PAUL BEISS, FAPMI
By Peter K. Johnson*
Paul Beiss, FAPMI, recently retired professor at the Institute for Materials Applications in Mechanical Engineering, Aachen, Germany, is an academician who enjoys the real-world, problemsolving side of industrial production. “We educate for industry and are totally dedicated to this mission,” he says. “Sixty-five percent of our faculty have five to 10 years of industrial experience.” His career in powder metallurgy (PM) happened solely by chance. “I was looking for a job in 1979 after receiving my doctorate from the Technical University of Aachen and accepted a position with Sintermetallwerk Krebsöge,” he says. “I had many options but Lothar Albano-Müller, the managing director, offered me the responsibility of commercializing a new vacuum sintering process for making fully dense high-speed steels.” Because his father was a commercial manager of a copper-base alloy firm, he was familiar with metals and technology from an early age. “Before I was 15 I was determined to become an engineer,” he says. “During school vacations I worked in the plant where my father was employed learning about melting, extrusion, and rod and wire drawing.” However, his university studies had not included any PM classes; his PhD thesis covered the thermal extrusion of copper. Nevertheless, he accepted the Krebsöge offer and there he mastered vacuum sintering in a hands-on environment making PM high-speed steel indexable inserts, trimming dies, and gun barrel blocks. He remembers the late PM pioneer Gerhard Zapf, who had retired as managing director, checking his progress during regular weekly visits to the plant. “Dr. Zapf was very curious about my work,” Beiss says. After managing the high-speed steel operation until 1983, he was named technical manager of the conventional PM–parts unit, which had 340 *Contributing editor
4
employees. The plant, which made iron and aluminum PM parts and bearings, had 35 compacting presses up to 250 mt, three mesh-belt sintering furnaces, and one walking-beam furnace. Within five years production at the plant doubled. Beiss was responsible for tooling design and fabrication, quality control, and equipment purchasing. In 1988 he was promoted to vice president of engineering for the entire Krebsöge Group under a new owner, MAAG Gear-Wheel & Machine Co. Ltd., Zurich, Switzerland. MAAG had purchased a majority interest in Krebsöge the previous year and expanded Beiss’s responsibilities to include PM plants in the U.S. and Canada. He remained in this position until 1991 when serious problems emerged at the PM parts operation in Bad Brückenau, Germany, where sales had plunged by 40 percent within the previous year. This plant was an important production center for automotive products such as oil-pump parts; camshaftdrive, crankshaft, and water-pump pulleys; and synchronizer hubs and rings. It had compacting presses up to 1,250 mt and employed 655 workers. Beiss was tapped to fix the problem and return the plant to profitability, a task he accomplished. He stayed at Krebsöge until 1994 when he left in order to begin his academic career at the Institute for Materials Science in Aachen by establishing a PM program. His duties covered teaching mechanical engineering students in materials science and PM, and introducing PM into R&D programs. “I focused on iron and steel,” he says. Currently the university annually graduates about 10 PM diploma engineering students who have been exposed to PM courses. Beiss has devoted a considerable research effort to axial, bending, torsion, and rolling-contactfatigue testing, as evidenced by his department’s 35 fatigue-testing machines, some of which were inijpm
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
NEWSMAKER_ NEWSMAKER 9/22/2010 9:56 AM Page 5
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NEWSMAKER: PAUL BEISS, FAPMI
house designed and built; other in-house built machines were even sold to PM-industry customers. He has also focused on the machining of green PM parts. Two doctoral students did their theses on machining and green-machining processes, which have been adopted by several industrial companies. His academic career and close ties to the PM industry have given Beiss a strategic platform to examine PM’s technology needs and barriers to growth. “The technology needs more shape capability and more levels in compacting presses and more complex parts with undercuts,” he suggests. As an example, he points to the compacting of cross holes in hard metals, a technique already underway in Europe. Yet another area that needs further development is that of higher-density PM parts capable of replacing forgings. “Many design engineers in Europe look at PM microstructures and will not buy porous parts,” he says.
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Beiss has been recognized internationally for his work. In addition to being named an APMI Fellow in 2008, he received the Skaupy Lecture Award from the German Joint Committee on PM and the Ivor Jenkins Award from the Institute of Materials, Minerals and Mining in England. He has authored more than 240 technical papers, about 70 percent of which deal with PM-related topics. Apart from PM, other research areas of his have been grey and vermicular cast iron, fatigue and failure analysis of metallic materials, structure–property relationships, and alloy development in tin-based sliding bearings. Here, the aim is to increase the strength of new alloys to the load-bearing capacity of aluminum–tin bearing alloys to make use of their superior adhesive wear performance. Several patents have been granted on these developments. Paul Beiss plans to continue consulting and lecturing two days a week to undergraduate and graduate students until 2014. ijpm
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NEWSMAKER_ NEWSMAKER 9/22/2010 9:56 AM Page 6
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THE POWDER EXPERTS Rutile Milled, Rutile Extra Fine, Titanium Powder, Ferro Titanium Powder
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CONSULTANTS' CORNER_ CONSULTANTS' CORNER 9/22/2010 9:57 AM Page 7
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CONSULTANTS’ CORNER
JAMES G. MARSDEN, FAPMI* Q A
Why is there no MPIF standard for checking “combined carbon”? I do not believe there is an ASTM standard for this either. I cannot speak for MPIF but I can give you several reasons why standards have not been established by the PM industry. For many of us who have spent several years of our lives studying microstructures and estimating the combined carbon of PM compacts in particular, it is easy to understand why no one has attempted to establish a definitive standard. Several charts have been produced over the years that give some guidelines on estimating combined carbon in iron–carbon alloys, but these are not standards. Probably foremost is the fact that the microstructures of PM steel compacts are not homogeneous and therefore the carbon distribution will vary from particle to particle and area to area throughout the entire cross section of the compact. Another point to consider is that not every parts manufacturer has the equipment and/or the personnel to perform reliable microstructural analysis. To my knowledge, and based on my experience, there are three different methods for checking the carbon content in a PM compact and each one results in a different conclusion. These methods include metallographic analysis, chemical analysis, and gas analysis, sometimes referred to as LECO analysis. Metallographic analysis requires the preparation and microscopic examination of the cross section to be analyzed. It also requires interpretation by a technician with an extensive background in microscopy since it is an estimate and does not give a specific carbon level. The one benefit that it does offer is that one can establish the location of the carbon in cases where there is decarburization or carburization of the compact during sintering. Limitations are that it is only feasible in iron–car-
bon and iron–copper– carbon alloys in which the microstructure consists of ferrite, lamellar pearlite and, in some cases, iron– carbides (hyper eutectoid steel) that form in the grain boundaries. When evaluating low-alloy steels, the presence of molybdenum retards the formation of the carbide platelets during cooling, and this produces a structure of randomly spaced carbide platelets (compared with a lamellar shape in iron–carbon alloys). One can guess at the carbon content but I have always found it not feasible to give an accurate analysis. It must also be remembered that adding alloying elements to the base iron will change the eutectoid composition. Instead of basing the analysis on 0.8 w/o C for the eutectoid, as you would with iron-base powders, it must be based on a eutectoid composition of ~0.6 w/o C. Chemical analysis will give the total carbon and the free carbon; therefore, the combined carbon can be determined by subtracting the free carbon from the total carbon. This method provides a reasonable estimate of what percentage of graphite has gone into solution in the base iron and what percentage has not. However, this method will not establish the location of the carbon and whether or not the compact has been carburized or decarburized during sintering or heat treatment. Gaseous analysis consists of burning a small sample of the compact, in the form of drillings, a very small section, and/or a powder sample. Burning the specimen will produce CO and CO2. A small amount of oxygen is added to these gases and then passed over a catalyst of platinum and silica so that the oxygen combines with the CO and converts it to CO2. Molecular oxygen can then be separated from the carbon by passing the CO2
*Consultant, Furnace & Atmosphere Service Technology, Inc. (F.A.S.T., Inc.), P.O. Box 43, Big Run, Pennsylvania 15715-0043, USA; Phone: 814-427-2228; E-mail:
[email protected]
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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CONSULTANTS' CORNER_ CONSULTANTS' CORNER 9/22/2010 9:57 AM Page 8
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CONSULTANTS’ CORNER
through an O2 molecular sieve. Once the oxygen is removed the total carbon content can be determined. This method will only give the total carbon, not the combined carbon content. If sampled correctly, it is possible to arrive at some indication of the carbon distribution by following this procedure. First, drillings from several areas at the top and bottom surfaces of the part (to a depth ~1.6 mm) are each analyzed for carbon content. Then, using the same holes, you drill into the core of the part and analyze these drillings for carbon content. It is important to insure that any contaminants, for example, drilling solutions, and quench oil are removed by washing in acetone before burning. This will give an indication of the carbon distribution throughout the cross section of the part, and whether or not the part has been decarburized or carburized during processing. When looking at the total picture it is not difficult to be convinced that it would be virtually impossible to develop a standard that would not only establish the combined carbon content in a compact but also identify the location and percentage of graphite that actually diffused into the base metal. In summary, the variables that must be considered are so numerous that if someone could outline a method that incorporated all these factors I am sure the standards committee would be most interested in listening.
Q
Our trials on stainless steel powders, both austenitic and martensitic grades, are utilized in the density range of 6.0 to 6.5 g/cm3. The sintering temperature was approximately 1,160°C (2,120°F) in an atmosphere of pure dissociated ammonia with a soak time of 40 min. All the parts exhibited a blue/black color after sintering. What went wrong and what is the correct dew point for sintering stainless steel compacts? First let me discuss the dew point for sintering stainless steels in dissociated ammonia. The furnace dew point for this atmosphere should be -29°C (-20°F) or lower. The dew point should be checked by placing a 6.4 mm stainless steel tube down the middle of the furnace into the hot zone (I am assuming this is a continuous belt furnace). Using a diaphragm pump, a filter, and a dew point analyzer, check the dew point in this zone of the furnace under normal operating conditions. I would also recommend checking the dew point of the gas at the dissociator. There should not be
A
8
much difference in dew point between these two locations. However, in relation to the blue/black surface oxide that is forming on the parts, I am sure there will be a considerable difference between the two locations with the dew point in the furnace being much higher than at the dissociator. The formation of the blue/black oxide is a strong indication of either air entering the furnace in the transition zone between the high heat and cooling zone, a water leak in the cooling zone, or that the parts are exiting the furnace at a temperature at which they cannot be handled without gloves. I tend to believe the first or second scenario is actually what is happening. Since you are sintering in an atmosphere of 75 v/o hydrogen/25 v/o nitrogen, there is ample hydrogen to combine with any air ingression into the furnace. However, if the leak is large enough, the oxygen in the air will attack the metal and form a surface oxide resembling the one you are describing. The temperature of the metal, when attacked, will dictate the color of the oxide and the color you describe is typical of this location in the furnace. One way to determine if the oxygen ingress is from air or water is to use an atmosphere of pure nitrogen (if you have the capability). If the dew point goes down to -40°C to -51°C (-40°F to -60°F) there is an air leak. However, if the dew point remains high, there is a water leak. I expect that you will find that an air leak rather than a water leak is causing the problem. If, after conducting the dew point tests, it proves to be an air leak, make sure that all the bolts on the flange area that connect the high-heat and cooling zones are tight, unless they are welded flanges. If that is the case then look for cracks in the welds. Another place the air may be coming from is loose joints in the piping from the dissociator to the furnace. You can determine this by leak testing the joints using a soap/water mixture. Air can be readily sucked into the furnace through loose joints in the line and, if you are injecting the atmosphere in the transition zone, it will attack the metal parts and form the observed blue/black oxide. If there are any questions or if I can be of any further assistance please feel free to contact me @ 814-427-2228. ijpm Readers are invited to send in questions for future issues. Submit your questions to: Consultants’ Corner, APMI International, 105 College Road East, Princeton, NJ 085406692; Fax (609) 987-8523; E-mail:
[email protected] Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Fang-Intro_Zheng et al 9/22/2010 9:58 AM Page 9
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PM TITANIUM
POWDER METALLURGY TITANIUM—CHALLENGES AND OPPORTUNITIES Z. Zak Fang*
Powder metallurgy (PM) titanium and its alloys are, in many ways, similar to ferrous and other nonferrous metals. There are conventional press-and-sinter manufacturing routes, and advanced processing technologies based on plasma atomization and hot isostatic pressing. The former is the least expensive approach while the latter results in highperformance materials. The difficulties in developing PM titanium and its alloys are, however, significantly more challenging as compared with most other PM materials. Similar to other PM materials, titanium competes with ingot metallurgy by offering a low-cost net- or near-net-shape alternative. The mechanical properties, corrosion resistance, and biocompatibility of titanium alloys are compelling and well established. However, ingot metallurgy titanium and its alloys find limited applications because of cost—more than four times higher than that of steels. Therefore, the PM route could offer an attractive alternative. Civilian applications of titanium are limited, hence the commercial market is significantly smaller (a fraction) of that of other metals. This reflects the high cost of titanium, regardless of the processes by which it is produced. The aerospace industry dominates the marketplace and this forces PM to compete and address challenges including the sensitivity of titanium powder to oxygen and the effect of porosity on fatigue properties. The combined effect of the requirements for low cost and high performance is therefore a trade-off between these entities. The conventional press-and-sinter approach offers low-cost materials with adequate static mechanical properties, while the hot isostatic pressing of prealloyed powders results in mechanical properties equivalent to those of ingot metallurgy materials at a significantly increased cost. How to achieve both low cost and high performance has been, and will remain, the key challenge in the PM processing of titanium and its alloys. Another reality, arguably due to the challenge cited above, is the reality that the market size for PM titanium is small, except for powders and isolated cases of parts manufacturing. Therefore, the question or challenge, phrased in a different way, is where to go from here and what are the future prospects for PM titanium? It is against this backdrop that this “Focus Issue” on PM titanium and its alloys is compiled. Obviously, one such attempt will not address *Associate Professor, University of Utah, Metallurgical Engineering Department, 135 S. 1460 East, Room 412, Salt Lake City, Utah 84112, USA; E-mail:
[email protected]
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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Fang-Intro_Zheng et al 9/22/2010 9:58 AM Page 10
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POWDER METALLURGY TITANIUM—CHALLENGES AND OPPORTUNITIES
all the concerns and questions regarding PM titanium; rather, it aims to provide a realistic review of the status, challenges, and opportunities. The articles are intended to highlight developments that are uniquely promising. First, a review of powder production methods is coauthored by McCracken that includes a brief introduction to different methods and, more important, insights into the pros and cons, economics, and the markets served by the different powder production methods. Following powder production, Qian takes us through a history of the sintering of titanium and titanium alloy powders. A fundamental understanding, with respect to the issue of titanium oxide on particle surfaces, the effects of vacuum and atmosphere, and interparticle diffusion, are particularly worth noting. Qian also cites a specific example of the commercial production of PM titanium automobile parts, albeit the only viable tonnage commercial production to date. In relation to parts manufacturing, German presents a convincing case that the powder injection molding of titanium is “ready for prime time” from a knowledge and technology-readiness perspective. The key hurdle that continues to prevent the metal injection molding of titanium from rapid growth is the high cost of titanium powder. The final article by Wang et al. focuses on mechanical properties and their dependence on microstructure, porosity, and oxygen content, all of which are unique to PM processing. This critical review, and a comparison of the mechanical properties of PM titanium, with ASTM Standards for ingot metallurgy titanium puts things in perspective, details future challenges and, hopefully, illustrates opportunities. Although the topics covered in this “Focus Issue” are not orchestrated to address any specific industry, the emphasis on low-cost methods for civilian applications is evident. To paraphrase a well-known cliché, the three most important factors in relation to titanium are cost, cost, and cost. The cost of titanium will not decrease until it is more widely used in civilian applications such as automobile parts and consumer products. The counter argument is that, if the cost of titanium is significantly reduced, non-military industries will then adopt titanium. In this chicken-or-egg dilemma, we predict PM technologies will play a crucial role in eventually bringing about a watershed event (products) that will cascade affordable titanium over the entire manufacturing spectrum. ijpm
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Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Randall M. German_Zheng et al 9/22/2010 11:54 AM Page 11
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PM TITANIUM
STATUS OF METAL POWDER INJECTION MOLDING OF TITANIUM Randall M. German, FAPMI*
INTRODUCTION Titanium production by the metal powder injection molding process (Ti-MIM) has been an area of heated debate. In 1997 Animesh Bose and I wrote a book on MIM/PIM at the request of the Metal Powder Industries Federation.1 As with all such books, reviews were performed on the manuscript to ensure accuracy. The reviewers complained that Ti-MIM was not possible, contrary to the detailed description given in the book. We begged to disagree, so on the front cover of the book we included a micrograph to show the structure possible with Ti-MIM, in 1997. Saying it is “not possible” fails to change the fact that Ti-MIM is technologically well advanced. TITANIUM STATUS Titanium is poised for significant activities by MIM, but early efforts did not properly balance technical sophistication and economics. To make the case, this status report will show that significant progress has been made in powder fabrication and in component fabrication. What often has been missing is the realization that markets for Ti-MIM require high-performance and high-value applications, areas that specify aerospace and medical quality. Demonstration components in Ti-MIM span areas that include the following, several of which are illustrated in Figures 1 to 7: • automotive gearshift knobs • toy components, including “transformer” hinges, train links and wheels • surgical tools, including scalpel holders • rifle and firearm components • watch cases, watch bands, watch clasps • eyeglasses components, even eyeglasses frames • cell phone hinges, knuckles • heat valves • golf clubs, ranging from putters to drivers • implant devices, including chemotherapy pumps • tooth-implant anchors • orthodontic brackets • jet-engine fasteners • cosmetic cases • decorative hardware for luggage and purses
The metal powder injection molding process (MIM or PIM) has been applied to titanium in various forms since the late 1980s. Powder production is well advanced with many offerings, giving a wide choice in particle size, particle shape, purity, and cost. Likewise, component production steps exist with many variants, but a general baseline process is identified here. There is much knowledge on how to fabricate titanium using MIM, but this knowledge is concentrated in the hands of a few fabricators. Against this baseline, emerging innovations can be assessed. Demonstration products have been shown for several applications, including watch cases, toy trains, cell phones, heat valves, dental implants, and medical surgical tools. This article rationalizes the technical and economic factors to show why research efforts in titanium injection molding are focused on reduced interstitial contents.
*Associate Dean of Engineering, Professor of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1326, USA; E-mail:
[email protected]
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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Randall M. German_Zheng et al 9/22/2010 11:54 AM Page 12
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STATUS OF METAL POWDER INJECTION MOLDING OF TITANIUM
Figure 1. Surgical scalpel holder fabricated using Ti-MIM Figure 5. Implant pump fabricated via Ti-MIM
Figure 2. Underside of a Ti-MIM golf putter Figure 6. Clasp on watch band formed using Ti-MIM
Figure 3. Example of a firearm component fabricated by Ti-MIM
Figure 7. Toy component fabricated by Ti-MIM
Figure 4. Cellular-telephone cover formed using Ti-MIM
12
Clearly the range of target applications for TiMIM is large, but the technology segregates into three groups: 1) Decorative items where mechanical and other properties are secondary to the marketing advantage—watch cases and golf putters are in this category 2) Mechanical components where mechanical and corrosion properties exceed those possible from a stainless steel—surgical tools that Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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can be repeatedly autoclaved without rusting are in this category 3) Demanding applications where titanium is critical to success—biomedical implants and aerospace components are in this category The decorative items are cost sensitive and often only make it to the demonstration phase in Ti-MIM due to cost; the marketing advantage from “titanium” does not justify the added cost. This was seen when some of the first cellular telephone hinges were made using Ti-MIM, but later switched to stainless steel. Also this happened 15 years ago when sunglasses frames and golf clubs were first injection molded from titanium. The marketing infatuation with titanium continues. Remember the titanium golf balls? No one really expected to find a metallic core and it seems marketing took some liberty with the white titania in the coating. However, several successes have occurred in titanium components where mechanical and corrosion properties are dominant, such as lightweight eyeglasses hinges. Now we are on the cusp of seeing Ti-MIM move into taxing applications where the added cost is justified. Accordingly, several groups are at work isolating the powders, processing steps, and property tradeoffs possible. This article provides an update on the status of Ti-MIM, with a prime focus on the tradeoffs being faced as the technology translates into production. As knowledge is gained, Ti-MIM turns its attention to the economics of powder fabrication and processing. TITANIUM POWDER COST FACTORS The thermodynamics of titanium reduction from ores is a well-explored topic and well over 30 research efforts were launched on this topic in the past ten years. A few of those efforts are coming to fruition with new powder options. However, powder cost is not low. Part of the problem can be traced to the 50-fold greater energy required to convert ore to metal for titanium vs. common metals like copper and iron. This alone makes the cost of titanium high. An additional factor is that titanium use is only 0.01% that of steels, so global production of titanium sponge (recently ~220,000 mt at an average price of nearly $13/kg) is over three times the commercial value of iron powder production, and most of the material goes into mill products, castings, and forgings, not into powder metallurgy Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
(PM) components. Even so, over 40 firms sell titanium powder, with prices that range from $30/kg upward.2 Powders required for Ti-MIM are made by several approaches, including gas atomization, hydride– dehydride, plasma atomization, and rotating electrode.3 Figure 8 is a scanning electron micrograph (SEM) of a typical spherical, -45 µm powder used in Ti-MIM. Although there has been much effort to innovate, current production costs ($10/kg powder) are high, and classification of the powder into the small sizes required for MIM results in prices from $40 to $220/kg, depending on the alloy, particle size, and purity level. Indeed, the most popular Ti-MIM powders tend to average ~$120/kg. This powder cost precludes Ti-MIM from being a low-cost option and drives the field toward highervalue applications. Consumer products exist, but tend to be small and highly valued for being lightweight—watch components, cellular-telephone components, eyeglasses components, and such. On the other hand, biomedical applications rely heavily on titanium because of density, strength, corrosion resistance, and biocompatibility. The three product categories—cosmetic, structural, demanding—require different price structures and powder attributes. The demanding applications require control over the impurities and that means every step in the PIM process must be monitored and controlled. The oxygen content in the final product is a reflection of the starting powder purity. To avoid contamination of the powder, the typical decision is to use a larger particle size than is normal in MIM to reduce the surface area and hence contamination, with attendant difficulties in sintering densification. At least
Figure 8. Spherical gas-atomized titanium alloy powder customized for Ti-MIM. SEM
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three titanium powder suppliers are recognized for producing powders with low interstitial levels meeting biomedical standards, and a fourth has announced plans to enter production in 2010. Their powders are priced in the $110 to $220/kg range, depending on purity, quantity, alloying, and particle size. Curiously, gas-atomized cobalt– chromium MIM powder sells at $160/kg. On a volume basis, titanium is lower in cost (density of 8.4 g/cm3 for cobalt–chromium vs. 4.5 g/cm3 for Ti-6Al-4V). Accordingly, titanium should displace cobalt–chromium in several biomedical applications. KEY FEATURES IN TITANIUM PM A few parameters dominate the mechanical properties of titanium when it is fabricated by PM—density, interstitial content, alloying, and microstructure. Residual pores degrade mechanical properties, so full density is desirable. Hot isostatic pressing (HIPing) is a common means to attain full density, since grain growth during the sintering of large particles degrades strength as full density is approached. Without full density the fracture toughness and fatigue strength suffer more than the tensile strength. A demonstration of this for titanium is given in Figure 9. This is a plot of tensile strength, yield strength, and fatigue strength for Ti-6Al-4V fabricated to different sintered-density levels using blended elemental powders.4 Between 94% and 100% of the pore-free density the yield strength increases 30% but fatigue strength increases fourfold. Yet, as illustrated in
Figure 9. Properties vs. sintered density for Ti-6Al-4V fabricated from blended elemental powders using the press-and-sinter PM process, showing property gains from the elimination of pores
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Figure 10, when sintering is used, concurrent grain growth degrades the strength, notwithstanding density gains. Thus, densification via HIPing after sintering is a typical step in Ti-MIM, but this adds to the production expense. The interstitial content has a significant effect on mechanical properties, biocompatibility, and corrosion resistance. For PM, oxygen is the usual focus. Interstitials of carbon, hydrogen, nitrogen, and oxygen increase strength and hardness, but decrease ductility. Typically, alloys are sorted by grade levels that correspond to the oxygen content. As an example of the sensitivity of titanium to oxygen, consider that the ASTM standard for unalloyed (CP or commercially pure) grade-1 titanium requires <1,800 ppm oxygen to give a tensile strength of 240 MPa and 24% elongation. At higher oxygen levels, such as for grade-4 titanium, the oxygen level can range up to 4,000 ppm and the tensile strength increases to 550 MPa, but the ductility declines to 15% elongation. Control of the interstitials is the largest difficulty with the PM processing of titanium. Indeed, since oxygen is the major contaminant, it is possible to specify an “oxygen equivalent” impurity level that combines the impurity effects into a single effective number. Again, interstitial control is a significant cost factor in Ti-MIM. Alloying is a third consideration. Most of the compositions used in MIM copy the wrought specifications. A large number of other compositions have been investigated using press-and-sinter, HIPing, and injection molding, but for Ti-MIM the
Figure 10. Example of the difficulty with sintering densification as seen from data for a Ti-12Mo alloy showing strength loss with densification at high sintering temperatures (5 h hold) due to rapid grain coarsening Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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prime focus is on CP Ti, Ti-6Al-4V, and Ti-6Al-7Nb. By far the most common alloy is Ti-6Al-4V. When this alloy is processed to full density with <2,000 ppm oxygen, the alloy delivers a tensile strength between 710 and 850 MPa with 12% elongation. The strength varies with the initial powder choice and processing details. This strength–ductility combination is below its wrought counterpart, namely, 950 MPa and 14% elongation, and reflects a coarse-grained microstructure concomitant with the time–temperature combinations required for densification. A few alloys have been developed in the context of MIM/PIM, but they have not seen much acceptance. For example, a liquid-phasesintered Ti-Cu alloy has been developed and tested for dental applications, but has not been commercialized. The last factor is associated with the microstructure, primarily grain size and phase relations after sintering. Due to microstructural coarsening during sintering, PM products tend toward the lower side of the mechanical-property range encountered with wrought materials. The relatively slow sintering densification kinetics associated with the large particle size (needed to reduce oxygen contamination) result in extensive grain growth. Accordingly, one option is to sinter to the closed-pore condition and rely on HIPing for final densification at a lower temperature where grain growth is slow. When this route is employed for MIM products, the final mechanical properties approach those attained in annealed wrought material. Strength levels approaching 975 MPa with 14% elongation-to-fracture are achieved. PROCESSING The PM processing of titanium by the pressand-sinter route was reported in the 1960s, but few products other than filters and fasteners reached production. Subsequent developments brought full density by HIPing using inert handling to deliver low interstitial levels. This was successful for high-performance applications and today constitutes the largest user of titanium alloy powder. Other efforts relied on lower-cost powders, such as sponge fines and blended elemental powders, in traditional press-and-sinter consolidation. A conference in 1980 documented considerable progress with the PM routes.5 The HIPed Ti-6Al-4V product delivered a tensile strength of 975 MPa with 14% elongation, similar to the corresponding wrought-alloy properties. Different post-consolidaVolume 46, Issue 5, 2010 International Journal of Powder Metallurgy
tion heat treatments enabled strength–ductility combinations that ranged up to 1,130 MPa and 9% elongation. The press-and-sinter properties were lower, since full density was not attained. Still the press-and-sinter route reached 920 MPa with 11% elongation at 98% of the pore-free density for Ti-64. However, fatigue properties and fracture toughness of the PM products were typically only 80% of the comparable wrought product. Using the base provided by press-and-sinter, TiMIM was demonstrated in the late 1980s. Early reports showed an impressive 1,000 MPa tensile strength, but with only 2% elongation. Even so, TiMIM reached production status in Japan by 1991. An early application gained notice when American sprinter Leroy Burrell posted a time of 9.88 s in the 100 m dash using ASICS shoes with Ti-MIM spikes. Subsequently, scientific studies expanded quickly to improve processing with a primary focus on oxygen control for improved ductility. A large body of literature emerged detailing the processing cycles required for titanium and its alloys. Representative references are detailed in an earlier article.3 After 20 years of activity, Ti-MIM is moving into applications where titanium is fully justified—dental, aerospace, medical, and chemical devices. Variants on the core technology are practiced by about 18 firms with several firms active in laboratory trials. Today Ti-MIM is a favorite research topic.6 Now that sintered mechanical properties rival handbook values for their cast/wrought counterparts, efforts are focused on lowering cost and becoming qualified for demanding biomedical and aerospace applications. Titanium is a significant portion of each market segment, so the issue is to convince designers to use MIM. As a metal, titanium provides the largest value in aircraft (outside the turbine) and second largest value in biomedical. The challenge is not in developing these markets, but in qualifying the Ti-MIM approach. After many experiments it is now possible to outline a baseline Ti-MIM process, as detailed in Table I. This gives examples of best practices for each of the steps. Many variants exist, but still this reflects the best current technology. Changes in the powder or binder result in differences in mixing, impurities, sintering, and other steps. Hence, this is a demonstration of what should work, but is not comprehensive with respect to the many options.
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TABLE I. BASELINE Ti-MIM PROCESSING Powder
Deagglomerated spheres formed by gas or plasma atomization 30 to 60 μm median particle size Tap density 60% to 62% of pycnometer density Initial oxygen level 0.15 w/o, initial carbon level 0.04 w/o
Binder
Majority low-molecular-weight paraffin wax or polyethylene glycol Backbone of 15% to 25% polypropylene or ethylene vinyl acetate 5% stearic acid as surfactant, lubricant, plasticizer
Mixing
Argon cover gas Room-temperature dry mix all ingredients at 65 v/o solids loading 30 min in high-shear mixer to peak temperature of 120°C to 185°C Viscosity 150 to 250 Pa·s at 500 1/s
Molding
Nozzle temperature 120°C to 180°C, mold temperature 30°C Injection temperature 160°C Injection pressure 30 MPa Green strength 10 MPa
Debinding
First-stage solvent removal of wax or polyethylene glycol at 60°C Penetration rate 2 mm/h Second-stage argon sweep gas heating in vacuum system Heat to 450°C, hold under vacuum to complete debinding Continue heating to presintering stage, near 900°C
Sintering
High vacuum, refractory-metal furnace, yttria or zirconia trays Peak temperature 1,250°C for 120 to 180 min Sintered density 95% of pore-free value
Densification
Hot isostatic pressing in argon without container Peak temperature 900°C, 100 MPa pressure, 60 min
Properties
0.20 to 0.22 w/o oxygen, 0.04 w/o carbon Tensile strength 900 MPa, elongation-to-fracture 12%
MARKET SITUATION As noted earlier, many demonstration components have been formed using Ti-MIM and a few companies are providing feedstock. Even so, the market for titanium by MIM is probably only 1% of the global MIM total and data on titanium use is spotty. For example, the Japan Powder Metallurgy Association reported total sales in 2007 for Ti-MIM at $4 million, mostly for surgical tools. According to statistics from EPMA, titanium powder sales are 1% of the overall MIM tonnage. However, since titanium powder is significantly higher in price than ferrous powders, this means the approximately 10,000 kg of powder per year results in sintered components that sell, on average, for $8 each with an average mass of 10 g. In other words, TiMIM is about $10 million in component sales globally or about 1% of the total market. As noted previously, a variety of products have been demonstrated using Ti-MIM. The more demanding applications require HIPing to ensure
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full density, which adds to the expense. This is on top of an already high powder cost and an extra cost for contamination control during processing. Unfortunately, for large-production-quantity projects, the cost advantage is not evident. Thus, as powder costs decrease there is ample opportunity for expanded applications. So the challenge is to qualify the MIM approach. Standards exist for mill products and a new standard is emerging for Ti-MIM. For medical applications the standard will require a low oxygen level, which is the most difficult aspect of the technology. SUMMARY There is nothing routine about Ti-MIM. The field is highly fragmented, with about two dozen different powder vendors reflecting several different powder production routes. Likewise, binders tend to emphasize lower-melting ingredients and easily extracted phases. Debinding by first-stage solvent Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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immersion is highly beneficial in reducing contamination. All reports show that contamination increases from powder to product, so the strategy is to optimize each step in sequence to minimize the increase in contamination. Certain binder ingredients appear to add more to the impurity burden, as do different thermal cycles, atmospheres, substrates, peak temperatures, and other variables. At this point, the technology base for Ti-MIM is well developed, and there are several powder vendors and several excellent technical demonstrations. What is missing is the economic justification. So, to the first question in Ti-MIM, “Can you make the part?”, the general answer is, yes. To the second question, “How much will it cost?”, there tends to be a barrier. Accordingly, little thinking is going on with respect to the most relevant issue, that being, “What is the value?” Clearly, aerospace and biomedical applications have an established high valuation for titanium, use a considerable amount of titanium, and so should be targets for product demonstrations. The future situation is favorable as many new powder-production operations step up to the titanium challenge.
ACKNOWLEDGEMENTS Considerable help was provided by individuals participating in the Ti-MIM field and a large number of comments were received to help make this report an accurate reflection of the status of TiMIM. My thanks to all of those who helped understand the situation, successes, barriers, and emerging strategy. REFERENCES 1. R.M. German and A. Bose, Injection Molding of Metals and Ceramics, Metal Powder Industries Federation, Princeton, NJ, 1996. 2. F.H. Froes and R.M. German, “Cost Reductions Prime Ti PIM for Growth,” Metal Powder Report, 2000, vol. 55, no. 6, pp. 12–21. 3. R.M. German, “Titanium Powder Injection Moulding: A Review of the Current Status of Materials, Processing, Properties, and Applications,” Powder Injection Moulding International, 2009, vol. 3, no. 4, pp. 21–37. 4. ASM Handbook Vol. 7: Powder Metallurgy Technologies and Applications, ASM International, Materials Park, OH, 1998. 5. F.H. Froes and J.E. Smugeresky, Powder Metallurgy of Titanium Alloys, Proc. of a Symposium, 1980, The Metallurgical Society of AIME, Warrendale, PA. 6. R.M. German, “R&D in Support of Powder Injection Molding: Status and Projections,” International Journal of Powder Metallurgy, 2007, vol. 43, no. 6, pp. 47–57. ijpm
NEW Powder Metal Parts Trays Glass reinforced thermoplastic trays designed specifically for the powder metal manufacturing process, in both automated and manual tray handling operations. Engineered to handle, move and store green parts in both pre-sintering and post-sintering operations. • Durable • No sharp edges • Oil/moisture resistant • Stack securely in two directions • Maximum number of holes in bottom for powder to escape • Eliminate oil-soaked cardboard and wooden pegboard disposal problems • More cost effective than pegboard and steel trays 12” x 16” with an open front section, and available in two heights; ½” and 2”. Tapered bosses at front of 2” high tray allow parts to be easily and uniformly removed from the tray. Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Molded Fiber Glass Tray Company 6175 US Highway 6 Linesville, PA 16424 Tel. 814-683-4500 x 230 Fax 814-683-4504 www.mfgtray.com
[email protected]
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AMETEK
AMETEK
AMETEK’s
AMETEK
WWWAMETEKMETALSCOM
Visit V isit us at BOOTH # 7 7
Fortezza da Basso Centre, Florence
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PM TITANIUM
REVIEW OF TITANIUMPOWDER-PRODUCTION METHODS
Colin G. McCracken*, Charles Motchenbacher** and Daniel P. Barbis***
INTRODUCTION Titanium-powder production can be subdivided into several categories based on powder morphology and shape, purity, and even cost; however, for the purposes of this review we classified titanium-powderproduction methods based on the primary powder-particle size. The coarsest titanium powders are the titanium sponge fines that originate as a by-product of the traditional titanium sponge-reduction process. In descending order of primary powder-particle size, titanium powders produced using the hydride–dehydride (HDH) process offer a wide range of particle sizes from coarse to fine. This process overlaps several other powder production methods that incorporate a liquid-phase change, such as the plasma rotating electrode process (PREP), plasma atomization (PA), and gas atomization (GA). Over the past decade several direct, continuous, and semi-continuous titanium-powder-production methods have been developed without the need for traditional ingot or wrought raw-material feedstock; these are referred to generically as “meltless” processes. The titanium powders produced utilizing these meltless processes exhibit fine primary particle sizes with high surface area:volume ratios. TITANIUM SPONGE FINES Currently all titanium ingot and wrought products originate from titanium sponge produced by the chemical reduction of titanium tetrachloride, either by pure sodium (Hunter process) or pure magnesium (Kroll process).1 This high-volume, sponge-manufacturing process also produces titanium sponge fines; a representative scanning electron micrograph (SEM) of fine titanium sponge powder is shown in Figure 1. This by-product has been utilized as coarse titanium powder suitable for conventional bulk PM processes such as press-and-sinter (P/S), and cold isostatic pressing/sintering (CIP/Sinter). These solid preforms, such as plate and billet, are suitable for conversion by conventional metallurgy into sheet, bars, and tubes. The typical particle-size distribution (PSD) of titanium (Kroll) sponge fines is normally in the range of 180–850 μm. This powder has limited application due to its coarse particle size and the high levels of metallic-salt residues such as sodium chloride and magnesium chloride.
Titanium and its alloys have been in industrial use for over half a century; however, applications have been restricted primarily to the aerospace industry, where high strength and light weight are the most critical parameters, and to the chemical industry, where titanium’s resistance to corrosion is the most critical parameter. Traditional titanium-production methods used to produce both titanium ingot and wrought products have inherently high manufacturing costs, due in part to the high affinity between titanium and its interstitial elements carbon, hydrogen, nitrogen, and oxygen. In contrast, the titanium-based powder metallurgy (PM) industry is still relatively young, and not only faces traditional issues with interstitial contamination, but also those associated with powder manufacturing and consolidation. The aim of this review is to document current titanium-powderproduction methods and to examine newer continuous direct-powder-synthesis methods currently under development and expansion.
*Director of Technology, **Technical Manager, Powders, ***R&D Powder Engineer, Reading Alloys Inc., An Ametek Company, 220 Old West Penn Avenue, Robesonia, Pennsylvania 19551, USA; E-mail:
[email protected]
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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REVIEW OF TITANIUM-POWDER-PRODUCTION METHODS
Figure 1. Representative micrograph of 180–850 μm titanium sponge powder produced by the Kroll process. SEM
HYDRIDE–DEHYDRIDE TITANIUM POWDER Titanium sponge cannot be crushed to a powder <850 μm due to its high ductility. If, however, the titanium sponge were embrittled, it could readily be crushed and milled into a finer powder. Cryogenic crushing has been successfully used on a limited scale to produce titanium sponge powders. The HDH process has been successfully scaled up to produce high volumes of crushed titanium sponge powder, as shown in Figures 2 and 3.2 When commercially pure (CP) titanium is heated above 350ºC and cooled in the presence of excess hydrogen, the α phase is converted into the stable δ titanium hydride (TiH2) phase. Titanium hydride contains ~4 w/o hydrogen and can be readily crushed and milled into a powder using conventional powder-sizing equipment. Once the titanium hydride powder has been sized to the desired PSD, it is then dehydrided at ~750ºC under high vacuum to liberate the hydrogen and to convert the titanium hydride powder back into titanium powder, as shown in Figure 4. During all titanium-powder-production methods care has to be taken to inert-gas shield all the powder-production equipment with argon as the titanium powder surface area:volume ratio increases and to avoid uncontrolled exothermic oxidation. Controlled passivation stages are required to reintroduce oxygen into the titanium powder in order to form the stable TiO2 passive film on the powder surface. Due to these repeated thermal/passivation cycles in both the hydride and dehydride operations the oxygen and nitrogen
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Figure 2. Representative micrograph of 150–250 μm HDH titanium sponge powder produced by the Hunter process. SEM
Figure 3. Representative micrograph of 75–150 μm HDH titanium sponge powder produced by the Kroll process. SEM
contents will increase when compared with the initial interstitial chemistry. Commercially available HDH titanium powders are normally produced in the size ranges of 150–300 μm, 45–150 μm and <45 μm, including intermediate screen fractions. There is no practical upper limit to the particle-size range for HDH titanium powder. In the lower particle-size range, controlling the chemistry and PSD of <45 μm titanium powder becomes cost prohibitive. This is due to a combination of sintering (ultrafine powder forms satellites), the generation of hard-sintered agglomerates, and the resulting higher oxygen levels due to the repeated passivation cycles. Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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REVIEW OF TITANIUM-POWDER-PRODUCTION METHODS
static pressing (HIPing), direct powder rolling (DPR), and vacuum plasma spraying (VPS).
Figure 4. Representative micrograph of <90 μm HDH CP titanium powder produced from wrought feedstock. SEM
PLASMA ROTATING ELECTRODE PROCESS POWDER The PREP technology was developed by Nuclear Metals, Inc., in 1988 for the production of reactive metal powders such as beryllium, molybdenum, titanium, and zirconium.4 A transferred arc plasma torch replaced the tungsten anode used in the rotating electrode process (REP) and created a “cleaner” heat source that operated at temperatures >10,000°C. The raw material feedstock is a high-purity electrode bar, 65–75 mm dia. and 250–1,520 mm long. An argon or helium plasma melts one end of the anode, which rotates at ~15,000 rpm. This causes molten droplets to undergo centrifugal acceleration away from the anode and the solidifying droplets minimize their surface energy by forming spheres, which undergo convective cooling at a rate <100°C/s, as shown in Figure 6. PREP is used primarily to produce high-strength Ti-6Al-4V alloy powder that is characterized as highly spherical with low levels of attached satellites, Figure 7. A typical PSD would be 100–400 μm. The main advantage of PREP powder is that the molten reactive metal droplets are never in contact with any other metal or refractory material. This condition reduces the likelihood of contamination and makes it possible to produce powders that are close to the feedstock chemistry. Also, because the droplets are dispersed and are moving radially away from each other, there is limited opportunity for interparticle collisions
Figure 5. Representative micrograph of 75–212 μm HDH Ti-6Al-4V powder produced from wrought feedstock. SEM
The HDH process can also be used to produce powders from α + β and β titanium alloys, yielding lower hydrogen levels of between ~2.5 w/o and ~2.0 w/o, respectively, in the corresponding hydride powders. In both the α + β and β titanium alloys, the α phase is converted into the δ phase, either within the α + β grains or at the β phase grain boundaries where small amounts of α phase accumulate.3 Figure 5 shows Ti-6Al-4V (α + β alloy) powder produced via the HDH process. The reduced PSD of HDH titanium powder compared with titanium sponge fines increases the number of PM powder-consolidation methods available; these include near-shape methods such as hot isoVolume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Figure 6. Transferred arc by plasma rotating electrode process5—schematic
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REVIEW OF TITANIUM-POWDER-PRODUCTION METHODS
Figure 7. Representative image of 250–400 μm PREP Ti-6Al-4V powder produced from wrought feedstock. SEM. Courtesy of Affinity International L.L.C.
Figure 8. High-frequency-induction PA using three argon plasma torch jets that converge on a titanium wire. Courtesy of Advanced Powders & Coatings
between droplets that may produce irregularly shaped powders or lead to powder clusters. The single-particle nature of PREP powders, along with their high level of sphericity, result in a high flow rate and the highest levels of powder-packing density compared with all other titanium powder types. The physical properties of large spherical powders make them ideally suited for near-netshape HIPing of large complex shapes that require limited surface machining. PLASMA ATOMIZATION Powder production using PA can be subdivided into two categories based on the type of plasma used, namely, DC arc or high-frequency induction. PA is a two-step process, first, shearing of a liquid metal stream and, second, freezing of the liquid droplets. During high-frequency-induction PA a titanium alloy wire is fed into the apex of three argon plasma torches, which melt the wire, as shown in Figure 8. Aerodynamic drag results in the formation of droplets, which are rapidly cooled during their free-fall flight in an argon atmosphere with a cooling rate in the range of 100°C– 1,000°C/s. The resulting spherical powders can have a wide PSD in the range of 25–250 μm, similar to PREP powders, and exhibit limited agglomeration and attached satellites, Figure 9. If angular powder feedstock is substituted for the titanium wire, a new powder morphology can be generated by the selective melting of the smaller powder particles, with careful control of the operating parameters, such as the plasma type (argon, helium, hydrogen, and nitrogen), power input, and powder-feed rate. In certain applica-
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Figure 9. Representative image of 45–106 μm PA CP titanium powder produced from wire feedstock. SEM. Courtesy of Advanced Powders & Coatings
tions such as VPS, high-density coatings can be achieved by using a PA powder containing ~50% spherical and ~50% angular powder particles. GAS ATOMIZATION Similar to the PA process, GA of titanium powders can be subdivided into two main groups based on the type of melting process used, i.e., induction drip melting or conventional titanium melting followed by GA. Both GA processes rely on gravity feed and rapid cooling during free fall to produce fine titanium powders. The induction drip process utilizes a solid feedstock bar while the bulk titanium melting process uses conventional vacuum induction melting (VIM), plasma arc meltVolume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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REVIEW OF TITANIUM-POWDER-PRODUCTION METHODS
Figure 10. Image of 45–106 μm GA Ti-6Al-4V powder. SEM
Figure 11. Image of <45 μm GA Ti-6Al-4V powder. SEM
ing (PAM), or electron beam cold hearth melting (EBCHM) of virgin ingot or wrought feedstock to produce the liquid stream suitable for GA. Electrode induction melting gas atomization (EIMGA) employs a crucible-free technique, with a prealloyed rod (electrode) ~40 mm dia. The conical induction coil is normally water cooled and coated with a ceramic to prevent sparking between the coil and the electrode. When the electrode is heated above its melting point the liquid titanium falls through the induction coil and is atomized with argon gas. The electrode rotates at 5 rpm to ensure even melting and is lowered slowly into the induction coil to maintain a continuous stream of liquid titanium. Since the electrode rotates slowly, the Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
rod does not need to be balanced, in contrast to the PREP process. Argon is the preferred gas for atomization at a pressure of 2–3 MPa with a flow rate of 10–15 m3/min. The resulting titanium powders have a narrower PSD (in the range of <180 μm) compared with both PREP and PA powders. Of particular interest is the <45 μm PSD range as it is ideally suited for metal injection molding (MIM) applications. Unlike both PREP and PA powders, GA powders can exhibit enhanced levels of particle interactions during free fall and solidification, resulting in the formation of numerous satellites and agglomerates, as shown in Figures 10 and 11. GA powders do, however, exhibit other technical limitations. During melting, either by the induction drip or the conventional melting process, contamination by fine, high-temperature ceramic particles can occur; the particles originate either from the coating on the induction-heating coils or from the spraying nozzle.6 In particular, yttrium is considered a GA powder contaminant and in Ti-6Al4V specifications such as ASTM F1580 (powder) and ASTM F1472 (wrought) this element is limited to 0.005 w/o max. Another technical limitation of GA powders is argon-gas entrapment. Under certain process conditions argon can cause thermally induced porosity (TIP) in the final component. This limitation is particularly important with coarse GA powders where the mean particle diameter is >75 μm.7 MELTLESS TITANIUM POWDERS All of the previously described titanium powder manufacturing processes originate from the conventional titanium sponge production route. The commercial production of titanium sponge involves the chlorination of natural and synthetically produced rutile (TiO2) in the presence of carbon to produce TiCl4, as represented by Equation 1. The titanium sponge is then produced by the reaction of either sodium or magnesium with the titanium tetrachloride (TiCl4). TiO2(s) + 2Cl2(g) + 2C(s) TiCl4(g) + 2CO(g) (1) This batch process results in titanium sponge contaminated with iron from the walls of the reaction vessel. It has long been the goal to develop a scalable alternative process that produces pure titanium directly and continuously from titanium precursors such as pigment-grade TiCl4 or pure TiO2
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the processes that are most advanced in terms of production quantities and that have recently received the most commercial attention.
Figure 12. Image of titanium powder from Armstrong process. SEM
produced via the sulphate route. Other titanium bearing precursors include fluotitanate salts, titanium sub-oxides/carbide mixtures, and heavier titanium tetrahalides. Twentythree different processes have been investigated to produce primary titanium, either as powder, sponge, or ingot.8 These precursors can be reduced either by electrolysis or metallothermic reduction with an alkali metal, alkali earth metal, aluminum, or hydrogen. After the production of pure titanium, the material must be treated to remove residual salts and other contaminants. Several of these processes have also been developed to produce titanium alloys by the addition of the corresponding alloying element chlorides or oxides for co-reduction into the final alloy. For the purpose of this review we will consider only two of
Armstrong Process The Armstrong process is a further development of the Hunter process, designed to run continuously to produce titanium powder. Gaseous TiCl4 is injected into a liquid stream of flowing sodium. The sodium reduces the TiCl4 into titanium metal plus sodium chloride. This mixture is then filtered and distilled to remove the excess sodium. The sodium chloride is removed by washing and drying to produce the titanium powder. Figure 12 shows the resulting titanium sponge powder morphology. The powder is also characterized by its low bulk density. The Armstrong process has also been used to produce prealloyed Ti-6Al-4V powder by the simultaneous injection of TiCl4, AlCl3, and VCl4 into the reducing sodium stream. In March 2010, International Titanium Powder (ITP) LLC announced plans to construct a commercial-scale plant, using the Armstrong process to target an annual titanium powder capacity of 1.82 M kg (4 M lb.). Production is expected to begin late in 2010 and reach full capacity in 2011. Previously in 2009 ITP entered into a partnership under the Title III program of the Defense Production Act to fund the building of a commercial-scale plant.9 Fray, Farthing and Chen (FFC) Cambridge Process In the FFC Cambridge process, fine TiO2 powder is pressed into solid thin-walled rectangular or circular-shaped cathodes that are immersed in a
Figure 13. FFC Cambridge Process—schematic. Courtesy of Metalysis Ltd
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Figure 14. Representative morphology of titanium powder produced from FFC Cambridge process. SEM. Courtesy of Metalysis Ltd
molten calcium chloride electrolytic cell at temperatures 800°C–1,000°C. When a 3.3V cathodic potential is applied, oxygen anions are transported through the electrolyte and are liberated at the graphite anode either as gaseous oxygen, carbon monoxide, and/or carbon dioxide, thus reducing the TiO2 into titanium metal. Although 3.3V is the theoretical minimum voltage needed for this electrolytic reaction to proceed, in practice higher voltages are employed. Figure 13 shows a schematic representation of the FFC Cambridge process, while Figure 14 illustrates the resulting titanium powder morphology and particle sizes.10 The FFC Cambridge process results in high calcium residues in the range of 1,500 ppm. This powder production route is currently being scaled up into a semi-continuous process by Metalysis Ltd, with the aim of producing hundreds of kg of titanium in early 2011.11 KEY POWDER-COST DRIVERS This review has outlined the main commercial titanium powder manufacturing processes. Currently titanium sponge fines offer the lowest manufacturing costs, since they are a by-product of the existing and well-established titanium sponge supply industry; the price range is typically $11/kg–$33/kg ($5/lb.–$15/lb.). PREP and PA powders reflect the highest manufacturing costs due to high raw-material (bar/wire) costs and low production throughput. These powders typically cost in the range of $407/kg–$1,210/kg ($185/lb.–$550/lb.) depending on alloy composition. GA powders are more widely available compared with PREP and PA powders and will normalVolume 46, Issue 5, 2010 International Journal of Powder Metallurgy
ly cost in the region of $165/kg–$330/kg ($75/lb.–$150/lb.) in high volumes, depending on the alloy composition.12 The HDH powder process can utilize a wider range of raw-material feedstock and supports the highest batch throughputs; HDH powder costs typically range $66/kg–$176/kg ($30/lb.–$80/lb.), depending on alloy composition. As the demand for titanium and titanium alloy powder continues to increase, the demand for lower cost “meltless” titanium powders will also increase. When these alternative powders become readily available in high volumes, and can be used to manufacture titanium components, the cost expectation will be $33/kg–$88/kg ($15/lb.– $40/lb.), depending on alloy composition. SUMMARY Titanium powders are continuing to find increasing new applications as near-shape and near-net-shape PM production methods continue to be developed and optimized. Titanium PM offers the opportunity to manufacture complex shapes more cost effectively than by conventional wrought metallurgy and can also be incorporated into metal matrix composites. As outlined in this review there are many different types of titanium powders; however, the number of suppliers is limited due to the requirement for specialized dedicated manufacturing equipment. In many cases the titanium alloy raw materials originate from the within the wrought-titanium industry, which focuses primarily on higher-cost aerospace-grade titanium alloy products. Also, not all of the different types of titanium powders included here are suitable for every PM production method, due to the inherent differences in powder morphology, available PSD, or purity. As the demand for titanium PM parts continues to expand, titanium powder users will evaluate other ways to lower production costs either by migrating to lower-cost titanium powder types or by blending different titanium powder types in order to optimize powder performance. In the future we will also see the emergence of new “meltless” titanium alloy powders, which will also offer the potential to expand the titanium PM industry. ACKNOWLEDGEMENTS The authors thank David Heraud, Tekna Advanced Materials Inc., and Luck England, Metalysis Ltd, for providing images of their titanium powders.
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REFERENCES 1. J.W. Kroll, “How Commercial Titanium And Zirconium Were Born”, Journal of The Franklin Institute, 1955, vol. 260, no. 3, pp. 179–183. 2. C.G. McCracken, “Production of Fine Titanium Powders via the Hydride–Dehydride (HDH) Process”, Powder Injection Moulding International, June 2008, vol. 2, no. 2, pp. 55–57. 3. S. Zhang, “Hydrogenation Behavior, Microstructure and Hydrogen Treatment For Titanium Alloys”, Progress in Hydrogen Treatment of Materials, International Association for Hydrogen Energy, edited by V.A. Goltsov, Donetsk State Tech. University, Donetsk, Ukraine, 2001, pp. 281–298. 4. P.R. Roberts, Ja.J. Airey, Jo.J. Airey, and J.E. Blout, “Method and Apparatus For Producing Fine Metal Powder”, U.S. Patent No. 4,824,478, April 25, 1989. 5. D. King, “Ultra-Clean Metal Powders for High Performance Applications”, Advanced Specialty Metals, www.asmpowders.com 6. R. Gerling and F.P. Schimansky, “Crucible- and CeramicFree Melting and Atomization of Ti-Based Alloys”, Powder Manufacturing and Processing, EURO PM2004, European Powder Metallurgy Association, Shrewsbury, UK, 2004. 7. U. Ackelid and M. Svensson, “Additive Manufacturing of Dense Metal Parts by Electron Beam Melting”, Novel Sintering Approaches, EURO PM2009, European Powder Metallurgy Association, Shrewsbury, UK, 2009. 8. D.S. van Vuuren, “A Critical Evaluation of Processes to Produce Primary Titanium”, The Journal of The South African Institute of Mining and Metallurgy, 2009, vol. 109, pp. 455–461. 9. F. Haflinch, “4M-lb. Titanium Powder Plant Planned in Illinois”, American Metal Market, http://amm.com/201003-29_16-24-08.html. 10. M. Bertolini, L. Shaw, K. Rao and L. England, “The FFC Cambridge Process for Production of Low Cost Titanium and Titanium Powders”, Proceedings of TITANIUM 2009, International Titanium Association, Broomfield, CO, 2009, on CD. 11. M. Bertolini, L. Shaw, K. Rao and L. England, “The FFC Cambridge Process for Production of Low Cost Titanium and Titanium Powders”, TMS 2010, Warrendale, PA, 2010, oral presentation. 12. Titanium Information Group, “Titanium Powder Suppliers & Processors,” Data Sheet No. 16, http://www.titanium infogroup.co.uk/documents/technical/40.pdf. ijpm
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Serving the world of powder metallurgy MPIF is the world’s largest PM trade association, growing the powder metallurgy industry for the benefit of its member companies and the greater PM industry through: • Annual PowderMet conference & trade exhibition • Educational seminars and e-learning courses • Development and promulgation of standards • Market development and public relations • The world’s most extensive PM publications department • Dissemination of information via a family of Web sites SINCE 1944, THE VOICE OF PM Metal Powder Industries Federation 105 College Road East Princeton, New Jersey 08540-6692 U.S.A. E-mail:
[email protected] • Web site: mpif.org VISIT ANY OTHER OF OUR FAMILY OF WEB SITES:
apmiinternational.org APMI International— A professional society
pickpm.com An information resource for PM designers & engineers
mimaweb.org Home of the Metal Injection Molding Association
pmdatabase.com A free online Global PM Property Database
cpmtweb.org Home of the Center for Powder Metallurgy Technology
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PM TITANIUM
COLD COMPACTION AND SINTERING OF TITANIUM AND ITS ALLOYS FOR NEAR-NET-SHAPE OR PREFORM FABRICATION Ma Qian*
INTRODUCTION Titanium and its alloys are advanced structural materials that possess an outstanding array of properties not readily achievable with other materials. However, structural applications have been limited to a few industries such as aerospace, chemical processing, power generation, and offshore drilling on a limited scale (2009 world titanium sponge output ~115,000 mt). The major reason has been their costaffordability. The high cost of a component arises from the cost of titanium as well as that of the manufacturing process, which in many cases determines the consumption level. Historically, titanium components are machined from wrought stock, with an average material utilization factor of 10%–15%.1,2 For example, aerospace parts makers currently buy about eight times as much titanium as is needed for the finished part utilizing ingot metallurgy and forging.3 The target set by Lockheed Martin for the F-35 joint strike fighter (JSF) program is to reduce this buy-to-fly ratio to 5:1,2 which still results in 80% scrap. The high material losses, coupled with machining costs and expensive starting stock, provide an attractive economic motive for near-net- or even net-shape processes.1 There are also compelling processing advantages as titanium is difficult to machine and not easy to recycle by remelting. Also, unlike other metals, molten titanium reacts with most gas atmospheres and with most metallic and non-metallic materials that would otherwise serve as crucible materials in melting. It is also challenging to maintain liquidmetal flow over severe changes in dimension or direction within the mold cavity.4 The skull technique, which operates through the maintenance of a solid layer of titanium between the crucible and molten metal, requires careful control of the melting process, where the arc is directed to the center of the charge to maintain the correct temperature gradient between the molten titanium and the crucible walls. To prevent atmospheric contamination, an argon atmosphere is sustained in the crucible and the mold. Casting, even to a simple shape, is still a
The conventional cold-compaction-and-sinter powder metallurgy (PM) approach offers an attractive solution to the near-net shape or preform fabrication of titanium and its alloys for cost reduction and/or improved chemical homogeneity and refined microstructures. However, the potential of the process is yet to be realized as a viable industrial approach. Kroll first compacted and sintered 14 titanium alloys in 1937, followed by significant efforts from DuPont during the formative years of the titanium industry in the U.S., and persistent efforts from many researchers thereafter. This article outlines a historical account of PM titanium and reviews the major characteristics of the approach for fabrication, including the mechanical properties obtainable from the process. Future directions to realize its commercial potential are recommended with respect to both currently available and emerging titanium powder products.
*Reader in Materials, The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia; E-mail:
[email protected]
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complicated endeavor.4 Conventional shape casting is thus impractical for titanium due to these adverse factors, which are unique to this metal. This reinforces the attractiveness of PM. In addition, there are improved constitutional (enhanced chemical homogeneity) and microstructural (finer grain size) advantages with titanium parts made from powders.5 In principle, a titanium component can be fabricated through a variety of PM techniques. These include cold compaction and sintering, sintering and hot working, direct powder rolling or extrusion, hot pressing and machining, hot isostatic pressing (HIPing), metal injection molding (MIM), and sintering. Some PM processes may be regarded as mainstream while others as second order. The cold-compaction-and-sintering process is technically the simplest and economically the most attractive near-net-shape or preform PM fabrication approach, compatible with non-fatiguecritical applications. This review discusses the major characteristics of the process and cites future directions for anticipated applications. HISTORY The first documented attempts to fabricate titanium alloys from powders were made in 1937 by Kroll,6 who switched from Hunter’s sodium reduction approach to the use of calcium and then to magnesium.7 The magnesium reduction approach allowed Kroll to produce ~0.5 kg batches of powder or sponge fines.7,8 Kroll subsequently compacted and sintered 14 binary titanium alloys in argon, with additions ranging from 2 w/o to 9 w/o. He then hot-rolled the sintered cylinders (19 mm dia.) into strips ~1 mm thick to assess their ductility.6 Kroll presented his magnesium-reduction approach at the 1940 autumn meeting of the Electrochemical Society in Ottawa.7,8 Kroll’s enthusiasm promoted a lengthy R&D program at the U.S. Bureau of Mines to industrialize the magnesium reduction process.7,8 Dean et al.9 were able to produce 7.5 kg batches of powder with a purity sufficient to be consolidated into ductile titanium and published a seminal paper in 1946.9 The results of this program provide an important basis for understanding and exploiting titanium PM. By 1947, the U.S. Bureau of Mines had successfully piloted several important modifications to the original Kroll process and produced 2 mt of titanium sponge.10 In 1948, based on the Bureau’s work, DuPont built the
30
world’s first sponge production plant in the U.S.10 and produced 3 mt of sponge (>99% pure) in the same year.11 This, together with the 1st Titanium Symposium in Washington, D.C., in 1948, which attracted 200 industrial, technical, government, and military leaders, marked “the birth of titanium in the United States as a metal of industrial stature.”7,10 Commercial production of titanium sponge followed suit in the UK, Japan, and the U.S.S.R in 1951,12 1952,13 and 1954,14 respectively, and titanium emerged as a tonnage structural metal in these countries. Following Dean et al.,9 Bickerdike and Sutcliffe15 produced titanium powders in 100 g batch sizes in the UK and fabricated dense bars by sintering the powder compacts in vacuum at 1,200°C for 16 h. A tensile elongation up to 33.2% was attained at room temperature after cold hammering the sintered material to essentially its pore-free density (4.49 g/cm3), followed by 2 h annealing at 800°C. Close attention was given to the influence of impurities, in particular carbon, iron, oxygen, nickel, and silicon. Goetzel16 reviewed titanium PM up to 1950. A wide variety of binary alloys were sintered that contained up to 20 w/o cobalt, chromium, molybdenum, tantalum, or tungsten, up to 5 w/o of aluminum, magnesium, or vanadium, and up to 1 w/o boron, beryllium, indium, or silicon. The sintered alloys were cold/hot worked or swaged under various conditions, and tested in both the worked and annealed states for tensile properties, hardness, and electrical resistivity. A multiyear R&D program focusing on the processing of titanium powder directly to commercialgrade mill products was undertaken by DuPont from 1950 to 1962. Using a commercial-size facility, mature technologies were developed throughout the 1950s to make sheet, bar, tubing, and other shapes by the direct powder rolling or extrusion of hydrostatically compacted billets, followed by sintering and annealing as necessary.10 Highquality products with superior surface finish were produced and a tonnage PM industry looked promising. The near-net-shape PM route for mill products was initiated to achieve cost reductions by negating the melting operation. Unfortunately, DuPont curtailed the activity in 1962 because of an unexpected technical issue;10 it was found that the residual chlorides in the mill products originating from the titanium powder volatilized rapidly during fusion welding, leading to a build-up on Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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the welding electrode that resulted in inconsistent weld quality.10 Consequently, the company concluded that their products would not be competitive vis-à-vis melted and wrought products.10 It was likely that DuPont’s products were adequate for some applications. To avoid the chlorineinduced weldability issue, the chlorine content needs to be <50 ppm, which, at that time, was judged to be impractical or too costly.10 Along with DuPont’s direct powder-conversion efforts, several cold-compacted-and-sintered commercially pure (CP) titanium PM parts were introduced to the market on a commercial basis by 1956. Examples included bearing housings, valve trims, and reverts, with some parts reportedly weighing up to 2.7 kg.17 It was clear to the titanium community that, at that time, CP titanium PM parts could be fabricated with properties equivalent to those of materials forged from ingots. In addition, the advantages of PM over arc-melting and forging were appreciated in product forms as a result of these developments. For example, it was established that to make a bearing housing of one pound by arc-melting and forging would require 3.6–4.5 kg of sponge while PM methods would require only 0.86 kg of starting material, or even less.17 The resulting end-cost savings were 20%–25%. These practical evaluations encouraged continued interest in titanium PM, despite a few severe supply-and-demand swings beginning in the late 1950s.10 A large number of titanium PM parts were fabricated from blended elemental (BE) or prealloyed powders by the mid-1980s.1,5,18,19 Additionally, the static and dynamic properties of various PM versions of Ti-6Al-4V were evaluated,1,18,19 reflecting different levels of residual chlorides. The results of these efforts confirmed that the use of quality powder allows for most of the mechanical properties of Ti-6Al-4V PM to equate with those of the cast and wrought levels,1,18,19 with the exception of fatigue properties. Other alloys were similarly fabricated via different PM methods and evaluated.18,19 Thus, by the mid-1980s titanium PM was established technically from a R&D perspective after almost four decades of persistent efforts, although industrialization remained sluggish. Notable industrial advances were made by the Toyota Central R&D Laboratories, Inc., from 1989 to 1998,20 and initiated PM titanium metal matrix composite (MMC) design in 1989; the Ti-TiB system was identified in 1990. After several years of Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
development, Toyota released a new mass-produced family car, the Altezza, in 1998 with Ti-TiB intake and exhaust engine valves.21 In 2000, the Yamaha Motor Co. followed suit and released a new mass-produced motorbike with similar Ti-TiB engine valves.22 Manufacture of these Ti-TiB engine valves starts with conventional cold compaction and sintering to fabricate Ti-TiB MMC billets from BE powder mixes. These billet preforms are then extruded and forged into near-net-shaped engine valves and finished by machining.21 Toyota has been manufacturing these PM-based titanium engine valves commercially since 2000 and currently produces ~10,000 pieces/month for Yamaha motorcycles and Toyota sports cars.22 This is the first tonnage application of titanium PM since Kroll. With improvements to the Kroll process and developments in melting techniques, the early 1990s saw the shutdown of two major Hunter sponge plants: RMI closed its Hunter sponge plant in the U.S. in 1992 and Deeside Titanium followed suit in 1993.23 Since then Hunter sponge has been essentially out of the market for structural applications and various powder products are now made from Kroll sponge. Honeywell remains a major Hunter sponge producer, with a facility in Salt Lake City that has a capacity of ~340 mt annually.24 The Hunter sponge produced by Honeywell is refined electrolytically to 99.999% purity and is the source of metal in integrated circuits.25 Several novel production processes have been under development over the last decade that produce spongy titanium powders as direct products.26 Of these, the Armstrong process is scheduled for commercial production in September 2010 with a subsequent scale-up to 2,000 mt of titanium powder annually in 2011.27 These novel powder products have the potential to enhance applications if they turn out to be competitive in the market. COLD COMPACTION The methods for compacting titanium powder are similar to those used for other ductile powders. In fact, high-purity titanium in the most ductile state is similar to annealed copper in terms of elastic modulus, hardness, elongation, and ultimate tensile strength. Table I lists property data for high-purity titanium, annealed copper, and iron. However, the properties of titanium are sensitive to the impurity level, in particular to nitrogen,
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oxygen, carbon, and iron. Nitrogen and oxygen originate from magnesium, TiCl4, argon gas, and air during sponge production while carbon and iron originate primarily from magnesium, TiCl4, the steel reactor vessels, and other processing facilities.28 Hardness is a convenient measure of the quality of a titanium sponge product and the influence of 15 common impurities on the hardness of titanium has been established. Sumitomo’s high-purity powder products (TILOP-45H) specify 14 impurities, Table III. Statistical analyses suggest that the Brinell hardness (HB) of titanium sponge obeys an approximate relationship with impurity levels given by:11,28 ______ ______ ______ HB =______ 196√w/o N + 158√w/o O + 45√w/o C + 20√w/o Fe + 57 (1) The relationship presumably applies to powder products that are made from the sponge. Highpurity iodide titanium produced by the de Boer–Fast approach7 from titanium sponge typically contains <0.1 w/o impurities, with each impurity falling in specific ranges: 0.007–0.008 w/o N, <0.05 w/o O, 0.005 w/o C, 0.002–0.006 w/o Fe, 0.003–0.009 w/o Mg, 0.001–0.0075 w/o Al, 0.0006–0.001 w/o Si, 0.001–0.005 w/o Mn, 0.0001–0.0004 w/o Ni, and <0.0001 w/o Cr.28 As with the green-shape formation of other metals, small parts can be compacted uniaxially in closed steel dies using standard presses. For large and complex parts, however, cold isostatic pressing (CIPing) may have to be used,29 in which process pressure is applied from multiple directions permitting increased green-shape-making capability with enhanced uniformity in green density. Figure 1 shows the relationship between compaction pressure (up to ~1,400 MPa) and the attendant green density for CP titanium powder TABLE I. PROPERTY DATA FOR HIGH-PURITY TITANIUM, ANNEALED COPPER, AND IRON* Property Brinell Hardness Vickers Hardness Yield Strength (MPa) Ultimate Tensile Strength (MPa) Elongation (%) Modulus of Elasticity (GPa)
Metal Ti
Cu
Fe
70 60 140 220 54 116
— 50 33.3 210 60 110
146 150 50 540 — 200
*Matweb Materials Property Data, http://www/matweb.com
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Figure 1. Pressing characteristics of sponge titanium powder (-595 μm with <15% -74 μm).9 Redrawn with permission from The Minerals, Metals & Materials Society (TMS)
products made from the Kroll process.9 The oxygen and nitrogen contents were estimated to be <0.1 w/o each, based on hardness.9 The increase in green density with compaction pressure is rapid up to 690 MPa and slows down thereafter. At 690 MPa, the powder compression ratio is ~3.5 to 1, resulting in >80% of the pore-free density.9 Figures 2 and 3 show the influence of nitrogen and oxygen on the compaction of CP titanium powder, plotted using data reported by Lim et al.30 Increasing nitrogen or oxygen levels lead to a significant reduction in green density because of the increased hardness of the powder. Nevertheless, the powder is still compactable with 1.51 w/o N or 1.34 w/o O. However, significant particle cracking was observed at high nitrogen or oxygen levels during compaction, rather than appreciable plastic deformation.30 Nitrogen showed a stronger effect than oxygen, consistent with Eq. (1). Owing to its sensitivity to oxygen and nitrogen, unlike other metal powders, the particle size of titanium powder may exert a unique effect on powder compaction since the propensity for picking up oxygen and nitrogen at temperatures ≥300°C increases with decreasing particle size. Powder morphology affects powder flow, tap density, and compaction behavior;29,31 and plays an important role in the consolidation process and parameter selection. Imam and Froes32 recently cited nine different titanium powder morphologies resulting from various processes, and a timeline for the development of processes to produce various types of titanium powder is given in Ref. 31. Of Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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Figure 2. Effect of nitrogen content on compaction of titanium powder at a compaction pressure of 927 MPa.30 Sumitomo hydride–dehydride TSP-100 powder was nitrided in pure nitrogen to increase nitrogen content. As-received powder particles are irregular in shape with a mean particle size 53 μm. Figures in brackets are oxygen contents of the powders
Figure 3. Effect of oxygen content on compaction of titanium powder at a compaction pressure of 927 MPa.30 Sumitomo hydride–dehydride TSP-100 powder was oxidized in air to increase oxygen content. As-received powder particles are irregular in shape with a mean particle size 53 μm. Figures in brackets are nitrogen contents of the powders
Figure 4. (a) dendritic “coral-like” titanium powder produced by International Titanium Powder (Armstrong process), (b) green density and sintered density (1 h at 1,300ºC) as a function of compaction pressure (Armstrong powder). Courtesy of Brian Fuller, International Titanium Powder
these, the Armstrong powder is a novel product that has a dendritic “coral-like” morphology, Figure 4(a). The powder appears to require the use of a high compaction pressure (≥1,100 MPa) to attain >80% of the pore-free density, Figure 4(b); this compares with a 600–800 MPa compactionpressure range for sponge fines or HDH powder. Compacts made from Armstrong powder are expected to exhibit excellent green strength because of strong particle interlocking. Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
The compaction behavior of a titanium-alloy powder mix is, in general, determined by the basemetal powder but may be affected by the form of the alloying additions. It has been found that blended elemental powder mixes are easier to press than mixes with master alloy powder additions.33 Brittle titanium hydride (TiH2) powder is harder than CP titanium powder but it can be readily pressed to >80% of the pore-free density, since the TiH2 powder particles fracture into finer
33
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COLD COMPACTION AND SINTERING OF TITANIUM AND ITS ALLOYS FOR NEAR-NET-SHAPE OR PREFORM FABRICATION
pieces at pressures >250 MPa.33 The high green density obtainable from cold pressing various types of titanium powder ensures adequate green strength, and therefore facilitates safe and rapid ejection from the die. Unlike aluminum,34 titanium powder exhibits minimal seizing and galling of steel dies when compacted. Internal lubrication or powder lubrication is thus not used for titanium PM, in order to avoid interfering with the subsequent sintering process as well as contamination.5,35 However, excessive friction during pressing may lead to an inhomogeneous green-density distribution, increased ejection forces and reduced die life.36 A recent assessment of the effect of powder lubrication on the cold compaction of CP titanium powder showed that lubrication has a distinct effect on both the frictional properties of the powder as well as the ability to achieve desired green densities.37 In contrast to powder lubrication, die-wall lubrication with commercial PM lubricants such as zinc and lithium stearates or Acrawax is generally acceptable in reducing both die wear and ejection forces. CONVENTIONAL SINTERING Titanium and its alloys are unique in their suitability for sintering.10 This is because the thin oxide and/or nitride layers on the powder surfaces can readily dissolve above 500°C–550°C10,15 or 700°C.28,38 Oxygen and nitrogen have different solubility limits in α-titanium and β-titanium. Table II summarizes solubility data extracted from binary-phase diagrams.39,40 In the sintering temperature range (1,200°C–1,350°C), the solubility of oxygen in β-titanium varies from 1.75 w/o to 2.25 w/o while that of nitrogen varies from 0.5 w/o to 0.9 w/o. Both oxygen and nitrogen increase the c value of the α-titanium lattice but the a value remains essentially constant.28 Reactions between titanium and oxygen or nitrogen are slow below 500°C but accelerate significantly above 500°C.28 In the case of titanium oxide, early work indicatTABLE II. SOLUBILITY LIMITS OF OXYGEN AND NITROGEN IN α-TITANIUM AND β-TITANIUM (ESTIMATED FROM Ti-O AND Ti-N PHASE DIAGRAMS39,40) Maximum Solubility Solubility in β-Titanium in β-Titanium from 1,200°C to 1,350°C (w/o) (w/o)
Element
Maximum Solubility in α-Titanium (w/o)
Oxygen
14.25 at ~600°C
2.75 at ~1,720°C
1.75–2.25
Nitrogen
7.6 at ~1,083°C
2.1 at ~1,995°C
0.5–0.9
34
ed that the oxide film on titanium powder disappears around 550°C in α-titanium.15 Subsequent work by Watanabe and Horikoshi41 revealed that it took about 60 min for the oxide film to disappear on loose titanium powder surfaces at 1,000°C (β phase). Mo et al.28 pointed out that significant dissolution of the oxide film in titanium began at about 700°C. The diffusion coefficient of oxygen in titanium is a few orders of magnitude faster than in TiO2 in both the α- and βphase regions.41 Hence, the oxide film may disappear in either region, depending on the heating rate. The solubility of oxygen in α-titanium is, however, several times greater than that in β-titanium over a wide temperature range (oxygen can stabilize α-titanium up to ~1,885°C).39 In consequence, no oxide or nitride barrier will exist at the isothermal sintering temperature (1,200°C– 1,350°C). Solid-state sintering of titanium powder compacts is thus essentially an interparticle-diffusion-bonding process.10 It should be noted that diffusion bonding is a commercially viable fabrication technique for titanium while it is not commonly used for other structural metals as the persistent oxide films on these metals prevent diffusion. Hollow fan or compressor blades used in aero engines are fabricated via the diffusion bonding of titanium sheets at Rolls-Royce.42 Because of the self-cleaning process that occurs on the surfaces of titanium powder particles during heating, the sintering of titanium is fundamentally a straightforward process. In practice, challenges encountered originate from either the impurities in the powder or the reactivity of titanium at elevated temperatures. Sintering Atmosphere The first sintering trials by Kroll in 1937 were carried out in argon at 6,667 Pa pressure.6 Subsequent work by Dean and co-workers9 concluded that sintering of sponge fines requires the use of a vacuum ~10-2 Pa to remove the hydrogen absorbed during leaching, distill residual magnesium, and protect titanium from oxygen and nitrogen. Helium protects the metal from oxygen and nitrogen but does not permit hydrogen and magnesium to be effectively removed.9 The various phenomena observed during the vacuum sintering of sponge fines are shown in Figure 5. Hydrogen can be removed above 600°C while the effective removal of magnesium requires temperatures >1,000°C. A thorough removal of the Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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and the oxygen and nitrogen contents after sintering in argon using an Oxynon furnace were lower.45 This highlights the difference between sintering in argon and vacuum. An in-depth understanding of the sintering behavior of titanium powder in argon compared with vacuum sintering is needed in relation to the influence of common impurities.
Figure 5. Effect of sintering temperature on weight loss, void space, hydrogen evolution and hardness. Samples were made from sponge powder with a maximum particle size of 595 μm and ≤15%–20% 75 μm particles compacted at 690 MPa.9 Redrawn with permission from The Minerals, Metals & Materials Society (TMS)
volatiles is essential to attain full densification. The removal of hydrogen, magnesium, and other volatiles gives rise to a weight loss. Although vacuum sintering is preferred from a densification perspective, it is a batch process. In addition, leaks at high sintering temperatures (~1,300°C) may occur unexpectedly leading to rejection of the entire workload. Thus production sintering of titanium and its alloys has been performed in argon in a continuous-belt sintering furnace. Examples are the sintering of titanium mill products in the 1950s10 and the sintering of Ti-TiB preforms since 2000.22 To ensure adequate protection against oxidation in argon, commercially pure argon must be further purified before entering the hot zone. This may be realized by allowing the pure argon to pass over heated titanium chips (800°C–1,000°C) or to pass through a separate tube containing titanium sponge preheated to a similar temperature.43 Alternatively, the oxygen partial pressure can be controlled through the use of a carbon-fiber belt and/or graphite walls in the furnace. The Oxynon furnace44 for hightemperature sintering (≥1,100°C) is one such example. It has been used for the sintering of titanium in argon since 2002. A recent comparison of the sintering behavior of three different CP Ti powders in argon and vacuum revealed that sintering in argon resulted in much lower tensile properties than did sintering in vacuum, although the sintered densities were similar Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Racking Materials for Sintering Titanium reacts with most materials at high sintering temperatures. As a result, it adheres to most support materials, resulting in contamination of the alloy and adhesion to the support. Therefore, an appropriate support material must be used for the sintering of titanium and its alloys. Unfortunately, options are limited to only two materials, namely, molybdenum, because of its low solubility in titanium and its ability to retain strength at sintering temperatures, and Y2O3 because of its inertness to titanium. In most cases, the use of Y2O3 plates or a Y2O3 coating is compatible with the solid-state sintering of titanium and its alloys. Liquid-phase sintering of titanium alloys can be more problematic in terms of the support material. A detailed assessment of the lining material used in the molten-metal pour tubes for gas atomization of titanium confirmed that Y2O3 was the most promising material.46 Even so, significant contamination of the powder by yttrium occurred, which exceeded the limit (50 ppm) required by ASTM F1472 for surgical implant applications. Accordingly, the use of Y2O3 has been excluded in the processing of titanium and its alloys for implant applications.46 This remains as a challenging issue. At this time, no other options are available as an alternative material to Y2O3. Sintering of CP Titanium As noted previously, the solid-state sintering of titanium is essentially a diffusion-bonding process. The hardness profile shown in Figure 5 implies that the sintering of CP titanium starts to develop at ~700°C in the α-phase region. Dilatometric studies of the sintering of titanium– nickel alloys show similar observations47,48 and confirm that oxide films on titanium powder surfaces do not need to be reduced by the atmosphere or disrupted by a chemical additive.35 Since diffusion bonding develops only when the oxide films have disappeared, the results generally support the conclusion that significant dissolution of the
35
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oxide films in titanium occurs at ~700°C.18,28 The other dissolution temperature invoked (500°C),15 appears to be too low. Isothermal sintering of CP titanium is normally conducted between 1,250°C and 1,350°C for 2 to 4 h. Sintered densities ~95% of the pore-free level are not difficult to achieve but levels ≥98% will require the use of fine powder (<45 µm) and/or a high compaction pressure.31,37,49 As in the sintering of any powder, a reduction in particle size increases the sintering rate and sintered density of titanium and its alloys because of the higher driving force and smaller pores present in the green state. Recent trials using titanium hydride (TiH2) powder rather than CP titanium powder to produce CP titanium by sintering have shown encouraging results.50 The use of TiH2 powder (<44 µm, compacted at 700 MPa) resulted in ≥99% of the pore-free density after 4 h at 1,200°C.50 This is a significant result compared with the sintering of CP titanium powder at 1,200°C which normally results 95%–96% of the pore-free density (<44 µm, compacted at 700 MPa). The activation energy for the self-diffusion of α-titanium ranges from 169 to 192 kJ mol-151 and from 131 to 328 kJ mol-1 for the self-diffusion of β-titanium.47 There is an increase in volume (5.5%) corresponding to the α → β transformation.28 In addition, titanium exhibits diffusional anisotropy over the α-titanium range, and it shows anomalous Arrhenius behavior in the β-titanium range.52,53 As a result, attempts have been made to take advantage of the α → β transformation to
enhance the sintering of titanium, for example, by cyclically heating the powder compacts around the transformation temperature 882°C.54 No clear consensus has resulted from these studies. Residual chlorides are known to have an important influence on the sintering of CP titanium.55,56 It has been found that even hot forging is unable to eliminate porosity in sintered compacts which contain chlorides at levels as low as 100 ppm.29 After forging, the pores become lenticular, resulting in an increase in stress intensity leading to lower tensile ductility and fatigue strength.29 The use of ultra-low-chloride grade titanium powder appears to be critical for full densification by solid-state sintering. Table III lists the specifications for five ultralow-chloride grade titanium powder products. Sintering of Ti-6Al-4V and Other Alloys Both prealloyed and blended elemental methods can be used to fabricate a titanium alloy. However, prealloyed powders are typically difficult to press due to their hardness. For example, in order to compact <149 µm prealloyed Ti-6Al-4V powder to 84% of the pore-free density, a compaction pressure of 965 MPa is necessary; this is essentially the yield strength of Ti-6Al-4V.49 In comparison, to compact <149 µm blended elemental powder to the same green density requires only 413 MPa.49 In addition, sintering of prealloyed powders requires higher sintering temperatures due to the absence of the positive effects arising from the diffusion of alloying elements.49 The conventional PM cold-compaction-and-sintering approach is thus not the best option for prealloyed powders. Hot
TABLE III. COMMERCIAL ULTRA-LOW-CHLORIDE TITANIUM POWDER PRODUCTS57,58 Product
w/o (minimum)
Particle Size (μm)
Chemical Components (ppm, maximum)
Ti
Fe
Cl
Mn
Mg
Si
Ni
Cr
Al
Na
K
N
C
H
O
Sumitomo TILOP-150H
99.98
40
10
5
10
5
10
5
5
0.1
0.1
200
200
100
800
-150
Sumitomo TILOP-45H
99.98
50
10
5
10
5
10
5
5
0.1
0.1
200
200
100
1,300
-150
Toho TC-150
_
500
30
100
10
200
_
_
_
_
_
200
200
300
1,500
-150
Toho TC-450
_
500
30
100
10
200
_
_
_
_
_
200
200
600
3,500
-45
Toho TC-150
_
500
30
100
10
200
_
_
_
_
_
200
200
700
3,500
-20
36
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isostatic pressing (HIPing), hot pressing (HP), or spark plasma sintering (SPS) are more suited to consolidating prealloyed powders. The blended elemental approach produces titanium alloys from mixtures of titanium and a master alloy powder (e.g., 60Al-40V), which can be cold compacted and then sintered. Control of the process variables can result in sintered densities ranging from 95% of the pore-free level to 99%.1 The approach is capable of producing acceptable mechanical properties. Table IV summarizes selected literature data on cold-compacted-andsintered Ti-6Al-4V from blended elemental powder mixes. In general, the tensile properties of powder compacts sintered to ≥97% of the pore-free density can meet the wrought specification. The chlorine levels in the titanium powders are also cited. Acceptable tensile properties were attained from titanium powder mixes containing chlorine at levels as high as 0.27 w/o.52 While mechanical properties are important, the main driving force in titanium R&D has been cost reduction.29,32 To reduce the high cost of CP titanium powder, the use of TiH2 powder has been
investigated for the fabrication of Ti-6Al4V.33,50,59–61 Table V lists the various powder mixes investigated by Ivasishin et al.,33 and the sintered densities are shown in Figure 6. This approach resulted in high sintered densities (≥98%), regardless of the form of the aluminum and vanadium additions (mix 5, elemental; mix 7, master alloy) while the use of CP titanium powder led to inferior densification (mixes 2, 4, and 6). Recent work by Fang50 has shown that the use of TiH2 powder (<44 µm) and 60Al-40V master alloy powder permits sintering to ≥99% of the pore-free density at 1,200°C for 4 h. Dilatometric experiments revealed that the sintering shrinkage of TiH2 was rapid in the α-titanium range following dehydrogenation and most of the densification occurred at this stage. The shrinkage curves of TiH2-6Al-4V indicate that sintering above the beta transus temperature is necessary in order to achieve full densification and a homogenous microstructure.50 Similarly, recent sintering trials of TiH2-10V-2Fe-3Al (Ti-1023) and TiH2-5Al-5V5Mo-3Cr (Ti-5553) have resulted in high sintered densities (≥97%) with excellent tensile proper-
TABLE IV. DENSITIES AND TENSILE PROPERTIES OF COLD-PRESSED-AND-SINTERED BLENDED ELEMENTAL Ti-6Al-4V Powder
Sintered Density (% PFD)*
0.2% YS (MPa)
UTS (MPa)
Elongation (%)
Oxygen (w/o)
Source
Sponge Fines, <149 μm, 0.13 w/o Cl, 0.13 w/o O; 4 h at 1,260°C
99
860
930
12.5–17
0.24
[1]
Sponge Fines, <149 μm, 0.13 w/o Cl, 0.13 w/o O; 4 h at 1,260°C
97–98
820
890
10–12.5
0.24
[1]
Sponge Fines, <149 μm, 0.13 w/o Cl, 0.13 w/o O; 4 h at 1,260°C
95–96
700
810
5–7.5
0.24
[1]
Crushed Sponge, <149 μm, 0.26 w/o NaCl; 0.1 w/o O
94
737
827
5.0
—
[5]
Sodium-Reduced Ti Powder, <149 μm, 0.18 w/o Cl, 0.096 w/o O
99
847
983
11.4–13.8
—
[59]
Ti Powder, 0.27 w/o Cl; held at 350ºC for 60 min
98.8
—
960
11
—
[77]
CP Ti Powder, <149 μm, 0.15 w/o Cl (estimated)
94
738
827
5.0
0.12
[49]
CP Ti Powder, <149 μm, 0.15 w/o Cl (estimated)
97.5
667
780
10.5
0.07
[49]
Ti Powder, <10 ppm Cl
—
800
910
8.3
—
[64]
ASTM B-348 Wrought Specification
100
830
980
10
≤ 0.2
* Pore-free density Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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TABLE V. POWDER MIX CONDITIONS FOR DATA IN FIGURE 633 Mix
Description
Method of Addition
1
Ti, <100 μm, 1 w/o Impurities, Including 0.29 w/o O
Elemental Powders Al: 98 w/o, -100 μm; V: 99 w/o, <100 μm
2
Ti, <100–200 μm, 0.7 w/o Impurities, Including 0.29 w/o O
Elemental Powders Al: 98 w/o, -100 μm; V: 99 w/o, <100 μm
3
TiH2, <100 μm, 1 w/o Impurities, Including 0.30 w/o O
Elemental Powders Al: 98 w/o, -100 μm; V: 99 w/o, <100 μm
4
Ti, <100 μm, 1 w/o Impurities, Including 0.29 w/o O
Elemental Powders Al: 95 w/o, -20 μm; V: 98 w/o, <40 μm
5
TiH2, <100 μm, 1 w/o Impurities, Including 0.30 w/o O
Elemental Powders Al: 95 w/o, -20 μm; V: 98 w/o, <40 μm
6
Ti, <100 μm, 1 w/o Impurities, Including 0.29 w/o O
Master Alloy Powders Ti-35Al: 98.5 w/o, -100 μm; V-25Al: 98.3 w/o, <100 μm
7
TiH2, <100 μm, 1 w/o Impurities, Including 0.30 w/o O
Master Alloy Powders Ti-35Al: 98.5 w/o, -100 μm; V-25Al: 98.3 w/o, <100 μm
Figure 6. Sintered density vs. compaction pressure for Ti-6Al-4V made from various powder mixes.33 Reprinted with permission from Springer Science + Business Media
ties.62 This is encouraging from a cost-reduction perspective. Two hypotheses have been advanced to explain the sintering of TiH2 powder compacts. The combination of dehydrogenation and densification in one process creates new dehydrided titanium surfaces, which are expected to facilitate diffusion bonding leading to rapid densification and high sintered densities.50 The other possible reason is that the oxide films are reduced by the hydrogen released
38
from the hydride,33,50 thereby enabling sintering to develop in the α-titanium region with a much reduced interparticle diffusion barrier at contact points where local reduction has occurred. The only concern over the use of TiH2 powder appears to be the massive release of hydrogen above 600°C. While achieving full densification is a constant goal in sintering, it should be noted that the grainboundary pinning effect of residual porosity can help to control the sintered microstructure. It has been shown that fully dense BE Ti-6Al-4V exhibits both a coarse beta grain size and large alpha plate dimensions because of the lack of pinning of the pores during sintering, while less dense Ti-6Al-4V displayed a low aspect ratio of the alpha structure.29 In the final stage of sintering, the correlation between porosity (assuming spherical pores), pore size, and grain size can be understood through the relation:63 gdp Vp = ___ RG
(2)
where Vp is the fractional porosity, g is a geometric constant, dp is the pore size, G is the grain size, and R is an attachment ratio parameter that measures the fraction of pores attached to grain boundaries. A large value of R favors a small grain size. For non-fatigue-critical applications, the pinning effect of the pores may offset their adverse effect. A detailed study of the pore surfaces of Ti-6Al-4V at >99% pore-free density after tensile testing and Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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fracture toughness testing showed extrusive slip lines on pore surfaces, suggesting that the pores played a role in accommodating plastic deformation, thereby increasing ductility.1 Apart from Ti-6Al-4V, other commercial alloys have been sintered, including Ti-3Al-2.5V, Ti-6Al6V-2Sn; Ti-6Al-6V-2Fe; Ti-5Al-5Mo-1.5Cr; Ti-5Al2Sn-2Zr-4Cr-4Mo; Ti-6Al-2Sn-4Zr-(2/6)Mo; Ti5Al-5V-5Mo-3Cr (Ti-5553), Ti-10V-2Fe-3Al; Ti-5Al2.5Fe, Ti-11.5Mo-6Zr-4.5Sn,29,35,49,56,59–64 and Ti6Al-7Nb.65 These alloys, including Ti-6Al-4V, are all sintered in the solid state. Of these, the Ti-6Al7Nb alloy has been developed to replace Ti-6Al-4V as a preferred bio-titanium material due to the toxicity of vanadium. The alloy can be fabricated by metal injection molding (MIM) and sintering to densities >97% of the pore-free level and mechanical properties comparable with those of wrought materials.65 To understand the sintering processes in highly alloyed titanium, the sintering of Ti-10V-2Fe-3Al has been investigated in detail over the temperature range of 1,200°C–1,350°C using dilatometry.66 The apparent activation energy for sintering (163±13 kJ/mol) was determined with respect to two different modes of introduction of aluminum and vanadium. It was concluded that the densification of this alloy is basically dictated by the selfdiffusion of β-titanium. NOVEL SINTERING PROCESSES Conventional sintering is versatile but, in most cases, is relatively slow. Spark plasma sintering and microwave sintering are two novel sintering processes that have proven to be effective for the consolidation of titanium powders. Each exhibits distinctive features and has the advantage of being capable of significantly reducing the time of the sintering cycle. Spark Plasma Sintering Spark plasma sintering (SPS) is a pressureassisted pulsed-current process. Because of the combined effects of pressure and the heat generated by the Joule effect from the high pulsed-current density (~107 A m-2, 10 V), powder compacts may be sintered in a short time period, often at a low temperature. Consolidation of CP titanium powder by SPS has demonstrated that the process can lead to rapid and full densification at a relatively low temperature.67,68 Figure 7 shows the relative density as Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
a function of the sintering temperature for CP titanium grade 1 (45 µm) and CP titanium grade 3 (45 µm) powder utilizing SPS in low vacuum (2 Pa).68 The powder was placed in a graphite die and pressed uniaxially at 60 MPa while a pulsed electric current was applied through a graphite punch. Heating rates up to 200°C/min were used and isothermal holding at each temperature was limited to 5 min. The compacts were disks 30 mm dia. × 5 mm thick. Full densification was attained at 950°C for 5 min, and 99% of the pore-free density was achieved after 5 min isothermal holding at 800°C. Because of rapid consolidation, all the sintered samples showed similar carbon, nitrogen, and oxygen contents to those of the starting powder. Figure 8 is a representative optical micrograph of fully dense, fine-grain CP titanium grade 1 consolidated at 950°C for 5 min. The mechanical properties of this material satisfy ASTM standards for CP titanium grade 1 and grade 3.68 Although not as versatile as conventional sintering, SPS is capable of rapidly consolidating a variety of symmetrical shapes such as gears, brake disks, sprockets, clutch pressure plates, and cams, in addition to billet preforms for subsequent hot working.69,70 Also, SPS is compatible with both blended elemental powder mixes and prealloyed powders. It is a novel and viable rapid-consolidation technique for titanium and its alloys in niche market applications. Microwave Sintering The effectiveness of microwave (MW) sintering
Figure 7. Consolidation of CP titanium grade 1 and CP titanium grade 3 powders by SPS as a function of sintering temperature.68 Reprinted with permission from Maney Publishing
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tering produced densification levels comparable with those of conventional vacuum sintering. However, MW radiation delivered a much faster heating rate (34°C/min) than did conventional vacuum heating, leading to a substantial reduction in the sintering cycle. This is attributed to three combined effects:76 (i) heat radiation from the MW susceptors at low temperatures, (ii) enhanced MW absorption due to transformation of the TiO2 film on the titanium powder particle surfaces to oxygen-deficient titanium oxides, which are MW absorbers, and (iii) volumetric heating of the titanium powder particles by eddy currents. More detailed studies are under way to assess the effectiveness of MW sintering of titanium compared with conventional vacuum sintering. Figure 8. Optical micrograph of CP titanium grade 1 powder (45 μm) consolidated by SPS at 900°C for 5 min.68 Reprinted with permission from Maney Publishing
has been demonstrated on many ceramic systems and a number of metallic and metal–ceramic systems but has been unproven for titanium PM until recently. One reason is that titanium is a paramagnetic metal coupling weakly with the magnetic field of the microwaves, which is principally responsible for the MW heating effect in magnetic oxides.71 The effectiveness of MW for the sintering of metal powders was first demonstrated on ferromagnetic metal powders such as cobalt and iron and their alloys.72 Recent work has confirmed that MW radiation is much more effective in heating ferromagnetic iron powders than in heating nonferromagnetic powders such as copper, gold, tin, and titanium.73 Limited evidence suggests that MW sintering is capable of producing titanium parts having a relatively dense core.74 A recent detailed comparative study of the MW sintering and conventional sintering of CP titanium powder compacts in high vacuum confirmed that MW radiation is effective for the consolidation of titanium with the assistance of MW susceptors.75 Green compacts were made from three different types of CP titanium powder under compaction pressures in the range 200 to 800 MPa and sintered at 1,200°C in a multimode cavity 3kW MW furnace operating at 2.45 GHz in high vacuum (HAMilab-HV3, Synotherm, China); the response is compared with that of conventional vacuum sintering, Figure 9. The highest density attained with MW sintering is 96.30% of the porefree density with AEE-titanium powder. Other than the samples compacted at 200 MPa, MW sin-
40
MECHANICAL PROPERTIES AND APPLICATIONS The mechanical properties of cold-compactedand-sintered titanium and its alloys are determined by the sintered density, microstructural uniformity, grain size, and impurity levels. Table VI lists the tensile properties of cold compacted and sintered CP Ti, Ti-3Al-2.5V, and Ti-6Al-6V-2Fe using BE powders and Ti-5Al-5V-5Mo-3Cr and Ti10V-2Fe-3Al using TiH2 powder and master alloy powders. The property data listed in Tables IV and VI show the capabilities of the cold-compactionand-sinter approach. In addition to cold-compacted-and-sintered Ti-6Al-4V, as-sintered Ti-5Al-5V5Mo-3Cr and Ti-10V-2Fe-3Al can also attain high strength (YS ≥940 MPa; UTS ≥1,030 MPa) and ductility (elongation 8%–12%).62 These properties are
Figure 9. Densification of titanium powder compacts by microwave and conventional sintering.75 AEE-Ti: <20 μm; SUMI-Ti: 45–63 μm; CERAC-Ti: 100–150 μm
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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TABLE VI. REPRESENTATIVE PROPERTIES OF PRESS-AND-SINTER CP Ti AND Ti ALLOYS* Description CP Ti CP Ti CP Ti CP Ti Ti-3Al-2.5V Ti-6Al-6V-2Fe Ti-5Al-5V-5Mo-3Cr Ti-5Al-5V-5Mo-3Cr Ti-10V-2Fe-3Al Ti-10V-2Fe-3Al
Density (% PFD)
Oxygen (ppm)
Yield Strength (MPa)
UTS (MPa)
Elongation (%, 25.4 mm)
Reduction in Area (%)
95 98 94 98 97.3 97.8 96 97.2
415 700 1,200 3,000 1,200 1,200 -
224 283 338 483 564 845 966 933 944 939
305 383 427 611 650 963 1,067 1,031 1,033 1,033
24.5 37.1 15.0 11.0 11.5 6.0 10.1 11.3 8.0 12.0
23 30 23 10 14 3.8 16.9 14.9 13.5 19.5
*Elemental powder additions49,56,62 acceptable for a wide range of non-fatigue-critical applications. The as-sintered Ti alloys are known to be inferior to their wrought counterparts under cyclic loading. The fatigue strength of as-sintered Ti-6Al-4V at 99.8% of the pore-free density is at the lower bound for wrought Ti-6Al-4V.29 Other mechanical properties such as fracture toughness and fatigue-crack-growth rate are, however, at a similar level to those of wrought titanium.29 Titanium has four major fields of application: military aerospace, commercial aerospace, industrial, and consumer/other.10,78,79 Unfortunately, the application of near-net-shape titanium parts produced by cold compaction and sintering is virtually zero.32 The only PM tonnage application remains Ti-TiB engine valves, with production at ~10,000 pieces/month.22 Due to their inferior fatigue properties, as-sintered titanium PM parts must focus on less demanding non-aerospace, non-fatigue-critical applications. It is encouraging to note that the worldwide consumption of titanium mill products in the non-aerospace field has now exceeded that for aerospace consumption.79 This should provide increased opportunities and incentives for the future of the titanium PM industry. FUTURE DIRECTIONS Cost reduction has been the primary driving force for titanium R&D. For titanium PM, while significant efforts are underway to develop and produce cost-affordable low-impurity-level titanium powder products, there are other issues that will be important to the future of the titanium PM industry: • Sintering in Argon: High-vacuum sintering is preferred for densification but it is a batch Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
process. Mass-production sintering will need to be carried out in a continuous sintering furnace. Can titanium PM parts be sintered to high densities (>98% pore-free density) in a short period (≤60 min) in argon? At present, little is known as past efforts have focused on vacuum sintering. As a starting point, a detailed assessment of argon sintering vs. high-vacuum sintering is necessary. • Titanium PM Alloys: Current commercial titanium alloys are sinterable. However, the isothermal-sintering process typically requires ~240 min to ensure good densification, which is less competitive for continuous production sintering. Work is needed to limit the isothermal-sintering time to ≤60 min, either through the development of effective sintering aids for existing commercial alloys, or through the design and development of rapid-sinterable titanium PM alloys. • High-Chloride-Content Titanium Powder: CP titanium grade 2 powder products (0.2 w/o O, 0.03 w/o N, 0.02 w/o C; 0.06 w/o Fe, 0.01 w/o Mg, 0.05 w/o Cl) made from Kroll sponge are presently sold at a market price ~$25/kg. High-chloride-content titanium powder products can be produced at a more affordable price as ~80% of the vacuum distillation efforts are to remove the 1–2 w/o MgCl2 deep in the sponge.28 There is a need to develop suitable sintering processes for the fabrication of quality titanium PM parts from high-chloride titanium powders. Progress has been encouraging and cold-compacted-and-sintered Ti-6Al-4V prepared from titanium powder mixes containing >0.36 w/o MgCl2 (0.27
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w/o Cl) attained an ultimate tensile strength of 960 MPa and a tensile elongation of 11%.77 • Novel Sintering Processes: Both SPS and MW have proven to be effective for the consolidation of titanium powder compacts. Both have the potential for niche market applications and should be exploited in near-net-shape or preform fabrication. • Titanium Metal Matrix Composites (MMCs): PM has advantages over other manufacturing approaches in composite design and fabrication. PM-based Ti-TiB engine valves are an encouraging example.21 A significant improvement in stiffness through composite design and fabrication by a PM approach is likely to open new markets for titanium PM. In addition, titanium MMCs have the potential to offer unique functional properties. Titanium PM composites represent an important direction in the future.80 • Titanium PM Applications: Owing to their inferior fatigue properties, titanium PM parts made by conventional cold compaction and sintering should focus on non-aerospace, non-fatigue-critical applications. To enhance titanium PM applications, it is important that property data for as-sintered titanium alloys be reliable and complete so that design engineers will be willing to specify titanium PM parts for potential new applications. Both innovation and coordinated efforts are needed for a viable and sustainable titanium PM industry in the near future. In summary, the attractiveness of titanium PM has been recognized for several decades, but its potential is yet to be realized as an industrial manufacturing approach. The major challenge to be overcome is economic. New titanium powder products from a few emerging meltless production processes may provide an improved basis for further development. Nevertheless, opportunities exist even with currently available titanium powder products made from Kroll sponge. The potential of cold compaction and sintering for near-netshape or preform fabrication will be realized for selective applications. ACKNOWLEDGEMENTS This work is supported by the Australian Research Council (ARC) and the ARC Center of Excellence for Design in Light Metals. The document delivery team at the Dorothy Hill Physical
42
Sciences and Engineering Library of The University of Queensland (UQ) is gratefully acknowledged for the availability of a significant number of the references. Ray Low, UQ, is thanked for drawing the authors attention to Ref. 77. REFERENCES 1. P.J. Anderson, V.M. Svoyatytsky, F.H. Froes, Y. Mahajan and D. Eylon, “Fracture Behavior of Blended Elemental P/M Titanium Alloys”, Modern Developments in Power Metallurgy, edited by H. Hausner, H.W. Antes and G.D. Smith, Metal Powder Industries Federation, Princeton, NJ, 1981, vol. 13, pp. 537–549. 2. J.E. Barnes, W. Peter and C.A. Blue, “Evaluation of Low Cost Titanium Alloy Products”, Mater. Sci. Forum, 2009, vol. 618–619, pp. 165–168. 3. “DuPont Comes up with Titanium Powder for Parts”, Metal Powder Report, 2006, vol. 61, no. 9, p. 4. 4. S. Abkowitz, J.J. Burke and R.H. Hiltz, Jr., Titanium in Industry: Technology of Structural Titanium, 1955, D. Van Nostrand, New York, NY. 5. G.I. Friedman, “Titanium Powder Metallurgy”, Inter. J. Powder Metall., 1970, vol. 6, no. 2, pp. 43–54. 6. W. Kroll, “Verformbare Legierungen des Titans”, Z Metallkunde, 1937, vol. 29, pp. 189–192. 7. T.W. Lippert, “Titanium in U.S.A.”, The Science, Technology, and Application of Titanium, edited by R.I. Jaffee and N.E. Promisel, Pergamon Press, Oxford, 1970, pp. 5–9. 8. Ibid., W.J. Kroll, “Preface”. 9. R.S. Dean, J.R. Long, F.S. Wartman and E.L. Anderson, “Preparation and Properties of Ductile Titanium”, Trans. Amer. Inst. Mining Metall. Engineers, 1946, vol. 166, pp. 369–381. 10. Titanium: Past, Present, and Future, National Materials Advisory Board, 1983, National Academy Press, Washington, D.C. 11. D.C. Li, H. Liu and D.L. Zhou, Titanium Smelting Technologies, 2009, Chemical Industry Press, Beijing, China. 12. G. Lütjering and J.C. Williams, Titanium, Second Edition, 2007, Springer, Berlin, Germany. 13. Osaka Titanium, http://www.osakati.co.jp/e/e_product/ titan/index.html [Accessed 17 August 2010]. 14. S.G. Glazunov, “Titanium in the U.S.S.R.”, The Science, Technology, and Application of Titanium, edited by R.I. Jaffee and N.E. Promisel, Pergamon, Oxford, UK, 1970, pp. 75–76. 15. R.L. Brickerdike and D.A. Sutcliffe, “The Tensile Strength of Titanium at Various Temperatures”, Powder Metallurgy, London, Her Majesty’s Stationery Office, 1951, vol. 9, pp. 153–159. 16. C.G. Goetzel, Treatise on Powder Metallurgy, 1950, Interscience Publishers, Inc., New York, NY, vol. II, pp. 692–707. 17. R.F. Bunshah, H. Margolin and I.B. Cadoff, “Titanium Powder Metallurgy (part two)”, Precision Metal Moulding, 1956, vol. 14, no. 6, pp. 42–43. 18. F.H. Froes and D. Eylon, “Powder Metallurgy of Titanium Alloys”, Titanium Science and Technology: Proceedings of the Fifth International Conference on Titanium, compiled by G. Lütjering, U. Zwicker and W. Bunk, Deutsche Gesellschaft für Metallkunde, Oberursel, Germany, 1985, vol. 1, pp.
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267–286. 19. F.H. Froes, D. Eylon and G. Friedman, “Titanium P/M technology”, ASM Handbook, Vol. 7: Powder Metallurgy Technology and Applications, ASM International, Metals Park, OH, 1984, pp. 748–755. 20. T. Saito, “A Cost-Effective P/M Titanium Matrix Composite for Automobile Use”, Advanced Performance Materials, 1995, vol. 2, pp. 121–144. 21. T. Saito, “The Automotive Application of Discontinuously Reinforced TiB-Ti Composites,” JOM, 2004, vol. 56, no. 5, pp. 33–36. 22. T. Saito, Private Communication, June 2010 23. A.D. Hartman, S.J. Gerdemann and J.S. Hansen, “Producing Lower-Cost Titanium for Automotive Applications”, JOM, 1998, vol. 50, no. 9, pp. 16–19. 24. J. Gambogi, “Titanium”, U.S. Geological Survey Minerals Yearbook, 1999, U.S. Government Printing Office, Washington, D.C., 2001, pp. 79.1–79.6. 25. Honeywell Electronic Materials, http://www51.honeywell. com/sm/em/common/documents/product-overview.pdf [Accessed September 2010]. 26. A. Woodfield, E. Ott, J. Blank, M. Peretti, D. Linger and L. Duke, “Meltless Ti—A New Light Metals Industry”, Mater. Sci. Forum, 2009, vol. 618–619, pp. 135–138. 27. International Titanium Association, Titanium Update Newsletter, 2008, issue V, p. 5, http://www.titanium.org/ files/ItemFileA4403.pdf [Accessed September 2010]. 28. W. Mo, G.Z. Deng and F.C. Luo, Titanium Metallurgy, Second Edition, 2007, Metallurgical Industry Press, Beijing, China, pp. 11–20, 48–49, 293–300. 29. F.H. Froes and D. Eylon, “Powder Metallurgy of Titanium Alloys”, Inter. Mater. Rev. 1990, vol. 35, no. 3, pp. 162–182. 30. J.B. Lim, C. Bettles, B.C. Muddle and N.K. Park, “Effects of Impurity Elements on Green Strength of Powder Compacts”, Materials Science Forum, 2010, vol. 654–656, pp. 811–814. 31. M. Qian, G.B. Schaffer and C.J. Bettles, “ Sintering of Titanium and Its Alloys”, Sintering of Advanced Materials, edited by Z.K. Fang, Woodhead Publishing Ltd., Cambridge, UK, 2010, pp. 323–354. 32. M.A. Imam and F.H. Froes, “TMS 2010 Symposium: CostAffordable Titanium III”, JOM, 2010, vol. 62, pp. 15–16. 33. O.M. Ivasishin, D.G. Savvakin, F.H. Froes, V.C. Mokson and K.A. Bondareva, “Synthesis of Alloy T-6Al-4V with Residual Porosity by a Powder Metallurgy Method”, Powder Metallurgy and Metal Ceramics, 2002, vol. 41, pp. 382–390. 34. M. Qian and G.B. Schaffer, “Sintering of Aluminium and Its Alloys”, Sintering of Advanced Materials, edited by Z.K. Fang, Woodhead Publishing Ltd., Cambridge, UK, 2010, pp. 289–322. 35. P.C. Eloff, “Sintering of Titanium”, ASM Handbook, Vol. 7: Powder Metallurgy Technology and Applications, ASM International, Metals Park, OH, 1984, pp. 393–395. 36. S.T. Hong, Y. Hovanski, C.A. Lavender and K.S. Weil, “Investigation of Die Stress Profiles During Powder Compaction Using Instrumented Die”, J. Mater. Eng. Perform., 2008, vol. 17, pp. 382–386. 37. Y. Hovanski, K.S. Weil and C.A. Lavender, “Developments in Die Pressing Strategies for Low-Cost Titanium Powders”, TMS 2009 138th Annual Meeting & Exhibition: Supplemental Proceedings, Volume 1: Materials Processing and Properties, 2009, TMS, Warrendale, PA, pp. 549–556. 38. G.S. Wang and R.Z. Tian, Application Technologies of Titanium, 2007, Central South University Press, Changsha,
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Hunan, China, pp. 394–396. 39. J.L. Murray and H.A. Wriedt, “The O-Ti (Oxygen-Titanium) System”, Phase Diagrams of Binary Titanium Alloys, ASM International, Metals Park, OH, 1987, pp. 211–229. 40. Calphad, http://www.calphad.com/titanium-nitrogen.html [Accessed August 2010]. 41. T. Watanabe and Y. Horikoshi, “The Sintering Phenomenon of Titanium Powders—A Discussion”, Inter. J. Powder Metall., 1976, vol. 12, no. 3, pp. 209–214. 42. M. Peters, J. Kumpfert, C.H. Ward and C. Leyens, “Titanium Alloys for Aerospace Applications”, Adv. Eng. Mater. 2003, vol. 5, no. 6, pp. 419–427. 43. D.S. Arensburger, V.S. Pugin and I.M. Fedorchenko, “Properties of Electrolytic and Reduced Titanium Powders and Sinterability of Porous Compacts from such Powders”, Powder Metallurgy and Metal Ceramics, 1968, vol. 7, pp. 362–367. 44. Kanto Yakin Kogyo Co., http://www.k-y-k.co.jp/en/ sanso_e.html [Accessed August 2010]. 45. D. Heaney and R.M. German, “Advances in the Sintering of Titanium Powders”, Proceedings of the PM2004 Powder Metallurgy World Congress (Vienna, Austria), vol. 4, European Powder Metallurgy Association, Shrewsbury, UK, 2004, pp. 222–227. 46. A.J. Heidloff, J.R. Rieken, I.E. Anderson, D. Byrd, J. Sears, M. Glynn and R.M. Ward, “Advanced Gas Atomization Processing for Ti and Ti Alloy Powder Manufacturing”, JOM, 2010, vol. 62, no. 5, 35–41. 47. B.B. Panigrahi, M.M. Godkhindi, K. Das, P.G. Mukunda and P. Ramakrishnan, “Sintering Kinetics of Micrometric Titanium Powder’, Mater. Sci. Eng. A, 2005, vol. 396, pp. 255–262. 48. B.B. Panigrahi, “Sintering Behaviour of Ti-2Ni and Ti-5Ni Elemental Powders’, Materials Letters, 2007, vol. 61, pp. 152–155. 49. S. Abkowitz, J.M. Siergiej and R.D. Regan, “Titanium P/M Preforms, Parts and Composites”, Modern Developments in Powder Metallurgy, edited by H.H. Hausner, Metal Powder Industries Federation, Princeton, NJ, 1971, vol. 4, pp. 501–511. 50. H. Wang, M. Lefler, Z.Z. Fang, T. Lei, S. Fang, J. Zhang and Q. Zhao, “Titanium and Titanium Alloy via Sintering of TiH2”, Key Engineering Materials, 2010, vol. 436, pp. 157–163. 51. C. Herzig, R. Willecke and K. Vieregge, “Self-diffusion and Fast Cobalt Impurity Diffusion in the Bulk and in Grainboundaries of Hexagonal Titanium”, Philosophical Magazine A, 1991, vol. 63, no.5, pp. 949–958. 52. M.C. Naik and R.P. Agarwala, “Anomalous Diffusion in Beta Zirconium, Beta Titanium, and Vanadium”, J. Phys. Chem. Solid, 1969, vol. 30, pp. 2,330–2,334. 53. B.B. Panigrahi and M.M. Godkhindi, “Sintering of Titanium: Effect of Particle Size”, Inter. J. Powder Metallurgy, 2006, vol. 42, no. 2, pp. 35–42. 54. K. Akechi and Z. Hara, “Increase of Sintering Rate of Titanium Powder during Cyclic Phase Transformation”, Powder Metallurgy, 1981, vol. 24, no. 1, pp. 41–46. 55. Y. Mahajan, D. Eylon, R. Bacon and F.H. Froes, “Microstructure Property Correlation in Cold Pressed and Sintered Elemental Ti-6Al-4V Powder Compacts”, Powder Metallurgy of Titanium Alloys, edited by F.H. Froes and J.E. Smugeresky, The Metallurgical Society of AIME, Warrendale, PA, 1980, pp. 189–202. 56. M.J. Donachie Jr., Titanium—A Technical Guide, Second Edition, ASM International, Materials Park, OH,
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2000, pp. 46–53. 57. Sumitomo, http://www.sumitomocorp.co.jp/titanium/ powder.html [Accessed August 2010] 58. Toho, http://www.toho-titanium.co.jp/en/products/tipowder_en.html [Accessed August 2010]. 59. V.A. Druz, V.S. Moxson, R. Chernenkoff, W.F. Jandeska Jr. and J. Lynn, “Blending: An Elemental Approach to Volume Titanium Manufacture”, Metal Powder Report, 2006, vol. 61, no. 10, pp. 16–21. 60. O.M. Ivasishin, D.G. Savvakin, X.O. Bondareva and O.I. Dekhtyar, “Synthesis of PM Titanium Alloys Using Titanium Hydride Powder: Mechanism of Densification”, Ti2003 Science and Technology, Volume I, edited by G. Lütjering and J. Albrecht, Wiley-VCH, Weinheim, Germany, 2004, pp. 495–502. 61. O.M. Ivasishin, D. Eylon, V.I. Bondarchuk and D.G. Savvakin, “Diffusion During Powder Metallurgy Synthesis of Titanium Alloys”, Defect and Diffusion Forum, 2008, vol. 277, pp. 177–185. 62. O.M. Ivasishin and D.G. Savvakin, “The Impact of Diffusion on Synthesis of High-Strength Titanium Alloys from Elemental Powder Blends”, Key Engineering Materials, 2010, vol. 436, pp. 113–121. 63. R.M. German, Sintering Theory and Practice, 1996, John Wiley & Sons, New York, NY. 64. A.D. Hanson, J.C. Runkle, R. Widmer and J.C. Hebeisen, “Titanium Near Net Shapes from Elemental Powder Blends”, Inter. J. Powder Metall. 1990, vol. 26, no. 2, pp.
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157–164. 65. E. Aust, W. Limberg, R. Gerling, B. Oger and T. Ebel, “Advanced TiAl6Nb7 Bone Screw Implant Fabricated by Metal Injection Moulding”, Advanced Engineering Materials, 2006, vol. 8, pp. 365–370. 66. Y.F. Yang, S.D. Luo, G.B. Schaffer and M. Qian, “Sintering of Ti-10V-2Fe-3Al by blended elemental method,” 7th Pacific Rim International Conference on Advanced Materials and Processing (PRICM-7), 2010, Cairns, Australia. 67. M. Eriksson, Z. Shen and M. Nygren, “Fast Densification and Deformation of Titanium Powder”, Powder Metallurgy, 2005, vol. 48, no. 3, pp. 231–236. 68. M. Zadra F. Casari, L. Girardini and A. Molinari, “Microstructure and Mechanical Properties of CP-Titanium Produced by Spark Plasma Sintering”, Powder Metallurgy, 2008, vol. 51, no.1, pp. 59–65. 69. K. Kondoh, T. Threrujirapapong, H. Imai, J. Umeda and B. Fugetsu, “Characteristics of Powder Metallurgy Pure Titanium Matrix Composite Reinforced with Multi-Wall Carbon Nanotubes”, Composites Science and Technology, 2009, vol. 69, pp. 1,077–1,081. 70. T. Threrujirapapong, K. Kondoh, H. Imai, J. Umeda and B. Fugetsu, “Mechanical Properties of a Titanium Matrix Composite Reinforced with Low Cost Carbon Black via Powder Metallurgy Processing”, Materials Transactions, 2009, vol. 50, pp. 2,757–2,762. 71. M. Tanaka, H. Kono and K. Maruyama, “Selective Heating Mechanism of Magnetic Metal Oxides by a Microwave Magnetic Field”, Phys. Rev. B, 2009, vol. 79, pp. 104,4201–104,420-5. 72. R. Roy, D. Agrawal, J. Cheng, and S. Gedevanishvili, “Full Sintering of Powdered-Metal Bodies in a Microwave Field,” Nature, 1999, vol. 399, p. 668. 73. V.D. Buchelnikov, D.V. Louzguine-Luzgin, G. Xie, S. Li, N. Yoshikawa, M. Sato, A.P. Anzulevich, I.V. Bychkov and A. Inoue, “Heating of Metallic Powders by Microwaves: Experiment and Theory”, J. Appl. Phys., 2008, vol. 104, pp. 113,505–113,514. 74. M.G. Kutty and S.B. Bhaduri, “Gradient Surface Porosity in Titanium Dental Implants: Relation between Processing Parameters and Microstructure”, J. Mater. Sci.: Mater. Med., 2004, vol. 15, pp. 145–150. 75. S. Luo, C. Bettles, M. Yan, G.B. Schaffer and M. Qian, “Microwave Sintering of Titanium”, Key Engineering Materials, 2010, vol. 436, pp. 141–147. 76. S. Luo, M. Yan, G.B. Schaffer and M. Qian, “Sintering of Titanium in Vacuum by Microwave Radiation”, 2010, submitted to Metall. Mater. Trans. A. 77. V.A. Duz, O.M. Ivasishin, V.S. Moxson, D.G. Savvakin and V.V. Telin, “Cost-Effective Titanium Alloy Powder Compositions and Method for Manufacturing Flat or Shaped Articles from These Powders”, U.S. Patent 2009/0252638 A1, October 8, 2009. 78. M. Holz, “European Titanium Market—Current and Future Scenario”, 2006, http://www.deutschetitan.com/ documents/ETM.pdf [Accessed September 2010]. 79. L. Hogan, E. McGinn and R. Kendall, “Research and Development in Titanium: Implications for a Titanium Metal Industry in Australia”, 2008, Australian Bureau of Agricultural and Resource Economics, Canberra, Australia, http://www.abare.gov.au/publications_html/energy/energy_08/titanium.pdf [Accessed September 2010]. 80. “Titanium MMC Gains Aerospace Contract”, Powder Metallurgy, 2007, vol. 50, no. 4, p. 285. ijpm
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PM TITANIUM
A CRITICAL REVIEW OF MECHANICAL PROPERTIES OF POWDER METALLURGY TITANIUM Hongtao Wang*, Z. Zak Fang** and Pei Sun*
INTRODUCTION Titanium is an excellent material for many applications in light of its combined properties of high strength, light weight, and corrosion resistance. However, practical applications have been severely limited due to its high intrinsic cost, particularly in cost-sensitive civilian applications such as automobiles. Therefore, cost reduction is a major driving force in R&D on particulate titanium production and processing. PM provides a viable approach for reducing the cost of titanium parts because of its near-net-shape capability. It has therefore been the focus of R&D worldwide in the past three decades. An extensive body of data has been reported and accumulated on the mechanical properties of PM Ti, especially during the 1980s and ‘90s, which demonstrated that PM is indeed a viable approach for manufacturing low-cost high-performance products. A series of comprehensive reviews by Eylon, Froes, and Donachie on the mechanical properties of PM Ti and their dependence on processing are available.1–3 Notwithstanding long-term efforts and the data that are available, concerns over the mechanical properties of PM Ti persist, especially for aerospace applications in which parts are required to have mechanical properties and performance comparable to IM parts. Critical properties such as fatigue strength are not allowed to deviate from those of IM Ti. As a convention, PM Ti is classified into two groups based on the processing route: blended elemental (BE) and prealloy (PA).2,4 BE parts possess the attribute of low cost, but their mechanical properties are, in general, lower than those of IM parts.1,2 In contrast, PA parts usually have satisfactory properties,1,2 while the cost is significantly higher than that of BE parts, reducing competitiveness with respect to cost reduction. Therefore, the challenge is the trade-off between cost and performance. The opportunity is to either reduce the cost of PA parts or increase the performance of BE parts. Based on recent developments, the latter may hold the more promise. This review provides an update and a critical assessment of the mechanical properties of PM Ti, focusing on the factors that affect these properties. In addition, developments over the last decade with respect
Mechanical properties are the primary concern in the development and application of powder metallurgy (PM) titanium and its alloys. Their mechanical properties are reviewed by comparing PM with ingot metallurgy (IM) and by examining the dependence of the mechanical properties on the microstructures that are unique to PM titanium. The effects of the most critical factors (porosity, oxygen content, and microstructure) on mechanical properties are discussed. Throughout this review, PM Ti refers generically to PM titanium and titanium alloys. IM Ti embraces ingot metallurgy titanium and titanium alloys. Static and dynamic properties are examined to illustrate the challenges as well as the opportunities for PM Ti. Selected recent PM Ti technologies and attendant mechanical properties are also assessed.
*Graduate Research Assistant, **Associate Professor, Department of Metallurgical Engineering, University of Utah, 135 S. 1460 E. Room 412, Salt Lake City, Utah 84112, USA; E-mail:
[email protected].
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A CRITICAL REVIEW OF MECHANICAL PROPERTIES OF POWDER METALLURGY TITANIUM
to titanium produced by advanced particulate processes are highlighted. MECHANICAL PROPERTIES Mechanical properties are usually assessed from two perspectives: static properties and dynamic properties. Static properties include yield strength, tensile strength, and ductility (elongation at fracture or percentage reduction in area). Dynamic properties typically refer to fatigue properties. Based on the available literature, mechanical properties are summarized in Figures 1 and 2 for Ti-6Al-4V. Figure 1 shows the tensile strength as a function of ductility of PM Ti-6Al-4V, including material produced by different processing routes.5–41 ASTM Standard B348 for IM is included for comparison. Most of the data in Figure 1 reflect information published during the 1980s and ‘90s.1 Data from the literature after 200011,16,18,22,27,28,32,36,38,40,41 are labeled separately for comparison. In addition, data from recent advanced/developing processes, including the sintering of titanium powder produced by the Armstrong process,38 powder injection molding (PIM),11,28,32 and direct sintering of TiH227 are included in Figure 1. As shown in Figure 1, there is scatter in the data over a broad range in both dimensions. The tensile strength varies between 900 and 1,100 MPa, while elongation changes significantly from 4% to 30%. These variations, especially in ductility, are attributed to differences in porosity and microstructure in the PM alloy. For example, Andersen et al.8
Figure 1. Ultimate tensile strength vs. elongation of PM Ti-6Al-4V produced by different routes5–41
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showed that the elongation of Ti-6Al-4V BE parts at 96% of the pore-free density was 6%. However, Sun et al.18 reported that a near-fully dense BE Ti6Al-4V alloy sheet (>99% pore-free density) with a uniform fine equiaxed microstructure exhibited an attractive combination of tensile strength (1,104 MPa) and elongation (19.7%). Further, Eylon et al.12 reported a high tensile elongation (22%) for Ti-6Al-4V PA parts; this was attributed to the fine and fully equiaxed alpha-phase microstructure. Compared with wrought materials (ASTM B348), PM Ti-6Al-4V achieves equal or higher tensile strength, but the ductility is usually lower than what is required. By examining data in the literature after 2000, it can be concluded that there is a trend toward improved strength/ductility combinations. In short, Figure 1 demonstrates that the overall static properties of PM Ti-6Al-4V can meet or surpass those specified in ASTM B348. Figure 2, based on the original chart by Froes et al.,1,2 illustrates the dynamic (S-N) properties of PM Ti-6Al-4V. The range in fatigue life of the BE and PA Ti-6Al-4V alloys is compared with that of IM Ti-6Al-4V. It is seen that the fatigue response of the PA alloy is comparable with that of IM Ti-6Al4V. In contrast, the BE alloy exhibits lower fatigue strength and life compared with the PA and IM alloys. The inferior fatigue properties of the BE alloy are attributed to residual porosity and/or contaminants in the sintered alloy. Both porosity8,37,42 and contaminants are known to be detrimental to fatigue performance because it has been confirmed that fatigue cracks usually initiate from pores at the surface of the specimen.8 Thus, the
Figure 2. Fatigue data-scatter bands of Ti-6Al-4V produced by BE, low-chloride BE, treated BE, and PA, compared with wrought annealed Ti-6Al-4V.1,2 Redrawn with permission from Maney Publishing Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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fatigue performance of BE alloy parts can be improved by using powder with a low chloride-contaminant level,33,34,37 Figure 2. One common feature in Figure 1 and Figure 2 is the scatter in both static and dynamic property data. Inconsistencies among the data reflect the use of different powders and different processing routes. Because the mechanical properties are functions of the microstructure, and the microstructure is determined by processing history, it is logical to infer that the differences in powders, in sintering conditions, or in consolidation processes, hot working, and post-sintering heat treatments, all contribute to diversity in the microstructures, which result in variations in mechanical properties, as shown in Figures 1 and 2. MECHANICAL PROPERTIES: EFFECT OF POROSITY, OXYGEN, AND MICROSTRUCTURE In addition to the microstructure, processing influences two other entities: porosity and oxygen content. Both are uniquely important in PM Ti. Oxygen content is one of the most important factors affecting IM Ti and PM Ti, but it is particularly important in PM Ti because fine powders are involved. Although the critical nature of these factors (porosity, oxygen, and microstructure) and their effect on mechanical properties is recognized, there has been no concerted effort (to the knowledge of the authors) to compile a comprehensive review. Porosity Porosity is the first issue that is uniquely important in PM Ti compared with IM Ti. The effects of porosity on static properties, including yield strength, tensile strength, and elongation are shown in Figure 3, based on data extracted from several references.1,5,8,15,17,37 It can be seen that, as expected, static properties improve essentially linearly with increasing relative density. Based on ASTM B348 for IM Ti, PM Ti products must have a relative density >98% to achieve similar staticproperty levels. The dependence of fatigue strength on sintered density is shown in Figure 4.1,2 Compared with static properties, the dynamic response is more sensitive to residual porosity. Figure 4 shows that even 1.0 v/o porosity (99% relative density) can significantly degrade fatigue strength. Consequently, only fully dense PM Ti meets the fatiguestrength requirement at the same level as IM Ti. Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Figure 3. Dependence of ultimate strength and elongation on sintered density of Ti-6Al-4V compacts1,5,8,15,17,37
Figure 4. Dependence of fatigue strength on sintered density of Ti-6Al-4V.1,2 Redrawn with permission from Maney Publishing
In general, the number of remaining pores depend on both the type of powder used and the subsequent process parameters. Full densification can usually be achieved with PA, while porosity
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always remains when BE is used, especially if the powder has a relatively high residual chlorine content. Numerous studies43–45 have shown that the residual chlorine content (0.12~0.15 w/o) has a strong effect on sintered density. During consolidation, chlorine volatilizes, creating pores with insoluble gas bubbles within the material; these pores cannot be eliminated, even by secondary pressing after sintering.1,2,44 Therefore, using lowchlorine powders is necessary for achieving highdensity PM Ti products.46 Consolidation processes also affect sintered density. The typical press-and-sinter process can result in sintered densities ranging from 95% to 99% of the pore-free level.1,2 Further treatment by hot isostatic pressing (HIPing) after sintering can lead to densities as high as 99.8% of the pore-free level.1,2,47-49 A secondary treatment after sintering, such as hot forging, hot rolling, and heat treatment, has also been used to improve the properties of PM Ti, especially fatigue properties.50,51 HIPing is also the most common consolidation technique to produce fully dense PM Ti parts when the PA approach is used.1,2 Another issue that affects the level of porosity is the initial green density. Green density usually depends on compaction pressure and particle size. Studies52 have shown that under similar sintering conditions, the sintered density can be significantly enhanced by using high compaction pressures, as shown in Figure 5. Higher compaction pressures usually yield a higher green density, and in turn a higher sintered density. Green density is also influenced by particle size. Controlling the particle-size distribution can yield a high green
Figure 5. Relationship between sintered density of Ti-6Al-4V and compaction pressure52
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density, and consequently a high sintered density.53,54 Cold isostatic pressing (CIPing) is also effective in boosting green density and the uniformity of density in green compacts. In summary, the effects of porosity on the mechanical properties of PM Ti, in particular fatigue resistance, are significant and critical. A reduction in the level of porosity can be achieved by using high-purity (low-chlorine) powders, increasing the green density, and utilizing appropriate sintering cycles, followed by secondary processing treatments. Oxygen Another important issue in the manufacture of PM Ti products is the control of oxygen content. Oxygen is the most common interstitial element in titanium and its alloys. Interstitial oxygen can increase strength but degrades ductility dramatically.55 Therefore, the oxygen content should be kept at a low level. Standards require that the oxygen level be <0.3 w/o for PM Ti-6Al-4V products (ASTM B817), and <0.2 w/o for IM Ti-6Al-4V products (ASTM B348). Due to its affinity for oxygen, PM Ti must be handled carefully. The effect of oxygen content on the mechanical properties of PM Ti-6Al-4V is illustrated in Figure 6 using data from the literature.5,6,9,18,20,22,25,33,34,37,40,41,56 In general, the effect of oxygen on strength is less critical compared with its effect on ductility. Ductility usually decreases dramatically as the oxygen content increases. High-oxygen-level PM Ti parts are unacceptable for many applications requiring ductility. Thus, controlling the oxygen level is a major challenge during the processing of PM Ti. The oxygen level in sintered PM Ti products depends on factors such as the powder-production method, particle size, powder processing and compaction method, sintering parameters, and the vacuum level during sintering. Usually, powders produced by melt-solidification processes such as gas atomization (GA), the rotating electrode process (REP), or the plasma rotating electrode process (PREP), have a lower oxygen content than hydride–dehydride (HDH) powders or sponge fines produced from titanium sponge without melting.52 Sponge fines and HDH powders are, however, preferred from an economic point of view. The dependence of the oxygen level on the particle size of HDH powder is shown in Figure 7.57 Larger particles exhibit lower oxygen content, but their sinVolume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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Figure 6. Effect of oxygen content on strength and elongation of sintered PM Ti-6Al-4V
keep the vacuum level as high as possible during sintering. Recently, there have been a number of developments that focus on improvement of the oxygen level in PM Ti. One approach is to use TiH2 instead of titanium as the starting powder for producing PM Ti via the BE route. Ivasishin56 compared the sintering of TiH2 with that of titanium powder. The results showed that the final oxygen content after direct sintering of TiH2 (0.21 w/o) was significantly lower than that in sintered titanium (0.39 w/o). This difference was attributed to the release of hydrogen from TiH2 during sintering with attendant cleaning of the starting material. Another strategy for reducing the oxygen level is to dope the powders with rare-earth elements. Liu et al.58,59 studied the effects of rare-earth elements on the properties of PM Ti and concluded that such additions can increase ductility in the sintered state because the rare-earth elements are able to scavenge oxygen from the titanium matrix. In summary, oxygen levels >0.3 w/o are detrimental to the ductility of PM Ti-6Al-4V. It is a challenge to control oxygen during the processing of PM Ti, especially with fine powders. However, with the development of new technologies for the production of cost-effective low-oxygen titanium powders (e.g., Armstrong powder60 and low-oxygen HDH powder61) and new sintering technologies, oxygen levels in PM Ti can be effectively minimized. Microstructures The microstructures of PM Ti are inherently different from those of IM Ti because of differences in the manufacturing steps. The microstructures of PM Ti can be tailored to be similar to those of IM Ti if post-sintering thermal and mechanical treatment processes are designed to do so. Since the microstructure depends on both composition and processing history, subsequent discussion focuses on the microstructures of Ti-6Al-4V produced by different processes, since this alloy has been studied most extensively.
Figure 7. Dependence of oxygen content on particle size of HDH powders57
terability becomes an issue with respect to achieving full density. Besides controlling the oxygen level in the starting powders, it is also important to avoid oxygen pick-up during processing and sintering. To this end, it is necessary to prevent exposure of the powder to air during processing and to Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
As-Sintered Microstructure When the BE approach is used, the sintering temperature should be higher than the beta transus temperature in order to provide particle bonding and compositional homogeneity.1 This is because the diffusivities of many alloying elements, including the self-diffusivity of titanium,
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are higher in the beta phase than in the alpha phase.62 As a result, a representative as-sintered microstructure of the alpha+beta alloys consists of an alpha plate colony structure (beta-transformed microstructure), Figure 8(a).1,2 Two characteristics of this microstructure that affect mechanical properties are the size of the lenticular alpha plates and the aspect ratio of the plates. Studies have shown that small lenticular alpha plates are beneficial to both tensile strength and fatigue strength.2 A low aspect ratio of alpha (equiaxed alpha morphology) is preferred for improving resistance to fatigue-crack initiation.2,63,64 Sintering cycles and post-sintering heat treatments are, therefore, often designed to refine the alpha phase and to reduce the aspect ratio of the alpha phase. For example, the beta-transformed microstructure is influenced strongly by the betaphase boundaries.65,66 The sintering process67 has
to be designed to inhibit beta-phase grain growth, thus keeping the beta grain size small during sintering, which subsequently results in an alpha structure with a low aspect ratio. Moreover, it has been shown that rapid cooling after sintering can also refine the alpha lamellae.28 As-HIPed Microstructures As-HIPed microstructures usually refer to those of PA material produced by HIPing. In contrast to the sintering of BE materials, HIPing is commonly carried out below the beta transus temperature (1,020°C) of Ti-6Al-4V.1,2 A recent study showed that HIPing at 930°C yielded an improved balance of mechanical properties of Ti-6Al-4V, compared with HIPing at higher or lower temperatures.40 The typical as-HIPed Ti-6Al-4V microstructure consists of alpha plates in a beta matrix, as illustrated in Figure 8(b).1,2 It should be noted that PA powders
Figure 8. Representative microstructures of Ti-6Al-4V produced by different processes.2 (a) as-sintered, (b) as-HIPed, (c) BUS treatment, (d) THP treatment. Optical micrographs. Reprinted with permission from Maney Publishing
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often exhibit a martensitic microstructure within the particles due to the production process, e.g., PREP.40,68,69 Thus, the HIPed microstructure of PA powders reflects the characteristics of the phase transformation from martensite to alpha+beta, giving rise to a fine microstructure in which the size of the alpha plates depends on the HIPing temperature.40 It has been shown40 that the microstructure coarsens with increasing temperature or time during HIPing. Significant coarsening was observed when the temperature was close to the beta transus.40 The aspect ratio of the alpha plates is determined predominantly by the amount of strain energy in the particles.2,70–74 Higher strain energy stored in the powders can result in a lower alpha-plate aspect ratio,70–74 which is preferred for enhanced mechanical properties, especially fatigue strength. Strain energy can be increased by deforming the powder before compaction or by applying high pressure during HIPing.14,26,75–78 Besides lamellar alpha, the presence of equiaxed alpha grains in the as-HIPed microstructure has been confirmed by recent research.40 The equiaxed alpha grains formed at particle–particle contacts where extensive local deformation occurred causing recrystallization. Post-Sintering Heat Treatment Both as-sintered and as-HIPed microstructures can be modified by subsequent heat treatment. For PM Ti-6Al-4V, the most successful heat treatment is the “broken-up structure (BUS)” treatment in which, after beta quenching, the alloy is subjected to extended annealing at low temperatures in the alpha+beta phase field, for example, at 850ºC.1,2 The BUS treatment yields a refined broken-up alpha phase in the matrix of beta, Figure 8(c).1,2 Such microstructures are beneficial in relation to both tensile and fatigue properties.1,2 The increased fatigue strength resulting from the BUS treatment is shown in Figures 2 and 4. Thermohydrogen Processing Thermohydrogen processing (THP) is an important treatment for titanium materials. The concept is to use hydrogen as a temporary alloying element to refine the microstructure. The alloying of titanium with hydrogen creates opportunities for modifying phase compositions, developing metastable phases, and controlling phase transformations.79 Typically, THP involves hydrogenation treatment, beta solution treatment at an elevated-temperaVolume 46, Issue 5, 2010 International Journal of Powder Metallurgy
ture, eutectoid decomposition at a moderate temperature, and finally a vacuum dehydrogenation step. Corresponding to each step, the phase transformations of Ti-6Al-4V during THP can be expressed as:80 (α+β)coarse + H → (α+γ) + β(H) → β(H) → (α+γ) → (α+β)fine
(1)
These phase changes can be used to enhance processability and to refine the microstructure and to improve the mechanical properties of both PM Ti and IM Ti. It should be noted that, since no mechanical working is required in THP, this method is particularly suitable for improving the microstructure of net-shape products.1,2,81 The principle of THP has also been used to produce titanium powder by the HDH process, and to increase compactibility and sinterability by using hydrogenated-titanium powder.82 Details of THP have been reviewed by Senkov and Froes.82–83 A representative microstructure of Ti-6Al-4V after THP is given in Figure 8(d).1,2 Such microstructures have been shown to enhance both tensile strength and fatigue behavior.1,2 Fatigue properties after THP are also included in Figures 2 and 4. However, a recent study has shown that the ductility of THP treated Ti-6Al-4V decreased, although the strength increased.81 In practice, this trade-off needs to be carefully balanced. Thermomechanical Processing Thermomechanical processing (TMP) is the most commonly used method for optimizing the microstructure and mechanical properties of IM Ti and the technology can be applied directly to modify the microstructures and boost the properties of PM Ti. Conventional TMP treatments involve cold and/or hot working followed by annealing. The effects of TMP on the microstructures and properties of titanium are documented in many publications2,84–86 and thus will not be included here. One point that should be emphasized is that the thermomechanical processing of PM Ti not only refines the microstructure but also helps to eliminate residual porosity,50 leading to significant improvements in both tensile and fatigue properties. ADVANCED TECHNOLOGIES TO ENHANCE MECHANICAL PROPERTIES In addition to the conventional BE and PA approaches, R&D has focused on non-convention-
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al advanced processes for manufacturing PM Ti. Although it is beyond the scope of this review to cover all these approaches, we have identified those developments for which published mechanical-property data exist. Armstrong-Powder Sintering Production of low-cost titanium powder has been a major concern for PM Ti because of the cost–performance trade-off. Of some thirty different new processes for making titanium powders developed over the past ten years,87 the Armstrong process has garnered more attention and has made most progress toward commercialization. Armstrong powders are produced by a continuous process involving the reduction of TiCl4 by sodium.60 It is similar to the traditional Hunter process except that the latter is a batch process. Armstrong powders exhibit a three-dimensional fine dendritic network with fine-particle sizes <10 μm. This dendritic coral-like morphology is viewed as beneficial for producing fine microstructures; however, it poses challenges in cold compaction or hot consolidation.88 Although the Armstrong process was introduced nearly a decade ago as a low-cost titanium-powder production method, there are only a limited number of publications that include data on the mechanical properties of sintered Armstrong powders. Yamamoto et al.38 recently reported tensile properties of press-and-sinter Ti-6Al-4V Armstrong powder. Up to 96.4% of the pore-free density was achieved, and mechanical property data are included in Figure 1. Compared with ASTM B348, the strength of the alloy is acceptable; however, its ductility is too low, which may be attributed to porosity and/or oxygen in the sintered condition. Another example of the sintering of Armstrong powder was reported by Eylon et al.88 The commercially pure (CP) titanium powder was compacted to give a fine grain size (~2 to 3 µm) by rapid heating and short-hold vacuum hot pressing. The product showed a comparable chemistry with that of CP Ti ASTM grade 2, but with significantly higher strength; this was attributed to the fine microstructure. The ductility was lower than that of CP Ti grade 2 but comparable with that of CP Ti grade 3. Powder Injection Molding The powder injection molding (PIM) of titanium powder is a cost-effective method for fabricating small-to-moderate-sized components with intri-
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cate geometries. One of the driving forces for the PIM of titanium is the demand for biocompatible materials in medical applications.89 The PIM of titanium is the subject of one of the articles in this “Focus Issue” by German, and other detailed publications are available.90–93 In general, the critical issue is to find a suitable polymeric binder that does not result in interstitial contaminants such as carbon, hydrogen, and oxygen during subsequent debinding and sintering.94 Limited data on the mechanical properties of PIM titanium components11,28,32 are included in Figure 1. PIM Ti components exhibit relatively low mechanical properties in comparison with those produced by other PM processes. This is linked to the issues of low sintered density and interstitial contamination. For example, Obasi et al.28 investigated the influence of processing parameters on the mechanical properties of PIM Ti-6Al-4V and found that at a relative density of 94.6%–98.0%, yield strength and tensile strength levels were 699–757 MPa and 801–861MPa, respectively. Sintering of TiH2 As noted previously, the direct sintering of TiH2 is a promising approach for producing low-cost PM Ti with a low oxygen content. The direct sintering of TiH2 powder was first reported in the 1980s by Yolton and Froes95 who consolidated hydrogenated Ti-6Al-4V powder by HIPing. Since 2000, there have been a number of publications on the vacuum sintering of TiH2.96–98 It was shown that a blend of TiH2 with a 10 w/o 60Al-40V master alloy powder can be sintered to 98.5%–99.5% of the pore-free density by press-and-sinter PM, in contrast to 90%–95% of the pore-free density when titanium powder was used. Similar results were reported by Wang and Fang.99 The primary advantages of using TiH2 powder instead of titanium powder are: (a) Compactability: Studies have demonstrated that TiH2 powder can be readily compacted due to reduced cold welding between TiH2 particles and fragmentation of the TiH2 powder during compaction.99,100 As a result, a high green density can be reached. (b) Oxygen Control: TiH2 helps to reduce the oxygen content in sintered PM Ti. Possible reasons are (i) the interstitial sites in the titanium lattice are occupied by hydrogen atoms, and this reduces the chance of dissolution of oxygen, and (ii) hydrogen released Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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during sintering is assumed to have a cleansing effect by removing oxygen from the particle surfaces.56 (c) Enhanced Sinterability: It is easier to sinter TiH2 to >99% of the pore-free density than sintering titanium powder; this may be the result of the defects formed during the dehydriding process, which accelerate mass transport for densification.27 Figures 1 and 2 show that the tensile and fatigue properties of PM Ti produced by sintering TiH2 powder27,101 are competitive with those of IM Ti. Nano-/Ultrafine PM Ti Nano-/ultrafine grain materials are expected to exhibit attractive mechanical properties, for example, high strength combined with reasonable ductility and toughness. Nano- or ultrafine grain materials are also expected to display superplasticity. Nanocrystalline titanium-base materials can be produced by the “severe plastic deformation” (SPD) method using IM Ti as the feedstock. PM Ti can also be subjected to SPD. Yapici et al.102 cited the mechanical properties of PM Ti-6Al-4V after processing by equal channel angular pressing (ECAP) in which PM Ti-6Al-4V was severely deformed at different temperatures from 550ºC to 800ºC. The alpha grain size was refined from 17 μm to <500 nm which increased the strength but reduced ductility. However, it was shown that, by controlling the processing conditions, high strength and acceptable ductility can be obtained. For example, a tensile strength of 1,257 MPa and an elongation of 9.1% were achieved after a twopass extrusion at 800ºC. In order to improve both strength and ductility, a harmonic nano/meso hybrid microstructure of CP Ti was recently designed and developed by mechanical milling and subsequent consolidation. Fujiwara et al.103 and Sekiguchi et al.104 showed that by controlling the mechanical milling conditions, it was possible to generate a shell/core microstructure in the titanium particles with nanograins as the shell and work-hardened coarse grains as the core. These powders were then consolidated by hot-roll sintering to obtain bulk titanium with a nano/meso hybrid network microstructure consisting of continuously connected shell and dispersed core regions. CP Ti fabricated by these processes exhibited not only high strength but also substantial plastic strain (UTS 750 MPa/elongation 20%). The study also showed Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
that the mechanical properties of the core-shell structure can be controlled by varying the grain sizes in the shell and core, and the fractional ratio between the shell and the core. This new microstructural methodology design for titanium is worthy of enhanced attention in the future. SUMMARY This article constitutes a critical review of the current status of the mechanical properties of PM Ti and the dependence on porosity, oxygen, and sintered microstructures. In general, a relative density >98% is required for PM Ti to achieve static strength and ductility equivalent to IM Ti. The effect of porosity on the dynamic properties of PM Ti is more critical. Only fully densified PM Ti can achieve the same fatigue strength as IM Ti. Oxygen is critical in relation to the ductility of PM Ti. The oxygen content should be controlled to <0.3 w/o for PM Ti-6Al-4V to avoid a severe decrease in ductility. The effects of microstructure on the mechanical properties of PM Ti-6Al-4V depend primarily on the size of the lenticular alpha plates and the aspect ratio of the alpha plates. Refinement of the alpha grain size and a reduction of the aspect ratio of the alpha phase are beneficial to both the tensile and fatigue properties of PM Ti-6Al-4V. Finescale microstructures can be achieved by optimizing the sintering cycle, by post-sintering heat treatments, or by thermomechanical processing. Finally, it is demonstrated that new PM Ti techniques show promise in either lowering the cost or improving the properties of PM Ti. REFERENCES 1. D. Eylon, F.H. Froes and S. Abkowitz, “Titanium Powder Metallurgy Alloys and Composites”, ASM Handbook Vol. 7: Powder Metal Technologies and Applications, ASM International, Materials Park, OH, 1998, pp. 874–886. 2. F.H. Froes and D. Eylon, “Powder Metallurgy of Titanium Alloys”, International Materials Reviews, 1990, vol. 35, no. 3, pp. 162–182. 3. M.J.J. Donachie, Titanium—A Technical Guide, 2nd edition, 2000, ASM International, Materials Park, OH. 4. C.A. Kelto, B.A. Kosmal and D. Eylon, “Titanium Powder Metallurgy—a Perspective”, Journal of Metals, 1980, vol. 32, no. 8, pp. 17–25. 5. S. Abkowitz, G.J. Kardys, S. Fujishiro, F.H. Froes and D. Eylon , “Titanium Alloy Shapes from Elemental Blend Powder and Tensile and Fatigue Properties of Low Chloride Compositions”, Titanium Net Shape Technologies: Proc. of a Symposium, edited by F.H. Froes and D. Eylon, 1984, The Metall. Soc. of AIME, Warrendale, PA, pp. 107–120. 6. S. Abkowitz, “Isotatic Pressing of Complex Shapes from Titanium and Titanium Alloys”, Titanium ‘80, Science and
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Plate”, Powder Metallurgy of Titanium Alloys, edited by F.H. Froes and J. Smugeresky, TMS-AIME, Warrendale, PA, 1980, pp. 151–162. M. Hagiwara, Y. Kaieda, Y. Kawabe and S. Miura, “Fatigue Property Enhancement of α-β Titanium Alloys by Blended Elemental P/M Approach”, ISIJ International, 1991, vol. 31, no. 8, pp. 922–930. J.P. Herteman, D. Eylon, and F.H. Froes, “Mechanical Properties of Advanced Titanium Powder Metallurgy Compacts”, Powder Metallurgy International, 1985, vol. 17, no. 3, pp. 116–118. O.M. Ivasishin, D.G. Savvakin, F.H. Froes and K.A. Bondareva, “Synthesis of Alloy Ti-6Al-4V with Low Residual Porosity by a Powder Metallurgy Method”, Powder Metallurgy and Metal Ceramics, 2002, vol. 41, no. 7–8, pp. 382–390. W.H. Kao, D. Eylon, C.F. Yolton and F.H. Froes, “Effect of Temporary Alloying by Hydrogen (Hydrovac) on the Vacuum Hot Pressing and Microstructure of Titanium Alloy Powder Compacts”, Progress in Powder Metallurgy, edited by J.M. Capus and D.L. Dyke, Metal Powder Industries Federation, Princeton, NJ, 1981, vol. 37, pp. 289–302. L. Levin, R.G. Vogt, D. Eylon and F.H. Froes, “Fatigue Resistance Improvement of Ti-6Al-4V by ThermoChemical Treatment”, Titanium, Science and Technology, edited by G. Lutjering, U. Zwicker and W. Bunk, Deutsche Gesellschaft für Metallkunde, Oberursel, Germany, 1985, pp. 2,107–2,114. Y. Mahajan, D. Eylon, R. Bacon and F.H. Froes, “Microstructure Property Correlation in Cold Pressed and Sintered Elemental Ti-6Al-4V Powder Compacts”, Powder Metallurgy of Titanium Alloys, edited by F.H. Froes and J.E. Smugeresky, The Metall. Soc. of AIME, Warrendale, PA, 1980, pp. 189–202. Y.R. Mahajan, D. Eylon, C.A. Kelto. T. Egerer and F.H. Froes, “Modification of Titanium Powder Metallurgy Alloy Microstructures by Strain Energizing and Rapid OmniDirectional Compaction”, Powder Metallurgy Intl., 1985, vol. 17, no. 2, pp. 75–78. V.S. Moxson, V.A. Duz, O. Ivasishin, C. Lavender and V.V. Telin, “Innovative Powder Metallurgy Process for Producing Low Cost Titanium Alloy Components”, Titanium 2008—Conference Proceedings (CD-ROM), International Titanium Association (ITA), Broomfield, CO, 2008. G.C. Obasi, O.M. Ferri, T. Ebel and R. Bormann, “Influence of Processing Parameters on Mechanical Properties of Ti-6Al-4V Alloy Fabricated by MIM”, Mats. Sci. and Eng. A, 2010, vol. 527, no. 16–17, pp. 3,929–3,935. J. Park, M.W. Toaz, D.H. Ro and E.N. Aqua, “Blended Elemental Powder Metallurgy of Titanium Alloys”, Titanium Net Shape Technologies: Proc. of a Symposium, edited by F.H. Froes and D. Eylon, 1984, The Metall. Soc. of AIME, Warrendale, PA, pp. 95–105. R.E. Peebles and C.A. Kelto, “Investigation of Methods for the Production of High Quality, Low Cost Titanium Alloy Powders”, Powder Metallurgy of Titanium Alloys, edited by F.H. Froes and J. Smugeresky, TMS-AIME, Warrendale, PA, 1980, pp. 47–58. R.E. Peebles and L.D. Parsons, “Study of Production Methods of Aerospace Quality Titanium Alloy Powder”, Titanium Net Shape Technologies: Proc. of a Symposium,
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Powders”, ibid., pp. 203–216. 45. V.A. Duz, V.S. Moxson, F.H. Froes, F. Sun and J.S. Montgomery, “Powder Metallurgy Ti-6Al-4V Components Produced from Low Cost Blended Elemental Powder by Hot Pressing”, Cost-Affordable Titanium, edited by F.H. Froes, M.A. Imam and D. Fray, TMS, Warrendale, PA, 2004, pp. 145–150. 46. D. Eylon, R.G. Vogt, and F.H. Froes, “Property Improvement of Low Chlorine Titanium Alloy Blended Elemental Powder Compacts by Microstructure Modification”, Progress in Powder Metallurgy, edited by E.A. Carlson and G. Gaines, Metal Powder Industries Federation, Princeton, NJ, 1986, vol. 42, pp. 625–634. 47. F.H. Froes, “Developments in Titanium Powder Metallurgy”, JOM, 1980, vol. 32, no. 2, pp. 47–54. 48. Powder Metallurgy of Titanium Alloys, edited by F.H. Froes and J.E. Smugeresky, The Metall. Soc. of AIME, Warrendale, PA, 1980. 49. Titanium Net Shape Technologies, edited by F.H. Froes and D. Eylon, The Metall. Soc. of AIME, Warrendale, PA, 1984. 50. B. Liu, Y. Liu, X-y. He, H-p. Tang and L-f. Chen, “Low Cycle Fatigue Improvement of Powder Metallurgy Titanium Alloy Through Thermomechanical Treatment”, Transactions of the Nonferrous Metals Society of China, 2008, vol. 18, no. 2, pp. 227–232. 51. D. Eylon and F.H. Froes, “Tensile and Fatigue Strength Improvement of Titanium PM Alloys Through Microstructural Refinement”, Strength of Metals and Alloys (ICSMA-8), edited by P.O. Kettunen, T.K. Lepistö and M.E. Lehtonen, Pergamon, Oxford, UK, 1988, pp. 527–533. 52. I.M. Robertson and G.B. Schaffer, “Review of Densification of Titanium Based Powder Systems in Press and Sinter Rrocessing”, Powder Metallurgy, 2010, vol. 53, no. 2, pp. 146–162. 53. J. Park, M.W. Toaz, D.H. Ro and E.N. Aqua, “Blended Elemental Powder Metallurgy of Titanium Alloys”, Titanium Net Shape Technologies: Proc. of a Symposium, edited by F.H. Froes and D. Eylon, 1984, The Metall. Soc. of AIME, Warrendale, PA, pp. 95–105. 54. B.S. Becker. and J.D. Bolton, “Corrosion Behaviour and Mechanical Properties of Functionally Gradient Materials Developed for Possible Hard-Tissue Applications”, Journal of Materials Science: Materials in Medicine, 1997, vol. 8, no. 12, pp. 793–797. 55. H. Conrad, “Effect of Interstitial Solutes on the Strength and Ductility of Titanium”, Progress in Materials Science, 1981, vol. 26, no. 2, pp. 123–403. 56. O.M. Ivasishin, “Cost-Effective Manufacturing of Titanium Parts with Powder Metallurgy Approach”, Materials Forum, 2005, vol. 29, pp. 1–8. 57. C. McCracken,”Production of Fine Titanium Powders via the Hydride–Dehydride (HDH) Process”, PIM International, 2008, vol. 2, no. 2, pp. 55–57. 58. Y.Liu, L. Chen, W. Wei, H. Tang, B. Liu and B. Huang, “Improvement of Ductility of Powder Metallurgy Titanium Alloys by Addition of Rare Earth Element”, J. Materials Science and Technology, 2006, vol. 22, no. 4, pp. 465–469. 59. Y. Liu, L.F. Chen, H.P. Tang, C.T. Liu, B. Liu and B.Y. Huang, “Design of Powder Metallurgy Titanium Alloys and Composites”, Mats. Sc. and Eng. A, 2006, vol. 418, no. 1–2, pp. 25–35. 60. G. Crowley, “How to Extract Low-Cost Titanium”,
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1985, vol. 2, pp. 1,073–1,078. 75. D. Eylon, P.R. Smith, S.W. Schwenker and F.H. Froes, “Status of Titanium Powder Metallurgy”, Industrial Applications of Titanium and Zirconium, edited by R.T. Webster, C.S. Young, ASTM International, Philadelphia, PA, 1984, pp. 48–65. 76. I. Weiss, F.H. Froes, D. Eylon and C.C. Chen, “Control of Microstructure and Properties of Ti-6Al-2Sn-4Zr-6Mo Powder Forgings”, Titanium Net Shape Technologies, edited by F.H. Froes and D. Eylon, The Metall. Soc. of AIME, Warrendale, PA, 1984, pp. 79–94. 77. Y.R. Mahajan, D. Eylon, and C. Kelto, “Evaluation of Ti10V-2Fe-3Al Powder Compacts Produced by the ROC Method”, Progress in Powder Metallurgy, edited by H.I. Sanderow, W.L. Giebelhausen and K.M. Kulkarni, Metal Powder Industries Federation, Princeton, NJ, 1985, vol. 41, pp. 163–171. 78. F.H. Froes, D. Eylon, G. Wirth, K.J. Grundhoff and W. Smarsly, “Fatigue Properties of Hot Isostatically Pressed Ti-6Al-4V Powders”, Met. Powder Rep., 1983, vol. 38, no. 1, pp. 36–49. 79. O.N. Senkov, J.J. Jonas and F.H. Froes, “Recent Advances in the Thermohydrogen Processing of Titanium Alloys”, JOM, 1996, vol. 48, no. 77, pp. 42-47. 80. T.Y. Fang. and W.H. Wang, “Microstructural Features of Thermochemical Processing in a Ti-6Al-4V Alloy”, Materials Chemistry and Physics, 1998, vol. 56, no. 1, pp. 35–47. 81. A. Guitar, G.. Vigna, and M.I. Luppo, “Microstructure and Tensile Properties After Thermohydrogen Processing of Ti6Al-4V”, Journal of the Mechanical Behavior of Biomedical Materials, 2009, vol. 2, no. 2, pp. 156–163. 82. F.H. Froes, O.N. Senkov, and J.I. Qazi, “Hydrogen as a Temporary Alloying Element in Titanium Alloys: Thermohydrogen Processing”, International Materials Reviews, 2004. vol. 49, pp. 227–245. 83. O.N. Senkov and F.H. Froes, “Thermohydrogen Processing of Titanium Alloys”, Int. J. of Hydrogen Energy, 1999, vol. 24, no. 6, pp. 565–576. 84. E.W. Collings, The Physical Metallurgy of Titanium Alloys, 1984, American Society for Metals, Metals Park, OH. 85. I. Weiss and S.L. Semiatin, “Thermomechanical Processing of Beta Titanium Alloys—an Overview”, Mats. Sci. and Eng. A, 1998, vol. 243, no. 1–2, pp. 46–65. 86. I. Weiss and S.L. Semiatin, “Thermomechanical Processing of Alpha Titanium Alloys—an Overview”, Mats. Sci. and Eng. A, 1999, vol. 263, no. 2, pp. 243–256. 87. E.H. Kraft, Summary of Emerging Titanium Cost Reduction Technologies, A Study by the United States Dept. of Energy (DOE) and Oak Ridge National Laboratory (ORNL), 2003. 88. D. Eylon, W.A. Ernst and D.P. Kramer, “Development of Ultra-Fine Microstructure in Titanium via Powder Metallurgy for Improved Ductility and Strength”, Materials Science Forum, edited by M. Cabibbo and S. Spigarelli, Trans Tech Publications Ltd., Switzerland, 2009, vol. 604–605, pp. 223–228. 89. T. Ebel, “Titanium and Titanium Alloys for Medical Applications: Opportunities and Challenges”, PIM International, 2008, vol. 2, no. 2, pp. 21–30. 90. R.M. German, “Titanium Powder Injection Moulding (TiPIM): A Review of the Current Status of Materials, Processing, Properties and Applications”, PIM International, 2009, vol. 3, no. 4, pp. 21–37.
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91. F.H. Froes and R.M. German, “Cost Reductions Prime Ti PIM for Growth”, Metal Powder Report, 2000, vol. 55, no. 6, pp. 12–14. 92. R.M. German, “R&D in Support of Powder Injection Molding: Status and Projections”, Int. J. Powder Metall., 2007, vol. 43, no. 6, pp. 47–57. 93. R.M. German and A. Bose, Injection Molding of Metals and Ceramics, 1997, Metal Powder Industries Federation, Princeton, NJ. 94. F.H. Froes, “Developments in Titanium P/M”, Metal Powder Report, in press. 95. C.F. Yolton and F.H. Froes, “Method for Producing Powder Metallurgy Articles”, U.S. Patent No. 4,219,357, August 26, 1980. 96. O.M. Ivasishin, D.G. Savvakin, V.S. Moxson, K.A. Bondareval and F.H. Froes, “Titanium Powder Metallurgy for Automotive Components”, Materials Technology, 2002, vol. 17, no. 1, pp. 20–25. 97. O.M. Ivasishin, V.M. Anokhin, A.N. Demidik and D.F. Savvakin, “Cost-Effective Blended Elemental Powder Metallurgy of Titanium Alloys for Transportation Application”, Key Engineering Materials, edited by F.H. Froes and E. Evangelista, Trans Tech Publications. Switzerland, 2000, vol. 188, pp. 55–62. 98. O.M. Ivasishin, D.G. Savvakin, V.S. Moxson, V.A. Duz, F.H. Froes and R. Davies, “Low-Cost PM Titanium Materials for Automotive Applications”, JOM, 2004, vol. 56, no. 11, p. 270. 99. H.T. Wang, M. Lefler, Z. Fang, T. Lei, S.M. Fang, J.M.
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Zhang and Q. Zhao, “Titanium and Titanium Alloy via Sintering of TiH2”, Key Engineering Materials, edited by M. Ashraf Imam, F. H. Froes and K.F. Dring, Trans Tech Publications, Switzerland, 2010, vol. 436, pp. 157–163. I.M. Robertson and G.B. Schaffer, “Comparison of Sintering of Titanium and Titanium Hydride Powders”, Powder Metallurgy, 2010, vol. 53, no. 1, pp. 12–19. O.M. Ivasishin, K.A. Bondareva, V.I. Bondarchuk, O.N. Gerasimchuk, D.G. Savvakin and B.A. Gryaznov, “Fatigue Resistance of Powder Metallurgy Ti–6Al–4V Alloy”, Strength of Materials, 2004, vol. 35, no. 3, pp. 225–230. G..G. Yapicia, I. Karamana, Z.P. Luob and H. Rack, “Microstructure and Mechanical Properties of Severely Deformed Powder Processed Ti-6Al-4V Using Equal Channel Angular Extrusion”, Scripta Materialia, 2003, vol. 49, no. 10, pp. 1,021–1,027. H. Fujiwara, M. Nakatani, T. Yoshida, Z. Zhang and K. Ameyama, “Outstanding Mechanical Properties in Materials with a Nano/Meso Hybrid Microstructure”, Materials Science Forum, edited by Y. Estrin and H.J. Maier, The Institution of Engineering and Technology, Trans Tech Publications, Switzerland, 2008, vol. 584–586, pp. 55–60. T. Sekiguchi, K. Ono, H. Fujiwara and K. Ameyama, “New Microstructure Design for Commercially Pure Titanium with Outstanding Mechanical Properties by Mechanical Milling and Hot Roll Sintering”, Mats. Trans., 2010, vol. 51, no. 1, pp. 39–45. ijpm
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PM INDUSTRY NEWS IN REVIEW The following items have appeared in PM Newsbytes since the previous issue of the Journal. To read a fuller treatment of any of these items, go to www.apmiinternational.org, login to the “Members Only” section, and click on “Expanded Stories from PM Newsbytes.”
Vale Sells Novamet Vale Inco, Toronto, Canada has sold Novamet Specialty Products Corporation, Wyckoff, N.J. to Palm Novamet Holdings, LLC, an affiliate of Palm International, Inc., LaVergne, Tenn. Novamet produces sized nickel powders, metallic flake, coated materials and specialty nickel oxide, and distributes Vale’s nickel powders and oxides in the United States. New High Corrosion Resistant Molybdenum Electrode Plansee High Performance Materials, Reutte, Austria, has launched a very high corrosionresistant molybdenum glassmelting electrode with high-temperature strength for use in the production of glass for the solar industry. Doped with small amounts of zirconium oxide (ZrO2), the new molybdenum electrode withstands harsh conditions during refining glass in the melting tank. First Quarter Sales Improve Miba AG, Laakirchen, Austria, reported a 32% sales increase to 98 million euros (about $118 million) for the first fiscal quarter 2010–2011. Sales have almost returned to the level of the strong first quarter of 2008 for its products, which include PM parts, bearings, coatings, friction products and automation equipment. MIM Company Opens Expanded Sales Unit Citing impressive growth, Indo-US MIM Tec (P) Ltd. has moved into a larger sales office in Princeton, N.J.
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The metal injection molding (MIM) parts maker, headquartered in Bangalore, India, recently opened investment casting and machining facilities.
association, the Metal Powder Industries Federation, has just released a new report titled, “Powder Metallurgy—Intrinsically Sustainable.”
CAMP Professors Partner with General Electric in N.Y. State Funded Program Professors Dan Goia and Dipankar Roy at Clarkson University’s Center for Advanced Materials Processing (CAMP) will participate in a General Electric program to improve sodium metal halide batteries for a new generation of cleaner locomotives and stationary applications. The New York State Energy Research and Development Authority is providing $2.5 million for the $5 million project.
Metal Powder Industry Rebounding After several years of declining shipments, mainly due to falling light-vehicle production in North America, the powder metallurgy industry has returned to a growth track, reports Michael E. Lutheran, MPIF president, at PowderMet2010, the 2010 International Conference on Powder Metallurgy & Particulate Materials in Hollywood, Florida. The industry began turning the corner slowly during the second half of 2009, and the trend has continued into the first quarter of 2010.
General Motors Revs Up ZeroWaste Program Worldwide As reported recently in the weekly newsletter of the National Council for Advanced Manufacturing, “62 General Motors plants are now zero-waste facilities by recycling or reusing more than 97 percent of the materials left from the manufacturing process that would otherwise be sent to landfills, the company said. The roughly 3 percent of remaining manufacturing waste is converted to energy at waste-to-energy facilities to help replace fossil fuels, GM said in a recent announcement. New PM Sustainability Report As part of its new Sustainability Initiative, aimed at defining the ways in which the powder metallurgy part-forming process is a “green” technology, the industry’s trade
Award-Winning Parts Winners of the 2010 Powder Metallurgy Design Excellence Awards Competition, sponsored by the Metal Powder Industries Federation, were announced in Hollywood, Florida, at PowderMet2010, the 2010 International Conference on Powder Metallurgy & Particulate Materials. Receiving grand prizes and awards of distinction, the winning parts are outstanding examples of powder metallurgy’s precision, performance, complexity, economy, and innovative design advantages. PM Industry Recognizes VVT Technology The Metal Powder Industries Federation has identified variable valve timing (VVT) as a Powder Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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Metallurgy Industry Landmark technology for its innovative and significant use of PM parts. The recognition was announced in Hollywood, Florida, at PowderMet2010, the 2010 International Conference on Powder Metallurgy & Particulate Materials. United States Metal Powders Focuses on Aluminum United States Metal Powders Incorporated, Flemington, N.J., has sold its remaining copperbased powder assets in New Jersey and Tennessee to American Chemet Corp. The company will focus on its aluminum and alloy powder companies, Ampal Inc., Palmerton, Pa., and Poudres Hermillon in France. Press Builder Launches PM Control Retrofit Cincinnati Incorporated, Cincinnati, Ohio, has introduced a new state-of-the-art microprocessor control with a sequence controller for its compacting presses built between 1978 and 1998, which number about 400 units. The new PM control retrofit system replaces obsolete electronics in older presses, bringing them up to current PC and CNC control standards. PowderMet2011 Call for Papers The technical program committee for the 2011 International Conference on Powder Metallurgy & Particulate Materials (PowderMet2011) has issued a “Call for Papers & Posters.” Sponsored by the Metal Powder Industries Federation in cooperation with APMI International, the conference will be held in San Francisco, May 18–21.
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
Technology Development Programs Aired Programs conducted by the Metal Powder Industries Federation (MPIF) to strengthen the PM industry were explained in the annual State of the North American PM Industry report presented by MPIF President Michael E. Lutheran at PowderMet2010. The programs cover technology assessment, applied research, educating design engineers, and standards development. PM2010 World Congress Technical Program The final technical program for the PM2010 Powder Metallurgy World Congress & Exhibition is available from the European Powder Metallurgy Association. The congress will be held October 10–14 in Florence, Italy. The latest date for pre-registration is September 30. Visit www.epma.com/pm2010 for registration details. Fuel Cell Partnership H.C. Starck Ceramics GmbH & Co. KG, Goslar, Germany, and Kerafol Keramische Folien GmbH, Eschenbach, Germany, have formed a partnership to manufacture, sell, and distribute high-temperature solid oxide-fuel cells doped with scandium. Starck will offer expertise in electrode manufacturing and Kerafol will contribute its experience with scandium-doped electrolytes. Powder Maker Reports Firm Demand Swedish powder maker Höganäs AB reports improving markets across most regions in the second quarter. Net sales soared 62 percent to MSEK 1,783 (about $247 million) while production increased 55 percent.
New Copper Powder Production Royal Metal Powders Inc., Maryville, Tenn., a division of American Chemet Corporation, has begun production of wateratomized copper powders, reports Michael E. Lutheran, president. The company expects to open its air-atomized production line during the fourth quarter of this year. PM Parent Firms Report Strong Gains GKN plc and Tomkins plc reported double-digit increases in sales and strong earnings for the first half of 2010. Both UK-based companies have major PM operations. SCM Expands Copper Powder Plant in China SCM Metal Products, Inc., Research Triangle Park, North Carolina, will expand its copper powder plant production capacity in Suzhou, China, in support of the growing market in Asia. Construction is scheduled for completion during the summer of 2011. Tungsten Mine Reopens Based on the recovery of worldwide tungsten prices, the board of directors of North American Tungsten Corporation Ltd. has approved the restarting of the Cantung tungsten mine in the Northwest Territories. Full production will begin in October. Outstanding Technical Paper Award Winner Named The MPIF Technical Board has announced that “On the Development of an Aluminum PM Alloy for ‘Press-Sinter-Size’ Technology” by D. Paul Bishop, Christopher D. Boland, Dalhousie University, and Rich L. Hexemer, Jr., Ian W. Donaldson, GKN Sinter Metals LLC, is the
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PM INDUSTRY NEWS IN REVIEW
recipient of the 2010 MPIF Howard I. Sanderow Outstanding Technical Paper Award. Nickel-Powder Distribution Agreement Continues Novamet Specialty Products Corporation, recently acquired by Palm Novamet Holdings, LLC, an affiliate of Palm Commodities International, Inc., La Vergne, Tenn., will continue operating and representing Vale’s nickel product line, including 123, 255 and CGNP powders. ACuPowder International LLC, Union, N.J., will remain a distributor reselling nickel powder grades 123 and 255 to the PM and related markets on behalf of Novamet. Hot Automotive Markets A study by Roland Berger Strategy Consultants GmbH, Munich, Germany, says turbochargers and electric motors in
China are the most promising auto product lines in the future, according to Automotive News. China’s demand for powertrain parts will grow to $33.14 billion by 2020, surpassing the U.S. by $6.5 billion, the study continues. New PM Nanocrystalline Aluminum Composites Working with the Army Research Laboratory (ARL) and Case Western Reserve University, Powdermet, Inc., Euclid, Ohio, has developed a non-cryomilling process for making PM aluminum composites with stable nanocrystalline grain structures. The PM materials offer lower weight, higher strength, and increased hardness compared to currently used aluminum alloys, without relying on rare-earth or other costly foreign-sourced alloying elements, the company says.
Screening Company Adds New Reps Elcan Industries, Mamaroneck, N.Y., has appointed new sales representatives to market the Virto/Elcan and Minox/Elcan screening machines. Allied Bulk will cover eastern Kansas, Illinois, and Missouri; Johnson Engineering & Sales Co. will cover Michigan; M.C. Schroeder will cover North Carolina, South Carolina, and Virginia; and Webb Process Equipment Co. will cover Northern Ohio. Miba Expands into Energy Products Miba AG, Laakirchen, Austria, has expanded its product mix by acquiring two Austrian suppliers of power electronics products, EBG and DAU. The Styria region companies have combined sales of approximately 30 million euros (about $38 million) and 130 employees. ijpm
WANTED: USED HIGH TEMPERATURE FURNACE Used high temperature furnace wanted having the following specifications: 2400° - 2600° Fahrenheit or higher pusher furnace, with hydrogen and nitrogen protective atmospheres; ideally capable of running 12” x 12” x 1” ceramic pusher plate design; requires 4” - 6” of working height; will consider all sizes and manufacturers. Contact-Robert Henkle at (414) 298-8140 or at
[email protected]
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MEETINGS AND CONFERENCES
2010
FORGING, SHEET METAL FORMING & POWDER METALLURGY – LINKING INDUSTRY & TECHNOLOGY October 20–22 Porto Alegre, Brazil www.senafor.com.br
PM2010 WORLD CONGRESS October 10–14 Florence, Italy www.epma.com/pm2010 7TH INTERNATIONAL SYMPOSIUM ON SUPERALLOY 718 & DERIVATIVES October 10–13 Pittsburgh, PA www.tms.org PM + ACE 2010 2ND INTERNATIONAL POWDER METALLURGY & ADVANCED CERAMICS EXHIBITION & CONFERENCE October 18–20 Shanghai, China www.China-PM-ACE.com/en
PM SINTERING SEMINAR December 7–8 Cleveland, OH MPIF*
2011 MIM2011 INTERNATIONAL CONFERENCE ON INJECTION MOLDING OF METALS, CERAMICS AND CARBIDES March 14–16 Lake Buena Vista (Orlando), FL MPIF*
(NOTE: NEW DATES & LOCATION) PowderMet2011: MPIF/APMI INTERNATIONAL CONFERENCE ON POWDER METALLURGY & PARTICULATE MATERIALS May 18–21 San Francisco, CA MPIF* INTERNATIONAL CONFERENCE ON TUNGSTEN, REFRACTORY & HARDMATERIALS VIII Co-located with PowderMet2011 May 18–21 San Francisco, CA MPIF* *Metal Powder Industries Federation 105 College Road East Princeton, New Jersey 08540-6692 USA (609) 452-7700 Fax (609) 987-8523 Visit www.mpif.org for updates and registration. Dates and locations may change
The U.S. Department of Commerce has identified PM as one of the nation’s “growth” technologies capable of enhancing the productivity of America’s manufacturing community. The Center for Powder Metallurgy Technology was founded in 1980 in order to foster the growth and vitality of PM. Since its inception as a “non-profit, cooperative technology foundation,” CPMT has been advancing PM technology through the collaborative research efforts of its member organizations: end users, parts fabricators, powder producers, and equipment & service providers. In addition, CPMT provides funding to academic institutions and students to help meet the industry’s need for scientifically trained personnel. Learn more about CPMT and its research projects and educational efforts at www.cpmtweb.org.
cpmtweb.org FOSTERING RESEARCH AND EDUCATION IN SUPPORT OF PM’S FUTURE Celebrating 30 years in R&D ~ Join us today
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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MEMBERSHIP APPLICATION Please type or print legibly.
I hereby apply for membership in APMI International. Name (First, Middle Initial, Last) Company
ANNUAL DUES: G United States, Canada and Mexico .............$105.00 G Overseas.......................................................$125.00 G Students (Full-Time Only)..................................$25.00 G Overseas Students (Full-Time Only) ..................$40.00 Payments by check or credit card are acceptable, in US Dollars, drawn on a US bank. Make check payable to APMI International. Upon receipt of full payment, membership will be processed.
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PLEASE CHECK APPROPRIATE BOX Level of Education G High School G Associate Degree G Some College G Bachelor’s Degree
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PLEASE CIRCLE APPROPRIATE NUMBERS (ONLY ONE IN EACH CATEGORY) Primary Job Function Company Primary Function 1 Company Management 1 PM Parts Manufacturer 2 Research & Development 2 Metal Powder Supplier 3 Engineering (incl. Design) 3 User of PM Parts or Products 4 Sales/Marketing 4 Equipment Mfg/Supplier (i.e., presses, furnaces, lab equip., 5 Metallurgical/Laboratory belts, atmospheres, services, etc.) 6 Production/Mfg/Maintenance 5 Consulting or Research 7 Technician 6 Educational Institution 8 Educator 7 MIM—Parts and Suppliers 9 Student 8 HIP/Advanced Particulate Products 10 Human Resources 9 Hardmetals 11 Accounting/IT 10 Other ______________________________ 12 Quality Assurance 13 Other ______________________________ APMI International 105 College Road East, Princeton, New Jersey 08540-6692 USA Phone: 609-452-7700 Fax: 609-987-8523
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For a complete list of benefits and an online application visit: www.apmiinternational.org
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PM BOOKSHELF
Volume 46, Issue 5, 2010 International Journal of Powder Metallurgy
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ADVERTISERS’ INDEX ADVERTISER
FAX
WEB SITE
PAGE
Ace Iron & Metal Co. Inc. ________________(269) 342-0185_________________________________________________________5 ACuPowder International, LLC ____________(908) 851-4597 _________www.acupowder.com _____________________________26 Ametek specialty metal products __________(724) 225-6622 _________www.ametekmetals.com ___________________________18 Centorr Vacuum Industries _______________(603) 595-9220 _________www.centorr.com ________________________________44 Global Titanium _______________________(313) 366-5305 _________www.globaltitanium.com ___________________________57 Hascor International Group ______________+10 210 225 6120_______www.hascor.com__________________________________6 Hoeganaes Corporation _________________(856) 786-2574 _________www.hoeganaes.com ________________Inside Front Cover Molded Fiber Glass Tray Company ________(814) 683-4504 _________www.mfgtray.com ________________________________17 North American Höganäs Inc. ____________(814) 479-2003 _________www.nah.com ____________________________________3 Rio Tinto Metal Powders/ Quebec Metal Powders Limited __________(734) 953-0082 _________www.qmp-powders.com ____________________Back Cover Robert Henkle ______________________________________________rhenkle@reinhartlaw.com __________________________60 SCM Metal Products, Inc.________________(919) 544-7996 _________www.scmmetals.com _________________Inside Back Cover Union Process ________________________(330) 929-3034 _________www.unionprocess.com ___________________________10
ADVERTISER’S REQUEST FOR INFORMATION FAX FORM Need more information on products or services seen in this issue?
Complete the form below and fax to the advertiser(s) of your choice. Fax numbers are listed in the advertisers’ index above.
To:___________________________________ Fax #: ______________________________________ Company: _________________________________________________________________________ Please send me more information on:_____________________________________________________ _________________________________________________________________________________ as advertised in the __________ issue of the International Journal of Powder Metallurgy. Please send information to: Name: Title: ________________________________________________________________________ Company: _________________________________________________________________________ Address:___________________________________________________________________________ City:____________________________ State:_______________ Postal Code: ___________________ Country:___________________________________________________________________________ Phone:___________________ Fax:___________________ E-Mail: ___________________________
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A COMMITTED BUSINESS & TECHNICAL PARTNER
Metal Powders
www.qmp-powders.com