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March/April 2010
Focus Issue: Microminiature Powder Injection Molding—Part I
46/2 Newsmaker Animesh Bose Materials for Medical and Dental Devices Metal and Ceramic Parts High-Strength 316L Stainless Steel Nitrided High-Performance Titanium Products
FRONT MATTER_ FRONT MATTER 3/2/2010 12:08 PM Page 15
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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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46/2 March/April 2010
Editor’s Note Newsmaker Animesh Bose PMT Spotlight On …Todd M. Jensen, PMTII Consultants’ Corner Joseph Tunick Strauss
FOCUS: Microminiature Powder Injection Molding—Part I 15 Materials for Microminiature Powder Injection Molded Medical and Dental Devices R.M. German
21 Metal and Ceramic Parts Fabricated by Microminiature Powder Injection Molding V. Piotter, T. Hanemann, R. Heldele, M. Mueller, T. Mueller, K. Plewa and A. Ruh
29 High-Strength Powder Injection Molded 316L Stainless Steel L.-H. Cheng and K.-S. Hwang
39 Nitriding Response of Microminiature Powder Injection Molded Titanium T. Osada and H. Miura
45 46 47 48
DEPARTMENTS PM Industry News in Review Meetings and Conferences PM Bookshelf Advertisers’ Index Cover: A zirconia dispenser screw (green and sintered) made via μPIM. Photo courtesy Volker Piotter, Karlsruhe Institute of Technology.
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:
[email protected]
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EDITOR’S NOTE
T
he downturn in the economy has not been as serious to the powder injection molding (PIM) industry as it has to conventional powder metallurgy (PM), and the prognosis for 2010 is upbeat. The PIM segment’s growth reflects strong exports to Asia and Europe, growth in traditional PIM markets, and increasing penetration into new markets as a result of opportunities for cost-effective manufacturing. PIM is most advantageous for small, complex-shaped components fabricated in high production volumes and there is a significant growth trend toward microminiature components in which individual features are measured in micrometers. Given this technological climate, Rand German has assembled a two-part focus in the Journal on microminiature powder injection molding (μPIM). This issue’s Part I focuses on materials for medical and dental devices, metal and ceramic parts, high-strength 316L stainless steel, and titanium products. As an example, the front cover displays a ZrO2 dispenser screw (used as an untwisting tool) in the green and sintered conditions. In Part II (May/June 2010), coverage will embrace full-density nanopowder agglomerate sintering and modeling/simulation in PIM and μPIM. The two focus issues are timely in that they bookend MIM2010, the International Conference on Injection Molding of Metals, Ceramics, and Carbides, sponsored by the Metal Injection Molding Association. It is also timely that Animesh Bose, co-chair of MIM2010, and a Fellow of APMI International, is this issue’s “Newsmaker.” Animesh is a frequent participant in APMI and MPIF activities and is recognized throughout the PM industry for his contributions to the science and technology of PIM, hardmetals, tungsten heavy alloys, and intermetallic compounds. In the “Consultants’ Corner” again, Joe Strauss tackles readers’ questions and issues in three diverse areas: reverse engineering of the metal matrix for diamond-grinding wheels; the origin of blistering in the sintering of silver-base contacts and electrodes; and increasing the specific surface area of catalytic oxide powders.
Alan Lawley Editor-in-Chief
Following R&D Magazine’s R&D Awards (see my comments in the November/December 2009 issue of the Journal) it is interesting to review the Wall Street Journal’s 2009 Technology Innovation Awards. Winners included a tool to identify new disease strains, an artificial hand, and paper-thin flexible speakers. In the “Materials and Other Base Technologies” category, the winner was QD Vision, Inc., for the development of a way to dramatically improve the color quality of LED lights by using semiconductor nanocrystals. The runner-up, Novelx, touts a scanning electron microscope capable of imaging nanoscale objects and materials—utilizing silicon processing technologies to miniaturize the technology inside the microscope. With μPIM, nanocrystals, and miniaturization, it really is a small world after all!
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Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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International: powder injection molding. If you wish to produce complex ceramic and metal products using the PIM process, then come to the leading international @?>=<;:<@9@8<7896<@85>:32810/.0-,8+*)8(*'&8%>86;$>896>8;??)*?)<;9>81##0".! 08;=6<7>89>=67*:*(8 and the required know-how from our PIM laboratory. With our expertise, you will be able to manufac9')>8>5=<>79:(8;7389*896>86<6>@98';:<9(&8?)>?;)>8;9>)<;:&8<7>=9<*7*:38=*?*7>79@&83><738;738
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NEWSMAKER
ANIMESH BOSE
By Peter K. Johnson*
Overcoming obstacles is a hallmark of the distinguished career of Animesh Bose, 2009 APMI Fellow and powder injection molding (PIM) expert. His first experience with metal powders happened in Kolkata (formerly Calcutta), India, on his seventh birthday when his parents gave him a science set that contained a magnet and iron filings. Metal particles dancing as they were attracted to the magnet fascinated him. He also fired clay dug up from his backyard garden and formed it into shapes in the family cooking oven. “Those two things stuck in my mind for a long time,” he said. Chemistry and metals took root in his imagination during his teenage years alongside a keen interest in cricket, field hockey, and soccer. When the time came, he decided on pursuing a metallurgical engineering degree from the Indian Institute of Technology (IIT) in Kharagpur, West Bengal. But he also considered staying heavily involved in sports, which almost crippled him. A sports-related injury that turned into a serious infection in two vertebrae put his academic career on hold for almost two years. He faced two crucial choices: delicate back surgery to replace the two vertebrae, a procedure that could paralyze him if anything went wrong, or long-term heavy doses of medication and absolute bed confinement. After choosing the surgery option, against the wishes of his family, he endured months of rehabilitation, including learning how to walk again. “I was happy just climbing a few stairs,” he said. As his illness thus quashed plans for participating in any active sports, he returned to IIT in 1975 to complete his bachelor’s degree and pursue a career in academics. He studied PM under Professor B.K. Dutta in his final year for a B. Tech (Honors) degree in metallurgical engineering. “PM was relatively new in India and caught my fancy,” he said. “And I never forgot using my mother’s oven to sinter ceramic (clay) shapes.” In 1982 Bose received a PhD under Professor P.G. *Contributing editor
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Mukunda, writing his thesis on vacuum sintering of bronze bearings. A fellowship from the Council for Scientific and Industrial Research and from IIT studying cutting tools and hardmetals was his next career move. In 1985, he was invited by Professor Randall German to come to Rensselaer Polytechnic Institute (RPI) in Troy, New York, as a visiting scientist. He stayed four years, working on metal injection molding (MIM), iron–nickel alloys, tungsten heavy metals, and intermetallic compounds. While presenting a paper on MIM at the 1988 International PM Conference in Orlando, Florida, he challenged the MIM industry to break out of its secrecy shell and consider working together to promote the spread of the technology and the development of new binders and materials. At RPI, his work on intermetallic compounds resulted in a patent on reactive sintering of nickel aluminides. His research on tungsten heavy alloys led to the development of several new high-strength, high-hardness heavy alloys through alloying with other refractory metals; several alloys were also patented. This development, along with MIM, allowed the processing of complex-shaped high-strength, high-hardness heavy alloys without the need for thermomechanical processing. Fond memories of RPI remain with him, especially sharing an apartment with graduate students and Professor German sponsoring him for a green card. “Deepak Madan helped me break into U.S. culture and Joe Strauss helped me buy my first car, an old Dodge Omni that he fixed up for me,” he says. Upstate New York’s bitter cold winter was the only major problem for him. Coming from a warm climate in India, he had never witnessed snow. One particularly severe storm that blew a large snowdrift against his apartment door and shut him inside forced a decision to relocate to a warmer climate. In 1989, after receiving his permanent residency, he ijpm
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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left Troy for Southwest Research Institute in San Antonio, Texas, to establish a new PM division. He earned three patents, which were placed under a U.S. government secrecy order, before joining Parmatech Corp., Petaluma, California, in 1992 as director of R&D. He expanded the company’s MIM materials base and helped train licensees of its technology in Brazil, France, Israel, Japan, and Switzerland. Being fiercely independent and entrepreneurial, in 1995 he co-founded Material Innovations in Petaluma to make MIM dental parts. The company was sold within six months. Parmatech was purchased by Carpenter Technology in 1995. Deciding that he would not enjoy working within a large corporate structure, Bose left Parmatech before the buyout. However, he agreed to help the new owner with the transition for 18 months as an independent contractor. It was a tough decision because his contract prevented him from working in the PIM business for five years. He founded Materials Processing in the same town and ran R&D programs on traditional PM materials while also joining forces with a start-up company, Powdermet, working on coated powders. “It was five long years away from the field that I loved,” he said. But he used the time well, coauthoring with Randall German the textbook Injection
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
Molding of Metals and Ceramics, which was published by MPIF. When his no-compete contract was about to expire, he moved to Fort Worth, Texas, and formed Materials Processing Inc. (MPI) in 1999. Initially he concentrated on government projects in heavy alloys and later moved into PIM R&D and manufacturing hardmetal MIM and advanced ceramic parts for the government, as well as parts for the aerospace and oilfield markets. In 2008, having been in business for almost a decade, he contemplated selling MPI and started discussions with several interested businesses. Before a sale was completed, an excellent opportunity came up with Advanced Metalworking Practices (AMP), LLC, in Indiana. “I was impressed by Ken Edwards, CEO of the parent company that had taken ownership of AMP,” he says. “I was really fond of all the people that worked at AMP” he adds. He joined AMP as general manager, leaving his family in Texas to run MPI until the sale was completed. However, MPI’s business suffered during one of the worse economic downturns in late 2008, and the proposed sale did not materialize. The strain of being away from his close-knit family (he traveled almost every weekend between Indiana and Texas) and the detrimental effect his protracted absences were
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having on his two children made him decide to leave AMP and return to Texas after nearly a year. He is currently working at MPI. Recently he teamed up with Trigon International Corporation, Bolingbrook, Illinois, a medical device company, to form a joint venture in MIM to be named Trigon Advanced Materials. This entity will make complex-shaped PIM wear-resistant parts from hardmaterials and will also expand into MIM medical applications. Bose and his wife Prarthana, who works alongside him in the business, have been married 24 years, in spite of never having met until their wedding day. Considering life as an experiment, he decided to follow this ancient Indian tradition that nourishes lifetime marriages. “When I saw my wife for the first time at the wedding ceremony it was as if we had known each other for ages,” he said. “She is my soulmate and a great encourager.” At times when the workload requires 10- to 12-hour workdays, it is a tremendous blessing to have her work side-by-side. They have two children: a son, Pinaki, a student at the University of Texas, and a daughter, Shree, who is in high school. Bose has given back to the PIM and PM industries for the career they have given him. He served on the APMI International Board of Directors, and co-chaired several international conferences and six MPIF conferences on tungsten, refractory, and hardmetals. He is the co-chair of the MIM 2010 International Conference on the Injection Molding of Metal, Ceramics and Carbides, in Long Beach, California. He has also been named co-chair of the International Conference on Tungsten, Refractory and Hardmaterials in 2011 in Chicago. He has published more than 115 technical papers, authored or co-authored three books, and is listed as the inventor or co-inventor on eight U.S. patents. He is also a Fellow of ASM International and a life member of the Powder Metallurgy Association of India (PMAI). When not pursuing technical activities, Bose enjoys collecting rare stamps and listening to old Hindi songs and Indian classical music. Told by his doctor in 1976 that he should consider himself lucky to one day be able to tie his own shoelaces, he feels fortunate indeed. Summing up his career, Bose is most pleased about his involvement in the growth of MIM technology worldwide. “MIM is going to be one of the major strongholds of the powder metallurgy industry,” he says. “It still has a tremendous potential.” ijpm
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Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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AMETEK
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PoP Centre Inaugurated in October 2009, the PoP Centre’s aim is to support our customers more effectively, in application and process development of metal powder components. The resources of the PoP center include FEA/CAD-design support, stateof-the art multi-level CNC compaction press, advanced CNC machining, CNC milling centre for rapid protyping, and component fatigue and tribology testing. These resources are complemented with the metal powder knowledge and experience of North American Höganäs.
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SPOTLIGHT ON_ SPOTLIGHT ON 3/2/2010 12:10 PM Page 9
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SPOTLIGHT ON ...
TODD M. JENSEN, PMTII Education: Diploma in Metallurgical Technology, Hennepin Technical College, 1994 BS, Industrial Management, University of Wisconsin, Stout, 2003 MBA, University of Wisconsin, Eau Claire, 2008 Why did you study powder metallurgy/particulate materials? I was first introduced to PM when I visited the Metallurgical/Powder Metal Technology program at Hennepin Technical College, Brooklyn Park, Minnesota. That is where I met Jeff Reinhart, the program’s instructor. He showed me the manufacturing process at the college’s laboratory and demonstrated the advantages of the process over other manufacturing technologies. I was amazed with it all, having never heard of PM. I left the school that day and immediately starting making calls trying to find out as much about the PM industry as I could. After some investigation, I could clearly see that PM was a viable, growing industry. I also spoke to alumni of the program and industry employers who gave the program great reviews. I enrolled in the program the same week! When did your interest in engineering/science begin? My father always seemed to find a way to make things work. At a young age, he taught me that there is a solution to every problem; you just have to find it. To me, problem solving is the essence of engineering and pursuing a profession in problem solving seemed like the logical choice. I am going to get some grief for saying this, but, as a kid, I can remember watching the TV show, Gilligan’s Island, in awe of the professor who made batteries for the radio out of coconuts. I was the only kid in my elementary school class who wanted to be a scientist. Growing up I had a chemistry set and microscope and,
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
in school, mathematics and science were always my easiest and most interesting subjects. What was your first job in PM? What did you do? My first job in PM was a part-time internship at FMS Corporation, Minneapolis. I updated shop-floor routings and made minor print changes. Describe your career path, companies worked for, and responsibilities. My first full-time position was as a metallurgical technical specialist for L.E. Jones, Menomonee, Michigan. The company is primarily a foundry and the PM operation was a pilot plant at the time. As it was a small operation, I got a chance to work in a variety of capacities including all the work in the metallurgical laboratory. I also set up compacting presses, trained operators, and set quality standards. My next two jobs were with companies that have since been acquired by larger organizations: Comtech, Waupun, Wisconsin, and Zenith Sintered Metals, Manitowoc, Wisconsin. Both positions were as a process engineer and both focused on high-volume production for the automotive industry. My responsibilities were primarily in the area of Lean Manufacturing improvements, driving waste out of the process, and improving margins on existing products. For the last 10 years, I have been with Phillips Plastics, working in their metal injection molding (MIM) business unit. I started as a process engineer and now I manage the technical group as well as the maintenance department. Senior Process Engineer Phillips Metal Injection Molding 422 Technology Drive East Menomonie, Wisconsin 54751 Phone: 715-233-4040 Fax: 715-233-4090 E-mail:
[email protected]
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SPOTLIGHT ON ...TODD M. JENSEN, PMTII
What gives you the most satisfaction in your career? It has been most satisfying for me to grow with the MIM business unit at Phillips Plastics. I started with the company just after the MIM facility was built in 1999 and I have had the privilege of working with a unique group of professionals to help create something special.
to recognize some areas of the business in which I needed to improve my understanding. The examination was a means to test my comprehension of the subject matter. Additionally, I felt that the two levels of certifications would add to my credibility as an engineer in the field of PM, both to my employers and to the customers I work with.
List your MPIF/APMI activities. I have served on the MIMA Board, the MIMA Standards Committee, and the APMI Membership Committee.
How have you benefited from PMT certification in your career? Studying for certification helped me explore areas of PM in which my depth of knowledge was deficient. I used the recommended study material to increase my understanding of the broader field of PM. Ultimately, having the knowledge and certification made me a more competitive candidate for positions in the industry. Many employers recognize PMT certification at both levels as evidence of a higher level of understanding of PM. Also, acquiring certification is a testament to an individual’s drive and commitment in reaching personal goals.
What major changes/trend(s) in the PM industry have you seen? Because I have spent the last 10 years in MIM, I can speak about trends in this subsector of the PM industry. We see many more customers coming to us with prior knowledge of MIM. In the past, we had to spend a substantial amount of time educating and explaining the MIM process and capabilities and had to work diligently with our sales force to find the right applications. This involved weeding out projects that were an obvious fit for other technologies, such as conventional press-and-sinter PM or screw machining. Now the applications tend to be a good fit, but customers are looking for tighter tolerances and higher-performance alloys. Why did you choose to pursue PMT certification? I chose to pursue PMTI and PMTII certification for two reasons. The path to certification helped to increase my knowledge of PM’s broad subject area. As I studied for the examination, I was able
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What are your current interests, hobbies, and activities outside of work? I have two boys, aged 8 and 9, and I love spending time with them. I hunt whitetail deer near my property in northern Wisconsin and enjoy trout fishing in the springtime and boating in the summer. I have recently started woodworking again, a hobby that I had to let slide over the last few years. ijpm Would you like to be featured here? Have you been PMT Certified for more than 2 years? Contact Dora Schember (
[email protected]) for more information.
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
CONSULTANTS' CORNER_ CONSULTANTS' CORNER 3/2/2010 12:11 PM Page 11
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CONSULTANTS’ CORNER
JOSEPH TUNICK STRAUSS* Q
We are trying to reverse engineer the metal matrix in an industrial diamond grinding wheel and have obtained chemical analyses via wet chemistry, inductively coupled plasma (ICP), atomic absorption (AA), and spark spectroscopy. The metal matrix appears to be a cobalt base alloy with additions of chromium, copper, iron, nickel, tin, titanium, tungsten, and other elements. We have approached several powder producers but they say this alloy is impossible to make. What makes an alloy impossible to atomize and where can we obtain this alloy in powder form? If a powder producer turns you down chances are they are telling you the truth; this alloy cannot be made via atomization. Not all alloys can be atomized into powder and the reasons can be technical and/or economic. For instance, incompatibility among the components, reactivity of the melt, temperature required to atomize, or required purity levels may limit or eliminate the feasibility of processing an alloy into powder by atomization. In addition, small quantities and narrow particlesize distributions may increase the cost to where the task is not economically feasible. In your particular case the alloy specified simply does not exist although your analyses have accurately determined the components and the composition. The analytical techniques employed provide an analysis of the bulk material—as if the material exists in a homogenous alloy. They do not account for the individual components existing as discrete entities. A diamond wheel bond matrix is not necessarily an alloy and, in fact, rarely is. Metal-bonded diamond tools are produced via powder metallurgy (PM). These materials are made by blending various metal powders with diamond powder and then subjecting this mixture to con-
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solidation and sintering processes. Some parts can be cold pressed and sintered while others require hot pressing. The components and composition of the “matrix” can be complex and will depend on the ultimate application. Many of the matrices used are cobalt based but additions are made to aid processing, vary the mechanical properties (e.g., ductility, toughness), and to vary the bond strength between the matrix and the diamond. Copper and tin are used to form a liquid phase to enhance densification and to increase thermal conductivity. Iron and nickel increase the ductility and toughness of cobaltbased matrix alloys. Carbide formers such as chromium, iron, and tungsten increase the bond strength between the diamond and the matrix (although the majority of the “bonding” is mechanical, some is chemical). Even titanium is added and can act as an oxygen getter and a carbide former. To make things even more confusing, most diamond tool matrix compositions are somewhat of an art rather than a science or an engineered product and not all the causes and effects of the various components and their contributions and interactions are understood. In order to properly reverse engineer the matrix another approach is necessary. This will involve quantitative microstructural analysis of the diamond tool. Essentially, you must look at the microstructure and identify the number of different entities (phases) and calculate their relative amounts. You must use a scanning electron microscope (SEM) since you will need to use its energy dispersive spectroscopy (EDS) function to identify the composition of these phases. The quantitative analysis of the microstructure will
*Engineer & President, HJE Company, Inc., 820 Quaker Road, Queensbury, New York 12804; Phone: 518-792-8733, Fax: 518-792-8735; E-mail:
[email protected]
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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CONSULTANTS’ CORNER
give you the relative amounts of each phase on a volume basis. This must be converted to a weight percent once you know the composition (and density) of each phase. In addition, you should measure the size of the entities, which will give an approximate measure of the particle size of the powder used in the process. Copper and tin may be distributed with a skeletal morphology since these elements probably were liquid during the processing (and their initial particle sizes are not too critical for that reason). Some of the other particles will have sintered and coarsened so one must look for prior particle boundaries if possible. Also, since the material contains diamond particles, metallographic preparation will not be simple. It may be easier to examine a fracture surface, which is not easy, either. This task is not trivial. Sample preparation is difficult and the number of separate entities that have to be counted is probably in the hundreds in order to get a statistically representative sampling.
Q
We press-and-sinter silver and silver-alloy parts for contacts and electrodes. Occasionally a decrease in density results after sintering and/or blistering. What is the origin of this problem? This phenomenon is also common in PM copper systems and several other materials, especially soft materials. There are two primary causes of a density decrease and blistering: evolution of a gas or expansion of an entrapped gas. Evolution of a gas means just that: gas evolves from where there was none before. In the case of a silver–copper alloy (or silver–tin and other alloys where the alloying addition forms an easily reducible oxide), the copper will cause the alloy powder to contain some oxygen in the form of an oxide (silver oxide is not stable at room temperature). If the powder is pressed into a compact and then sintered in a hydrogen atmosphere, hydrogen diffuses into the part. When it reaches the oxide it reduces the oxide. Where there was an oxide there is now a clean metal surface and a molecule (or more) of water (H2O). In the early stages of sintering this may not be a problem as the water molecule can escape via the interconnected pores of the compact. However, if the material around the water molecules has sintered, there is no way for the gas to escape. Hydrogen (especially atomic hydrogen) is a small entity and can diffuse readily through the silver or copper matrix. A water mole-
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cule is huge in comparison and cannot diffuse through the matrix. With no place to go, and with continued heating, the water vapor expands and its pressure can exceed the yield strength of the surrounding matrix causing the material to expand. The resulting material has numerous pores and an attendant low density. Sintering in a hydrogen-free atmosphere can help eliminate this situation. A pressed part can also expand on sintering if entrapped gas cannot escape. If one presses a part to >92% of the pore-free density then the pores do not exist in an interconnected structure—some will be isolated. These pores contain air. In some material, notably clean copper (and indium, lead, and tin), gold, platinum, and silver, pressing at room temperature can result in a significant amount of metallurgical bonding. So it is possible to press these materials to such an extent that a green part is formed with isolated pores that are surrounded by fully dense material. Upon heating, the gas (air) in these pores cannot escape and the gas molecules (nitrogen and oxygen) cannot diffuse through the metal matrix. As the temperature increases the gas pressure in these pores can exceed the yield strength of the surrounding metal and cause the part to dilate. One could press the part in a vacuum but this would be difficult, if not expensive, in production. Rather, the part must not be pressed to such high densities where isolated pores are formed. In this case, the final density of the part will be higher if the part is pressed to a lower density (a density where there is a minimum of isolated pores in the green state).
Q
For a catalytic substrate application we buy an oxide powder with a mean particle size ~10 µm and a D95 ~25 µm. We wish to increase the specific surface area from the current 90 m2/g. We milled this powder and reduced the mean particle size to ~5 µm and D95 to ~15 µm but we did not significantly change the specific surface area. What did we do wrong? You did not do anything wrong but you are not going to increase the specific surface area (SSA) of this powder by reducing its particle size. Although this powder is relatively fine with respect to most metal powders used in the PM industry, this regime of particle size does not correspond to the high specific surface area of 90 m2/g, based on its particle-size distribution. If the individual powder particles of your powder were solid smooth
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Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
CONSULTANTS' CORNER_ CONSULTANTS' CORNER 3/2/2010 12:11 PM Page 13
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CONSULTANTS’ CORNER
spheres the expected SSA for this size distribution would be at least two orders of magnitude less than what you are currently measuring. This implies that there is significantly more surface area in your material than just the external surfaces of the powder. Your powder particles are most likely agglomerates of extremely fine particles. When you measure particle size you are actually measuring the external size of the stable agglomerates and these agglomerates have a high internal surface area. The method used to measure SSA (BET, gas adsorption) can measure this internal surface area, as long as an interconnected porous network exists. Particle-size determination by laser scattering techniques is not affected by these internal surfaces. Although you did manage to significantly decrease the particle size by milling, you did not appreciably increase the SSA since the majority of
the surface area is internal to the particles. Think of it this way: A kitchen sponge has a high internal surface area. The contribution of the external surface to the total surface area is probably negligible. If you cut the sponge in half you have certainly decreased the size of the sponge but you have not significantly increased the specific surface area. There is probably nothing you can do to this powder to increase its SSA as this property is related to the original manufacturing process. I would suggest contacting the manufacturer to see if they produce a powder grade with increased SSA. 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]
MOLDING METALS INTO MEDALS year after year after year... 2009 MPIF Award of Distinction Hand Tool/Recreation Category Where Imagination & Design Take Shape For nearly 40 years, Parmatech has refined PIM technology by molding metals into award winning products.
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Preview of Part II of focus on μPIM in next issue The Journal will follow up on the MIM2010 conference in the May/June 2010 issue with Part II of the focus on μPIM. Following are the articles to be included in that issue:
MICROMINIATURE POWDER INJECTION MOLDING—PART II Full-Density Nanopowder Agglomerate Sintering of Powder Injection Molded Iron–Nickel J.-S. Lee, B.-H. Cha and W.-K. You A Review of Computer Simulations in Powder Injection Molding S.J. Park, S. Ahn, T.G. Kang, S.-T. Chung, Y.-S. Kwon, S.H. Chung, S.-G. Kim, S. Kim, S.V. Atre, S. Lee and R.M. German Characterization and Simulation of Macroscale Mold-Filling Defects in Microminiature Powder Injection Molding S.G. Laddha, C. Wu, S.-J. Park, S. Lee, S. Ahn, R.M. German and S.V. Atre Sintering of Powder Injection Molded 316L Stainless Steel: Experimental Investigation and Simulation X. Kong, T. Barriere, J.C. Gelin and C. Quinard
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Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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MICROMINIATURE POWDER INJECTION MOLDING—PART I
MATERIALS FOR MICROMINIATURE POWDER INJECTION MOLDED MEDICAL AND DENTAL DEVICES Randall M. German*
INTRODUCTION One of the key successes of PIM has come from its penetration into the medical and dental fields. This started with dental orthodontic brackets, moved to surgical and dental tools and, in recent years, has included implants out of both metals and ceramics. Today there is serious discussion of replacement components, such as heart valves, using PIM. This maturation has come slowly since considerable concern arises over the difference between PIM and standard materials. For example, surface depletion of high-vapor-pressure species, such as chromium during the sintering of stainless steel, creates significant concerns over a loss of corrosion resistance and biocompatibility in a sintered device.1–3 Thus, through meeting bulk chemistry specifications, special care is required to ensure that the performance is equivalent to previously qualified materials. For μPIM components, the ratio of surface area to bulk makes it imperative to secure the highest possible passivity, so conservative materials selection is the routine approach. In the companion paper by Cheng and Hwang the surface-area effect is turned into an advantage as a means to convert a traditional PIM 316L stainless steel into a high-strength option useful in notebook computers. With respect to medical applications, the PIM materials spectrum is covered in detail by Johnson.4 The 300-grade austenitic stainless steels are probably the most advanced in terms of acceptance, followed by 400-grade stainless steels, and 600-grade precipitation-hardening stainless steels. The PIM strength and ductility properties are comparable with those specified for medical and dental devices, even though the impact and fatigue properties are inferior. Thus, stainless steels reflect early successes for surgical tools and a few temporary implants.5 Ceramics have progressed to implants such as heart-pacemaker capsules where most of the successes have been with alumina or zirconia.6–8 A few other metals are used to a lesser extent; these include cobalt–chromium alloys, nitinol, titanium alloys, and tungsten.4,9–13
Powder injection molding (PIM) is most successful for small, complex-shaped components fabricated in high production volumes. A significant growth trend is toward microminiature components, in which individual features are measured in micrometers. These are destined for minimally invasive surgical tools, medical and dental implants, and components for various intervention strategies. The required component specifications put a demand on PIM in terms of available powders. This contribution reviews the status of the materials used for microminiature powder injection molding (µPIM) medical and dental devices by considering powder availability, cost, and performance attributes. The seed for this analysis comes from discussions at a workshop that focused on the scientific and technological barriers in microminiature molding. Companion papers in this and the subsequent issue of the International Journal of Powder Metallurgy provide an overall perspective on µPIM and its enormous potential in relation to medical and dental devices.
*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 2, 2010 International Journal of Powder Metallurgy
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MATERIALS FOR MICROMINIATURE POWDER INJECTION MOLDED MEDICAL AND DENTAL DEVICES
Figure 1. Factors affecting material choice for μPIM in medical and dental devices
Tungsten is used for cauterization applications, but not in implants. Beyond these materials, most of the remaining candidates, such as glass-ceramics, gold, hydroxyapatite, molybdenum, niobium, platinum, and tantalum, have peripheral interest due to cost or property limitations. The trend in μPIM medical and dental devices is toward smaller components.12,14,15 As with all efforts, cost is an overarching concern. Thus, a summary assessment of the materials for μPIM is provided in Figure 1, based on consideration of the following: • medical and dental application acceptance • powder injection molding issues, namely, sinterability • powder availability, and • powder cost. As will be shown, only a few materials tend to be successful in μPIM when targeted at medical and dental applications. CONSTRAINTS µPIM identifies a size scale in which the component has features measured in micrometers and a component mass down to 0.002 g. Today individual features reach into the 10 to 15 μm range. The largest dimension tends to range up to 10 mm. The medical field is well recognized as being attractive for PIM vendors.16,17 Accordingly, the medical arena is now the largest market segment for PIM in North America. This is in dramatic contrast to Asia where the emphasis is on computers, consumer products, and cellular telephone parts. The
16
European emphasis is on automotive components. Several dental and medical μPIM components are under active investigation, including cochlear ear implants, small surgical forceps and scissors, arthroscopic surgery components, dental tools, implantable defibrillator and heart-pacemaker components, filters, stents, valves, dental implants, transducers, orthodontic brackets, and several tools for minimally invasive surgery or robotic surgery techniques. Cost reductions of 15-fold are reported in moving to μPIM for small medical devices, such as biopsy forceps.18 Thus, an important target for μPIM is in components for minimally invasive medical and dental applications. With respect to μPIM medical and dental devices, most of the efforts have focused on just three materials—alumina, zirconia, and stainless steel. The properties of μPIM materials tend to be attractive, since invariably strength increases as the cross section and grain size decrease. For example, a report by Piotter et al. in this issue cites μPIM zirconia with an astounding strength of 3 GPa. So, from the viewpoint of static properties, μPIM materials are highly successful, and case studies have established biocompatibility, especially for zirconia. In a survey of available powders, only the following were generally available in submicrometer particle sizes: • Elemental powders—copper, cobalt, iron, molybdenum, platinum, nickel, silver, tantalum, tungsten • Compound powders—ZrO2, Al2O3, WC • Alloy powders—special orders only With respect to powder availability, the ceramic compositions are available in submicrometer particle sizes. Since design specifications are now reaching to 10 μm, to properly fill out such a feature requires a particle size at least 10% of the feature size, or below 1 μm. Small metallic powders are often fabricated by evaporation–condensation or chemical-precipitation techniques, so they are not available in alloys. Alloys are extremely difficult to atomize into this size range, although elemental powders (such as copper, iron, nickel, tantalum, tungsten) are available. Alloy powders are currently fabricated in the micrometer-size range, so this limits feature sizes attainable in the μPIM component. New atomization routes for submicrometer powders have been examined, but so far viability has not been demonstrated. Most submicrometer powders are expensive. Further, test Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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MATERIALS FOR MICROMINIATURE POWDER INJECTION MOLDED MEDICAL AND DENTAL DEVICES
data on biocompatibility is largely absent, and as particle size decreases there is increasing concern over impurity effects that might influence behavior. Compounds are available in several ceramics, but most of the focus is on zirconia and alumina. In many of these powders agglomeration is a problem and in most cases there is no specification for agglomeration or purity. ASSESSMENT For μPIM medical and dental device applications, materials selection narrows for microminiature devices. For devices with features in the 1 mm range, all of the common PIM materials are possible. As the feature size decreases to 0.1 mm the required particle size falls below 10 μm and only ceramics and stainless steels are available. Below this feature-size range, zirconia and alumina are the remaining candidates, and of these, zirconia provides the better properties. For alumina, the chief concerns are low strength, the absence of ductility, low fracture toughness, and only modest success in the medical and dental fields. Cobalt–chromium alloys are widely accepted in partial dentures, implants, and surgical tools, with good biocompatibility, but there is not a supply of small powders. Further, the integrity of sintered materials during long-term use is not established. Stainless steel powders are now fabricated down to the micrometer size range, but during sintering the preferential evaporation of chromium creates concern over biocompatibility. Current prices are high. Indeed, Figure 2 plots the powder cost per cm3 (sintered) for 72 different PIM materials representing different vendors, compositions, and powder characteristics. The median cost of powder is $0.14/cm3 and, ignoring gold alloys, 90% of the powders being offered for PIM are under $0.57/cm3. Some of the novel submicrometer powders, offered at $10/cm3, exceed the current range used in PIM. Titanium is typically not fabricated as a small powder because the accompanying impurity levels take the sintered product out of specification. The proposed ASTM PIM standard for Ti-6Al-4V sets the maximum oxygen level after sintering at 0.2 w/o. In a companion paper in this issue, Osada and Miura show promise using surface nitridation for titanium. For the typical 25 to 35 μm particlesize range, spherical powder with a low oxygen content (<0.2 w/o) tends to sell for $100/kg, but submicrometer powder price increases to $2,000/kg. Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
Figure 2. Cost distribution of 72 PIM powders normalized on a unit volume basis
Likewise, zirconia is one of the fully accepted materials used in μPIM, but it represents an expensive material that requires special sintering cycles. Although access to small powders inhibits some of the envisioned applications for μPIM, at the same time this represents a challenge on how to fabricate submicrometer alloy powders for both research and for commercial investments. RECOMMENDATIONS AND CONCLUSIONS In engineering materials there is a well-recognized inverse relation between cost and use. Materials with a high cost, such as diamond, see limited use, while materials with a low cost, such as steel, are prevalent.19 This is illustrated by the scatter plot shown in Figure 3. Some of the new powders discussed here are being offered at a price range rivaling diamond, and accordingly consumption will be limited to the most demanding applications. Although μPIM is still a small field, the inverse relation is evident. Only a few materials are in use, and these tend to be available, lower in cost, familiar to PIM, and accepted by the medical and dental fields. When viewed in these terms the inventory becomes small with only alumina, stainless steel, and zirconia. Other materials are often discussed, but issues such as cost, availability, processing, and contamination constitute significant barriers. ACKNOWLEDGEMENTS Funding for the “Workshop on Scientific Issues
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Figure 3. Relative use of various engineering materials vs. relative price, showing the natural tendency to use more of the lower-cost materials
for Medical and Dental Applications of Micro/Nano Powder Injection Molding—Molding, Sintering, Modeling, and Commercial Applications” was provided by the National Science Foundation, OISE0738021, and the Korea–U.S. Science Cooperation Center. The workshop was held March 2009, Lake Buena Vista, Orlando, Florida, in collaboration with the Metal Powder Industries Federation (MPIF). Thanks are due the several speakers and participants who helped to add to the vitality of the program. James Adams, MPIF, was most helpful in making the venture successful. REFERENCES 1. R.M. German and D. Kubish, “Evaluation of Injection Molded 17-4 PH Stainless Steel Using Water Atomized Powder”, Int. J. Powder Metall., 1993, vol. 29, no. 1, pp. 47–62. 2. S. Sunada, T. Yamamoto, S. Tanaka, N. Kada and K. Majima, “Influence of Sintering Atmosphere on the Corrosion Behavior of the Type 410 Stainless Steel Prepared by MIM Process”, Journal of the Japan Society of Powder and Powder Metallurgy, 2007, vol. 54, pp. 529–531. 3. S. Sunada, T. Yamamoto, K. Majima and N. Nunomura, “Comparison of Corrosion Behavior Among I/M, P/M, and MIM SUS304 Stainless Steels by Electrochemical Method”, Journal of the Japan Society of Powder and Powder Metallurgy, 2005, vol. 52, pp. 551–562. 4. J.L. Johnson, “Mass Production of Medical Devices by Metal Injection Molding”, Medical Device and Diagnostic
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Industry, 2002, vol. 24, no. 11, pp. 48–53. 5. S. Banerjee, “Structure–Property Relationship of Metal Injection Molded 17-4 PH Orthodontic Parts”, Powder Injection Molding Symposium 1992, edited by P.H. Booker, J. Gaspervich and R.M. German, Metal Powder Industries Federation, Princeton, NJ, 1992, pp. 181–192. 6. I.H. Lee, H.C. Kim, S.T. Chung, Y.S. Kwon and S.H. Baek, “Zirconia Post for Dental Application by Micro PIM”, Workshop on Medical Applications for Microminiature Powder Injection Molding, edited by R.M. German, Metal Powder Industries Federation, Princeton, NJ, 2009, on CD. 7. Y.S. Kwon, S.T. Chung, I.H. Lee and S.H. Baek, “Tips for Dental Application by Micro PIM”, ibid. 8. P.L. Divya, A. Singhal, D.K. Pattanayak and T.R.R. Mohan, “Injection Moulding of Titanium Metal and AWPMMA Composite Powders”, Trends in Biomaterials and Artificial Organs, 2005, vol. 18, pp. 247–253. 9. T. Deguchi, M. Ito, A. Obasta, Y. Koh, T. Yamagishi and Y. Oshida, “Trial Production of Titanium Orthodontic Brackets Fabricated by Metal injection Molding (MIM) with Sinterings”, Journal of Dental Research, 1996, vol. 75, pp. 1,491–1,496. 10. Y. Xu and H. Nomura, “Homogenizing Analysis for Sintering of Bio-Titanium Alloy (Ti-5Al-2.5Fe) in MIM Process”, Journal of the Japan Society of Powder and Powder Metallurgy, 2001, vol. 48, pp. 1,089–1,096. 11. E. Baril, L.P. Lefebvre, Y. Thomas and F. Ilinca, “Foam Coated MIM Gives New Edge to Titanium Implants”, Metal Powder Report, 2008, vol. 63, no. 7, pp. 46–55. 12. B. Williams, “Powder Injection Moulding in the Medical and Dental Sectors”, Powder Injection Moulding International, 2007, vol. 1, pp. 12–19. 13. Y. Thomas, E. Baril, F. Ilinca and J.F. Hetu, “Development of Titanium Dental Implant by MIM: Experiments and Simulation”, Advances in Powder Metallurgy and Particulate Materials—2009, compiled by T.J. Jesberger and S.J. Mashl, Metal Powder Industries Federation, Princeton, NJ, 2009, vol. 1, part 4, pp. 81–93. 14. R.M. German, “Medical and Dental Applications for Microminiature Powder Injection Moulding—A Roadmap for Growth”, Powder Injection Moulding International, 2009, vol. 3, no. 2, pp. 21–29. 15. P. Imgrund, F. Petzoldt and V. Friederici, “Micro MIM for Medical Applications”, Proc. Europe PM 2008, European Powder Metallurgy Association, Shrewsbury, UK, 2008, vol. 2, pp. 305–310, on CD. 16. T.A. Tomlin, “Metal Injection Molding: Medical Applications”, Int. J. of Powder Metall., 2000, vol. 36, no. 3, pp. 53–57. 17. R. Tandon, “Development of Co-Cr-Mo (F-75) Via Metal Injection Molding”, P/M Science and Technology Briefs, 1999, vol. 1, no. 2, pp. 13–16. 18. Y. Li, and H. He, “Biopsy Forceps for Minimally Invasive Surgery by Metal Injection Molding”, PM Asia 2009, Elsevier, Shanghai, China, 2009, on CD. 19. R.M. German, Powder Metallurgy of Iron and Steel, John Wiley and Sons, New York, NY, 1998. ijpm
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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June 27–30 The Westin Diplomat Hollywood (Ft. Lauderdale), Florida
2010 International Conference on Powder Metallurgy & Particulate Materials For complete program and registration information contact: METAL POWDER INDUSTRIES FEDERATION ~ APMI INTERNATIONAL INTERNATIONAL 105 College Road East, Princeton, New Jersey 08540 USA Tel: 609-452-7700 ~ Fax: 609-987-8523 ~ www.mpif.org
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MICROMINIATURE POWDER INJECTION MOLDING—PART I
METAL AND CERAMIC PARTS FABRICATED BY MICROMINIATURE POWDER INJECTION MOLDING
Volker Piotter*, Thomas Hanemann**, Richard Heldele***, Marcus Mueller***, Tobias Mueller****, Klaus Plewa*****, and Andreas Ruh*** INTRODUCTION Microsystems engineering offers a number of fabrication technologies to improve existing methodologies or to manufacture entirely new products that are pervading in diverse ways markets such as information technology, life sciences, vehicle and energy technology, mechanical engineering, and physical- and chemical-process technology. These products are based primarily on fabrication methods to generate micro components from silicon or plastics. Many applications, for example, small gears and counters, chemistry, telecommunications, and biology, require highly resistive micro components made of metals or ceramics. The potential of these materials is well known from precision technology applications where forces, torques, wear, corrosion, and high temperatures are present, or where a small thermal expansion is required. Appropriate fabrication methods will have to be further developed in order to make metals and ceramics usable in microsystems engineering. From an economic point of view, high production efficiency for medium and large numbers of pieces is mandatory without ruling out rapid prototyping. A production method meeting both these demands is powder injection molding1 (PIM) which can be performed as low-pressure powder injection molding (L-PIM) for small series and prototypes, and as high-pressure powder injection molding (H-PIM) for large scale production.2 The present contribution outlines the status of R&D in the PIM of micro components from metals and ceramics. Unique micro specific features in relation to feedstock development, tooling, and processing steps will be highlighted. Some laboratory models will be presented to demonstrate that a number of industry sectors may benefit from this economically efficient production technology.
Microminiature powder injection molding (µPIM) is a promising variant of powder injection molding (PIM) with development profiting by the demand for highly resistive micro products made of metal or ceramic materials. μPIM involves low-pressure or high-pressure injection molding; the former is commonly used for prototypes and small runs while the latter reflects medium and large-scale production. In relation to feedstock improvement, current research focuses on the exploitation of additives and the application of ultrafine powders. In terms of the μPIM shaping process, primary challenges are further miniaturization and the production of micro components with increased complexity. Presently, variants of multi-component μPIM are under development and the process parameters have to be adjusted for metals and ceramics to address issues of disconnection in micro areas. Examples are shaft–wheel combinations consisting of different couples of ceramics or metals. Depending on the powders and process parameters, mobile or immobile joints can be fabricated.
*Head of Process Development Unit, **Head of Materials Development Unit, ***Senior Scientist, ****PhD Student, and *****Head of Operations and Maintenance Unit , Karlsruhe Institute of Technology, Institute for Materials Research III, P.O. Box 3640, 76021 Karlsruhe, Germany; E-mail:
[email protected]
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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METAL AND CERAMIC PARTS FABRICATED BY MICROMINIATURE POWDER INJECTION MOLDING
LOW-PRESSURE POWDER INJECTION MOLDING L-PIM is a variant of the PIM process and is characterized by markedly lower equipment and tooling costs compared with standard H-PIM. It therefore represents an attractive alternative for product development, prototyping, and small production numbers of net-shape-formed complex parts.3,4 While H-PIM typically requires pressures of >50 MPa, the pressure employed in L-PIM is limited to 0.1–1 MPa. To achieve complete mold filling with these low injection pressures the feedstock viscosity must be correspondingly low (two to three orders of magnitude lower compared to that with H-PIM). This is realized by using low-viscosity paraffin or wax instead of a high-viscosity polymeric binder. Paraffin-based binders offer a beneficial melting range, are inexpensive, non toxic, and decompose without boiling. Because of the different binder systems, there are small differences between L-PIM and H-PIM in relation to feedstock preparation, injection molding machinery, and the debinding process. Due to the low feedstock viscosity and low injection pressure, wear of the mold is much less critical than in H-PIM so that even soft molds, for example, silicone rubber molds, can be applied in L-PIM. Silicone molds offer additional advantages since fabrication is simple, fast, and inexpensive. They permit the replication of fine structures with details in the sub-micrometer range. A high mechanical elasticity and low adhesion forces facilitate demolding of complex-shaped parts. To a certain degree even undercuts can be realized in one single tool. The use of soft tools is limited by the fact that only a moderate injection pressures can be applied, otherwise deformation of the mold inserts and the molded part occurs. It is possible to prepare the L-PIM feedstocks such that the starting powders and solids loading are consistent with H-PIM. Hence, when the organic fraction is removed by debinding, parts fabricated by the alternative routes are similar. For sintering, the same schedule can be applied, resulting in components with the same microstructure and, in principle, the same properties. An easy transfer from prototypes as functional models and small series (L-PIM) to large series production (H-PIM) is thus possible. It is the inherently low feedstock viscosity which makes L-PIM particularly attractive in the manufacturing of micro components. Even very narrow
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cavities can be filled homogeneously under mild injection conditions, thus preventing powder– binder separation. Compared with macroscopic parts, green bodies of micro components are more easily debindered as their small wall thickness and large surface area-to-volume ratio result in short diffusion paths for the liquid binder. Although the two technologies are closely related, in the case of L-PIM some additional effects may occur during removal of the low-viscosity paraffin binder. It was observed that a film of liquid binder forms during the early stages of debinding, decreasing the surface roughness and, in general, being capable of reducing the defect density at or near the surface. This effect is of importance in the case of micro components. Owing to their large surface fraction, the significance of surface defects as a strengthlimiting factor increases with decreasing size. There is no realistic way to improve the surface quality of components with features in the micro range by means of post-sinter finishing processes. Therefore, defect healing mechanisms during debinding may offer the only practical way to obtain micro components free of surface defects. By investigating processing–microstructure–property relations in L-PIM ZrO2 micro samples it was established that the three-point-bending strength increases from ~2,000 MPa to >3,000 MPa for samples in which defect healing had taken place, in particular smoother surfaces and rounded edges.5 Examples of two micro components manufactured by L-PIM are shown in Figure 1. Due to its excellent mechanical properties and sintering behavior, ZrO2 is the material of choice for highly loaded structural applications. Other advanced ceramic materials, for example, Si3N4 (Figure 1(b)), can also be fabricated by L-PIM.
Figure 1. L-PIM and sintered components of ceramic micro turbine: (a) ZrO2 (3 mol. % Y2O3 stabilized), (b) sintered reaction-bonded silicon nitride (SRBSN) nozzle plate. SEM Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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METAL AND CERAMIC PARTS FABRICATED BY MICROMINIATURE POWDER INJECTION MOLDING
HIGH-PRESSURE POWDER INJECTION MOLDING Powder Injection Molding Feedstock for Micro Components The μPIM of components reflects demanding requirements in the selection of feedstock components (e.g., binder formulation, additives, ultrafine powders) and the compounding technology.6,7 As an example, the development of a typical ZrO2 feedstock, with different binder additives to achieve optimum powder content, is now described. Because of its excellent sintered properties, a commercial ZrO2 powder (Tosoh Corporation) containing 3 mol. % Y2O3 (type TZ-3YS-E, average particle size: 0.44 μm, specific surface area 6.6 m²/g, density 6.05 g/cm³) was selected for a systematic screening of the effect of the torque of polyethyleneglycol–alkylethers with different lengths of the polar glycol units (tradename Brij®) during compounding of highly filled feedstocks (Table I). Compounding was performed in a measuring mixer–kneader system (Brabender W50 EHT, volume 55 cm³, compounding temperature 125°C, blade rotational speed 30 rpm). The binder consisted of commercially available low-density polyethylene (Lyondell-Basell Industries, 50 v/o) plasticized with a paraffin wax (Sasol Wax GmbH, (50-x)v/o and x v/o dispersant). For comparison, the Brij concentration was set at 4.4 mg/m², based on the total ZrO2 surface area. Figure 2, generated by Origin software, shows the change in the equilibrium torque with ZrO2 loading and the length of the polar glycol-based units in the different Brij dispersants. At low ZrO2 loads up to 20 v/o, with a large particle-to-particle separation, no significant influence of the Brij structure can be detected. At higher loads the molecular extension of the dispersant molecules attached to the inorganic ceramic particles causes an increase in the inner friction indicated by the increasing equilibrium torque. With the exception of the torque values for 45 v/o zirconia, the order TABLE I. MOLECULAR CHARACTERISTICS OF DISPERSANTS
Brij 72 Brij 76 Brij 78 Brij 700
Molecular Formula
Calculated Molecular Length (nm)
C18H37(OCH2CH2)2OH C18H37(OCH2CH2)10OH C18H37(OCH2CH2)20OH C18H37(OCH2CH2)100OH
3.1 5.7 9.2 35.3
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
Figure 2. Influence of ZrO2 loading and glycol unit length on equilibrium torque
of the torque values follows the molecular extension of the different Brij molecules. Using Brij 700 at 50 v/o zirconia, the equilibrium torque increases disproportionately and feedstocks with higher solid loads could not be realized. In contrast, the use of Brij 72 enables a feedstock with 60 v/o ZrO2; unfortunately, the corresponding viscosity was too high for replication using micro ceramic injection molding. Therefore, these feedstock systems are usually applied with powder loadings of 50 v/o or slightly higher. Microminiature Powder Injection Molding In addition to the optimization of μPIM feedstocks, current R&D activities focus on enhancement of the shaping process, including debinding and sintering. Two experimental series, one dealing with miniaturized devices and the other with parts of increased complexity, are now considered. One of the major objectives of μPIM research is the miniaturization of micro parts produced by PIM. To achieve smaller part sizes, filigree mold inserts are required. Therefore, a microstructured mold insert was produced by combining X-ray lithography and electroforming (LIGA technique). A full description of the fabrication process can be found elsewhere.8 This specific mold insert featured different test structures, such as gear wheels or nozzle plates with different sizes, to investigate the accuracy and smallest details replicable by PIM. To avoid time-consuming and expensive tool engineering, the structures were molded on a supporting plate which was removed by mill cutting and grinding in the green state. After debinding
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and sintering the finished micro parts were examined utilizing scanning electron microscopy (SEM). Past experiments showed that gear wheels with a 560 μm OD could be replicated by PIM. With the new mold insert the size of the gear wheels could be reduced to 275 μm OD (Figure 3). Smallest details that could be replicated are the channel structures of the nozzle plates (Figure 4) with a minimum width ~7 μm (Figure 5). In addition to the miniaturization of micro parts9, efforts have been made to increase the complexity of structures replicable by PIM.10 For this purpose, a tool concept for the production of a screw-like part was conceived. This so-called untwisting tool, Figure 6 (generated by CorelDraw software), was adapted from the production of polymer parts and adjusted to the special requirements of PIM. Basically, injection molding of the untwisting tool can be described as follows: • The feedstock is injected in the screw cavity. • After cooling down, the green part is pulled back under synchronical rotation and thereby removed from the cavity. • The tool is opened and the finished green part ejected. A more detailed description of the process and results can be found elsewhere.11 A first screw design was fitted by using a conical shaft, a half-rounded thread profile, and only a few threads to ensure a limited loosening of the torque during demolding of the green part. Filling simulations generated by the filling simulation software (Moldflow) were carried out to determine the best positions for the injection gates (Figure 7).
Due to our extensive preliminary studies the injection molding process was not too exacting. The screw could be molded using a ZrO2 feedstock that was developed in-house (Figure 8). In a second step, a new and more complex
Figure 4. ZrO2 nozzle plate. SEM
Figure 5. Trench in nozzle plate: width ~7 μm. SEM
Figure 3. ZrO2 gear wheel with ~275 μm OD. SEM
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Figure 6. Untwisting tool—schematic Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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design was created to demonstrate the possibilities of the untwisting concept. The revised screw design featured more threads, steeper thread flanks, and a cylindrical shaft. Again, injection molding was not a critical issue. In addition to using the ZrO2 feedstock, a feedstock containing 17-4PH stainless steel powder was evaluated. Both materials could be processed successfully (Figure 9 and Figure 10).
Figure 7. Simulation of mold filling to determine injection gate positions
Figure 8. ZrO2 dispenser screw: green (right) and sintered (left)
Figure 9. Sintered ZrO2 dispenser screw, revised design Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
Multi-Component Injection Molding Multi-component injection molding is a wellestablished molding process in the plastics industry. Its most important advantages are the reduction of mounting costs and the possibility of producing multi-functional parts.12 Similarly, micro devices would be increasingly attractive if several features can be joined, for example, if the devices consist of different components. Mounting and assembly of the components are crucial processes in the fabrication of microsystems technology devices. Fortunately, the number of steps can be reduced by multi-component injection molding. The use of two-component μPIM has been realized for metallic systems13 and for ceramic materials.14 The challenge in the process is not only the fabrication of the complex injection molding tools but also the adjustment of the thermal processes and the selection of materials.15 The choice of materials and the adjustment of debinding and sintering parameters are crucial since dimensional changes due to thermal expansion and shrinkage have to be considered. This can be demonstrated on a shaft-to-collar connection. If the shrinkage of the gear wheel is too high in comparison with that of the axle, the stresses may become too high and cracks may occur. In contrast, if the shrinkage of the gear wheel is too low, the components may not join together. The latter case can be utilized when a mobile connection is desired which permits the gear wheel to rotate. However, with a fixed shaftto-collar connection, shrinkage of the gear wheel must not be lower than that of the axle. Preliminary studies on the different materials are therefore essential. The studies include the shrinkage behavior of the powders, the production of trial batches of feedstocks, and characterization of the feedstocks. Finally, injection molding of
Figure 10. Sintered 17-4PH dispenser screw, revised design
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samples is necessary in order to evaluate the processability of the feedstocks and to determine mold filling and shrinkage by subsequent sintering tests. To facilitate the injection molding of two-component micro parts, a special molding tool and
Figure 11. Green shaft-to-collar connection formed by two-component μPIM. SEM
Figure 12. Interface of fixed shaft-to-collar connection. SEM
Figure 13. Interface of mobile shaft-to-collar connection. SEM
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process control were developed. A rotatable index plate enables the transport of the first component to a second cavity where the second component is injected through the center hole of the gear wheel to form a two-component green body, as illustrated in Figure 11. Using different materials and process parameters, two different features can be obtained, namely, fixed or movable connections. In principle, there are several factors which influence the formation of fixed or loose connections. Primary considerations are the behavior of the ceramic powders and the feedstocks used for two-component μPIM and the parameters in the thermal processes. It was shown that ZrO2 powder (TZ-3YS-E, Tosoh Corporation), in combination with Al2O3 powder (CT 3000 SG, Almatis GmbH) are promising in the fabrication of fixed connections (Figure 12). A combination of ZrO2 (PYT05.0-005H, Unitec Ceramics Limited) and Al2O3 (RC UFX-DBM, Baikowski Malakoff Inc.) has the potential for connections16 (Figure 13). Figure 12 shows the interface of a sintered shaft-to-collar connection. The two components are closely bonded, which results in an acceptable adhesive strength. For loose connections several requirements are essential in order to avoid any bonding at the interface (Figure 13). Apart from a suitable material, the thermal process, especially the sintering parameters, had to be varied. Reduction of the sintering time by decreasing the dwell time and enhancing the heating rate at high temperature are requirements for achieving movable junctions. Initially, movable connections could only be produced by interrupting the sintering process and by applying circular motions. A modification of the gating concept of the injection molding tool was found to be essential for the production of mobile connections without additional interruptions and mechanical motions. SUMMARY AND OUTLOOK μPIM has the potential to be a key technology for fabriacting moderate or large series of highly loaded micro components of variable complex geometries. Using micrometer or sub-micrometer powders, it is possible to fabricate precision components from metal alloys or ceramics. Based on LPIM, a microfabrication method suited for the production of microcomponent prototypes is available. The development of environmentally friendly Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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water-soluble binders and the screening of tailored dispersants reflect current major R&D efforts in feedstock improvement. Further trends in the development of micro powder injection molding technology are evident, for example, increasing the spectrum of materials to open up new areas of application. In particular, this applies to μPIM used to process metals such as titanium or tungsten. A further trend is the fabrication of multi-component parts from metals or ceramics by two-component μPIM or inmold-labeling using PIM feedstocks. ACKNOWLEDGEMENT The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) under the collaborative research center SFB 499 and by projects funded by the EU Commission. REFERENCES 1. D. Löhe and J. Haußelt, Advanced Micro and Nanosystems, Vol. 3 Microengineering of Metals and Ceramics, edited by H. Baltes, O. Brand, G.K. Fedder, C. Hierold, J. Korvink and O. Tabata, Wiley VCH-Verlag, Weinheim, 2005, chapters 10 to 12. 2. R.M. German, “Divergences in Global Powder Injection Moulding,” Powder Injection Moulding Intl., 2008, vol. 2, no. 1, pp. 45–49. 3. W. Bauer, J. Haußelt, L. Merz, M. Müller, G. Örlygsson and S. Rath, “Micro Ceramic Injection Molding,” Advanced Micro and Nanosystems Vol. 3: Microengineering in Metals and Ceramics, Part I, edited by D. Löhe and J. Haußelt, Wiley-VCH, Weinheim, Germany, 2005, pp. 325–356. 4. W. Bauer, M. Gomez, V. Valcarcel, C. Cerecedo, F. Guitian, M. Peltsman and J.E. Zorzi, “Advancing Components with Low-Pressure Injection Moulding”, Ceramic Industry, 2006, vol. 156, no. 5, pp. 22–26. 5. M. Müller, J. Rögner, B. Okolo, W. Bauer and H.-J. Ritzhaupt-Kleissl, “Factors Influencing the Mechanical Properties of Moulded Zirconia Micro Parts”, Proc. 10th ECerS Conf., edited by J.G. Heinrich and C. Aneziris, Göller Verlag, Baden-Baden, Germany, 2007, pp. 1,291–1,296. 6. W.J. Tseng, “Influence of Surfactant on Rheological Behaviors of Injection Molded-Alumina Suspensions”, Mater. Sci. Eng. A, 2008, vol. 289, pp. 116–122. 7. T. Hanemann and K. Honnef, “Process Chain Development for the Realization of Zirconia Microparts Using Composite Reaction Molding”, Ceramics Intern., 2009, vol. 35, pp. 269–275. 8. M. Guttmann, J. Schulz and V. Saile, “Lithographic Fabrication of Mold Inserts”, Advanced Micro and Nanosystems, Vol. 3, Microengineering of Metals and Ceramics, edited by H. Baltes, O. Brand, G.K. Fedder, C. Hierold, J. Korvink and O. Tabata, Wiley-VCH, Weinheim, Germany, 2005, pp. 187–219. 9. F. Petzoldt, “Micro Powder Injection Molding—Challenges and Opportunities”, Powder Injection Moulding Intl., 2008,
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vol. 2, no. 1, pp. 37–41. 10. V. Piotter, H.-J. Ritzhaupt-Kleissl, A. Ruh and J. Hausselt, “Manufacturing of Versatile Ceramic or Metal Micro Components by Powder Injection Molding”, Proceedings of 4M2008 Conference, Whittles Publishing, Dunbeath, UK, 2008, pp. 69–72 11. T. Mueller, V. Piotter, K. Plewa, J. Prokop, H.-J. Ritzhaupt-Kleissl and J. Hausselt, “Complex Shaped Micro Component Produced by Powder Injection Molding,” 4M/ICOMM 2009 Conference Proceedings, edited by V. Saile, K. Ehmann and S. Dimov, The Charlesworth Group, Wakefield, UK, 2009, pp. 103–106. 12. W. Michaeli and D. Opfermann, “Micro Assembly Injection Moulding—Potential Application in Medical Science”, Proceedings of 4M2005 Conference, Elsevier, BV, The Netherlands, 2005, pp. 79–82. 13. P. Imgrund, A. Rota, F. Petzoldt and A. Simchi, “Manufacturing of Multi-Functional Micro Parts by Two-
Component Metal Injection Moulding”, Int. J. Adv. Manuf. Technol., 2007, vol. 33, pp. 176–186. 14. V. Piotter, G. Finnah, B. Zeep, R. Ruprecht and J. Hausselt, “Metal and Ceramic Micro Components Made by Powder Injection Moulding”, Materials Science Forum, Trans Tech Publications Inc., 2007, vols. 534–536, pp. 373–376. 15. A. Ruh, A.-M. Dieckmann, R. Heldele, V. Piotter, R. Ruprecht, C. Munzinger, J. Fleischer and J. Haußelt, “Production of Two-Material Micro Assemblies by TwoComponent Powder Iinjection Molding and SinterJoining”, Microsyst. Technol., 2008, vol. 14, pp. 1,805–1,811. 16. A. Ruh, T. Hanemann, R. Heldele, V. Piotter, H.-J. Ritzhaupt-Kleissl and J. Hausselt, “Development of TwoComponent Micro Powder Injection Molding (2CMicroPIM)—Characteristics of Applicable Materials”, Int. J. Appl. Ceram. Technol., in press. ijpm
HOT ISOSTATIC PROCESSING SERVICES FOR PRODUCTION AND RESEARCH PROGRAMS ISO 9001, AS9100 REGISTERED
• CASTING DENSIFICATION • Improved Properties • Reduced Rejection Rate • Reduced Scrape Rate
• POWDER CONSOLIDATION • PRESSURE BRAZING • DIFFUSION BONDING • CERAMICS
KITTYHAWK PRODUCTS
11651 MONARCH ST. • GARDEN GROVE, CA 92841 Tel. (714) 895-5024 Fax (714) 893-8709 www.kittyhawkinc.com E-mail:
[email protected]
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MICROMINIATURE POWDER INJECTION MOLDING—PART I
HIGH-STRENGTH POWDER INJECTION MOLDED 316L STAINLESS STEEL Li-Hui Cheng* and Kuen-Shyang Hwang**
INTRODUCTION Powder injection molding (PIM) technology has been used widely as a manufacturing process for structural parts with complicated shapes. Of the available materials, 316L stainless steel is frequently employed. To obtain the full benefits of 316L, parts are typically sintered in vacuum or hydrogen at temperatures ~1,350°C in order to attain high sintered densities. With a sintered density >7.50 g/cm3 (94.9% of the pore-free value) and a carbon content <0.03 w/o, the material exhibits excellent corrosion resistance. However, strength and hardness are relatively low due to significant grain growth during sintering and the attendant austenitic structure. Typical properties of sintered PIM 316L are a density of 7.60 g/cm3, tensile strength 520 MPa, elongation 50%, and HRB 67.1 These low values for hardness and strength impose limitations on further promoting applications. For example, microminiature parts produced by microminiature powder injection molding (μPIM) for biopsy tools, cellular phones, and notebook computers require high strength because of the small load-bearing areas. Thus, improving the strength and hardness of 316L has become a focus area in current PIM research. To alleviate these problems, dispersoids, such as oxides and carbides, are often added to stainless steel powders.2,3 Tongsri et al.2 added alumina particles to 316L, and although the hardness was improved, the density, strength, and elongation decreased. Lal and Upadhayaya3 demonstrated that the density and hardness of 316L increased when 2.9 μm yttria powder was added at the expense of corrosion resistance due to the presence of interfaces between the dispersoids and matrix. Liquid-phase-forming materials, such as tin and nickel boride,4,5 have also been used to sinter stainless steel parts at low temperatures and to improve density, hardness, and strength, but the second phases formed at the grain boundaries changed the fracture behavior. One approach that does not degrade the corrosion resistance and mechanical properties is to refine the grain size by using fine powders which have a high driving force for sintering, thus lowering the sintering temperature that can be used to attain a high density. Lowtemperature sintering limits grain growth.
Powder injection molded (PIM) 316L stainless steels are widely used in the form of small components with complicated shapes. In order to attain a high sintered density, 316L is generally sintered at high temperatures and the attendant grain growth and austenitic structure limit hardness and strength compared with lowalloy steels. These disadvantages restrict its use in microminiature structural parts that require wear resistance, corrosion resistance, and high strength, such as biopsy tools, cellular phones, and notebook computers. To address this problem, stainless steels can be sintered in dissociated ammonia and cooled rapidly to obtain high levels of dissolved nitrogen without forming chromium nitrides. However, the nitrogen content is limited at the regular sintering temperature of ~1,350°C. To circumvent this limitation, fine 316L powder (D50 = 4.1 μm) was sintered at a low temperature of 1,120°C for 2 h in dissociated ammonia to achieve a high sintered density of 7.70 g/cm3 (97.2% of the pore-free density) and a nitrogen content of 0.29 w/o. Hardness and tensile strength were 95 HRB and 740 MPa, respectively, and compare with values of 55 HRB and 450 MPa for regular 316L powders (D50 = 12.0 μm) sintered at the standard temperature of 1,350°C. Subsequent hot isostatic pressing (HIPing) in nitrogen resulted in a nitrogen content of 0.79 w/o and hardness and tensile strength levels of 100 HRB and 800 MPa.
*Graduate Student, **Professor, Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Road, Sec. 4, Taipei, 106, Taiwan, R.O.C.; E-mail:
[email protected]
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HIGH-STRENGTH POWDER INJECTION MOLDED 316L STAINLESS STEEL
Strengthening by grain refinement has also been utilized in combination with nitrogen-solution strengthening for wrought austenitic stainless steels by Schino and Kenny.6 The yield strength, hardness, and elastic modulus can be improved significantly without reducing ductility. Since nitrogen is a strong austenite stabilizer, is a potent solid-solution strengthener, and improves pitting corrosion, sintering in a nitrogen-containing atmosphere7–10 should enhance properties. In addition, 316L has a maximum nitrogen solubility ~0.4 w/o at 1,140°C.9,10 Because of its limited solubility in 316L, the hardening effect of nitrogen is limited. However, it has been demonstrated that the amount of nitrogen dissolved in austenitic stainless steel can be enhanced when high pressure is applied.11 Rawers et al. prepared high-nitrogen stainless steels by melting under high pressure in nitrogen in a HIPing furnace,12,13 resulting in a nitrogen content of 1.0 w/o. Based on these observations, it is possible that the nitrogen content in sintered 316L can be increased if sintered compacts are HIPed in a nitrogen atmosphere. Since only limited information has been reported on the properties of 316L using fine powders, low-temperature sintering, and HIPing in nitrogen, the objective of this study was to produce compacts containing fine grains and a high nitrogen content to enhance tensile strength and hardness. In this study, fine (D50 = 4.1 μm) and regular (D50 = 12.0 μm) 316L stainless powders were used to prepare PIM specimens. Since the size difference of the powders may influence the processing of the part, solvent and thermal debinding rates were examined. The effects of sintering temperatures and sintering atmosphere on the sintered properties were also evaluated. Finally, the effect of HIPing in nitrogen on the properties of compacts sintered in dissociated ammonia was investigated. EXPERIMENTAL PROCEDURE Fine and regular 316L powders with mean particle sizes of 4.1 μm and 12.0 μm, respectively, were used in this study to compare their effects on sintered properties. The powders were produced by water atomization, and thus were irregular in shape. The powder characteristics are given in Table I. To prepare the PIM specimens, the metal powder was kneaded with 7 w/o wax-based binder, which consisted of paraffin wax (PW), stearic acid
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(SA), and polyethylene (PE), using a Σ-blade kneading machine. After kneading, the feedstock was molded into 4 × 10 × 100 mm plates and barshaped tensile specimens using an injection molding machine (320C, Arburg GmbH, Lossburg, Germany). The molded flat plates were solvent debound in heptane at 40°C to remove the soluble binders, PW and SA. The binder removal rate was calculated from the equation: Amount of Soluble Binder Removed = Weight Loss after Solvent Debinding ____________________________________ Total Weight of PW and SA
(1)
To assess whether the dimensional change during solvent debinding was influenced by the powder size, the in situ length change of the rectangular plates was monitored using a self-designed laser dilatometer. The setup and details of the instrument can be found elsewhere.14,15 The temperature of the solvent bath in the laser dilatometer was controlled at 40±0.2°C. The thermal debinding rates of the specimens prepared with fine and regular powders were also compared. Solvent debound specimens were heated at a rate of 5°C/min in hydrogen to 450°C, 475°C, 500°C, 515°C, and 535°C and then cooled immediately, after which the weight losses were measured. The binder removal rate during the thermal debinding stage was calculated from the equation: Amount of Binder Removed = Weight Loss after Thermal Debinding ____________________________________ Total Weight of Remaining Binder
(2)
TABLE I. CHARACTERISTICS OF FINE AND REGULAR 316L POWDERS Characteristic
Fine Powder
Regular Powder
Particle-Size Distribution (μm) (laser scattering)
D10 = 2.4 D50 = 4.1 D90 = 8.2
D10 = 4.4 D50 = 12.0 D90 = 30.0
Pycnometer Density (g/cm3)
7.92
7.92
16.58 12.23 2.12 0.030 0.086 0.333 ATMIX Corporation
16.55 12.56 2.09 0.027 0.043 0.390 ATMIX Corporation
Element (w/o): Cr Ni Mo C N O Supplier
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After solvent and thermal debinding, the specimens were sintered at 1,120°C, 1,190°C, 1,350°C, and 1,370°C for 2 h in hydrogen, vacuum, or dissociated ammonia (75 v/o H2/25 v/o N2), and then cooled to room temperature at a rate of 6°C/min (measured over the range 800°C to 500°C). To further increase the density and nitrogen content, the specimens prepared from fine powders and sintered at 1,120°C in dissociated ammonia, and those prepared from regular powders and sintered at 1,370°C in dissociated ammonia , were HIPed at 1,250°C in nitrogen under a pressure of 100 MPa. The sintered density was measured using the Archimedes method (MPIF Standard 54). To measure grain size, the line intercept method was used (ASTM E112-96). The hardness of the sintered specimens was measured using a Rockwell hardness tester (ARK600, Mitutoyo Corp., Tokyo, Japan). Tensile properties were determined using a Universal tensile tester (AG-10TE, Shimadzu Corp., Kyoto, Japan). Corrosion resistance was examined following MPIF Standard 62. Flat specimens were immersed in a 2 w/o H2SO4 solution at room temperature for 24 h. The weight loss was measured and converted to units of g/dm2/day. To assess the extent of organic binder removal during debinding and sintering, the carbon content was determined by means of a carbon analyzer (EMIA-220V, HORIBA, Ltd., Kyoto, Japan). The nitrogen and oxygen contents were measured utilizing an oxygen/nitrogen analyzer (TC-136, LECO Corp., St. Joseph, Michigan). Since the nitrogen levels of the specimens on the surface and in the center may differ after sintering in dissociated ammonia and HIPing in nitrogen, the distribution of the nitrogen was also examined. To construct the nitrogen profile, specimens were milled layer by layer using a milling machine at a feed rate of 0.04 mm per cut. The chips from each layer were collected and their nitrogen content measured. The microindentation hardness from the surface to the center was also measured using a Vickers micro-hardness tester (HM-112, Mitutoyo) to examine the effect of nitrogen on hardness. RESULTS AND DISCUSSION Solvent and Thermal Debinding Figure 1 shows the amount of soluble binder removed during solvent debinding. The specimens prepared from both fine and regular powders exhibited similar binder removal rates. The in situ Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
Figure 1. Binder removed during solvent debinding, showing similar solventdebinding rates for both fine and regular powders
Figure 2. Length change of molded specimens fabricated from regular and fine powders during solvent debinding at 40°C
length change (Figure 2), indicated that the length changes of the specimens fabricated from fine powder were smaller than those of the specimens fabricated from regular powder. This is attributed to the higher number of powder particle contacts and the resulting larger interparticle friction in the fine powder. The amount of the backbone binder (PE) removed during thermal debinding was also measured; the results are shown in Figure 3. The two specimens showed similar thermal debinding rates. The similarity in the solvent and thermal debinding rates of the fine and regular-size powder specimens was not unexpected. Previous studies
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resistance to atmospheric hydrogen to flow into the pores was larger in the fine-powder specimens. Sintering Figure 4 shows that after sintering at 1,120°C, the fine powder specimens reached a density of 7.70 g/cm3. As the sintering temperature was increased to 1,370°C, the sintered density further increased to 7.82 g/cm3. In contrast, the specimens fabricated from regular 316L powder attained a sintered density of only 7.52 g/cm3, the higher sintering temperature notwithstanding. Grain size in both the fine and regular powder specimens increased as the sintering temperature increased, as shown in Figure 5. The grain size of Figure 3. Binder removed during thermal debinding showing similar thermal debinding rates for specimens fabricated from fine and regular powders
on the debinding rates of fine and coarse iron powders also show comparable results.16,17 The primary reason for the similarity is that during solvent debinding, most pores are formed inside the binder due to the extraction of the soluble binder components, and to the small difference in pore sizes in both specimens, irrespective of the size of the interstices among the 316L powder particles. Moreover, the pores are too large to influence the diffusion of the small paraffin-wax molecules.16 When solvent debound compacts were subsequently heated for thermal debinding, the remaining binder melted and most of the binder was redistributed in powder-particle-contact regions and fine pores due to capillarity forces. Accordingly, the pore size is related to the size of the interstices among the powder particles, but the size of these pores is usually in the micron range so that a permeation mechanism dominates.18 Since the pressure builds up when the binder decomposes, the gas evolved permeates out of the material. However, this does not preclude counterflow of hydrogen into the inner pores and reaction with the residual carbon. In fact, it was observed that the carbon content of the thermally debound specimens was different; a higher carbon residue (0.18 w/o) was present after the fine-powder specimens were heated to 550°C, compared with a 0.12 w/o carbon residue in the regular-powder specimens. This implies that a reduced reaction occurred between the carbon residues and the hydrogen atmosphere during thermal debinding in the fine powder specimens, primarily because the
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Figure 4. Density as a function of sintering temperature for fine and regular powder specimens
Figure 5. Grain size as a function of sintering temperature for fine- and regularpowder specimens
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HIGH-STRENGTH POWDER INJECTION MOLDED 316L STAINLESS STEEL
the specimens fabricated from fine powders remained small (~10 μm), when sintered at 1,120°C for 2 h. It increased to 68 μm as the sintering temperature increased to 1,370°C. Since the density of the specimens fabricated from fine powder was high when sintered at 1,120°C, this suggests that fewer pores were present to impede grain growth as the sintering temperature rose above 1,120°C. Thus, the grain size increased significantly in the fine-powder specimens sintered at high temperatures. In contrast, in the specimens fabricated from regular powders, a high level of porosity existed when the sintering temperature was low. As a result, the pores in these specimens can impede grain growth and, accordingly, the
Figure 6. Carbon content as a function of sintering temperature
Figure 7. Nitrogen content as a function of sintering temperature
Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
grain size was small. After sintering at 1,370°C, the grain size of the specimens fabricated from regular 316L powder was smaller than that in the specimens fabricated from fine powders. Since the properties of PIM 316L are influenced by the levels of carbon and nitrogen, the content of these elements in sintered compacts was measured; the results are shown in Figures 6 and 7, respectively. The carbon content decreased with increasing sintering temperature, irrespective of the atmosphere, because the equilibrium carbon content was lower at the high sintering temperatures. However, it should be noted that the carbon content in the fine-powder specimens was higher than that in the regular-powder specimens, probably because more carbon was present initially in the fine-powder specimens after thermal debinding. Another explanation could be that the density of the fine-powder specimens was already high after sintering at 1,120°C, and this made it more difficult for the dissolved carbon or carbon soot to be transported out of the material. Figure 7 shows that when 316L was sintered in pure hydrogen or vacuum, the nitrogen content was low. When the atmosphere was changed to dissociated ammonia, the nitrogen contents of the fine-powder specimens sintered at 1,120°C and 1,370°C were 0.29 w/o and 0.14 w/o, respectively. Since the solubility of nitrogen in 316L is maximized at 1,140°C,9,10 the nitrogen content decreased when the sintering temperature exceeded 1,140°C. The same trend was observed in the specimens fabricated from regular powders. When sintering at 1,120°C, the nitrogen content of the specimens fabricated from regular 316L powder was 0.45 w/o, which was higher than that of the specimens using fine 316L powder. When sintered at 1,370°C, its nitrogen content was the same as that of the fine-powder specimens. To understand what causes the differences in nitrogen content when fine- and regular-powder specimens are sintered at 1,120°C, the nitrogen profile from the surface to the center of the specimens was determined, as shown in Figure 8. For the fine-powder specimens, the nitrogen content decreased from the surface to the center. In contrast, the regular-powder specimens showed a uniform nitrogen content throughout the sintered compact. This suggests that only a small number of interconnected pores were present in the highdensity fine-powder specimens sintered at 1,120°C, leading to inhibition of the inward perme-
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Figure 8. Depth profile of nitrogen after sintering at 1,120°C
Figure 9. Hardness as a function of sintering temperature in dissociated ammonia
Figure 10. Tensile strength as a function of sintering temperature
Figure 11. Elongation as a function of sintering temperature
ation of the nitrogen. In contrast, the density of the regular-powder specimens sintered at 1,120°C was low, which allowed the nitrogen to flow into the specimens through the interconnected pores, and thus saturate the compact with nitrogen to the solubility limit. Mechanical Properties Figures 9, 10, and 11 show the hardness, tensile strength, and elongation of the sintered specimens, respectively. When the fine-powder specimens were sintered in hydrogen or vacuum, the grain size increased as the sintering temperature increased; as a result, the hardness and tensile
34
strength decreased. For the specimens fabricated from regular powders, the hardness and tensile strength increased slightly with increasing sintering temperature because the sintered density increased, the adverse effect of grain growth notwithstanding. Using a sintering atmosphere of dissociated ammonia, the hardness and tensile strength of both specimens improved, due primarily to the solid-solution strengthening effect of the nitrogen. Since the nitrogen content was high and the grain size was small in the fine-powder specimens sintered at 1,120°C in dissociated ammonia, the hardness and tensile strength reached 95 HRB and 740 MPa, respectively. These values are signifVolume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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HIGH-STRENGTH POWDER INJECTION MOLDED 316L STAINLESS STEEL
TABLE II. CORROSION-RESISTANCE RATING OF FINE- AND REGULAR-POWDER SPECIMENS SINTERED AT 1,120°C AND 1,370°C IN DISSOCIATED AMMONIA OR VACUUM Powder Type
Sintering Temperature (°C)
Atmosphere
C (w/o)
N (w/o)
O (w/o)
Weight Loss (g/dm2/day)
Corrosion Resistance Rating
Regular Fine Regular
1,370 1,120 1,370 (continuous furnace) 1,120 (continuous furnace) 1,370 1,120
NH3 NH3 NH3
0.003 0.025 0.012
0.135 0.287 0.182
0.120 0.220 0.252
0.520 0.738 0.004
2 2 0
NH3
0.026
0.292
0.362
0.025
1
Vacuum Vacuum
0.009 0.031
0.002 0.035
0.196 0.310
<0.001 0.020
0 0
Fine Regular Fine
icantly higher than the corresponding levels obtained by using regular 316L powder and regular sintering conditions of 1,350°C in pure hydrogen or vacuum (55 HRB and 450 MPa). In terms of elongation, both specimens showed improvement as the sintering temperature increased, since the density and grain size increased. It was noted that the elongation of the specimens sintered in dissociated ammonia was lower than that of the specimens sintered in hydrogen or vacuum. This is attributed to the solutioning of nitrogen, which inhibited grain growth. The preceding results show that a smaller grain size, higher nitrogen content, and higher sintered density result in enhanced hardness and strength in PIM 316L. When sintering fine–316L powder compacts at 1,120°C in dissociated ammonia, a high sintered density (7.70 g/cm3), a high nitrogen content (0.29 w/o), and a small grain size (~10 μm) resulted. The attendant hardness of 95 HRB, tensile strength of 740 MPa, and elongation of 36% reflect significantly improved mechanical properties compared with those of a typical PIM 316L.1 Corrosion Resistance Table II shows the corrosion resistance of specimens fabricated from fine powders and sintered at 1,120°C, and those fabricated from regular powders and sintered at 1,370°C. When sintered in vacuum, all the specimens are rated 0 (MPIF standard 62). When an atmosphere of dissociated ammonia was used, all the specimens are rated 2. Since the corrosion resistance of the 316L specimens sintered in dissociated ammonia can be improved by fast cooling, a separate group of fineand regular-powder specimens was sintered in dissociated ammonia using an industrial walkingVolume 46, Issue 2, 2010 International Journal of Powder Metallurgy
beam furnace with a high cooling rate. The regular-powder specimens achieved a rating of 0 and the fine-powder specimens attained a rating of 1 due to its high nitrogen content. The mechanical properties of these fast-cooled specimens are similar to those reported in Figures 9, 10, and 11. HIPing To further improve mechanical properties, the fine-powder specimens sintered at 1,120°C and the regular-powder specimens sintered at 1,370°C in dissociated ammonia were HIPed at 1,250°C under a pressure of 100 MPa using nitrogen as the pressurizing medium. Table III shows that the nitrogen content can be increased to 0.79 and 0.74 w/o for fine- and regular-powder specimens, respectively. For the fine-powder specimens, the sintered density increased from 7.70 to 7.80 g/cm3 after HIPing. For the regular-powder specimens, the density increased from 7.51 to 7.80 g/cm3. As a result, few pores were present to impede the grain growth. The grain size increased slightly from 10 to 25 μm in the fine-powder specimens, and in regular-powder specimens it
TABLE III. PROPERTIES BEFORE AND AFTER HIPing
Density (g/cm3) Grain Size (μm) Nitrogen (w/o) Hardness (HRB) Tensile Strength (MPa) Elongation (%)
Fine Powder (1,120°C)
Regular Powder (1,370°C)
As-sintered As-sintered+ in NH3 N2 HIPed
As-sintered As-sintered+ in NH3 N2 HIPed
7.70 10 0.29 95 740 36
7.80 25 0.79 100 800 12
7.51 58 0.14 74 530 45
7.80 65 0.74 96 705 15
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increased from 58 to 65 μm. Since the graingrowth effect was minimal, the hardness and tensile strength increased significantly due to the high HIPed density and high nitrogen content. For specimens fabricated using fine powders and sintered in dissociated ammonia, the hardness increased from 95 to 100 HRB, and the tensile strength increased from 740 to 800 MPa. For the specimens fabricated from regular powders, the hardness increased from 74 to 96 HRB, and the tensile strength increased from 530 to 705 MPa. To understand whether or not the nitrogen was uniformly distributed in the matrix or formed a nitride layer, microindentation hardness measurements were taken from the surface to the center of the specimens before and after HIPing. The results cited in Figure 12(a) suggest that nitrogen can diffuse through the sintered compacts during HIPing, despite the high compact density of 7.70 g/cm3 and high hardness. Figure 12(b) confirms that the hardness profiles are consistent with the nitrogen profiles, particularly for the fine-powder specimens. It was observed that a small amount of chromium nitride had precipitated at grain boundaries. This was due primarily to the slow cooling rate in the HIPing cycle. To further improve the microstructure and corrosion resistance of the HIPed PIM 316L products, the cooling rate in the HIPing process needs to be increased. CONCLUSIONS Since the fine 316L powder has a higher driving force for sintering, a high sintered density of 7.70 g/cm3 can be obtained at a low sintering temperature of 1,120°C. The low-temperature sintering provides two benefits: a high nitrogen content of ~0.29 w/o when sintered in dissociated ammonia, and a fine grain size. Specimens thus sintered can achieve a hardness of 95 HRB, a tensile strength of 740 MPa, and an elongation of 36%. These mechanical properties are much improved compared with those of a typical PIM 316L. When compacts are further HIPed using nitrogen as the pressurizing medium, the nitrogen content increased to ~0.79 w/o, and the density increased to 7.80 g/cm3. The resulting hardness increased to 100 HRB, and the tensile strength increased to 800 MPa. These results demonstrate that a combination of fine powder, low-temperature sintering in dissociated ammonia, and post-sinter HIPing is a viable route for fabricating high-strength PIM 316L, thus broadening its application.
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Figure 12. (a) Nitrogen profile from the surface to the center and (b) microindentation hardness profile before and after HIPing
ACKNOWLEDGEMENT The authors thank the Taiwan Powder Technologies Co. for their support of this work under contract 96-SA-73. REFERENCES 1. Materials Standards for Metal Injection Molded Parts, 2007 Edition, Metal Powder Industries Federation, Princeton, NJ, pp. 18–19. 2. R. Tongsri, S. Asavavisithchai, C. Mateepithukdharm, T. Piyarattanatrai and P. Wangyao, “Effect of Powder Mixture Conditions on Mechanical Properties of Sintered Al2O3-SS 316L Composites under Vacuum Atmosphere”, J. Metals,
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3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13. 14.
15.
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Materials and Minerals, Metallurgy and Materials Science Research Inst., Thailand, 2007, vol. 17, no. 1, pp.81–85. S. Lal and G.S. Upadhyaya, “Effect of Phosphorus and Silica Addition on the Sintered Properties of 316L Austenitic Stainless Steel and its Composites Containing 4vol% Yittria”, J. Mater. Sci., 1989, vol. 24, pp. 3,069–3,075. O. Coovattanachai, N. Tosangthum, M. Morakotjinda, T. Yotkaew, A. Daraphan, R. Krataitong, B. Vetayanugual and R. Tongsri, “Performance Improvement of P/M 316L by Addition of Liquid Phase Forming Powder”, Mater. Sci. Eng. A, 2007, vol. 445, pp. 440–445. H.Ö. Gülsoy and S. Salman, “Microstructures and Mechanical Properties of Injection Molded 17-4PH Stainless Steel Powder with Nickel Boride Additions”, J. Mater. Sci., 2005, vol. 40, pp. 3,415–3,421. A.D. Schino and J.M. Kenny, “Grain Refinement Strengthening of a Micro-crystalline High Nitrogen Austenitic Stainless Steel”, Mater. Lett., 2003, vol. 57, pp. 1,830–1,834. J. Rawers, F. Croydon, R. Krabbe and N. Duttlinger, “Nitrogen Enhanced Stainless Steel by Powder Injection Molding”, Int. J. Powder Metall., 1996, vol. 32, no. 4, pp. 319–322. R. Tandon, J.W. Simmons, B.S. Covino, Jr. and J.H. Russel, “Mechanical and Corrosion Properties of NitrogenAlloyed Stainless Steels Consolidated by MIM”, Int. J. Powder Metall., 1998, vol. 34, no. 8, pp. 47–54. G. Lei, R.M. German and H.S. Nayar, “Influence of Sintering Variables on the Corrosion Resistance of 316L Stainless Steel”, Powder Metall. Int., 1983, vol. 15, no. 2, pp. 70–76. K.S. Hwang and Y.W. Hsueh, “Post-Sintering Thermal Treatment of Nitrogen Containing Pressed and Sintered and PIM Stainless Steels”, Powder Metall., 2007, vol. 50, pp. 165–171. J.W. Simmons, “Overview: High-Nitrogen Alloying of Stainless Steels”, Mater. Sci. Eng. A, 1996, vol. 207, pp. 159–169. J. Rawers, G. Asai, R. Doan, and J. Dunning, “Mechanical and Microstructural Properties of Nitrogen-High Pressure Melted Fe-Cr-Ni Alloys”, J. Mater. Res., 1992, vol. 7, pp. 1,083–1,092. J.C. Rawers, “High Carbon-Nitrogen Iron alloy”, J. Mater. Sci., 1999, vol. 34, pp. 941–944. H.K. Lin and K.S. Hwang, “In Situ Dimensional Changes of Powder Injection-Molded Compacts During Solvent Debinding”, Acta Mater., 1998, vol. 46, pp. 4,303–4,309. S.C. Hu and K.S. Hwang, “Length Change and Deformation of Powder Injection-Molded Compacts during Solvent Debinding”, Metall. Mater. Trans. A, 2002, vol. 31A, pp. 1,473–1,478. G.J. Shu and K.S. Hwang, “High Density Powder Injection Molded Compacts Prepared from a Feedstock Containing Coarse Powders”, Mater. Trans., 2004, vol. 45, no. 10, pp. 165–171. K.S. Hwang and G.J. Shu, “Solvent Debinding Behavior of Powder Injection Molded Components Prepared from Powders with Different Particle Sizes”, Metall. Mater. Trans. A, 2005, vol. 36A, pp. 161–167. R.M. German, “Theory of Thermal Debinding”, Int. J. Powder Metall., 1987, vol. 23, pp. 237–245. ijpm
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MICROMINIATURE POWDER INJECTION MOLDING—PART I
NITRIDING RESPONSE OF MICROMINIATURE POWDER INJECTION MOLDED TITANIUM Toshiko Osada* and Hideshi Miura**
INTRODUCTION Advances in metal injection molding (MIM) technology embrace miniaturization and sophisticated features of products for various industrial applications. The term μMIM can be defined as very small fine-structured MIM products with dimensions <1 mm. μMIM has focused recently on the application of microsystems technology.1–3 The MIM process exhibits many different features compared with conventional powder metallurgy (PM). Only limited research work has been done in relation to wear and surface treatments,4,5 even though MIM parts are being used increasingly for dry sliding-wear-resistant metal parts in diverse industries. With the increasing miniaturization of devices and components, the surface-to-volume ratio increases and wear is inevitable when two surfaces undergo sliding under load. Although it is impossible to prevent wear, it must be controlled so that surface properties become increasingly important. In general, the surface treatment of metal products such as carburizing, coating, and nitriding, is effective in achieving a high surface hardness and wear resistance. These surface treatments can be applied industrially to most metals and alloys. Gas nitriding is a viable surface treatment to obtain high hardness and wear resistance. For example, titanium and its alloys are materials used in the MIM process, but have a low hardness compared with stainless steels. However, titanium reacts with nitrogen during sintering in an atmosphere of nitrogen gas and TiN is formed on the surface of sintered compacts. TiN is stable in all environments and exhibits high hardness, but limited machinability. Thus, the MIM process coupled with gas nitriding is attractive in net-shape manufacturing of TiN. High quality and high performance of μMIM products are attractive. The properties of the final products are strongly influenced by the debinding and sintering processes. The aim of this study was to investigate the gas-nitriding mechanisms operative in sintered titanium parts produced by MIM. In the MIM process, gas nitriding can be performed continuously subsequent to debinding and sintering. In the case of μMIM, the specimen is small enough and the specific surface
Microminiature powder injection molding (μPIM) offers many advantages in the manufacture of minute parts. In this study, gasnitriding response was evaluated for various sizes of titanium specimens in order to increase functionality. Micro-dumbbell specimens exhibited a nitrogen content about five times as high as that of block specimens. The μPIM products were small enough that the surface treatment can contribute significantly to an improvement in properties by converting titanium to TiN. Thus, gas nitriding is an effective surface treatment for highly functional μPIM titanium products.
*Assistant Professor, **Professor, Kyushu University, Department of Intelligent Machinery & Systems, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; E-mail: osada@ mech.kyushu-u.ac.jp,
[email protected]
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area is large enough that the mechanical and functional properties may be readily changed by a surface treatment. Moreover, the small size of these titanium compacts may permit the formation of TiN throughout the compact by nitriding. Therefore, the effect of nitriding on the properties of μMIM products was also investigated. EXPERIMENTAL PROCEDURE Materials used in this study were a gasatomized titanium powder (Osaka Titanium Technologies Co, Ltd., TILOP-45) and a wax-based binder. Table I gives the composition and size of the powder; Figure 1 and Figure 2 show the morphology of the powder and the particle-size distribution, respectively. The titanium powder and binder were mixed at a 65 v/o solids loading by kneading. The feedstock was then injection molded utilizing an injection molding machine. In order to investigate the effect of nitriding on the specimen size, three types of specimens with different thicknesses were prepared. Their width and length were both 5 mm, and the thicknesses were 1, 2.5, and 3.5 mm, coded as S-, M-, and L-type specimens. Thus, the specific surfaces of the S, M, and L specimens were 2.9, 1.5, and 1.4 mm2/mm3, respectively. Thermal debinding, sintering, and nitriding were performed continuously in a vacuum and gas-atmosphere furnace. Sintering was performed at 1,150°C, 1,200°C, and 1,250°C for 2 h. Nitriding parameters of time and partial pressure in the furnace were investigated. The nitriding temperature was 1,250°C, and the times were 0, 3, and 5 h. The partial pressure of nitrogen in the furnace was controlled at 6.9% and 9.8% (conditions A and B), respectively. The effects of the nitriding condition on the nitrogen content and microstructure of the specimens were qualified. Two different sizes of specimens were prepared; the larger one was a block specimen of rectangular shape, 9 mm in width and thickness × 40 mm in length. The smaller specimen was a micro-dumbbell specimen, illustrated in Figure 3, with a width and thickness of 0.2 and 0.15 mm, respectively.
Two types of debinding were examined. For the large-size block specimen, solvent debinding was performed with heptane followed by thermal
Figure 1. Representative image of titanium powder. SEM
Figure 2. Particle-size distribution of titanium powder
TABLE I. TITANIUM POWDER COMPOSITION* AND SIZE Element (w/o)
Mean Diameter (μm)
Fe
O
C
N
H
0.050
0.112
0.007
0.013
0.004
*Balance titanium
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23 Figure 3. Geometry of micro-dumbbell specimen. Dimensions in mm Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
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Figure 4. Relative sintered density of specimens in the absence of nitriding
Figure 5. Weight increase as a function of sintering temperature and specimen thickness
Figure 6. Weight increase as a function of nitriding time and specimen thickness Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
debinding. For the micro-dumbbell specimens, only thermal debinding was performed. The specimens were sintered in the vacuum furnace, followed by nitriding. Sintering was performed at 1,200°C for 1.5 h, after which nitriding was performed continuously. The nitriding temperature was 1,200°C and the time was changed from 0 to 1, to 3, and to 5 h. The nitrogen content of the specimens was evaluated by means of the weight increase of the specimen. Microstructures and impurities in the sintered-and-nitrided specimens were evaluated by means of optical microscopy (OM) and elemental analysis using electron microprobe analysis (EMPA). The Vickers microindentation hardness was determined and the hardness profile from the surface to the inside of the specimens was determined. RESULTS AND DISCUSSION Figure 4 shows the relative sintered density for the different specimen sizes in the absence of nitriding. Clearly, the sintered density increases with an increase in the sintering temperature. The thinner specimen shows a slightly higher sintered density than the thicker specimen. Generally the relative density was >97% of the pore-free level, irrespective of the thickness. Figure 5 shows the relationship between the weight increase and the sintering temperature of the nitrided specimens. The weight increase is a measure of the increase in nitrogen content as a result of nitriding. The nitrogen content increases with decreasing specimen thickness and decreasing sintering temperature. The specimen sintered at the low temperature shows little grain growth and the attendant grain boundaries provide preferred paths for the inward diffusion of nitrogen, resulting in an enhanced nitrogen content. Figure 6 shows the relationship between the weight increase and specimen thickness for the two nitriding times. Weight increases are enhanced with decreasing specimen thickness and by increasing the nitriding time. The relationship between weight increase and specimen thickness for different nitriding conditions (partial pressure of nitrogen) is shown in Figure 7. The nitrogen content increases with decreasing specimen thickness. The nitrogen content achieved by nitriding under condition B, (nitrogen partial pressure 9.8%), is larger than that achieved by nitriding under condition A
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Figure 7. Weight increase as a function of nitrogen partial pressure and specimen thickness
(nitrogen partial pressure 6.9%). The partial pressure of nitrogen for condition B is 1.4 times as high as that for condition A and the nitrogen content in each size specimen nitrided under condition B is ~1.6 times as high as that under condition A. Thus, the nitrogen content is influenced by the nitrogen concentration in the furnace. Figure 8 shows the microstructure of the cross section of a block specimen. A TiN layer is present on the surface of the specimen. α-phase and an acicular α-phase are visible at the inside of the TiN layer. There are voids at the boundary between the α-phase and acicular α-phase (delineated by arrows). Similar microstructures of TiN, α-phase, acicular α-phase, and voids were observed in the
micro-dumbbell specimens. However, the area fraction of voids in the micro-dumbbell specimen was larger than that in the block specimen. The specific mechanism for void generation has not been clarified; it may be a Kirkendall void process associated with the unbalanced diffusion of nitrogen. A further detailed investigation of this phenomenon is needed. Figure 9 shows the nitriding depth as a function of nitriding time in a block specimen. The nitriding depth increased with an increase in the nitriding time and reached 100 μm after nitriding for 3 h. Thus, the entire cross section of the micro-dumbbell specimen contains nitrogen. Figure 10 shows the relationship between nitrogen content (elemental analysis) and the nitriding time of the block and micro-dumbbell specimens. In both specimens the nitrogen content increased with increasing nitriding time. The nitrogen content of the micro-dumbbell specimen was five times as high as that in the block specimen. This is due to the high specific surface area of the titanium in the micro-dumbbell specimen, compared with the block specimen. Figure 11 shows the Vickers microindentation hardness (HV) from the surface to the inside of the specimen. Since the TiN layer was thin and brittle, the hardness test was performed in the α-phase. Without nitriding, the block specimen exhibits a constant value of HV. For the nitrided specimens, higher HV values were obtained in the vicinity of the surface. However, the HV value decreased at a depth ~0.1 mm from the surface; it then remained
Figure 8. Microstructure in cross section of block specimen
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Figure 9. Nitriding depth vs. nitriding time (block specimen)
Figure 11. Hardness profile in block and micro dumbbell specimens
Figure 10. Nitrogen content of sintered specimens
Figure 12. Line analysis of nitrogen (EMPA)
constant. In contrast, the micro-dumbbell specimens exhibit a different hardness profile from that of the block specimens. The specimen thickness is ~0.2 mm and the center of the specimen is 0.1 mm from the surface. Micro-dumbbell specimens show relatively higher HV values at the center compared with the block specimens. From these observations it can be concluded that the microstructure and nitrogen content of μMIM titanium compacts are sensitive to the nitriding treatment, which may lead to enhanced properties and performance. Figure 12 shows the nitrogen distribution from the surface to the interior of the micro-dumbbell and block specimens determined utilizing EMPA. A similar pattern to the microindentation hardness exists: the nitrogen content at the surface is higher than that at the center of both specimens. Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
However, the nitrogen content of the micro-dumbbell specimens is significantly higher than that in the block specimens. This resulted in a higher microindentation hardness even in the inside of the specimens. CONCLUSION Nitrogen content increased with decreasing specimen thickness, longer nitriding times, and a higher partial pressure of nitrogen. TiN and α-phase were present within the nitrided specimens. Because μMIM specimens respond to nitriding, the microindentation hardness is much higher than that of conventional MIM block specimens. The nitrogen contents of micro specimens is also higher than that of block specimens. The microstructure and nitrogen content of μMIM
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titanium compacts are strongly influenced by nitriding, which can lead to a higher performance. REFERENCES 1. S. Rath, L. Merz, V. Piotter, R. Ruprecht and J. Hausselt, “Feedstocks and Microparts Made by Powder Injection Molding”, Advances in Powder Metallurgy & Particulate Materials—2003, compiled by R. Lawcock and M. Wright, Metal Powder Industries Federation, Princeton, NJ, 2003, part 8, pp. 45–51. 2. T. Gietzelt, O. Jacobi, V. Piotter, R. Ruprechet and J. Hausselt, “Development and Characterization of Micro
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Parts Made by PIM”, ibid., pp. 160–171. 3. K. Nishiyabu, S. Matsuzaki, S. Tanaka and V. Piotter, “Micro Evaluation Methodology for Micro Metal Injection Molding”, ibid., pp. 172–189. 4. K. Kameo, K. Nishiyabu, K. Friedrich, S. Tanaka and T. Tanimoto, “Sliding Wear Behavior of Stainless Steel Parts Made by Metal Injection Molding (MIM)”, Wear, 2006, vol. 260, no. 6, pp. 674–686. 5. C. Kanchanomai,, B. Saengwichian and A. Manonukul, “Delamination Wear of Metal Injection Moulded 316L Stainless Steel,” Wear, 2009, vol. 267, no. 9–10, pp. 1,665–1,672. 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.”
New Standards Released New 2010 editions of Standard Test Methods for Metal Powders and Powder Metallurgy Products, and Standard 35, Materials Standards for PM Self-Lubricating Bearings have been issued by the Metal Powder Industries Federation. The 130-page standard on test methods contains 39 individual standards on terminology and recommended methods for testing metal powders, PM parts, metal injection molded parts, metallic filters, and PM equipment. The 28-page self-lubricating bearings standard has a new material section on diffusion-alloyed iron-bronze bearings, new information on oil impregnation efficiency, revised information for bronze bearings and data table modifications. Sandvik Offers Rapid-Production Option Sandvik Materials Technology, Sandviken, Sweden, has invested in direct metal laser sintering to shorten the time to produce prototypes for medical-device applications. Through this investment and the enhanced capabilities it brings, the company has strengthened its position as a strategic partner to medical technology companies. OM Group Buys Advanced Battery Maker OM Group, Inc., Cleveland, Ohio, has agreed to buy EaglePicher Technologies LLC, Joplin, Mo., a manufacturer of batteries, battery management systems, and energetic devices for the defense, aerospace, and medical industries. OM reports Volume 46, Issue 2, 2010 International Journal of Powder Metallurgy
a $171.9 million sales price and expects to finalize the acquisition during the first quarter of 2010. New PM Video Released Powder Metallurgy: The Preferred Metal-Forming Solution, a new video showcasing the fabrication capabilities of the various technologies known collectively as powder metallurgy (PM), has just been released on DVD by the Metal Powder Industries Federation (MPIF). The 13-minute production, built on the theme, “Every day, in some way, PM touches your life,” uses dozens of examples of actual components manufactured for many different applications to illustrate the benefits PM offers parts designers and engineers. The DVD is available for purchase online through the MPIF Publications Department (www.mpif.org). Powder-Core Business Sold Arnold Magnetic Technologies Corp., Rochester, N.Y., sold its powder-core business in Shenzhen, China, to Micrometals, Inc., Anaheim, Calif. Terms of the sale were not announced. MIM Company Opening East Coast Operation Parmatech Corporation, Petaluma, Calif., reports that ParmatechProform Corp. will open a new 25,000 sq. ft. metal injection molding (MIM) manufacturing plant in East Providence, R.I. Scheduled to be onstream during the summer of 2010, the facility will serve as Proform’s future headquarters.
New Domestic Source of Electrolytic Copper Powder SCM Metal Products, Inc., Raleigh, N.C., has expanded its product offerings to include electrolytic copper powder, to be imported from an undisclosed source overseas. The U.S. has had no domestic producer of electrolytic copper powder after the closing of the AMAX powder plant in Carteret, N.J., during the mid-1980s. PM Tool Steel Marketing Agreements Crucible Industries LLC, Syracuse, N.Y., has signed sales and marketing agreements for its Crucible Particle Metallurgy PM tool steels with Latrobe Specialty Steel Distribution, Latrobe, Pa., and Robert Zapp Werkstofftechnik GmbH, Ratingen, Germany. Latrobe Specialty Steel will sell CPM products in North America and Robert Zapp will sell the products internationally except for North America and Japan. Press Maker Increases Plant Investments Taking advantage of the excess capacity created by the economic downturn, Cincinnati Incorporated, Ohio, is investing 10 percent of 2009 and 2010 sales to retool its plant and implement Lean practices. As a manufacturer of compacting presses, laser cutting systems, press brakes, and shears, the company has rebuilt its major machine tools with new controls, drives, and spindles.
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MEETINGS AND CONFERENCES
2010 MIM2010: INTERNATIONAL CONFERENCE ON INJECTION MOLDING OF METAL, CERAMICS AND CARBIDES MARCH 29–31 Long Beach, CA MPIF* POWTECH 2010 April 27–29 Nuremberg, Germany www.powtech.de ITSC 2010 INTERNATIONAL THERMAL SPRAY CONFERENCE & EXPOSITION May 3–5 Singapore http://asmcommunity.asmin ternational.org/content/ Events/ITSC/ LAM LASER ADDITIVE MANUFACTURING WORKSHOP May 11–12 Houston, TX http://www.laserinstitute.org/ conferences/lam/conference SMST 2010 THE INTERNATIONAL CONFERENCE ON SHAPE MEMORY AND SUPERELASTIC TECHNOLOGIES May 16–20 Pacific Grove, CA www.asminternational.org INTERNATIONAL SYMPOSIUM ON SURFACE HARDENING OF CORROSION RESISTANT ALLOYS May 25–26 Cleveland, OH www.asminternational.org
NANOMATERIALS June 8–10 Bad Gastein, Austria www.nanoconsulting.de AEROMAT 2010 June 20–24 Bellevue, WA www.asminternational.org PowderMet2010: MPIF/APMI INTERNATIONAL CONFERENCE ON POWDER METALLURGY & PARTICULATE MATERIALS June 27–30 Hollywood (Ft. Lauderdale), FL MPIF* BASIC PM SHORT COURSE July 25–28 State College, PA MPIF* 1st TMS-ABM INTERNATIONAL MATERIALS CONFERENCE July 26–30 Rio de Janeiro, Brazil www.tms.org MICROSCOPY & MICROANALYSIS 2010 August 1–5 Portland, OR www.microscopy.org PRICM 7 7th PACIFIC RIM INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS AND PROCESSING August 1–5 Cairns, Australia www.materialsaustralia.com.a u/scripts/cgiip.exe/WServic e=MA/ccms.r?PageID=19070
ILASS 2010 23rd ANNUAL CONFERENCE ON LIQUID ATOMIZATION AND SPRAY SYSTEMS September 6–8 Brno, Czech Republic www.ilasseurope2010.org PM SINTERING SEMINAR September TBA MPIF* TITANIUM 2010 October 3–5 Orlando, FL www.titanium.org 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 FORGING, SHEET METAL FORMING & POWDER METALLURGY – LINKING INDUSTRY & TECHNOLOGY October 20–22 Porto Alegre, Brazil www.senafor.com.br
2011 PowderMet2011: MPIF/APMI INTERNATIONAL CONFERENCE ON POWDER METALLURGY & PARTICULATE MATERIALS June 19–22 Chicago, IL 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
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PM BOOKSHELF
Volume 46, Issue 2, 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 ___________________________37 ADVANCED METALWORKING ____________(317) 843-9359 _______www.advancedmetalworking.com _________________38 AMETEK SPECIALTY METAL PRODUCTS ___(724) 225-6622 _______www.ametekmetals.com _________________________7 ARBURG GmbH + Co KG ________________(860) 667-6522 _______www.arburg.com _______________________________3 ELNIK SYSTEMS ______________________(973) 239-6066 _______www.elnik.com ________________________________27 GLOBAL TITANIUM ____________________(313) 366-5305 _______www.globaltitanium.com ________________________44 HOEGANAES CORPORATION _____________(856) 786-2574 _______www.hoeganaes.com ___________INSIDE FRONT COVER KITTYHAWK PRODUCTS ________________(714) 895-5024 _______www.kittyhawkinc.com__________________________28 NORTH AMERICAN HÖGANÄS INC. _______(814) 479-2003 _______www.nah.com __________________________________8 PARMATECH _________________________(707) 778-2262 _______www.parmatech.com ___________________________13 PIM INTERNATIONAL___________________44 (0)1743 469909 ____www.pim-international.com______________________20 RIO TINTO METAL POWDERS/ QUEBEC METAL POWDERS LIMITED_____(734) 953-0082 _______www.qmp-powders.com ________________BACK COVER SCM METAL PRODUCTS, INC. ___________(919) 544-7996 _______www.scmmetals.com ____________INSIDE BACK COVER UNION PROCESS ______________________(330) 929-3034 _______www.unionprocess.com __________________________6
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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