J.L. Johnson, L.K. Tan, R. Bollina, P. Suri and R.M. German

Advances in Powder Metallurgy and Particulate Materials, R. Lawcock and M. Wright (eds.), MPIF, Princeton, NJ, 2003, pp. 8.234-8.244.

Abstract

Bi-material components can be processed by MIM by two-color injection molding and co-sintering, but compositions and sintering cycles must be optimized to minimize shrinkage mismatch while providing the desired properties. The effects of sintering temperature on the sintered density, hardness, and mechanical properties of M2 tool steel and boron-containing 316L stainless steel are investigated. The compatibility of co-sintering these materials is predicted based on calculations of the thermal stress and in situ strength of the component during sintering. This prediction is verified by successful Bi-MIM processing of 316L-0.5B stainless steel/M2 tool steel components for applications that require a combination of toughness and wear resistance.

Conclusions

Boron additions of 0.5 wt.% enhance the sintering of 316L stainless steel such that it has similar densification behavior to M2 tool steel. Strength, density, and hardness after sintering 316L-0.5B at 1245°C are all improved in comparison to 316L sintered at 1330°C. Ductility is reduced to 15%, but this is still acceptable for many applications. M2 sintered at 1245°C shows high strength and can achieve a hardness of 59 HRC after heat treating. The hoop stresses generated during co-sintering of M2 and 316L0.5B concentric rings do not exceed the in situ strengths of the materials, allowing for the fabrication of demonstration Bi-MIM components with a unique combination of wear resistance and toughness.

J.L. Johnson, L.K. Tan, P. Suri and R.M. German

JOM, vol. 55, no. 10, 2003, pp. 30-34.

Abstract

Powder injection molding can be used to fabricate bi-material components that provide unique functionality, such as a combination of toughness and wear resistance. Successful processing of these components requires minimization of internal stresses during sintering. The stresses generated during co-sintering of concentric rings are analyzed, compared to the materials’ strengths, and correlated with defects. The results provide guidelines for determining the compatibility of various materials and the effect of component geometry.

Conclusions

Successful processing of bi-material components requires that the interfacial strength exceed the maximum radial stress and that the in situ material strength exceed the maximum tensile hoop stress. These requirements can be met if the following conditions exist.

1. Metallurgical bonding between materials Using materials with similar compositions and powder characteristics can ensure good bonding. Small modifications (less than 25%) to the powder can be used to tailor properties. This ease of modifying compositions is one of the main advantages of using PIM technology in producing metallic parts.
2. Shrinkage match between the two materials Co-sintering materials requires that they have similar densification behavior. Minor chemistry changes can significantly alter a material’s shrinkage behavior, making it more compatible for co-sintering. Shrinkage can also be adjusted by modifying the solids loading of the feedstock.
3. Component geometry that minimizes internal stresses Performing stress calculations on a component can identify potential design changes to reduce the likelihood of cracking. Some design changes may not affect the external dimensions of the part, making them relatively easy to implement.

J.L. Johnson and D.F. Heaney

Medical Device Materials III, R. Venugopalan and M. Wu (eds.), ASM International, Materials Park, OH, 2006, p. 99-103.

Abstract

Metal injection molding of gas- and water-atomized Co-28Cr-6Mo powders is evaluated. Sintering is conducted in different atmospheres to evaluate their effects on sintering response and carbon, nitrogen, and oxygen contents. The effects of hot isostatic pressing and heat treat on the mechanical properties are investigated. Properties are correlated to the interstitial content and microstructure. Optimized processing gives mechanical properties that exceed ASTM requirements for cast and wrought Co-28Cr6Mo.

Conclusions

Both water-atomized and gas-atomized Co-28Cr-6Mo powders can be injection molded and sintered to a closed pore condition. Sintering in a nitrogen or a hydrogen/nitrogen atmosphere results in nitrogenization, which can compensate for low carbon levels and increase the strength. Static mechanical properties of as-sintered Co-28Cr-6Mo exceed ASTM F75 requirements for cast material. Mechanical properties exceeding ASTM F1537 requirements for wrought material can be achieved after HIP of the water-atomized powder, and after HIP and heat treat of the gas-atomized powder. Solution heat treating keeps the nitrogen from segregating to the grain boundaries, but has a mixed effect on mechanical properties depending on the powder source. Impurities, such as manganese and silicon, in the water-atomized powder produce inclusions in the final microstructure, which may hinder fatigue properties.

J.L. Johnson, L.K. Tan, R. Bollina, P. Suri and R.M. German

Powder Metall. vol. 48, no. 2, 2005, pp. 123-128.

Abstract

Powder characteristics that enable metal injection molding of high conductivity copper heat sinks are investigated. Gas-atomized, water-atomized, oxide-reduced, and jet-milled copper powders are characterized in terms of particle size, packing density, mixing torque, and impurity content. The sintering kinetics and rate of oxide reduction are investigated using die pressed samples. Densities of 93 to 96% of theoretical can be achieved for all of the powders. Oxide reduction is complete by about 900°C. One of the water-atomized copper powders is injection molded to produce a demonstration heat sink. Thermal and electrical conductivities are measured and related to iron content and porosity. Overall impurity contents for typical powders are about 0.15 wt.%, resulting in thermal conductivities of 280 to 320 W/(m⋅K) for MIM copper.

Conclusions

Oxide-reduced, water-atomized, gas-atomized, and jet-milled copper powders can all be processed to give densities near 95% of theoretical with slow heating rates in hydrogen and sufficient holds for oxide reduction. Higher green densities and higher heating rates lead to swelling especially for water-atomized powders. Reduction of copper oxides occurs primarily between 700 and 900°C and has an activation energy of about 100 kJ/mol. Pore swelling can still hinder achieving densities above 95% of theoretical, even if oxygen levels are reduced to less than 200 ppm before pore closure.

Impurities are a main factor affecting electrical and thermal conductivity. The thermal conductivity dropped significantly for powders that contained 50 ppm or more iron. The iron content is likely representative of the concentration of other impurities. Typical thermal conductivities as determined by both thermal diffusivity and electrical resistivity measurements were in the 280-320 W/(m⋅K) range. Elimination of the remaining porosity is estimated to give a thermal conductivity of 350 W/(m⋅K), which is typical of commercially pure cast copper alloys containing small amounts of silicon, tin, zinc, aluminum, and phosphorus.

R.M. German and J.L. Johnson

Inter. J. of Powder Metall., vol. 43, no. 5, 2007, pp. 55-63.

Abstract

Powder injection molding (PIM) has been applied to copper and copper alloys for several years. Many powder and process variants have been demonstrated, and recent work has been directed to applications associated with heat dissipation in electronic systems. This focuses attention on unalloyed copper with high thermal and electrical conductivity and on bronze for aesthetic non-structural uses. This paper provides a brief history of the field and the ensuing rationalization of the powder, process, and properties to the application. Fundamentally, PIM of copper requires a balance between the optimal processing options that deliver the desired properties and the conflicting dictates of low-cost processing. A key to success often is tied to oxygen control in the copper powder.

Conclusions

None of the seven copper powders tested here emerged as clearly the best choice for PIM. All of the powders achieved from 93% to 96% of the pore-free density. The jet-milled copper powder had the lowest impurity level and gave the highest sintered thermal conductivity. The gas-atomized and 25 µm water-atomized powders had the highest solids loadings, which generally promotes ease of PIM processing; however, a less spherical particle shape is often preferred for component shape retention during debinding. Evaluation of the dimensional uniformity differences from these powders is a future task. Additionally, as PIM moves into thermal management applications the obvious cost issues arise as different powders and processing options are considered. However, it is clear that the foundation in powders, processing, and design are in place to make PIM copper a successful growth area.

J.L. Johnson

Handbook of Metal Injection Molding, Second Edition, D.F. Heaney (ed.), Woodhead Publishing, Duxford, UK, 2019, pp. 461-498.

Abstract

This chapter begins with an overview of heat dissipation in electronics and the need for thermal management materials. Methods of measuring thermal properties are reviewed. The characteristics and preparation of suitable powders for MIM Cu, W–Cu, and Mo–Cu are discussed. Examples of binders and mixing techniques for preparing injection molding feedstock from these powders are described. Specific issues with molding Cu, W–Cu, and Mo–Cu feedstocks into heat sink components are discussed. Processing conditions for debinding and sintering or infiltrating such components are described with a special focus on densification and control of the oxygen content, microstructure, and dimensions. Factors affecting the thermal properties, such as impurities and porosity, are discussed. Examples of MIM heat sink components are provided.

Conclusions

High thermal conductivity materials, Cu, W–Cu, and Mo–Cu can be processed by metal injection molding. Copper can be solid-state sintered to near full density, but care is needed to avoid hydrogen swelling. Near full density can be achieved for W–Cu and Mo–Cu by either liquid-phase sintering or infiltration. The commercial availability of composite W–Cu powders with excellent sintering behavior makes liquid-phase sintering preferable for W–Cu. Because of the poor molding characteristics of fine Mo powders and their poor densification during liquid-phase sintering, infiltration techniques are recommended for Mo–Cu. With proper powder selection and good control of sintering cycles and impurities, thermal properties close to model predictions can be achieved. Metal injection molding allows fabrication of heat sink geometries that are difficult to produce with other metal-working technologies. Novel structures, such as a heat pipe with a high conductivity casing surrounding a porous wick, can be directly fabricated into complex shapes.

Ultra Fine Specialty Products continues to expand its production of highly spherical, gas atomized powders in a wide array of alloys used in metal additive manufacturing and metal injection molding. This year, Ultra Fine has also installed capabilities to produce pilot quantities of gas atomized powders, which will enable the development of custom alloys for powder bed fusion and binder jet printing. The pilot equipment will also enable development of technology to narrow particle size distributions, improve sphericity, and reduce satellites. Upon successful evaluation, quantities can be readily scaled to high volumes on existing production equipment.

John L. Johnson

MIM 2022 Conference, February 21-23, 2022, West Palm Beach, FL

Abstract

Due to a combination of strength, wear resistance, corrosion resistance, and low magnetic permeability, Co-28Cr-6Mo is used in diverse applications ranging from smart phones to surgical implants. Historically cast to ASTM F75 specifications, many Co-28Cr-6Mo components are suitable for metal injection molding. Achieving F75 properties in the as-sintered state requires control of the interstitial content and microstructure, which are dependent on the starting powder chemistry and the sintering atmosphere as shown from sintering studies of metal injection molded F75 powders produced by gas- and water-atomization. The effects of hot isostatic pressing and heat treatment to further improve the mechanical properties are also investigated.

John L. Johnson

MIM 2023 Conference, February 27-March 1, 2023, Costa Mesa, CA

Abstract

Ferromagnetic materials are used in almost all modern electronic equipment. They are also used for magnetic shielding. Metal injection molding enables fabrication of complex magnetic components for advanced applications while also avoiding particle deformation that reduces magnetic properties. Many different ferromagnetic powders can be produced by gas atomization. Some common alloys include Sendust (Fe-Si-Al), molybdenum permalloy (Ni-Fe-Mo), and permendur (Fe-Co-V). The effects of different particle size distributions and post-atomization treatments of these ferromagnetic powders on their magnetic properties, including permeability, coercivity, and saturation induction, are investigated.

John L. Johnson

MIM 2024 Conference, February 26-28, 2024, Raleigh, NC

Abstract

Nickel-containing alloys exhibit a combination of strength and resistance to surface degradation that make them useful for many applications, especially in the aerospace and chemical processing industries. Corrosion resistant alloy powders with a wide range of nickel contents can be produced for metal injection molding (MIM) by gas atomization. Examples include 316L, HX, 718, 625, and 270. Sintering conditions to achieve maximum densities for these alloys are reviewed. The mechanical properties of these corrosion resistant alloys processed by MIM are compared to equivalent cast and wrought alloys. Their corrosion resistance is tested in nitric acid, hydrochloric acid, bleach, sodium hydroxide, and sulfuric acid. While 316L and 270 perform well under oxidizing and reducing conditions, respectively, the superalloys, especially 625, perform well in both types of corrosive environments.