Densification and microstructural evolution during co-sintering of Ni-base superalloy powders
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re the major class of materials used for high-temperature applications in aircraft, marine, and land-based power systems. The successful application of these alloys is due to their high long-time creep strength and stability at elevated temperatures combined with their outstanding resistance to corrosion.[1] Nowadays, wrought components made from these alloys have gain considerable acceptance in industry. Nevertheless, many of components are highly complex shapes that are very expensive to produce due to extensive machining.[2] Conventional ingot metallurgy suffers compositional segregation due to its inherently slow solidification rate and thereby does not yield material suitable for high-performance applications. The desire to extend further the elevated-temperature capabilities has made the powder metallurgical route increasingly attractive. In this process, improved compositional homogeneity is achieved by dramatically increasing the solidification rate. Hot isostatic pressing of atomized superalloy powders, which has been a viable commercial process for over 25 years, yields fine grain structure with an equiaxed microstructure.[3] Another motivation for using powder metallurgy is the need to improve material use and thus conserve strategic elements.[1] This process also has a large cost-saving potential. Recently, metal injection molding (MIM) of superalloys for manufacturing gas turbine engine components has gained interest.[2–6] This owe to the ability of the MIM process to be cost-effective in the production of complex-shaped parts. Recent enhancements of the MIM process are ‘‘twocolor’’ and ‘‘co-injection’’ molding of metallic alloys, A. SIMCHI, Associate Professor, is with the Department of Materials Science and Engineering and Institute for Nanoscience and Nanotechnology, Sharif University of Technology, 14588 Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted December 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
which allow manufacturing of complex-shaped bimetallic parts in a single production route.[7] This process is a combination of two existing techniques: MIM and the ‘‘twocolor injection’’ molding process.[8] In this method, a twin-barreled injection-molding machine is used. This enables injection of two different feedstocks either simultaneously or in subsequent stages in one molding machine through a single injection route.[9] Alternatively, the two feedstocks can be injected through two different gates, thus making up two well-defined areas of the composed green part. The molded green part is composed of two different materials, depending on the injection process. For instance, the process can be controlled in a way that one of the barrels forms the skin of the component while the second barrel forms the core.[10] The result is a complex-shaped part with two different compositions in the core and at the skin, e.g., a hard material at the skin and a softer material in the core. The two-body component is then carefully debinded and sintered to form an intricate bimetal part.
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