On the advantages of using powder metallurgy in new light metal alloy design
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G the attractive properties offered by alloys based on light metals such as aluminum, resistance to corrosion, low density, and high strength to weight ratio are perhaps the most important. Unfortunately, wear resistance, tensile behavior, and high-temperature service applications are not particularly favorable. With this in mind, it is reasonable to assume that an improvement in any of these aspects that could enhance use of these alloys would be vital to overall global attempts to lower emissions, particularly in the automotive and aerospace industries. Perhaps the most versatile processing technique when considering both ability to modify alloy chemistry and at the same time produce a near-net-shape product is powder metallurgy (P/M). Fundamentally, the process follows procedures developed historically by the ceramics sector and therefore the most important operating parameters include particle size distribution, blending techniques (including use of dispersants), pressing, and ultimately sintering.[1] G.J. KIPOUROS, Professor, Materials Engineering, and Director, Minerals Engineering Centre, W.F. CALEY, Professor, Materials Engineering, and D.P. BISHOP, Professor and Chair, Materials Engineering Program, Department of Process Engineering and Applied Science, are with Dalhousie University, Halifax, NS, Canada B3J 2X4. Contact e-mail: georges. [email protected] This article is based on a presentation made in the symposium entitled ‘‘Fourth International Alloy Conference,’’ which occurred in Kos, Greece, from June 26 to July 1, 2005, and was sponsored by Engineering Conferences International (ECI) and co-sponsored by Lawrence Livermore National Laboratory and Naval Research Laboratory, United Kingdom. METALLURGICAL AND MATERIALS TRANSACTIONS A
To be commercially useful, the latter should ideally produce a product of suitable final density (.95 pct) so that postworking (e.g., forging) is not required. Of the P/M techniques available to enhance selected properties of light metal alloys, incorporation of a secondary strengthening phase (e.g., SiC in aluminum) and macro-microalloying is the most amenable to P/M processing. Whereas significant attention has been directed toward the former,[2,3,4] it is macro-microalloying that may well offer the most versatility. With this in mind, the authors embarked upon a research initiative in the early 1990s that has led to the development of some promising advances in the quest to raise the profile of P/M in the light metals sector. These works involve mineral dissociation, diffusion of suitable alloying elements, and various sintering regimes including reaction sintering and super solidus liquid phase sintering. Therefore, the research involves both equilibrium (thermodynamic) and kinetic (diffusion) considerations. A. Thermodynamic Considerations To provide a reasonable selection of candidate minerals for either direct incorporation into a matrix (Metal Matrix Composites (MMC)) or as a source(s) for macro-microalloying through dissociation/reaction sintering/elemental diffusion of m
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