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Metal Matrix Composites




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A novel, near-net shape, low-cost sinter-forging approach to processing particle-reinforced metal matrix composites for high performance applications produced strong, fatigue-resistant connecting rods. Nikilesh Chawla* Arizona State University Tempe, Arizona High strength-to-weight ratios, en- hanced mechanical and thermal prop- erties, and tailorability make metal ma- trix composites (MMCs) very attractive for automotive applications. Particle-reinforced MMCs, such as SiC particles in an aluminum alloy matrix, are particularly attractive because of their lower cost, relative isotropy, and ease of fabrica- tion relative to their continuous-fiber reinforced counterparts. An important application for Al/SiCp compos- ites is in the connecting rod, which requires high fatigue resistance at temperatures as high as 150°C (300°F). A lighter connecting rod would provide a 12 to 20% reduction in secondary shaking force, a 0.5 to 1% improvement in fuel economy (with lightweight piston and pin), a 15 to 20% increase in peak RPM, lower bearing width (package im- provement), and better bearing and crankshaft durability. This article describes the sinter forging process for producing particle-reinforced aluminum com- posites for automotive applications. It includes results of microstructural characterization and mechanical properties of the composites. Sinter-forging At Arizona State University, we have examined the microstructure and properties of a particle re- inforced composite fabricated by a low cost, novel sinter-forging technique. In this process, the powder mixture of SiC and Al is cold compacted, sintered, and forged to nearly full density. Its main advantage is that sinter-forging produces a near- net shape component, and machining operations and material waste are minimized. The mechanical behavior and microstructure of the composite were characterized and com- pared to materials of similar composition *Member of ASM International processed by the hot- pressing + extrusion tech- nique. The low-cost, sinter-forged com- posites studied have tensile and fatigue properties that are comparable to those of ma- terials produced by extrusion. The aluminum and alloy powders in this study were gas-atom- ized (Valimet Inc.), while a SiC abrasive grade powder (Saint-Gobain) served as the particle rein- forcement. The alloy powder consisted of a mix- ture of pure aluminum powder, Al-50 wt% Cu prealloyed powder, and Al-50 wt% Mg prealloyed powder. The prealloyed powders enabled en- hanced sinterability and compositional homo- geneity in the matrix. The composite contained 20 vol.% SiC and a bulk alloy composition of 3.7 wt% Cu, 1.8 wt% Mg, and balance of Al, consis- tent with the composition of the 2080 Al alloy. In the sinter-forging process, the powder mix- ture was blended in a V-cone blender and cold- compacted at a pressure of 480 MPa (70 ksi). The powder compact was then sintered in a nitrogen atmosphere at 565°C (1050°F) for 60 minutes to achieve partial densification, followed by closed- die forging at 482°C (900°F) to achieve near-full density (Metaldyne Inc., Fort Wayne, Ind.). The nominal total strain achieved during forging, through the thickness of the preform, was approximately 25%. The forged composite was compared to a composite consolidated via powder metallurgy processing and extrusion (Alcoa Tech- nical Center, Alcoa, Pa.). The extruded composite ...





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