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Metal Matrix Composites

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A novel, near-net shape, low-costsinter-forging approach to processingparticle-reinforced metal matrix composites for high performance applications produced strong, fatigue-resistant connecting rods.Nikilesh Chawla*Arizona State UniversityTempe, ArizonaHigh strength-to-weight ratios, en-hanced mechanical and thermal prop-erties, and tailorability make metal ma-trix composites (MMCs) very attractivefor automotive applications. Particle-reinforcedMMCs, such as SiC particles in an aluminum alloymatrix, are particularly attractive because of theirlower cost, relative isotropy, and ease of fabrica-tion relative to their continuous-fiber reinforcedcounterparts.An important application for Al/SiCpcompos-ites is in the connecting rod, which requires highfatigue resistance at temperatures as high as 150°C(300°F). A lighter connecting rod would providea 12 to 20% reduction in secondary shaking force,a 0.5 to 1% improvement in fuel economy (withlightweight piston and pin), a 15 to 20% increasein peak RPM, lower bearing width (package im-provement), and better bearing and crankshaftdurability.This article describes the sinter forging processfor producing particle-reinforced aluminum com-posites for automotive applications. It includesresults of microstructural characterization andmechanical properties of the composites.Sinter-forgingAt Arizona State University, we have examinedthe microstructure and properties of a particle re-inforced composite fabricated by a low cost, novelsinter-forging technique. In this process, thepowder mixture of SiC and Al is cold compacted,sintered, and forged to nearly full density. Its mainadvantage is that sinter-forging produces a near-net shape component, and machining operationsand material waste are minimized. The mechanical behavior and microstructureof the composite were characterized and com-pared to materials of similar composition *Member of ASM Internationalprocessed by the hot-pressing + extrusion tech-nique. The low-cost,sinter-forged com-posites studied have tensile and fatigue propertiesthat are comparableto those of ma-terials produced byextrusion.The aluminum andalloy powders in thisstudy were gas-atom-ized (Valimet Inc.), whilea SiC abrasive gradepowder (Saint-Gobain)served as the particle rein-forcement. The alloy powder consisted of a mix-ture of pure aluminum powder, Al-50 wt% Cuprealloyed powder, and Al-50 wt% Mg prealloyedpowder. The prealloyed powders enabled en-hanced sinterability and compositional homo-geneity in the matrix. The composite contained20 vol.% SiC and a bulk alloy composition of 3.7wt% 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). Thepowder compact was then sintered in a nitrogenatmosphere at 565°C (1050°F) for 60 minutes toachieve partial densification, followed by closed-die forging at 482°C (900°F) to achieve near-fulldensity (Metaldyne Inc., Fort Wayne, Ind.). The nominal total strain achieved duringforging, through the thickness of the preform, wasapproximately 25%. The forged composite wascompared to a composite consolidated via powdermetallurgy processing and extrusion (Alcoa Tech-nical Center, Alcoa, Pa.). The extruded compositehad a similar nominal matrix composition andvolume fraction of SiC as the forged material, al-though the average SiC particle size in the ex-truded composite was slightly larger (36 Pm forextruded vs. 29 Pm for forged). A T6 heat treat-ment (peak-aging) was applied to the specimens,which entailed a solution heat treatment at 493°C(920°F) for two hours, followed by a water quenchto room temperature, and finally aging at 175°C(350°F) for 24 hours to achieve peak hardness. Microstructure characterizationThe forged microstructure of the composite,shown in Fig. 1, exhibited some preferential align-IN AUTOMOTIVE APPLICATIONS METAL MATRIXCOMPOSITESSinter-forgedparticle-reinforced aluminumcompositeconnecting rodsmay someday beselected for automotiveengines. Thisimage shows theFord 3.5-literDuratec 35 V-6engine, whichfeatures a lightweightdie-castaluminumblock. Imagecourtesy FordMotor Co.ADVANCED MATERIALS & PROCESSES/JULY 2006 29ment of SiC particles perpendicular to the forgingdirection. SiC particles have a platelet morphologywith a mostly equiaxed shape. The SiC particlestended to align in such a way that the smallest di-mension was parallel to the forging direction. A comparison with the extruded 2080/SiCpcomposite, with similar particle size, indicatedthat the alignment in the sinter-forged compos-ites was not as significant as that produced by ex-trusion. This is to be expected, because a muchlarger amount of plastic deformation is inducedduring extrusion. The morphology of the aluminum grains wasalso affected by forging. The grains appeared tobe “pancake-shaped,” Fig. 1, with the major axisof the grain perpendicular to the forging axis. Theobserved anisotropy appeared to be a direct result of the constraint on lateral deformation provided by the closed die forging, which pre-vented the grains from deforming equally in alldirections.Tensile behaviorA comparison of the tensile behavior of thesinter-forged 2080/SiC/20p-T6 composite withthat of the extruded materials is shown in Fig. 2.The sinter-forged materials had a slightly higherYoung’s modulus and tensile strength, but loweroverall ductility. Overaging the sinter-forged ma-terial at 150°C (300°F) for 24 hours did not changethe tensile properties significantly. Under tensile loading, a significant fraction ofthe stress is initially borne by the reinforcement,because the reinforcing phase is much stiffer thanthe matrix. Thus, the composite typically has ahigher Young’s modulus than the unreinforcedalloy. The load-bearing capacity of the compositeis greatly weakened by processing-induced par-ticle fracture, such as during the extrusion process.For this reason, the sinter-forged materials hada slightly higher modulus than that of the ex-truded material. The incorporation of particles into the matrixresults in an increase in work-hardening over theunreinforced alloy because of the lower matrixvolume. When the matrix is significantly work-hardened, it is placed under great constraint, preventing strain relaxation. This causes void nucleation and propagation to be


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