Plastic deformation behavior of aluminum casting alloys A356/357
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8/10/04
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Plastic Deformation Behavior of Aluminum Casting Alloys A356/357 Q.G. WANG The plastic deformation behavior of aluminum casting alloys A356 and A357 has been investigated at various solidification rates with or without Sr modification using monotonic tensile and multiloop tensile and compression testing. The results indicate that at low plastic strains, the eutectic particle aspect ratio and matrix strength dominate the work hardening, while at large plastic strains, the hardening rate depends on secondary dendrite arm spacing (SDAS). For the alloys studied, the average internal stresses increase very rapidly at small plastic strains and gradually saturate at large plastic strains. Elongated eutectic particles, small SDAS, or high matrix strength result in a high saturation value. The difference in the internal stresses, due to different microstructural features, determines the rate of eutectic particle cracking and, in turn, the tensile instability of the alloys. The higher the internal stresses, the higher the damage rate of particle cracking and then the lower the Young’s modulus. The fracture strain of alloys A356/357 corresponds to the critical amount of damage by particle cracking locally or globally, irrespective of the fineness of the microstructure. In the coarse structure (large SDAS), this critical amount of damage is easily reached, due to the clusters of large and elongated particles, leading to alloy fracture before global necking. However, in the alloy with the small SDAS, the critical amount of damage is postponed until global necking takes place due to the small and round particles. Current models for dispersion hardening can be used to calculate the stresses induced in the particles. The calculations agree well with the results inferred from the experimental results.
I. INTRODUCTION
WITH the current demand of weight reduction for improved product performance and fuel economy, aluminum casting alloys A356/357 are increasingly being used in critical structural applications in automotive and aerospace industries such as engine blocks, cylinder heads, chassis, suspension systems, etc. As many of these applications involve high stresses, the plastic deformation and tensile properties of the alloys are critical in both design and manufacturing. The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on the microstructure, which is simply comprised of age-hardenable aluminum dendritic matrix, mainly strengthened by Mg/Si precipitates,[1] and a dispersion of eutectic silicon and Ferich intermetallic particles. The initial yield stress is largely determined by the Mg/Si precipitates in aluminum matrix formed during aging. The large strain behavior, however, involves a strong interaction of plastic flow with eutectic silicon and Fe-rich intermetallic particles that locate in aluminum dendritic cell and grain boundaries (Figures 1 and 2).[2] The tensile fracture of alloys A356 and A357 is initiated by the cracking of eutectic silicon[
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