Thixotropic behavior and structure evolution of A356 alloy in the semisolid state

Abstract

The study on the behavior of metals in the semisolid state was pioneered by David Spencer at MIT in 1971. He applied a shear strain on solidifying Sn-15 pct Pb alloy, and discovered a remarkable reduction in shear stress comparing with those that were not sheared during solidification. The grain structure of the sheared alloy was nondendritic, as opposed to the dendritic structure of not-sheared alloy. Since this early work, semisolid processing (SSP) has become a widely studied and accepted process. Comparing with conventional cast materials, SSP materials have less segregation and finer grain size. Practically, SSP can increase part-forming rate, reduce the thermal shock imposed on the mold, increase mold life, etc. A356 Al-Si alloy has the following characteristics, i.e. good castability and mechanical properties, light weight and a better oxidation resistance, and is widely used. The solidus and liquidus of the alloy are 830 K and 886 K, respectively, which provide a considerably wide semisolid regime that is suitable for the studies on the semisolid behaviors. The evolution of nondendritic structure and the thixotropic behavior of A356 alloy during semisolid processing were studied. During the process, semisolid slurry was produced using mechanical stirring technique.