The Influence of the Casting Methods and Variables on the Microstructural Properties of A356-SiC Nanocomposite

In a comprehensive study, effects of casting methods such as gravity sand casting, squeeze casting and composite casting in semi-solid state and casting variables (i.e., cooling rate, stirring speed and shear forces) on the microstructure, porosity formation and mechanical properties of A356-SiC nanocomposites were investigated thoroughly. First, a step-like sample was used to make the sand casting samples to study the effects of different cooling rates that each step experienced on the properties of nanocomposite, where squeeze casting and composite casting samples were produced in a metallic mold. Once the samples were produced and prepared for the studies, grain size, shape factor, dendritic and globular microstructure, amount of eutectic phase and mechanism of porosity formation, mechanical properties and fracture surface of the samples were analyzed precisely and discussed. In addition, the effects of stirring forces, time and temperature, used in the squeeze casting and the compo-casting methods, on the morphology of the dendritic arms of aluminum matrix composites were investigated and compared with the microstructure of castings produced by conventional gravity casting. It was discovered that switching from the gravity sand casting process to the squeeze casting process delivers a less dendritic and more refined microstructure in the nanocomposite. However, composite castings produced in the semi-solid state by applying high stirring shear forces for 80 s produce the finest and the most globular and spherical microstructure. This optimized globularization condition provides desired microstructure and mechanical properties for this type of nanocomposite. The dominant failure mechanism in the composite casting samples produced in semi-solid state was ductile, where it was a mix of ductile/brittle for the squeeze casting and brittle for the sand casting samples. Finally, the effects of globularization temperature on the nucleation and growth of primary alpha grains were studied.