Effect of Section Size as a Measure of Cooling Rate on the Solidified Microstructure and Mechanical Properties of Sand-Cast Al-Si Eutectic Alloy

Abstract

Several factors contribute to the development of structure and properties of aluminiumalloy castings. This study investigated the singular effect of cooling rate on the as-cast structure andmechanical properties of an aluminum-silicon eutectic alloy, keeping other factors such as pouringtemperature, melt treatments, physical and thermal properties of the mould, and alloy compositionconstant. The rate of cooling was varied by employing different casting section sizes, based on thevariation of rate of heat extraction given by solidification time as predicted by the Chvorinov’s rule.Four test bars of section sizes 10, 20, 30, and 40 mm respectively were cast in sand mould using thesame gating system. Spectrometric analysis of the alloy formulated revealed that it could be specifiedapproximately as Al-12.8Si-1.0Cu alloy. The study showed that as section size decreased from 40mm to 10 mm; the solidification time reduced (i.e. the cooling rate increased), the microstructure gotfiner, the silicon flakes became more uniformly distributed, and the mechanical properties generallyimproved. The tensile strength, ductility, and hardness all increased in the order of decreasing sectionsize, i.e. increasing cooling rate. The mechanical properties were found to be linearly correlated withsection size or cooling rate. Whereas the elongations were lower than values for pure aluminium, thestrength and hardness were significantly higher than values for the pure metal. It is concluded thatthe cooling rate modifies the microstructure and improves the mechanical properties of as-cast Al–Sieutectic alloys