Advanced light metals casting development: Solidification of aluminium alloy A356

Abstract

Microstructural reactions during solidification of aluminium alloy A356 were analysed using cooling curve analysis to determine the evolution in fraction of solid. The experiments involved recording the temperature/time data under different cooling rates so that the various phase changes associated with solidification could be identified in both unmodified and strontium modified versions of the alloy. Once collected, the cooling curve data were processed to calculate the first derivative together with a baseline cooling curve (cooling curve in the absence of any transformations). Comparison of the baseline cooling curve with the first derivative data allowed accurate identification of start temperatures of the various liquid to solid transformations and characterisation of fraction solidified during the cooling process. Strontium modification altered the morphology of A356, decreased the precipitation temperature of new phases (2°C to 11°C), and increased the solid fraction of the beginning of eutectic and Mg₂Si precipitation (0.01 to 0.05). Cooling rate appeared to have no effect on solid fraction evolution but an increase in cooling rate reduced solidus temperature (48°C to 75°C).