Effect of T4 heat treatment on microstructure and hardness of A356 alloy refined by Ga + In + Sn mixed alloy

• Phase transition cooling (PTC) is available to cool A356 alloy for fine microstructure.
• Finding dendritic eutectic Si and Si precipitation in as-cast microstructure.
• Typical as-cast microstructure can shorten solution time for good modification of eutectic Si.
• Prolonging time to compensate reduced temperature in solution treatment was impracticable.
• The inner-relationship between solidification and heat treatment process was interpreted.

A356 alloy was solidified with cooling rate enhanced by phase transition cooling (PTC) of Ga + In + Sn mixed alloy, and then solution treated at 540 °C, 500 °C and 450 °C for different time, followed by quenching and immediately natural aging. Evolution of the corresponding microstructure and hardness with the solution temperature and time were studied. The as-cast sample was hardened due to the refined microstructures, the large solubility of Si- and Mg-clusters in Al matrix and the nano-scale Si precipitates. The modification of eutectic Si was verified to depend on the initially solidified morphology of Si, the solution temperature and time, for example, the Si particles subjected to high cooling rate and high solution temperature become more readily spheroidized. The age hardening, from the hardness tests and differential scanning calorimetry (DSC) results, was ascribed to be associated with the Guinier–Preston (GP) zones. It can be concluded that an optimal combination of solidification with heat treatment may achieve the better mechanical property but with the less product cost.