Effect of vacuum degree and T6 treatment on the microstructure and mechanical properties of Al-Si-Cu alloy die castings

This study investigated the effect of T6 treatment on the microstructure and mechanical properties of a die-cast Al–Si–Cu alloy at three vacuum degrees: 1013, 200, and 100 mbar. Corresponding microstructure and mechanical properties were performed. At the as-cast state, the eutectic Si sizes increased unexpectedly with increasing vacuum degree because oxides can act as heterogeneous nucleation sites for the eutectic Si at atmospheric pressure. Also, the increasing vacuum level from 1013 mbar to 100 mbar decreased pore size and porosity, which was the main reason for 12% and 55% increases in tensile strength and elongation, respectively. After T6 treatment, the decreasing pore size and porosity prevented the formation of blister. In addition, numerous θ′ phases were precipitated to strengthen the α-Al grains, evidently improving the tensile strength. The fractography of alloys under 1013 mbar revealed transgranular fractures because the spheroidized eutectic Si reduced the stress concentration at the grain boundaries. In contrast, the fractography of the alloys under 200 and 100 mbar showed coexistence of transgranular fractures and intergranular fractures due to the eutectic Si phase with coarse morphology presented at the grain boundaries.