Effect of Mn addition on the age-hardening behavior of an Al-(9-10)%Si-0.3%Mg die casting alloy in T5 and T6 heat treatment

This study investigated the effect of Mn adding on the T5 and T6 age-hardening behaviors of Al-(9.0-10.0)%Si-0.3%Mg (equivalent to A360 alloy) and Al-10%Si-2%Cu-0.3%Mg (equivalent to A383 alloy) die casting alloys using a hardness measurement, an electron probe micro analyzer (EPMA) and thermodynamic software. When 0.4%Mn was added, the hardness of the T5 heat-treated Al-Si-Mg die casting alloy increases because the Mn addition suppresses the formation of the π-Al₈FeMg₃Si₆ phase during solidification and distributes Mg into the a solid solution. This mechanism named as π-phase suppression mechanism promotes precipitation strengthening. In the T6 heat treatment, the hardness of the alloys hardly increased when Mn was added. In the Al-Si-Cu-Mg alloys, the hardening by the 0.5%Mn addition was negligible compared to that of the Al-Si-Mg alloys regardless of T5 or T6 treatment. Therefore, the hardening by Mn addition is the characteristic phenomenon in the T5 heat-treated Al-Si-Mg high-pressure die casting alloy.