Microstructure evolution and mechanical properties of low-silicon cast aluminium alloys with varying Mn contents

The influences of Mn level (0-0.8 wt%) on the microstructure and mechanical properties of as-cast and T6 heat-treated low-silicon cast aluminium alloys were investigated. The results indicated that the addition of 0.5% Mn completely transformed the needle-like beta-Fe phase in as-cast condition into a small-sized pi-Fe phase, and the pi-Fe phase was fully dissolved after T6 heat treatment. The as-cast and T6 heat-treated alloys with 0.5% Mn achieved the best combination of strength and ductility. However, an excess addition of Mn reduced the mechanical properties due to the occurrence of coarse pi-Fe phase (as-cast state), alpha-Al(Fe, Mn)Si phase (T6 heat-treated state) and eutectic Si. With the increase of Mn content from 0 to 0.8%, the fracture modes of the as-cast alloys changed from intergranular + cleavage fracture to intergranular + quasi-cleavage fracture; The fracture modes of the T6 heat-treated alloys underwent a transition from intergranular + cleavage fracture to quasi-cleavage fracture and then to intergranular + cleavage fracture.