Synergistic effects of manganese micro-alloying on the microstructure, mechanical properties, and corrosion resistance of a T6-treated 356-0.4Fe alloy

Micro-alloying has been widely used to alleviate the detrimental effects of Fe-rich intermetallic particles (IMPs) in cast Al alloys. In our present work, a series of 356-0.4Fe alloys with distinctive Mn doping were prepared, and their microstructure, mechanical properties and corrosion resistance were systematically investigated after T6 treatment. The results show that with increasing Mn content, the Fe-rich IMPs witnessed a considerable change from needle-like β-Al 5 FeSi into Chinese-script or skeleton-like α-Al 15 (Fe, Mn) 3 Si 2 phases, while the area fraction of these precipitates increased with the increase of Mn content. Moreover, the addition of Mn element has the potential to refine grains, as which 0.35 wt.% addition exhibited the most obvious effect. With gradual increasing Mn content to 0.50 wt.%, it led to a substantial improvement in mechanical properties compared to the Mn-free alloy. Electrochemical tests indicated that the alloy containing 0.35 wt.% Mn showed the best overall corrosion resistance, characterized by a relatively low corrosion current density and the highest charge-transfer resistance. This behavior is likely associated with a more favorable distribution of Fe-rich IMPs and a reduced tendency for localized micro-galvanic corrosion. In contrast, the 0.50 wt.% Mn alloy showed a deterioration in corrosion resistance, which may be related to the formation of coarse Fe-rich IMPs and the resulting increase in local electrochemical heterogeneity. Within the investigated composition range and processing conditions, an Mn addition of about 0.35 wt.% provided the best balance between microstructural control, mechanical performance, and corrosion resistance.