Effects of Cu content and heat treatment process on microstructures and mechanical properties of Al−Si−Mg−Mn−xCu cast aluminum alloys

The effects of Cu content and heat treatment process on the microstructures and mechanical properties of a series of Al−Si−Mg−Mn−xCu cast aluminum alloys prepared by the vacuum die casting process were investigated by three-dimensional X-ray microscopy, optical microscopy, scanning electron microscopy, transmission electron microscopy and microhardness testing. It was found that the number density and size of gas porosities increase with increasing Cu content. However, the Cu addition will promote the formation of Cu-containing primary phases (Q-Al5Cu2Mg8Si6 and θ-Al2Cu) during the solidification, which will improve the properties of the alloys. Five different primary phases were observed, namely eutectic Si, α-Al(Fe, Mn)Si, β-Mg2Si, Q-Al5Cu2Mg8Si6, and θ-Al2Cu phases. With increasing the Cu content, the θ phase area fraction increases significantly, while the α-Al(Fe, Mn)Si phase area fraction decreases initially, followed by a slight increase, with the Q phase area fraction displaying the opposite trend relative to α-Al(Fe, Mn)Si phase. These primary phases present different evolution rules during heat treatment process. During subsequent aging, the synergic effect of precipitating Q' and θ' phases can significantly increase the alloy hardening response.