Microstructure Evolution and Corresponding Mechanism of Al–Si–Mg–Cu Alloy with Addition of Mn/Mn+Cr

The influence of Mn and Mn+Cr additions on α-Al dendrites, intermetallic phases, and eutectic Si in Al–Si–Mg–Cu alloys was examined. Thermodynamic calculations were utilized to elucidate the mechanism of microstructural evolution in these modified alloys. It was observed that the addition of Mn and Mn+Cr transformed the β-Al₅FeSi and π-Al₈FeMg₃Si₆ phases in the base alloy to α-Al₁₅(FeMn)₃Si₂/α-Al₁₅(FeMnCr)₃Si₂ and nonequilibrium π-Al₈(FeMn)Mg₃Si₆ phases, respectively. The combined addition of 0.3% Mn and 0.3% Cr enhanced the refinement of α-Al secondary dendrites and α-Fe intermetallic compounds, as well as facilitated the dissolution and fragmentation of the α-Fe phase during heat treatment. The diffusion of Cr into the α-Al matrix during solution treatment acted as a driving force for the dissolution and fragmentation of intermetallic compounds. Furthermore, the additions of Mn and Mn+Cr led to the refinement of eutectic Si, dissolution of π-Fe phases, and precipitation of fine dispersoids in T6 heat-treated alloys. This study is anticipated to serve as a reference for the application of transition metal elements in Al–Si–Mg–Cu alloys.