Effect of Cu Content on the Microstructures of As-Cast and Homogenized Al-Cu-Mg-Ag Alloys

The microstructures of as-cast and homogenized Al-Cu-Mg-Ag alloys with 1.98, 3.66 and 5.16 wt.% Cu contents (Alloy 1, Alloy 2, Alloy 3, respectively) were investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and calculation of phase diagram. The results indicate that the second phases in the as-cast alloys consist of θ (Al 2 Cu), ternary α (Al)-θ (Al 2 Cu)-S(Al 2 CuMg) eutectic and Fe-enriched phase. The invariant reactions are L→α (Al) + θ (Al 2 Cu) at 542°C and L→α (Al) + θ (Al 2 Cu) + S(Al 2 CuMg) at 505°C during solidification. An increase of Cu content promotes the formation of θ (Al 2 Cu) phase, therefore leading to the total second phases increasing in the as-cast alloys. The ternary eutectic completely dissolves into the matrix after the first-step homogenization at 490°C, and θ (Al 2 Cu) phase dissolves subsequently after the second-step homogenization at 510°C. It is suggested that the proper homogenization treatments are 490°C/24 h for Alloy 1, 490°C/24 h + 510°C/24 h for Alloy 2 and 490°C/24 h + 510°C/48 h for Alloy 3.