Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

In this study, Cu was added into the high-pressure die-casting Al-5.5 Mg-0.7 Mn（wt%） alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5 Mg-0.7 Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al;CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al;CuMg precipitates with a size of 15–20 nm × 2–4 nm were generated in the T5-treated Al-5.5 Mg-0.7 Mn-x Cu（x = 0.5, 0.8, 1.5 wt%） alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al;CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5 Mg-0.7 Mn-0.8 Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5 Mg-0.7 Mn-0.8 Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.