Mechanical and Thermal Properties of Cu-Alloyed AZ91 Magnesium Alloy Subjected to T4 and T6 Heat Treatments

In this study, AZ91 magnesium alloy was used as the matrix material, and Cu powder was added at contents of 1 and 2 wt.% as an alloying addition. The AZ91–Cu alloys were fabricated by gravity casting combined with mechanical stirring. Subsequently, T4 solution treatment and T6 artificial aging treatment were conducted to investigate the effects of Cu content and heat treatment conditions on the microstructure, mechanical properties, and thermal conductivity. The results showed that Cu addition promoted the formation of Al4Cu9 intermetallic compounds. After T4 treatment, part of the β-Mg17Al12 phase dissolved into the α-Mg matrix, resulting in a more homogeneous microstructure. After T6 treatment, fine second phases re-precipitated, leading to a precipitation strengthening effect. In terms of mechanical properties, the T6-treated AZ91-2Cu alloy exhibited the highest hardness, yield strength, and ultimate tensile strength, reaching 83.73 HV, 117.94 MPa, and 173.82 MPa, respectively. In contrast, the highest thermal conductivity of 64.74 W/(m·K) was obtained for the as-cast AZ91-2Cu alloy. Therefore, although T6 aging maximized the mechanical strength, it did not simultaneously maximize thermal conductivity, demonstrating a trade-off between mechanical strengthening and thermal transport performance.