Study on microstructure, mechanical properties and thermal conductivity of vacuum-assisted high pressure die casting Mg-5Zn-xCu-0.5Zr alloy

This study investigates the effects of Cu content and heat treatment on the microstructure, mechanical properties, and thermal conductivity (TC) of vacuum-assisted high pressure die cast (HPDC) Mg-5Zn-xCu-0.5Zr alloys (x = 0, 0.5, 1, and 2 wt%). The results indicate that the addition of Cu leads to the formation of the MgZnCu phase and refines the microstructure of the alloy. The strength and elongation of the alloy increase and then decrease with increasing Cu content. Among the alloys, the Mg-5Zn-1Cu-0.5Zr alloy exhibited the highest ultimate tensile strength (231 MPa), yield strength (127 MPa), and elongation (7.48 %) compared to the others. At higher Cu content, the formation of a reticulated MgZnCu phase, which is prone to fracture during tensile testing, results in a deterioration of tensile properties. Heat treatment (400 °C × 16 h + 200 °C × 12 h) transforms the brittle reticulated MgZnCu phase into fine particles, reduces its detrimental effect on the matrix, and promotes the precipitation of solid solution elements within the matrix. As a result, the Mg-5Zn-1Cu-0.5Zr-T6 alloy demonstrates a combination of excellent mechanical properties and TC, with a tensile strength of 254 MPa, elongation of 11.76 %, and TC of 117.64 W/(k·m). The variation in TC and mechanical properties is discussed based on microstructure observations.