Discussion of the influence of Al content on the microstructure and mechanical properties of Mg-xAl-Zn alloys fabricated by wire arc additive manufacturing

Aluminum (Al) acts as the primary alloying element in Mg-Al-Zn alloys, so its content is of crucial importance for the samples fabricated by wire arc additive manufacturing (WAAM). However, microstructure and property fluctuations of the alloys induced by varying WAAM processes hinder the extracting of consistent Al content-microstructure-property relationships from existing studies. In this study, Mg-xAl-Zn (x = 3, 6, 9, 12) alloys were fabricated via WAAM using consistent process parameters to focus on the influence of Al content alone. The results indicate that all of the four alloys primarily consist of α-Mg grains and Mg₁₇Al₁₂ phase. As Al content increases, the α-Mg grain size decreases from 43.1 μm in Mg-3Al-Zn to 19.7 μm in Mg-12Al-Zn. The content of Mg₁₇Al₁₂ phase increases from near 0% in Mg-3Al-Zn to 7.78% in Mg-12Al-Zn, and a sudden increase observed when Al content reached 12 wt%. The microhardness and yield strength of WAAM Mg-xAl-Zn alloys increase linearly with the increasing of Al content, which is attributed to the synergistic effects of grain refinement strengthening, solid solution strengthening, and second-phase strengthening induced by Al content. However, when the Al content reaches 9 wt%, the elongation and tensile strength decreased because of the excessive Mg₁₇Al₁₂ phase. Among the investigated alloys, Mg-6Al-Zn achieves the optimal strength-ductility balance, with a tensile strength of 266.3 ± 0.4 MPa and an elongation of 13.9 ± 1.4%. Thus, alloys with an Al content of approximately 6 wt% exhibit favorable performance and can be used without heat treatment, while those with Al content exceeding 9 wt% require heat treatment to optimize microstructure and improve ductility.