Microstructural evolution and strengthening mechanism of Mg-Gd-Y-Zn-Zr alloy fabricated by wire arc additive manufacturing: Effects of heat treatment on mechanical properties

Abstract

Thin-walled Mg-8.5Gd-2.5Y-1.8Zn-0.5Zr (GWZ932) alloy component was successfully fabricated by the cold metal transfer (CMT) based wire-arc additive manufacturing (WAAM) process. Considering the significant influence of secondary phases on mechanical properties, we regulated the secondary phases through heat treatment. As-deposited sample exhibits the typical layered microstructure with alternating coarse/fine grains, along with a lot of Mg₃(RE,Zn) eutectic phases at grain boundaries (GBs). After solid-solution treatment, these eutectic phases are transformed into coarse X phase at GBs and fine lamellar 14H long period ordered stacking (14H-LPSO) phases in grain interiors. Further peak-aging induces amounts of prismatic β′ phases precipitated in the α-Mg matrix, which produces the strong precipitation hardening effect. Note that prismatic β′ phases are perpendicular to basal 14H-LPSO phase in space, which can form a closed space that blocks dislocation motions more effectively. Thus, peak-aged sample exhibits a high tensile strength of (314 ± 3) MPa and an acceptable ductility of 4.3% ± 0.6%, which outperforms most Mg-Gd-Y series alloys prepared by WAAM reported previously. Our work provides a basis for forming thin-walled Mg-Gd-Y series components with high strength via WAAM process, but the deposition process should be further optimized.