Effect of heat treatment on microstructure and mechanical properties of Mg-6Gd-3Y-0.5Zr alloy fabricated by wire arc additive manufacturing

Heat treatment is necessary for GW63K (Mg-6Gd-3Y-0.5Zr, wt%) alloy fabricated by wire are additive manufacturing (WAAM) to improve the mechanical properties before final applications. Considering the unique solidification process and microstructure of WAAMed GW63K alloys, the heat treatment parameters must be different from the cast or forged ones. Thus, the present paper investigated the influence of solution temperature and time on the microstructure and mechanical properties of WAAMed GW63K, as well as the aging temperature and time. It was found that the as-deposited alloy exhibited fine equiaxed α-Mg grains with an average size of 13 μm and few eutectic phase Mg24(Gd, Y)5 around the grains. The proper temperature of solution treatment was 480℃ and the proper time was 0.5 h, lower and shorter than cast ones. After solution treatment, the average size of α-Mg grains almost remained the same and nearly all of the eutectic phase dissolved. To exert precipitation strengthening to a larger extent, we conduct aging treatment at 175°C, 200°C, and 225°C and draw their age-hardening curves. Besides, a superior strength /ductility trade-off was obtained peak aging at 200°C for 80 h, with the ultimate tensile strength of 337±8 MPa, yielding strength of 231±2 MPa, and elongation of 8.9±1.0%. This outstanding strength can be accounted for by the refinement strengthening of fine microstructure inherited from as-deposited GW63K alloy and precipitation strengthening from dense β′ precipitates with a remarkable plate aspect ratio. Besides, suitable β′ precipitates could support a favorable work hardening effect and compatible deformation capability.