The role of zinc addition on the microstructural evolution and mechanical properties of wire-arc directed energy deposited Mg-Gd-Y-(Zn)-Zr alloys

Wire-arc directed energy deposition (WA-DED) demonstrates significant potential in the rapid and free forming of high-strength magnesium rare-earth (Mg-RE) alloys. This highlights a growing demand for tailored wire alloy compositions to promote engineering applications of this technology. In this work, the role of zinc addition on WA-DED processed Mg-Gd-Y based alloys has been thoroughly investigated by utilizing Mg-6Gd-3Y-0.5Zr (wt%, GW63K) and Mg-6Gd-3Y-0.5Zn-0.5Zr (wt%, GWZ631K) alloy wires as model materials. The microstructure evolution of these two alloys during deposition and heat treatment is revealed by multi-scale microstructure characterization techniques. The influences of Zn addition on mechanical properties of these deposited alloys are investigated comparatively. Obtained results indicate that addition of Zn element into Mg-Gd-Y based alloys promotes phase transformation between eutectic phases and long period stacking order (LPSO) phases during the WA-DED deposition process owing to the multiple thermal cycling. More importantly, the addition of Zn element increases the tendency of solidification shrinkage of the deposited metal. During high-temperature solid solution treatment, the presence of Zn is conducive to improving the thermal stability of the as-deposited grain boundaries and thus suppressing grain coarsening. After optimized T6 treatments, the strength of GW63K alloys increases to 374 +/- 7 MPa, and is obviously higher than that (315 +/- 6 MPa) of GWZ631K alloys. The addition of Zn element weakens the precipitation strengthening effect. This can be attributed to the formation of LPSO phases, which consumes a substantial amount of RE elements, and in turn decreases the density of aging precipitated nanoscale beta<acute accent> phases. This work promotes the understanding of the non-equilibrium solidified microstructure evolution of multi-component Mg-RE alloys and guides the optimization of alloy composition of WA-DED specific wire materials.