Excellent isotropic mechanical properties of directed energy deposited Mg-Gd-Y-Zr alloys via establishing homogeneous equiaxed grains embedded with dispersed nano-precipitation

Magnesium rare-earth (Mg-RE) alloys have broad prospects for lightweight applications in the aerospace industry. Rapid manufacturing of large-sized Mg-RE alloy components is essential to promote their engineering applications. In this work, a directed energy deposition employing an ultrasonic frequency pulsed arc as thermal energy is applied to fabricate a high-quality formed Mg-6Gd-3Y-0.5Zr (wt%) single-pass multilayer component. The microstructural evolution during the deposition and post-forming heat treatment is investigated by electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy. Microhardness measurement, room-temperature uniaxial tensile test, and fractography are carried out to evaluate the mechanical performance. The obtained results show that the as-deposited Mg-6Gd-3Y-0.5Zr alloy possesses a homogeneous equiaxed α-Mg grains structure (avg. diameter 12 µm) and excellent isotropic tensile properties along the building and traveling directions. After solution + aging treatment, the average diameter of grains increases to 26 µm, accompanied by distinct coarsening of several grains. Moreover, high-density nanoscale β′ particles (avg. size 7.8 ± 1.2 nm) are introduced into the α-Mg matrix grains, which gives rise to a significant precipitation strengthening effect. Compared with the as-deposited alloy, the heat-treated Mg-6Gd-3Y-0.5Zr alloy shows a remarkably enhanced ultimate tensile strength of 345 ± 7 MPa and a slightly decreased elongation of 5.2 ± 0.8%.