Laser powder bed fusion of Mg-6Gd-3Y-0.2Zr alloy: Excellent printability, heterogeneous microstructure and dedicated direct aging heat treatment

As a universal casting Mg-RE alloy, Mg-6Gd-3Y-Zr (GW63K, wt.%) alloy exhibits superior strength-ductility synergy and holds significant potential for engineering applications. In this study, the GW63K alloy is produced using the laser powder bed fusion (LPBF) additive manufacturing (AM) process for the first time. The printability, microstructure characteristics, and post-heat treatment conditions of the GW63K alloy are systematically investigated. The as-built GW63K samples demonstrate high relative densities exceeding 99.6% and exhibit no macroscopic and microscopic cracking across a wide range of process parameters, indicating excellent printability. An exceptional heterogeneous microstructure is observed in the as-built GW63K alloy, comprising coarse columnar grains, fine equiaxed grains with an average grain size of 21.72 µm, uniformly distributed nano-sized Mg24(Gd,Y)5 secondary phase, and numerous dislocations. Consequently, the as-built GW63K alloy displays enhanced tensile strengths and ductility compared to the as-cast alloy, with yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) values of 218 ± 4 MPa, 284 ± 5 MPa and 11.9 ± 1.6% respectively. Additionally, due to the absence of coarse micron-sized secondary phase, a specific direct aging (T5) heat treatment regime at 200 °C for 128 h is optimized for the as-built GW63K alloy to introduce dense and dispersed β' aging precipitates. This T5 treatment surpasses the conventional solution plus aging (T6) heat treatment in enhancing mechanical properties. The LPBF-T5 GW63K alloy exhibits YS, UTS and EL values of 293 ± 6 MPa, 359 ± 4 MPa and 2.9 ± 0.7%, respectively. Notably, the YS of the LPBF-T5 alloy represents the highest value for the GW63K alloy, even surpassing that of the extrusion-T5 alloy. This study indicates that the GW63K alloy is a highly promising material for manufacturing near-net-shape high-strength Mg alloy components with intricate geometries using LPBF.