Mg-Gd based alloys are an important class of high-performance Mg alloys. In this study, three Mg-Gd alloys with different gadolinium (Gd) contents: Mg-9.54Gd-0.40Zr (wt.%, G10 K), Mg-15.11Gd-0.35Zr (wt.%, G15 K) and Mg-19.67Gd-0.33Zr (wt.%, G20 K) were prepared by semicontinuous casting and subsequent solution and aging heat treatments. The role of Gd content on microstructures and mechanical properties of the Mg-Gd-Zr alloy is studied. All three as-cast alloys exhibit eutectic phases of Mg₅Gd, with the amount increasing as the Gd content rises. Mg₅Gd disappears after the solution heat treatment (the G10 K alloy solution-treated at 480 °C for 4 h, the G15 K alloy at 500 °C for 12 h and the G20 K alloy at 520 °C for 24 h, respectively). Aging heat treatment at 200 °C for 64 h after solution introduces numerous prismatic β′ precipitates, with a significant increase in their area number density corresponding to increased Gd content. Additionally, the morphology of the β′ precipitates exhibits distinct variations: the G10 K alloy is characterized by an enhanced aspect ratio. Consequently, the peak-aged G10 K alloy demonstrates superior strength-ductility synergy, with a yield strength (YS) of 216 ± 1 MPa, an ultimate tensile strength (UTS) of 363 ± 1 MPa, and an elongation (EL) of 8.7 ± 0.6%. This study suggests that plasticity diminishes and precipitation strengthening is limited when the gadolinium content exceeds 15 wt.%.

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.

Laser powder bed fusion (LPBF) of Mg alloys mainly focuses on the traditional commercial casting Mg alloys such as AZ91D, ZK60 and WE43, which usually display relatively low tensile strengths. Herein we developed a novel high-strength Mg-12Gd-2Y-1Zn-0.5Mn (wt.%, GWZ1221M) alloy for the LPBF additive manufacturing process, and the evolution of microstructure and mechanical properties from the as-built state to LPBF-T4 and LPBF-T6 states was systematically investigated. The as-built GWZ1221M alloy exhibited fine equiaxed grains with an average grain size of only 4.3 ± 2.2 µm, while the as-cast alloy displayed typical coarse dendrite grains (178.2 ± 73.6 µm). Thus, the as-built alloy showed significantly higher tensile strengths than the as-cast counterpart, and its yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) were 315 ± 8 MPa, 340 ± 7 MPa and 2.7 ± 0.5% respectively. Solution treatment transformed hard and brittle β-(Mg,Zn)₃(Gd,Y) phase into basal X phase and lamellar long period stacking ordered (LPSO) with better plastic deformability, leading to the improvement of EL. Then peak-aging heat treatment introduced numerous nano-sized prismatic β′ precipitates inside grains, resulting in the enhancement of YS. Finally, the LPBF-T6 alloy achieved appreciably high strength with YS, UTS and EL of 320 ± 3 MPa, 395 ± 4 MPa and 2.1 ± 0.4% respectively. Both as-built and LPBF-T6 GWZ1221M alloys showed remarkably higher tensile strengths than the as-cast counterparts and as-built commercial Mg alloys, highlighting the great potential of high-strength as-built Mg-Gd based alloys for structural applications.