Arc-mode-governed arc behavior, microstructure, and mechanical properties in cold metal transfer-based wire arc additive manufacturing of Mg–Gd–Y–Zn–Zr alloy

Wire arc additive manufacturing (WAAM) provides a viable route for fabricating large-scale rare-earth (RE) magnesium-alloy components. In WAAM, the cold metal transfer (CMT) mode strongly affects arc behavior, droplet transfer, and heat-input distribution. In this study, Mg–9Gd–4Y–1Zn–0.5Zr (wt%) alloy specimens were fabricated using four CMT-derived modes, namely, CMT, CMT Pulse (CMT-P), CMT Advanced (CMT-ADV), and CMT Pulse Advanced (CMT-PADV), and their effects on arc behavior, bead geometry, microstructure, and mechanical properties were systematically investigated. Stable deposition was achieved under all four modes, but clear differences were observed in average heat input and the within-cycle distribution of arc energy and droplet transfer. CMT exhibited relatively gentle arc action and stable short-circuit transfer, CMT-P showed the strongest arc spreading capability, and CMT-ADV and CMT-PADV displayed more pronounced expansion–contraction modulation characteristics. Polarity switching in CMT-ADV and CMT-PADV reduced the difference in eutectic-phase fraction between the fine- and coarse-grained regions. The as-deposited microstructures under all four modes were dominated by equiaxed grains and comprised similar phase constituents, predominantly including the α-Mg matrix and RE-rich secondary phases such as the (Mg,Zn)3(Gd,Y) eutectic and blocky RE-rich cubic phases. CMT, CMT-P, CMT-ADV, and CMT-PADV had average grain sizes of 22.14 ± 8.04, 17.41 ± 7.10, 16.08 ± 6.46, and 17.30 ± 7.21 μm, respectively. CMT-ADV specimens exhibited the highest tensile strength (264 ± 5 MPa), whereas CMT-PADV specimens showed the highest elongation (6.37 ± 0.52%) and weakest anisotropy. These results provide guidance for the process-mode selection and performance optimization in WAAM for RE magnesium alloys.