Coupled oxidation and solidification control in wire-arc deposited Mg–Gd–Y–Zr alloy through local shielding and polarity modulation

Oxide entrainment and uncontrolled thermal histories limit open-arc deposition of reactive magnesium–rare-earth (Mg–RE) alloys. Here, Mg–9.54Gd–1.82Y–0.44Zr wire was deposited by wire-arc additive manufacturing (WAAM) using cold metal transfer (CMT), local trailing Ar shielding, and CMT-Advance polarity modulation. Relative to unprotected deposition, porosity decreased from 0.065% to 0.040%, while the X-ray computed tomography (CT)-detectable oxide-related defect fraction decreased from 0.02300% to 0.00047%. A three-dimensional arch-bead heat-transfer model was calibrated against complete thermocouple histories. Peak-temperature errors were 0.09–1.07%, and 0–100 s root-mean-square errors (RMSEs) were 3.37–10.02 °C. Model-consistent net heat inputs were 226.7, 103.9, 113.9, and 107.1 J mm⁻¹ for CMT and CMT-Advance positive-electrode/negative-electrode (EP/EN) ratios of 0, 1, and 5, respectively. CMT-Advance increased the thermal gradient (G) and cooling intensity at the stable section, while the ratio of thermal gradient to interface velocity (G/R) increased strongly towards the substrate. Electron backscatter diffraction (EBSD) showed weak textures and a representative minimum mean grain size under EP/EN = 0. X-ray diffraction (XRD) was dominated by α-Mg and contained weak reflections indexed to Mg₂₄(Gd,Y)₅. Scanning electron microscopy (SEM) of the fracture surfaces showed dimpled rupture in all conditions, with a more uniform ductile morphology after shielding. The shielded CMT-Advance condition achieved 154.8 ± 2.8 MPa yield strength, 250.6 ± 0.9 MPa ultimate tensile strength, and 10.0 ± 0.2% elongation. These results separate oxygen-supply control from polarity-dependent heat input and residual-film transport within a process–microstructure–property framework.