Improving surface formability and microstructural uniformity of ZM6 magnesium alloy fabricated by CMT arc additive manufacturing via preheating

Wire arc additive manufacturing (WAAM) of ZM6 magnesium alloy holds broad application prospects in the aerospace field. Cold metal transfer (CMT) WAAM suppresses distortion but suffers from poor surface finish and microstructural inhomogeneity due to low heat input. Based on the improvement of weld pool fluidity by preheating, preheating was applied to improve surface quality, microstructural uniformity and mechanical properties. Mechanisms related to arc stability, oxidation, pore escape, and microstructural evolution were analyzed. At preheating temperatures of 350 °C and above, smooth near-vertical walls with 78% effective utilization ratio were obtained. At 400 °C, porosity and pore size were low; at 450 °C porosity further dropped but pore size increased via bubble coalescence. Preheating at 350–450 °C yielded uniform grains, with size difference (≤2 μm) across build height. Optimal strength-ductility occurred at 400 °C, with vertical/horizontal strengths of 201/200 MPa and mixed ductile-brittle fracture. Preheating stabilized arc by enhanced thermionic emission; at 400 °C, residual stress was minimized without twins. Grain growth followed parabolic law, rate constant rising with temperature. This study demonstrates a novel preheating approach that enables simultaneous optimization of surface finish, microstructural homogeneity, and mechanical properties in CMT WAAM ZM6 alloy, offering practical guidance for high-quality additive manufacturing of Mg-RE alloys.