The preparation of large-scale magnesium (Mg) alloy parts by wire arc additive manufacturing (WAAM) has broad application prospects, including automotive and aerospace industries. The chemical composition of Mg alloy wires plays a critical role in determining mechanical properties of WAAM Mg alloys. However, types of Mg alloy wires for WAAM need to be extended, in order to improve mechanical properties. Therefore, in the present work, a novel ATZM31 Mg alloy wire has been prepared and applied to the cold metal transfer (CMT)-WAAM process. This study focuses on understanding the forming quality, microstructure evolution, and mechanical properties of the ATZM31 alloy thin-wall component fabricated by WAAM. The results show that the Mg alloy thin-wall component possesses satisfactory formability, with minor sidewall roughness. The ATZM31 thin-wall component is mainly composed of columnar dendrites and equiaxed dendrites of the α-Mg phase, with the η-Al₈Mn₅ phase distributes dispersedly at grain boundaries. The area fraction of the η-Al₈Mn₅ phase is estimated to be ∼0.21% based on the statistical analysis of SEM images. Due to different cooling behaviors, the distribution of grain size along the build direction of the thin-walled component is uneven. The average grain size is ∼46 µm, ∼74 µm and ∼61 µm at the bottom, middle and top of the ATZM31 alloy thin-wall component, respectively. From the substrate to the top of the ATZM31 alloy thin-wall component, the hardness decreases gradually. The ultimate tensile strength along the deposition direction and build direction are ∼225 MPa and ∼214 MPa, respectively, without pronounced anisotropy. The ATZM31 alloy thin-wall component fabricated by WAAM exhibits a comparable ultimate tensile strength to forged AZ31 Mg alloys and weaker anisotropy than wrought Mg alloys.

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