In-situ CO₂-induced synthesis of T2-Al₂MgC₂ phase enabled high-performance wire arc additive-manufactured AZ91D Mg-based alloy

The chemical composition of Mg-based welding wires plays a decisive role in determining the microstructure and mechanical properties of wire arc additive-manufactured (WAAM) components. Herein, an innovative CO₂-induced wire fabrication strategy was proposed to achieve the in-situ synthesis of T2-Al₂MgC₂ phase within the AZ91D Mg-based alloy. The fine T2-Al₂MgC₂ phase was synthesized through the in-situ reaction between CO₂ and AZ91D Mg-based alloy, after which Mg-based welding wires were prepared through hot extrusion and used for WAAM of thin-walled structures. Microstructural evolution, mechanical properties, and the refinement mechanism of the T2-Al₂MgC₂ phase during the WAAM process were investigated. The as-built WAAM components were characterized by a predominantly equiaxed microstructure and an average grain size of approximately 14.7 μm. Grain refinement was attributed to the orientation relationship (0001)_T2−Al2MgC2//(0001)_α-Mg, which enabled T2-Al₂MgC₂ to act as effective heterogeneous nuclei for α-Mg. MgO nanoparticles were synchronously formed to impede grain-boundary migration and suppress the coarsening of the β-Mg₁₇Al₁₂. The horizontal direction of the components exhibited an ultimate tensile strength of 320.6 MPa and an elongation of 9.2 %, while the vertical direction showed corresponding values of 269.2 MPa and 6.8 %, respectively. Furthermore, the components exhibited pronounced quasi-cleavage fracture characteristics. This work provides a new pathway for developing high-performance Mg-based welding wires suitable for the WAAM process.