Dependence of mechanical properties on microstructural partitioning driven by non-uniform rare-earth distribution in laser-CMT hybrid welded Mg-Al-Zn alloy

The insufficient mechanical properties in welded magnesium alloy joints, which are inherently due to weak metallic bonds, pose a considerable challenge that restricts their widespread application in lightweight aircraft structures. This paper focuses on the enhancement of mechanical properties in laser-CMT hybrid-welded joints of a newly developed corrosion-resistant magnesium alloy. The WE43 magnesium alloy filler wire with rare-earth (RE) was innovatively introduced to modify the solidification process and strengthening phase formation. The microstructural partitioning characteristics and mechanical properties driven by non-uniform rare-earth distribution were investigated by comparing the Mg-Al-Zn alloy welded joints with AZ31 filler wire and WE43 filler wire. A systematic investigation was conducted to elucidate the microstructural partitioning characteristics, RE segregation behavior, and associated strengthening mechanisms. Results reveal that RE addition significantly alters the solidification behavior, refining the arc zone average grain size to 14.4 µm (a 53.2% reduction). Furthermore, distinct macroscopic RE segregation—characterized by strip-like bands near the fusion line and plate-like accumulations at the pool bottom—was driven by the preferential precipitation of high-density Al₂(Y, Nd) phases migrating under coupled gravitational settling and Marangoni convection. Under optimized parameters, the ultimate tensile strength peaked at 290 MPa, significantly outperforming the AZ31B-reinforced joint (253 MPa). Quantitative deconvolution demonstrates that while precipitation strengthening dictates the RE-reinforced matrix (∼51.0%), the global yield strength is governed by the “weakest link” RE-rich segregation zone, where precipitate coarsening renders grain boundary strengthening primary (42.6%). Ultimately, these coarse precipitates induce local strain localization, triggering a ductile to mixed-mode quasi-cleavage fracture transition.