Enhanced mechanical properties and equal-strength matching in deep penetration welding of rolled AZ31 Mg alloy through synergistic oscillating laser and CMT techniques

To overcome the “bottleneck” of deep penetration welding of rolled AZ31 Mg alloy, this work innovatively employed an oscillating laser-CMT hybrid welding (OLCMT) strategy to achieve equal-strength matching joints. Through systematic optimization of process parameters, the optimal welding conditions were identified as a laser power of 4KW, a welding speed of 1.8 m/min, and a wire feeding speed of 4 m/min. Experimental results demonstrated that the circular oscillation (CW) laser not only widened the weld with but also effectively eliminated dendrites and significantly promoted equiaxed grain refinement and microstructural homogenization. This resulted in a refined microstructure consisting of fine equiaxed α-Mg grains (~10 μm), intergranular α+β divorced eutectic and a small quantity of Al₁₁Mn₄ particles. In contrast, the linear (Lin) oscillating laser failed to improve the microstructure and instead caused the formation of undesirable snowflake-shaped structures. The enhanced molten pool dynamics induced by the CW oscillating laser generated strong convection and periodic vortices, which facilitated the remelting of columnar structures and dendrites, eliminated dendrite boundaries and arms and increased the availability of nucleation sites for equiaxed grain formation. Consequently, the CW-OLCMTW weld exhibited superior mechanical properties: average microhardness of 62 HV0.2, average yield strength (TYS) of 115 MPa, average ultimate tensile strength (UTS) of 253 MPa and average plastic elongation (PE) of 22%. Moreover, the CW-OLCMTW deep penetration joint achieved actual equal-strength matching ("S" ̅_"r" "=0.981" ) and superior-plastic matching (("A" _"r" ) ̅"=1.7" ) relative to the rolled AZ31 plate, validating the effectiveness of the proposed OLCMTW strategy.