Microstructure and Mechanical Properties of Single-Pass Ultrahigh-Power Laser-Metal Inert Gas Hybrid Welding of 20-mm-Thick Steel

Single-pass ultrahigh-power laser-MIG hybrid welding was employed to join 20-mm-thick Q355 steel plates. Beyond obtaining a sound full-penetration joint, this work focuses on the through-thickness microstructural heterogeneity generated by the coupling of the ultrahigh-power laser keyhole, arc-assisted filler transfer, and non-uniform thermal cycles. A defect-free joint was achieved under the optimized parameters of 20 kW laser power, 350 A welding current, and 1.5 m/min welding speed. The upper region, dominated by arc heat input and filler-metal mixing, mainly consisted of ferritic structures, whereas the middle and lower laser-dominated regions were primarily martensitic because of rapid cooling and limited downward diffusion of filler elements. Thermal simulation further revealed distinct t_8/5 values along the penetration direction, providing quantitative support for the observed phase evolution. EBSD analysis indicated that the laser-dominated region exhibited higher local misorientation and dislocation density, corresponding to thermal strain accumulation and residual-stress strengthening. As a result, microhardness increased along the depth direction and reached 435.5 HV0.2 in the lower layer, while all joints maintained base-metal-level tensile strength. These results provide mechanistic insight into the relationship among thermal cycle, filler diffusion limitation, phase transformation, and mechanical heterogeneity in ultrahigh-power laser-MIG hybrid welded thick low-alloy steel joints.