Laser–MIG hybrid welding is increasingly used in shipbuilding for medium-thick steel plates, where assembly-induced gaps and misalignment often compromise weld quality. In this study, a three-dimensional transient numerical model is established to simulate the molten pool dynamics during full-penetration laser–MIG hybrid welding of 10CrNiCu steel under varying gaps and misalignment conditions. The model incorporates coupled heat transfer, fluid flow, keyhole behavior, and droplet transfer and is validated against experimental weld profiles. The simulation results reveal that increasing gap size broadens the heat distribution, reduces keyhole depth, and decreases the bridging capacity of the filler metal. Larger misalignment enhances the step effect, promotes gravity-driven downward flow of liquid metal, and increases keyhole instability and porosity risk. The adaptability to assembly errors is further assessed. As welding current increases, the gap tolerance slightly improves from 1.61 mm to 1.65 mm, whereas the misalignment tolerance markedly decreases from 3.00 mm to 1.82 mm due to the earlier onset of porosity defects. These findings provide quantitative guidance for optimizing welding parameters to accommodate realistic assembly variations in marine steel fabrication.

In this study, a three-dimensional transient multi-physics coupled numerical model of narrow-gap tungsten inert gas (TIG) welding of TC4 alloy is established, and the heat transfer, flow behavior of the liquid metal in the molten pool, and the formation mechanism of sidewall lack-of-fusion defects are quantitatively investigated. The results show that, along the groove width direction, the temperature and flow velocity of the liquid metal gradually decrease from the center toward the sidewall. Along the welding direction, the fluid velocity presents multi-peak fluctuation characteristics. As the weld pass increases, the maximum velocity of the liquid metal significantly increases, enhancing the overall fluidity of the molten pool, and the temperature distribution becomes more uniform due to the weakened geometric constraints of the sidewall and the inter-pass heat accumulation effect. Furthermore, excessively high welding speeds (0.2 m/min) or insufficient welding currents (150 A) drastically reduce the per-unit-length heat input, resulting in diminished molten pool volume and decreased maximum liquid metal velocities of 0.075 m/s and 0.1 m/s, respectively. Under these conditions, lateral driving forces generated by the Marangoni effect are insufficient to overcome the viscous resistance of the molten metal. Consequently, the liquid metal solidifies prior to reaching the sidewall, inducing pronounced sidewall lack-of-fusion defects and localized humping on the weld top surface.