Mechanistic insights into the oxidation-driven alteration of molten pool surface dynamics under laser irradiation in spot welding

The dynamic behavior of the molten pool surface during laser spot welding significantly impacts forming quality. This study combines numerical simulations and experiments to comprehensively investigate the effect of surface oxidation in an air atmosphere on the molten pool flow behavior and solidification process during laser spot welding. Throughout the laser processing, the metal surface undergoes three stages: solid-phase heating, molten pool expansion, and molten pool solidification. The durations of solid-phase heating and molten pool expansion are similar in both argon and air, but the solidification time in air is 63.7% longer than in argon. During the early stages of processing in air, the expansion rate of the molten pool increases rapidly, with the expansion rate in air reaching twice that in argon. During the solidification process in air, within 0.3–1.2 ms after the laser is turned off, strong oscillations appear on the surface of the molten pool, and it is observed that the contraction rate of the molten pool during solidification slows down. This oscillatory behavior is the main cause of the ripple-like texture on the solidified surface in air and leads to a significant increase in roughness. During solidification, rapid oxidation and the subsequent drop in surface temperature change the temperature dependence of surface tension. The surface tension's temperature coefficient shifts from negative to positive, reversing the Marangoni flow direction from outward to inward. With extended solidification time and gravity's influence, this reduces the final residual height.