Enhanced heat transfer characteristics and microstructure of laser-welded 304 stainless steel through beam vibration and heat sink-assisted coupling effects

This article introduces a novel composite laser welding technology and its application to the 304 stainless steel. Through related experiments, the study analyzes the dynamic changes of the molten pool and the evolution of microscopic structure during 304 stainless steel laser welding after the introduction of a heat sink device and the use of a vibrating laser beam. High-speed imaging shows that as the cooling temperature decreases, the plasma plume exhibits deflection and weakening. This phenomenon originates from the combined effects of directional adjustment of the Marangoni force, weakening of recoil pressure, and thermal stress constraints. Meanwhile, the cooling process reduces the characteristic length of the molten pool, with an 8.54% reduction in molten pool length observed at −8 °C cooling compared to conventional welding. Additionally, as the cooling temperature decreases, the macroscopic morphology of the weld transitions from a full arc shape to a flatter form, significantly improving surface smoothness. Metallographic observations indicate that the heat sink can refine the grain size from relatively coarse, directionally grown columnar or dendritic grains to uniform fine equiaxed grains. Porosity defects appear under both conventional conditions and −8 °C cooling conditions. Within the observable cooling range, −6 °C is identified as the optimal cooling temperature that balances microstructural refinement and defect-free welding.