Effect of noncontact ultrasonic assistance on the microstructure and mechanical properties of 304LN stainless steel joints during local dry underwater laser welding

This study investigates local dry underwater laser welding (ULDW) of 304LN stainless steel and systematically evaluates how the horn-to-molten-pool (H-to-P) distance in a non-contact ultrasonic configuration affects weld formation. The roles of the aqueous environment and ultrasound in tailoring the weld microstructure and mechanical properties are further clarified. A stable local dry cavity was generated at a water depth of 50 mm using a dual-layer gas-curtain drainage nozzle. Under identical processing parameters, onshore welding, ULDW, and non-contact ultrasonic-assisted ULDW (ULDW-U) were systematically compared. The results indicate that ultrasound intensifies molten-pool convection through acoustic streaming and cavitation-driven micro-jets, facilitating downward molten-metal transport, markedly improving root-side bead formation, and mitigating incomplete penetration. Under the present experimental conditions, the optimum joint performance was obtained at an H-to-P distance of 30 mm. Microstructural and EBSD analyses indicate that the water environment promotes columnar-grain coarsening, whereas ultrasonic assistance refines the microstructure and weakens texture intensity, reducing the area-weighted mean columnar grain size from 124.83 μm to 100.53 μm. Concurrently, ultrasound homogenizes the distribution of δ-ferrite and modifies its morphology. Mechanically, the ULDW-U joint achieves a tensile strength of 693.04 MPa and an elongation of 52.52%, representing increases of 9.94% and 50.53%, respectively, relative to the ULDW joint (630.38 MPa and 34.89%) and approaching the performance of onshore welds. The impact toughness is further enhanced by 17.55% to 152.81 J/cm². These findings demonstrate that non-contact ultrasonic assistance can substantially improve joint quality in local dry underwater laser welding implemented with a drainage nozzle under the present experimental conditions.