Microstructure and mechanical properties of SUS304 weldments manufactured by ultrasonic vibration assisted local dry underwater welding

To address the key manufacturing challenges such as severe columnar crystal growth trend and heterogeneous composition of molten pool, which caused by the rapid water cooling in local dry underwater welding (LDUW), the ultrasonic vibration assisted local dry underwater welding (U-LDUW) was innovatively proposed in this study. The phase structure and distribution, grain structure characteristic, microhardness, and tensile property of SUS304 weldments fabricated by LDUW and U-LDUW were investigated and compared comprehensively. The obtained results demonstrated that the stirring action on the molten pool was realized by the external ultrasonic vibration, which accelerated the ferrite morphology evolution from skeleton to lath, and more ferrites were remained in the austenite matrix. Compared with LDUW, the ultrasonic vibration facilitated to break the dendritic crystals, with the ultrasonic power of 800 W, the grain refinement was the maximum 32.1% and the proportion of the small-dimension (less than 20 µm) grains was increased from 53% to 67%. The ultrasonic vibration also contributed to the grain boundary transformation from low-angle to high-angle and increased the proportion of the recrystallized grains from 10% to 15%. With the increase of ultrasonic power, the microhardness, ultimate tensile strength, and elongation were increased first and then decreased, that of U-LDUW weldment were increased by 16.3%, 23.3% and 55.5% than that of LDUW. The results of this work provide a novel process method to enhance the comprehensive performance of LDUW weldments and expand the application of LDUW in repairing large-scale underwater structures.