Effect mechanism of arc oscillation on microstructure and mechanical performance of SUS304 weld seams manufactured by local dry underwater double pulsed MIG welding

An innovative local dry underwater double pulsed MIG (LDU-DPMIG) technique was successfully applied to fabricate SUS304 weld seams, in which the stirring effect on the molten pool was realized via a double pulsed current waveform. The weld formation, phase morphology and distribution, grain structure characteristics, microhardness, and tensile properties of underwater weld seams were comprehensively analyzed in this study, especially the mechanism differences between local dry underwater single pulsed MIG (LDU-SPMIG) and LDU-DPMIG were revealed. The results demonstrated that the stirring effect caused by arc oscillation contributed to the enhancement of the molten pool fluidity during the solidification process, which effectively facilitated the smooth elements allocations between austenite and ferrite and the evolution of δ-ferrite phase structure from skeleton to lath. In addition, the grain structure in the DPMIG weld centre transformed from slender columnar dendrites to fine equiaxed dendrites. Due to the pronounced stirring effects in uniform composition distribution, ferrite morphology evolution, grain refinement, and increased twin activity, DPMIG weld seams exhibited an attractive enhancement in weld mechanical performances, with the optimal low frequency of 3 Hz, the average microhardness, ultimate tensile strength, and elongation enhanced by the highest 18.1%, 8.9%, and 16.3% compared with SPMIG weld seam. These results are expected to offer a novel approach to regulate and control the microstructure and mechanical performance of underwater weld seams and facilitate the application of LDU-DPMIG in ocean engineering equipment.