Effect of backward median current on stability, microstructure, and mechanical properties of SUS304 weldments fabricated by local dry underwater MIG welding.

Local dry underwater backward median pulse MIG welding (LDU-BMPMIG) was proposed in this study. The effects of different backward median currents on welding process stability, weld forming, microstructure, and mechanical properties of weldments were investigated comprehensively. The results reveal that the backward median current waveform effectively improved the welding stability. By calculating the approximate entropy of the voltage signal during welding, it is proved that the welding stability increases first and then decreases with the increase of the backward median current. The welding process stability was the most satisfied with the backward median current of 200 A. The quality of weld formation was consistent with welding stability. LDU-BMPMIG weldments exhibited fewer macroscopic defects and better overall morphology compared with LDU-PMIG weldment. The weldment microstructure was mainly composed of γ-austenite and δ-ferrite with different morphologies. The satisfied stability of the welding process contributed to improving the microstructure uniformity. The variation trend of tensile strength for LDU-BMPMIG weldments was approximately kept consistent with welding process stability. The weldment prepared by 200 A showed considerable mechanical properties with minimal fluctuation, which reached 97.58% of maximum average tensile strength, which meant the most excellent compared to the others. The experimental results can improve the stability of local dry underwater welding and provide a theoretical basis and technical support for the application of underwater welding. [ABSTRACT FROM AUTHOR]