Effect of frequency on welding stability, microstructure, and mechanical performance of SUS304 welded by local dry underwater fast-frequency pulsed MIG

Abstract

A new local dry underwater fast-frequency pulsed MIG (LDU-FFPMIG) welding method was developed for fabricating SUS304 weldments. The effect of the fast-frequency single pulsed (FFSP) waveform at different fast-frequency pulsed (FFP) frequencies (10 kHz–30 kHz) on welding stability, microstructure and mechanical performance were studied, with the low-frequency pulsed (LFP) waveform serving as a comparative reference. The findings indicated that the FFSP waveform, especially at 20 kHz, enhanced the welding stability, improved the microstructure and strengthened the mechanical performance. Compared to the LFP waveform, the FFSP waveform at 20 kHz increased the droplet transfer frequency by 150 %. Additionally, the FFSP waveform at 20 kHz exhibited an optimal stirring effect on the molten pool, which significantly facilitated the transformation of skeletal ferrite into lathy ferrite and refined grains. The standard deviations of the weld element distribution obtained by the FFSP waveform at 20 kHz were reduced by 18.3 % (Fe), 29.9 % (Cr) and 30.1 % (Ni), respectively. Furthermore, with the increase of FFP frequency, the mechanical performance of weldments first increased (10 kHz–20 kHz) and then decreased (20 kHz–30 kHz). The microhardness (220 HV), tensile strength (765 MPa) and elongation (36.1 %) of weldments prepared by the FFSP waveform at 20 kHz increased by 20.2 %, 19.7 % and 43.3 %, respectively, compared to weldments prepared by the LFP waveform. This study was conducive to providing a novel method for the marine engineering field to improve the quality of SUS304.