Key mechanisms and process features of the metal transfer and cavity evolution during underwater submerged arc welding

The key mechanisms and process features of the inner physical processes and quality improvement for the novel underwater submerged arc welding (USAW) were revealed with the employment of X-ray observation. Based on the investigation of metal transfer, cavity, and behaviors of the weld pool and flux in USAW, three kinds of droplet transfer mode were confirmed, namely the indirect transfer, direct transfer, and surface tension transfer, and the proportions of them were 81.8%,12.9% and 5.3%, respectively. Compared with underwater wet welding (UWW), the average droplet transfer period was greatly reduced from 198.5 ms to 102.8 ms, and the mean sizes were also decreased from 3.2 mm to 2.2 mm in USAW. Because far more gases were generated in USAW (4394.2 ml/min) than in UWW (755.7 ml/min), the leading gas flow dragging force acting on the droplet was much stronger in USAW, which repelled the smaller droplet to detach from the wire tip within shorter time than it did in UWW. In addition, smooth and uniform weld seam without obvious defects was produced by USAW, of which the width and reinforcement were increased and reduced by 22% and 14%, respectively. And the 4.6 mm thickness of heat-affected zone for the joint of USAW was 59% wider than that of the UWW joint, indicating the cooling rate was effectively reduced in USAW, which consequently help acquiring sound quality joints in underwater welding. [Display omitted] •X-ray observation is conducted to investigate the physical phenomenon of the USAW.•The droplets are mainly repelled to transfer into the weld pool.•The dynamics of droplets are greatly influenced by the shape of cavity.•Metal transfer mechanism of the USAW is discussed from the aspect of force state.