Influence of pressures in a hyperbaric environment on the penetration during pulsed laser welding of 304 stainless steels and its influence mechanism

Developing laser welding (LW) technology in a hyperbaric environment is significant for manufacturing and maintaining nuclear power facilities, offshore engineering structures, etc. However, the penetration is significantly reduced during LW in a hyperbaric environment, which has not been solved yet. Previous research has shown that compared with the penetration of LW in a hyperbaric argon (Ar) environment, the penetration of 304 stainless steels attained through LW in a hyperbaric environment can be remarkably increased when using helium (He) as the shielding gas on same conditions. Hence, the plasma and molten pool behaviors in the LW process of 304 stainless steels in a hyperbaric He atmosphere were systematically explored. Furthermore, the effect of the growth of ambient pressure on the process of pulsed laser spot welding of 304 stainless steels was analyzed. It was found that when ambient pressure gradually rose from 0.1 to 1.8 MPa, the height and the intensity of characteristic plasmas above the molten pool gradually increased. In the initial stage of welding, the expansion velocity of plasmas under 1.8 MPa was about 1/4 of that under 0.6 MPa; the growth rate of the molten pool depth under 1.8 MPa was about 1/13 of that under normal pressure (0.1 MPa). The increase in ambient pressure led to the great decrease in weld penetration. Under experimental conditions in this paper, the weld penetration at the ambient pressure of 1.8 MPa was only 25% of that at normal pressure (0.1 MPa).