In the process of laser deep penetration welding, the laser beam interacts with the material to produce photo plasma, metal vapor, plume, and welding defects such as spatter and porosity. Among them, the root cause of small porosity is the instability of small holes, the collapse of the small hole, and the gas that enters the deep penetration welding hole is too late to escape during the solidification of the weld pool, which eventually affects the stability of the welding process. In order to study the influence of gas protection modes on the laser deep welding process of 15 mm thick 316H stainless steel and reduce the influence of photo plasma, metal vapor, plume, etc., on the welding process, this paper carries out a study on the influence law of different types of gas protection modes on the laser deep penetration welding process. This paper provides a theoretical basis for the application of the laser deep penetration welding technology in 316H stainless steel plates.

First, the gas protection device is specially designed, and 5 gas channels are designed, namely molten pool protection gas (MP), transverse blowing protection gas 1 (TB1), transverse blowing protection gas 2 (TB2), side blowing protection gas (SB), and tail dragging protection gas (ST) (Fig. 2). Second, seven different types of gas protection modes are established. Finally, the key process parameters such as laser power are kept unchanged, and only the gas mode is changed. The welding test is carried out successively from the gas protection mode A to G. Meanwhile, the images of plume and metal vapor in the welding process under different gas protection modes are observed with the help of visual high-speed photography (Fig. 6). Furthermore, the cross-sections of 7 beads are compared and analyzed, and the cross-sectional morphologies of beads are indirectly used to characterize the suppression effect of different gas protection modes on the plume glow during welding.

The addition of SB has little effect on weld penetration depth and width. The cross-sectional weld morphology of bead 7 is regular, the weld penetration depth reaches 10.1 mm, and the weld width reaches 2.5 mm (Fig. 4). However, from the observation of high-speed photography, it is found that the addition of SB has a blowing effect on the narrow and long plume similar to the laser beam focusing shape, and can reduce the spatter during the welding process (Fig. 6). Among them, the bead 6 has the worst shape (compared with the other 6 wbeads), especially irregular shape appears at the upper end of the weld, and it is tilted toward one side near the upper end of the weld. Combined with the gas mode F corresponding to bead 6, the interference of TB1 (Ar) and TB2 (Air) in this gas mode leads to drastic changes in the transient plume flow state, resulting in turbulence in the air flow near the weld pool. This results in poor deformation of the surface weld (Fig. 4). Under the G type gas protection mode, key welding process parameters such as laser power, welding speed, and defocusing amount are optimized again, achieving double-side formation from single-pass welding of 316H stainless steel (Fig. 8). The microstructures of the base metal, weld metal, and heat-affected zone are all austenitic metal. The lowest impact energy of a single sample is 198 J, which is far greater than the minimum 90 J requirement. There is a shear lip on the impact fracture of the weld, and obvious plastic deformation occurs.

The addition of SB has almost no effect on the cross-sectional morphology of the bead, and the weld penetration depth and width are basically the same. However, from the observation of high-speed photography, it is found that the addition of SB has a blowing effect on the narrow plume similar to the laser beam focusing shape, and can reduce the splash during the welding process. Finally, it is determined that the gas protection mode of type G is the optimal gas configuration in the process of laser deep penetration welding. Under the G type gas protection mode, the key welding process parameters such as laser power, welding speed, and defocusing amount are further optimized to achieve double-side formation from single-pass welding of 316H stainless steel. The weld forming is excellent, and the relevant indicators meet the technical requirements. The investigation here provides a theoretical basis and a practical experience for the application and development of laser deep penetration welding technology in 316H stainless steel medium plates.