Marine maintenance operations, such as repair jobs for offshore oil platforms and pipelines, often require welding in a hyperbaric underwater environment. Laser welding can provide a high welding speed and a narrow heat-affected zone, but the influence of increasing ambient pressure on the technique is not well-studied.
Long et al. performed pulsed laser welding on a commonly used stainless steel SAE 304 under high-pressure. The setup included a glass-metal, half-sandwiched sample enclosed in a vessel where pressure varied from 0.1 to 1.8 MPa. A high-speed camera was used to record morphological changes, while a spectrometer measured the spectral signals of plasmas near the sample surface.
As the pressure in the vessel increased, the researchers noticed a monotonic decrease in weld penetration. At 1.8 MPa, welding depth was only a quarter of that under normal pressure. When comparing helium and argon as shielding gases, the authors found that applying helium with higher ionization energy led to a weaker laser-induced plasma and larger penetration under hyperbaric environment.
Their analysis revealed that, during the initial stage of laser welding, higher pressure significantly reduces the expansion velocity of plasma plumes and the growth of the molten pool depth. After the height of plasma plumes gradually stabilized, their volume, height and brightness in this quasi-steady stage rose with increasing pressure. The hyperbaric environment also increased the shielding effect of plasmas on laser energy, meaning less energy can reach the surface of the sample, decreasing weld penetration.
The authors hope their results can guide efforts to solve this issue. One possibility is to regulate the recoil pressure at the gas-liquid interface by adding appropriate alloying elements to promote the emergence of a keyhole effect.
Source: “The influence of pressures in a hyperbaric environment on the penetration during pulsed laser welding of 304 stainless steels and its influence mechanism,” by Jian Long, Lin-Jie Zhang, Liang-Liang Zhang, Jie Ning, Xian-Qing Yin, Jian-Xun Zhang, and Suck-Joo Na, Journal of Laser Applications (2020). The article can be accessed at http://doi.org/10.2351/7.0000156.
