Physical understanding of keyhole and weld pool dynamics in laser welding under different water pressures

Underwater laser welding, an efficient and labor-saving in-situ repair technology, is prospective in the field of underwater environment manufacturing. It simplifies the tedious process of disassembly and transportation when the underwater facilities are usually repaired ashore. However, the keyhole and weld pool dynamics in laser welding under different water pressure are little understood. In this study, we developed a mathematical model of underwater laser welding process with arbitrary ambient pressures for the first time. The underlying physics of the underwater laser welding process was explored, and the keyhole and weld pool dynamics between the underwater environment and the ground were compared. Results showed that a keyhole similar to that in the atmosphere is also formed in the water environment. And the keyhole was oscillated. The surface of the weld pool in contact with water cooled quickly, forming a special morphology of the weld pool. The flow in the weld pool was mainly driven by the recoil pressure. The temperature of the surface of the keyhole increased with the increase of water depth. That is because the laser beam energy density is large enough to heat the local metal to the boiling point which increases with the ambient pressure increase. It means that the weld pool was able to absorb more energy before evaporation. Then, less energy is applied to induce the recoil pressure to deepen the keyhole. As a result, a shallower but more stable keyhole is formed in underwater laser welding process under 5 MPa pressure. This study can provide better understanding of the heat and mass transfer behaviors in underwater laser welding. (C) 2019 Elsevier Ltd. All rights reserved.