Plasma Characteristics in Underwater Laser Welding via In Situ Observation and Optical Emission Spectroscopy

During underwater laser welding, the coupling among water cooling, gas–liquid interface disturbances, and metal-vapor recoil pressure means that the relationships among plasma evolution, the thermal state of the keyhole, and spatter behavior remain insufficiently understood. In this study, 304NG stainless steel was welded in air and under local dry underwater conditions using a 10 kW continuous-wave fiber laser with a wavelength of 1070 nm to investigate the effects of laser power on the energy state and process stability of underwater welding. The underwater experiments were conducted in deionized water, with the workpiece positioned approximately 100 mm below the free water surface and without additional pressurization. High-speed imaging, infrared thermography, and spectroscopy were employed to characterize the transient evolution of the plasma and spatter, the apparent thermal state near the keyhole opening, and the spectral characteristics of the underwater plasma, respectively, while the plasma excitation temperature was calculated using the Boltzmann multi-line fitting method. The results showed that as the laser power increased from 2000 to 4000 W, the plasma size and temporal persistence increased significantly in both air and underwater environments; the transverse width of the underwater plasma at T0 + 0.8 ms increased from approximately 2.7 to 5.8 mm, while the average maximum apparent temperature near the keyhole opening increased from approximately 2477 to 3071 °C. Meanwhile, the intensities of the characteristic Fe I lines increased overall, and the plasma excitation temperature increased from 5977 to 6510 K, consistent with the expansion of the plasma-emitting region, the enhanced persistence of the high-temperature core, and the increase in the apparent temperature near the keyhole opening. This study aims to provide a new systematic understanding and technical insights into plasma evolution during underwater laser welding.