Spectroscopic studies of plume/plasma in different gas environments

Under high power densities in laser welding, vaporised material is ejected from the keyhole and forms a plume/plasma above the weld pool. From previous studies, differences in the plume formation and extent of ionisation have been observed depending on the type of laser employed. In this study a comparison between CO₂ and Nd:YAG laser welding has been performed using the same energy density (~ 1.24MW/cm², produced using 3.5kW of power and a focal spot size of 0.55mm) under He, Ar and N₂ gas environments and vacuum. Spectroscopy measurements were made to investigate the temporal evolution of the plume temperature as a function of the atmosphere and plume/plasma control gases. The Boltzmann-plot method was used to analyse the temperature in the CO₂ plasma, while the Nd:YAG plume spectrum was fitted to blackbody radiation curves for the temperature calculation. The temperatures of both the CO₂ laser-generated metallic vapour and the Nd:YAG laser-generated plume remain quite stable, despite different gas environments and welding speeds. Plume/plasma evolution has also been recorded with a high-speed camera at 9000 frames/second and these results have been correlated with the characteristics of the weld shape.