Effects of different gas environments on Co2 and Nd:YAG laser welding process efficiencies

When laser welding mild steel under high power densities, vaporised material is ejected from the keyhole and forms a plume/plasma above the weld pool. In previous studies on plume formation and extent of ionisation, the influence of the laser wavelength and the gas environment has been observed. In this study a comparison between CO2 and Nd:YAG laser welding has been performed using the same energy density (similar to 1.24MW/cm(2), produced using 3.5kW of power and a focal spot size of 0.6mm) in He, Ar and N-2 gas environments and in vacuum. Plume/plasma evolution was recorded with high-speed video at 9000 frames/second and these images have been correlated with the characteristics of the weld cross-section. The fusion and heat-affected zone profiles have been measured to analyse the melting efficiency at different processing speeds. The temperatures and electron densities in the plume/plasma have also been calculated by spectroscopic methods to estimate the losses caused by the plume/plasma development. By analysing the differences in the weld shape profiles and the plume/plasma behaviour, the temporal evolution of the laser welding process efficiency was also obtained.