Mechanism governing nitrogen absorption by steel weld metal during laser welding

The existence of monatomic nitrogen in the plasma just over the keyhole during CO2 laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO2 laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO2 laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO2 laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO2 laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.