Alloying element vaporization during laser spot welding of stainless steel

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Abstract
Alloying element loss from the weld pool during laser spot welding of stainless steel was investigated experimentally and theoretically. The experimental work involved determination of work-piece weight loss and metal vapour composition for various welding conditions. The transient temperature and velocity fields in the weld pool were numerically simulated. The vaporization rates of the alloying elements were modelled using the computed temperature profiles. The fusion zone geometry could be predicted from the transient heat transfer and fluid flow model for various welding conditions. The laser power and the pulse duration were the most important variables in determining the transient temperature profiles. The velocity of the liquid metal in the weld pool increased with time during heating and convection played an increasingly important role in the heat transfer. The peak temperature and velocity increased significantly with laser power density and pulse duration. At very high power densities, the computed temperatures were higher than the boiling point of 304 stainless steel. As a result, evaporation of alloying elements was caused by both the total pressure and the concentration gradients. The calculations showed that the vaporization occurred mainly from a small region under the laser beam where the temperatures were very high. The computed vapour loss was found to be lower than the measured mass loss because of the ejection of tiny metal droplets owing to the recoil force exerted by the metal vapours. The ejection of metal droplets has been predicted by computations and verified by experiments.