Study of bulk defects creation generated during the solidification of Nd:YAG laser spot welds. Application to tantalum welding

The significant problems in deep laser spot welding are formation of welding defects, and particularly porosities in the case of tantalum joining. In this study, we investigate and model porosities forming and trapping at the end of a laser pulse, during the solidification stage. According to experimental conditions, keyhole refilling and metal cooling modes may induce bulk defects (porosities). These defects have been studied experimentally by means of microscopy and flash radiography. Two types of porosities were observed: small bubbles and large voids. Small bubbles are round shape porosities which appear mostly within the solidified metal. Large voids have un-definite shapes and are generated in the bottom of the welded zone, as if the molten metal have not had enough time to fill up the keyhole completely before it solidified. We elaborated a numerical model for the comprehension of small bubbles trapping by comparing the calculated spot weld solidification time to the rising time of bubbles in the liquid phase. In this model, cooling is calculated on melt layers that refill the keyhole linearly with time. The results allow to predict the size of bubbles which may be trapped in the welded zone. We also studied the melt flow back into the keyhole (with FLUENT program) in order to explain the formation of the observed large voids. This model uses Navier-Stokes equation and takes into account surface tensions and heat transfer. It is shown that, due to surface tension effects, the upper part of the keyhole is occluded rapidly by the melt. Some vapor is trapped and generate a large void when the metal solidifies. These two models are in good agreement with experimental observations, in particular with X-ray radiographs achieved during the solidification of tantalum spot welds. (author)