Effects of Pulse Shape Modulation in Nd:YAG Laser Beam Welding on the Weld Pool Flow and Solidification

Laser beam welding is a field of growing importance to the industry. As a result of extensive and continuous development of laser beam technology, pulsed Nd:YAG laser beam sources have been introduced. Pulsed laser welding offers the advantage of very low heat input to the workpiece, resulting in low distortion and the ability to join heat sensitive components. Further improvements to the method consider pulse shape modulation in Nd:YAG laser beam sources resulting in improved weld pool dynamics. Nd:YAG laser beam welding with pulse shape modulation is studied both experimentally and theoretically. Observational results on the interaction between time-dependent heat fluxes and flows in the weld pool as well as on solidification of the molten material are presented. In welds produced with modulated laser beam pulse shape, improved material flow and finer microstructure have been observed. Using an axisymmetric 2D model for heat transfer coupled with surface tension driven flow of molten metals undergoing solid-liquid and evaporation phase transitions in the weld pool, it is shown that the time modulation of the pulse power influences the melting front and flow velocity which together with the predicted undercooling may explain the fine-grain structure of the resolidified welds that have no cracks inside.