Physical mechanisms of conduction-to-keyhole transition in laser welding and additive manufacturing processes

Thermo-hydrodynamic phenomena which take place during laser welding or additive manufacturing processes as laser powder bed fusion, have been investigated for years, but recent advances in X-ray images and in situ analysis have highlighted new findings that are still under debate. Conduction-to-keyhole transition, and more broadly, keyhole dynamics, are typical cases, where complex coupling between hydrodynamic and optical problems are involved. In this paper, a keyhole and melt pool model is developed with the software COMSOL Multiphysics (R), where laser energy deposition is computed self-consistently thanks to a ray tracing algorithm. The model successfully reproduces experimental findings published in the literature and helps to analyze accurately the role played by the beam trapping phenomenon during the conduction-to-keyhole transition, in both spot welding (i.e., stationary laser illumination) and welding configurations (i.e., with scanning speed). In particular, it is shown that depending on the welding speed, multiple reflections might be either a stabilizing or a destabilizing factor. Understanding these mechanisms is thus a prerequisite for controlling the stability of the melt pools during the joining or the additive manufacturing processes.