Modeling of keyhole formation in laser and laser arc hybrid welding based on CFD simulations

Volume-of-Fluid (VOF) method is used to calculate the free surface shape of the keyhole using a unique ray-tracing algorithm which can take into account the effects of multiple reflections, laser heat source shape (Gaussian, etc.), laser beam profile, metal vapor shear stress, metal vapor heat source, sulfur contents, etc. on the formation of keyhole and subsequently their effects on molten pool behavior, fusion zone shape, etc. The model can be used alone for laser welding and it can also be combined with arc heat source models such as gas metal arc welding (GMAW) to simulate the laser-GMA hybrid welding. Salient features of the model like ray tracing, search of free surface cells, multiple reflections and laser material interaction using simplified Fresnel's reflection model by the Hagen-Rubens relation are discussed in little detail. Other than keyhole formation, factors considered in the simulations include buoyancy force, Marangoni force, recoil pressure and especially pore generation is simulated by means of an adiabatic bubble model, which also can lead to the phenomenon of keyhole collapse. The model is applied to laser welding of 6 KW and representative simulation results are presented to show the effects of the factors mentioned above, in the laser welding and their contribution in forming keyhole, bead profile and other important welding variables.