The joint quality of deep-penetration laser beam welding is related to the keyhole behaviour, e.g. keyhole-induced porosities. In this paper, a model which considered the existence of three phases, including plasma gas, liquid metal and solid metal, was proposed to describe the keyhole phenomena of laser welding. The forces of interaction of fluid dynamics in the keyhole and molten pool were modelled using the CFD software, and an adaptive heat source model was proposed for the absorption of laser energy. The molten pool and keyhole phenomena of laser beam welding were simulated using the developed model, as well as the formation of keyhole-induced porosities. It was found that the keyhole depth self-fluctuates in continuous laser welding, and the bubbles formed from keyhole collapse and shrinkage are the cause of keyhole-induced porosity.

The deep penetration laser welding process involves complex physical phenomena and widely concerned areas. In this paper, a transient multiphase full coupled model rigorously incorporating the physical process and presenting the effect of oxygen on weld pool is proposed. The phenomena of inverse Bremsstrahlung absorption, scattering and plasma induced refraction are discussed to renew the heat source, which consists of incident laser energy and vapor plume. A series of CO2 and fiber laser welding experiments are executed to validate the model. The scattering phenomenon and the interaction between the scattering behavior and different oxygen environments are discussed. The effects of oxygen on the molten pool and keyhole are investigated with the proposed model. The investigation proves that oxygen presence will improve the laser energy focusing state by attenuating the Mie scattering phenomenon and laser's refraction, and suppress the flow intersection on the rear keyhole wall by changing the direction of Marangoni convection. The weld width will be reduced, weld penetration will be increased, and the porosity in the molten pool will be suppressed eventually.