Effects of energy density attenuation on the stability of keyhole and molten pool during deep penetration laser welding process: A combined numerical and experimental study

The energy density attenuation of focused laser beam could induce inconsistent thermodynamic behaviors from near field to far field, which leads to the deterioration of the welding stability. However, the influence of energy density attenuation on welding process remains unclear. In this work, a set of innovative characterizing methods are presented to quantitatively evaluate the features of the front keyhole wall and molten pool dynamics. The energy density attenuation and correlative phenomena are numerically studied using a multiphase model. This model incorporates an improved ray-tracing method which can describe the actual beam profile more accurately. Based on experimental and numerical results, a unique melting behavior, i.e., the alternate melting between keyhole wall and keyhole bottom due to the energy density attenuation, is discovered. This melting behavior is primarily caused by movement of protrusions along the front keyhole wall, which can change the energy absorption efficiency significantly. Besides, it is found that the fluctuation amplitude of molten pool is closely related to this melting behavior. By sustaining protrusion movement process, the melting behavior can be regulated and hence improve the molten pool stability. Finally, an optimization method by adjusting the beam defocus and inclination is proposed and testified to improve the welding stability.