A unified model for coupling mesoscopic dynamics of keyhole, metal vapor, arc plasma, and weld pool in laser-arc hybrid welding

Although building a model of laser-arc hybrid welding has attracted substantial attention in recent decades, many problems remain. In particular, existing models failed to reproduce the highly transient (down to several tens of nanoseconds) physical interactions between solid, liquid, vapor plume and plasma occurring in the welding process. These interactions control the key physical processes of laser-arc hybrid welding and play dominant roles in process defect formation and final joint quality. Development of a unified model for laser-arc hybrid welding capable of modeling the multiphase physical interactions is faced with three major challenges. The first is how to decouple the highly correlated and mesoscopic interactions between the solid, liquid, vapor plume and plasma phases. The second and third are respectively the numerical treatments of the nonlinear discontinuous sheath layer between the arc plasma and the weld pool and the Knudsen layer between the vapor plume and the weld pool. In this study, we have developed a three-dimensional unified mathematical model that couples the mesoscopic dynamics of the arc plasma, keyhole, metal vapor, and weld pool in the laser-TIG hybrid welding process. To decouple the highly correlated gas, liquid, solid and plasma interactions, we proposed a novel approach in which the electromagnetic field in the workpiece (including solid and liquid phases) and outside the workpiece (gaseous phase and plasma) were continuously modeled globally, but in which the heat transfer and fluid flow are modeled separately within and outside of the workpiece and then coupled through boundary conditions. To treat the nonlinear interface between the arc plasma and weld pool outside the keyhole as well as between the vapor plume/plasma and the weld pool inside the keyhole, we proposed a novel ghost-fluid-based multiple timescale stepping algorithm. The results indicate that our model can be used to predict the time-dependent mesoscopic dynamics of vapor plume, plasma, keyhole and weld pool in a coupled manner. Good agreement was obtained between the numerical predictions and experimental results and literature data.