Numerical modeling of coupled arc plasma, metal transfer and molten pool evolution for underwater flux-cored arc welding

Based on multiphase flow and volume of fluid (VOF) methods, a 3D transient numerical model coupling arc, droplet, and molten pool for underwater wet self-shielded flux-cored arc welding (FCAW) was firstly developed. Multiple coupled physical fields including the temperature, velocity, and electromagnetic fields were numerically calculated. The simulation results indicate that the arc is always distributed at the bottom of the droplet and subject to it, and the temperature difference of the arc in a metal transfer period may reach up to 10,000 K. Meanwhile, the temperature of the liquid metal varies most significantly before and after the droplet necking, and the flow field of the droplet displays alternating patterns of convection and vortex, with the flow velocity varying from 0.3 to 1.4 m/s. After the droplet transferring into the molten pool, the liquid metal diffuses around firstly, and then flows back to the center due to the electromagnetic force and the central concave deformation. Besides, it is also found that as the welding goes on, the melt width does not increase constantly, which might even shrink in particular stage as a result of the cooling effect of surrounding environment and the metal backflow. In terms of the molten pool deformation, the results show that the latter droplet can bring a larger transient concave deformation than the former, while it does not accumulate to the stable deformation. In order to verify the reliability of the numerical model, underwater welding experiments and high-speed photography were carried out, the simulated arc and droplet are in good agreement with the experimental ones, and the molten pool morphologies also fit well, with a slight error of 0.4 mm in the melt width and 0.3 mm in the melt penetration.