Numerical simulation of arc, metal transfer and its impingement on weld pool in variable polarity gas metal arc welding

A unified numerical model of the variable polarity gas metal arc welding process is developed based on the solution of magnetohydrodynamic equations within the framework of the phase-field algorithm. The droplet formation, necking, detachment, transfer through the arc plasma, and impingement onto the weld pool are investigated at different electrode positive (EP) and electrode negative (EN) regions of the current pulse regime under the condition of one droplet per pulse. The current density, electromagnetic force, surface tension, transient melt-flow velocity, and Joule heating effects are calculated. The numerical results indicate that the arc plasma generated in the EN current phase has a constricted arc shape at the electrode tip compared with the typical bell shape arc in the EP current phase. At the same welding current during the pulse cycle, the arc plasma temperature in the EN current phase is greater than that in the EP current phase. The model shows that before the droplet detaches, the surface tension forces are dominant, whereas during the EP current phase, the electromagnetic forces strongly intensify along the liquid bridge, thereby causing droplet detachment. In addition, the phenomenon of finger-like shape penetration due to the momentum transferred by a falling droplet to the bottom of the weld pool is simulated. The predicted results well agree with the high-speed images and weld pool parameters, accordingly validating the formulated model.