Impact of Welding Current on Weld Formation in Variable Polarity Plasma Arc Welding: A Numerical and Experimental Analysis

The characteristics of a variable polarity plasma arc (VPPA) and the keyhole behavior significantly influence weld formation. This study investigates the impact mechanism of welding current on weld formation by examining both arc thermal-force output and keyhole behavior through a combination of numerical analysis and experimental methods. A three-dimensional transient arc model with alternating loading of electrode negative (EN) and electrode positive (EP) polarity arcs is developed based on magnetohydrodynamics and is enhanced by user-defined scalars (UDS). The analysis of the arc characteristics reveals that the arc in the EN phase exhibits a larger arc penetration force and keyhole digging effect, while a divergence of the arc occurs in the EP phase. The thermal force of the arc exhibits periodic variation with changes in arc polarity. EN and EP arcs associated with "critical current difference" have minimal thermal fluctuations, minimal fluctuations in the keyhole dimensions (the keyhole long-axis size and keyhole area fluctuation ranges are 4.5-5.2 mm and 78-83 mm2, respectively), and the best keyhole stability and weld bead formation. Otherwise, the fluctuation of the keyhole long-axis size and keyhole area can be very large, which may lead to an unstable keyhole molten pool and poor weld formation.