Influence of the metal flow in the keyhole molten pool on the molten pool stability in continuous variable polarity plasma arc keyhole vertical-up welding

Abstract

The metal flow of the keyhole molten pool affects weld pool stability and weld formation vitally during continuous variable polarity plasma arc welding (VPPAW). A novel “keyhole inside alternating oscillation” (KIAO) thermal-mechanical coupling model was developed to study the influence of keyhole molten pool metal flow on molten pool stability. This model can achieve the oscillation effect of heat-force on the molten pool during the electrode negative (EN) and electrode positive (EP) phase. A heat source, with keyhole channel backward bending, was used to create a keyhole backward deviation along the workpiece thickness. An asymmetric arc pressure source was adopted to ensure asymmetric distribution of the arc pressure. The metal flow of the stable and cut molten pool are calculated in continuous VPPAW process, and the conditions needed for the stable existence of the molten pool are obtained. The molten pool must have sufficient liquid metal which needs to flow as follows: in the front of the upper molten pool, a part of the metal flows in the form of “two-way rotating confluence” along the keyhole side wall to the rear of the lower molten pool, and part of the metal flows backwards to the rear of the upper molten pool. The metal at the rear of the lower molten pool flows upwards along the keyhole side wall. Under the combined action of these flow behaviors, the expansion speed of the leading edge of the keyhole is approximately equal to the shrinkage speed of the keyhole trailing edge, and the keyhole molten pool is stable. The stability of the bottom molten pool has a significant impact on the overall molten pool stability. The calculated fusion line and the offset distance between the central axis of the keyhole exit and the welding torch corresponded with the measured results.