Elucidation of arc coupling mechanism in plasma-MIG hybrid welding process through spectroscopic measurement of 3D distributions of plasma temperature and iron vapor concentration

This study aims to clarify the mechanism of arc coupling phenomenon in plasma-MIG hybrid welding through the tomographically spectroscopic measurement of 3D distributions of plasma temperature and iron vapor concentration. The arc coupling strongly depends on arc current waveforms, so both the cases using DC MIG current and pulse MIG current were performed for the comparison. As a result, in the case using DC MIG current, two arcs were regularly connected, because the high temperature region between two arcs with an enough electrical conductivity was widely formed, retaining the current conduction between arcs. In the case using pulse MIG current, the two arcs were strongly connected only in the upslope duration of pulse MIG current. Both arcs were weakly connected or disconnected in other durations. The arc connection is thought to be governed by the balance between the stiffness and deflection of both arcs. In the case using pulse MIG current, two arcs were not connected in the downslope duration of pulse MIG current, although the pulse MIG current was similar with that in the upslope duration. In the upslope duration, the MIG arc is thought to be constricted because of the thermal pinch effect to increase the current density, which enhances the stiffness of MIG arc to prompt the arc connection. In contrast, the pulse MIG current density becomes lower in the downslope duration, which deflects the MIG arc backward to disconnect the arcs. The plasma current increased by 35 A during the arc connection, which increased the plasma arc temperature below the nozzle by 1000 K. The result implies that the arc coupling affects the distributions of the arc pressure and shear force by plasma arc acting on the keyhole inner wall, governing the keyhole stability as well as the weld pool formation process.