Effect of current waveform in MIG arc on weld bead formation in plasma-MIG hybrid welding

The version of record of this article, first published in International Journal of Advanced Manufacturing Technology, is available online at Publisher’s website: https://doi.org/10.1007/s00170-024-13684-8.Plasma-metal inert gas (MIG) hybrid welding enables to join thick steel plates in single pass. However, arc coupling occurring between the plasma and MIG arcs disturbs its heat source characteristics, lowering the welding quality. This arc coupling phenomenon is not yet understood due to the complexity. This study aims to clarify the effect of current waveform of arc on weld bead formation according to the arc coupling in plasma-MIG hybrid welding. The metal transfer characteristics and bottom side weld pool were observed for direct current (DC) and pulse-MIG current waveforms. In addition, Ni element was used for visualizing the transport process of high-temperature molten metal provided by MIG welding within the weld pool. From these results, the effects of differences in MIG arc current waveforms on heat and mass transport processes within the weld pool and also on weld bead formation on the bottom side through changes in the occurrence of arc coupling were discussed. As a result, it was clarified that the droplets after detachment from the wire were transferred to the weld pool surface under the wire tip for DC MIG current, while those were transferred along the wire axis to the weld pool surface behind the keyhole for pulse-MIG current. When the droplet was transferred to the weld pool region with the forward flow such as the pulse-MIG current case, the heat was transported to the bottom side together with the counter-clockwise eddy behind the keyhole, strongly contributing to increasing the penetration depth. In the case of pulse-MIG current, the plasma arc is oscillated due to the arc coupling. According to this oscillation, the accumulation of molten metal behind the keyhole is prevented to suppress the humping bead formation. Consequently, pulse-MIG current was found to be suitable for increasing the penetration depth and suppressing humping bead formation on the bottom side comparing with DC MIG current

Effect of oxygen in shielding gas on weldability in plasma-GMA hybrid welding process of high-tensile strength steel

The version of record of this article, first published in International Journal of Advanced Manufacturing Technology, is available online at Publisher’s website: https://doi.org/10.1007/s00170-024-14100-x.This study aims to clarify the effect of oxygen in shielding gas on weldability in the plasma-GMA (Gas Metal Arc) hybrid welding process of high-tensile strength steel plates. The difference in keyhole profile and bead formation, when the GMA shielding gas was pure Ar, Ar + 2% O₂ or Ar + 20% CO₂, was investigated for plate thicknesses of 6 and 9 mm for the first time. It was found that the weld beads were in good condition for 6 mm thickness plates for all shielding gases, which implied that the window of welding conditions for this thickness is wide. In contrast, for 9 mm thickness plates, a fully penetrated weld bead was achieved only in Ar + 20% CO₂, and weld bead penetration in Ar + 20% CO₂ is higher than in pure Ar and Ar + 2% O₂ in the same welding condition. Due to decreased surface tension caused by sufficiently increased oxygen absorbed into the weld pool, the keyhole diameter increased to penetrate the bottom side of the plate, and the depressing weld pool surface under GMA allowed the heat input from the GMA to be directly applied to a deeper position. Consequently, the plasma-GMA hybrid welding process with Ar + 20% CO₂ achieved a complete penetration for a plate of 9 mm thickness, owing to the effects of both phenomena. It proved a potential to increase penetrability in welding thicker plates by controlling oxygen content in shielding gas of GMA adequately