Observation of hybrid (cw Nd:YAG laser + MIG) welding phenomenon in AA 5083 butt joints with different gap condition

In recent time there has been vigorous development of laser hybrid welding technology for applications in numerous industrial fields. Many of those research achievements have already been reported to relevant technology users. However, there are many issues still requiring further research and development, including more comprehensive understanding of the various welding phenomena in laser hybrid welding. In the present study, therefore, the laser hybrid welding process was examined using high speed video in order to understand the relationships between bead formation and the relative locations of the laser beam, arc and molten wire droplet during aluminium butt joint welds. It is apparent from the results of experiments and observations of high speed images that the heat input delivered to the plate is dependent on the nature of the leading heat source and also the joint condition used in hybrid set-up. In particular the presence of a gap in the joint changes the heat transfer efficiency and electrical circuit conditions. These aspects affect the stability of the welding process; therefore, a joint stabilisation mechanism is needed before laser irradiation if the process instabilities are to be overcome. From this perspective a metal inert gas (MIG) leading hybrid (MIG + laser) process is more suitable for butt joints with gaps larger than the beam diameter, even though the laser leading hybrid (laser + MIG) configuration makes better use of heat input in bead on plate (BOP) welding. Synergistic effects of the two heat sources are maximised when the laser beam is located between the arc centre and the impact point of the molten droplet within the weld puddle. It is understood that the final bead shape in hybrid welding is influenced by the volume of the molten pool before the impingement of the laser beam. That is, a critical depth of molten material needs to form before being irradiated by the laser beam in order to maximise the coupling effects of the respective heat inputs. The bead shape was also found to be dependant on other features of the welding process such as gap condition, leading heat source, motion of molten wire in the gap and preheating effects.