Effects of the shielding gas and laser wavelength in laser welding magnesium alloy sheets

摘要

To clarify differences in welding characteristics depending on any difference in the laser wavelength during laser welding of magnesium wrought alloys, welding phenomena found during CO_2 laser and Nd:YAG laser welding of AZ31 wrought alloy are compared, and the welding mechanisms are examined. The effects of the type of shielding gas and whether or not any shielding is performed are compared on the surface and back of welds by bead-on-plate welding of AZ31 wrought alloy with a continuous wave CO_2 laser and continuous wave Nd:YAG laser. The tensile strength and microstructures of laser-welded butt joints, the selection of suitable welding conditions, and corresponding mechanisms are also investigated. The results obtained may be summarised as follows:1 During CO_2 laser welding with helium centre shielding and argon back shielding, sound weld beads are obtained. By argon centre shielding without back shielding, however, the bead shape deteriorates. The results obtained during analysis of high-speed video camera images show this to be an effect due to the shielding gas and laser-induced metal plasma plume. Under the effect of the plasma reaction force on the bead back, grooves in the bead are covered over.2 On the other hand, during Nd:YAG laser welding, centre shielding with argon is sufficient to produce sound beads, which are also formed even without back shielding. This is attributable to the fact that Nd:YAG laser welding is less likely to lead to the shielding gas and metal being converted into plasma than CO_2 laser welding. The alloy Mg component vaporised by welding accordingly spreads around the weld bead, is itself oxidised, and prevents oxidation of welds.3 The results obtained during the EPMA analysis of microparticles adhering to the bead back suggest that Mg is itself oxidised by reacting with ambient oxygen during vaporisation. Under the conditions investigated in this study, Nd:YAG laser welding also ensures a sufficiently high tensile strength. This is achieved in Nd:YAG laser welding through the Mg metal vapour generated during the welding process providing adequate protective shielding against ambient air in such a way as to prevent bead oxidation.