Development of an all-position YAG laser butt welding process with addition of filler wire

摘要

This article initially describes observations with a highspeed camera of the plasma plume and droplet behaviour during YAG laser irradiation in various welding positions on filler wire supplied in air. The characteristics of all-position YAG laser butt welding with a gap with addition of filler wire to the workpiece surface are then examined. Based on these results, a technology for all-position YAG laser butt welding of fixed pipelines with a gap with addition of filler wire to the workpiece surface is elaborated. The results obtained may be summarised as follows: When the YAG laser is irradiated perpendicularly to filler wire supplied in air, the plasma plume is generated at an angle of around 40 deg to the filler wire. The droplets, being susceptible to the evaporation recoil force of the plasma plume, are scattered in the opposite direction to the plasma plume. When the YAG laser is irradiated as a rectangularly modulated beam, the plasma plume is initially generated in a perpendicular direction to the filler wire, be coming inclined after several msec and being extinguished simultaneously as YAG laser emission pauses. This phenomenon is repeated in synchrony with the rectangular modulation frequency. Although the droplet scattering trajectory is susceptible to the effect of gravity, the relationship between the plasma plume generation direction and droplet scattering onset direction relative to the filler wire is insensitive to the effect of the laser irradiation position. During YAG laser butt welding with addition of filler wire, the excess laser beam remaining after filler wire melting and evaporation is irradiated on the base metal to generate a plasma plume emanating from the filler wire and a plasma plume emanating from the keyhole. The filler wire supplied to the workpiece surface is reduced to droplet form by the evaporation recoil force and fed inside the gap, being thereafter continuously stirred by the molten metal flow in the depth direction along the keyhole and transported to the weld back and being virtually uniformly distributed inside the molten metal. The rectangularly modulated beam with a higher peak power than the continuous beam, through having a greater driving force, is better able to transport the molten metal and thus provides good scope for welding of thicker plates even in the overhead position. Filler wire can be effectively supplied to the pipeline surface during full girth YAG laser butt welding of fixed pipelines with a gap width of 1 mm.