Increase in Weld Penetration Depth by Two Nd: YAG Laser Beams Combined

Abstract

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.