Authors:
Yousuke Kawahito
Keisuke Kinoshita
Naoyuki Matsumoto
Seiji Katayama
Joining and Welding Research Institute (JWRI), Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

This study was performed to obtain a fundamental knowledge of the interaction between a near-infrared laser beam and an induced plume during welding. The plume was characterized by spectroscopy, and the effect and mechanism of the laser-induced plume on the refraction and/or attenuation were investigated by high-speed video observation of the plume and the probe laser, and power meter measurement of the fiber probe laser beam of 1090 nm wavelength which passed horizontally through the plume formed during bead-on-plate welding of an 8-mm-thick type 304 plate with a 1.5 kW yttrium-aluminum-garnet (YAG) laser beam. The plume induced by YAG laser at the focus position grew about 20 mm toward the incident laser beam and was identified to be nonionized metallic vapor of 3600 K in average temperature on the basis of the spectroscopic analyses. The high-speed observation images and the power measuring results revealed that the rapid movement and the low brightness of the probe laser beam, seen after the plume, were caused by refraction due to density difference between the plume and its environment and Rayleigh scattering due to ultrafine particles, respectively. The maximum refraction and attenuation were 1.2 mrad and 3%, which were much lower than the beam divergence and the same levels as power variation in the incident YAG laser beam under argon (Ar) shielding gas, respectively. Moreover, the YAG laser-induced plume hardly affected the reduction in weld penetration under the defocused conditions since the penetration of type 304 weld made without Ar shielding gas was deeper than that produced with Ar shielding gas although the former plume was longer than the latter one. This reason was interpreted in terms of the greater effect of surface tension-driven melt flows. Consequently, the wavelengths of near-infrared laser beams were desirable for laser welding in the case of the laser-induced plume of less than 20 mm height, owing to their weak optical interaction to the plume.