Effect of atmosphere above specimen on welding results during remote welding

Remote welding has already been applied in the automobile industry [S. Ream, Proceedings of the 69th Laser Materials Processing Conference, Tokyo, Japan, 2007, pp. 1–14: M. H. Rhee et al., Proceedings of the 68th Laser Materials Processing Conference, Osaka, Japan, 2007, pp. 141–146), owing to many advantages. The authors revealed that the weld bead changed from full to partial penetration during remote welding of a steel sheet at the focal position without shielding gas (S. Oiwa et al., Proceedings of the Eighth International Welding Symposium, Kyoto, Japan, 2008, p. 36: S. Oiwa et al., Proceedings of LAMP2009, Kobe, Japan, 2009, pp. 9–128]. It was attributed to the phenomena that a laser-induced plume ascended high and an incident laser beam was apparently refracted and defocused by its interaction to the tall plume. It was also considered that the interaction zone between the plume and the laser beam above the specimen was heated spatially and consequently, its refractive index distribution affected the beam shape and focusing conditions. In this study, therefore, the laser-induced plume and low refractive index zones above the specimen during laser welding using focusing optics of long focal distance were simultaneously observed with high-speed video cameras and a Michelson interferometer, respectively. As a result, it was confirmed that the interaction zones with small refractive index values ascended high together with the growth of a plume and exerted a great effect on welding results in the atmosphere without a fan or shielding gas flow. Laser welding using a fan was also confirmed to be beneficial to the production of a stable penetration weld by suppressing the rise of a plume and a zone with low refractive index values. Furthermore, it was found that the refractive index distribution inside the plume and the peripheral atmosphere had such an inclination as to decrease their values toward the beam central axis.