Recently 10 kW fiber lasers with high beam quality have been developed to achieve the power density of about 1 MW/mm2. A plume induced during welding with such an ultra-high power density laser was in a weakly ionized state. The objectives of this research are to obtain a fundamental knowledge of interaction between a laser beam and an induced plume and to evaluate the effect of this ultra-high power density laser on the weld penetration. The observation result of the plume induced during bead-on-plate welding of a 20 mm thick Type 304 plate with 10 kW laser showed the growth of about 12 mm height at the repetition cycle of around 0.5 ms. The interaction was investigated by the power meter measurement or the high-speed behavior observation of a fiber laser probe beam (wavelength : 1.09 μm), which passed vertically through the plume. The attenuation of the probe laser beam was measured to be about 4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering due to ultrafine particles and partly by absorption and reflection due to spatters. The probe beam observed was refracted at 0.6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fiber laser beam. Moreover, a stable laser welding process could be produced at such ultra-high power density that 12 mm deep penetration was obtained even if the laser peak power was decreased 1 ms periodically from 10 kW to 8.5 kW. It was consequently considered that the interaction between a focused 10 kW fiber laser beam and a weakly ionized plume was too small to exert the reduction in weld penetration.
