This study was undertaken to obtain a fundamental knowledge of the generation behavior and internal state of a plume induced during bead-on-plate welding of a 20 mm thick Type 304 stainless steel with a 10 kW fiber laser beam of a 130 μm spot diameter, on the basis of 40,000 f/s high-speed video observation and spectroscopic analysis. This 0.9 MW/mm2 ultra-high power density laser produced a 12 mm deep partial-penetration weld with 1.3 mm narrow bead width at 3 m/min welding speed. According to the high-speed observation pictures, laser-induced plumes were repeatedly generated from a keyhole at the interval of about 0.5 ms period to reach 12 mm height at the maximum. The plume was also characterized by bright blue emission. The spectroscopy indicated that the plume was emitted from the line spectra of neutral atoms such as iron (Fe), chromium (Cr) and manganese (Mn), which were alloying elements of Type 304. However, ionized spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and ionization degree of the laser-induced plumes were calculated to be 6,000 K and 0.02, respectively, by the Bolzman plots and Saha's equation. Consequently, the plume induced with a 10 kW fiber laser of extremely high power density was judged to be in a state of weakly ionized plasma.
