Optical Interaction between Laser Beam and Induced Plume in the Ultra-High Power Density Fiber Laser Welding of Stainless Stee
2008; Osaka University; Volume: 37; Issue: 2 Linguagem: Inglês
Autores
Yousuke Kawahito, Masami Mizutani, Seiji Katayama,
Tópico(s)Laser Material Processing Techniques
ResumoThe objective of this research is to obtain a fundamental knowledge of the optical interactions in a 10-kW fiber laser beam of 0.9-MW/mm 2 power density and the plume or plasma induced during bead-on-plate welding of 20-mm-thick Type 304 stainless steel plate, on the basis of high-speed video observations, spectroscopic analysis and laser probe measurements. According to the high-speed observation results, the laser-induced plume was repeatedly generated from a keyhole at intervals of about 0.5-ms. The spectroscopy indicated that a bright plume emitted the line spectra of neutral atoms of alloying elements in Type 304 such as iron (Fe), chromium (Cr) and manganese (Mn). The temperature and the ionization degree of the laser-induced plume were calculated to be approximately 6,000 K and 0.02, respectively, by the Bolzman plots and via Saha’s equation. Furthermore, the probe laser passed horizontally through the plume was refracted at 0.6-mrad angle on average, which was much lower than the 90-mrad divergence of the focused fiber laser beam. The attenuation of the probe laser was measured to be about 4 %, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering. Subsequently, a stable laser welding process could be established at such an ultra-high power density that 11.5-mm-deep penetration was obtained even if the laser peak power was modulated 1-ms-periodically from 10 kW to 8.5 kW. It was consequently considered that the optical interaction between the 10-kW fiber laser beam and the weakly-ionized plume was too small to exert a reduction in weld penetration.
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