Characterisation of plasma induced during high power fibre laser welding of stainless steel

The objective of this research is to obtain a fundamental knowledge of generation behaviour and ionised state of a plume or plasma induced during bead on plate welding of a 20 mm thick type 304 stainless steel plate with a 10 kW fibre laser beam of 0.9 MW mm 22 power density, on the basis of 10 000 to 40 000 flames s(-1) high speed video observation and spectroscopic analysis. The high power fibre laser produced a partial penetration weld of 12 mm in depth at 50 mm s(-1) welding speed. According to the high speed observation pictures, the laser induced plume was repeatedly generated from a keyhole at the interval of about 0.5 ms period to reach 12 mm in maximum height. The spectroscopy indicated the line spectra of neutral atoms of alloying elements of type 304 such as iron (Fe), chromium (Cr) and manganese (Mn). However, ionised spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and the ionisation degree of the laser induced plume were calculated to be approximately 6000 K and 0.02 respectively, by the Bolzman plots and Saha's equation. Consequently, the plume induced with the 10 kW fibre laser beam of the ultra high power density was judged to be weakly ionised plasma from these experimental results.