Plasma plume oscillations during welding of thin metal sheets with a CW CO2 laser

An analysis is presented of the oscillations of keyhole pressure and plasma radiation emitted during welding with a continuous wave (CW) CO2 laser. We lding was done with a CW CO2 laser, Photon Sources VFA 2500, operating at t he power of 1.75 kW. The welded materials were mild and stainless steel she ets, 0.8-2 mm thick. The shielding gas was argon or helium. Oscillations of plasma radiation were registered in monochromatic or broad band radiation with the use of a photomultiplier or photodiode and pressure variations wit h a microphone in the frequency range of 20-2 x 10(4) Hz. It has been found that the optical signal from the plasma plume is closely connected with th e acoustic signal and that the source of the acoustic signal is the pulsati ng movement of the plasma plume. Spectral analysis of the measured oscillat ions shows differences in power spectra depending on the welding conditions . Generally, two intrinsic frequency peaks in the range of 0.5-4 kHz are al ways present but the amplitude, frequency and width of the peaks depend on the material and welding conditions. The results show that the optical and acoustic signals emitted during the welding process can be useful for proce ss monitoring. The behaviour of the observed oscillations is characteristic for deterministic chaos. Considerable regularization of the process was ob served as an effect of modulation of the laser beam. The modulation factor (Pmax - P-min) / P-max was equal to 0.2 and the modulation frequency was 2 kHz. In this case, the intense peak corresponding to the modulation frequen cy was observed in the power spectrum together with smaller peaks correspon ding to the harmonic frequencies.