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

Abstract

An analysis is presented of the oscillations of keyhole pressureand plasma radiation emitted during welding with a continuous wave (CW)CO2 laser. Welding was done with a CW CO2 laser, Photon Sources VFA2500, operating at the power of 1.75 kW. The welded materials were mildand stainless steel sheets, 0.8-2 mm thick. The shielding gas was argonor helium. Oscillations of plasma radiation were registered inmonochromatic or broad band radiation with the use of a photomultiplier orphotodiode and pressure variations with a microphone in the frequencyrange of 20-2×104 Hz. It has been found that the optical signalfrom the plasma plume is closely connected with the acoustic signal andthat the source of the acoustic signal is the pulsating movement of theplasma plume. Spectral analysis of the measured oscillations showsdifferences in power spectra depending on the welding conditions.Generally, two intrinsic frequency peaks in the range of 0.5-4 kHz arealways present but the amplitude, frequency and width of the peaks dependon the material and welding conditions. The results show that the opticaland acoustic signals emitted during the welding process can be useful forprocess monitoring. The behaviour of the observed oscillations ischaracteristic for deterministic chaos. Considerable regularization of theprocess was observed as an effect of modulation of the laser beam. Themodulation factor (Pmax -Pmin )/Pmax was equal to 0.2 and themodulation frequency was 2 kHz. In this case, the intense peakcorresponding to the modulation frequency was observed in the powerspectrum together with smaller peaks corresponding to the harmonicfrequencies.