Oscillations of Keyhole Pressure and Plasma Radiation During cw CO_2 Laser Welding

摘要

The analysis of the fluctuations of pressure and plasma radiation observed during welding with a cw CO_2 laser is presented. Welding was done with a continuous wave CO_2 laser, Photon Sources VFA 2500, operating at the power of 1.75-2 kW. The welded materials were mild and stainless steel sheets 0.8-2 mm thick. Shielding gas was argon or helium. Plasma fluctuations were registered in monochromatic radiation with the use of a monochromator and hotomultiplier while the pressure variations - with a microphone i nthe frequency range of 20-2x1-~4 Hz. The results show that the optical and acoustic signals emitted during the welding process can be used for process monitoring. The most promising method is a frequency analysis based on the Fourier techniques. The Fourier spectra (power spectrum densities - PSD) of pressure waves and plasma emission change with the welding condition and hence contain information about the process of welding. Power spectra of both signals and their coherence are studied in dependence on the welding conditions. Generally two intrinsic frequency peaks are always present but the relative peak amplitude, exact frequency and width depend on welding conditions. In the case of mild steel 0.8 mm thick when helium is used as the shielding gas these peaks correspond to the frequencies of approx 1.3 kHz and approx 3.7 kHz in the PSD of both signals, optical and acoustic. The clear dependence of the measured signals on process parameters shows that they can be used to find the best welding conditions.