Plasma plume oscillations during welding of thin metal sheets with a CW CO₂laser

Abstract

An analysis is presented of the oscillations of keyhole pressure and plasma radiation emitted during welding with a continuous wave (CW) CO2 laser. Welding was done with a CW CO2 laser, Photon Sources VFA 2500, operating at the power of 1.75 kW. The welded materials were mild and stainless steel sheets, 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 with a microphone in the frequency range of 20-2×104 Hz. It has been found that the optical signal from the plasma plume is closely connected with the acoustic signal and that the source of the acoustic signal is the pulsating movement of the plasma plume. Spectral analysis of the measured oscillations shows differences in power spectra depending on the welding conditions. Generally, two intrinsic frequency peaks in the range of 0.5-4 kHz are always 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 process monitoring. The behaviour of the observed oscillations is characteristic for deterministic chaos. Considerable regularization of the process was observed as an effect of modulation of the laser beam. The modulation factor (Pmax -Pmin )/Pmax was equal to 0.2 and the modulation frequency was 2 kHz. In this case, the intense peak corresponding to the modulation frequency was observed in the power spectrum together with smaller peaks corresponding to the harmonic frequencies.

Properties of CO2 laser-welded butt joints of dissimilar magnesium alloys

“…3). The value of frequency of oscillations (less than 100 Hz) suggests that observed oscillations are not associated with oscillations of the keyhole (usually of order of some kHz as it has been shown by Szymanski et al, 2001).…”

Time-Resolved Absorptance and Melt Pool Dynamics during Intense Laser Irradiation of a Metal

“…These oscillations have been shown to correlate to weld penetration depth, and could therefore be used for in situ weld monitoring of weld quality 41,42,44 . Significant computational efforts have gone into calculating the oscillatory mode eigenfrequencies of the keyhole in radial, axial, and azimuthal directions 32,39,40,46 , which result from instabilities around a balance of pressures that wish to push the keyhole open (ablation and gas flow) and those that wish to close it (surface tension, hydrostatic pressure, and fluid flow). These calculations generally agree with observations of oscillations below 10 kHz.…”

A Method for Detecting Defects in Laser Weldings for the Automotive Industry

“…To keep the keyhole open, the metal vapor pressure inside the keyhole must be slightly hight than the ambient pressure. The excess pressure is necessary to balance the pressure due to surface tension which is given by p = γ/r, where γ is the surface tension coefficient and r is the radius of keyhole, see (Szymanski et al, 2001). This pressure is balanced mainly by the pressure driven by the flow of the metal vapor through the keyhole and partly by the ablation pressure.…”