Forced oscillations of the keyhole in penetration laser beam welding

The dynamic behaviour of the keyhole in penetration laser beam welding is essential for the entire welding process, and is closely connected to welding seam defects and splash generation. To improve understanding of the process and to help interpret measured process signals, forced oscillations of the keyhole due to fluctuations of the CW laser power are theoretically studied. It is shown that even very small laser power fluctuations can lead to strong keyhole oscillations dominated by eigenfrequency response. Because the governing differential equation is nonlinear, bifurcation effects can occur which tend to broaden eigenfrequency peaks in the keyhole vibration spectrum. Calculated Fourier spectra are in good agreement with corresponding measurements of the ultraviolet and near infrared light emission during welding with an industrial CW laser.