Monitoring of cw YAG laser welding using optical and acoustic signals

In this study, we propose a technique to detect the weld morphology between deep penetration mode and shallow penetration mode in CW 4kW YAG laser welding by monitoring an optical and acoustic emissions. Both signals were analyzed by Fast Fourier Transform (FFT) program and their frequency spectra were obtained. Mild steel, stainless steel and aluminum alloy were used for workpiece. When the weld morphology changed from deep penetration mode to shallow penetration mode in mild steel, main frequency components of the acoustic spectrum, which appeared in 1 to 10 kHz, became smaller and a flat spectrum was obtained. The similar change of the main frequency components was observed for FFT spectra of optical emission intensity. This phenomenon was also observed for other two materials examined. We found that the intensity ratio of lower frequency components versus higher frequency components of the spectrum correlated well to the aspect ratio of welding bead. The ratio of the optical emission intensity measured in the range of 23 - 25 kHz to that measured in the range of 2 - 4 kHz showed a good correlation with the aspect ratio, and the ratio of 32 - 35 kHz to 2 - 4 kHz correlated well to the aspect ratio for the acoustic emission. Thus we could evaluate whether the welding was deep penetration mode or shallow penetration mode by analyzing the frequency components of the signals in YAG laser welding.