In metal-welding, there are two types of welding: deep penetration mode (keyhole-type) and shallow penetration mode (heat conduction-type). In this paper, we propose a technique to detect the transition between these two types in CO2 laser welding by monitoring optical and acoustic emissions from a laser-irradiated point. Stainless steel, mild steel and an aluminium alloy were used for the work piece. Fast Fourier transform analysis was performed to obtain the frequency spectra of the optical and acoustic emissions. With deep penetration mode welding the acoustic emissions around 1-3 kHz appeared strongly. When the welding changed to shallow penetration mode, the main frequency components appeared in a higher frequency range. The main frequency components of the emission shifted from a lower frequency range to a higher frequency range according to the decrease in penetration depth. This phenomenon was observed for all three materials examined. Representative bands of both higher and lower frequency ranges were searched. As a result, the ratio of the emission intensity measured in the range of 12-15 kHz to that measured in the range of 0-3 kHz showed a good correlation with the aspect ratio. We could evaluate the transition between the two types of welding by taking the emission intensity ratio. [References: 12]
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