Sound is generated during laser spot welding; this is useful for monitoring the welding operation. In the present experiments, the sound is detected by a microphone for inspecting the generation of porosity defect in YAG laser spot welding. Laser beam conditions are the energy of 2.0 J and pulse duration of 5 ms; the welding is done on plates of stainless steel of 1.0 mm in thickness by single pulse. The Short Time Fourier Transform (STFT) analysis is applied for obtaining the time dependence of the generated sound. Though great difference cannot be observed in the sound signals detected by the microphone, the clear difference can be observed by the STFT. The intensity of 11-15 kHz is lower in the welding with defect than in without defect. This is also confirmed in the welding by laser beams of 2 J, 4 ms, 2 J, 3 ms and 1.8 J, 3 ms. The result shows a possibility of detecting the welding defect by the sound analysis in the production systems.

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