Relationship between airborne acoustic and optical emissions during laser welding

This paper describes experimental work directed at understanding the relationship between acoustic and optical emissions from the laser welding process. Laser melds were performed and the emissions were measured with a microphone and photodiode and recorded with a data acquisition system. A time-domain correlation analysis of the acoustic and optical signals revealed them to be highly related at a phase shift corresponding to the delay time for sound to propagate from the weld area to the microphone. A moving average model which predicted the acoustic signal from light signal measurements was fitted to experimental data using least squares regression. The acoustic signal predicted by the model was found to correspond fairly well to the experimental acoustic data. In addition, the least squares model indicated a very simple relationship between the two signals: the sound pressure predictions were proportional to the time derivative of the light signal samples. This relationship was explained by physical analysis of the mechanism for generation of sound and light signals by the plume.