Resonant acoustic emission during laser welding of metals

We observed sharp resonances at certain frequencies in the acoustic and optical emission from metals during laser welding with a modulated CO2 laser beam. Strong emission is observed at high-order harmonics of the laser modulation frequency. Harmonics whose frequencies overlap with bands of vibrational frequencies corresponding to eigenmodes of the keyhole are greatly enhanced, relative to other harmonics whose frequencies lie outside these bands. The keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluctuations induced by changes in the interaction of laser radiation with the walls of the keyhole. When a CW beam is used, random fluctuations with frequencies within these allowed bands are amplified to produce a spectrum consisting of a large number of overlapping but discrete frequency components. When the laser beam is modulated, a forced response is elicited resulting in a much simplified emission spectrum. These results offer some new insights into the physical mechanisms that convert laser radiation to heat in laser welding, drilling and cutting applications.