Acoustic emission from modulated laser beam welding of materials

Acoustic emission during the laser welding of metals with a modulated CO2 laser beam has been investigated. Sharp resonances at many frequencies in the acoustic emission spectra have been observed. This was most noticeable when the average laser power was high enough to produce full penetration welds whose acoustic emission at high harmonics with frequencies that overlap with bands of vibrational frequencies corresponding to eigenmodes of the keyhole are greatly enhanced. Lower harmonics were not clearly observed. However, in partial penetration welding, many harmonic components were observed. These results are not affected by a surface coating of the metal as in the galvanized steel sheet sample. This observation indicates that the keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluctuations induced by changes in the interaction of the laser radiation with the walls of the keyhole. When a continuous wave (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.