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 frequen cies in the acoustic emission spectra have been observed. This was mos t noticeable when the average laser power was high enough to produce f ull penetration welds whose acoustic emission at high harmonics with f requencies that overlap with bands of vibrational frequencies correspo nding to eigenmodes of the keyhole are greatly enhanced. Lower harmoni cs were not clearly observed. However, in partial penetration welding, many harmonic components were observed. These results are not affecte d by a surface coating of the metal as in the galvanized steel sheet s ample. This observation indicates that the keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluc tuations induced by changes in the interaction of the laser radiation with the walls of the keyhole. When a continuous wave (CW) beam is use d, random fluctuations with frequencies within these allowed bands are amplified to produce a spectrum consisting of a large number of overl apping but discrete frequency components. When the laser beam is modul ated, a forced response is elicited resulting in a much simplified emi ssion spectrum.