MELT POOL DYNAMICS DURING LASER-WELDING

The dynamics of the melt pool and keyhole was investigated during CO2 laser welding using high-speed video photography and the laser reflect ometer technique. A low-power argon laser beam, focused on the weld po ol, provided illumination to obtain a direct image of the weld pool su rface. The near-surface plasma emission background was decreased by us ing a narrow-bandwidth interference filter centred at the argon laser wavelength (514 nm). A variation in the shape of the keyhole opening w ith a characteristic frequency higher than 1 kHz was observed both dur ing spot welding and during welding with a moving beam. For the case o f spot welding with a 20 ms laser pulse, long-wavelength (about 1 mm) oscillations of the weld pool were observed with a frequency during th e laser pulse and the first 5 ms after the laser pulse in the range 20 0-500 Hz. In the time interval starting at 25 ms and ending at approxi mately 40 ms from the beginning of the laser pulse, the long-wave osci llation frequency increased up to 1.3 kHz. The solidification time was determined to be approximately equal to the pulse duration for the sp ot welding. Surface deformation during cooling was also observed. This information is used to develop a model illustrating the dynamics of t he post-pulse weld pool.