The crucial factor of laser welding is the laser energy conversion. For a better understanding of the process, the interaction process between the laser beam and keyhole wall was investigated by observing the keyhole wall evaporation during high-power fiber laser welding. The results show that the evaporation vapor, induced by the laser beam, discretely distributed on the keyhole wall. A tiny 'hollow' zone was observed at the spot center-action region on the FKW. The evaporation vapor induced by the spot center moved downward along the front keyhole wall (FKW) with a period of about 0.3~0.75 ms, which indicates that the keyhole formation is reminiscent of a periodical laser drilling process on the FKW. The evaporation vapor on the keyhole wall suggest the assumption that the laser energy coupling mode in the keyhole was multiple-reflection, and the keyhole depth was mainly determined by the drilling behavior induced by the first absorption on the FKW.
To clarify the effect of the laser-induced plume on weld penetration during high-power fiber laser welding, this paper investigates the change in weld penetration, plume behavior and temperature, particles present in the plume, and the attenuation of the horizontal probe laser. Results show that the weld penetration had an improvement of about 20% as the plume was blown away by using a supersonic cross jet. The plume temperature at a height of 5 mm was approximately 5078 K. The weld penetration, plume temperature, and plume attenuation to a probe laser decreased as plume height increased. A plume height of approximately 80 mm resulted in a shape similar to that of a focused laser beam; a cone of white smoke composed of numerous particles appeared around the plume. Calculated and measured results indicate that the interaction of laser particles in the plume accounts for the primary impact on laser-plume interaction. It is found that, if the plume height increases, the weld penetration decreases correspondingly due to the increasing attenuation of the high-power fiber laser beam by the particles in the plume.
The laser-induced plume is an essential physical phenomenon during high power fiber laser welding. Its physical nature can be investigated by diagnosing its temperature. In order to overcome the lack of sufficient line spectra in the temperature diagnosis, the continuous spectrum method (based on the continuous spectrum and Wien's displacement law) is thus developed in this research. The optical spectra emitted by the plume or plasma were detected by using a SP-2500i spectrometer during bead-on-plate welding with an IPG YLS-6000 fiber laser or fiber laser-arc hybrid. The rationality of the continuous spectrum method was evaluated by comparing the plasma temperatures diagnosed by applying the continuous spectrum and line spectra. The results reveal that the continuous spectrum method used in temperature diagnosis is rational. When the line spectra cannot be detected due to an increase in detected height, the plume temperature can be still obtained by using the continuous spectrum method.
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