Study On The Characteristics Of Plasma In Laser Welding Under Vacuum Conditions

Currently, with the rapid developments of high power and high quality laser, laser deep penetration welding is used more and more widely in the aerospace, shipbuilding and automobile manufacturing. But “the shielding effect” of laser induced plasma plume has limited the applications of laser welding in deep penetration. In recent years, Japanese scholars have put forward a new method of laser welding under vacuum conditions, bringing a new direction for laser welding. Laser welding under vacuum conditions is studied in this paper fristly in China,and the characteristics of laser induced plasma plume under vacuum condition s are focused on.At first, the characteristics of laser welding process under vacuum conditions are studied in this paper, and the influence of the ambient pressure, the welding speed and the defocused distance on weld beam formation in partial penetration are analysised. The studies find that the weld penetration increases apparently with a decrease in the ambient pressure and the influence of ambient pressure on penetration is strong when the welding speed is small. Under vacuum conditions, a big negative defocused distance is suitable for a deep penetration and a good weld beam formation, and when the welding speed is small, the negative defocused distance should be enlarged. Under the optimized process parameters, our studies get a parallel welding seam with extremely depth-to-width ratio which is very similar to electron beam welded seam.Secondly, the observations with a CCD camera of laser welding process show that the laser induced plasma is suppressed under vacuum conditions. Using the MATLAB software to process the laser induced plasma images, the studies find the height and brightness gradient of plasma images decrease with the decreasing of ambient pressure, and analysis the plasma attenuation of laser energy in quality under vacuum conditions. Our studies also find that the molten pool length increases with the decreasing ambient pressure. And our studies use an experiment that plasma plume jet was sedimented on a cold glass plate to demonstrate that the increase of plasma plume jet velocity changes the molten pool liquidity of laser welding under vacuum conditions.Thirdly, the spectral characteristics of laser induced plasma are studied in this paper by spectroscopic analyses. The studies find that the electron temperature calculated by Boltzmann plot and the electron density calculat ed by Stark broadening effect decrease with the reduction of ambient pressure. And the laser power dominated by inverse bremsstrahlung absorption also decrease s with the reduction of ambient pressure. Through the above analysis, our studies demonstrate that the energy deposition process in laser welding is changed by ambient pressure. At atmospheric pressure the laser is absorbed by a combination of Fresnel effect and inverse bremsstrahlung plasma process. Under vacuum conditions, the laser light is mainly absorbed by Fresnel reflection.Finally, the plume attenuation of laser radiation during laser welding in quantitativeness is studied in this paper by probe laser measurement. This study finds that the extinction effect and the refraction effect of laser induced plasma plume decrease with the reduction of ambient pressure. This research demonstrates that we can get a good formation of weld, because the energy attenuation of laser radiation during laser welding is suppressed under vacuum conditions.