Research On Plasma Plume And Molten Pool Behavior In Fiber Laser Welding Under Subatmospheric Pressure

High power laser has a great potential in thick plate welding application owing to its high efficiency and exellent welding quality. With further increasement of the plate thickness, the higher laser power and beam quality are required. Moreover, the defects in deep penetration welding process are also one of the key problems to restrict high quality manufacture. Not only the welding quality but also the penetration can be obviously increased in the laser welding in vacuum. However, the dimensional limitation of vacuum chamber is the main obstacle for the thick plate laser welding of large-scale structure. To conquer such an obstacle, a movable half-sealed mini chamber was proposed in this study and was placed on the molten pool to produce a local subatmospheric pressure for laser welding.The local low vacuum chamber or local subatmospheric chamber was proposed not only to break the size restriction of the full vacuum chamber, but also to enhance the suppressing effect on plasma plume and metallic vapor. For better understanding the characteristics of the laser weld bead formation and its variation rule in laser welding under subatmospheric pressures, it has important therotical significance and practical value to do the research on the behavior of plasma plume, metallic vapor, melt pool and keyhole, as well as the mechanism of their interaction with laser.Firstly, based on an experimental platform for fiber laser welding with a fully sealed subatmospheric pressure chamber, a series of laser welding experiments were implemented. Secondly, the behavior of plasma plume under subatmospheric pressure was observed. A spectrum diagnosis system was used to analyze the changes of plasma plume spectrum. The relationship between plasma plume temperature and ambient pressure was discussed. The extinction of fiber laser radiaton by plasma plume under different subatmospheric pressures was obtained by solving Rayleigh approximation equation. Researches on the behavior of molten pool and keyhole were made under different ambient pressures, as well as the mechanism restraining weld spatter and weld porosity. Finally, based on the researches above, the effect of plasma plume and molten pool behavior on weld formation was studied with a movable local subatmospheric pressure chamber. The results obtained are listed as follows:There is a critical pressure around 20 k Pa for the effect of ambient pressure on welding penetration and weld cross-section characteristics. When the ambient pressure ranges from 101 to 20 k Pa, the welding penetration increases slightly and the weld cross-section tends to be “Y” shape. As the ambient pressure keeps decreasing to below 20 k Pa, the welding penetration increases greatly and faster while the pressure gets lower. Compared with welding in atmosphere pressure, the welding penetration becomes twice deeper under ambient pressure of 3 k Pa and the weld cross-section tends to be “I” shape.A regression model was proposed, which can be decribed as, to illustrate the relation among the welding penetration, the ambient pressure, the laser power and the welding speed. The value of R square is 0.989 by variance analysis. The equation shows that the welding penetration increases with decreasing ambient pressure and welding speed. And it increases with higher laser power. The welding penetration has exponent relation to the ambient pressure when the laser power and welding speed remain unchanged.The restraining mechanism of ambient pressure on plasma plume was revealed, i.e. the diffusive plasma plume at the outer part, the condensed plasma plume in the central part and the residual plasma plume near the keyhole are depressed in the pressure range 101~80 k Pa, 80~20 k Pa, 20~3 k Pa respectively. The extinction of fiber laser radiation is 11% when welding in atmosphere while it goes down to 1% when the ambient pressure decreases to 3 k Pa. The residual plasma plume is the main part to influence the laser beam. The extinction efficiency of laser energy can be calculated withThe behavior of molten pool and keyhole was observed with high-speed camera under different ambient pressures. The swing amplitude of plasma plume decreases with dropping ambient pressure and tends to be stable when the ambient pressure is lower than 10 k Pa. Correspondingly, the surface of molten pool and the behavior of keyhole gets stable. As a result, the weld quality gets better. Under subatmospheric pressure, the weld spatter gets less due to depressed fluctuation on keyhole back wall. The weld porosity is also reduced due to the change of the molten pool flowing behavior and the bubble escape speed under lower ambient pressure, because the bubble escape path gets shorter and its escape position in molten pool is changed.The structure of local subatmospheric pressure chamber was optimized based on the gas hydrodynamics theory. The effect of chamber size and suction location on the pressure distribution and the gas flow state in chamber was analyzed. Compared with the fully sealed chamber, a deeper and sound weld bead can be obtained in a local subatmospheric pressure chamber. The suction flow will reduce the residual plasma plume and increase the laser power density on the plate, as well as stir the molten pool to increase the energy absorption of the keyhole.