A3D Numerical Study On Dynamics Of Compressible Vapour/Plasma In Keyhole During Deep Penetration Laser Welding

The physics of deep penetration laser welding process are very complicated, andcurrently the dynamics of compressible vapour/plasma in keyhole during the deeppenetration laser welding are still unclear. Supported by the Natural Science Foundation ofChina and so on, this thesis theoretically investigates the dynamics of compressiblevapour/plasma in keyhole during the deep penetration laser welding by numericalsimulation. The major results are listed below:(1) Coupling considers the major important physical factors, such as heat transfer andfluid flow of weld pool, the thermodynamic effects such as evaporation, melting andsolidification, and the compressibility of the vapour/plasma in the keyhole, athree-dimensional mathematical model is developed to simulate the dynamics ofcompressible vapour/plasma in keyhole during the deep penetration laser welding.(2) A code is developed to solve the three-dimension compressible Euler equationwith independent intellectual property rights. The software can attain some physical statesof the compressible gas at all speeds, including density, pressure, velocity and internalenergy and so on, and the result can reach second order accuracy. Meanwhile, someclassical1D,2D,3D cases including high speed, low speed and complex boundarycondition cases are used to comprehensively validate the code.(3) The dynamics of compressible vapour/plasma in keyhole during the304stainless steel deep penetration laser welding is numerically simulated using the modeland software. The evolutions of three-dimensional distribution of density, pressure,velocity and mach number field changing with time are attained. A304stainless steel fiberlaser welding process with laser power of2kW, welding speed of3m/min, and spot radiusof0.25mm is simulated and the dynamics of vapour/plasma in keyhole are deeply studied.The results show that, in the current welding process:1) the vapour/plasma in keyhole isvery unstable, and the distributions of temperature, density, pressure, velocity and machnumber field are far from uniform, and vary greatly with time;2) the density ofvapour/plasma in keyhole is about0.1~0.3kg/m3;3) the vapour/plasma at the bottom andcenter part of the keyhole may run downward because of the high pressure zone produced at the center part of the keyhole;4) the mach number at the opening and the bottom of thekeyhole is higher;5) the max velocity of the vapour/plasma in the keyhole can reachseveral hundred meter per second, and the oscillation frequency of the max velocity isabout3kHz~6kHz, which is basely consistent with the experiment.