Fluid Flow Analysis And Mechanism Research On Heat And Mass Transfer During Deep-penetration Laser Welding

Laser welding has been widely used in the fields of automotive,aerospace,electronic packaging and shipbuilding,because of its advantages such as accurate energy control,narrow heat affected zone and low residual stress.Laser welding shows fast speed,which greatly improves production efficiency.The laser beam energy can be controlled precisely,thereby effectively reducing the formation of intermetallic compounds in laser welding of dissimilar metals.Laser welding is a complicated physical and chemical process.The quality of weld is determined by fluid flow,heat transfer,mass transfer between metals and gas protection during welding process.However,the microscopic mechanism on the laser welding is still not full studied.Therefore,the thermo-dynamic behaviors of keyhole and molten pool,and the heat and mass transfer mechanism in dissimilar metal welding are studied in this thesis.The mass distribution of vapor plasma ejected from keyhole,shielding gas and air in the upper three-dimensional space of the workpiece is analyzed.Relevant welding experiments are performed to verify the accuracy of the develpoped model.A three-dimensional transient multi-phase flow model for continuous deep-penetration laser spot welding of the similar metal is established,using an adaptive heat source with variable keyhole shape.Rayleigh scattering of laser is considered in the heat source model,and the conversation of mass transfer caused by evaporation,momentum loss due to solidification are also considered in the model.Recoil pressure,surface tension,buoyancy force and hydrostatic pressure are taken into account for the dynamic analysis of keyhole and molten pool.Thermal parameters such as density,dynamic viscosity,thermal conductivity and specific heat are considered as temperature-dependent.The evolution process of keyhole and molten pool is simulated,and the principle of heat transfer and molten pool flow is studied.Furthermore,the effects of spatter generation,keyhole instability and variation of laser power on the welding process are analyzed in detail.Finally,a spot welding experiment is conducted to verify the correctness of the calculation results.A three-dimensional transient model of fluid flow and heat transfer is established for continuous deep-penetration laser welding.The welding mode includes two types: partial penetration keyhold mode and full penetration keyhold mode.The treating techniques of heat source,evaporation and solidification of the melted metal are the same as that of spot welding.The evolution process of keyhole and molten pool is similar to that of spot welding.The difference arise from fact that the heat source moves forward with the moving keyhole at welding speed,and transfers heat to the workpiece.At the front of the keyhole,the melted metal flows around the edge of the keyhole to the end of the keyhole,and re-solidifies to form a weld.According to welding experiments,the calculated results are in good agreement with the experimental results.A three dimensional transient multiphase flow-mass transport model for continuous deep-penetration laser welding of dissimilar metals is established,and the mass transport follows Ficks law.It is found that the thermal-flow characteristics of the keyhole and molten pool are similar to those of continuous welding in partial penetration mode.The difference is that the fluid flow not only affects the heat transfer,but also affects the mass transfer between dissimilar metals.Convection and diffusion are considered as two basic methods of mass transport between metals.In addition,the keyhole formation in the lower metal region also affects mass transport between metals.The melted metal in the lower region is sequeeded into the upper metal region to resolidify due to recoil pressure,and a large amount of high-concentration metal in lower region may appear in the upper region,which is likely to affect the welding quality.The effects of fluid flow,diffusion and keyhole formation on the thickness of intermetallic reaction layer and diffusion of elements in weld are studied during dissimilar metal welding.However,if the thickness of intermetallic reaction layer is too large,it is possible for new intermetallic compounds to be formed,which makes the welded joint brittle and hard and affects the mechanical properties of welded joints.Furthermore,the effects of laser power and welding speed on the thickness of intermetallic reaction layer and diffusion of elements in weld are also analyzed in depth.Finally,the welded joints are analyzed using optical microscopy(OM),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The results show that the simulation results are in good agreement with the experimental results.The species transport model on shielding gas is established during deep-penetration laser welding process.The turbulence model is used to analyze the fluid flow,and the mass transport follows Ficks law.The species transport process of shield gas,air and metal vapor plasma ejected from the keyhole is calculated,and the mass distribution of shield gas on the surface of workpiece and in the direction of height is obtained.The effects of changes in shielding gas flow,position and inclination of shielding gas pipe on mass distribution are analyzed.The optimized parameters obtained by the simulation are used to perform welding experiments on different materials.The results show that the shielding gas can effectively protect the molten pool and weld.Based on the above research,the basic theoretical framework of fluid flow,heat and mass transfer during deep-penetration laser welding is constructed.Numerical analysis provides theoretical basis for laser welding process,and the research has important guiding significance for engineering practice.