Study On Multiphase Coupled Flow Field Behavior During Laser-MIG Hybrid Welding Of Invar Alloy

With the accelerated research and development of domestic large aircraft,hybrid welding,which couples advanced laser beam and traditional arc source into one process,has shown great advantages and became more and more widely used in mold welding field of Invar alloy with large size and thickness.However,hybrid welding involves too many process parameters to be optimized,besides,the dynamic behavior of weld pool and keyhole severely affects the quality of weld seam.Therefore,using the method combined with numerical simulation and experimental,the temperature field distribution,fluid flow and the dynamic evolution of weld pool and keyhole in laser-MIG hybrid welding of Invar alloy are investigated in this paper,which lays foundation for understanding the physical phenomena accompanying hybrid welding and optimizing the process parameters for laser-MIG hybrid welding of Invar alloy.Firstly,a hybrid welding experiment of Invar alloy with three layers and three passes is conducted.Through the comprehensive analysis of the macroscopic morphology,microstructure and mechanical properties of welded joint,the best process parameters of 19.05 mm thickness Invar alloy hybrid welding is obtained.According to these parameters,a single-pass hybrid welding experiment of 7mm thickness Invar alloy is designed,which is used to verify the accuracy of simulation results under the corresponding parameters.Secondly,based on the coupling characteristics of laser,MIG arc and droplet,a rational three-dimensional mathematical and numerical model is established on the basis of mass,momentum and energy conservation equations.The method of attaching UDF source items is used to address the problems such as energy input,thermal and momentum boundary conditions,latent heat absorption and release caused by phase change.Darcy porous model with certain permeability is used to calculate the solid-liquid interface.The VOF equation in multiphase flow model is adopted to track the free interface of molten pool,and the addition of momentum such as recoil pressure of metal vapor is completed at this interface.Thirdly,beam tracing model is used as laser heat input,which takes into account the Fresnel energy absorption of keyhole wall to the light.Double ellipsoid model is used as arc heat input,and double ellipse model is used as arc pressure.Droplet transfer is regarded as high-temperature liquid metal uniformly inflow from a specific area above the workpiece,which falls into molten pool in spherical transitional form.The model used takes full account of the complex phenomena of transient evolution of keyhole,heat convection and conduction in weld pool,melting,solidification and evaporation of workpiece,as well as gravity,surface tension,Marangoni force,electromagnetic force,arc pressure,recoil pressure and the impact of droplet.Finally,the 7mm thickness Invar alloy laser-MIG hybrid welding is simulated.The distribution of temperature field,flow pattern of fluid,dynamic evolution of weld pool and keyhole are obtained.In addition,the factors that affect the stability of keyhole are discussed,and the influence of laser power,welding current and welding speed on formation of keyhole and weld pool are explored.The simulation results are compared and found in good agreement with the experimental results under corresponding parameters,which verify the accuracy of the mathematical model established in this paper and the rationality of laser,arc,droplet heat source model.