Study On Molten Pool Flow Of Aluminum Alloy T Joints By Laser +GMAW Hybrid Heat Source Welding

As a lightweight material,aluminum alloy has many advantages,such as small density,high specific strength,corrosion resistance and weldability.It has been widely used in automotive,rail transportation,aerospace and other fields.Due to the special thermal and physical properties of aluminum alloys,cracks,pores and undercuts often occur during welding.As a new welding process,laser+GMAW hybrid heat source welding combines the advantages of laser welding and GMAW welding,which has the characteristics of fast welding speed,deep melting,low assembly requirements for welding parts,stable welding process,small deformation and so on.Compared with single laser welding or GMAW welding,it can obviously reduce welding defects and so on.The great potential of high quality and high efficiency welding of alloy.However,the hybrid welding process involves many parameters and is difficult to optimize.Welding defects still occur when the parameters are not matched.In the welding process,the fluid flow in the weld pool is closely related to the weld formation.Therefore,with the help of experimental testing and numerical simulation,the study of the dynamic behavior of aluminum alloy T joint weld pool is of great significance for the comprehensive understanding of the internal mechanism of the aluminum alloy composite welding and the popularization and application of the complex joint.In this paper,X ray nondestructive testing is used to detect porosity in laser+GMAW hybrid welding of aluminum alloy.Based on the principles of heat transfer,optical and hydrodynamics,the interaction mechanism of gas,liquid,solid three phase coupling and droplets,the three interaction mechanism of small holes and molten pool,and the geometrical characteristics of the joint are taken into consideration.A numerical model of fluid flow in the three-dimensional transient molten pool of aluminum alloy hybrid welding is established based on the FLUENT software.A cone heat source is used to describe the heat input of the laser,and the double ellipsoid heat source is used to describe the heat input of the arc.A time function is used to simulate the discontinuous entry of droplets into the molten pool.The influence of the arc pressure,the laser induced reaction force,the droplet impact force,the gravity,the surface tension and the dynamic change of the small hole on the flow of the molten pool is considered during the calculation.The fluid flow and temperature field of molten pool in different joints were studied by using the established model.Based on the experimental detection and numerical calculation results,the paper discusses the mechanism of hybrid welding of aluminum alloy T joint,and draws the following conclusions:In the process of aluminum alloy hybrid heat source surfacing,the laser caused the small hole to be in the unstable state,and constantly closed and open periodically,and each small hole closed will form a closed bubble.The liquid metal flow pattern near the rear wall of keyhole also varies with the depth of the hole,and clockwise eddy tends to occur near the pores in the molten pool.With the increase of welding current,the number of pores in the weld decreases.When the current increases to 200 A,the porosity decreases to 6.6%.However,the basic flow pattern of liquid metal did not change significantly with 160 A.In the process of aluminum alloy T joint hybrid welding,the bubbles in the T joint pool are far more distant than the vertical floating in the plate welded pool,and the particularity of the self gravity action and the position of the T joint pool is not conducive to the floating of the bubbles,and the porosity is high,and most of them occur near the fusion line near the vertical plate.When the welding current is increased from 180 A to 240 A,the volume of the molten pool becomes larger,the temperature in the molten pool increases,the viscosity of the metal drops,the flow speed increases to the maximum peak velocity of 2.5m/s,and the flow pattern tends to be complex.The cooling rate of the weld pool is prolonged,and the porosity of the weld decreases to 4.66%.