Numerical Simulation Of Weld Pool Thermodynamics Behavior In High-speed TIG-MIG Hybrid Welding

With the development of modern manufacturing,there is an increasing demand for high welding efficiency and low cost without sacrificing weld quality.It is an important way to raise productivity and lower production cost by increasing the welding speed.However,if the welding speed is beyond a certain limit,the undercut defect will occur in metal-inert gas welding(MIG),which will seriously deteriorate the performance of welded joints and becomes the bottleneck of welding speed and productivity improvement.Experiments show that the introduction of TIG arc,can improve the stability of MIG arc and restrain undercut defect in high-speed TIG-MIG hybrid welding.However,the suppression mechanism of undercut defect is still unclear.It is difficult to quantitatively describe the influence of TIG arc on MIG arc,thermodynamics behavior and liquid metal flow state of weld pool only through experiments in high-speed TIG-MIG welding.Therefore,there is a pressing need to establish the mathematical and physical model to quantitatively analyze the thermodynamics distribution,heat and mass transfer in the weld pool based on the interaction between two arcs and weld pool behavior for high-speed TIG-MIG welding.It is of great theoretical and practical significance to reveal the suppression mechanism of undercut defect in depth for optimizing TIG-MIG hybrid welding and achieving low-cost,high-efficiency arc welding.In this thesis,the data of arc shape and droplet transfer is collected through the TIG-MIG hybrid welding experiment.A mathematic model of two arcs deflection is proposed and verified based on the experimental results.Considering the influence of arc deflection and weld pool surface deformation on arc heat flux distribution,an integrated model of arc pressure,arc shear stress,electromagnetic force and droplet transfer in hybrid welding is proposed.A three-dimensional transient computational fluid dynamic(CFD)model is developed using commercial software ANSYS FLUENT to quantitatively analyzing the difference on the effect of heat and forces distribution on weld pool temperature field and flow field between high-speed MIG welding and TIG-MIG hybrid welding.The results show that the momentum of droplets is absorbed effectively by the thicker liquid metal layer(0.8 mm)located in front of weld pool in TIG-MIG hybrid welding compared with MIG welding.In addition,due to preheating of leading TIG and common thermodynamics action of two arcs,the width of the weld pool in the fore part is smaller and the heat distribution on the weld pool surface is more uniform.Besides,backward flow velocity of liquid metal in the weld pool is decreased and the horizontal spreading trend of liquid metal is increased.So the weld toes can be fully filled by the liquid metal,which effectively restrain the formation of undercut defect.The effects of different distance between two welding torches(5-10 mm),TIG current(100 A-180 A)and welding speed(1.0-2.0 m/min)on temperature field and flow field in the weld pool are analyzed,respectively.The results show that with the increasing of torch distance,the accumulation of high-temperature liquid metal under the arc decreases and the velocity of backward flow increases.The undercut defect will be induced since the lack of liquid metal filling at the weld toe.Therefore,the distance between two welding torches should be controlled at 5 mm.With the increasing of TIG current,the backward word flow velocity increases.And after TIG current is 180 A,humping defect appears in weld bead.So the TIG current should not be too large.With the increasing of welding speed,the backward flow velocity and the cooling rate of liquid metal increases,which is easy to induce undercut defect.Backward flow velocity of liquid metal can be reduced by changing TIG current,torch inclination and other parameters,which can improve the formation of weld beam.