| Gas tungsten arc welding(GTAW)has the advantages of stable welding process,high welding quality,strong adaptability,good operating environment and low welding cost,so it has been widely used in the manufacturing of modern thin-walled structures(such as welded steel pipes).Increasing welding speed is the main way to improve the welding production efficiency.However,when the welding speed and welding current exceed a certain critical value,the weld formation will become worse,and the weld formation defects such as undercut and hump weld bead will appear.Therefore,based on the self-developed external compound magnetic field device,the TIG arc and weld pool behavior were controlled by the external magnetic field,so the weld bead defects in high-speed TIG was suppressed.However,the regulating mechanism of the external composite magnetic field on the arc and weld pool behavior is still unclear,which needs to be further analyzed.In this thesis,a three-dimensional unified model of arc-weld pool was established for fixed-point TIG based on computational fluid dynamics and electromagnetic theory.The "Local thermodynamic equilibrium-diffusion approximation" was used to deal with the non local heat balance region between the arc and the work-piece,the generation and distribution of metal vapor and its influence on the arc characteristics during TIG welding were simulated.Considering the ferromagnetic characteristics of the workpiece material(409L),the distribution of the external magnetic field in the arc and workpiece under different excitation currents was calculated and verified with the experimental results.The integrated numerical model of "arc-weld pool" under the action of external magnetic field was established,and the characteristics containing temperature field,flow field,potential field,current density,self-induced electromagnetic force and external electromagnetic force in TIG welding process with and without magnetic field were analyzed.It was found that the physical fields was no longer distributed symmetrically with respect to the axis of the tungsten electrode,but inclined to one side of the tungsten electrode.The applied electromagnetic force led to the difference of the resultant force of the arc and the molten pool in the front,back,left and right directions.The difference led to the periodic left-right deflection of the arc in the perpendicular direction of the weld direction,and the arc always leaned forward along the forward welding direction.The temperature field of the molten pool under the action of magnetic field is no longer symmetric about the axis of tungsten pole.The liquid metal on the workpiece surface flows to the side of arc deflection under the action of arc shear force and pressure,and then flows back to the center of the weld along the solid-liquid interface at a flow rate of about 0.28m/s.In addition,considering the influence of metal vapor,the generation and distribution of metal vapor and its influence on the arc characteristics in the process of TIG welding were calculated.It was found that the metal vapor presented asymmetric distribution,and the periodic arc swing reduced the surface temperature of the molten pool,the amount of metal vapor produced on the surface of the molten pool and the diffusion of metal vapor into the arc when the external magnetic field was applied.Finally,the magnetic field assisted high-speed TIG welding experiment was carried out using the 4mm thick 409L stainless steel.The shape of the welding arc was measured in the TIG process and the simulation results agree well with the experimental results. |
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