Numerical Simulation Of Micro-plasma Arc Under Transverse Magnetic Field And Study Of Molten Pool Forming Characteristics

Micro-plasma arc welding is a kind of low current welding technology,which is suitable for the repair and the additive manufacturing of thin-walled parts in the fields of automobile manufacturing,aerospace,etc.However,due to the thin-walled features and the accumulation of reciprocating heat during the manufacturing process,the cladding will appear to "downward collapse" on both sides of workpiece,resulting in the forming accuracy is lowered and the comprehensive performance is deteriorated.In this paper,the method of using external transverse magnetic field to assist the welding process is proposed,and the non-contact electromagnetic force is used to control the heat input and force input of the arc to the molten pool,so as to improve the appearance and the performance of the cladding,and to provide a theoretical basis for the technology of magnetic field assisted arc additive manufacturing.According to the equations of Magnetohydrodynamics,a three-dimensional numerical model of micro-plasma arc was established and solved by multi-physics coupling analysis software COMSOL.The simulation results show that the temperature distribution of the arc center is in shape of "brush" from the tungsten needle to the workpiece as a whole,and the arc temperature distribution under the nozzle is in shape of "bell",and the temperature on the workpiece conforms to the Gaussian distribution;the arc plasma has a large velocity inside the nozzle.After leaving the nozzle,its direction gradually changes from vertical downward in the nozzle to the surrounding direction when reaching the workpiece;the current flows out from the surface of the workpiece and flows into the lower end face of the tungsten electrode through the arc column,and the maximum value is obtained near the lower end face of the tungsten electrode;the flux density distribution of the arc is in shape of "lobes",and the Lorentz force generated by self-induction is inclined downward from all sides to the center of the arc.In order to investigate the influence of the external transverse magnetic field on the micro-plasma arc,the external magnetic field was simplified into the background field to join the model,and the additional electromagnetic force was added to the momentum equation in the form of source term,and the four fields coupling model of electric-magnetic-thermal-flow was established.The simulation results show that the arc plasma inside the nozzle does not change significantly when the steady magnetic field is applied,and the arc plasma under the nozzle is pushed to the x negative direction by the Lorentz force,which causes the arc temperature,plasma speed and current density to be deflected.The molten pool moves with arc deflection,and the heat input and arc pressure of the arc to the workpiece decrease;the arc oscillates back and forth under the action of the electromagnetic force when the alternating magnetic field is applied,but there is a certain "delay" between the arc oscillates and the magnetic field,and the higher the frequency,the higher the "delay".The molten pool does not move with the arc oscillating.Compared with the non-magnetic and applied steady magnetic field,the arc heat input to the molten pool is smaller and more dispersed in one cycle,and the overall value of the arc pressure is reduced.The fusion welding and cladding tests under different transverse magnetic fields were carried out on the thin-walled parts.The arc shape captured during the test is in good agreement with the simulation results.As the strength of the steady magnetic field increases,the fusion penetration and the fusion width decrease,and the residual height increases when the steady magnetic field is applied;as the frequency of the alternating magnetic field increases,the fusion penetration and the residual height increase,and the fusion width decreases when the alternating magnetic field is applied.Through metallographic analysis,it is found that the steady magnetic field has a certain effect on improving the microstructure of the weld,and the alternating magnetic field has better effect on improving the microstructure of the weld.Through metallographic analysis,it is found that the steady magnetic field has a certain effect on improving the microstructure of the weld line,but the alternating magnetic field has better effect for that.The multi-layer cladding test was carried out on thin-walled parts,and it was found that both the applied steady magnetic field and the alternating magnetic field can inhibit the phenomenon that the weld line " downward collapse " on both sides.