Numerical Simulation Of Arc-droplet-pool Coupling Behavior In Magnetically Controlled High Efficiency MIG Welding

MIG(Metal Inert Gas)welding is widely used in welding production,although increasing the current can significantly improve the welding efficiency,the unstable droplet transfer form hinders the application of high current MIG welding in actual production.In order to solve this problem,the controllable electromagnetic force is introduced into the welding process by applying an alternating axial magnetic field of different frequencies.It acts on the arc,droplet,and molten pool,strengthens the arc stiffness,controls droplet transfer,and stirs the liquid metal in the molten pool to achieve the stable welding process and improve welding formation.Due to the limited testing methods,it is very difficult to directly measure the key physical parameters of the arc and droplet.So as to further study the coupling behavior of high current MIG welding,this paper reasonably simplifies the complex welding physical process.The numerical simulation method based on magneto hydrodynamics is used to solve the mass,momentum,energy,and metal vapor transport equations to obtain the temperature field,electric field,flow field,and metal vapor distribution in the entire computational domain.The effect of different frequency axial magnetic fields on the coupling behavior in high current MIG welding is studied,which lays a theoretical foundation for the application of magnetically controlled high efficiency MIG welding.Based on the different welding currents,the droplet transfer behavior of MIG welding is studied by photography system and electrical signal acquisition system,and welding voltage is 40 V,welding wire stick out length is 15 mm.Research shows at 380 A,the droplet transfer form is shot transfer,and the transfer frequency is about 300 Hz.With the current increasing to 400A,the liquid cone formed by melting welding wire becomes unstable and deviates from the wire axis under the action of unbalanced electromagnetic force,the droplet transfer form is unstable rotating jet transfer,the rotation frequency is about 400 Hz.When the current increases to 500 A,the liquid cone is deflected greatly,with a deflection angle of 45°,and the liquid flow beam transitions to the molten pool in the rotating pattern.The arc oscillates due to the movement of the anode spot caused by the liquid cone in the same direction as the electromagnetic force and spot pressure.On the basis of welding current 400 A,the axial magnetic field with different frequencies is applied,it is found that the arc shrinks in different degree no matter how the magnetic field frequency changes.After applying DC magnetic field,the liquid cone rotates in one direction,and the maximum rotation deflection angle increases from 35°to 43°,and it is helically compressed in the axial direction.After 100 Hz alternating magnetic field is applied,the rotation direction of the liquid cone is periodically changed with the frequency of the applied magnetic field,and the rotation angle decreases obviously at the moment when the magnetic field direction changes.After adding 200 Hz alternating magnetic field,liquid cone shows two cases of strengthening rotation and suppressing rotation,the rotation deflection angle decreases significantly at the instant of suppressing rotation.With the addition of 500Hz alternating magnetic field,the effect on the liquid cone decreases significantly.To clarify the influence of metal vapor for the physical parameters,a two-dimensional numerical model considering the arc,droplet and molten pool coupled of MIG welding is established,and the effect of metal vapor for the coupling behavior is investigated.The calculation results show that when the droplets begin to form,the metal vapor mainly collects at the end of the welding wire,and distributes along the arc axis under the action of sinking and convection.The arc temperature is greatly reduced and the low temperature zone appears in the center of the arc.At the same time,the maximum current density increases from 1.6E+7 A/m~2to 2.6E+7 A/m~2in the center of arc,and the current density expands on the surface of molten pool.After considering the metal vapor,the arc heat is no longer concentrated on the center of the molten pool,but it is dispersed on both sides,the peak temperature of the molten pool decreases,the downward flow trend of liquid metal weakens,and the trend of flow to the boundary on both sides is strengthened,and the calculated penetration size is more consistent with the experimental measurement.At the same time,the dynamic coupling process of arc,droplet and molten pool in 400A is analyzed by 3D model.It is found that motion of the liquid cone belongs to self-magnetic rotation,and the unbalance of electromagnetic force is the key factor to promote the high-speed rotation of the liquid cone,while the force driving the high-speed rotation of the arc is provided by the magnetic field of the liquid cone.The impact of the droplet and arc plasma flow force causes the concave deformation in the molten pool surface,which in turn acts on the arc and droplet.Considering the effect of external magnetic field on this model.The results show that the axial magnetic field with different frequencies shrinks the arc and increases the arc temperature in different degrees.The electromagnetic force generated by the external DC axial magnetic field causes the arc plasma to produce a circumferential velocity component,which expands outwards under the action of the centrifugal force,and flows from the anode to cathode in the form of a spiral.The position of peak temperature at the bottom of the arc is extended,which increases the action range of the arc on the surface of the molten pool.At the same time,the circumferential electromagnetic force caused by DC axial magnetic field diffuses the liquid metal around,accelerates the heat dissipation of the molten metal,and makes the molten pool shallower and wider.After adding 100Hz alternating magnetic field,the applied electromagnetic force begins to change direction periodically,and the transient liquid stream transitions along the wire axis in the direction of magnetic field.After adding 200Hz alternating axial magnetic field,the rotating radius of the liquid stream beam decreases greatly,the digging effect of the molten pool is obvious,the molten metal is squeezed more seriously,the penetration depth increases greatly,and the molten width narrows.