Research On Porosity Suppression And Weld Pool Behaviors In Magnetic-field-assisted Laser-MIG Hybrid Welding Of Aluminum Alloy

Aluminum alloy,owing to its low density and high strength,is widely used in rail transit,aerospace and ship manufacturing industries.Welding technology,as a key link in the process of industrial manufacturing,is of great significance to the forming and manufacturing of structural parts.The frequent collapse of keyhole and the fast cooling rate of molten pool lead to serious porosity defects in the laser welded joint,which seriously damages the service life of the welded joint.Although the laser-arc hybrid welding technology can suppress the keyhole-induced porosity of aluminum alloy,it is difficult to obtain the non-penetrated welded joints with large deep penetration,high quality and no obvious porosity defects.Thus,the 6082 aluminum alloy lap-butt joint of medium-thickness plate for rail train was used as material,and a novel magnetic-field-assisted laser-MIG hybrid welding method was proposed.To suppress keyhole porosity defects of aluminum alloy laser-arc hybrid welded joint,the Lorentz force induced by the magnetic field was used to control the weld pool dynamic behavior.In this study,the effect of external magnetic field on the appearance of aluminum alloy laser-MIG hybrid welded lap-butt joint was carried out.The results showed that the impact of magnetic field on arc shape and droplet transfer in hybrid welding process reduced fishscale features,and the increase of dark marks on weld surface.In addition,with the increase of magnetic flux density,the weld depth and height increased while the weld width decreased.It was mainly attributed to changes in heat transfer caused by liquid flow in the molten pool.The magnetic field promoted the backward flow of melts in weld pool and the more laser energy absorbed by the keyhole and the droplet heat was transferred toward the top part in the rear region of weld pool.In this case,less heat was transferred to both sides of the molten pool,leading to the increase of the weld height.Moreover,the increase of weld depth was mainly attributed to the increase of keyhole stability beacasue the absorption of laser energy at the keyhole bottom was increased.To point out the influence of magnetic field on molten pool flow and keyhole dynamic behavior in laser-MIG hybrid welding,a heat-flow coupled model on magnetic-field-assisted laser-arc hybrid welding was established.In this model,the impact of magnetic field on the arc heat source distribution and current density was considered.The model was verified to be reliable.The simulation results showed that the Lorentz force produced by the interaction between the magnetic field and the arc current in the weld pool was the main driving force,which drives the liquid flow behavior.In the front and middle regions of the molten pool,the larger backward Lorentz force component promoted the backward flow of the liquid,and inhibits the forward flow of the liquid from the rear region of the molten pool and the generation of vortics.The change of liquid flow behavior in the molten pool,and the decrease of the droplet dropping velocity and the backward of the droplet dropping position reduce deformation of the keyhole rear wall.Additionally,the increasing keyhole width reduced probability of keyhole collapse,leading to improving stability of keyhole.The applied transverse magnetic field could significantly inhibit the keyhole-induced porosity defects of the aluminum alloy laser-MIG hybrid welded lap-butt joint.The porosity rate was decreased from 11.2% to 1.5% when the magnetic flux density was 24 m T.Additionally,the suppression mechanism of porosity defect was mainly attributed to the reducement of bubbles formation and promote of bubbles escape.First,the improvement of keyhole stability reduced the probability of bubble formation caused by keyhole collapse.Second,the liquids flowed backward and upward in the weld pool and vortex was disappearance under the effect of Lorentz force,which was conducive to the escape of bubbles.Therefore,the porosity rate was decreased.The applied magnetic field also had an important effect on the microstructure and mechanical property of laser-MIG hybrid welded lap-butt joint.The Lorentz force induced by external magnetic field changed the liquid flowing behavior at the front edge of the solid-liquid interface,leading to dendrite breakage.This promoted the transformation from columnar crystals to equiaxed crystals in the weld.However,the equiaxed grain size and proportion of large angle grain boundaries in weld center were increased when the magnetic field was employed because the Joule heat generated by the magnetic field provided more energy for grain growth.In addition,compared to that without magnetic field,the average tensile strength and elongation of the welded joint are increased by 13.5% and 33.8%,respectively,under magnetic field of 24 m T.The study supplements the theory of magnetic-field-assisted welding method,and provides more guides on the application of laser-arc hybrid welding technology.