Study On Plasma Characteristics And Melting Behavior Of Aluminum Alloy Laser-MIG Hybrid Welding With Alternating Magnetic Field

Aluminum alloy welding is widely used in aerospace equipment,urban rail transit,ship superstructure and many other production and our daily life.However,many disadvantages exist in traditional arc welding of aluminum alloys,such as microstructure coarsening,welding deformation,high porosity,strength reduction caused by element burning loss and poor welding adaptability.In recent years,many researches have been devoted to the laser-MIG hybrid welding,which has solved the problems related to aluminum alloy welding to a certain extent.Hybrid welding has many advantages such as less heat input,better welding adaptability and so on.External energy fields were usually applied In order to further improve the positive effect of laser-MIG hybrid welding.In this thesis,alternating magnetic field was applied,the homogenization heat input of arc caused by the magnetic field and magnetic stirring effect were used to further refine the microstructure and reduce the porosity so as to improve the joint quality of laser-MIG composite welding of aluminum alloy.The alternating magnetic field assisted laser-MIG welding platform of aluminum alloy was built,and the magnetic field assisted laser-MIG hybrid welding experiment was carried out.With the increase of magnetic field intensity,small splashes were gradually generated on both sides of the weld with 10 mT and 20 mT magnetic field intensity,and there were traces of molten pool being pulled on the weld surface when the magnetic field intensity was 30 mT,which severely affect the weld forming.In addition,with the increase of magnetic field intensity from 10 mT to 30 mT,the convective heat transfer effect of molten pool gradually increases,and the sectional area of weld increases by 16.12%,23.41% and 28.7%,respectively.The arc has different degree of shrinkage with magnetic field of different intensity.With the increase of magnetic field intensity from 0mT to 30 mT,the proportion of arc plasma area is 9.81%,8.22%,6.72% and 5.67%,respectively.In the center part of the bottom of the arc,the arc temperature without alternating magnetic field is 12812 K.With the addition of the alternating magnetic field,the arc plasma oscillates periodically.The heat loss caused by oscillating arc and the short center stay time decrease the temperature of the high temperature zone in the center to different degrees.When the magnetic field intensity is 10 MT and 20 MT,the center area at the bottom is always covered by arc,and the arc temperature is 11965 K and 12474 K respectively.When the magnetic field intensity is 30 MT,the arc swing is too large,and the heat loss is more serious.The central arc temperature is 10552 K.As for the temperature of both sides of the arc,it mainly depends on two aspects,one is the heat loss caused by the magnetic oscillating arc,the other is the temperature compensation caused by the migration of high temperature zone.As the magnetic field strength increases,the arc's swing increases,which increases the heat loss of the arc.But at the same time,the migration of high temperature zone caused by arc swing can bring more temperature compensation,which makes the arc temperature distribution more uniform,and improves the utilization efficiency of arc energy on the whole.The droplet transition and arc oscillations in alternating magnetic field assisted laser-MIG hybrid welding were studied by high speed camera.The results showed that the time that droplet growth process took under three different alternating magnetic field intensity was 4.40%,7.33% and 8.65% higher than that without alternating magnetic field,respectively,and time the process of droplet necking to falling off took was 13.71%,23.41% and 30.27% less than that without alternating magnetic field.The overall droplet transition frequency was increased by 3.9%,6.9% and 9.72%respectively.With the increase of the magnetic field intensity,the arc has different degrees of contraction,the higher the intensity was applied,the more obvious arc contraction occurred.The arc swing also increases with the increase of magnetic field intensity.The fluidity of molten pool also increases with the increase of magnetic field intensity,reducing the burning loss of Mg element to a certain extent.The microstructure refinement was not obvious with 10 mT magnetic field intensity.When the magnetic field intensity was 20 mT and 30 mT,the microstructure is obviously refined.EBSD analysis showed that the dendrite near the fusion line was broken while alternating magnetic field was applied,which is conducive to the formation of new nucleated particles and thus to refine the microstructure.With the application of alternating magnetic fields of 10 mT,20 mT and 30 mT intensity,the tensile strength of the joint is increased by 3.49%,12.2% and 13.95% respectively due to the microstructure refinement and the inhibition of Mg element burning loss.