Experimental And Numerical Research On Influence Of Magnetic Field On Molten Pool Flow And Solidification Structure Of Aluminum Alloy Laser Welding

Aluminum alloys have been widely applied in automobiles,high-speed trains,airplanes,etc.due to their high specific strength and low density.And laser welding has attracted attentions in aluminum alloy welding because of its small heat input,low residual stress,and fast welding speed.However,because of the special physicochemical properties of high reflectivity,large thermal expansion coefficient,low liquid surface tension,and so on,aluminum alloy laser welding is prone to problems such as porosity and poor forming,which reduces the mechanical properties of the joint and then limits its application in the industry.Magnetic field-assisted welding technology has obvious effectiveness in suppressing defects and improving weld morphology and microstructure.It has much potential in laser welding of aluminum alloys.During the magnetic field-assisted aluminum alloy laser welding process,the influence of the magnetic field on the molten pool is extremely complicated.The existing research focuses on the utilization of magnetic fields to suppress defects and improve weld properties.Less research on the influence mechanism of magnetic field on the weld morphology and microstructure is done,and the effect of the magnetic field on the flow and solidification behaviors of the molten pool is not well understood.Therefore,this paper has conducted in-depth experimental and numerical simulation researches on the influence mechanism of magnetic field on the weld profile and microstructure of aluminum alloy laser welding from the perspectives of molten pool flow and solidification behaviors.The main research work is as follows:(1)The magnetic field-assisted laser welding experimental platform has been built,and the influences of the key process parameters(laser power,welding speed,and magnetic flux density)on the cross-sectional shape of the weld,microstructure and mechanical properties of the weld are studied.Studies have shown that the magnetic field can reduce the width and increase the penetration depth and middle width of the weld,thereinto the magnetic field has the most significant influence on the middle width.The magnetic field can significantly change the shape of the columnar crystal structure of the weld and the width of the transition zone.The tensile strength of the joint can be improved obviously with suitable process conditions.(2)A three-dimensional transient molten pool flow model has been established.The model considers Lorentz force generated by liquid metal under the action of a magnetic field and the free interface of gas and liquid.The heat source model with the real-time variation of beam energy distribution with the depth of keyhole is adopted.By considering the thermoelectric current at the dendritic solid-liquid interface,thermoelectric magnetic force and thermoelectric magnetic flow caused by the magnetic field,and coupling thermoelectric magnetic flow and dendritic growth process,a coupled model based on the phase-field method including the thermoelectric current and the magnetic field is proposed.A numerical model for describing the molten pool solidification of aluminum alloy laser welding under the influence of the magnetic field is established.(3)The influence mechanism of magnetic field on the cross-sectional morphology of the weld of aluminum alloy laser welding has been studied based on the perspective of the influence of magnetic field on the flow behavior of the molten pool.The distributions of current density caused by the liquid motion and the Lorentz force in the molten pool under the influence of a magnetic field,and the flow field are simulated.The direction of current density is simultaneously perpendicular to the directions of the magnetic field and the liquid flow,and the Lorentz force is opposite to the direction of liquid flow.As the magnetic field intensity increases,the flow velocity of the molten pool decreases remarkably.On the upper surface of the molten pool,the flow velocity from the center to the edge of the molten pool decreases,which reduces the convective heat transfer from the center to the edge of the molten pool,and the melt width is reduced.At the same time,the upward flow velocity near the wall surface of the keyhole is reduced,so that the convective heat transfer from the lower part to the upper part of the molten pool is reduced,and heat is accumulated at the lower part,so that the penetration depth and the middle width of the weld shape are increased.The simulation results are in good agreement with the experimental results.(4)The influence mechanism of the magnetic field on the solidification structure of aluminum alloy laser welding has been studied based on the perspective of the influence of thermoelectric magnetic flow on the solidification behavior of the molten pool.The distributions of thermoelectric current and thermoelectric magnetic force,and the thermoelectric magnetic flow near the dendritic solid-liquid interface are simulated.It is found that the thermoelectric current density is larger in the vicinity of the solid-liquid interface.Thermoelectric current density decreases sharply with the distance away from the solid-liquid interface increasing.The thermoelectric magnetic force caused by the interaction between the magnetic field and the thermoelectric current is larger near the interface,and it decreases sharply away from the interface.The thermoelectric magnetic force can induce liquid flow in the interdendritic region and the maximum flow velocity reaches a few millimeters per second,which is sufficient to affect the solute distribution,thereby affecting the dendritic morphology.The magnetic field can cause more branches of the columnar crystals near the fusion line and reduce solute segregation.The simulation results are in good agreement with the experimental results.