| Ferritic stainless steel has broad application prospects because of its low cost and superior performance.However,in the welding process of ferritic stainless steel,it is easy to appear the phenomenon of weld microstructure coarsening,which significantly reduces the strength and toughness of welded joints,and becomes more severe with the increase of plate thickness.Therefore,the welding of medium and thick plates usually adopts the groove-filling austenitic welding wire,that is,the method of compensation alloy is used to improve,such as SMAW,GMAW,etc.,but the disadvantage is low efficiency,many consumables,high cost;With the development of high-efficiency welding technology of medium and thick plate,electron beam welding,laser welding,and other autogenous welding technologies have been applied to solve the problem of welding efficiency of medium and thick plate.Still,the joint performance is often not satisfactory,and the equipment is complex,high cost,difficult to be applied in practice.Based on this,the main research objectives of this thesis are to achieve high efficiency(autogenous,deep melting)and high-quality welding of 6mm thick AISI444 ferritic stainless steel.The following research conducted: The welding power source and magnetic field generation device are developed,and the high-frequency electromagnetic pulse deep penetration TIG welding system is established.The effect of high-frequency sine wave electromagnetic pulse on the microstructure and properties of TIG welded joints was studied by means of material analysis method and performance testing equipment,and the research idea was further optimized.The effects of axial alternating magnetic field on the weld appearance,keyhole effect,microstructure,and mechanical properties of high-frequency pulsed deep penetration TIG were studied,and the mechanism of the magnetic field was analyzed.The result of the magnetic field on the temperature field and flow field of high-frequency electromagnetic pulse deep penetration TIG pool was studied by numerical simulation,and the mechanism of the magnetic field was further demonstrated.The main achievements are as follows:(1)The high-frequency electromagnetic pulse deep penetration TIG welding system was established.The high-frequency sine wave pulse welding power source was developed by the LC resonance principle,and its arc thrust and thermal efficiency were doubled.The high frequency pulse deep penetration TIG welding power source was developed using the finite bipolar soft switching principle.The pulse current frequency could reach 20 k Hz.The IGBT AC field excitation power source is created using single-chip microcomputer control technology.Its technical indexes are as follows: excitation current 0-40 A,excitation frequency 0-2k Hz;COMSOL software was used for magnetic field simulation.Combined with previous experience,the magnetic head was designed and manufactured.(2)Under the same parameters,compared with constant-current TIG,the weld penetration and weld width of ferrite stainless steel high-frequency sinusoidal pulse TIG increased,the arc pressure increased by 37.3%,and the arc thermal efficiency increased by one time.Still,the increase of arc energy led to the arc shape dispersion.Although the thermal efficiency of the high-frequency sine wave pulsed TIG arc is increased,the main effect of the thermal efficiency is to increase the weld width,and how to achieve greater penetration still needs to be further studied.After the magnetic field is applied,the liquid metal is affected by the electromagnetic force to form circulation,which hinders the downward flow of the liquid metal and reduces the penetration of the weld.With the increase of magnetic field intensity,the weld penetration will gradually recover.Reasonable magnetic field parameters will promote the refinement of weld microstructure.The average grain size of the weld zone is the most refined at 3A-60 Hz,which is refined by 17.71% compared with no magnetic field.The joint strength is consistent with its microstructure.When the exciting current is 3A,and the exciting frequency is 60 Hz,the tensile strength is the best,which is 461.7 MPa,reaching 94.8% of the base metal,which is 14.26%higher than that without a magnetic field,and the elongation is 28.71%.The content of dimples in the fracture morphology is the highest.Grain refinement is the main reason for improving weld strength and hardness.It is found that the effect of magnetic field is closely related to magnetic field strength.Reasonable matching of magnetic field intensity and excitation frequency plays an active role in refining weld grain.Therefore,the magnetic field parameters should be changed to excitation frequency and magnetic field strength for research.(3)High-frequency pulse deep penetration TIG welding of 6mm thick ferritic stainless steel test plate to achieve full penetration under the premise of autogenous welding.After applying the a magnetic field,the electromagnetic force promotes the formation of circulation in the middle of the molten pool.With the increase of magnetic field intensity and excitation frequency,the weld appearance changes gradually from "funnel shape" to "fan shape",the penetration depth decreases gradually,and the average grain size in the weld zone decreases first and then increases.At 60Hz-35 m T,the average grain size in the weld zone is the smallest,only 235.1 μm,refined by 37.9% compared with no magnetic field.With the increase of magnetic field intensity,the more intense liquid phase convection,resulting in the smaller critical nucleation radius of the second phase,the gradually uniform distribution of elements,the size of σ phase,Laves phase,and α phase gradually decreased,the number gradually increased,and gradually tended to be dispersed.At 0Hz-0m T,{001}<100> texture exists in the weld zone of the joint,and its maximum pole density is 7.08.At 30Hz-35 m T,the {001}<100>texture of the joint weld zone is weakened,and its density is reduced to 5.83.Under the premise of complete penetration of the weld,the maximum tensile strength is 662.8 MPa when the magnetic field parameter is 30Hz-35 m T,which is 20.55% higher than that of the joint without magnetic field,and the tensile fracture morphology appears large and deep dimpled and micropores.Under the premise of complete penetration of the weld,when the magnetic field parameter is 30Hz-35 m T,the impact absorbed energy in the weld zone of the joint reaches the maximum value of 43.3 J,which is 133% higher than that without a magnetic field,and the impact fracture morphology appears dimpled.(4)After the application of a magnetic field,the liquid phase of ferritic stainless steel high-frequency pulse deep penetration TIG molten pool will be affected by ampere force,thermoelectric force,and lorentz force,which intensifies the liquid phase flow,reducing the temperature gradient of the molten pool,increases the nucleation rate,grain refinement,with the increase of excitation frequency and magnetic field intensity of the effect,first enhanced and then weakened.With the growth of magnetic field intensity,the distribution of elements in the molten pool becomes more uniform,the size of σ phase precipitated at the grain boundary decreases,and the Laves phase and α phase gradually show a dispersion distribution.The application of magnetic field gradually reduces the critical nucleation radius of the second phase.Finally,it results in the decrease of the size and increase of the number of Laves phase and α phase.At the same time,the influence of liquid flow on the mass transfer process,the change of heat flow direction,and the reduction of temperature gradient at the solidification front will promote the diverse orientation of the weld grain.In addition,the influence of the thermoelectric force on the solid and liquid phases also leads to the fracture and dissociation of the ferrite phase columnar crystals,further refining the microstructure.After a magnetic field is applied,the joint stress concentration decreases,the grain refinement,the second phase size decreases,the dispersion distribution,and the grain orientation diversification are the reasons for the joint performance strengthening.(5)Heat and mass transfer of high-frequency pulsed deep penetration TIG molten pool under magnetic field was revealed by numerical simulation.It is found that the maximum temperature in the molten pool decreases from 3441 K to 3120 K after the magnetic field is applied,which reduces the temperature gradient in the molten pool.The flow direction of liquid metal near the keyhole in the molten pool changes from vertically down along the hole to a certain angle with the vertical.The maximum flow velocity increases and the flow becomes more intense.The simulation results agree with the experimental results,which further verify the mechanism analysis under the magnetic field. |
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