Effect Of External Magnetic Field On Microstructure And Properties Of Fine Grain Steel Welds

In order to further meet the needs of modern industry for advanced manufacturing,researchers have researched and developed fine grain steel as a new generation of steel materials by relying on science and technology.Due to its high strength and toughness,fine grain steels are widely used in pressure vessels,automobiles,bridges,ships and other important structural parts.Due to the ultrafine grain structure of fine grain steel,the sharp growth and coarsening of grain in weld zone and heat affected zone after welding leads to the deterioration of mechanical properties of fine grain steel.To solve the above problems,some scholars put forward that arc plasma has good electrical conductivity,which provides the possibility of external magnetic field for it.The external longitudinal magnetic field acts on arc,droplet and pool simultaneously,effectively promoting droplet transition,changing arc shape and stirring pool,and achieving the effect of improving weld formation,inhibiting weld defect,refining grain and changing dendrite growth direction.Therefore,it is of great significance to study the influence of external longitudinal magnetic field on microstructure and mechanical properties of fine grain steel welds under different magnetic field parameters.Through the observation of the high-speed camera acquisition system,it is found that the external longitudinal DC magnetic field makes the CMT arc rotate in a directional way,the top of the arc contracts,the bottom of the arc expands,the arc shape changes from "cone" to "bell shape",and the arc radius increases with the increase of the excitation current.Extra longitudinal alternating magnetic field makes the CMT arc reciprocating and rotating along the axis.Compared with the absence of magnetic field,the arc is obviously compressed,the arc acting radius increases.With the increase of excitation current,the transition frequency of CMT droplet increases firstly and then decreases.When the longitudinal alternating magnetic field is applied,the excitation current is changed when the magnetic field frequency is100 Hz.The droplet transition frequency increases with the increase of excitation current.When the excitation current is kept at 7A,the magnetic field frequency is changed,and the droplet transition frequency decreases with the increase of magnetic field frequency.In the welding process,the fluctuation of current and voltage is basically the same as the droplet transition frequency.The external longitudinal magnetic field can effectively control the frequency of short-circuit transition in MIG welding: when the DC magnetic field is applied,the short-circuit transition frequency increases by 52.9% compared with that without magnetic field when the excitation current is 4A;When an alternating magnetic field is applied,the excitation current is9 A and the magnetic field frequency is 100 Hz,the short-circuit transition frequency increases by 73.5% compared with that in the absence of magnetic field.Under the action of the external longitudinal magnetic field,the necking is divided into upper and lower parts,forming two circumferential electromagnetic forces respectively,which are in opposite directions,so that the liquid bridge is twisted off at the necking section.Therefore,the external axial magnetic field can effectively promote the droplet to break at the necking part during the short circuit,thus improving the droplet transition frequency.During MAG welding and CMT welding,with the increase of excitation current and magnetic field frequency,the number and size of proeutectoid ferrite and side lath ferrite in weld microstructure decrease gradually,while the number and size of acicular ferrite increase gradually,and granular bainite with fine size appears.After grain refinement,the grain size becomes smaller and the number increases.The increase of grain boundary proportion hinders the dislocation movement and improves the strength and toughness of the weld.As the magnetic field parameters continue to increase,the microstructure of weld joint appears coarsening.In the study of the influence law of different magnetic field parameters on the mechanical properties of MAG welding joints,when the excitation current is 4A,the impact value of the weld reaches a larger value of 50.6J,which is 14.2% higher than that without magnetic field.When the longitudinal alternating magnetic field is applied and the magnetic field parameter is 100 Hz and 9A,the impact value of the welded joint is 49.9J,which is 12.6% higher than that without magnetic field.With the increase of excitation current and frequency of magnetic field,the mechanical properties of weld microstructure decrease.In CMT welding technology,when the excitation current of external longitudinal DC magnetic field is 3A,the tensile strength of the weld reaches the maximum value of 543.5MPa and the impact value of the weld is 44.8J.When the excitation current of the applied longitudinal alternating magnetic field is 7A and the frequency of the magnetic field is 100 Hz,the tensile strength of the weld reaches a maximum value of 550 MPa,and the impact value of the weld is 46.1J.With the continuous increase of the excitation current and the frequency of the magnetic field,the mechanical properties of the weld joint decrease gradually.The addition of longitudinal alternating magnetic field can effectively refine the grain,disrupt the dendrite growth direction and increase the grain orientation distribution types.In the absence of magnetic field,the grain size is mainly distributed in the range of 0?m to 3?m,with a small amount of large grains over 6?m,and the average effective grain diameter is 1.69?m.When the longitudinal alternating magnetic field is applied at 100 Hz and 7A,the grains with grain size less than 1?m increase obviously,and the grains with grain size larger than 6?m disappear almost,and the average effective grain diameter is 1.37?m.Compared with no magnetic field,the average effective grain diameter decreased by 18.9%.In summary,the effects of external longitudinal DC magnetic field and alternating magnetic field on droplet transition,arc morphology,microstructure and mechanical properties of welded joints during MAG and CMT welding were systematically studied.The effects of external longitudinal magnetic field on droplet transition,arc shape,microstructure and mechanical properties of welded joints were systematically studied.