Study On Arc Motion And Weld Grain Growth Mechanism Of Medium And Large Diameter Hollow Stud Welding Assisted By Magnetic Field

In view of the problems of pores,local non-fusion,and in fluxing of molten pools into hollow studs during the welding process of medium and large diameter Q235 hollow studs and 30CrNi3MoV steel.In this paper,a constant longitudinal magnetic field and alternating rotating magnetic field are used to assist welding,so that the arc rotates and undergoes a certain deflection at the same time.The deflected arc promotes the expansion of the molten pool and reduces the melted metal entering the hollow stud,that is,the generation of"inner surge".This paper mainly studies the influence of alternating magnetic field on the arc motion and appearance forming of hollow stud welding,and the influence of constant longitudinal magnetic field and alternating magnetic field on the growth and mechanical properties of weld grains.Firstly,the law of arc motion caused by the magnetic field parameters of the alternating rotating magnetic field is studied.The results show that the arc speed of the alternating rotating magnetic field is significantly higher than that of the constant longitudinal magnetic field.Due to the air resistance,the arc rotation frequency always lags behind the magnetic field rotation frequency.When the excitation current is 0.65a,the arc rotation frequency increases with the increase of the magnetic field rotation frequency,and the arc rotation radius gradually decreases.When the frequency is 60Hz,the stud forming is beautiful and there is basically no"inrush",Too fast frequency will lead to too much splash and the forming quality of the joint will be reduced.When the magnetic field frequency is 60Hz,with the continuous increase of the excitation current,the arc rotation radius increases,the expansion of the molten pool reduces the inrush height in the center of the stud,and the excessive excitation current reduces the forming quality of the joint.Secondly,the influence of the excitation current of the constant pair longitudinal magnetic field on the microstructure change and mechanical properties of the joint weld area is studied.Within a certain range,with the gradual increase of the exciting current,the grains at the weld are fined significantly.When the exciting current is further increased,the grains of the weld begin to coarse.Compared with welded joints without magnetic field,the large-angle grain boundary ratio of the joint weld area increases from 24.27%to 34.2%.when the excitation current is 0.45A,the large-angle grain boundary helps to resist crack propagation and improve the toughness of the material.the shear strength of the joint is increased from 218MPa to 323MPa,and the impact toughness is increased from 27.12 J·cm^-2 upgraded to 34.28J·cm^-2.The influence of the magnetic field parameters of the rotating magnetic field on the microstructure change and mechanical properties of the stud welding seam area was further studied.Under the conditions of welding current 800A and welding time 1300ms.With the increasing frequency of magnetic field,the grains gradually refine,and the splash of the melting pool at too fast frequency affects the forming quality more.With the gradual increase of the exciting current,the weld grains show a trend of refinement first and then coarse.Compared with welded joints without magnetic field,the excitation current is 0.65A,magnetic field frequency of 60Hz,the large-angle grain boundary ratio of the weld area of the joint weld area increases from 24.27%to 41.2%at,the shear strength of the joint is increased from 218MPa to 323MPa,and the impact toughness is increased from 27.12 J·cm^-2upgraded to 34.28J·cm^-2.