Research On Electromagnetic Attractive Forming Of Ferromagnetic Materials

Electromagnetic attractive forming(EMAF)is a new kind of electromagnetic forming method which uses the attraction between the discharge coil and metal blank to complete the deformation.Compared with the traditional electromagnetic repulsive forming,EMAF made electromagnetic bulging of fine tube and repair of dent possible due to different direction of forming force,and expanded the application field of electromagnetic forming technology.There are very fewer reports about EMAF due to technologies confidential.As a new research direction of electromagnetic forming,EMAF of single-frequency discharge of ferromagnetic materials was studied in this thesis systematically by theoretical analysis,numerical simulation and experiments combination method.It could be achieved repulsive and attractive forming in electromagnetic forming of single-frequency,the kind of forming was related to parameters such as discharge frequency and material type.According to Maxwell’s equations,the formula of magnetic force was deduced on the basis of a simplified model of electromagnetic forming of single-frequency discharge system,and the magnetic force of ferromagnetic materials including magnetic volume pressure and magnetic surface pressure was expressed.The electromagnetic field coupling model of ferromagnetic steel sheet system was established by using ANSYS/Multiphysics module.The simulation results showed that the position of the straight line formed by the center point of magnetic lines at different moments was related to the direction of magnetic volume pressure.Magnetic volume pressure and magnetic surface pressure were decaying with time,and were un-uniform along the steel sheet radial direction.The peak of magnetic surface pressure was later than that of magnetic volume pressure,and its distribution was related to the magnetic field and the current,the maximum value lay in the central part of the coil radial direction.The amplitude of the magnetic repulsion of non-ferromagnetic material was much larger than that of the attraction.The period of the induced current and the discharge current in the same direction of ferromagnetic materials was larger than that of non-ferromagnetic materials,making the attraction longer in ferromagnetic materials.The discharge voltage and discharge frequency were proportional to the amplitude of the magnetic force.Magnetic surface pressure was moved toward the center of the sheet and increased by the combination of concentrator and solenoid coil.Based on the finite element analysis results,the effect of process parameters on forming of ferromagnetic steel sheet was analyzed by EMAF.The experimental results showed that the constraint mold with inner diameter less than 50mm could produce attractive forming.The constraint mold with greater than 50mm produced repulsive forming,therefore,a appropriate inner diameter of constraint mold could effectively control the attractive forming.There was a corresponding discharge interval(the discharge voltage of 13-turn coil was 2600～3600V).When over this range,the sheet produced repulsive forming.Discharge frequency was an important factor in EMAF,when the discharge frequency was 3103 Hz,it would not produce attractive forming;when the discharge frequency was 2080Hz,the voltage range of the attractive forming was 2600～3600V,the maximum depth was 1.54mm;When the discharge frequency was 1725Hz,the voltage range was 4200～4500V,the maximum depth was 1.64mm.The use of solenoid coil with concentrator could get a greater depth in the lower discharge energy.The use of multiple discharges was conducive to increasing the attractive effect.It could be achieved to produce dent and repair dent by EMAF,the use of multiple discharges could get a good repair effect,the quality of repair under the combination of concentrator and solenoid coil was better than that of plate coil.