Design And Implementation Of Tube Bulging By Anattractive Electromagnetic Force

Electromagnetic forming can effectively increase the ductility of the material and improve the forming limit of the material due to the extremely high deformation rate.It is the key technology to solve the forming problem of light alloy such as aluminum alloy and magnesium alloy at room temperature.It is of great significance for accelerating the lightweight process,reducing greenhouse gas emissions,saving energy and protecting the environment.Electromagnetic forming has the advantages of simple tooling and high forming efficiency,but it is difficult to meet the forming requirements of complex parts.The main reason is that the traditional electromagnetic forming exerts the electromagnetic force on the workpiece just in a single way which is always the repulsive force to drive the workpiece to be formed away from the forming coil.This not only severely limits the utilization efficiency of the forming coil,but also greatly limits the tooling form of the coil and die.Especially for the forming of the small tubes,electromagnetic forming is difficult to meet its requirements.For this reason,the small size tubes are selected as the research objects in this paper,then,an electromagnetic bulging method of the small tubes based on the attractive electromagnetic force has been proposed,and the effective control mechanism of the attracted electromagnetic force and the feasibility of forming workpiece have been studied and verified.Firstly,the coupling relationship between coil current,axial magnetic field,workpiece inductive induced eddy current and its electromagnetic force in the bulging process is analyzed in detail using full coupling finite element simulation model based on the traditional repulsive electromagnetic bulging of tubes.It is found that even in the traditional repulsive electromagnetic bulging,an extremely transient attractive process also exists.It is only limited by single discharge current mode of the traditional electromagnetic bulging so that this process is extremely short and the resulting attractive amplitude is too small to drive the material to deform.Further analysis found that the electromagnetic force on the tube is regulated by both the coil current magnitude and its rate of change.When the change rate of coil current is greater than zero,the electromagnetic force between coil and tube is exclusive,conversely,the electromagnetic force between the two is attracted.Therefore,the idea of generating sufficient attractive electromagnetic force by optimizing current waveform is put forward and its implementation is theoretically analyzed in this paper.That is,two sets of power supply systems with different pulse widths(wide and narrow)are inversely superimposed to get the pulse currents with slow rising edge and steep falling edge in the forming coil so that the tube is subjected to a large attractive electromagnetic force.Then,the simulation model of tube bulging under attractive electromagnetic force based on full coupling model is established,and the parameter design of wide pulse width and narrow pulse width is analyzed in detail.The results show that for such an annealed 1060 aluminum tube with a 28 mm inner diameter and a 1 mm thickness tube,the pulse width time of wide pulse power supply should be greater than 1.2 ms in order to generate enough attractive electromagnetic force and inhibit the subsequent repulsive force process.At the same time,under the premise of matching with the wide pulse width power supply,the discharge voltage of the narrow pulse width power supply can be appropriately increased.In addition,it is also necessary to minimize the inductance parameters in the discharge circuit.Thus,an experimental scheme of electromagnetic expansion of tube based on attractive electromagnetic force is developed,the high strength forming coil is designed and wounded,and the matching experimental tooling is designed and processed.Rely on Wuhan National High Magnetic Field Center of Huazhong University of Science and Technology,the corresponding experimental research was carried out.In the experiment,two sets of power supply systems with maximum discharge energy of 1 MJ and 50 kJ are used.For an annealed 1060 aluminum tube with a 28 mm inner diameter and a 1 mm thickness,when the discharge voltage of wide and narrow pulse power supply is respectively 6 kV and 9.5 kV,the tube has a visible attractive bulging effect,and the expansion amount is 0.2 mm.When the discharge voltage respectively goes up to 10 kV and 11 kV,the expansion of the tube reaches 2.55 mm,and the strain in the center area of the tube has already exceeded 0.15.Simultaneously,the transient process of workpiece deformation in the experiment get a detailed analysis combined with the full coupling simulation model mentioned above.The results show that the experimental results are in line with theoretical expectations.The above work fully verifies the feasibility of the tube bulging with attractive electromagnetic,shows the potential of the electromagnetic forming technology,and explores a new way for the further application of the electromagnetic forming technology on the small tube.