Multi-physics Field Coupled Simulation And Experimental Study For Electromagnetic Impulse Forming

In this paper, a new method named electromagnetic incremental forming (EMIF) has been proposed based on the single point incremental sheet forming and electromagnetic forming (EMF). The principle of the new method can be described that the working coil is used to replace the rigid tool in the incremental sheet forming. The working coil is moved along a special trajectory and small discharge energy to cause workpiece local deformation in a high speed by magnet force. Finally, all local deformations accumulate into large parts. In addition, EMF is a complex Multi-physics coupling forming process. Numerical simulation offers an opportunity to overcome the problem. However, modeling approaches found for EMF process in the literature are mainly restricted to2D model. For real industrial applications, the modeling of3D forming operation becomes crucial for an effective process design. Thus, the whole paper can be divided into the following sections:In order to propose a new strong coupling method which can be used for complex3D models, three-dimensional finite element models are established and sequential coupling method are used to analyze electromagnetic sheet free bulging. The Arbitrary Lagrangian-Eulerian (ALE) algorithm or the Morphing technologies are used in air meshes to make them change regularly with the deformation of the workpiece and avoid the occurrence of distorted meshes. The dynamic and distortionless hexahedral meshes can ensure the accuracy of date transfers between the magnetic field module and structure field module. Therefore, the computational accuracy and computational efficiency have been improved. The comparison between the simulation results and the experimental ones indicates the deformation laws in simulation results agree with the experimental one. For the complex coil structure, the remeshing technology is used to make the air meshes change with the workpiece deformation. Then, the3D finite element model is established to analyze the distribution of the magnetic forces and work principle of the uniform pressure electromagnetic actuator. The magnetic forces on the sheet are input into the software, ANSYS/MECHANICAL, which is used to analyze the deformation law of the sheet. And, the reason for coil failure has been proposed based on the force on coil.The loose coupling method and sequence coupling method are both used to investigate how the second half current wave affects the simulation result. It is found that the effect of the second current pulse on sheet deformation increases with the increasing of the discharge frequency. The sequential coupling method may or may not obtain higher simulation accuracy than the loose coupling method if the first current pulse just be considered. However, the sequential coupling method always can obtain accurate simulation results whatever in a high or low current frequency if the second current are also considered. In addition, the sequential coupling method is used to analyze the effect of the ratio of coil to die dimension and current frequency on sheet forming. It exist two regions with great thickness thinning in the final sheet shapes. If the ratio of coil and die dimention is close to1, the sheet could obtain the maximum value thickness thinning. And there exists optimum frequency which corresponding to the maximum displacement in sheet forming. However, the effect factors, such as sheet thickness and current damping exponent, play a very great role in choosing the optimum current frequency. Thus, it is not reasonable to choose the optimum frequency according to the basic experience that the skin depth should be equal to or slight below the wall thickness.Based on the experiment and the2D and3D finite element models, it is deeply analyzed the forming process of electromagnetic incremental sheet and tube forming. To produce large sheet parts using EMIF, it is found that:1) the forming quality of sheet side surface is better in a high discharge voltage than that in the low discharge voltage. However, a high air pressure is created to hinder sheet deformation during the forming process, which will cause the appearance of concave on the sheet if the discharge voltage exceeds a critical value (U=1700V);2) Two consecutive discharges in a fixed position are needed to produce large parts using EMIF technology;3) There exists optimum number of discharge regions (N=4), which corresponds to the best overlap ratio in two adjacent discharge regions and best forming quality. For the deformation of large-straight tube, the technology liked "birth-death element" is used to indirectly describe the movement of the coil. The simulaiton results are better agreement with the the experimental values of single, twice and triple discharging. Then the effect factors named overlap ratio of discharge regions, discharge voltage, forming sequence and the dimensional of die structure on tube homogeneous deformation are analyzed. The results demonstrate that this new technology is feasible to produce large sheet and tube parts.