Systematic investigation of deformation behavior of tubes in a three-coil electromagnetic forming process

The multi-coil electromagnetic forming technology developed in recent years has shown unique advantages in improving the forming ability and controllability for both tubes and sheet metals. In this work, a three-coil tube forming process is developed to improve the forming uniformity and achieve controllable forming, and the deformation behavior of the tube and corresponding deformation mechanism are systematically explored by both numerical and experimental methods. A forming process window with the voltages of the subcoils as variables is established to reveal the flexibility of the three-coil system in the shape control of deformed tubes, and to quickly evaluate the forming shape under specific discharge conditions. Meanwhile, it is demonstrated that there exists an optimum discharge voltage of the middle coil to achieve the maximum homogeneous deformation range when the discharge voltage of the upper and lower coils is fixed, while the forming height basically linearly increases with the discharge voltage of the upper and lower coils when the discharge voltage of the middle coil is fixed. Moreover, it is found that the forming uniformity of tubes can be further improved from 23.7 to 34.9 mm by increasing the distance between the upper and lower coils, and the same polarity can generate a 17.5 times stronger Lorentz force in the tube center that is preferable for higher forming efficiency. These results have certain significance for understanding the multi-coil forming process of tubes and expanding the application of electromagnetic tube forming.