Design of Multi-Layer and Multi-Turn Tooling Coils for Electromagnetic Pulse Forming Applications

Electromagnetic repulsive forming is a high-strain rate process used for shaping components made of sheet metal, tubes, joining, and explosive welding. A current pulse through the tooling coil creates a strong magnetic field that interacts with induced currents in the conductive workpiece, causing significant plastic deformation. Conventionally, single-turn coils are being used to form the workpieces electromagnetically. These coils, however, have very low efficiency due to the high discharge frequency of the current and are not suitable for forming thick workpieces. It is well known that about 95 percent of the workpiece’s induced current flows through a layer of about 2-3 skin layers depth in the material. The single-turn coil also has very low inductance, leading to high peak currents and excessive stress on the coil. It also leads to a very high discharge frequency of the coil current, leading to poor efficiency. Multi-turn coils are, therefore, used to form thick workpieces as they have considerably high inductance. However, if the thickness of the workpiece becomes large, the capacitor’s voltage has to be increased to increase the energy, which leads to high hoop stress in the coil leading to its failure. In this paper, the authors have studied the design of multi-turn and multi-layer coils and their feasibility in forming thick workpieces. The skin depth of the material becomes a critical parameter, and thus its effect on the coil design and deformation efficiency has been considered in the present work.