Two-step electromagnetic forming with a low-rigidity die: A novel strategy in reducing high-velocity rebound for flatness parts

Achieving high-precision forming of flatness parts via traditional electromagnetic forming (EMF) technology has long been a persistent technical bottleneck. The key reason for this difficulty lies in the high deformation velocity of sheet materials: after the sheet contacts the die, its deformation kinetic energy cannot be dissipated in a timely manner, readily giving rise to rebound that compromises the die-fitting precision of formed parts. Nevertheless, existing rebound reduction strategies for EMF are generally inefficient and unsustainable. To address these generic challenges, this study first proposes a two-step EMF method with a low-rigidity die. The process initiates with the first-step EMF of the sheet using a flat coil, followed by the second-step EMF utilizing a profiling coil and a low-rigidity die. On this basis, a collision mechanical response model was theoretically established by combining a bilinear elastoplastic model with a single-degree-of-freedom linear vibration system, revealing that the mechanical properties of different die materials lead to varied rebound effects. Subsequently, decoupled numerical simulations were conducted to analyze the deformation behaviors of the sheet and the die under the two-step EMF with a low-rigidity die, and to elucidate the corresponding rebound reduction mechanisms. The reduction in collision velocity, the appropriate distribution of the forming force field, and the bidirectional flow behavior of the sheet material are the core factors enabling the two-step EMF process to suppress rebound. Moreover, compared with the traditional steel die, low-rigidity die mitigates rebound by lowering collision velocity and dissipating more viscous damping energy, and its strain recovery effect renders the proposed strategy sustainable. The simulation results were validated through systematic experiments. The experimental results demonstrate that the proposed method can reduce the rebound height from 9.34 mm in single EMF with a steel die to 0.79 mm. Finally, by increasing the pre-discharge voltage and introducing a cushion block to restrict the rebound deformation space at the dome region of pre-bulged parts, the rebound height was further reduced to 0.31 mm. This work breaks through the limitations of traditional EMF in the processing of flatness parts and achieves high-velocity rebound reduction via two technical advancements: reduced collision velocity and the adoption of energy-absorbing die. It lays a scientific foundation for the development of high-efficiency, low-cost, and sustainable high-precision EMF strategies with industrialization potential.