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Description
Electromagnetic forming (EMF), a critical application of pulsed magnetic field technology, is extensively utilized in manufacturing lightweight automotive components and high-precision aerospace structures. The process involves rapidly discharging high-energy pulsed currents from a capacitor bank into a coil, generating a transient, intense magnetic field. When a conductive workpiece is placed within this dynamic magnetic field, eddy currents are induced on its surface. The interaction between these eddy currents and the pulsed magnetic field produces electromagnetic forces, causing plastic deformation of the workpiece. Unlike conventional quasi-static forming, EMF operates at high strain rates, offering advantages such as reduced springback and enhanced material formability. However, Unlike other high-strain-rate techniques, such as explosive forming, laser shock forming, and vaporizing foil actuators, EMF introduces multifactorial effects, including thermal, electroplastic, and magnetoplastic phenomena due to the pulsed magnetic field and induced currents. These effects complicate the isolation of mechanical deformation from electromagnetic influences. Consequently, decoupling high-strain-rate deformation from electromagnetic effects is essential to elucidate their distinct impacts on macro- and microstructural material properties, advancing both the efficiency and applicability of EMF.
To achieve this decoupling, this study investigates the electromagnetic expansion of 6061 aluminum alloy rings (inner diameter: 108 mm, outer diameter: 118 mm, height: 8 mm). An electromagnetic expansion platform was established, comprising a discharge coil, coil framework, and end plates. The coil (inner diameter: 72 mm, outer diameter: 96 mm, height: 30 mm) was wound onto the framework and secured with end plates and bolts. A magnetic shielding device was designed to enable shielding-driven expansion. The shielding mechanism leverages the high conductivity of the ring material to induce eddy currents, which generate opposing magnetic fields that attenuate internal magnetic flux penetration. Simultaneously, the aluminum alloy ring expansion is driven by the electromagnetic force generated from the interaction between the induced eddy currents and the pulsed magnetic field in the shielding ring. Key design considerations included the skin effect: shielding ring thickness significantly smaller than the skin depth reduces eddy current efficacy, while excessive thickness diminishes expansion efficiency and increases costs. Through theoretical calculations and COMSOL simulations, an optimized red copper shielding ring (inner diameter: 98 mm, outer diameter: 106 mm, height: 15 mm) paired with an embedded copper plate structure was designed to balance magnetic shielding and forming efficiency. Then, conventional and shielding-driven electromagnetic expansions were experimentally compared. Plastic strain and residual stress elimination rates were analyzed. Microstructural characterization via Electron Backscatter Diffraction (EBSD) evaluated grain size, texture, and microscopic deformation. By comparing the macroscopic residual stress elimination and microstructural changes between conventional electromagnetic expansion and shielding-driven expansion, the study successfully decouples the effects of the magnetic field and its associated phenomena on the macro- and microstructural properties of aluminum alloy rings.
COMSOL simulations and experimental data validated the shielding device’s efficacy, confirming its ability to decouple electromagnetic effects and high-speed deformation. This decoupling provides critical insights into the distinct roles of electromagnetic phenomena in altering material properties, establishing a theoretical foundation for optimizing high-efficiency EMF processes.
