Although the mechanical cold-expansion process may improve the fatigue life of holes in workpieces to some extent, various inherent limitations of this process restrict further improvement of the fatigue life of holes. To overcome the limitations of the mechanical cold-expansion process, a new electromagnetic cold expansion process with a dual-stage coil system, which expands holes by a radial pulsed electromagnetic force, is presented in this paper. The basic principle and implementation of the process are analyzed in detail. Then a series of experiments for cold expansion of holes in sheets made of 2A12-T4 aluminum alloy, is carried out to validate the proposed method. Results show that it can significantly improve fatigue life and increase the fatigue limit of specimens from 60 MPa to about 127 MPa. Compared with the mechanical cold expansion processes, the fatigue life has doubled in high maximum nominal stress, which is mainly attributed to non-contact processing and more uniform residual stress distribution. Moreover, it can be concluded that the new process can strengthen different types of holes (i.e. special-shaped, small) that are difficult to be processed by conventional cold expansion processes.

Wuhan National High Magnetic Field Center (WHMFC) at Huazhong University of Science and Technology is one of the top-class research centers in the world, which can offer pulsed fields up to 90.6 T with different field waveforms for scientific research and has passed the final evaluation of the Chinese government in 2014. This paper will give a brief introduction of the facility and the development status of pulsed magnetic fields research at WHMFC. In addition, it will describe the application development of pulsed magnetic fields in both scientific and industrial research.

A pulsed electromagnetic blankholder system consisting of a pulsed magnet driven by a capacitor bank and a conductive ring as a blank holder has been proposed, designed, and analyzed for electromagnetic forming. The blank holder force (BHF) supplied by the system is a huge repulsive electromagnetic force generated in the blank holder by the interaction between the pulsed magnetic field and the induced eddy current. The electromagnetic performance of the system has been investigated in detail based on finite element method, and simulation results show that the system can generate a pulsed BHF ranging from 0 to 1000 kN at different discharge voltages with a rise time of more than 5 ms, which fulfills the common requirements for electromagnetic forming process.

In order to overcome the limitations of the existing electromagnetic forming systems for deep drawing, a dual-coil system has been developed for deep drawing of sheet metal with large drawing ratio. In addition to the conventional driving coil that generates the axial Lorentz force on the workpiece, a radial inward force in the flange region is generated by a second coil that is energized by an individual power supply. The axial force pushes the sheet into the die in axial direction, while the radial Lorentz force enhances the material flow of the flange. The effectiveness of the novel system is validated by a series of experiments for deep drawing a cup of AA1060-H24 with drawing ratio of 3.25. It is demonstrated that with the dual-coil system the material flow of the flange can be significantly enhanced, and the maximum forming depth without failure is greatly increased from 8.44 mm to 20.28 mm. The new electromagnetic forming method is expected to break through the bottleneck of deep drawing and enable major advances in forming metallic parts and components that are difficult to form, by developing driving coils with advanced design.

How to reduce the temperature rise of coil winding is an important issue for improving the production and lifetime performance of the tool coil in electromagnetic forming system. To address the problem, this paper proposes a new discharge circuit configuration with a crowbar circuit to control the discharge current flowing in the tool coil and then to reduce the Joule heating generated without affecting the forming efficiency. A detailed numerical analysis of the mechanism of the temperature rise reduction was first carried out, and then numerical simulation and experimental investigation of the effects of the crowbar circuit on the coil current and surface temperature rise were investigated. The simulation results show that the Joule heating in the coil winding can be reduced from 4.62 kJ to 2.07 kJ when a crowbar circuit with a resistor of 0.3 Ω was applied, and the corresponding temperature rise in the coil can also be effectively reduced. Meanwhile, the experimental results show good agreements with the simulation results, further verifying the effectiveness of the proposed method.