Li Qiu

Effects of Current Frequency on Electromagnetic Sheet Metal Forming Process

In the paper, the effects of current frequency on the electromagnetic sheet metal forming process are investigated using an efficient finite element model, which couples analysis of circuit, electromagnetic, and mechanical equations. Based on the initial electrical and structural parameters of the system, the model calculates the pulsed current flowing through the coil, the consequent magnetic force acting on the metal sheet, and finally the generated sheet deformation. The effects of current frequency on the maximum displacement in axial direction of the sheet are analyzed for two sheets by changing the capacitance of capacitor bank, while keeping the stored energy constant. The results show that there exist two optimum frequencies that produce relatively large sheet deformation and the optimum frequencies are related with the thickness of the sheet.

Design and Experiments of a High Field Electromagnetic Forming System

The concept of capacity coefficient is introduced to evaluate the processing capacity of electromagnetic forming (EMF). An EMF system design method, including capacity coefficient design, inductance design and strength design, is developed by a finite element method. The geometry and size of the driving coil are optimized by the capacity coefficient design. The inductance design is aimed at obtaining a reasonable pulse width of EMF. Finally, the strength design of the driving coil is presented with respect to the reinforcement. With this design method, a high strength driving coil is designed and wound. The height, inner radius, and outer radius of the driving coil are 25 mm, 5 mm, and 50 mm respectively. The number of turns is 40, resulting in reasonable pulse width of 380 . Experiments with different driving coils and pulse width were carried out. The results show that the processing capability of EMF is improved due to the high strength driving coil.

The Electromagnetic Flanging of a Large-Scale Sheet Workpiece

In this paper, an electromagnetic forming (EMF) system with an energy of 200 kJ (25 kV, 640 μF) was designed and fabricated to flange a large-scale aluminum alloy sheet with bore, whose outer diameter, bore diameter, and sheet thickness are 640 mm, 180 mm, and 5 mm, respectively. The stress distribution of the midplane of the coil was calculated to check the coil structural strength. And a multiphysics coupled finite element model, which involves the coupling of circuit, electromagnetic field, deformation field, and thermal field, was built to assess the forming capacity of the EMF system. Furthermore, the experimental results of electromagnetic flanging in the case of 155 kJ are presented and compared with the numerical results. Both the simulation forming depth 87 mm and experiment forming depth 90 mm show that the EMF system is effective to form the large-scale sheet workpiece.

The Development of High Performance Pulsed Magnets of the Prototype Facility of WHMFC

Coils for pulsed magnets in the range of 50-75 T are developed with optimized combinations of conductors and reinforcement for the prototype laboratory of the Wuhan High Magnetic Field Center (WHMFC). The basic design tool is a Pulsed Magnet Design Software package (PMDS) for calculating the capacitor discharge, conductor heating, mechanical stresses and the residual stress, including plastic deformation and winding tension. A 1 MJ capacitor bank is equipped with a thyristor switch, a mechanical switch, polarity reversal switches, a current limiting inductor and a diode crowbar. The design and the test results of the magnets, of the protection inductance and of the capacitor bank are presented and discussed in this paper.

Finite Element Analysis for Stress and Magnetic Field of a 40 kA Protection Inductor

The protection inductor serves for limiting the peak current in order to protect the thyristor switch of the pulsed magnetic field facility in case of a short circuit. Because of the high current and strong magnetic field, the Lorentz force in the protection inductor is large. This paper describes the calculation of the inductance and the optimization of the stresses in the protection inductor. A finite element analysis with the ANSYS software is used to calculate the magnetic field and stresses in the protection inductor. A three-dimensional static finite element analysis model of the inductor has been built for calculating the stresses in the copper coils and stainless steel rings as well as the static inductance. Furthermore, a harmonic finite element analysis model has been built to analyse effects such as the influence of eddy currents in the copper wires and induced current in the stainless steel rings on stress and inductance. Eddy currents cause an uneven distribution of stresses; induced current can decrease the stresses in the copper coils but increase those in the stainless steel rings. Both effects reduce the inductance. The typical maximum stresses in our design are 70 MPa in the copper coils and 210 MPa in the stainless steel rings; these are both below the yield strength of these materials. The inductance is 1.05 mH at the frequency of 50 Hz. The protection inductor has been manufactured according to the design and the performance testing has been successfully completed.

25 kV/40 kA Protection Inductor for Capacitor Bank of the Wuhan Pulsed High Magnetic Field Facility

A 25 kV/40 kA protection inductor with low stray field was designed and the first prototype was fabricated. The protection inductor protected the thyristor switch in the capacitor bank, limit the current at 40 kA in case of a short circuit. It was designed as a toroidal system consisting of 12 coils evenly distributed in the perimeter of a circle. The inductance could be changed easily by adjusting the number of coils. The structure of coils was optimized to withstand the great electromagnetic force produced by the 40 kA current. The coils were wound by a 4 45 mm copper strip standing on edge. Each coil was externally reinforced by a stainless steel ring tightly enclosing the coil. The first prototype was fabricated. Its inductance and magnetic field were tested. The prototype has been mounted in the 1 MJ module and is now in operation. In this paper, the structure design, fabrication and testing of a new protection inductor are presented. The testing results are discussed. Tested inductance was 1.05 mH and direct resistance was 16 . The first prototype was proved to be successful and 13 more protection inductors will be made after some improvements.

Numerical Analysis of the Workpiece Velocity in Electromagnetic Forming Process

The effect of the motional electromagnetic force in the electromagnetic forming circuit on the workpiece velocity is analyzed. The differential equations of unconsidering and considering the motional electromagnetic force in the electromagnetic forming circuit are solved numerically. The results without considering the motional electromagnetic force are unavailable because they violate the law of conservation of energy, while the results with considering the motional electromagnetic force can accurately reflect the electromagnetic forming process. Furthermore, it is found that the electrical energy transforms into the kinetic energy due to the motional electromagnetic force.

Electrical and Thermal Modeling of Pulsed Magnets Using Finite Element Analysis

A precise and efficient finite element model has been developed to solve the coupling of the magnetic field and heating in pulsed coils energized by a capacitor bank. The magnetic field and the current density distribution in each conductor layer of the coil are simulated, taking into account the eddy current, magneto-resistance and heat conduction. The temperature distribution in the conductor during the discharge is also calculated. Eddy currents and magneto-resistance result in a large temperature gradient across the layers. The calculated pulsed field waveform is in good agreement with experimental results for a large range of pulsed coils; this provides useful insight for optimized coil design.

Effect of Workpiece Motion on Forming Velocity in Electromagnetic Forming

The effect of workpiece motion on the forming velocity is analysed by the finite element method. To study the two factors of workpiece displacement and motional electromotive force, a static model, an incomplete motional model and a complete motional model are created. The incomplete motional model is simulated by the finite element software COMSOL, while the complete motional model is simulated by another finite element software Flux. To ensure the correctness of the model, the static model is created by both softwares. For the specific system treated in this paper, the results show that when the workpiece velocity is below 100 m/s, the workpiece displacement has only a small effect on the forming velocity. But when the workpiece velocity is above 200 m/s, the effect of the workpiece displacement on the forming velocity must be taken into account in the finite element model of the electromagnetic forming process.

Space-Time-Controlled Multi-Stage Pulsed Magnetic Field Forming

Electromagnetic forming (EMF) is a high strain-rate forming method where a pulsed electromagnetic force is applied to a conductive metallic workpiece. To improve the performance of the EMF system, the current problems which restrict its extensive application have been analyzed. To this end, a space-time-controlled EMF technology with multi-stage and multi-direction coils system has been developed. In our new EMF system, the magnetic field generated by driving coils is much higher than in conventional EMF due to introducing design methods developed for non-destructive pulsed high field magnets. This technology enables the forming of complex, large-scale sheets and tubes that may be difficult to deform by conventional methods, as well as controlling particular properties of the work pieces.