Zhongyu Zhou

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.

Development of Space-Time-Controlled Multi-Stage Pulsed Magnetic Field Forming and Manufacturing Technology at the WHMFC*

In November 2011, the Project of Basic Research of Forming by Space-Time-Controlled Multi-Stage Pulsed Magnetic Field (Stic-Must-PMF) was supported by the National Basic Research Program of China (973 Project, 2011.11-2016.08). It is aimed at achieving breakthroughs in manufacturing technology to solve current problems in forming largescale and complex sheet and tube parts and components, imposed by the limitations of existing equipment and materials forming properties. The objective of our research group focuses on the design principles and structural layout optimization of Stic-Must-PMF facility. And this paper will report the development of Stic-Must-PMF forming and manufacturing technology at the Wuhan National High Magnetic Field Center (WHMFC) including numerical modeling, experimental setup and experimental studies.

Effect of Electromagnetic Ring Expansion on the Mechanical Property of A5083 Aluminum Alloy

Electromagnetic forming (EMF) is a promising metal-forming process for complex geometrical shape and light-weight construction. Ring expansion is one of the basic EMFs wherein the workpiece is under nearly uniform tensile stress. The final strains of the rings at different discharge voltages are measured. It is found that the strain increases with the voltage. The strain can exceed the maximum strain in the ASTM standard tension test when the voltage is higher than 8 kV, and it has the largest improvement of 53% at 9 kV. The Vickers hardness is also measured at each voltage. Also, a rising trend is observed as the voltage increases. The highest hardness is found at 9 kV with a value of 110.1. It has an increase of 46% compared to the ring without expansion. Finally, the microstructure of the expanded ring is observed with the electron backscatter diffraction technique.

Online Feature Parameter Extraction for Pulsed Magnets

For pulsed magnets, knowing the real-time performance of the resistance and inductance is significant for both science and engineering. Investigation on the resistance and inductance performance with experimental results will deepen the understanding of electromagnetic physics in pulsed magnets. More importantly, the resistance and inductance values can be used as the thermal stability or the structure stability criterions for pulsed magnets. In this paper, an online extraction method is presented to obtain the inductance and resistance.

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.

Evaluation Indexes of Reinforcement for Optimizing Pulsed Magnet Design

To evaluate the efficiency of reinforcing layers in pulsed magnets with different reinforcing materials, reinforcement methods, and sizes by the same standard, evaluation indexes of reinforcement, Reinforcement Ratio and (RR) material Utilization Ratio (UR), are introduced. The RR is used to evaluate the reinforcing ability of a reinforcing layer, and the UR is used to indicate the material utilization ratio of a reinforcing layer. The approximate analytical formulas of RR and UR are derived in the approximation of zero axial stress. Both the approximate formulas can reflect the effects of the radius, thickness of reinforcing layer, and the anisotropy of the reinforcing materials. Two examples about optimizing the magnet design using the indexes are discussed. One is about poly-layer reinforcement scheme for pulsed magnets, which can utilize reinforcement material effectively. The indexes are used to determine the thickness of each layer with different reinforcing materials in this scheme. The other is about dividing conductor layer into thinner layers, which is an effective method for reducing the peak stress in the reinforcing layer, the indexes are applied to optimize the number of layer division.

High Temperature Magnetization Measurement System in Pulsed Magnetic Field

As a scientific experiment platform, a novel high temperature magnetization measurement system in pulsed magnetic field has been developed. In this paper, the thermal insulation design, measurement system and magnet are presented. A thermal analysis and a heating test have been carried out in the temperature range of 300 K-1060 K. Using the thermal insulation, the temperature of the pick-up coils can be maintained below 450 K when the sample is heated to 1060 K and the magnet is immersed in 77 K liquid nitrogen. Finally, a magnetization measurement of La2/3Ca1/3MnO3 in a 15 T pulsed magnetic field at the temperature of 300 K, 350 K and 520 K has been carried out. It is evident that the magnetization of the sample significantly decreases with a rise of temperature.