Electromagnetic forming is a high-energy rate processing technology that uses pulsed electromagnetic force to achieve rapid plastic forming of lightweight alloys, which can significantly increase the forming limit of materials and improve forming performance. Suction electromagnetic forming is one of the unique applications, using two different pulse width currents to drive the same coil to generate attractive electromagnetic forces, suitable for plate dent repair, micro pipe forming and other fields. The single-coil structure requires very strict parameter matching of the two currents and the energy utilization efficiency is extremely low.
In order to solve this problem, Researchers Xiong Qi, Yang Meng, Zhou Lijun, Song Xianqi and Li Zhe of the School of Electrical and New Energy of China Three Gorges University and the Hubei Provincial Transmission Line Engineering Technology Research Center of China Three Gorges University wrote in the 10th issue of Transactions of Electrical Engineering in 2021, proposing a two-coil structure suction electromagnetic forming scheme in which two currents each drive a coil, and conduct simulation verification.
However, during the study, it was found that the double coil structure brought a new eddy current density competition phenomenon, which will cause forming defects. In order to eliminate this defect, the researchers further studied the causes and mechanisms of eddy current competition, and finally eliminated the forming defect in the AA1060-H28 aluminum alloy plate double coil suction electromagnetic forming scheme with a diameter of 200mm by adjusting the structural parameters of the coil. The relevant research results can deepen the understanding of the electromagnetic forming process of attracting plates, which is of great significance for expanding the application of electromagnetic forming technology in plate processing.
Electromagnetic Forming (EMF) is a high-speed forming method that generates a pulsed magnetic field by pulsed large currents in the coil, and then induces the eddy current density in the metal material, and the eddy current density and the magnetic field work together to produce a Lorentz force, thereby driving the plastic deformation of the workpiece. This technology can significantly improve the forming limit of lightweight alloys such as aluminum and magnesium, inhibit wrinkles, reduce rebound, and has great potential in the processing of lightweight alloys. At present, this technology is widely used in the electromagnetic expansion of metal pipe fittings and plates, electromagnetic compression, electromagnetic stamping and electromagnetic welding and other processes.
Usually, electromagnetic forming uses a solenoid coil as the driving coil, and the electromagnetic force is generated by the combination of the pulsed magnetic field generated by it and the induced eddy current density generated on the workpiece. Theoretically, the direction and magnitude of the electromagnetic force can be regulated by only changing the drive current parameters, which is far more flexible than traditional mechanical forces.
However, a large number of current studies are still generally focused on the formation of repulsive electromagnetic forces. This is mainly due to the fact that the existing electromagnetic forming current loading method is very single, the eddy current density on the workpiece and the magnetic field are highly coupled, and under the action of The Law of Lenz, the electromagnetic force generated by it is almost all repulsive. This limitation not only does not give play to the characteristics of flexible regulation of electromagnetic force, but also greatly restricts the application scenarios of electromagnetic forming technology.
Therefore, Deng Jianghua et al. studied the influence of the flat coil discharge waveform on the electromagnetic force on the workpiece, and proposed a scheme to use the pulse current waveform of the slow rising edge and the accelerated falling edge to generate an attractive electromagnetic force. Based on this research, Cao Quanliang et al. and Xiong Qi have successively carried out electromagnetic attraction forming studies on plates and small pipe fittings. They generated long and short pulse width currents through the dual power supply system, drove the same coil, generated a current waveform that met the above characteristics, and then used the inducing electromagnetic force generated for aluminum alloy forming, and verified its feasibility through experiments. These studies fully reflect the characteristics of flexible regulation of electromagnetic forming, and expand the application of electromagnetic forming in the field of dent repair and micro pipe fittings.
However, the single-coil scheme requires that the discharge parameters of the two power supplies be tightly matched, otherwise it is difficult to generate a synthetic current waveform that meets the requirements. At the same time, its energy use efficiency is also low. Ouyang Shaowei et al. proposed to use a dual coil model with internal and external coaxial placement to improve this problem. The two coils are connected to two sets of power supplies, and pass through different pulse width currents, long pulse width current to provide a relatively slow change of the background magnetic field, with a short pulse width current generated by the rapidly changing magnetic field to provide the required induction eddy current density, and then produce the desired attraction, and through simulation and experiments to verify the feasibility of the pipe attraction expansion.
Fig. 1 Double coil suction plate forming system
The dual-coil solution is more flexible than the single-coil solution, but it also introduces new problems. In the single-coil scheme, the induced eddy current density direction generated on the workpiece is consistent at the same time. However, in the two-coil scheme, there is a corresponding induced eddy density in the opposite direction at different positions of the workpiece at the same time, which in turn leads to inconsistent distribution of electromagnetic forces, and when the overall area of the workpiece is attractive, the local area still has a repulsive force and eventually leads to dents on the formed part. This eddy current competition phenomenon is an inherent property of the two-coil scheme and cannot be ignored, and how to eliminate its impact on the forming effect also needs to be studied urgently.
In order to solve this problem, the researchers of China Three Gorges University systematically studied the causes and mechanisms of eddy current competition, and studied the eddy current density phenomenon generated by the coils of various structural parameters by establishing the electromagnetic forming simulation model of the plate attraction under the dual coil scheme, and tried to elucidate the relationship between the vortex density and the coil structure, and analyzed its impact on the forming effect.
Fig. 2 Simulation model of double coil suction plate forming
Figure 3 Plate attractive forming simulation algorithm flow
They finally found that by changing the number of layers of coil L (through the long pulse width current), the influence of eddy current competition on the shape of the form can be reduced, the axial displacement of the plate can be effectively improved, and the dent of the plate can be eliminated, and the forming effect can be improved: 1) increase the number of layers of the coil L, which can effectively increase the magnetic flux density on the plate, thereby increasing the axial displacement of the plate, effectively avoiding the occurrence of the dent area on the plate; 2) the axial layer number of coil L is greater than the axial layer of coil S, or increasing the current of coil L. In order to make the magnetic flux density on the plate meet the requirements of attractive forming; 3) The radial layer number of coil L should be slightly greater than the radial layer number of coil S, which can better optimize the uniformity of the attractive forming of the plate.
The researchers confirmed that Scheme 2 can effectively avoid the dent phenomenon caused by the eddy current density competition of the two-coil suction plate electromagnetic forming, providing a better embodiment for the electromagnetic forming of the double coil suction plate. However, it should be noted that the pulse width of the pulse current used in this scheme is long, and the Joule heating caused by it cannot be ignored, so the temperature rise of the coil needs to be considered in practical applications so as not to affect the service life. At the same time, because the temperature rise of the coil will also change with environmental factors, usage and changes in parameters such as thermodynamic properties of the material, there are many influencing factors, and further research will be done on this part in the future.
The above research results were published in the 10th issue of Transactions of China Electrotechnical Society in 2021, with the title of "Eddy Current Competition Problems in the Electromagnetic Forming Process of Double-Coil Attracted Plates", and the authors are Xiong Qi, Yang Meng and so on.