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The generation and optimization of attractive force between the coil and the workpiece is of significance for expanding the application of electromagnetic forming. In this work, a method using a lo...

In order to meet the requirement of manufacturing of tubular parts in variant applications, the study of deformation behavior of tube in electromagnetic expansion has remarkable necessity. Especially, in the electromagnetic expansion of short tube (the tube length shorter than the coil length), the deformation behavior is more complex than the existing studies of electromagnetic expansion of long tubes. In this paper, the effect of different discharging current pulse width on deformation behavior of AA6061 aluminum short tube has been studied and discussed, according to the waveforms of radial Lorentz force, velocity and displacement. Results showed that when the discharging current has short pulse width, the deformed tube has concave profile, while when the discharging current has long pulse width, the displacement of tube middle gradually exceeds that of tube end during the forming process, which means that the deformed tube changes to convex profile.

The Lorentz force distribution on a tube is the dominant factor that determines the tube deformation behavior in the electromagnetic forming process. However, the existing forming technologies in this field are mainly based on a single-coil system, which results in a relatively fixed Lorentz force distribution and makes it difficult to control the forming process or the final tube shape. To solve this problem, a three-coil forming system composed of three sub-coils and two pulsed power supplies is developed for tube forming in this work. Both numerical simulation and experimental results show that the three-coil system can be easily applied to alter the Lorentz force distribution through adjusting the discharge voltage of sub-coils, providing an effective way to improve the deformation uniformity and control the deformation shape. It is found that an AA6061-O aluminum alloy tube with an inner diameter of 76 mm and a thickness of 2 mm can be deformed with a homogeneous deformation range of about 23.56 mm via the three-coil system in the experiments, while the range is only about 11.48 mm under the same conditions when using the conventional single-coil system. Meanwhile, it is demonstrated that the three-coil system can achieve a variety of tube shapes including concave and convex profiles with different uniformity. The developed method and obtained results are of good value for expanding the application of electromagnetic tube forming.

In the majority of electromagnetic tube forming system, the distribution of Lorentz force acting on tube is restricted and has poor controllability, which leads to the tube being unable to meet the requirement of manufacturing flexibly. A new electromagnetic tube-forming method based on the two-stage coils system, which consists of two coils and two independent capacitor banks, has been proposed to improve the distribution mode and controllability of Lorentz force. With higher deformation depth and less occurrence of cracking compared with the conventional one single-coil system; this method has been proved to be effective in improving the deformation behavior of tube in electromagnetic forming through experiments. However, the deformation behavior of tube in this forming method still needs further studies. In this paper, the effect of pulse width of middle-coil current on AA1060 aluminum tube deformation behavior under two-stage coil system is investigated through simulation model, which is based on the combination of current filament method and finite element method. Results show that the short pulse width of the middle-coil current can achieve larger deformation depth of tube under relatively small discharging energy, and the long pulse width of the middle-coil current has good performance in deformed profile. Moreover, the thickness reduction in the case of long pulse width of middle-coil current is less than the case of short pulse width of the middle-coil current under equal deformation depth of tube.

The electromagnetic force between the tube and the coil is generally dominated by repulsive Lorentz force in the existing electromagnetic tube forming systems. In these cases, the coil has to be placed inside the tube for tube bulging, which makes it difficult to achieve when the inner space of the tube is not large enough to accommodate the coil. To solve this problem, a new electromagnetic tube bulging process with an external dual-coil system was developed in this work. According to both numerical and experimental studies, it has been demonstrated that the AA6061-O aluminum alloy tube with a diameter of 33 mm and a thickness of 1 mm can be attracted toward the coil, and the maximum deformation is more than 4 mm after a single discharge. Meanwhile, the tube deformation can be significantly increased by multiple discharges using the attractive forming process. Furthermore, a series of simulations and experiments were carried out to systematically investigate the attractive forming behavior of the tube under different discharge conditions, showing that the discharge voltages of the two coils and the current waveform controlled by a crowbar circuit have a great impact on the tube forming.

In existing electromagnetic sheet metal forming processes, the sheet metal is typically deformed away from the coil under repulsive Lorentz forces. This deformation mode makes it difficult to apply the forming technology to dent repair without disassembly of the automobile body and aircraft fuselage. To solve this problem, a new electromagnetic attractive forming process with a low-frequency discharge current was developed in this work. Different from previous attractive forming methods, only a single coil and a single power supply are required that is low cost and easy to implement. Both numerical and experimental studies have demonstrated that an AA1060-H24 aluminum alloy sheet with a thickness of 1 mm can be effectively attracted towards the coil with a maximum deformation of more than 8 mm after a single discharge. More importantly, the mechanism of the attractive forming process has been clarified in detail through the analysis of the Lorentz force characteristics and deformation process of sheet metal. It has been pointed out that the current waveform after the first peak plays a vital role in improving the attractive force and the attractive forming process, which can be optimized by introducing a crowbar circuit. Meanwhile, it is found that the deformed shape of sheet metal caused by a repulsive force helps to facilitate the subsequent reverse deformation under an attractive force. This is also an important reason why an obvious attractive forming can be achieved, although the attractive force is much smaller than the repulsive force during the forming process. The obtained results have the significance for fundamental understanding of electromagnetic forming process, and further applications can be expected using the developed forming method.

Improving the strength and controllability of the magnetic force that acts on a workpiece is important to enhance the forming ability and outcome during the electromagnetic forming (EMF) process. For this purpose, the present article introduces the concept of a background magnetic field (BMF) to enhance or reverse the forming force and proposes a dual-coil EMF system to generate a BMF for tube forming. Relative to the conventional electromagnetic tube forming system with a single coil, the magnetic force and forming capability can be improved with the aid of a BMF. In addition, a BMF can be used to change the magnetic force direction to induce bulging in small tubes by mounting the two coils outside these tubes. To validate the feasibility of this method and the proposed system, experimental and simulation studies were performed to investigate the bulging behavior of aluminum alloy tubes (AA6061-O). The results show that with a BMF, the deformation depth of the tube can be significantly increased, and the optimal strength of the BMF can be determined. Moreover, a BMF can improve the morphology of the workpieces by enlarging the forming areas and reducing their thickness reduction. Furthermore, tube bulging induced by attractive forces can be realized by using a BMF. The presented method and results are significant for EMF system design and practical applications.