Qi Xiong

Electromagnetic pulse welding (EMPW) is a process that achieves metallurgical bonding through the collision of metal plates driven by a Lorentz force. EMPW has several advantages, including high welding efficiency, low overall welding temperature, and no pollution. It is suitable for a wide range of metal plate connections. Single H-type and double H-type coils are commonly used in plate welding due to their simple structure, small size, and convenient assembly. The majority of existing research has focused on the influence of process parameters on the quality of plate welding, with a paucity of comparative studies on the welding performance of these two coils. In this paper, two types of coils are employed to weld a Cu-Al plate using the finite element method, with the initial relative velocity of collision, energy utilization rate, and effective welding width analyzed. The results indicate that, in terms of the initial relative velocity of the collision, the double H-type coil will exceed the single H-type coil as the distance between the plates increases. Furthermore, the double H-type coil can apply Lorentz force to two plates simultaneously, resulting in a higher energy utilization rate. In terms of effective weld width, in the case of a small distance between plates, with the increase of discharge voltage, a single H-type coil can also achieve a better welding performance. The above relevant conclusions will provide some guidance in choosing which H-type coils to use in electromagnetic pulse welding of plate.

As an important energy conversion component in electromagnetic-forming technology, the coil is subjected to great internal stress and is easy to break. The geometric structure and winding process of the forming coil draw on the research results of pulsed magnets. However, the two use conditions are different. It is very important to clarify the force difference between the two for the design of the forming coil. In this paper, the numerical model of an aluminum alloy (AA1060-O) is established, and the difference in force between the pulse magnet and forming coil with the same size in time and space under different working conditions is analyzed. A two-dimensional fully coupled finite element model consisting of circuit, magnetic field, and solid mechanics is established and used to determine the key parts of the coil force. It is found that the von Mises stress of the forming coil is greater than that of the pulsed magnet under the same circuit parameters and geometric structure. In the electromagnetic forming of the tube, the glass fiber is subjected to a large stress. In addition, the stress of glass fiber under the condition of tube necking is about 2 times that of pulsed magnet. When the voltage is increased, the failure of the middle part of the glass fiber causes the coil to break. In the electromagnetic forming of the sheet, the coil skeleton is subjected to large stress, and its upper end failure causes the coil to break. Therefore, new design ideas for forming coils under different working conditions are proposed.

A high-temperature superconducting (HTS) motor was designed, which employs the HTS wires in the rotor field windings. To reduce the welding joint resistance of the wires, a simple and efficient welding method is developed, and the welding process suitable for 2G and 1G HTS wires is summarized as follows. First, the welding process was obtained by the tests, the results show that the welding temperature is chosen to be 220°C, the lap length is chosen to be 160mm, and the resistance of the joint can reach 10 ^-9 Ω. Second, the resistance influenced by the radius was analyzed by the finite element method. The results show that there might be little decrease in the critical current of the joints. Last, a 2G HTS magnet was manufactured to verify whether the joints made by the method can meet the requirements of bending deformation in the HTS magnet winding process. The results show that the welding process can satisfy the requirements, and provide an important reference for subsequent work.

In recent years we have seen the rapid development of electromagnetic flanging of tubes, which uses coils to generate electromagnetic forces to achieve the deformation with high speed but without contact. However, the electromagnetic force decays rapidly with the increase of distance, resulting in strict requirements of geometrical matching between the coils and the tubes. Usually, new coils should be fabricated for tubes with new sizes, which is inconvenient and uneconomical. Therefore, a more flexible and economical method is proposed in this article, which introduces a solenoid field shaper into the existing electromagnetic flanging system. By adjusting the structure and the position of the field shaper, the distribution of electromagnetic forces can be reshaped to form tubes with various sizes, without changing the coil, whose cost is much higher than a field shaper. The principle of this method is introduced in detail. Then an electromagnetic-structure coupled finite element simulation model is established to calculate the forming process. The results show that when forming an A1060-O aluminum alloy tube with an inner diameter of 110mm, the discharge voltage can be tuned down from 8.5kV, without field shaper, to 7.11kV, with field shaper. That means the energy consumption of the system can be saved by 30%, and the manufacturing process of the field shaper is simpler than that of the forming coil. What’s more, when forming tubes with different sizes, the new method shows higher effectiveness, greater flexibility, and lower cost than the traditional way.

Electromagnetic tube expansion has been given much attention due to its superior cold forming properties. The traditional electromagnetic coil is of single structure, which results in substantial tube wall thinning and mechanical strength reduction. This shortcoming restricts further development of the electromagnetic tube expansion technology in various industrial applications. This paper proposes an axial-radial electromagnetic force bidirectional loading method to increase the axial fluidity of the material and eliminate the wall thinning issue that currently exists in the traditional method. Furthermore, the performances of three proposed structures for the axial compression electromagnetic tube expansion using three-coil, two-coil and one-coil are analyzed and compared. In this context, the electromagnetic force distribution and the wall thinning of the tube during the expansion process are analyzed based on finite element modelling. Results show that with the proposed modified structure of the traditional single coil, the axial electromagnetic force can be substantially increased which results in reducing the tube wall thinning significantly. The proposed technique in this paper paves the way for further development and more industrial applications of the electromagnetic expansion technology.

Due to the fact that geometrical structure of tube electromagnetic bulging process with field shaper does not conform to the axisymmetric property, three-dimensional (3D) model is usually used to analyze the process which is complicated and time consuming. Therefore, on the basis of understanding the working principle of the field shaper, a two-dimensional (2D) model with current constraint is proposed in this paper to analyze the electromagnetic bulging process of a tube fitted with a field shaper. COMSOL finite element analysis software is used to develop three models for performance analysis and comparison, these are: conventional 3D model and the proposed 2D model with and without current constraint. Simulation results show that the induced eddy current density, magnetic flux density and electromagnetic body density of the two-dimensional model with current constraint are in good agreement with the results of the three-dimensional model. This provides an alternative and effective way to analyze the performance of the tube electromagnetic bulging with field shaper using a simple but yet reliable 2D model.

The attractive electromagnetic bulging is a novel technology which has unique advantages in the formation of small tube fittings. To generate the attractive electromagnetic force, a dual-frequency current method has been proposed in the existing research on the attraction bulging of the small tube fittings. This method, however, has a flaw that the tube fittings are subjected to a large repulsion electromagnetic force before the attraction, which will lead to a negligible effect on the final forming results. To solve this problem, a non-uniform solenoid coil structure is proposed in this paper to suppress the repulsion to improve the attraction. On this basis, a finite element model is built to analyze the process of an aluminum tube with an inner diameter of 20 mm and an outer diameter of 22 mm and the thickness of 1 mm. The influences of the coil structure on the magnetic induction intensity $B$ , eddy current density $J_{e}$ and the stress of the coil have been studied. The results show that the expansion of the tube can be improved when the non-uniform solenoid coil is used. The maximum expansion in this dissertation is achieved with a 28 turns non-uniform solenoid, and its expansion uniformity standard deviation is only 0.156. These simulations have a significant outcome to the design of the coils and the application of electromagnetic bulging in small tube fittings.