What Is Electromagnetic Forming?

Electromagnetic forming refers to the process of forming metal using magnetic force. The capacitor and the control switch form a discharge loop. The instantaneous current generates a strong magnetic field through the working coil. At the same time, the induced current and magnetic field are generated in the metal workpiece, and the workpiece is shaped under the force of the magnetic field.

Electromagnetic forming technology is an emerging high-energy forming technology. It is a forming method that uses an instantaneous high-voltage pulse magnetic field to force the blank to be formed at high speed under the impact of electromagnetic force. Electromagnetic forming belongs to high-energy (high-rate) forming technology. There are many types of high-energy (high-rate) forming technology, but electromagnetic forming eliminates the danger of explosive forming, which is more convenient than electro-hydraulic forming. From the end of the 1950s, electromagnetic forming has developed rapidly at home and abroad, and has become a new process method for metal plastic processing, which has been highly valued by various industrial countries. Now it has been widely used in many fields such as machinery, electronics, automobile industry, light chemical industry and instrumentation, aerospace, weapon industry and so on. The application prospect is very broad ^[1]

The capacitor and the control switch form a discharge loop. The instantaneous current generates a strong magnetic field through the working coil. At the same time, the induced current and magnetic field are generated in the metal workpiece, and the workpiece is shaped under the force of the magnetic field.

Research on electromagnetic forming technology began in the United States in the 1960s. In the 1920s, when physicist Kaptilap did experiments in a pulsed magnetic field, he found that the metal coils that form the pulsed magnetic field are prone to swell and burst. This phenomenon has inspired people to think about the principle of electromagnetic forming. In 1958, General Electric Company exhibited the world's first electromagnetic forming machine at the Second International Peace Atomic Energy Conference in Geneva. In 1962, Brower and Harrey of the United States invented an electromagnetic forming machine for industrial production. Since then, electromagnetic forming has attracted wide attention and attention from various industrial countries. The research on electromagnetic forming technology has achieved many application results, among which the United States and the former Soviet Union are in a leading position in this field. In the early 1970s, experts from the former Soviet Union studied the influence of blank deformation during the discharge process on the parameters of the discharge circuit of the processing coil and the blank system, and pointed out that the RLC circuit can only be applied approximately when the deformation is small; The ultimate deformation degree of the ribs and the hemispherical shape during the comparison are compared. It is pointed out that the ultimate deformation degree of the aluminum alloy, brass and other electromagnetic forming is higher than the ultimate deformation degree of the static forming. It is caused by more uniform distribution and reduced material strengthening. In 1979, the magnetic field distribution of the flat coil was studied, and it was pointed out that the unevenness of the distribution (weak center, strongest at 1/2 radius of the coil) caused the center of the blank. The main cause of insufficient stamping. In the mid 1960s, electromagnetic forming machines with energy storage of 50kJ, 200kJ, and 400kJ appeared. In the mid 1970s, more than 400 electromagnetic forming machines were running on various production lines. By the mid-1980s, electromagnetic forming had been widely used in the United States, the former Soviet Union, and Japan.

In 1994, Makoto Marata also researched the method of electromagnetic tube bulging using direct electrode contact. Through experimental analysis, the effect of working conditions on the current and tube deformation was studied. The elastoplastic analysis of its bulging process was carried out using the finite element method .

The research of electromagnetic forming technology in China began in the 1960s and was interrupted during the Cultural Revolution. In the late 1970s, Harbin Institute of Technology began to study the basic theory and technology of electromagnetic forming, and based on the experimental device, successfully developed China's first electromagnetic forming machine for production in 1986. At present, many universities and research institutes in China have carried out research on electromagnetic forming technology and applied it to actual production.

There are many application methods of electromagnetic forming in industrial manufacturing, which can be widely used for tube bulging, shrinking, punching, flanging and connection, sheet blanking, stamping and forming, assembly assembly, powder compaction, electromagnetic Riveting and storage of radioactive materials.

Electromagnetic forming of pipes

Tube forming is one of the most widely used aspects of electromagnetic forming technology. There are mainly free expansion of tube blanks, die forming, tube alignment, tube segment flanging, flaring and local shrinkage of tube blanks, tube necking, and special-shaped tube forming. During the electromagnetic forming, the deformation of the tube blank is uniform and the deformation hardening is not significant, so the formability of the material is improved. Compared with the static stamping, the electromagnetic forming method can increase the bulging coefficient by 30% -70%. Thin wall thickness or even cracking is the main problem of tube bulging. Now the process has been applied to the necking of some important parts and its correction.

The processing of pipes can also be subdivided into inward compression forming and outward expansion forming. When the workpiece is inside the coil and outside the mold, the workpiece will be compressed inward under the effect of electromagnetic force. This method can be used for processing such as necking of pipes. On the contrary, when the workpiece is outside the coil and inside the mold, the workpiece is bulged outward. This method is often used for the processing of bulging and flanging of pipes.

Electromagnetic blanking

When the coil of the electromagnetic punching device is discharged, the magnetic field force causes the driving piece to move downward, thereby driving the slider assembly. The punch is punched on the workpiece driven by the slider. Compared with ordinary punching, electromagnetic punching is simple in forming equipment and molds, easy to use, and has a high forming rate, which belongs to high-speed forming. Due to the fast forming speed, the quality of the cross-section of the workpiece is good, the end surface is smooth and smooth, there are no rounded bands, and there are almost no fracture bands and burrs. Therefore, electromagnetic blanking is better than ordinary blanking. If it can be applied to actual industrial production, it will bring huge economic benefits.

Compared with ordinary punching, electromagnetic punching has simple forming equipment and molds, and is easy to use. It has high forming efficiency and belongs to high-speed forming. Due to the fast forming speed, the cross-section quality of its workpiece is good, the cross-section is smooth and smooth, and there are no rounded corners. Fractures and burrs. It can be concluded that electromagnetic blanking is better than ordinary blanking, and its practical application in industrial production can bring huge economic benefits.

Electromagnetic riveting

Electromagnetic riveting is a riveting method developed based on electromagnetic forming technology. When the discharge switch is closed, a rapidly changing impulse current flows in the primary coil, generating a strong magnetic field around the coil. The magnetic field causes the magnetic pole coil coupled with the primary coil to generate an induced current, which in turn generates an eddy current magnetic field. The two magnetic fields interact to generate a strong eddy current repulsive force, which is the input force of the amplifier. This force is continuously reflected and transmitted as it propagates through the amplifier, and is output A waveform and peak, the changed stress is transmitted to the rivet, so that the rivet completes plastic deformation in a short time. Electromagnetic riveting is a kind of impact loading, high loading rate, large strain force, and the deformation of the material is different from quasi-static loading such as pressing riveting. Therefore, electromagnetic riveting has technical advantages that other riveting methods cannot replace. In the early 1980s, China began to study electromagnetic riveting technology, and has successfully developed fixed and portable electromagnetic riveting equipment. However, these riveting equipment use high voltage (4kV-10kV), which makes the equipment bulky, high cost, poor safety and reliability, and high discharge frequency. High discharge frequency leads to short rivet forming time, high strain rate of the material, micro-cracks are prone to occur in the hoe, and people's fear of high voltage limits the application of this advanced process. Research on electromagnetic riveting technology began in the early 1970s. By the end of the 1980s, the technology had become a key technology to solve the riveting problem in the aviation industry. In order to eliminate the micro-crack and shear failure of the rivet head caused by the excessive strain rate during high-voltage riveting, the United States began to study low-voltage electromagnetic riveting technology in the late 1980s, and applied for a patent for low-voltage riveting. Equipment began to be used on Boeing 747, A320 and other aircraft. The low-voltage riveting method solves many problems that high-voltage riveting cannot solve, making the electromagnetic riveting technology widely used soon.

Electromagnetic welding

Although there are few reports on the application of electromagnetic forming in welding, electromagnetic forming can be applied to welding when certain conditions are met. Such as welding between tubes and plates, welding between tubes and pipes, and welding between thin plates and thick plates. The conditions required for welding include a clean surface, which is conducive to the discharge of air in the gap when welding, with sufficient energy and a suitable frequency (to make the moving part reach a certain speed), and the moving part must be at an angle (not (Vertical) impact on the surface of the stationary member, etc. The realization of welding lies in the high-speed impact that causes the material surface to violently deform (especially when impacted at a certain angle), resulting in high temperature or even melting, so that the two pieces of material are welded together or connected by diffusion, thereby achieving the welding of the material.

(1) Non-mechanical contact processing. Electromagnetic force is the driving force of workpiece deformation. It is different from general mechanical force. When the workpiece deforms, the force applying device does not need to make direct contact with the workpiece, so there is no mechanical scratch on the surface of the workpiece, and no lubricant is needed. The surface quality of the workpiece is good. Electromagnetic forming uses a magnetic field as a medium to apply pressure to the blank. The magnetic field can penetrate non-conducting materials to achieve non-contact processing. It can directly process workpieces with non-metal coatings or polished surfaces. The surface quality of the formed parts is high.

(2) The deformation of the workpiece originates from the effect of the magnetic field on the charged particles inside the workpiece. Therefore, the deformation of the workpiece is uniform, the residual stress is small, the fatigue strength is high, the service life is long, and the mechanical, physical, and chemical properties of the part are not affected after processing, and no heat treatment is required. Electromagnetic forming is a high-energy forming method. Compared with conventional stamping, electromagnetic forming can effectively improve the plastic deformation ability of the material. Therefore, it is an ideal forming method for difficult-to-form materials with poor plasticity.

(3) High machining accuracy. The control of electromagnetic force is accurate, and the error can be within 0.5%. During electromagnetic forming, the part is filmed at a high speed, and the impact force between the part and the mold is very large. This not only helps to improve the filming property of the part, but also effectively reduces the springback of the part and significantly improves the forming accuracy of the part.

(4) The processing efficiency is high, the time is short, and the cost is low, which facilitates the automation of production. The electromagnetic forming method can be used to complete the parts that can be completed in multiple steps in the conventional forming method in one process, which is conducive to the realization of the composite process. Therefore, it can effectively shorten the production cycle and reduce costs.