Research of Dynamic Characteristics in Electromagnetic Attraction Forming of AZ31 Magnesium Alloy Tube
Xiong Qi1,3, Zhu Xinhui1,2, Zhao Xiang1,2, Fan Liping4
1. Hubei Provincial Engineering Technology Research Center for Power Transmission Line China Three Gorges University Yichang 443002 China; 2. College of Electrical Engineering & New Energy China Three Gorges University Yichang 443002 China; 3. Wuhan National High Magnetic Field Center Huazhong University of Science and Technology Wuhan 430074 China; 4. Yichang Power Supply Company State Grid Hubei Electric Power Corporation Yichang 443002 China
Abstract：Electromagnetic forming (EMF) technology can effectively improve the forming limit of light alloys at room temperature, suppress the spring-back of the workpiece, and reduce wrinkling. It has been widely used in the plastic processing of aluminum alloy workpieces. However, compared with aluminum alloys, magnesium alloys have lower electrical conductivity and higher yield strength, so conventional EMF technology can not satisfy the plastic processing of magnesium alloy workpieces at room temperature. Recently, some scholars used high-conductivity alloys as driving plates or heating devices to assist in forming, but the tooling was complex and the forming quality was difficult to guarantee. It was not satisfied small tube forming even more. To address these issues, this paper proposes an electromagnetic attraction forming method for magnesium alloy tubes at room temperature. By establishing a finite element simulation model, the feasibility of the forming scheme, the discharge and electromagnetic parameters and the deformation of workpiece were studied in the artic. Firstly, AZ31B magnesium alloy tube with an inner diameter of 10 mm, a length of 100 mm and a thickness of 2 mm are selected, and the dual-frequency discharge current method is used to generate attractive forces to drive the forming of the tube. Secondly, due to the extremely fast and complex electromagnetic forming, it is necessary to use simulation to explore the relationship between the parameters in the attractive electromagnetic forming process in detail. Finally, a finite element model considering the influence of tube displacement and deformation rate has been developed, the method realizes the full coupling between the electromagnetic fields and workpiece deformation. Simulation results show that the process of electromagnetic forming can be divided into the following three stages: the stage before the short pulse current is turned on; the deformation stage; the vibration stage. Changing the long-pulse voltage Us or the short-pulse voltage Uf can change the time-space distribution of electromagnetic parameters at point O, and ultimately regulate the motion state of the tube. During the short-pulse current conduction stage, the electromagnetic parameters at point O have opposite signs, the tube is subjected to a repulsive force. When Us is 9.4 kV and Uf is 13 kV, the tube is radially contracted by 0.27 mm, the final deformation area dent marks will appear and the deformation flatness will be poor. In the deformation stage, the electromagnetic parameters at point O have the same sign, and the tube is deformed by the attractive force. When Uf is 13 kV, Us increases from 4.4 kV to 8.4 kV, the action time of the attractive force in the deformation stage will become longer. The final radial expansion displacement of the tube will become larger. When Us is 8.4 kV and Uf is raised from 13 kV to 23 kV, the electromagnetic force and radial expansion velocity of point O in the deformation stage both increase. However, the final displacement at 23 kV is only 5.84 mm, much lower than 7.13 mm at 21 kV. To study this phenomenon, Uf was raised from 21 kV to 23 kV (0.5 kV intervals). In the vibration stage, the peak shrinkage radial velocity at 21 kV and 21.5 kV were 12.8 m/s and 13.4 m/s, respectively, and the peak shrinkage velocity increased to 32.9 m/s at 22.5 kV. At this time, the repulsive force will dominate the motion state of the tube in the vibration stage, so the final deformation effect is not good. Through the simulation analysis, the following conclusions can be drawn: 1) In practical engineering, it should be considered whether Us will cause necking in the workpiece before the short pulse is turned on. 2) Since Us has a lower upper limit, increasing Us is more effective than increasing Uf. It can improve the deformation effect of the tube fitting. 3) The upper limit of Uf will be determined by the motion state of the tube fitting in the vibration stage, that is, the velocity movement trend of the tube fitting in this stage should be roughly the same. If the shrinkage speed is significantly increased, Uf cannot be further increased.
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