Effect Of Induced Current And High-Strain Rate On The Mechanical Property Of Aluminium Alloy In Electromagnetic Forming

The need to improve fuel economy and protect environment has led to extensive application of lightweight structural materials as represented by aluminum alloys in the field of advanced manufacturing. Electromagnetic forming (EMF) technology provides a new way for room-temperature processing of complex-shape parts from lightweight, hard-forming materials. Electromagnetic forming process is a kind of high strain rate forming process which use Lorentz Force generated by the pulse magnetic field to form the work-piece. The deformation behavior of aluminum alloys under electromagnetic forming is different from low-rate deformation and conventional high-rate deformation. EMF is a complicated multi-field process which contains mechanical, electromagnetic and thermal problems, with many factors affecting the behavior of the deformation. In this paper, electromagnetic ring expansion and electroplastic experiment is introduced to study the effect of induced current and high strain rate on the deformation mechanism and microstructural evolution in the EMF. The research results are beneficial to the comprehension of many kinds of physical phenomena in the high-speed electromagnetic plastic forming process and provides scientific theory and technical experiences to implement complex net shapes.First of all, the multiphysics problem in the EMF including the electromagnetic field, organization field and temperature field is analyzed using the electromagnetic ring expansion model. In this part, the driving circuit, the electromagnetic process of the forming coil and the metal ring are described in detail, the equations of the stress and motion of the metal ring are deduced using the simplified model, and the joule heating and the deformation heating is studied. Combination in the process of electromagnetic bulging ring between the electromagnetic field, structure field and temperature field of mutual contact and mutual influence, using COMSOL field coupling analysis software, using direct coupling method of electromagnetic bulge loop systems is a2d graphic simulation modeling, to physical field coupling analysis under the action of electromagnetic bulge loop process. Combined the interaction of the multi-physical field, this paper uses COMSOL Multiphysics to build a2D plane model of the electromagnetic ring expansion to study the multiphysics problem with the direct coupling method.In order to separate the effect of the induced current in EMF on the mechanical behavior of aluminum alloy, this paper introduces an electroplastic experiment system. The system can study the deformation behavior and mechanical properties of the aluminum alloy in quasi static condition without the effect of the high strain rate. The electroplastic system, which is based on the electronic universal testing machine and with the power supply of a square wave generator using phase shifting ZVZCS full bridge converter, can carry out the uniaxial tensile test with current stimulation and make real-time measurements of the tensile stress, strain and temperature. With the experiment system, the effects of the current characteristic and the tensile speed on the mechanical properties of the aluminum alloy are tudied. From the results, it is found that the ductility of the aluminum alloy can be improved by the current stimulation. When the current with a amplitude of60A/mm2, a period of20s and a pulse width of1.5s is introduced and the tensile speed is1.25mm/min, the fracture strain of the aluminum alloy can reach34.44%, and has a100.35%improvement compared to the standard tensile experiment. It can be deduced that the ductility improvement of the alloy is a combined effect of electric current and temperature by analyzing the experiment results under different conditions. In terms of the flow stress, the current mainly influence the electricity recoverable drop, and the temperature lead to the irreversible decline of the flow stress due to the softening effect. Microscopic studies show that the number of10μm～20μm grain obviously increases, and the number of low angle grain boundary especially the orientation under5°degrees dramatically increases. This change may result in the promoted plasticity under current stimulation.On the basis of the basic theory of electromagnetic ring expansion, the experiment system which consists of the power supply system, forming coil, measurement system is set up. The capacitor power supply is employed as the power system of electromagnetic ring expansion device. According to the function and the parameters of each module of the capacitor power supply, the capacitor, the discharge switch and the freewheel diode are designed. The forming coil is winded with the winding method of pulse magnet to make the forming coil with high thermodynamic and mechanical strength. In the measurement system, the discharge current is measured using the Rogowski coil, and an indirect method is used to measure the induced current in the metal ring. In addition, based on the electromagnetic change characteristics of electromagnetic ring expansion, the electromagnetic displacement measurement system based on printed circuit board (PCB) is designed and developed, so that the metal ring of the displacement and velocity information can be collected and measured.With the experiment system, the rings made of aluminum A5083are test in electromagnetic ring expansion experiment under different circuit parameters and voltage. The experimental results show that the diameter of the expanded ring increases with the increase of the discharge voltage, and compared with standard tensile experiment, the maximum strain of the expanded rings have distinct promotion under each parameters. When the discharge voltage is10500V, the strain of the aluminum ring can reach26.37%with the forming coil No.2. Rather than the fracture strain in the standard tensile test, it has a61.62%improvement. Due to the experimental results under different parameters, it is found a positive correlation between the ultimate strain of the metal rings and the strain rate. With the comparison between the numerical analysis results of the electromagnetic ring expansion model and the experiment results, it is shown that the quasi static tensile stress-strain relationship is more suitable to the electromagnetic ring expansion rather than the C-S constitutive model. And the flow stress is improved in the ring expansion with the combined effect of the induced current, high strain rate and the temperature. In the microscopic research, the EBSD observation shows that the deformation mechanism of the electromagnetic ring expansion differs from that of the uniaxial tensile in quasi static condition. Due to the high strain rate, the plastic deformation in the electromagnetic ring expansion can not been completed in time through the sliding and rotation between the grain boundaries, and is mainly completed by the intracrystalline deformation. This mechanism make the grain in the expansion ring more flattened, and produce a large number of low angle grain boundary which leads to the promotion of the strain limit.