| Electromagnetic forming(EMF),which uses repulsive Lorentz force between the discharging current and induced current to deform a metal workpiece,is a promising high-speed forming method and has received much attention in the last decades.It is demonstrated to have the advantages of improving ductility of materials,reducing the springback,and improving the surface quality of formed parts.However,it is a complicated process that involves the coupling of electromagnetic field,thermal field,and mechanical field.The deformation process of the workpiece is affected by high deforming velocity,high temperature,and high current at the same time.The deformation behavior of the material in this process is more complicated than that of the quasi-static tensile test and other traditional high-speed forming processes.The lack of mechanical properties and deformation mechanism in the EMF process limits the further development and industrial application of this technology.In this paper,the electromagnetic ring expanding system is taken as the research object.The thermal effect caused by eddy current and high-strain-rate effect in the process of EMF is investigated by combining the means of theory analysis,numerical analysis,experimental test,and microstructure analysis.The deformation behavior of materials under multiple effects and its influencing mechanism is illustrated.Firstly,theoretical analysis of the multi-physics field of electromagnetic ring expansion is done.The theoretical equations of the circuit,electromagnetic field,temperature field,and structure field in the electromagnetic forming process are deduced,and the coupling effect among the physical fields are analyzed theoretically.A finite element simulation model is built based on the theoretical analysis of each physical field.The distribution characteristics and variation rules of each physical field are analyzed numerically.Secondly,the experimental platform is constructed with a measurement system of discharge current,eddy current,workpiece displacement,and temperature.The experiment results are compared with the simulation results,and the reliability of the simulation model and the feasibility of the measurement system are verified.The stress-strain curve of aluminum alloy in the electromagnetic ring expansion is calculated according to the measured results.The calculated stress-strain curve is applied to the simulation model to improve the accuracy of the simulation.The study of thermal effect can be divided into two aspects: the mechanism of thermal effect and the temperature range of thermal effect.For the research of mechanism,a liquid nitrogen cooling experiment is done to eliminate the thermal effect in the workpiece while deforming.The experimental results show that the elimination of the thermal effect causes the fracture strain of aluminum alloy to decrease from 24.7% to 19.9% at the same deformation velocity.It shows that the thermal effect plays a significant role in the improvement of material ductility in EMF.EBSD and TEM tests indicate that the main mechanism of thermal effect is dynamic recovery.In order to further explore the temperature range of thermal effect,an electromagnetic ring expansion experiment with multi-sized workpiece is designed.The deformation temperature varies from 144 to 474?.The fracture strain first increases and then decreases with increasing temperature,and the optimum deforming temperature range is about160 to 200?.The maximum fracture strain of aluminum can reach 30.8% while deforming temperature is 189?.Experiments with a temperature above 400? result in a sharp decrease of the fracture strain and obvious softening of the workpiece.The mechanism of ductility enhancement of aluminum alloy is proved to be dynamic recovery under optimum deforming temperature range.The process is gradually replaced by dynamic recrystallization as the temperature increase,and lead to grain growth and softening.An eddy current shielded electromagnetic ring expansion device is developed to study the high-strain-rate effect in EMF.The optimum coil structure with field sharper is chosen through simulation.More than 90% eddy current density is shielded,and the maximum temperature rise of the workpiece is approximately 1.4?.The discharge coil is fabricated according to the designed structure and a suitable measurement of eddy current and displacement is applied to the system.The experiment results show that the increase of strain rate can significantly improve the ductility,but still lower than the EMF process while deforming at the same velocity.Higher velocity can improve the ductility to the same level as that of EMF,but it requires greater speed and energy.By comparing the macroscopic and microscopic results of room temperature experiment,low-temperature experiment,and eddy current shielded experiment,the thermal effect,electric effect,and high-strain-rate effect can be decoupled.The results of macroscopic fracture strain indicate that the improvement of ductility in EMF is mainly caused by the highstrain-rate effect and thermal effect of eddy current,while the electric effect has no obvious effect on ductility.Microstructure indicates that thermal effect can promote the elongation and refinement of grain during plastic deformation and induce the dynamic recovery;the electric effect is the main factor affecting the grain orientation and microstructure of aluminum alloy in EMF,and it improves the grain size uniformity;the high-strain-rate effect can enhance grain refinement and lead to a small amount of dynamic recovery due to local accumulation of plastic deformation heat. |
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