| Electromagnetic forming is a special forming technology that uses Lorentz force to realize metal forming.It has the characteristics of high speed,non-contact and single-mold.Compared with traditional quasi-static forming methods,it can significantly improve the forming limit of workpieces,restrain wrinkling and reduce springback.It has a broad application prospect in the manufacturing of lightweight sheets or tubes in the fields of aerospace and automobile.However,electromagnetic forming technology involves harsh working conditions such as strong magnetic field,high voltage,large current,and high stress,which poses severe challenges to the mechanical and thermal properties of forming equipment in practical applications.Among them,the tool coil is a key component of energy conversion during the forming process.The Joule heating and temperature rise caused by the large current during the discharge process are one of the vital factors that determine the efficiency and lifetime of the tool coil.It is also a big problem that needs to be solved to realize the large-scale industrial application of electromagnetic forming.Around the theme,in order to reduce the temperature rise of the coil and keep the forming depth unchanged during the electromagnetic forming process to improve its efficiency,this thesis systematically carried out numerical simulation and experimental research on the deformation of the electromagnetic forming workpiece and the thermal characteristic of the coil under the adjustable damping coefficient.The main research content and results are as follows:First,the physical models of the circuits,electromagnetic fields and structural fields involved in the electromagnetic forming system and their coupling relationship are analyzed.Based on the finite element simulation software COMSOL Multiphysics,the 2-D axisymmetric multiphysics coupled numerical model for electromagnetic forming of 5052-O aluminum alloy sheet is established and solved.On this basis,the corresponding electromagnetic forming experiment platform is built,and the effectiveness of the numerical model was verified by comparing and analyzing the simulation and experimental results of discharge current and deformation displacement.Secondly,based on the exploration of the internal relationship among discharge current,deformation behavior of workpiece and thermal characteristics of the tool coil,a new Joule heating reduction method of the tool coil based on the adjustment of current damping coefficient is proposed.It can be realized just by introducing additional resistance into the original electromagnetic forming circuit.It has the advantages of simple structure,low cost and convenient analysis.Simulation and experimental studies show that this method can significantly reduce the Joule heating and temperature rise in the coil by more than 50% while ensuring the forming depth of sheet metal unchanged.Finally,the performance of the damping coefficient adjustment method under different parameters is systematically analyzed.On the one hand,the versatility of the damping coefficient adjustment method to realize coil cooling under high frequency current is further demonstrated through simulation and experiments.On the other hand,the effect of initial line resistance and capacitance on the deformation of the sheet metal and the cooling performance of the tool coil is clarified.The research indicates that smaller initial line resistance is not always a better choice without external resistance adjustment.It will significantly increase the Joule of the tool coil when the initial resistance is rather small.And it is proved that there is an optimal discharge frequency for the damping coefficient adjustment method to optimize the forming of workpiece and Joule heating of the tool coil. |
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