| Materials represented by aluminum alloys with light weight and high strength have great application value for aerospace and automobile manufacturing.The widespread application of aluminum alloys has become an important way to realize light-weighting,energy-saving and emission-reduction.However,problems including wrinkling,springback and fracture always occur when forming aluminum alloys parts with traditional quasistatic technologies,due to their poor plasticity at room temperature.To improve the forming performances of aluminum alloys,researchers have proposed several high speed forming methods based on different excitation sources,among which electromagnetic forming(EMF)technology has attracted much attentions due to its simple forming system as well as the convenient implement of energy control and automatic production.According to researches,EMF is beneficial in improving plasticity and forming limit,preventing wrinkling and reducing springback by the effect of high forming speed.However,challenges of precious shape controlling and mold sticking should be overcame when forming cylindrical cup by EMF,where only one single mold is used.To solve these problems,a new forming method named electromagnetic-force-driven sheet metal stamping(EFDS)process is proposed in this paper by combining the existing EMF technology and traditional quasistatic stamping method.During the forming process,Lorentz force is induced in a driver plate and drives a punch to realize the stamping process of metal sheet.This method has the advantages of high forming speed and precious shape-controlling.On this basis,systematic analysis on the forming performance of workpiece is carried out by both numerical simulations and experiments.Relevant achievements are able to make the efficient forming of aluminum alloys cylindrical cup with large depth accessible,which have great theoretical value and practical significance.On the aspect of numerical simulation,a finite element-circuit coupled numerical model is established based on the improved current filament method(CFM),aiming to solve the problems of distorted air meshes that occur in electromagnetic-structural coupled finite element model when dealing with large or complicated deformation.This model provides an efficient way for the analysis on deformation characteristics and regularity of workpiece.First of all,CFM based on the equivalent circuit is applied to predict coil current and calculate electromagnetic force in workpiece.Considering the fact that conventional CFM fails to take into account the skin effect of coil,an improved CFM model is developed by further dividing each turn of coil into several current filaments.Results show that the calculation accuracy of electromagnetic force using improved CFM is increased compared to conventional CFM,by correcting the distribution of mutual inductance gradients between current filaments.On this basis,a finite element-circuit coupled model is established by combining the electromagnetic and mechanical calculation,which are based on the improved CFM and finite element method(FEM),respectively.Precious predictions and analysis on forming behavior of aluminum alloys cylindrical cup with large depth are accessible using this model.On the aspect of experimental study,a set of forming system of EFDS is built for the systematic analysis on forming performances of workpiece under different discharge parameters,where AA5052-O metal sheet with drawing ratio of 2.0 is applied.Firstly,deformation characteristics of workpiece are studied by both simulations and experiments,in which forming conditions such as discharge capacitances and topology of discharge circuit varies.As a result,the effects of oscillation frequency and mode,the number of current halfwaves on the forming depth of workpiece are clarified.Meanwhile,the effect mechanisms are revealed according to the change regulation of electromagnetic force and its impulse.Secondly,the validity of EFDS is verified by experiments,in which an aluminum alloy cylindrical cup of 66.4mm height under the drawing ratio of 2 is obtained after 2 times of discharges,where the total discharge energy is 32 kJ.Compared to the existing electromagnetic-pulse-assisted stamping methods,many advantages are achieved.For instances,the discharge times and energy are reduced apparently,and the forming procedure is greatly simplified as well.Finally,the effect of auxiliary coil on the improvement of thickness distribution in workpiece is preliminarily analyzed.Results show that the flowing of flange is strengthened by the radial Lorentz force generated by auxiliary coil,which results in the reduction of maximum thinning rate from 9.06% to 6.56%.This dual-coil system is beneficial for the cylindrical cup forming with larger drawing ratio. |
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