Numerical Prediction And Experimental Investigation On Electromagnetic Forming Limit Diagram Of Aluminum Alloy Sheet

In recent years,in the face of the increasingly severe environmental pollution and energy shortages,the demand for aluminum alloy in automotive,aerospace and other fields is increasing.However,compared with low carbon steel,aluminum alloy has poor formability at room temperature,which restricts its application in industry to a great extent.Some studies have shown that electromagnetic forming,as a high strain rate forming process,can significantly improve the formability of aluminum alloy.As the main judgment basis of sheet metal forming performance,forming limit diagram（FLD）can be used to predict the failure of sheet metal in the forming process.At present,the electromagnetic forming limit diagram（EMFLD）of aluminum alloy sheet is mostly obtained through experiments.This method is not only economical and time-consuming,but also difficult to obtain the electromagnetic forming limit of aluminum alloy sheet at constant strain rate.In this paper,the 1mm thick AA5182-O aluminum alloy sheet was taken as the research object.Based on the electromagnetic hole-flanging experiment and simulation,the constitutive model of AA5182-O aluminum alloy was inversely identified,and the electromagnetic forming limit of AA5182-O aluminum alloy sheet was determined by the combination of theoretical derivation,numerical simulation and experimental verification.Firstly,based on uniaxial tensile experiment,electromagnetic hole-flanging experiment and numerical simulation,the Johnson-Cook（J-C）、Huh-Kang（H-K）、Allen-Rule-Jones（A-R-J）and Cowper-Symonds（C-S）constitutive models of AA5182-O aluminum alloy sheet under high strain rate were inversely identified.The accuracy of the constitutive model was verified by comparing the simulated values of height and thickness with the experimental values,and the flow stress curves of sheet under different strain rates were analyzed.The research shows that the reverse identification method can effectively establish an accurate high strain rate constitutive model,and the prediction of flow stress curve by different constitutive models is very different.Secondly,based on the identified hardening model and the modified Marciniak-Kuczynski（M-K）model considering normal compressive stress,the forming limit of AA5182-O aluminum alloy sheet at high strain rate was predicted by Newton iteration method in Matlab software.The influence of normal compressive stress₃and initial defect coefficient₀on forming limit diagram was discussed by taking C-S model as an example.It is found that the constitutive model directly affects the prediction of sheet metal forming limit at high strain rate.When J-C and A-R-J models are used,the FLD shows strain rate insensitivity.When H-K constitutive model is used,the FLD shows strain rate positive correlation within a certain strain rate range,while when C-S constitutive model is used,the FLD shows significant strain rate positive correlation.In addition,the prediction results of forming limit curve will be significantly affected by normal compressive stress and initial defect coefficient.When the strain rate is constant,the forming limit curve increases with the increase of normal compressive stress and initial defect coefficient.Finally,based on the racetrack coil,a new type of single strain sample and near plane strain sample were designed.Numerical simulation of electromagnetic forming for AA5182-O aluminum alloy sheet was carried out in LS-DYNA.The numerical electromagnetic forming limit strain of AA5182-O aluminum alloy sheet was obtained based on strain difference criterion,and the dynamic deformation process of AA5182-O aluminum alloy was discussed and analyzed.The quasi-static and electromagnetic forming limit diagrams of AA5182-O aluminum alloy were obtained by uniaxial tensile test,cup bulging test and electromagnetic forming test respectively.The results show that the forming limit of AA5182-O aluminum alloy under electromagnetic forming condition is significantly higher than that under quasi-static forming condition.The M-K theoretical prediction results and numerical simulation results of AA5182-O aluminum alloy forming limit were compared with the experimental results.It is found that the M-K model can accurately predict the quasi-static forming limit and electromagnetic forming limit of AA5182-O aluminum alloy.