Modelling Based Study On Mechanism Of Electromagnetic Incremental Forming Of Thin-walled Aluminum Component

Large thin-walled aluminum components(LTWACs)are crucial parts for high-end equipments in aviation and aerospace fields.Electromagnetic incremental forming(EMIF)technology,which combines the technical superiority of electromagnetic forming(EMF)and incremental forming,provides a promising way to improve the forming limit and the forming flexibility of this type of components.Thus,EMIF becomes an advanced plastic forming technology urgently needed to be developed.The key factor for the application of this technology is to realize the precision control of forming process and obtain the desirable shape and properties of the target component.Nevertheless,electromagnetic incremental forming is a multi physical field coupled process under complicated time-space-varying boundary conditions.In this process,the sheet metal undergoes a wide range strain rate varying with sudden acceleration and decelaration.Thus,the evolution of micro-defects within a wide strain rate range is complicated and interacts with macro stress responses.These factors make the prediction of macro-micro deformation behaviors and forming quality control of this component a challenging task.To this end,a systematic and in-depth investigation on the macro-micro mechanisms of the EMIF process of LTWACs has been carried out by theoretical modeling,finite element(FE)modeling and experiment.The main research contents and results obtained are as follows.The dynamic stress response rules,the evolution rules and mechanisms of micro-defects within a wide strain rate range have been revealed by high strain rate mechanical tests and microstructure characterization experiments.The significant three-stages varying characteristic of strain rate sensitivity(SRS)has been obtained,i.e.the negative SRS in quasi-static regimes,notably increased SRS at low strains and high strain rates,negative SRS at high strains and high strain rates.Through thermally-activated dislocation movements based theoretical analyses,the control mechanisms of these three stages are determined as dynamic strain ageing,strain hardening caused by strong dislocation interactions(viz.dislocation cells,dislocation bands and in-situ subgrains)and flow softening brought by micro-defects evolution.It is found that the main featured micro-defects in high strain rate tensile and compressive deformation processes are voids and adiabatic shear bands(ASBs).The volume fraction of voids and the width of ASBs are two key parameters which characterize the intensity of strain localization and influence the flow softening ratio of material.Additionally,the inertial effect in high strain rates disperses the second phase particles and stabilizes the defects,broadens the strain range of inhomogeneous deformation and thus improves the forming limit of material.By introducing evolution equations of the volume fraction of voids and the width of adiabatic shear bands into the formulation of structure-related athermal stress within a wide strain rate range,a unified model for predicting the stress responses and micro-defects evolution is established on the basis of Kocks model.The proposed model captures the competition between the strain hardening brought by thermally-activated dislocation movements and flow softening caused by micro-defects evolution during high strain rate deformation of aluminum alloy.In this way,the coupled precision prediction of micro-defects evolution and stress responses within a wide strain rate range is realized.The linear dependence of plastic stress drop on the electroplastic energy density and the functioning threshold of electroplasticity(EP)effect of aluminum alloy are revealed during plastic deformation with current charging.By introducing electroplastic energy density,Joule thermal expansion strain and rate-dependent factor,the prediction model of the elasto-plastic stress reponses is established in high strain rate deformation with current charging.Applied to EMF process,the quantitative effects of high-density pulsed current on the elasto-plastic flow behaviors and the plastic dynamics behaviors of aluminum alloy under wide-range strain rate and current intensity variations are clarified.In this way,the accurate prediction of the effect of high-density pulsed current during EMF of aluminum alloy is realized.The EP stress responses model during current-carrying dynamic deformation of aluminum alloy is proposed within a wide strain rate range by introducing electroplastic energy density and Joule heat expansion items into high strain rate constitutive model.Applying the model to electromagnetic forming process,the quantitative effect rules of high-density pulsed current on the elasto-plastic flow and plastic kinetic behaviors are revealed under a wide-range strain rate change,and thus the problem of accurate prediction of the EP effect during EMIF of aluminum alloy is solved.The through-process macro-micro coupled three dimensional FE model of EMIF of 5A06 LTWAC is established by implementing the high strain rate material model for coupling stress responses and micro-defect evolution and the prediction model of the high-density pulsed current effect.The reliability of established FE model is verified from both macro and micro perspective via comparing the strain and defect distribution of experimental component and the simulated one.Based on the established FE model,the effect rules of the process parameters(discharging parameters,station geometric parameters and discharging route)on the macro plastic flow and micro-defects evolution are investigated.Meanwhile,the correlations between process parameters and forming quality indices(macro-defects index and the distribution homogeneity of both macro deformation and micro-defects)are clarified.According to the above correlations,the optimized process scheme of the EMIF of LTWAC is proposed,which provides an important foundation for the precision forming of LTWACs by EMIF.