| Pre-deformed aluminum alloy has been widely used in aviation,aerospace,transportation,3C and other fields.Pre-deformation can improve aging precipitation and significantly improve mechanical properties of aluminum alloy,but also reduce material plasticity.The study of warm/hot deformation behavior and subsequent aging hardening response of pre-deformed aluminum alloy is helpful to the development of the formability/performance synergy process of this kind of alloy.Based on different pre-strain 2219 aluminum alloy as experimental objects,the warm/hot deformation behavior and subsequent age hardening behavior and microstructure evolution are studied.A constitutive model based on physical mechanism is established based on the results of macro and micro experiments,the constitutive model is embedded into the finite element software to establish the stamping simulation model of small test parts.The constitutive model and process were verified by simulation and sample production.The main research contents and results are as follows:(1)Aluminum copper alloys often need pre-deformation to realize their performance potential,but pre-deformation will improve the strength and reduce the plasticity of the alloy,then affect the forming limit and forming accuracy.The thermal deformation behavior,artificial aging hardening behavior and microstructure evolution of 2219 aluminum alloy with different pre-strain were studied by various macro and micro experiments.The results show that the increasing temperature can significantly reduce the deformation resistance,and the higher the temperature and pre-strain,the more obvious the effect is.The dislocation recovery during heating and holding,the reduction of dislocation strengthening at high temperature and the decrease of saturation dislocation density lead to the decrease of deformation resistance.The elongation after fracture of pre-deformed alloy can be improved at 200℃,but it decreases when the temperature is higher than 250℃,which is caused by local thinning during warm deformation.Higher dynamic recovery rate results in a significant decrease in uniform elongation.When the deformation temperature is not higher than 250 ℃,the subsequent peak aging properties of the alloy can be retained completely,and the hardness is consistent with the traditional T8 state.However,when the deformation temperature is higher than 300℃,the peak aging hardness decreases significantly,which is caused by the rapid nucleation and growth of θ’ phase and excessive recovery of dislocation at high temperature.Compared with room temperature deformation,no PLC effect is found at 200℃ and above,which is beneficial to the stable deformation and surface morphology evolution.In summary,under the condition of rapid heating and deformation,the deformation temperature of small pre-deformed Al-Cu alloy should not be higher than 250℃,otherwise the mechanical properties after forming cannot be guaranteed.When the pre-strain is larger or the structure of parts is relatively complex,the deformation temperature should be reduced to obtain higher forming limit.(2)A certain amount of aluminum alloy sheets are supplied in heavily cold-worked(HCW)state.Large pre-deformation can greatly improve the strength while significantly degrade the ductility of metals,leading to a low formability.A fast heating and deformation method is developed to enhance the formability without appreciable sacrifice of dislocation hardening in 2219 aluminum alloy.The method first uses electromagnetic induction heating to heat the sample to 100-350℃,and then conducts rapid deformation,which takes less than 8s to reduce the processing time at elevated temperature.The microstructure evolution of large pre-deformed alloy during fast heat anddeformation and subsequent artificial aging is observed in detail by transmission electron microscopy,scanning electron microscope and electron backscatter diffraction.Compared with room temperature tests,the yield strength is reduced by～150 MPa while the elongation to fracture is increased by ～3 times at200℃.The strength of 120℃ peak aging sample is 80 MPa higher than that of T8 state,and the plasticity is basically the same.This is due to the high dislocation densities,large pre-deformation dislocation configuration and the bimodal distribution of θ’/θ’’ phase still keep in the alloy after a short period of thermal deformation.Although deformation above 250 ℃ could further improve the formability,it results in a considerable loss of both strength and ductility,due to the substantial precipitate coarsening and dislocation recovery during deformation.Our findings reveal that fast heat and deformation at appropriate temperature can effectively improve the formability and prevent the dislocation recovery as well as precipitate coarsening in large pre-deformed alloy.The new process can improve the formability of large pre-deformed alloy and ensure excellent mechanical properties,which is a potential method for the warm forming of pre-deformed alloy.(3)A constitutive equation based on dislocation density and configuration evolution was established to describe the warm deformation behavior of 2219 aluminum alloy with small/large pre-deformation.The two models have good fitting and prediction effects.The microscopic parameters in the model can describe the microstructure changes during the warm deformation process.The small pre-deformation model can accurately predict instability fracture strain by adjusting dislocation parameters.(4)Finite element simulation and sample forming verification are carried out.Using UMAT subroutine interface of Abaqus,the constitutive model is defined.The simulation and trial production of curved bending parts and three-point bending experiments are carried out,and the high consistency between them verifies the accuracy of the model and the feasibility of the process.Simulation and trial production results show that the proposed warm forming process can significantly improve the forming limit,reduce deformation resistance and springback of pre-deformed Al-Cu alloy. |
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