| Automobile lightweighting is becoming one of the main trends in the automobile industry development in recent years.Aluminum alloy has been regarded as one of the most ideal materials for automotive lightweighting technology due to its high specific strength,rich reserves and good recycling performance.7075 aluminum alloy belongs to the high strength 7XXX aluminum alloys,with the tensile strength higher than 500 MPa.However,due to the low plastic elongation of 7075 aluminum alloy at room temperature,traditional cold stamping has numerous deficiencies in the forming process(e.g.the material is easily fractured),resulting in its limited application in automobile field.The adoption of aluminum hot stamping process provides an effective solution for improving stamping formability of the high strength aluminum alloys.This technique combines stamping process and heat treatment process together,which can improve the formability of aluminum alloys without damaging the mechanical properties.Nowadays,there are still many scientific and engineering problems in the aluminum hot stamping process,especially for high strength aluminum alloys.In this study,the thermal deformation behavior and microstructure evolution of 7075 aluminum alloy was studied,then a unified viscoplastic constitutive model considering microstructure evolution was established.Under the condition of deformation temperatures ranging from 300 ℃ to 450 ℃ and strain rates ranging from 0.01 s-1 to 10 s-1,Gleeble-3500 thermal simulation machine was used to study the thermal deformation behavior of 7075 aluminum alloy.The influences of deformation temperature,strain and strain rate on the flow stress and microstructure evolution of the material were analyzed.The results showed that the plastic elongation was best at 400 ℃,and the process of uniaxial thermal tensile deformation can be divided into four stages:elastic deformation,work hardening,flow softening.and rupture.The average grain size of specimen increased with increasing temperature,and decreased with increasing strain and strain rate.Based on the uniaxial thermal tensile experiment of 7075 aluminum alloy,a unified viscoplastic constitutive model considering microstructure evolution was established.Stress,dislocation density,strain rate and deformation temperature,etc.,the typical variables in the process of thermal plastic deformation,were coupled in the constitutive model,reflecting the essential flow law of the 7075 aluminum alloy at high temperature.The optimal material constants of the constitutive model were obtained by using genetic optimization algorithm.The predictions of the constitutive model were examined by various statistic indexes,which reveals that the model could predict the flow stress of 7075 aluminum alloy under the condition of deformation temperatures ranging from 300 ℃ to 450 ℃ and strain rates ranging from 0.01 s-1 to 10 s-1.Meanwhile,the evolution behavior of microstructure can be accurately described by the model.An experimental device for testing forming limit curves(FLC)at high temperatures is designed and manufactured.Using this device,the forming limits of 7075 aluminum alloy at different deformation temperatures and strain rates are determined,and the influences of process parameters on the forming limit of 7075 aluminum alloy are analyzed.Based on the consideration of the first principal stress,the hydrostatic stress and the equivalent stress,the influence of multiaxial stress state on the damage evolution is introduced,then the uniaxial constitutive model is extended to the plane stress condition(a multi-axial constitutive model).Fixing the material constants of the uniaxial unified viscoplastic constitutive model,the parameters of the multi-axial constitutive model are optimized using the obtained FLC curve data.The multi-axial constitutive model was used to predict the FLC curve on different deformation conditions.The interfacial heat transfer behavior of the 7075 aluminum alloy sheet under hot stamping process was studied using a flat die.The experiments have shown that cooling rate increased with increasing contact pressure and use of lubricant.By analyzing the room temperature mechanical properties of the specimen after artificial aging,it was found that when the contact pressure is higher than 5 MPa,the key cooling rate of the material after the solution treatment can be guaranted and the good mechanical properties of the material can be achieved.The interfacial heat transfer coefficient of 7075 aluminum alloy and H13 steel was solved using a numerical method.Results indicated the heat transfer coefficient increased with increasing contact pressure and use of lubricant.The finite element simulation results of the interfacial heat transfer experiments matched well with the experimental results,which verified that the interfacial heat transfer coefficient can be used in the finite element model to precisely predict the heat transfer behavior of aluminum alloy in hot stamping process.Based on the unified viscoplastic constitutive model and the interfacial heat transfer coefficient,finite element models for 7075 aluminum alloy hot stamping process were established.Using the developed finite element software user subroutine(VUMAT),the finite element simulation model of the uniaxial thermal tensile experiment and the isothermal ball bulging experiment were setted up.As for the uniaxial thermal tensile model,the reliability of the finite element model is validated by the load-displacement data measured in the experiment,and the evolution of the strain field and the damage field in the process of tensile deformation were analyzed.The isothermal bulb bulging model was verified by comparing the thickness distributions of the experimental and simulation results.The influence of hot stamping process parameters on the formability of the 7075 aluminum alloy sheet was studied,and the damage field evolution of the specimens during the forming process was analyzed as well.A hot stamping simulation model for the door beam was built.The initial temperatures of the dies were set to room temperature to simulate the quenching process.The thickness analysis verifies the reliability of the thermo-mechanical coupled finite element model.The distributions of the strain field,the temperature field and the thickness field in the parts’ forming process were analyzed.Through the analysis and verification of the above three different finite element models,it was demonstrated that,in the hot stamping simulation of aluminum alloy,the damage evolution and cracking defects of the specimen forming can be accurately forecasted by the established unified viscoplastic constitutive model and the interfacial heat transfer coefficients,providing a theoretical guidance for the application of the aluminum hot stamping process. |
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