Research On High Strength Aluminum Alloy AA7075 Deformation Behavior And Key Technology Of Hot Forming Process With Synchronous Quenching

The hot stamping with synchronous quenching process is a new technology recently developed to improve the room temperature formability of heat treatable strengthening aluminum alloy and avoid the distortion during heat treatment after forming.It shows great advantages in controlling the forming accuracy and mechanical properties.And the key technical issuses related to this process such as the solutionized blank deformation behavior and heat treatment should be researched.The high strength aluminum alloy AA7075 has been widely applied in manufacturing airplane structure components as skins,frame,girder and et al.Experimental,theoretical and numerical analysis mehods were applied to systematically study the hot flow behavior,microstructure evolution and deformation behavior at elevated temperature and under natural ageing status and other related key technical issues for the AA7075 high strength aluminum alloy in the hot stamping with synchronous quenching process.The researches and conclusions are as following:?1?In AA7075 the hot stamping with synchronous quenching process application conditions of which the temperature range is 200-480?and strain rate range is 0.01-10 s^-1,hot uniaxial tensile tests were carried out to obtain the flow curves.The effects of temperature and strain rate were obtained.And modified phenomenological constitutive models were established to forecast the flow stress durin the hot stamping with synchronous quenching process.It shows that:?1?the modified Arrhenius type equations and Zerilli-Armstrong model can predict flow stress of the stable stage and hardening stage well respectively;?2?the modified model based on Johnson-Cook model and Misio?ek model can predict the plastic flow stress over both stages while the modified Misio?ek model is more suitable for describing the AA7075 hot flow behavior which can provide acurrate prediction with less parameters;?3?based on the continuum damage mechanic,constitutive equations considering the relative dislocation density and damage evolution was established to predict the flow stress in hardening,steady flow and damage stages.In the actual application,the constitutive model can be determined according to the requirement for predicting different flow behaviors of AA7075 in the hot stamping with synchronous quenching process.?2?Conclusions as following are alos obtained according to the study of AA7075 flow behaviors in the hot stamping with synchronous quenching process.Under 280?,the dislocation strengthening led to improved flow stress showing no strain rate effects.Above 360?,the flow stress increases with strain rate and stabilized when the balance between the dynamic recovery or recrystallization and strain strengthening reached.The temperature effect is obvious at the same strain rate.By Scanning electron microscope observation,the fracture behavior in the tensile process was discussed.The dimple depth reduced with increasing temperature and decreasing strain rate which indicating improved plasticity.And the ductility reached peak at 400?.When the temperature increased to 480?,the intergranular fracture enhanced and the ductility reduced due to the decreased bonding force between grains.?3?Electron backscatter diffraction and optical microscope were utilized to study the microstructure evolution in the AA7075 hot stamping with synchronous quenching process.It shows that the strain rate can raise the small angle grain boundary and subgrain quantity which motivate the dynamic recrystallization.At high strain rate,the grain was stretched.And uncoordinated deformation appeared which resulted in necking and low ductility due to local recrystallization and refined grains.The elevated temperature contributes to the dynamic recovery and recrystallization and the reduction of small angle grain boundary.At a given strain rate,the flow behavior is determined by the high temperature grain growth coarsening,strain strengthening,dynamic recovery and recrystallization.With increased strain,the ratio of small angle grain boundary,the dynamic recrystallization increased while the average grain size reduced,and the damage accumulated leading to dropped flow stress.?4?Quantitative analysis was performed to measure the grain size and dynamic recrystallization fraction under diffent forming conditions in the AA7075 hot stamping with synchronous quenching process.And critical strains of recrystallization were determined by plotting work hardening rate curve.The effects of temperatures,strain rates and strains on the microstructure evolution were discussed.And the empirical meodels,Yada model and Avrami equation were modified and calibrated to forcast the grain size and dynamic recrystallization fraction which provide consistent prediction with the experimental measurements.?5?Hot forming limit diagrams of AA7075 aluminum alloy in the AA7075 hot stamping with synchronous quenching process under the temperature range of 200-480?and strain rate range of 0.01-10 s^-1 were established by carrying out Nakazima test.To predict the forming limits under different deformation conditions in the AA7075 hot stamping with synchronous quenching process,based on the Marciniak-Kuzynski theory and the modified Misio?ek constitutive model,forming limit diagram prediction model was established considering of the strain rate effects in the groove section and taking the thickness ratio1)₍?8? as the flow instability creterion.Also a plane stress continuum damage mechanic model for predicting the forming limit diagram was calibrated by introducing dissipative potential function,stress state weight coefficients and multiaxial stress damage index based on the continuum damage mechanic constitutive equations.Under different strain paths,the grain size,dislocation density and texture orientation changed after hot deformation.Also,the fracture mechanic varied with temperature and strain rate leading to the forming limit prediction accuracy loss of the plane stress continuum damage mechanic model.And the results showed that the modified Marciniak-Kuzynski theory model can provide better predictions for AA7075 forming diagrams under different termpratures and strain rates by comparing the experimental forming limit diagrams which can be used in predicting the AA7075 forming limits in the hot stamping with synchronous quenching process,?6?Uniaxial tensile test,3-point bending test,Nakazima forming limit test and limit drawing ratio test were carried out to study the deformation behavior evolution of AA7075-W under different ageing times after hot stamping with synchronous quenching process.A constitutve model with voce hardening was calibrated to calculate the flow curves under different natural ages.The formability under natural ageing times were discussed.And it is concluded that the following forming step should be performed in 8 hours after hot stamping with synchronous quenching process.?7?With the hot tensile test results and the calibrated constitutive model,the material properties were defined for the thermos-mechanical coupled finite element analysis for the hot stamping with synchronous quenching process.With an optimized finite element model,the hot stamping with synchronous quenching process of an automotive B-pillar part was simulated.The forming results were evaluated by comparing the predicted strain distribution with the forming limit curves calculated with the Marciniak-Kuzynski model.And optimized process parameters were obtained for the hot stamping with synchronous quenching process.The hot stamping with synchronous quenching process was carried out to forming the actual B-pillar part.The formed part met the design requirement.It verified the effectiveness of the hot flow behavior characterization,the constitutive model and the forming limit diagram prediction model.The feasibility of the hot stamping with synchronous quenching process in forming complicate shape parts with controlled dimention accuracy and material properties is also demonstrated.The high strength aluminum alloy AA7075 flow behavior in the hot stamping with synchronous quenching process and relevant key technologies obtained in this study can provide new solutions for AA7075 product manufacturing process with coordinated shape accuracy and material properties which shows promoting effect for precision forming technology in aeronautics and astronautics and other areas.