Servo Hot Stamping Process And Property Regulation Of High Strength Aluminum Alloy Automotive Parts

Automotive lightweight is a national major science and technology demand,and also an important development direction of automobiles.One of the main methods for reducing the weight of automotive is applying lightweight materials,such as aluminum alloys,to the car body.However,when a aluminum alloy sheet is formed at room temperature,the defects such as cracks and wrinkles are easily caused because of the poor formability.By using the hot stamping process,the aluminum alloy has a better material flowability and the forming performance is improved to some extent.Since aluminum alloys have large heat dissipation coefficient,there is a rapid temperature drop and uneven temperature distribution when aluminum alloys at high temperature.Servo hot stamping process is used to control the stamping speed accurately according to the mechanical properties of the sheet at high temperature,which can further improve the forming performance of the material.In this paper,the servo hot stamping process of 7075-T6 high-strength aluminum alloy is studied by finite element simulation and experiments.The main contents are as follows:(1)The 7075-T6 high-strength aluminum alloy sheet is used in this paper,and the true stress-strain curves under different temperatures and different strain rates are obtained by isothermal uniaxial tensile test.The effect of temperature and strain rate on the true stress-strain curve is analyzed.It is found that the strain rate has a significant effect on the flow stress at high temperature.Based on the viscoplastic constitutive model coupled with dislocation density,a high-temperature constitutive model for high-strength aluminum alloys is established.The material constants of the high-temperature constitutive model are calculated and optimized by using the forward Euler integral method and genetic algorithm.The fitness function is used to verify the model’s accuracy.Based on this model,the true stress-strain curves of 7075-T6 high-strength aluminum alloy at other temperatures are predicted,which is beneficial to the aluminum alloy servo hot stamping finite element model.(2)A servo speed control method based on jerk is proposed,by which continuous adjustment of displacement,velocity,acceleration and jerk can be achieved.According to different jerk,six servo process curves are designed,and the effects of different servo process curves on the hot forming performance of cylinder and square box parts are analyzed by ABAQUS finite element numerical analysis software.The results show that in the cylinder part servo hot stamping,the forming performance under servo process curve 3,4 and 5 is the best.In the square box part servo hot stamping,the forming performance under the servo process curve 4 and 5 is the best.(3)The high-strength aluminum alloy B pillar servo hot stamping finite element model is established,which includes gravity loading,forming,pressure keeping,cooling and edge punching.And the influence of different servo process curves on the forming performance of high-strength aluminum alloy B pillar is analyzed.Under the servo process curve 3,4 and 5,the change of forming force is gentler and the maximum value is smaller,which indicates that the servo process can reduce the tonnage requirements of the press.Under the servo process curve 4 and 5,the thinning of the sheet at the danger point is smaller,the temperature decreased more slowly,the change of the maximum plane principal stress and strain is also more gradual,and the forming performance of the sheet becomes better.(4)The servo hot stamping test for high-strength aluminum alloy B pillar is done,and the influence of the servo hot stamping process on the performance of specimens is analyzed.Under the servo process curves 2 and 3,the specimens are cracked at the dangerous point;under the servo process curve 6,there is a noticeable necking at the dangerous point;under the servo process curves 4 and 5,the specimens are not cracked.Under the servo process curve 5,the specimen has the best forming accuracy(deviation is less than 2.187mm),the thickness of dangerous section is higher(thickness is greater than 1.38mm),and the hardness is also higher(HV is greater than 57.8HV).The experimental results show that using a reasonable servo stamping speed can effectively control the forming performance,geometric accuracy and mechanical properties of high-strength aluminum alloy parts with complex profiles.