Investigation On Thermal Forming Limit Diagram And Thermal Fracture Criterion Of 7075 Aluminum Alloy

With the continuous improvement of energy saving and emission reduction standards,the process of lightweight of vehicles has continued to advance.Naturally,aluminum alloys are more and more widely used in automobile manufacturing due to their high specific strength and light weight.At the same time,the solution-quenching-forming integrated hot stamping process of aluminum alloy effectively solves the problems of springback and cracking when forming aluminum alloy parts,and creates conditions for the production of complex and high-strength structural components of aluminum alloy sheet.However,the optimization of the hot stamping process parameters and the design of the mold surface need to be carried out with the help of finite element methods.Therefore,an accurate material model is particularly important to accurately predict the behavior of necking,flow and fracture of the metal at high temperature.In this paper,based on 7075 aluminum alloy sheet,and a series of investigations were carried mainly concrete on the forming limit,constitutive model and fracture model of 7075 aluminum alloy at high temperature.Using a biaxial stretching device,combined with induction heating and a DIC strain measurement system,a system for measuring thermal forming limit diagram of sheet metal under hot stamping conditions has been developed.Based on this system,a series of cross-shaped and unbiaxial tensile specimens with different linear strain paths are designed.Finally,the thermal forming limit diagrams of 7075 aluminum alloy under hot stamping conditions were obtained.Compared with the thermal forming limit diagrams obtained by other traditional experimental methods,the reliability of the method was finally proved.The high temperature unbiaxial tensile experiments at different temperatures and strain rates were used to study the high temperature flow behavior of 7075 aluminum alloy,and the true stressstrain curves under different conditions were obtained.And the Misiolek constitutive model is used to characterize the high temperature flow behavior of 7075 aluminum alloy,and a modified form of temperature and strain rate is proposed.The results show that the model has a good predictive ability.The DF2012 model is used to predict the high temperature fracture behavior of 7075 aluminum alloy.Through high temperature tensile tests of aluminum alloy specimens with different stress states,combining with the finite element simulation method to solve the material parameters of the fracture model at different temperatures,the two-dimensional fracture path and three-dimensional fracture surface were obtained.Using this fracture model to simulate the high-temperature box deep drawing of 7075 aluminum alloy,the results show that the fracture model can accurately predict the fracture behavior of 7075 aluminum alloy sheet.