Study On The Ductile Damage Model Of 7075 Aluminum Alloy At Elevated Temperature

Growing concerns about the global environment,particularly CO₂ emissions from cars,are driving the use of low-density lightweight alloys and thinner advanced high-strength steels in the automotive industry.High-strength aluminum alloy is one of the development directions of automobile lightweight.Aluminum alloys can reduce body weight by more than 40%or improve fuel efficiency by about 30%compared to steel and offer advantages in terms of weight and strength.There are a number of issues that need to be faced before aluminum alloys can be successfully used in the automotive industry,poor formability and premature failure at room temperature.In case of improving the formability of aluminum alloys,forming of aluminum alloys at elevated temperature is more promising.In the study,the stress-strain curves and fracture behavior of 7075 aluminum alloy at different temperatures and strain rates were studied.The high temperature flow stress-strain models and fracture models for 7075 aluminum alloy were established.The hot stamping test of the T-shaped part was carried out,and the numerical simulation of the forming process was implemented.The main research contents and results of this study are as follows:（1）Tensile test of 7075 aluminum alloy at room temperature.First,in order to obtain the mechanical properties of 7075 aluminum alloy at room temperature,tensile tests were performed with standard dog-bone specimens,which provided stress-strain data for simulations at room temperature.Tensile tests were then performed on the specimens under different stress states to determine the effect of the stress state on the fracture of the 7075 aluminum alloy.（2）In order to obtain the flow stress-strain curve at high temperature,the hot compression test of 7075 aluminum alloy was carried out at the deformation temperature of 573-723 K and the strain rate of 0.01-5 s^-1.High temperature compression tests are affected by temperature rise and friction.The original stress-strain data were modified to obtain the stress-strain curve under ideal test conditions.Flow stress-strain curves were predicted by three flow stress-strain models.The three flow stress-strain models are the Johnson-Cook（JC）model,the modified Johnson-Cook（MJC）model and the two-stage constitutive models.（3）Hot tensile tests were conducted on 7075 aluminum alloy at deformation temperatures of 623-723 K and strain rates of 0.01-1 s^-1to investigate the effects of temperature and strain rate on fracture.Temperature and strain rate were coupled together through the Z parameter.And the high temperature fracture model was established considering Z parameters.Under high temperature conditions,dynamic recrystallization occurred in aluminum alloys.（4）A material user subroutine for numerically analyzing the hot stamping process of AA7075 was proposed in ABAQUS software.The calibrated the two-stage constitutive models and fracture models were embedded in the finite element simulation software.With the model,the effect of strain,dynamic recrystallization,and strain rate on recrystallization and deformation behavior were considered.The established model accurately predicted the damage position of the T-shaped workpiece and verified the validity and reliability of the established model.