Investigation On Damage Coupled Viscoplastic Constitutive Model Of High Strength AA7075 And Forming Simulation In Hot Stamping Process

Automotive lightweight is one of the main ways to achieve energy saving and emission reduction.High strength aluminum alloy automotive structural parts can not only meet the strength requirements,but also reduce the weight of automobile body to a large extent,and become one of the most ideal materials in automotive lightweight technology.Among them,AA7075 aluminum alloy not only has the characteristics of low density,good corrosion resistance,but also has high specific strength and specific stiffness,so it is gradually used in the manufacturing of automotive structural parts.However,the forming property of AA7075 aluminum alloy can be improved by hot stamping process because of its poor formability,easy springback and fracture at room temperature.At present,there are still some key problems in the hot stamping process of aluminum alloys,such as the hot deformation behavior of sheet metal,the forming quality of hot stamping and the evolution law of microstructure of hot forming parts.In this paper,AA7075 aluminium alloy hot deformation behavior and microstructure evolution rule was studied,based on damage evolution mechanism of the viscoplastic constitutive model.It revealed the hot stamping process in the hot deformation behavior and microstructure evolution of materials.The typical hot stamping process of T-shape component was simulated.The main contents and related conclusions are as follows:(1)The hot deformation behavior and microstructure evolution of AA7075 aluminum alloy were revealed.The hot deformation behavior of AA7075 aluminum alloy was investigated by Gleeble 3500C thermal simulation experimental machine.The stress-strain curves at different deformation temperatures(350?,400? and 450?)and different strain rates(0.01 s-1,0.1 s-1 and 1 s-1)were obtained.The effects of deformation temperature and strain rate on the flow stress of AA7075 aluminum alloy at high temperature are illustrated.The results show that the flow stress increases with the increase of strain rate,while the flow stress decreases with the increase of temperature.Fracture strain decreases with increasing temperature and increases with increasing strain rate.By analyzing the fracture morphology of AA7075 aluminum alloy at 350? and 450?,the damage evolution mechanism of hot deformation was further revealed:the fracture mechanism at 350? is typical ductile fracture,and the quasi cleavage fracture at 450?.(2)The viscoplastic model coupled with damage of AA7075 aluminum alloy was studied.Based on the uniaxial thermal tensile test of AA7075 aluminum alloy,the dislocation density evolution equation was modified.The viscoplastic constitutive model of coupling damage was established by considering the effects of deformation temperature,strain rate,damage,grain size and dislocation density.The model parameters were calibrated based on genetic algorithm.Compared with the experimental results,the model can well describe the hot deformation behavior of AA7075 aluminum alloy at high temperature.(3)The hot deformation and microstructure evolution of AA7075 aluminum alloy were simulated.Based on ABAQUS/VUMAT platform,the finite element model of AA7075 aluminum alloy uniaxial hot tension was established.By comparing the load-displacement and grain size evolution between simulation and experiment,the reliability of the finite element model was verified,and the damage evolution law of AA7075 aluminum alloy in hot tensile process was predicted.(4)The finite element simulation of typical AA7075 aluminum alloy T-shaped parts hot stamping was carried out.Based on the hot stamping experiment of T-shaped parts,the finite element model of T-shaped parts was established.The distribution of temperature of and plastic strain in hot stamping process of T-shaped parts was analyzed,and the evolution of microstructure of T-parts was predicted accurately.The high temperature point was mainly located in the center of the T-shaped parts.The plastic strain mainly occurs in the rounded corner of the punch and concave die,and the strain of punch fillet was larger than that of die fillet.Due to the large strain of sheet material at the corner of punch and concave die,the thinning rate in the punch corner of T-shaped parts was higher.During the hot forming process of T-shaped parts,the damage was mainly distributed in the punch fillet.The dislocation density evolution of T-shaped parts mainly occurs at the rounded corners of punch and concave dies.When the deformation temperature increases,the dislocation density of T-shaped parts tends to decrease.