Study On Hot Tensile Deformation Behavior And Microstructural Mechanism Of 2195 Al-Li Alloy

2195 Al-Li alloy is widely used in the manufacture of spacecraft storage tanks abroad because of its advantages such as low density,high strength,good thermal stability,and high corrosion resistance which effectively improves the carrying capacity of the spacecraft.The domestic application of forming spacecraft large-scale devices with 2195Al-Li alloy medium-thick plate is still in the research stage and the understanding of its thermoplasticity and microstructural mechanism is still insufficient.Therefore,developing an in-depth study on the hot tensile deformation behavior and microstructural mechanism of 2195 Al-Li alloy medium-thick plates is very important to provide a theoretical basis for the formulation of the hot forming process.The high-temperature tensile experiments of 2195 Al-Li alloy were carried out on Gleeble-3800 thermal simulation test machine and the stress-strain curves of the test material under deformation conditions of 420～520℃ and 0.01～1s^-1were obtained.The results show that the flow stress decreases with the increase of the deformation temperature and the decrease of the strain rate.The elongation first increases and then decreases with the increase of the deformation temperature,and reaches the maximum at440℃.The constitutive equation of high temperature tensile deformation of 2195 Al-Li alloy was established according to the test data.The changes of fracture micro morphology and grain structure under different deformation parameters were studied through the SEM and EBSD experiments.The results reveal that,with the increase of deformation temperature,the fracture surface gradually changes from ductile fracture to brittle fracture,and the degree of dynamic recrystallization continues to increase with the increasing temperature.The grain size is smaller at lower deformation temperature,while grain growth occurs at higher deformation temperature,and the coarsening of the grains causes the decrease of the elongation.Finally,the relationship of elongation and the proportion of coarse grains was obtained according to the experimental results.The evolution of dislocations,sub-grains and precipitations under different deformation conditions were studied through TEM experiments.The influence of precipitation behavior on the dynamic recrystallization were analyzed.The study shows that,at 440℃,there are a large number ofθ°andδ°phases in the matrix under different strains,which pin dislocations and hinder the migration of grain boundaries.Therefore,a large strain（0.65）is required for obvious dynamic recrystallization at 440℃.When the deformation temperature rises to 480℃ and 500℃,theθ°phase completely dissolved,and only a small amount ofδ°phases remains,the pinning effect of the precipitated phase is significantly weakened,the migration ability of dislocation and grain boundary enhanced.Therefore,at 500℃,significant dynamic recrystallization occurs at a small strain（0.4）,and a grain growth happens at larger strain（0.48）.The thermal processing diagram of the experimental material under different strains was established based on the dynamic material model and the flow stress obtained from the thermal tensile experiment.The distribution characteristics of the power dissipation value and the rheological instability zone under different strains were analyzed,and the microstructure evolution under different deformation conditions was studied using the metallographic structure diagram.The results show that within the range of experimental parameters,the higher power dissipation value is obtained at the lower strain rate of0.01s^-1at different strains.Under the lower strains,the instability area is mainly concentrated on the deformation conditions of high strain rate and low deformation temperature.When the strain increases to 0.3,the instability area is mainly concentrated on 520℃.Based on the results of thermal processing diagrams and thermoplastic analysis,the optimal deformation process range of 2195 Al-Li alloy under high temperature tensile conditions was finally obtained:430～440℃-0.01～0.05s^-1,460～480℃-0.01～0.1 s^-1.