Effect Of Medium Temperature Deformation And Heat Treatment On Microstructure And Properties Of 2195 Al-Li Alloy

2195 Al-Li Alloy has excellent properties such as low density,high specific strength and specific rigidity,and is an important development direction of structural materials for transition ring of launch vehicle storage tank.However,lithium is very active.Al-Li alloys have many precipitated second phases,which lead to very complex evolution of structure and properties after forming and heat treatment.There are some problems such as non-uniform structure and low comprehensive properties during conventional thermal processing.In this paper,a new idea for adjusting the structure and properties of alloy by medium temperature deformation and heat treatment is put forward.Basic experiments of thermal compression simulation and heat treatment of 2195 Al-Li alloy and simulation experiments of forming and heat treatment of Al-Li alloy ring are carried out.The main research work and achievements in this paper are as follows:(1)Recrystallization regularity of 2195 Al-Li Alloy at medium temperature(160-330℃)was studied and recrystallization grain size model was established.Deformation interval of refined grain structure of2195 Al-Li Alloy was obtained.The results show that the recrystallized grain size decreases with decreasing deformation temperature and increasing deformation amount.When the deformation temperature is330℃,the coarse grains can not be refined effectively only by recrystallization.When the deformation temperature is lower than 240 ℃and the deformation amount is more than 30%,uniform,fine and equiaxed recrystallization structure can be obtained,and the grain size is concentrated in 30-40 μm;In addition,under the same deformation parameters,the grain equiaxing tendency of pre-aging state and annealing state after medium temperature deformation is obvious.The recrystallization grain can be further refined by increasing the heating rate,and the grain size is concentrated in 15 μm.(2)The evolution rule of precipitation phase and properties under the synergistic action of medium temperature deformation and heat treatment was studied,and the better T8 heat treatment regime of 2195 Al-Li alloy at medium temperature was obtained.The results show that the solution temperature is 550 ℃,the structure is overheated and the strength and elongation decrease obviously.With the increase of cold deformation amount,the strength of the alloy increases,and the strength increases by30 MPa with 3% cold deformation.The hardening phases for peak aging under different aging regimes(143 ～170 ℃)are all T1 phases.With the increase of aging temperature,the density of precipitated phase is the highest and the size is reduced.The current system was 160 ℃×24h,the length of T1 phase is about 89 nm,and there is a narrow PFZ at the grain boundary,which is about 48.3 nm.Under the better T8 heat treatment,the tensile strength,yield strength and elongation of the alloy are 549 MPa,508 MPa and 8.0%.(3)Simulated experiments on forming and heat treatment of 2195 AlLi alloy ring were carried out.The results show that the medium temperature process accumulates a large amount of storage energy in the form of high-density dislocations,which is helpful to promote recrystallization and crush coarse second-phase particles.Compared with the high temperature process at 480 ℃,the grain structure refined obviously after the combination of medium temperature process and T8 heat treatment,and the grain size concentrates on 40-50 μm,the second phase particles are well distributed,resulting in good structure uniformity,significantly improved three-dimensional mechanical properties,average increase of axial strength by 12 MPa,average increase of radial and circumferential strength by 20 MPa,and increase of radial elongation by55.5%(increased by 2.5%).