| 7050 aluminum alloy is a typical high-strength Al-Zn-Cu-Mg alloy,which has a wide application prospect in aerospace,rail transportation,national defense and other fields.At present,there are few researches on backward extrusion forming and subsequent solution aging heat treatment process of 7050 aluminum alloy,and the influence rule of process parameters of backward extrusion and solution aging on microstructure,mechanical properties and corrosion resistance is not clear yet,which is difficult to provide theoretical basis for the actual production of reverse extrusion and heat treatment of medium-high strength aluminum alloy.In this paper,a large size 7050 aluminum alloy cup forming device was designed,combined with experiment and numerical simulation.Effect of extrusion temperature on the microstructure,mechanical properties and corrosion resistance of extrusion direction(ED)and lateral(TD)was studied.The solution and aging heat treatment of different systems were carried out,the change of grain and second phase and aging precipitation behavior during solution process were analyzed,and the feasibility of further improving mechanical properties and corrosion resistance of reverse extrusion parts was studied.The research results are of great significance to the formulation of high strength aluminum alloy backward extrusion and solution aging process,and enrich and develop 7xxx aluminum alloy extrusion technology,which has important engineering application value.The main work and conclusions of this paper are as follows:(1)Backward extrusion experiment and numerical simulation under different extrusion temperatures were carried out for large-size 7050 aluminum alloy cup parts.The numerical simulation results show that the equivalent strain and temperature field increase with the increase of extrusion temperature.The microstructure and mechanical properties of the back extruded parts in the ED and TD directions were analyzed.It was found that the grains in the ED and TD directions were fibrous and the texture was mainly deformed when the extrusion temperature was lower.There are more elongated S phase and a small amount of coarse Fe phase and fine MgZn2 phase in ED direction,and a second phase like round in TD direction.With the increase of extrusion temperature,the dynamic recrystallization and grain growth became more obvious,which reduced the grain elongation caused by plastic deformation.The proportion of Cube and Goss recrystallization texture increased,and more fine second phase appeared in the matrix.With the increase of extrusion temperature,the tensile strength of the backward extrusion parts increases,and the elongation and intergranular corrosion resistance change little in ED direction,while the elongation decreases in TD direction,but the intergranular corrosion resistance increases.(2)Heat treatment of backward extrusion 7050 aluminum alloy with different solution time was carried out to study the effect of solution time on microstructure,mechanical properties and friction and wear resistance.The results show that with the increase of solution time,the grain growth in ED direction is obvious,but the grain growth in TD direction is slight.After solution for 15 min,recrystallized grains appeared in the sample at 350℃.Recrystallized grains appeared in the sample at 400℃ after solution for 60 min.No recrystallized grains were found in the specimens with backward extrusion temperature of 450℃ after 120 min solution.When the solution time is short,the second phase with larger size remains in the matrix.With the increase of solution time,more of the second phase dissolves into the matrix,but the large Fe phase does not dissolve.The tensile strength of the backward extruded parts increases with the extension of solution time.When solution time is 3 min,the tensile strength is longer than that without heat treatment,and the tensile strength increases significantly with solution time.When the solution time is 3 min,the elongation of the specimens is the highest.With the extension of solution time,the elongation fluctuates greatly.The friction coefficient increases firstly and then decreases with the solution time increasing,and reaches the maximum value when the solution time is 60 min.The grinding loss weight is the lowest when the solution time is 15 min,that is,the wear resistance is the best when the solution time is 15 min.With the increase of extrusion temperature,friction coefficient decreases and wear resistance increases.(3)The effects of different aging regimes on the precipitation and corrosion resistance of backward extrusion parts obtained at 350℃ were studied.The results show that the precipitated phase of T6 sample is dominated by η’phase and contains a small amount of GP region.The precipitated phase has the smallest size,the worst GBPs continuity,the smallest PFZ width,and the worst corrosion resistance and stress corrosion resistance.In T73 sample,the number of η phase is the largest,the size of precipitated phase in grain is larger,GP region disappears,GBPs shows continuous network distribution,PFZ width is larger,and corrosion resistance and stress corrosion resistance are improved.η phase and η’phase precipitate inside the grain of T74 sample.The η phase at grain boundary is mainly plate shape,the number of η phase is small,GBPs continuity is the worst,PFZ width is the largest,and the corrosion resistance and stress corrosion resistance are the best.In acidic corrosive medium,T74 sample has the worst corrosion resistance,while T6 sample has the best corrosion resistance. |
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