| As an important lightweight and high-strength structural material,7055 aluminum alloy has been widely used in aerospace and other fields.At present,the alloy with large-scale sheet still has a problem of poor uniformity along the thickness direction,and it is necessary to systematically study the influence of processing and heat treatment processes on its deformation behavior,microstructure and properties.Therefore,this paper takes 7055 aluminum alloy semi-continuous ingot and solid solution pre-stretched plate as the research material,the hot deformation behavior,rolling process and aging system under different processing techniques of 7055 aluminum alloy were studied through thermal simulation deformation and heat treatment.The test instruments using in this paper are scanning electron microscopy?SEM?,electron backscatter diffraction?EBSD?,Transmission electron microscopy?TEM?and three-dimensional atom probe?3D AP?and other characterization techniques.This paper focusing on research the influence of the processing technology and the deformation processing method to improve the uniformity of the thick plate,and the influence of the subsequent heat treatment on the properties of the alloy which provide a theoretical basis for solving the problem of unevenness of the alloy thick plate.The main findings are as follows:?1?The thermal deformation behavior of 7055 aluminum alloy under traditional thermal deformation process?NTMP?and two-stage thermal deformation process?DTMT?was studied by thermal simulation experiment.The flow stress curves under different deformation processes were obtained.The microstructure of the DTMT process on the flow stress,activation energy and thermal processing diagram,and the microstructure and recrystallization mechanism of the alloy under different processes were discussed.The results show that the flow stress and recrystallization activation energy of DTMT process are higher than NTMP,and the suitable processing region of NTMP is strain rate of 0.01s-1-0.1s-1 and temperature of 430°C-450°C;It is processed in a region of DTMP where the strain rate is 0.01s-1-0.08s-1 and the temperature is405°C-450°C.Under each deformation process,the degree of recrystallization of the alloy increases with the increase of temperature;when the compression conditions are certain,the percentage of recrystallization ratio of DTMT is higher than that of NTMP,which is caused by more sub-grains in DTMT.The difference in microstructure before deformation of the alloy leads to different recrystallization types.Both processes of recrystallization types are mainly discontinuous dynamic recrystallization?DDRX?and continuous dynamic recrystallization?CDRX?,but the proportion of DDRX in DTMT is higher than NTMP.In addition,there is geometric dynamic recrystallization?GDRX?in NTMP and meta-dynamic recrystal ization?MDRX?in DTMT.?2?The thermal deformation behavior of 7055 aluminum alloy under high temperature solid solution thermal deformation process?HTMP?was studied by thermal simulation compression.The influence of cooling rate on the microstructure and flow stress of the alloy was analyzed.The flow stress under each deformation condition was investigated.Thermal processing map was established based on the corrected data after temperature correction.Through the microstructure analysis,the influence of deformation conditions on the microstructure was studied.The results show the number,size and peak flow stress of precipitated phase decrease after high temperature solid solution with the increase of cooling rate;the ratio of continuous dynamic recrystallization?CDRX?decreases gradually,and the recrystallization ratio decreases gradually.When the cooling rate is constant,the main recrystallization type gradually changes from CRDX to discontinuous dynamic recrystallization?DDRX?as the strain rate increases.As the strain increases,the instability zone gradually expands;the most suitable processing zone is the domain that strain rate of 0.01s-1-0.1s-1 and the temperature of 380°C-430°C.?3?Based on the actual production process of aluminum alloy thick plates,the effects of rolling mode?synchronous rolling and asynchronous rolling?a nd allometric ratio on the uniformity of high-direction deformation of sheet metal in single pass deformation were studied.The effect of the method?synchronous rolling and asynchronous rolling?and the rolling process?pass reduction setting?on the uniformity of the sheet was explored in multi-pass rolling deformation.The results show that the asynchronous rolling increase the strain of the core of the sheet and the degree of deformation of the core of the sheet increases significantly with the increase of the isokinetic ratio.After multiple passes of asynchronous rolling,the high direction of the thick plate and the lateral uniformity of the surface layer are obviously increased;however,the lateral uniformity of the core is worse than the synchronous rolling,which is caused by the exchange of the upper and lower rolls during the asynchronous rolling process.Non-alternating asynchronous rolling can significantly reduce the high-direction uniformity of the slab,but the difference between the upper and lower surfaces of the slab increases after rolling in this way.The ratio of the different reductions of the pass has an effect on the final uniformity of the sheet.Within the rolling capacity of the mill,increasing the pass reductio n is beneficial to obtain a more uniform thick plate performance.?4?The effects of heating rate,cooling method,regression regime?regression temperature and time?on the properties of the alloy were investigated.The properties and microstructure of the alloy were characterized at each stage of aging.The results show that the slow temperature rise helps to obtain good mechanical properties,while the cooling rate has no obvious effect on the properties of the alloy.The regression treatment regime that can meet the production requirements of the factory is 175°C/1.5 h.The pre-aging stage is mainly the precipitation and growth of GP zone and?',the regression treatment stage is re-dissolution of GP zone and coarsening of?',and the re-aging stage is the precipitation and growth of GP,?'and?.The pre-aging temperature has a greater influence on the final precipitate phase size than the re-aging temperature;the most suitable RRA regime for the actual production of the plant is120°C/24h+175°C/1.5h+120°C/24h with heating rate of 35°C/h and air cooling.As the aging progresses,the Al content in the precipitated phase decreases first and then increases,and the Zn and Mg content increases first and then decreases,while the Cu content increases.As the aging progresses,Zn/Mg gradually decreases in the precipitated phase due to the different diffusion coefficients of solid solution elements in Al,and Cu/Zn gradually increases.As the size of the precipitated phase increases,Zn/Mg gradually increases and Cu/Zn gradually decreases.This is mainly due to the difference in diffusion coefficients of solid solution elements in Al.After treatment at different cooling rates,the performance of the alloy increases first and then decreases with the increase of the cooling rate,which is mainly due to the influence of the cooling rate on the precipitation phase of the aloy. |
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