| As light structural materials,7X50 series aluminum alloys have been widely used in fields of aerospace,transportation and weapon industries due to their excellent comprehensive properties,such as excellent specific strength,high specific stiffness,good fracture toughness and corrosion resistance.In decades,the as-demanded section size and thickness of 7X50 plates are increasing due to the large-scale and integration of aviation components.The comprehensive properties of these alloys are closely related to their precipitation characteristics,which are directly affected by quenching processes following solid solution treatment.Therefore,quenching processes are the key point to determine comprehensive properties of 7X50 alloys with large section size.In recent years,numbers of studies have been performed on optimizing quenching processes to control properties of Al-Zn-Mg-Cu super-high-strength aluminum alloys.However,researches on heterogeneous precipitation behavior in quenching or cooling processes are not systematic,and due to the limitation of thermocouple installation method,there may be some deviations in measured data of quenching temperature field,which can not truly and accurately reflect the heat transfer process on quenching surfaces.Therefore,in present work,different quenching processes after solid solution treatment,including isothermal treatment followed by water cooling,Jominy water-spray quenching and room temperature air cooling were applied to study their effects on precipitation behaviors,quenching temperature field and quenching stress field in hot-rolled 7050 and 7B50 plates.Time-Temperature-Properties(TTP)curves of 7050 and 7B50 alloys were plotted to study their quench sensitivity.Besides,effects of different isothermal processes on heterogeneous/homogeneous precipitation behaviors were discussed while,hardenability of 7B50 alloy was studied and quench factor analysis(QFA)method was used to predict the property distribution along the thickness direction of 7B50 thick plates after quenching and aging.In addition,synthetic surface heat transfer coefficient(SSHTC)curves of quenching surfaces were calculated using the inverse heat transfer theory based on measured cooling curves at different distances from the quenching surface of the modified Jominy specimen of 7B50 alloy.Using the SSHTC curves as heat transfer boundary conditions,coupling analysis of temperature field,stress field and strain field during on-line quenching processes of 7B50 thick plates was carried out.Principal conclusions are as follows:(1)Electrical conductivity data of specimens in water-cooled temper treated by different isothermal regimes following solid solution treatment can truly reflect the degree of dissolution of 7X50 alloys during the isothermal processes.The TTP curve with 0.5%transformed fraction of 7050 alloy plotted by electrical conductivity in water-cooled temper indicates that the nose temperature is 330℃ and the corresponding incubation period is 1.25 s.While the TTP curve with 0.5%transformed fraction of 7050 alloy plotted by properties(electrical conductivity or hardness)in T6 temper indicates that the nose temperature is also 330℃ but the corresponding incubation period is shorter,which is only 0.62 s.(2)When 7X50 alloys are isothermal treated at 180～400℃ after solid solution treatment,the exponent n in Johnson-Mehl-Avrami equation is always close to 1 until the transformed fraction rises to 60%.It indicates that the nucleation mechanism and growth mode of precipitates do not change in this period,which are classical nucleation mechanism and growth mode.(3)Electrical conductivity of 7X50 alloys in water-cooled temper after isothermal treated at 180℃ and 200℃ for a short time following solid solution treatment is lower than that of the standard specimen in as-quenched temper,which is directly quenched after solid solution treatment.Meanwhile,hardness in natural aging temper of these isothermal specimens is higher than that of the standard specimen.Therefore,the isothermal treatment in this temperature range for a short period can promote formation of strengthening precipitates.Not only heterogeneous η precipitates,but also some strengthening precipitates such as GP zones and η’ precipitates are present in matrix of the specimens in natural aging temper after isothermal treatment and then water cooling.When the average size of strengthening precipitates is 3-5 nm in matix of 7X50 alloys in natural aging temper,the effect of precipitation hardening is remarkable.(4)The modified Jominy specimen and its supporting experimental equipment designed in this study can accurately determine cooling curves at different positions in the specimen during water-spray quenching,which can provide a reliable basis for predicting and simulating quenching temperature field,stress field and properties distribution in workpieces with large section size after quenching and aging.(5)When the modified Jominy specimen of 7B50 alloy is spray quenched with water at 9℃ after solid solution treatment,the hardened depth is about 70 mm in natural aging temper while it decreases to 60 mm after natural aging for 50 days followed by T6 artificial peak aging treatment with the corresponding average cooling rates in quench sensitive temperature range increasing from 1.62℃·s-1 to 2.05℃·s-1,respectively.When the quench factor analysis method is used to predict hardness distribution of thick plates in T6 temper,the deviation between predicted and measured hardness is less than 2.7%when distance from the quenching surface is less than 65 mm,showing excellent precision of prediction results.(6)During quenching processes of 7B50 alloy from its solid solution temperature,heterogeneous precipitates firstly nucleate on grain or subgrain boundaries and then nucleate on A13Zr particles or dislocations in matrix.Heterogeneous precipitates have been found on grain/subgrain boundaries when the average cooling rate in quench sensitive temperature range is as high as 981℃·s-1,and heterogeneous η precipitates have been found nucleation on Al3Zr particles when the average cooling rate is 37.2℃·s-1.(7)The synthetic surface heat transfer coefficient(SSHTC)is calculated using the inverse heat transfer theory based on measured cooling curves at different positions of the modified Jominy specimen of 7B50 alloy during water-spray quenching with water at 20℃and 9℃.Peak-values of the SSHTC are both obtained at 0.4 s in spray quenching processes,which are 69 kW·m-2·K-1 and 135 kW·m-2·K-1,the corresponding temperatures of the quenching surface are 160℃ and 80℃ respectively,and then the value of the SSHTC drops rapidly with increasing of quenching time.(8)When the modified Jominy specimen of 7B50 alloy is spray quenched by water at 20℃ from its solid solution temperature,the "temperature plateau" appears in the cooling curve at center point of the quenching surface in the initial stage of water-spray quenching.The temperature range of this plateau is 160～170℃ and the plateau lasts about 3 s.During the temperature plateau" period,heat transfer mechanism on the quenching surface rapidly transforms from nucleate boiling regime to single-phase convective regime,which is the main reason for the "temperature plateau" phenomenon.(9)Distributions of residual stress in as-quenched 7B50 thick plates with size of 5000 mm×1500 mm×80 mm are calculated by coupling analysis of temperature field,stress field and strain field with the heat transfer boundary condition which is the SSHTC data of the surface spray quenched by water at 20℃.The maximum compressive residual stress appears on quenching surfaces of the plate,which are σx=-313 MPa and σy=-283 MPa,while the maximum tensile residual stress appears at center of the plate,which are σx=192 MPa and σy=127 MPa.The compressive residual stress changes to the tensile one at 15 mm from quenching surfaces. |
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