Effects Of Solution Treatment And Prestretching On Microstructures And Properties Of Al-Zn-Mg-Cu Alloy

The high strength/weight ratio of Al-Zn-Mg-Cu alloys make them important lightweight materials for manufacturing aircraft structural components.The proper heat treatment of these alloys involves solution treatment,quenching and aging to obtain the best combination of strength,fracture toughness and corrosion properties.Controlling the process parameters to produce the optimum microstructures and properties for Al-Zn-MgCu alloys has always been a focus of research in industry and academia.For solution treatment,most researchers have attempted to increase the solution temperature or prolong the solution time to maximize the supersaturation of solute elements.They have focused less on the dissolution of precipitates,recovery and recrystallization that take place during the heating process.For thick plates,pre-stretching is necessary to reduce the quench-induced residual stress.However,its influence on the formation of GP zones during natural aging is rarely reported and it is still controversial whether the prestretching has a positive or negative effect on the precipitation behavior during artificial aging in Al-Zn-Mg-Cu alloys.Two commercial Al-Zn-Mg-Cu alloys,AA7075 and AA7085,were used in the present work.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD),small-angle X-ray scattering(SAXS),hardness test,electrical conductivity(EC)test,tensile test,and corrosion resistance test were used to characterize the effects of process parameters on the microstructures and mechanical properties.Critical parameters such as heating rate to the solution heat treat temperature,quenching rate and pre-stretching were studied.The purpose of this work is hopefully to provide guidance for optimizing the heat treatment processes of Al-Zn-Mg-Cu alloys.This work systemmatically explores: 1)the influence of the dissolution of coarse precipitates on recovery,recrystallization and the final properties by means of studying the microstructural evolution during the heating process of solution treatment at different heaing rates;2)the effect of defects such as vacancies and dislocations introduced by pre-stretching on the formation of GP zones during natural aging by applying the excess vacancy model;3)the effect of pre-stretching on the precipitation behavior and mechanical property of AA7085 alloy by combining the excess vacancy model and KWN model.The conclusions are summarized as follows:(1)The heating rate to the solution heat treatment temperature significantly affects the grain structures of Al-Zn-Mg-Cu alloys.Both fast heating and slow heating produced elongated grains in AA7075 and AA7085 alloy,but the grains of the fast-heated sample are much finer than that of the slow-heated sample.It is attributed to the lower amount of recovery and higher amount of η phase left during the fast heating process.Less recovery results in a higher driving force and more sites for the nucleation of recrystallization.The remaining η phase,small constituents and dispersoids exert a pinning effect on(sub)grain boundaries and hinder the grain growth.(2)The tensile properties and corrosion resistance of Al-Zn-Mg-Cu alloy are controlled by the microstructures.The fast-heated AA7075 and AA7085 alloys have weaker textures and slightly elongated grains after solution treatment,while the slowheated samples have stronger textures and seriously elongated grains.After artificial aging,therefore,the anisotropy index,Δr,and variation in tensile strength of the fastheated AA7075 and AA7085 alloy tested along different directions is smaller than that of the slow-heated samples.The resistance to stress corrosion and intergranular corrosion of the fast-heated alloy is also better than that of the slow-heated alloys since the area fraction of precipitates on grain boundaries for the fast-heated alloy with finer grains is higher than that of the slow-heated alloy with coarser grains.(3)The precipitation strengthening of Al-Zn-Mg-Cu alloys is significantly affected by the quenching rate.For AA7075 and AA7085 alloys,the quenching rate of both cold water and 60℃ water are greater than their critical quenching rates,so the decomposition of supersaturated solid solution did not occur during the quenching process.However,the quenching rate of boiling water and still air is lower than the critical quenching rate,resulting in the formation of equilibrium η phase within grains and on grain boundaries as well as the precipitate free zones.The samples quenched into cold water and 60℃ water obtain the best mechanical properties after artificial aging.The formation of equilibrium η phase during quenching into boiling water and air consumes the solutes for the precipitation of η′,causing reductions in hardness and strength.(4)During the natural aging process,the pre-stretching reduces the incubation time for nucleation of GP zones and the age hardening rate.These are related to the dislocations and excess vacancies introduced by the pre-stretching.The vacancy concentration calculated by an excess vacancy model and the dislocation density after stretching increases with the pre-stretching amount.In the initial stage of natural aging,incubation time for nucleation of GP zones is reduced since the diffusion of solutes is accelerated by excess vacancies and dislocations.However,the dislocations introduced by pre-stretching act as vacancy sinks and reduce the vacancy concentration.The decrease in vacancy concentration leads to the reduction of age hardening rate during natural aging.(5)The formation of GPI zones and η′ phase significantly increase the hardness of AA7085 alloy during the heating process of artificial aging,but the age hardening rate of the unstretched alloy is higher than that of the stretched alloy.This is due to a lower amount of vacancy sinks(e.g.,dislocations)in the unstretched alloy,causing a higher vacancy concentration in the heating stage.The excess vacancies accelerate the diffusion of solute atoms,promoting the formation of GPI zones and η′ phase.However,in the prestretched alloy,vacancies have reached an equilibrium concentration due to the presence of dislocations and their effect on diffusion rates and formation of GPI zones and η′ is significantly reduced.Thus,the unstretched alloy has a higher hardness value after heating to the artificial aging temperature.During artificial aging,dislocations introduced by the pre-stretch act as heterogeneous nucleation sites for η phase.The precipitation ofη phase consumes the solutes needed for η′ phase,causing the stretched alloy to have a larger average precipitate size but a smaller hardness value.