Experimental And Model Study On The Effect Of Zn Content On Deformation And Precipitation Behaviors Of Al-Zn-Mg-Cu Alloy Following Thermal/mechanical Processing

Ultra-high strength Al-Zn-Mg-Cu alloys with high Zn content have been of great interest as key structural materials in the aerospace and transport industries owing to their high specific strength,high specific stiffness.The properties of Al-Zn-Mg-Cu alloy products prepared by multistage hot working such as rolling and forging are greatly influenced by their chemical compositions.Moreover,the dynamic microstructural variations during deformation and static microstructural variations during inter passes holding will also determine the properties of final products.However,there is still limited research on relationship between element contents and dynamic,static softening behavior of Al-Zn-Mg-Cu alloys during hot deformation at home and abroad.Particularly,the theoretical and physically-based modeling works used to predict the microstructure and properties evolutions of Al-Zn-Mg-Cu alloys after multistage hot working are destitute.In this works,Al-Zn-Mg-Cu alloys with different Zn contents were selected as the research material.By utilizing advanced Gleeble thermomechanical simulator,a series of single-pass,double-pass isothermal hot deformation tests were carried out.The functions of Zn content on the kinetics and mechanisms of dynamic,static softening of Al-Zn-Mg-Cu alloys during thermomechanical processing were also explored by c ombining advanced microstructure characterization techniques,i.e.,SEM,EBSD and TEM.Further,the interactions among precipitation,recovery and recrystallization were also discussed.Based on experimental results,an empirical model and a quantitative physically-based ISV model were developed to simulate the static softening behavior of Al-Zn-Mg-Cu alloys.Because of the significant effects of precipitation evolutions on dynamic,static softening behavior,the functions of Zn content on precipitation kinetics of Al-Zn-Mg-Cu alloys during isothermal and non-isothermal heat treatments was studied by in-situ electrical resistivity characterization technique.And an improved ISV physical model covering precipitation evolution,electrical and mechanical responses was developed.The main results are as follows:（1）On the basis of single-pass isothermal hot compression tests and microstructure characterization experiment s,the dynamic softening kinetics and corresponding mechanisms of Al-Zn-Mg-Cu alloy during hot deformation were investigated.The results showed that with the increase of Zn conte nt,the work hardening rate of Al-Zn-Mg-Cu alloys during the early stage of deformation was accelerated.And the peak stress and hot deformation activation energy increas ed frst and then decreased slightly.The deformation activation energy for 6.25%alloy,7.93%alloy and 10.11%alloy were estimated to be 158.22 k J/mol,164.27 k J/mol and163.38 k J/mol respectively.Under deformation condition with high Z value,the dynamic softening behavior of Al-Zn-Mg-Cu alloys accelerated gradually because that the increased Zn content accelerated the process of dynamic precipitation which was mainly responsible for the dynamic softening.While at low Z value,dynamic softening behavior of Al-Zn-Mg-Cu alloys decelerated with increasing Zn content when dynamic recovery and dynamic recrystallization were the predominant softening mechanisms.The addition of Zn inhibited the progress of dynamic recovery and dynamic recrystallization resulting and then decelerated dynamic softening.（2）Based on double-pass isothermal hot compression tests and m icrostructure characterization experiments,the static softening kinetics and corresponding mechanisms of Al-Zn-Mg-Cu alloys during hot deformation were investigated.The results showed that at deformation temperature of 300°C,the static softening curve plateau was observed,both the duration of static softening plateau and softening fraction decreased with increasing Zn content.The static softening plateau of 6.25%alloy,7.93%alloy and 10.11%alloy appeared when static softening fraction value reached 34%,30%and 22%respectively.The plateaus of them appeared between the holding time approximate 2-300 s,2-100 s and 1-10 s.When deformation temperature increased to 400°C,the static softening curves appeared approximate typical sigmoidal shape with slight influence from Zn content.The static softening mechanisms were found to be the functions of static recovery and precipitates coarsening at 300°C.The increased Zn addition mainly affected static softening mechanisms by forming precipitates.At deformation temperature of 400°C,the static softening mechanism was controlled by static recovery and static recrystallization simultaneously.The static softening fraction of the studied Al-Zn-Mg-Cu alloys reached about 70%after holding for 1000 s.（3）A simplified empirical model of static softening was investigated based on JMAK model which coupling static recovery and static recrystal lization.The estimated static softening activation energy for 6.25%alloy,7.93%alloy and 10.11%alloy were 124.02 k J/mol,132.16 k J/mol and 139.73 k J/mol respectively.On this basis,combined with the quantitative microstructure characterization results,an integrated physically-based model coupling recovery,recrystallization and precipitates coarsening was developed.The evolutions of stress,microstructure and static softening fraction during post-deformation holding could be predicted reasonably by the integrated physically-based model.A special attention was paid to model the functions of various alloy solute contents（i.e.,Zn,Mg,Cu and Zr）on the precipitation coarsening,static recovery and recrystallization kinetic s in Al-Zn-Mg-Cu alloys.The results showed that the different precipitation behaviors due to the addition of various alloying elements have a considerable influence on recovery,recrystallization and coupled static softening process.The solid solution atoms remaining in the matrix such as Zn,Mg,Cu and Zr inhibited the static recrystallization process by hindering the movement of grain boundaries.The inhibition ability of various solid solution atoms depended on their binding energy and the effective diffusivity at grain boundaries.Zr atoms had the strongest inhibition ability,followed by Cu and Mg atoms,and Zn atoms had the weakest inhibition ability.（4）The functions of Zn content on precipitation kinetics and evolutions of Al-Zn-Mg-Cu alloys during isothermal and non-isothermal heat treatments were studied scientifically by utilizing in-situ electrical resistivity monitoring technique,thermodynamic calculation,hardness test and TEM characterization.The results show that the formation of Mg Zn₂ nucleus requires soluble Zn,Mg atoms,and the ratio of Zn and Mg atoms in Mg Zn₂ nucleus was about 1.33.The directional growth of plate-like Mg Zn₂ precipitates such as GPII zones andη’phase needs to absorb a large number of soluble Zn atoms.The diffusion flux of soluble Zn controlled the crystallographic microstructures evolution of Mg Zn₂ precipitates during nucleation,growth and transition processes.Increasing Zn content in Al-Zn-Mg-Cu alloys greatly increases the concentration of soluble Zn in Al matrix,and then promotes the nucleation,growth and transition processes of Mg Zn₂ precipitates.The Al-Zn-Mg-Cu alloys with high Zn content tends to form a precipitates population with high density,large size and developed structure during aging treatment.（5）According to the quantitative results of obseved precipitation microstructures,an improved ISV model based on classical KWN model covering precipitation evolution,electrical and mechanical responses of Al-Zn-Mg-Cu alloys was developed.The role of Zn element in nucleation,growth and coarsening of Mg Zn₂ precipitates has been studied theoretically.The results show that the improved ISV model considering the intrinsic characteristics of precipitates such as crystallographic orientation,morphology,component,and distribution owns the notable advantages to simultaneously rationalize the observed microstructural characteristics,mechanical and electrical properties following heat treatment of Al-Zn-Mg-Cu alloys.Increasing Zn content in Al-Zn-Mg-Cu alloys introduces more potential nucleation sites for Mg Zn₂ precipitates and reduces the activation energy for nucleation,and then increase the density of Mg Zn₂ nucleus.The growth of Mg Zn₂ precipitates was controlled by the diffusion of soluble atoms in the Al matrix,and Zn element was the main controlling element.The increase of diffusion flux of soluble Zn caused by increasing Zn content would promote the growth of Mg Zn₂ precipitates.A certain correlation between electrical resistivity and strength for Al-Zn-Mg-Cu alloys was quantificationally established based on the improved ISV model.