Basic Study Related To The Key Processing Factor For High Strength Al-Zn-Mg-Cu Aluminum Alloy Plate

As the key materials of the aerospace industry and national defence industry,Al-Zn-Mg-Cu aluminum alloy is well kown as high strength,high toughness and resistance to stress corrosion cracking.At present,it is difficulty in processing and forming for the thick plate of Al-Zn-Mg-Cu aluminum alloy.The microstructures and properties are inhomogeneous in different layers along the thickness direction.The fundamental reason is that the basic research is weak in the preparation of processing theory and technology of high strength aluminum alloys.The lack of basic research restricted severely the development and application of high strength aluminum alloys.Therefore,in order to optimize processing technology and perfect property,it is very important to strengthen the research on the forming of high strength aluminum alloy plate.In order to study the hot deformation behavior and optimize the processing parameters of solid solution treatment of high strength aluminum alloy,the fourth generation of high strength aluminum alloy 7085 was used as the raw material in our work.The evolution microstructure and properties of the materials were investigated under different hot deformation and solid solution treatment conditions through isothermal hot compression,solid solution treatment and mechanical properties tests combined with the microstructure characterization techniques of metallurgical microscope?OM?,scanning electron microscope?SEM?,electron backscatter diffraction?EBSD?,transmission electron microscope?TEM?and X-ray diffraction analysis?XRD?.The main conclusions of this research are listed as follows:?1?The hot deformation behavior and hot work formability of 7085 aluminum alloy were investigated by means of hot compression tests.The effect of deformation heating and friction on the flow stress was discussed and thus some reasonable corrections of flow stress curves were preformed.The constitutive equation of hot deformation was also established.The relative softening decreased with increasing temperature with low strain rates.Moreover,the occurrence of flow instability may be reponsible for a continuous flow softening behavior at strain rate of 10.0s^-1 during isothermal compression.The processing maps at different strains exhibit that the former unstable region is transformed into a new safe deformation with low temperature and low strain rate.Microstructure characterization reveals that the coarsening of particles may be responsible for the transformation of processing map from instability to safe domain.The superimposed processing maps at different strains exhibit the optimized deformation parameters with the temperature range of 390⁴50? and strain rate of lower than 0.1s^-1.?2?Effects of strain rate on critical condition of dynamic recrystallization and dynamic recrystallization mechanism were investigated at 450? with the highest efficiency of power dissipation.The flow stress curves exhibit the same character of single peak at the early stage of deformation??<0.3?.Microstructure characterization reveals that dynamic recrystallization occurred preferentially at triple junctions and grain boundaries.Three types of orientation relationship were found at triple junctions between new grains and deformed grains.After the strain exceeds a critical value???0.3?,the flow stress increases monotonically with increasing strain at the strain rate of 0.001s^-1,while it reaches a steady state in the case of high strain rate(0.1s^-1).The microstructure observation exhibits that the recovery promotes continuous recrystallization and the original grains are subdivided by sub-boundaries to generate the network structure at the strain rate of 0.001s^-1.With increasing strain rate,due to the absence of recovery,a sufficient driving force can accelerate the bulging of grain boundary and induce a necklace structure.In this case,continuous recrystallization can be impeded.?3?Influence of heating rate on the microstructure,second-phase particles,texture evolution and mechanical property of cold-rolled 7085 aluminum alloy were studied during solid solution treatment.Heating rate can control the recrystallized grain size and volume fraction of recrystallization.At a given solution temperature,recrystallized grain size decreased with increasing heating rate.In the case of low heating rate?1?/min?,a coarse and elongated grain structure can be observed,while a fine equiaxed grain structure forms and distributes homogeneously at rapid heating rate??30?/min?.At lower solution temperarures?<450??,the volume fraction of recrystallization decreases as the heating rate increases.With the increasing solution temperature,the variation of volume fraction exhibits an inverse tendency.The dissolution of MgZn2 particles can be retarded with a rapid heating rate.At a slow heating rate,recrystallized textures are mainly composed of {001} <100>,{125} <5 5 1> and the residual rolling textures,while the recrystallized texture consists of random texture and rolling texture with a rapid heating rate.The tensile properties are improved with an inceasing heating rate during the solid solution treatment,which will provide a guide for the actual industrial production.