Studies On Forging Forming, Microstructure And Properties Of High-strength7085Aluminum Alloy

The large-scale, integral and high performance high strength aluminum alloy forgings have been required in aerospace to achieve light weight and high reliability service performance.The novel high-strength AA7085forgings were the vital structural material for domestic large aircraft. At present, it is difficult to simultaneous acquire both precise forged forming and high performance of high-strength AA7085forgings. It is important to systematic investigate the forged forming behavior, microstructure and properties of AA7085in order to achieve integration shape and properties.In order to meet the requirement of integration shape and properties of AA7085,the present work investigated the hot deformation behavior and processing maps of AA7085with different initial states, evolution of recovery and recrystalliztion microstructure in hot deformation and solution treatment of AA7085, the influence of recovery and recrystalliztion on precipitation and properties, the simulation and experiment of multi-directional forging and isothermal die forging of AA7085wheels, the influnece of multi-stages aging treatment on microstructure and properties of AA7085by using the hot compression testing, mechanical testing, slow strain rate stress corrosion testing and electrochemical corrosion testing combiles with microstructural observation(OM,SEM,TEM) and finite element simulation. The main conclusions are stated as following:(1) Effects of initial states (as-homogenzied and as-deformation annealed) on hot deformation behavior and processing map were investigated. The peak flow stresses decrease with increasing deformation temperature and a decrease of strain rate in both initial states. The peak flow stresses of the alloy after deformation-annealed are significantly higher than that of the alloy after homogenization treatment under the similar compression condition, especially at low deformation temperature. The peak flow stresses for the alloys after homogenization and deformation-annealed can be represented by the Zener-Hollomon parameter (Z) in the hyperbolic-sine equation with the hot deformation activation energy of187.4kJ/mol and318.9kJ/mol, respectively. On the basis of the flow stress data, the strain rate sensitivity of the material is evaluated and used for establishing the power dissipation maps and instability maps. Under the same deformation conditions, the value of peak efficiency power dissipation of the alloy after deformation-annealed is higher than that of the alloy after homogenization treatment. The flow instability region of the alloy after deformation-annealing is wider than that of the alloy after homogenization treatment. The differences of both initial states alloys on hot deformation behaviors and processing maps are mainly attributed to second particles dynamic precipitation characteristic.(2) Influences of hot deformation parameters on deformation and solution process recrystallization were studied.The subgrain size increases and the mainly deformation mechanism is transformed from dynamic recovey to dynamic recrystallization with the increases of deformation temperature and the decreases of strain rate. The volume fraction of static recrystallization increases with decreasing deformation. temperature and the extension of solution treatment temperature as well as solution treatment time. The deformation-solution treatment microstucture maps are established on the basis of the evolution of dynamic and static recrystallization. The deformation-solution recovery zones for the alloy are obtained.(3) Effects of recovery and recrystallization on aging precipitation and properties were investgatied. The distribution of aging precipitation is more uniform with the decrease of the volume fraction of recrystallization and refinement of the substructure. With the decrease the volume fraction of recrystallization and subgrain size, the strength and stress corrosion cracking (SCC) resistance are improved.The dominated fracture mode is transformed from intergranular fracture to transgranular dimple fracture.(4)Combined heat-machanical coupling model and recrystallization map, the effect of preheating temperature, single deformation amount on temperature and strain fields of multi-directional AA7085forging, and microstructure and properties along the thickness direction were inverstigated.Increasing the forging temperature and reducing the amount of single forging can improve the temperature and strain uniformity under forging thickness direction. The strength and SCC resistance increase with forgings thickness direction from the surface to the center due to decreases sub-grain size. (5) The thermal coupled rigid-plastic finite element3D model of isothermal die forging AA7085wheels has been established. The metal forming filling behavior, preform billet and hot process parameters were investgated. The optimized preform billet and hot process parameters were obtained. The filling integrity, well flow lines and non-recrystallization AA7085wheel forgings were obtained by combined isothermal slow rate die forming and close molding.(6) Effects of mult-stages aging on microstructure and properties of AA7085were investgated. Retrogression and Re-ageing (RRA) treatment increases SCC resistance and keeps approximate strength compared with the T6aging treatment. The strength of Double Retrogression and Re-ageing (DRRA) is close to RRA and the exfoliation corrosion and SCC is similar to T74. In the two-stages aging process, with the increasing second stage aging temperature and aging time, the tensile strength decreases while SCC resistance increases. The three-stages aging treatment (two-stages aging plus T6aging treatment) can simultaneously improves strength and SCC resistance compared to the two-stages aging.