| The 7xxx-series high-strength aluminium alloys have become the main structural materials for aerospace today due to their high specific strength and specific stiffness,good fracture toughness and corrosion resistance,and excellent formability.As the service performance requirements of aerospace vehicles for high manoeuvrability,high overload,high fatigue,and high reliability continue to rise,higher requirements are placed on the overall performance of the 7xxx-series aluminium alloys used to manufacture their structural components.As the precipitation-reinforced alloy,the mechanical properties of 7xxx-series alloys are greatly influenced by the grain size and the morphology and distribution of the reinforced phases.The residual crystalline phases generated by segregation during casting are easily inherited to the final part,resulting in stress concentration and becoming the crack sources and crack propagation channels.This reduces the fracture toughness of the alloy.In addition,the increase in strength of 7xxx aluminium alloys is often accompanied by a reduction in plasticity,and they are prone to various forms of local corrosion.The simultaneous increase in strength,toughness,and corrosion resistance of the alloy is a serious challenge for material processing,and how to improve the comprehensive performance of the 7xxx-series aluminum alloys has become an urgent problem to be solved in the manufacture of aerospace high-strength aluminum alloy components.Therefore,this thesis took the high hardenability 7A85 aluminium alloy as the research object,and proposed a new process based on high-temperature multi-directional forging,intermediate severe plastic deformation in conjunction with retrogression and reaging（RRA）heat treatment.The reduction of residual crystalline phases and the evolution of second-phase particles and properties of 7A85 aluminum alloy were studied,and a new preparation technology of 7xxx-series aluminum alloys with high strength,toughness,and corrosion resistance was obtained.The main research works are as follows.（1）The influence mechanism of high-temperature multidirectional forging processes on the residual crystalline phase and the uniformity of grains and precipitates of 7A85 aluminum alloy were investigated.High-temperature multidirectional forging could effectively reduce the coarse residual crystalline phase inside the forgings.With the increase of upsetting-stretching times from four-upsetting and three-stretching to seven-upsetting and six-stretching,the area fraction of coarse residual crystalline phase decreased from 9.7%to 2.1%.Meanwhile,the matrix precipitates were diffusely distributed with an increase in density,the precipitate-free zone（PFZ）width of grain boundary decreased,and and the precipitates at the center and edge were more uniform.Mainly due to the high temperature multi-directional deformation conditions,the lattice distortion and the movement of vacancies and dislocations are intensified,and the solute atoms are more active,which promotes the dissolution of residual crystalline phases,improves the solid solubility of the alloy,and enhances the driving force for aging precipitation.With the increase of upsetting-stretching times,the dynamic recrystallization is more adequate,which solved the problem of grain coarsening and directional protrusion of forgings.The size and anisotropy of the three-dimensional（3D）grains were reduced,and the uniformity of the grains at the center and edge was improved.（2）The influences of different deformation temperatures and deformation degrees on the residual crystalline phase fragmentation,grain refinement,and precipitation behavior of 7A85 aluminum alloy were studied,and the dislocation density model of 7A85 aluminum alloy under different deformation processes was constructed.The coarse aggregated crystalline phases still existed under the room-temperature deformation of 10%and the high-temperature deformation of 60%,while they were fully broken under the cryogenic deformation of 20%.Upon increasing the deformation amount at-196～250°C,the grain size of the alloy after aging first decreased and then increased,and increasing the deformation amount at 480°C,the grain was gradually refined;lowering the deformation temperature under the same deformation degree,the grains were gradually refined.Decreasing the deformation temperature and increasing the deformation amount can introduce a mass of dislocations and facilitate the fragmentation of coarse residual crystalline phases,as well as providing nucleation sites for static recrystallization during subsequent solution treatment and refining grains.However,the excessive deformation storage energy drives the grain growth.The residual crystalline phase of the alloy was significantly reduced,the grains were fine and equiaxed after aging,and the precipitates were dense under the 3D cryogenic deformation of 20%for one pass.（3）The effects of single-stage,double,and RRA treatments based on the cryogenic severe plastic deformation process on the microstructure and comprehensive properties of 7A85 aluminum alloy were investigated.The results showed that the continuously distributed grain boundary precipitates under the single-stage treatment led to lower yield strength（YS）,conductivity,fracture toughness,and corrosion resistance of the alloy;while the increase of matrix incoherentηphase and the spheroidization of grain boundary precipitates under double and RRA treatments reduce the stress concentration at grain boundaries and the strength difference between matrix and grain boundary,and cut off the corrosion channel,thus significantly improving the conductivity,fracture toughness,and corrosion resistance of the alloy.However,the strength loss of the alloy under double-stage treatment was large.On balance,the best aging treatment to match 7A85 aluminum alloy after 3D cryogenic deformation was RRA temper（120°C/24 h+180°C/0.5 h+water quenching+120°C/24h）.（4）The whole process test of 7A85 aluminum alloy with intermediate 3D deformation in combination with RRA heat treatment process was carried out to reveal the effect and mechanism of different deformation temperatures on the microstructure and properties of the alloy.The results showed that conducting the intermediate 3D deformation process could improve the strength and fracture toughness of the alloy,and maintain its good corrosion resistance under the synergistic RRA heat treatment.With the decrease of the deformation temperature,the fracture toughness of the alloy first decreased and then increased.Under 3D cryogenic deformation,the grain boundary strength increased,and the strength difference between the matrix and grain boundary reduced.At the same time,the particle fragmentation was sufficient,which reduces the source of particle-induced cracking and improves the fracture toughness of the alloy;the dense precipitates improved the 3D ultimate tensile strength（UTS）of the alloy,and the finely equiaxed grains and curved grain boundaries allow for a more tortuous crack extension path.The 3D elongation（EL）increased simultaneously and the anisotropy decreased.（5）A new preparation process of high-strength,high-toughness,and corrosion-resistance 7A85 aluminum alloy with"high-temperature multi-directional forging homogeneous cog-down+intermediate 3D cryogenic severe plastic deformation+synergetic RRA heat treatment"was proposed.The optimal comprehensive properties of the alloy were obtained with this process,and its average 3D UTS,average 3D YS,average 3D EL,and fracture toughness under this process were 545 MPa,476 MPa,16%,and 42.49MPa·m1/2,respectively,which were 2.3%,0.2%,18.5%,and 21.4%higher than those of the unintermediated deformed alloy.The process simultaneously improved the strength,plasticity,and toughness of the alloy and maintained its good corrosion resistance,thus solving the contradiction between the strength,toughness,and corrosion resistance of 7A85 aluminum alloy,and developing a new preparation technology for 7xxx-series aluminum alloys with high strength,toughness,and corrosion resistance. |
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