| Al-Li alloys are characterized by low density,high specific strength, high specific elastic modulus and rigidity,excellent resistance to extension of fatigue cracks,high mechanical properties and fairly good weldability. However,for their poor plasticity and formability at ambient temperature, cracking trend,distortion led by residual stress,the applications of Al-Li alloys are immensely limited.The techniques of superplastic forming/ diffusion bonding enable fabrication of delicate complicated-shaped structure with high integral rigidity.Applying this technique to Al-Li alloys is extremely important for weight loss of national fourth-stage fighter to enhance its technical and tactical performances.The recrystallization behaviors of two well-developed series of alloys——Al-Cu-Li and Al-Mg-Li were investigated by discontinuous-and continuous-recrystallization-based processing rounties,respectively.The principles and techniques of grain refinement were supposed.Main conclusions are listed in the following.1)A thermo-mechanical processing routine characterized by severe-aging, homogeneous shear strain rolling and rapid recrystallization annealing were suggested by breaking through the traditional idea of low-temperature-aging. A uniform microstructure with grain size of 10μm was obtained by applying the technique to an AA2195 alloy.The initial as-rolled sheet with a certain thickness were severe-aged at 400～450℃for 24 hours,then rolled without lubrication for the case of small final gauge,or rolled with good lubrication and higher reduction per pass for the case of large final gauge,and followed by a rapid recrystallization annealing.2)By investigation of influence of severe-aging on size,fraction and distribution of second phase particles,models of layered-microstructure led by continuously-distributed particles and equiaxed grain structure formed by coarse particles stimulating nucleation were supposed.A technique of high temperature severe-aging to form particles of high fraction and large size was also developed.High temperature of severe-aging benefits formation of higher fraction coarse spheric particles,thus,a fine equiaxed grain structure after rolling and recrystallization,while low temperature is not capable to remove the continuously distributed particles on grain boundaries, and thus the elongated grain structure is obtained.The optimal severe-aging process is 400-450℃/24h,followed by furnace cooling.3)A 3D crystal plastic finite element model was designed to investigate evolution of orientation and stored energy of zones near various shaped particles.The criterion and prerequisite of effective recrystallization sites are suggested.Smaller 3D radius of curvature of rigid particle brings higher orientation gradient and stored energy.For a certain single crystal containing a certain shaped particle,the sites of maximal orientation variation and plastic work locate on the internal tangent points between eigen octahendron composed of {111} crystallographic planes and the particle.To be a successful nucleus,the subgrains near particle must be characterized by advantages of difference of orientation and stored energy.The spheric particle is the most effective to stimulating recrystallization.4)The relationship between rolling process parameters and final grain structure was investigated.The cause of heterogeneity of grain structure along sheet normal direction is found to be shear gradient and 'heredity' effect of rolling structure.To reduce the damage of such structure to superplasticity,a homogeneous shear strain rolling was supposed that is realized by reducing friction,increasing reduction per pass and cross roiling.5)The optimal elongation 630%of AA2195 alloy in present investigation is obtained by the condition of T=490℃,(?)=10-3s-1.The deformation mechanisms to equiaxed and elongated grain structures are also discussed.6)By comparing various methods of grain refinement,a novel severe plastic deformation one——shear pressing was proposed.The grain size of Al-Mg-Li alloy reached 5.2μm only by being strained toε=2.3.More slip systems were involved in because the final stable orientation was changed, thus,the plastic energy were used to change shape of grains more effectively.7)To avoid the error introduced by dependence of q index on crystallographic orientation,a new method to evalue recrysallization fraction based on EBSD pattern quality index were suggested.
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