Evolution Of Microstructure And Properties Of Al-Zn-Mg-Cu Alloy During Asymmetric High Shear Rolling

Al-Zn-Mg-Cu series high-strength aluminum alloy has excellent comprehensive performance,so it is widely used in aerospace,rail transit,ships and other fields.With the advancement of the technology and manufacturing,higher requirements have been put on the comprehensive performance of Al-Zn-Mg-Cu alloy.The Al-Zn-Mg-Cu alloy ingot has many casting defects,which it is still difficult to eliminate by the homogenization heat treatment.The conventional rolled Al-Zn-Mg-Cu alloy thick plate has some defects,such as insufficient deformation at the center,uneven and coarse microstructure and a large number of coarse second phases,which seriously affects the comprehensive performance of the plate.In the thesis,the effect of asymmetric strong shear rolling on the microstructure and properties of Al-Zn-Mg-Cu alloy thick plates at different thicknesses are studied through experiments.It is found that asymmetric strong shear rolling can increase shear deformation.The defects in the core of the plate are eliminated,the microstructure and texture of the plate are improved,and the comprehensive performance of the plate is greatly increased.The main conclusions are as follows:The homogenization process of Al-Zn-Mg-Cu alloy ingot was determined by differential scanning calorimetry（DSC）:470℃×24h.Comparing the microstructure of the surface with that of the core of 7055 aluminum alloy ingot,it is found that the grain size of the ingot core is larger than the surface,and the second phases of the ingot include the network structure T phase（containing Al-Zn-Mg-Cu element phase）,the gray-black needle-like or irregular shape Fe-rich phase（Al₇Cu₂Fe）,the bright white elliptical S phase（Al₂Cu Mg）and the dispersedηphase（Mg Zn₂）,the number and size of the second phase in the core of the ingot are larger than that of the surface.After homogenization heat treatment（470℃×24h）,the number and size of the second phase on the surface and the core of 7055aluminum alloy have been reduced,but there are still a number of S phases and Fe-rich phases with larger sizes,and the alloy microstructure is uneven.Al-Zn-Mg-Cu alloy thick plates prepared by conventional rolling process（reduction percentageΔh=50%）and asymmetric strong shear rolling process（reduction percentageΔh=50%,misalignment amount⊿d=4,different speed ratio W₂/W₁=1.3）,respectively.It was found that deformation in the conventional process rolling plate was mainly concentrated on the surface,the size of the second phase in the core was large,and its texture characteristics were still mainly plane strain,and the uniformity of the plate was poor.The shear deformation is larger for asymmetric strong shear rolling,and the second phase of the core is severely broken,its Cube cubic texture（{011}<100>）and R-Cube rotating cubic texture({001}<110>are stronger,and the uniformity of the plate is improved.After heat treatment（475℃×2h solid solution+120℃×24h aging）,the grain size of the core of the conventional process rolled plate is larger,the number of coarse second phases is larger,and the plate uniformity and comprehensive performance are poor.The grains at different thicknesses of the asymmetric strong shear rolling plate are obviously refined,the size of the remaining second phase is little.The uniformity of the plate along the thickness direction is improved.The mechanical properties,elongation,hardness and stress corrosion resistance of the plate have also been greatly improved.By changing the reduction percentage,the different speed ratio and the misalignment amount,the influence of the asymmetric strong shear rolling process parameters on the microstructure of the Al-Zn-Mg-Cu alloy thick plate was studied.It was found that with the increase of the asymmetric rolling process parameters（reduction percentage,different speed ratio and misalignment amount）,the deformation stress of the core and surface of the plate increases,the deformation storage energy becomes larger.The proportion of dynamic recrystallization is increased,the degree of second phase crushing is improved,the difference between the second phase of the core,the upper and lower surfaces is reduced,and the uniformity of the plate in the thickness direction is improved.After heat treatment（475℃×2h solid solution+120℃×24h aging）,the grain size and the second phase size and number at different thickness of the plate are greatly reduced.The structural difference between the core of the plate,the upper and lower surfaces is improved,and the uniformity of the rolled plate along the thickness direction is improved,the optimum parameters of the snake rolling process areΔh=50%,⊿d=4,W₂/W₁=1.3.In this case.The smallest grain size on the upper surface is about 17.5μm,the grain size difference between the core and the surface is small,and the microstructure uniformity is the best.The maximum yield strength,tensile strength and elongation of the core of the plate are 572MPa,638MPa,20.1%,the maximum electrical conductivity is about 38.3%IACS.The optimal processing parameters of asymmetric strong shear rolling are reduction percentageΔh=50%,misalignment amount⊿d=4,different speed ratio W₂/W₁=1.3.