| 7B50 alloy,an ultra-high-strength aluminum alloy in 7xxx series aluminum alloys,is characterized with relatively high strength,good comprehensive properties and hardenability.Its composition is similar to that of 7150 aluminum alloy.Thick plates are main products of 7B50 alloy widely used as light and high-strength structural material in the aviation field.With rapid development of China’s aviation industry,not only the demand for 7B50 alloy thick plates has increased dramatically,but also the plates must be combined with high strength,high plasticity,high toughness and good corrosion resistance simultaneously and homogeneously.However,at present these thick plates still have some problems of structure and property inhomogeneous and anisotropy.Therefore,the above problems:were carried out in this study,which mainly included hot rolling deformation of thick plates,final heat treatment for strengthening,and effects of trace element Zr on microstructure and properties of 7B50 alloy plates in aspect of inhomogeneous and anisotropy.The purpose is to reveal the formation rules of the structure and properties inhomogeneous and anisotropy,and provide some experimental basis for controlling of the production process of this kind of thick plate.The results show that:(1)The anisotropy of mechanical properties of 7B50-T7751 alloy plates shows that differences of mechanical properties are obvious through out the thickness direction of the plate,which the highest strength presents in the central layer,i.e.1/2 thickness layer;the lowest strength presents in the 1/4 thickness layer,elongation decreases from the surface to the thickness center,and reverse plane anisotropy appears near the surface layer(i.e.the transverse strength is higher than the longitudinal one).Microstructures inhomogeneous of 7B50-T7751 alloy plates is characterized with the size of undissolved crystalline phases increase from the surface layer to the central layer,indicating that the distribution of the undissolved crystalline phases is more and more inhomogeneous.The microstructure with fibrous structure becomes wider and wider along the thickness direction and the volume fraction of recrystallization decreases gradually rom the surface layer to the central layer,meanwhile recrystallized grains with large size are easy to appear in the central layer.(2)The flow stress equation for semi-continuous casting ingots of 7B50 alloy isε=1.282×105[sinh(0.0101σ)]4.9145 exp(-134157/RT)after homogenization treatment,which recommends that the hot rolling temperature should be close to 420℃ and the maximum reduction per pass of smooth roll hot rolling should be lower than 40mm.During hot rolling processes with multi-passes and lower reduction rate,deformation is centralized near the surface layer.With reduction rate increasing,the deformation in the central layer becomes bigger and closer to that in the surface layer,the extent of deformation along the thickness direction is more uniform and the final mechanical properties are more uniform.The lower the reduction rate per pass is,the more obvious the reverse plane anisotropy of the 80mm thickness 7B50-T77 alloy plate is.(3)During the hot rolling processes of semi-continuous casting ingots,increasing the deformation rate of hot rolling is beneficial to the elongation of the plate after solution and aging treatment.However,when the deformation rate is about 70%,the plates can get the highest strength in longitudinal,transverse and thickness direction,and the reverse plane anisotropy appears with the deformation rate increasing.With the hot rolling deformation rate increasing after solution treatment,recrystallized grains are firstly formed on as-cast grain boundaries and clearly delineated the morphology of the original as-cast grains after deformation;then recrystallized grains are formed at the intersections of secondary dendrites in the original as-cast grains and clearly delineated the morphology of secondary dendrites in the original as-cast grains;finally the number and size of recrystallized grains gradually increase,and the grains tend to be distributed along the rolling direction.(4)With the gradual deepening of the degree of solution treatment,the coarse Al2CuMg phases decrease gradually.Taking into account the problems of energy consumption and man-hour,the solution treatment is recommended to be a two-stage process of 470℃×2h+480℃×2h.The strength of 80mm thickness 7B50-T77 alloy plates increases gradually with the deepening of solution treatment.The strength of 1/4 thickness layer increases the most obviously,and it is higher than that of other thickness layers.However,the elongation gradually decreases and reverse plane anisotropy becomes more and more obvious near the surface layer of the plate.(5)From the surface layer to the thickness center of 80mm thickness plates of 7B50 alloy,the intensity of Brass,S,and Cube textures increase greatly.After solution and aging treatment,the intensity of Cube texture near the surface layer is the highest,the volume fraction of R-cube and Cube textures near the surface layer has little changed,the volume fraction of S,R and Brass textures reduce significantly in the 1/4 thickness layer,while the volume fraction of S,R and Brass textures remains stable and the volume fraction of Brass texture increases slightly.(6)When trace element Zr is added into high-purity 7B50 alloy,the mass fraction range which starts to significantly refine the grains of semi-continuously casting ingots is 0.063%～0.094%.The main mechanism of refining grains of ingot by Zr element is to increase the number of grains,which has no obvious effect on the spaces between secondary dendrite arms.However,with the Zr element content increasing,the hindering effect on recrystallization of the plates is gradually enhanced,and the varieties and quantities of the deformation textures of thick plates in hot-rolled temper are gradually increased.After solution treatment,all deformation textures tend to be weaker,but the quantity of the recrystallization textures doesn’t change monotonically with the Zr element content increasing.Instead,the recrystallization texture is more obvious in a suitable range of Zr element content.When the mass fraction of Zr element is less than 0.16%,the mechanical properties of 7B50 alloy thick plates are increased with the Zr element content increasing.However,when the content exceeds this mass fraction,the primary phases containing Zr element appear obviously,which leads to properties of the thick plates decreasing.The reverse plane anisotropy appears in high-purity 7B50 alloy plates when the mass fraction of Zr element is 0.034%～0.094%and the hot rolling deformation is 78.6%.(7)The reverse plane anisotropy appearing near the surface layer of 80mm thickness 7B50-T77 alloy plates is not caused by Brass texture.One of the main reasons is the formation of mixed thin strips microstructures along the rolling direction which consist of recrystallized grains and fine sub-grains.The high density sub-grains and adjacent large angle grain boundaries in these thin strips can promote coordination deformation which can lead to the increase of longitudinal elongation and the slight decrease in strength. |
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