| Al-Zn-Mg-Cu-Zr alloy is a kind of high strength and high toughness aluminum alloy that is now widely used.It is an indispensable structural material in the field of aircraft manufacture.The mechanical properties of the alloy can be significantly improved by adding small amount of Sc to Al-Zn-Mg-Cu-Zr alloy.Creep-aging forming is a new forming technology.It is especially suitable for manufacturing aircraft wings and integral panels that usually have complex bending contour and internal structure.So it has a very good developing prospect.However,there are still many problems,which are needed to be studied,in the practical application of creep-aging forming technology.For this,the author of this paper prepared three types of Al-Zn-Mg-Cu-Zr alloy plates with different Sc contents.And the effects of Sc,stress and strain states on the creep-aging hardening behavior,springback phenomenon,tensile property and the anisotropy of the alloys were comparatively studied by carrying out creep-ageing at 120~oC for the alloy plates by means of three-point bending loading.The effect of Sc and creep-aging on the fatigue property of the alloy plates were also preliminarily investigated.The main studies and conclusions are as follows.1.The as-cast,homogenization and as-extruded microstructures of the prepared alloys were systematically analyzed by using metallographic microscope and scanning electron microscope.The results show that the as-cast microstructures of the alloys can be transformed into fine equiaxed crystal from coarse dendritic structure by adding small amount of Sc to Al-Zn-Mg-Cu-Zr alloy.And the microstructures of the alloys become finer with the increase of Sc content.Homogenizing treatment eliminated the dendritic segregation in the alloys,but the grains of the alloys are still very fine.Extruding deformation makes the grains of the alloys into a fibrous structure along extrusion direction,while the grain structure in vertical extrusion direction is still fine and uniformly distributed.2.Effects of Sc,stress and strain state on the creep-aging hardening behavior of Al-Zn-Mg-Cu-Zr alloy were comparatively studied.The results indicate that small amount of Sc addition to Al-Zn-Mg-Cu-Zr alloy increases the initial and peak hardness of the alloy.Compared with stress-free aging,creep-aging reduces the aging hardness of the alloys.When creep-aging is performed under the state of tensile stress,the aging hardness of the alloy is all higher than that of the alloys that creep-aged under the state of compressive stress during creep-aging.Strain state has some effect on the creep-aging hardening behavior of the alloys.When creep-aging is performed under the state of elastic-plastic strain,the initial hardness of the alloys is obviously higher than that of the alloys that creep-aged under the state of elastic strain,but the age-hardening rate of the alloys is relatively lower.As a result,the peak-aging hardness of the alloys that creep-aged under the state of elastic-plastic is similar to that of the alloy that creep-aged under the state of elastic strain when peak-aging is reached.3.Effects of Sc and strain state on the springback phenomenon of the creep-aged Al-Zn-Mg-Cu-Zr alloy plates were investigated.The results display that the Al-Zn-Mg-Cu-Zr alloys with Sc addition have a relatively higher springback rate after the creep-aging is completed.The springback rate of the alloys that creep-aged under elastic-plastic strain state is significantly lower than that of the alloys that creep-aged under elastic strain state.4.Effects of Sc and creep-aging on the tensile property and the anisotropy of Al-Zn-Mg-Cu-Zr alloy were investigated.The results reveal that Sc addition improve tensile property of both stress-free aged and creep-aged Al-Zn-Mg-Cu-Zr alloys.Creep-aging decreases the tensile property of the alloys compared with stress-free aging.The tensile properties of both stress-free aged and creep-aged alloys all display obvious anisotropy.And Sc addition has no significant function for improving the anisotropy of the tensile property of the alloy.The effect of different strain states on the anisotropy of the tensile property of the creep-aged alloys is not the same.Elastic strain state has no effect on the anisotropy of the tensile property of the alloys.But the anisotropy of the tensile property of the alloys is obviously decreased when the alloy is creep-aged under the state of elastic-plastic strain.5.Effects of Sc and creep-aging on the fatigue property of Al-Zn-Mg-Cu-Zr alloy were preliminarily investigated.The results show that Sc addition improves the fatigue property of the alloy to some extent,and proper high content of Sc is helpful to improve the fatigue property of the alloy.Compared with stress-free aging,creep-aging decreases the fatigue property of the alloy. |
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