| With the development of aerospace industry, property requirement of aluminum alloys increase constantly. Therefore, the need for new alloys has become more and more strong. In this paper, the parameters of Al-Zn-Mg-Cu-Zr alloy extrusion process are determined by the method of dynamic upsetting test. The profiles of Al-Zn-Mg-Cu-Zr alloy were prepared for the first time. With the methods of using optical microscopy (OM), x-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM), electronic probe microanalysis (EPMA), electronic back-scattered diffraction technique (EBSD), transmission electron microscopy (TEM) and tensile test, the hot deformation process, micro structure and mechanical properties after different heat treatments of alloys are studied. On this basis, the effect of Er and Sc addition to alloy profile on microstructure and mechanical properties are studied too. Microstructure and fracture behavior of Al-Zn-Mg-Cu-Zr alloy forging after different aging tempers are discussed.When the deformation temperature of Al-Zn-Mg-Cu-Zr alloy is below 350℃, there is dynamic precipitation behavior durng deformation because of low temperature. Futher deformation of the alloy will be difficult due to small precipitates in the alloy. When the deformation temperature is above 400℃, there is no dynamic precipitation behavior in the alloy. The results of dynamic upsetting test show that the appropriate foring temperature of Al-Zn-Mg-Cu-Zr alloy is range of 400℃-420℃, and the maximum deformation is 60%.The results show that the main precipitation sequence of Al-Zn-Mg-Cu-Zr alloy profile at 120℃is:SSS→GPI zones→η' phase→ηphase. There are two main precipitation sequence of alloy at 160℃:(1) SSS→GPzones (GPI+GPII)→η' phase→ηphase; (2) SSS→VRC→η' phase→ηphase. Suitable single aging treatment is aging at 135℃for 20h. In this condition, the tensile strength, yield strength, elongation and electrical conductivity of alloy profile is 626MPa,593.5MP,11.8% and 36.1%IACS, respectively. The phases ofη' are the dominant precipitates in the alloy under this condition.Suitable two-step aging treatment (T74) is aging at 165℃for 6h followed by 120℃for 4h. In this condition, the tensile strength, yield strength, elongation and electrical conductivity of alloy profile is 566MPa,540MP,10.9% and 40.8%IACS, respectively. Compared to 7050-T7451 profile with similar size and electrical conductivity, the tensile strength, yield strength and elongation of alloy profile increase by 6.3%,14.6% and 7.4%, respectively. After two-step aging treatment, the major precipitates in the alloy profile areη' phases andηphases. Discontinuous grain boundary precipitates and narrow precipitate-free zones (PFZ) along the grain boundary are observed clearly. With the increase of the second aging temperature and time, the width of PFZ does not increase significantly.Considering the match of strength and toughness, stress corrosion cracking (SCC) resistance and process operation, suitable RRA treatment is pre-aging at 120℃for 24h, retrogression at 180℃for 45min, and then re-aging at 120℃for 24h. In this condition, the tensile strength, yield strength, elongation and electrical conductivity of alloy profile is 613.5MPa,599MP,11.1% and 39.2%IACS, respectively. Compared to T6 temper, the elongation decreases slightly and the SCC resistance increases obviously with similar strength. At the beginning of retrogression treatment, most of GPI zones and smallη' phases have dissolved back to Al matrix, therefore, strength and hardness of the alloy decrease to the minimum value. With the increase of retrogression time, the main phases areη' phases and GPII zones with better stability of high temperature. The strength and hardness of the alloy increase in this stage. With futher extension of retrogression time, part ofη' phases transform toηphases. The strength and hardness of the alloy decrease slightly. In the stage of reaging, GPI zones and smallη' phases precipitate from Al matrix because of low aging temperature. The strength and hardness of the alloy increase slightly in this stage. The precipitation conditions of grain boundaries are similar to two-step aging treatment, which are sparsely distributed precipitates and narrow PFZ.With the addition of 0.5% Er, most of element Er aggregates at grain boundary during solidification and results in ternary Al8CuEr phases in Al-Zn-Mg-Cu-Zr alloy profile. The formation mechanism of Al8Cu4Er phases is investigated and dissolution temperature of AlgCu4Er phases is about 575℃. Because remainder Al8Cu4Er phases will happen crack due to the piling up of dislocation, the strength of Er-containing alloy profile decreases. However, the difference of potential between matrix and grain boundary decreases because of Al8Cu4Er phases at grain boundary. The corrosion resistance is improved significantly.With the addition of 0.2% Sc, elements Sc and Zr are shown to be more effective in refining as-cast and extruded grain size, inhibiting recrystallization and grain growth, and improving mechanical properties because of the newly formed Al3(Sc,Zr) particles in the Al-Zn-Mg-Cu-Zr alloy profile. In addition, a ternary W phase composed of elements Al, Cu and Sc can be observed in the alloy profile.After two-step aging treatment, the precipitates of matrix grain boundaries grow significantly and obvious precipitate free zones (PFZ) can be found. The fracture mechanisms of Al-Zn-Mg-Cu-Zr alloy forging in various aging treatments are different, which are mainly determined by different precipitation conditions. The fracture mechanism of alloy forging after T6 treatment is shearing and intergranular dimple mixed fracture. When the second aging temperature is 160℃in two-step aging treatment, the fracture mechanism of alloy forging is shearing and transgranular dimple mixed fracture. When the second aging temperature is 170℃, however, the fracture mechanism of alloy forging is transgranular dimple fracture. Suitable two-step aging temper is aging at 160℃for 6h followed by 120℃for 6h. At this condition, the tensile strength, yield strength, elongation in the longitudinal direction and electrical conductivity of alloy forging is 541MPa,514MPa,10.3% and 38%IACS respectively. Meanwhile, the L-T toughness fracture is 43.7MPa-m1/2 and the exfoliation corrosion test is EA degree. |
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