| Novel Al-Zn-Mg-Sc-Zr alloy has been developed by adding a small amount of scandium and zirconium on the base of medium-strength weldable Al-Zn-Mg alloy. The alloy has high strength, good corrosion resistance and welding performance, belonging to a new era of high-strength weldable structure material. Al-Zn-Mg-Sc-Zr alloy can be applied in aerospace, nuclear energy and ship industry.Al-5.4Zn-2Mg-0.25Cu-0.1Zr-0.25Sc alloy which was studied in this paper has been homogenized, hot rolled, intermediate annealed and cold rolled into plates with thickness of2mm. The investigation of the different heat treatment on the microstructure and mechanical properties、 Electrical conductivity of the above alloys has been studied. On the basis of this alloy, the microstructure and mechanical properties of the alloy under solution treatment, single-aging, two-stage aging, the intergranular corrosion, exfoliation corrosion and stress corrosion behaviour of the alloy have been investigated. As well as the corrosion process and its mechanism have been discussed. The evolution of electrochemical impedance spectroscopy (EIS) features and the relation with exfoliation corrosion under different aging regimes of the alloy in EXCO solution have been also analyzed.The favorable solution treatment of Sc-bearing Al-Zn-Mg-Mn-Zr alloy is470℃/60min solution, when improve the solution temperature, prolonged solution time, the second phase dissolved in the alloy and the supersaturation increases.During aging Sc-containing high-strength Al-Zn-Mg-Zr alloy precipitates as the main phase and phase, with the aging time, the intragranular phase becomes coarser precipitates on the grain boundary also appears discontinuous phase and the PFZ become wider. The favorable peak single-ageing treatment is120℃/24h, At this condition, tensile strength, yield strength,elongation and electrical conductivity of the alloy is581MPa,561MPa, respectively. Alloy desolventizing order:saturated solid solution(SSS)'Guinier Preston district (G.P district)'metastable phase (MgZn2) phase'equilibrium phase (MgZn2). After the two-stage aging crystal dispersed within the metastable strengthening phases, grain boundaries are not continuous equilibrium phase, accompanied by a certain width PFZ.The suitable two-step ageing treatment of the alloy is-ageing at120℃for6h followed by140℃for20h, At this condition, tensile strength, yield strength,elongation and electrical conductivity of the alloy is548MPa,527MPa,12.3%,38.2%IACS, respectively,compared with the single peak aging, while the loss of tensile strength is33MPa, but the conductivity increased3.9%IACS in value, the corrosion resistance of alloy is improved.The intergranular corrosion and exfoliation corrosion behavior of Al-Zn-Mg-Zr alloy containing Sc under different aging conditions were studied according to GB/T7998-2005and GB/T22639-2008. And IM6ex electrochemical workstation was employed to study the polarization of the alloy. The results showed that the intergranular corrosion resistance and exfoliation corrosion resistance of artificial aging alloys were better than those of natural aging alloys, and with increasing aging time, the corrosion resistance of Al-Zn-Mg-Zr alloy containing Sc turned better. However the alloy after two-stage aging got the best exfoliation corrosion resistance.By analyzing the impedance spectrum of alloy in EXCO solution, the experimental results show that natural aging state alloy from corrosion current was lower than that of artificial aging state. With the extension of artificial aging time and temperature, the corrosion current of the alloy decreased significantly, alloy corrosion sensitivity significantly reduced. The corrosion current and corrosion sensitivity of two-stage treated alloys is the minimum. Alloy impedance spectrum showed that with the extension of soaking time, different aging state alloy fitting Rpo value increases gradually, the Rct values gradually decline. Under the same soak time the Rpo value of two-stage aging state alloy is the maximum, value Rct is the minimum. Its corrosion resistance is best.The stress corrosion cracking (SCC) susceptibility of Al-Zn-Mg-Zr alloy containing Sc was investigated by slow strain rate tension (SSRT) tests according to GB/T15970.7-2000. The results showed that strain rate had severe influence on the SCC susceptibility of Al-Zn-Mg-Zr alloy containing Sc. At lower strain rate, the alloy was more sensitive to stress corrosion than those at higher strain rate, the SCC susceptibility index of T6aged alloys was larger than that of T73aged alloys, i.e. the T73aged alloys had better stress corrosion resistance than the T6aged alloys. |
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