Rra Heat Treatment Of Al - Zinc - Mg - Cu Alloy Organization, The Influence Of The Fatigue Performance And Corrosion Resistance

7000aluminum alloys are widely used in the commercial aircraft structures due to the combination of low density, high strength and ductility. T6-treated alloy had high strength, but its corrosion resistance was low. T76treatment increased the SCC(stress corrosion cracking) resistance of the alloy and decreased the strength by10-15%compared to T6treatment. The optimum RRA treatment not only improved both the EXCO(exfoliation corrosion) and SCC resistance but also obtained a tensile strength as high as T6treatment.The effect of retrogression and re-aging (RRA) treatment on mechanical properties, EXCO resistance, SCC resistance and fatigue crack propagation(FCP) resistance of Al-Zn-Mg-Cu alloy has been investigated. Besides, the relationship between the special microstructure of the RRA-treated alloy and the properties has been investigated. Conclusions have been summarized as follows:(1) According to the analysis of DSC, single aging, RRA and RR analysis of Al-Zn-Mg-Cu alloy, the specific RRA treatment has been confirmed as follows:retrogression temperature was170℃, the first and third aging treatment were100℃/24h, the retrogression time when the hardness reached the apex in retrogression curve was10min.(2) Both of retrogressing and reaging time had influence on the mechanical properties of the Al-Zn-Mg-Cu alloy, but the effect of retrogressing time was more. After different RRA treatments, the peak value of the tensile strength, yield strength were623MPa、554MPa、11.2%respectively.(3) The longest fracture time(125h) appeared in the RRA3sample, which was pre-aged for24h at100℃, retrogressed for120min at170℃and re-aged for24h at100℃, demonstrated the best SCC resistance in RRA3sample. Besides, the lowest Icorr and the intergranular corrosion depth in RRA3sample are132μA·cm-2、87nm respectively.(4) The RRA3sample which was pre-aged for24h at100℃, retrogressed for120min at170℃and re-aged for24h at100℃presented higher fatigue crack propogation(FCP) resistance than RRA2sample which was pre-aged for 24h at100℃, retrogressed for10min at170℃and re-aged for24h at100℃. When applied stress intensity factor range(ΔK) raised to33MPa· m1/2, the fatigue failure happenned in RRA3sample. It presented that the RRA3sample had higher FCP resistance at elevated ΔK. Plastic induced crack closure and microcrack-induced crack contributed to the lower crack growth rate of Al-Zn-Mg-Cu alloy.(5) Transmission Electron Microscope (TEM) observation suggested that the RRA3sample were associated with spaced η particles distributing on grain boundaries which contributed to the better SCC and EXCO resistance of Al-Zn-Mg-Cu alloy.