| Al-Zn-Mg-Cu aluminum alloys have been widely used as the structural materials in the aerospace industry due to their high specific strength, low price and excellent processability. In recent years, with the rapid development of the aerospace industry and the national defense modernization, there is a great demand for large aircraft, which make it desirable to use Al-Zn-Mg-Cu plates with high properties. This work is based on "National Defense Key Research Project" titled as "Research of X X X X aluminum alloy plate". In order to provide conferences for the optimization of hot working and heat treatment processes and the improvement of properties, the heat treatment process, microstructure and properties of an Al-Zn-Mg-Cu alloy were systematically studied by means of hot working simulation, tension, microhardness, conductance measurement, fatigue crack propagation test, exfoliation corrosion test, stress corrosion test, optical microscopy (OM), electronic microscopy (SEM, TEM), differential scanning calorimetry (DSC), electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD, Micro-XRD). In addition, the results were analyzed and discussed theoretically.The experimental results indicated that the eutectic structures consisting of η phase (MgZn2), S phase (Al2CuMg) and T phase (AlZnMgCu) were found to form in the grain boundary of as cast Al-Zn-Mg-Cu alloy due to the segregation of Zn, Mg and Cu, and the melting point of the eutectic structure was475.2℃. The primary eutectic structure of the alloy homogenized at465℃for24h dissolved into the matrix and the residual particles were Al7Cu2Fe. The triangular Al18Mg3Cr2phase (E phase) which distributed mainly in matrix formed during solidification. Moreover, there were no obvious changes on the E precipitates during the homogenization. It was found that the major existing form of erbium in Al-Cu-Mg-Er alloy was Al8Cu4Er phase, which formed in grain boundary during solidification. Al8Cu4Er particles could not be eliminated during homogenization and were crushed up along with the grain boundaries after hot rolling. A few secondary Al3Er precipitates were observed in the matrix in this investigation.The hot deformation behavior of Al-Zn-Mg-Cu alloy was studied and the results showed that when the deformation temperature was below390℃, the dominative restoring mechanism was dynamic recovery. However, when the temperature was420℃, dynamic recrystallization has occurred. The suitable hot working temperature range of the alloy was330℃-400℃with a strain rate of0.1s-1. Based on the results, the hot deformation activation energy was calculated which was192.6KJ/mol, and the constitutive equation was also given. The optimal T761ageing treatment was125℃/3h+170℃/10h. Under this condition, the tensile strength, yield strength, elongation and conductivity were498MPa,429MPa,12.1%and40.2%IACS, respectively. The optimal RRA treatment of the alloy was120℃/25h+190℃/10min+120℃/25h, and under this condition the tensile strength, yield strength, elongation and conductivity were554MPa,507MPa,16.5%and35.4%IACS, which were closely comparable to those of T6-treated alloy. The sheet of T761-treated Al-Zn-Mg-Cu alloy could meet the needs of AMS-4085B, and was satisfactorily applied to a large aircraft.It was found that the fatigue crack propagation (FCP) rate of RRA-treated Al-Zn-Mg-Cu alloy was slower than that of T761-treated alloy at a stress ratio of0.1with a sine-wave loading frequency of lOHz. The formation of more secondary cracks led to a more complex crack propagation path, thereby reducing the fatigue crack driving force and enhancing the crack propagation resistance in the RRA-treated alloy with shearable precipitates. For the T761-treated alloy with coarse η’precipitates and wide PFZs, secondary cracks were found to propagate along the grain boundaries, which was associated with the higher FCP rate.Short crack propagation of T761-treated Al-Zn-Mg-Cu alloy was investigated by EBSD analysis and it was found that the formation of zigzag crack was ascribed to the high misorientation of adjacent grains. The plastic deformation zone surrounding the long crack tip of T761-treated Al-Zn-Mg-Cu alloy was analyzed by Micro-XRD, and the dissolution of precipitates was not observed. The results of EBSD analysis showed that long crack tended to propagate along grain boundary, therefore, the dissolution of n precipitates on grain boundary was not found on the condition of reciprocating motion of dislocations.The fatigue crack propagation mode of T761-treated Al-Zn-Mg-Cu alloy was revealed and a crystallographic model of the transition of crack propagation mode was given. It was showed that short crack tended to transgranularly propagate in the alloy, whereas intergranular long crack propagation was observed. Short crack propagation was predominated by single shear, and the effect of PFZs on the crack propagation could be ignored in this stage. However, long crack propagation was predominated by duplex slip mechanism. Moreover, the presence of wide PFZs accelerated the growth of long crack which propagated along the grain boundary.The research showed that the exfoliation corrosion resistances of RRA-treated, T761-treated and T6-treated Al-Zn-Mg-Cu alloys were decreased in turn, and the degrees of exfoliation corrosion were EA-, EA and EB, respectively. The enhanced corrosion resistance of T761-treated alloy and RRA-treated alloy was ascribed to the discontinued GBPs, whereas the continued GBPs presented in T6-treated alloy resulted in the severe corrosion. Compared with the T761-treated alloy, the anode dissolution of GBPs in RRA-treated alloy was abated due to the higher copper content of GBPs, which led to the enhanced corrosion resistance of RRA-treated alloy. The stress corrosion resistance of the alloy under various RRA treatments on the corrosion condition of482MPa was also studied and the results showed that the alloy treated at120℃/25h+190℃/10min+120℃/25h had the longest fracture time of260h due to the higher yield strength of507MPa. |
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