Nano-scale precipitate evolution and mechanical properties of 7085 aluminum alloy during thermal exposure

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Abstract

As a new generation of Al-Zn-Mg-Cu alloy, 7085 aluminum alloy is a promising structural material in the field of aerospace industry. However, research on its thermal stability is still lacking. In the present work, thermal exposure was carried out on the T7452-treated 7085 aluminum alloy under different temperatures (100 °C, 125 °C, 150 °C and 175 °C) for 500 h. Variations of tensile properties and hardness were exhibited. The microstructure, nano-scale precipitates and fracture characteristics of the alloy were investigated using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that with the increase of exposure temperature, the strength and hardness increase first and then decrease while the elongation and the reduction of area increase continuously as compared to those of the non-thermal exposed alloy. The transformation from η′ phase to η phase during thermal exposure occurs continuously during thermal exposure. In addition, as the exposure temperature increases, the average dimensions of precipitates and the average spacing of neighbor precipitates become larger. The influence of precipitates on mechanical properties of the alloy is discussed.

Introduction

The applications of Al-Zn-Mg-Cu alloys in aerospace field as large structural components have been limited due to their high quench sensitivity [1], [2], [3]. 7085 aluminum alloy belongs to Al-Zn-Mg-Cu alloys which offers significant improvements in the shortcoming by increasing both the Zn content and Zn/Mg weight ratio [4], [5], [6], [7]. T7452 treatment (two-stage aging) has been proved to further improve the fracture toughness and stress corrosion resistance of 7085 aluminum alloy [8], [9]. During the last decade, 7085-T7452 aluminum alloy plates have been mainly used as large structural components in advanced aircrafts due to their excellent properties [3]. Thus, many investigations have been carried out on the microstructures and properties of 7085 aluminum alloy [10], [11], [12].

During applications as structure airframe parts, the 7085-T7452 aluminum alloy is inevitably exposed to slightly elevated temperature (83–177 °C) during service [13], [14]. The thermal exposure often results from aerodynamic heating or due to heat generated by engines, electrical equipment and auxiliary power units [14], [15]. The microstructure and mechanical properties of the 7085 aluminum alloy under various conditions significantly affect the quality and reliability of the aerospace parts [14]. Therefore, it is significant to investigate the effect of thermal exposure on microstructure and mechanical properties evolution of aluminum alloys.

There have been some previous reports concerning thermal exposure on microstructures and mechanical properties of Al-Zn-Mg-Cu alloys [15], [16], [17]. For example, Shen et al. investigated the microstructures of 7050 aluminum alloy under different thermal exposure conditions. They found that both the precipitates and precipitate free zone became larger with the increase of thermal exposure temperature [16]. Zheng et al. researched the degradation of precipitation hardening in peak-aged 7075 alloy subjected to thermal exposure. Their results showed that, during the thermal exposure at 300 °C, nano-scale fine precipitates dissolved and formed coarse Mg(Zn, Cu)2 on the surface of Al18Mg3Cr2 dispersoid particles [15]. However, in contrast to the previous works for 7050 and 7075 alloys, there is only limited information concerning 7085 aluminum alloy. Although the mechanical properties of 7085-T7452 die forgings and 7085-T7651 plate thermally exposed at elevated temperature (180 °C, 230 °C and 290 °C) have been studied by Jabra and coauthors [18], yet there has been no report about the microstructural features of 7085 alloy during thermal exposure. Therefore, it is a vital requirement to understand the relationship between mechanical behavior and microstructure after thermal exposure from both experimental and theoretical viewpoints.

In the present work, the hardness and mechanical properties of thick-plate 7085 aluminum alloy after T7452 treatment have been characterized and the effects of different thermal exposure temperatures (100 °C, 125 °C, 150 °C and 175 °C) on microstructure and mechanical properties of 7085-T7452 plate were studied. The strengthening mechanism and relationship between the microstructure and mechanical properties after the thermal exposure treatment were also discussed.

Section snippets

Materials

7085 aluminum alloy forging with a thickness of 220 mm was studied in the present work. The chemical composition of this alloy is (wt%): Al-7.56Zn-1.50Mg-1.45Cu-0.12Zr-0.06Fe-0.02Si. We have carried out the T7452 heat treatment to the alloy, which consists of a 470 °C/6 h solution treatment, a 5% cold compression, followed by a two-step ageing treatment with 120 °C for 6 h and 160 °C for 10 h. Specimens were all taken from the region of the 7085-T7452 plates at the position of 1/2 in thickness.

Microstructures of 7085 aluminum alloy after two-stage aging (T7452) treatment

Fig. 1 shows microstructure of the as-received 7085 aluminum alloy forging after T7452 treatment. The three-dimensional microstructure is shown in Fig. 1(a). The microstructure of the alloy is a mixture of elongated un-recrystallized grains and equiaxed recrystallized grains distributed along the deformation direction, showing that partial recrystallization occurs. Fig. 1(b) presents the SEM backscattered electron image of the alloy. A large number of impurity-phase particles with size of

Discussion

The precipitation sequence of the second phases in Al-Zn-Mg-Cu alloys has been generally accepted as [30], [41], [42], [43], [44], [45], [46]: supersaturated solid solution (SSS) → GP zones → metastable η′ → stable η phase. In the present study, the main precipitates in the alloy are η′ and η after treated by T7452. The process of thermal exposure is considered to be treated as continued aging. A number of fine GP zones and η′ phases generate from the residual solute atoms in the matrix while

Conclusions

The microstructure characteristics of precipitates, hardness and mechanical properties of the novel 7085-T7452 aluminum alloy plate at various thermal exposure temperatures are investigated in this study. The following conclusions can be drawn:

  • (1)

    With the increase of thermal exposure temperature, the strength and hardness of the alloy increase first and then decrease. The alloy achieves the highest strength with 487 MPa of YS and 509 MPa of UTS after thermal exposure at 100 °C.

  • (2)

    As exposure

Acknowledgement

The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (51271147 and 51201135) and the Aviation Science Foundation of China (2016ZE53046).

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