Effects of pre-stretching and ageing on the strength and fracture toughness of aluminum alloy 7050

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Abstract

The effects of the pre-stretching and ageing on the microstructure, strength and fracture toughness of the 7050 aluminum alloy were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile test and the plane-strain fracture toughness test. The results indicate that the peak aged 7050 aluminum alloy possesses a higher strength, but its fracture toughness is poor. The high temperature and a subsequent low temperature ageing improve the fracture toughness by 8% while the strength is decreased by 3.5%. The fracture toughness of the over aged 7050 aluminum alloy is enhanced by 29% with a decrease in the strength of 7%. The retrogression and reaging provides an optimum combination of the strength and fracture toughness, which improves the fracture toughness by 17% without sacrificing the strength. After the pre-stretching, the precipitates within grains become coarse for all the ageing tempers which results in a lower strength. While the strength difference between the matrix and the grain boundary become lower and the area fraction of the grain boundary precipitates become smaller, leading to the higher fracture toughness.

Research highlights

► The 7xxx series aluminum alloy has been extensively used for manufacturing of aircraft structural wing components where a good compromise between the strength and fracture toughness is required. ► The strength can be improved through the appropriate thermo-mechanical treatments, but it is difficult to keep the fracture toughness at a high level simultaneously. ► In this paper, the effects of pre-stretching, peak ageing, high temperature and a subsequent low temperature ageing, over ageing, and retrogression and reaging on the tensile property and fracture toughness of the 7050 aluminum alloy are studied. ► These results are contributed to get an improved comprehensive property of the 7xxx series aluminum alloy.

Introduction

The 7xxx series aluminum alloy has been extensively used for manufacturing aircraft structural wing components where a good compromise between the strength and fracture toughness is required [1], [2], [3]. The strength can be improved through the appropriate thermo-mechanical treatments, but it is difficult to keep the fracture toughness at a high level simultaneously. Therefore, many investigations have been carried out on the fracture toughness of the 7xxx series aluminum alloy, which show that the fracture toughness is influenced by a range of microstructural features, such as the coarse particles [4], [5], the ageing precipitates [6], [7], the configuration of the precipitation free zone (PFZ) [8], [9] and the grain size [10], [11], [12].

Generally, the pre-stretching is applied to the 7xxx series alloy in the as-quenched state to relieve quench-induced internal stress. It is widely recognized that the pre-stretching results in the generation of the dislocations [13], [14]. Waterloo and co-workers [15] proposed that the nucleation of the equilibrium phase on the dislocation network led to a lower strength. Ageing is a primary and key step to obtain the improved mechanical properties for the 7xxx series aluminum alloy [16]. Although the peak aged alloy can obtain the peak strength, its corrosion resistance and fracture toughness is poor. To increase the corrosion resistance and fracture toughness, over ageing is developed. However, the strength of the over aged 7xxx series aluminum alloy is decreased by 5–10% compared to that of the peak aged alloy. Wang and Ma [17] proposed a novel heat treatment that included pre-stretching, high temperature ageing and subsequent low temperature ageing. The strength under this novel temper was slightly higher than that of the T76 one but the elongation increased obviously. The retrogression and reaging has a positive influence not only on the stress corrosion cracking resistance but also on the fracture toughness without sacrificing strength [18]. In a word, the microstructures and properties of the 7xxx series aluminum alloy are strictly influenced by the characteristics of the pre-stretching and ageing.

In this paper, the effects of pre-stretching, peak ageing (T6), high temperature and a subsequent low temperature ageing (HLA), over ageing (T74), and retrogression and reaging (RRA) on the tensile property and fracture toughness of the 7050 aluminum alloy are studied. These results are contributed to get an improved comprehensive property of the 7xxx series aluminum alloy.

Section snippets

Experimental procedure

The present study was carried out on the 80 mm thick hot rolled 7050 aluminum alloy plate with chemical composition of Al–6.06% Zn–2.20% Mg–2.12% Cu–0.11% Zr–0.08% Fe–0.04% Si (wt.%). The plates were solution treated at 473 °C for 1 h, quenched in room temperature water, pre-stretched along the rolling direction by 0% and 2.3%, and then aged at different temperatures. The heat treatment procedures were listed in details in Table 1.

The samples for metallographic observation were prepared through a

Results

Fig. 1 presents the optical micrographs of the 7050 aluminum alloy treated at different conditions. Large elongated areas are the recrystallized regions and the small grains are subgrains. The volume fraction of the recrystallized regions is less than 10%. The coarse second phase particles shown as white are presented as clusters of fragmented along the rolling direction. The optical microstructure of the 7050 aluminum alloy treated at different conditions is similar to that of the peak aged

Mechanical property

It is well known that in the alloys strengthened by precipitation hardening the increment of the yield stress is induced by the hindering effect of the second phase particles on the movement of the dislocations. The increment of the yield stress caused by the shearable particles is as follows [20]:τc=1.44ΓGb3/2fDb1/2GWhen the particles are unshearable, the increment of the yield stress caused by Orowan mechanism, which can be given in the following form:τc=0.85f2G2π1νbln(1.57D/r0)D(11.57f/2

Conclusion

  • (1)

    The peak aged samples possess a small radius of the precipitates, which lead to the high strength but low fracture toughness. The matrix precipitates become coarse and the area fraction of the grain boundary precipitates become lower for the high temperature and a subsequent low temperature aged samples and the over aged samples, which improves the fracture toughness by 8% and 29%, while decrease the strength by 3.5% and 7%, respectively. The matrix precipitates of the retrogression and reaged

Acknowledgments

The authors would like to appreciate the financial supports from State Key Fundamental Research Program of China (no. 2005CB623706) and National Natural Science Foundation of China (no. 50230310 and no. 50905188).

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