J. Cent. South Univ. (2016) 23: 1858-1862
DOI: 10.1007/s11771-016-3240-8
Effect of a novel three-step aging on strength, stress corrosion cracking and microstructure of AA7085
CHEN Song-yi(陈送义)1, 2, 3, CHEN Kang-hua(陈康华)2, 3, DONG Peng-xuan(董朋轩)2, 3,
YE Sheng-ping(叶升平)2, 3, HUANG Lan-ping(黄兰萍)2, 3, YANG Dai-jun(阳代军)4
1. Light Alloy Research Institute, Central South University, Changsha 410083, China;
2. Collaborative Innovation Center of Advanced Nonferrous Structural Materials and Manufacturing,
Central South University, Changsha 410083, China;
3. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;
4. Capital Aerospace Machinery Company, Beijing 100076, China
Central South University Press and Springer-Verlag Berlin Heidelberg 2016
Abstract: The influence of a novel three-step aging on strength, stress corrosion cracking (SCC) and microstructure of AA7085 was investigated by tensile testing and slow strain rate testing combined with transmission electron microscopy (TEM). The results indicate that with the increase of second-step aging time of two-step aging, the mechanical properties increase first and then decrease, while the SCC resistance increases. Compared with two-step aging, three-step aging treatment improves SCC resistance and the strength increases by about 5%. The effects of novel three-step aging on strength and SCC resistance are explained by the role of matrix precipitates and grain boundary precipitates, respectively.
Key words: 7085 aluminum alloy; three-step aging; strength; stress corrosion cracking; microstructure
1 Introduction
AA7085 with high fracture toughness and slow quench sensitivity has been developed by Alcoa as the high-strength thick-plate alloy. The superiority of AA7085 may be ascribed to the relatively high zinc coupled with low copper and magnesium in comparison with the others [1-4]. The combination of high strength and stress corrosion cracking (SCC) resistance was contradictory and it is significantly affected by composition, heat treatment, etc [5-9]. For example, Zn and Mg increased the strength while decreased the corrosion resistance. T6 temper provided high strength but was susceptible to SCC. Many efforts have been put forward to increase strength and SCC resistance. Retrogression and reaging (RRA) [10-13] decreased the SCC susceptibility and kept the strength levels similar to T6 temper. Repetitive- RRA could further improve SCC resistance with retention of strength compared to RRA temper [14-15]. However, the retrogression temperature was in the range of 180-260 °C for several minutes which was difficult to be applied on thick plate or forging [16-17]. A novel three-step aging temper has been proposed for the Al-Zn-Mg-Cu thick plate [18]. Compared with two-step aging, the novel three-step aging increased SCC resistance combined with fracture toughness, but there is little information available of AA7085, especially the relationship between microstructure and properties. The purpose of the work is to investigate the effect of multi-step (three-step aging) on strength, SCC and microstructure of AA7085.
2 Experimental
The material used in the present work was thick hot- forged AA7085 (7.5%Zn, 1.5%Cu, 1.6%Mg, 0.12%Zr, 0.06%Fe, 0.02%Si and balance Al, mass fraction).
Tensile properties testing was performed on smooth plate specimens by an Instron 3369 testing machine at room temperature with a tensile speed of 2 mm/min. The gauge length and width of the specimens were 25 mm and 6 mm, respectively.
The SCC susceptibility test was performed according to the GB/T 15970.7—2000. The SCC susceptibility was evaluated using the slow strain rate test (SSRT) with a strain rate of 3.3×10-7 s-1 in air and in 3 % NaCl + 0.5% H2O2 aqueous solution. Rectangular tensile specimens with a gauge length of 30 mm and a width of 6 mm were used. The susceptibility to SCC was calculated by the ratio of loss of elongation. The expression was defined as follows: ISSRT=ICorr/IAir, where ICorr.is the elongation in air, and IAir is the elongation in corrosion solution.
Microstructures were studied by transmission electron microscope (JEOL-2100F) operated at 200 kV. Thin foils for TEM were prepared by mechanical polishing to 100 μm and final twin-jet electro polishing in the solution of 25% HNO3+75% CH3OH at -25 °C.
3 Results
3.1 Effect of two-step aging on properties of AA7085
Mechanical properties of the alloy after two-step aging with pre-aging at 110 °C for 6 h are shown in Fig. 1. With the increase of aging time, the ultimate tensile strength (UTS) and yield strength (YS) increase first and then decrease. While the UTS and YS are significantly decreased with the increase of aging temperature. For example, under the condition of aging temperature at 150 °C and aging time from 2 h to 24 h, the UTS and YS decrease from 544 MPa and 507 MPa to 511 MPa and 476 MPa, decreased by 6.07 % and 6.11%, respectively. While under the condition of aging temperature at 170 °C with aging time from 2 h to 24 h, the UTS and YS decrease by 19.2% and 29.6%, respectively.
Fig. 1 Effect of two-step aging on tensile strength of AA7085:
Figure 2 depicts the results of slow strain rate testing in air and in 3% NaCl+0.5% H2O2 under two-step aging heat treatment. Compared with the properties of the samples tested in air and in corrosion solution, it is found that both tensile strength and elongation are lower in the 3% NaCl+0.5% H2O2 solution than in air for the corresponding samples. It is indicated that the alloy has SCC susceptibility. ISSRT results of AA7085 with various two-step aging treatments are given in Table 1. It is revealed that the SCC resistance increases with increasing aging temperature and time.
3.2 Effect of three-step aging on properties of AA7085
The hardness and electrical conductivity of the alloy after three-step aging are shown in Fig. 3. The hardness decreases while the electrical conductivity increases with increasing second-step aging time regardless of two-step aging or three-step aging. Meanwhile, the hardness of the alloy after three-step aging is higher than that of two-step aging (increases to about HV5-10, increased by 3%-5%)(Fig. 3(a)). The conductivity of the alloy after three-step aging is higher than that of two-step aging, increased by about 2%-3% (Fig. 3(b)).
Fig. 2 SSRT curves of AA7085 under two-step aging heat treatment:
Table1 SSRT results of AA7085 with various two-step aging heat treatments
Fig. 3 Effect of three-step aging on hardness and conductivity of AA7085:
The mechanical properties of the alloy after three-step aging are shown in Fig. 4. The mechanical properties of the alloy decrease with the second step aging time regardless of two-step aging or three-step aging. The strength of the alloy after three-step aging is higher than that of the corresponding two-step aging. Under two-step aging of 110 °C, 6 h+160 °C, 2 h, tensile strength and yield strength are 546 MPa and 512 MPa, respectively. Under 110 °C, 6 h+160 °C, 2 h +120 °C, 24 h, tensile strength and yield strength increase to 567 MPa and 542 MPa, increased by 3.8% and 5.8%, respectively. However, under two-step aging of 110 °C, 6 h+160 °C, 24 h, the tensile strength and yield strength are 468 MPa and 416 MPa, respectively, while tensile strength and yield strength under three aging (110 °C, 6 h+160 °C, 24 h+120 °C, 24 h) are 476 MPa and 426 MPa, increased by 1.7% and 2.4%, respectively. The results indicate that the strength of the alloy after three-step aging is increased by about 2%-5% compared to two-step aging.
Fig. 4 Effect of three-step aging on strength of AA7085:
Figure 5 depicts the results of slow strain rate testing in air and in 3% NaCl+0.5% H2O2 under different aging heat treatments. Compared with the properties of the samples tested in air and in corrosion solution, it is found that both tensile strength and elongation are lower in 3% NaCl+0.5% H2O2 solution than in air for the corresponding samples. ISSRT results of AA7085 with various three-step aging treatment are given in Table 2. Three-step aging improves the stress corrosion resistance compared with the two-step aging.
The TEM microstructure and electron diffraction pattern of AA7085 under 110 °C, 6 h+160 °C, 24 h and 110 °C, 6 h+160 °C, 24 h+120 °C, 24 h are shown in Fig. 6. Compared with the two-step aging, after the third step of 120 °C, 24 h, the amount of fine intra-granular precipitates increases, and precipitates free zone is broadened (Fig. 6(b)). The diffraction pattern of the alloy under 110 °C, 6 h+160 °C, 24 h is shown in Fig. 6(c). Diffraction spots appear in the 1/3{220} and 2/3{220}, which indicates the spot with η phase (Fig. 6(c)). After 110 °C, 6 h+160 °C, 24 h+120 °C, 24 h, the diffraction spots change, besides in the 1/3{220} and 2/3{220} appearing at the diffraction, which indicates that thestability of the η phase, and the spots in the 1/3{311} and 2/3{311} show the presence of a GPI zone (Fig. 6(d)).
Fig. 5 SSRT curves of AA7085 with various three-step aging:
Table 2 SSRT results of AA7085 with various three-step aging heat treatments
4 Discussion
For Al-Zn-Mg-Cu alloy, the precipitates such as G. P. zones or η phase, contain most of Cu [19-20], and Cu increases the stability of G. P. zones [20]. It was also reported that the Cu content of precipitate increases with increasing Cu alloy content [21]. In the relatively high zinc coupled with low copper AA7085 in comparison with AA7075 and AA7050, as a consequence, less of the Cu enters into the precipitates and part of the solute atoms dissolves at the second aging temperature of 160 °C [22-24]. Finer precipitates appear after further aging treatment (Fig. 6). It is the equivalent of a low-temperature process for regression (regression temperature usually occurs at 180 °C) compared to RRA treatment. The hardness and mechanical properties of three-step aging are higher than those of the corresponding two-step aging. It can be attributed to more GPI zone precipitated during the third step aging treatment. For the two-step aging treatment, the stress corrosion resistance increases with the increase of aging temperature and aging time, which is attributed to the size of GPBs becoming coarsening and more discontinuous. These explanations can also be used to illustrate the fact that three-step aging has higher stress corrosion resistance than that of two-step aging process.
Fig. 6 Effect of three-step aging on microstructure of AA7085:
5 Conclusions
1) With increasing second aging time of two-step aging, the strength of the alloy increases first and then decreases, while the SCC resistance increases.
2) Compared with the two-step aging, three-step aging treatment not only improves SCC resistances but also increases the strength by 5%, which is attributed to coarse GPBs and precipitated GPI zone, respectively.
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(Edited by YANG Bing)
Foundation item: Project(2012CB619502) supported by the National Basic Research Program of China; Project(2016YFB0300800) supported by the National Key Research and Development Program of China; Project(CALT201507) supported by the CALT Research Innovation Partnership Fund, China; Project(HPCM-201403) supported by the State Key Laboratory of High Performance Complex Manufacturing, China
Received date: 2015-06-30; Accepted date: 2016-01-18
Corresponding author: CHEN Song-yi, PhD; Tel: +86-731-88830714; E-mail: sychen08@csu.edu.cn
