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Acta Metall Sin  2018, Vol. 54 Issue (1): 100-108    DOI: 10.11900/0412.1961.2017.00203
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Effect of Non-Isothermal Retrogression and Re-Ageing on Microstructure and Properties of Al-8Zn-2Mg-2Cu Alloy Thick Plate
Di FENG1(), Xinming ZHANG2, Hongmei CHEN1, Yunxue JIN1, Guoying WANG1
1 School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China
2 School of Materials Science and Engineering, Central South University, Changsha 410083, China
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Abstract  

7000 series Al alloy has been widely used in aeronautical structural materials because of its high strength, high stress corrosion cracking resistance and good fatigue resistance when treated by retrogression and re-ageing (RRA). Al-8Zn-2Mg-2Cu (mass fraction, %) thick plate is supposed to manufacture the aircraft wing in Chinese big plane project. Due to the non-isothermal environment in the process of heat treatment for aluminum alloy thick plate, the influence of coupling effect of non-isothermal retrogression temperature field and time on the microstructure and properties of Al-8Zn-2Mg-2Cu alloy thick plate was investigated by the non-isothermal kinetic model, mechanical properties tests, electrical conductivity test and TEM observations. The results show that, the electrical conductivity increases while the hardness and strength decrease with the non-isothermal retrogression time increasing. The optimized non-isothermal retrogression and re-ageing (NRRA) treatment makes the size distribution range of MgZn2 phase wider. Therefore,the logical matching between the dislocation cutting off mechanism and the dislocation by-passing mechanism effectively reduces the loss of hardness. Meanwhile, the electrical conductivity is significantly improved. After the treatment of 105 ℃, 24 h (pre-ageing) and non-isothermal regression (120 min) with slow heating rate and 120 ℃, 24 h re-ageing, the Al-8Zn-2Mg-2Cu alloy thick plate possesses an excellent comprehensive performance than those of T6 and T73 states. The tensile strength, yield strength and electrical conductivity are 620 MPa, 593 MPa and 21.1 MS/m, respectively. The NRRA treatment with slow heating rate is more suitable for the ageing treatment of thick plate.

Key words:  Al-Zn-Mg-Cu alloy      thick plate      non-isothermal retrogression and re-ageing      ageing kinetics     
Received:  25 May 2017     
ZTFLH:  TG146.2  
Fund: Supported by National Basic Research Program of China (No.2012CB619500), Natural Science Foundation of Jiangsu Province (No.BK20160560), Natural Science Foundation for Colleges and Universities of Jiangsu Province (No.16KJB430010), Priority Academic Program Development of Jiangsu Higher Education Institutions

Cite this article: 

Di FENG, Xinming ZHANG, Hongmei CHEN, Yunxue JIN, Guoying WANG. Effect of Non-Isothermal Retrogression and Re-Ageing on Microstructure and Properties of Al-8Zn-2Mg-2Cu Alloy Thick Plate. Acta Metall Sin, 2018, 54(1): 100-108.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00203     OR     https://www.ams.org.cn/EN/Y2018/V54/I1/100

Fig.1  Schematic of non-isothermal retrogression of Al-8Zn-2Mg-2Cu alloy thick plate (WQ—water quenching, AQ—air cooling)
Treatment Pre-ageing Retrogression Re-ageing
NRRA-i 105 ℃, 24 h 5 ℃min-1 and 190 ℃, i min 120 ℃, 24 h
T6 120 ℃, 24 h - -
T73 120 ℃, 6 h - 160 ℃, 24 h
Table 1  Parameters of ageing treatment
Fig.2  Ageing temperature-time T(t) curves of Al-8Zn-2Mg-2Cu alloy 30 mm thick plate (retrogression stage)
Fig.3  Hardness and electrical conductivity of Al-8Zn-2Mg-2Cu alloy 30 mm thick plate under different ageing treatments
Fig.4  Tensile properties of Al-8Zn-2Mg-2Cu alloy thick plate under different ageing treatments (σb—tensile strength, σ0.2—yield strength)
Fig.5  Bright field TEM images and corresponding SAED patterns along [001]Al (insets) of precipitates in matrix of Al-8Zn-2Mg-2Cu alloy thick plate under the ageing treatments of T6 (a), T73 (b), NRRA-95 (c), NRRA-120 (d), NRRA-160 (e) and schematic diagram of the diffraction patterns (f)[19]
Fig.6  Bright field TEM images of precipitates in grain boundary of Al-8Zn-2Mg-2Cu alloy thick plate under the ageing treatments of T6 (a), T73 (b), NRRA-95 (c), NRRA-120 (d) and NRRA-160 (e)
Fig.7  Scheil integral (Sc) -time curve of central layer in Al-8Zn-2Mg-2Cu alloy thick plate
Fig.8  Size distribution cartogram of intragranular precipitates under different ageing treatments
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