Effect of Quenching Rate on Stress Corrosion Cracking Susceptibility of 7136 Aluminum Alloy

doi:10.11900/0412.1961.2021.00053

Acta Metall Sin

2022, Vol. 58

Issue (9): 1118-1128 DOI: 10.11900/0412.1961.2021.00053

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Effect of Quenching Rate on Stress Corrosion Cracking Susceptibility of 7136 Aluminum Alloy

MA Zhimin¹^,²^,³, DENG Yunlai¹^,³, LIU Jia², LIU Shengdan¹^,³(

), LIU Honglei⁴

^1.School of Materials Science and Engineering, Central South University, Changsha 410083, China
^2.Baotou Vocational and Technical College, Baotou 014030, China
^3.Key Laboratory of Non-Ferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, China
^4.Northeast Light Alloy Company Ltd., Harbin 150060, China

Cite this article:

MA Zhimin, DENG Yunlai, LIU Jia, LIU Shengdan, LIU Honglei. Effect of Quenching Rate on Stress Corrosion Cracking Susceptibility of 7136 Aluminum Alloy. Acta Metall Sin, 2022, 58(9): 1118-1128.

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Abstract

7xxx series aluminum alloys are well-known structural materials, and they have been used in various fields, such as aerospace and vehicle, owing to their low density, high strength, and good formability. However, they are susceptible to stress corrosion cracking (SCC). SCC reduces the service life of these alloys and limits their application. With the development of the aerospace and vehicle industries, high-strength 7xxx series aluminum alloys are manufactured as semiproducts with large sections, such as thick plates, to avoid welding and jointing defaults. Quenching is a critical step for producing thick plates because their properties, such as SCC susceptibility, are sensitive to the quenching rate, and the quenching rate is generally lower at the center layer than at the surface layer during quenching. In this study, the effect of quenching rate on the SCC susceptibility of 7136 aluminum alloy was investigated using immersion end-quenching technique and slow strain rate tensile (SSRT) test. The strength, elongation, and fracture time of the samples after SSRT in oil and NaCl solution were obtained, and the crack features on and near the fracture surface were examined using SEM. SCC susceptibility was evaluated using the reduction rates of strength, elongation, fracture time, and stress corrosion sensitivity index (I_SSRT). The mechanism is discussed herein based on microstructural examination using SEM, EBSD, STEM in a HAADF mode, and EDS. Results show that the SCC susceptibility of the alloy first increases and then decreases with the decrease in the quenching rate. The SCC susceptibility is the highest at the quenching rate of approximately 5.3°C/s with the Ithe number and size of quench-induced precipitates increase gradually, and the precipitate free zones (PFZs) near grain boundary (GB) and subgrain boundary (SGB) widen; and the contents of Zn and Mg in precipitates at grain boundaries increase rapidly when the quenching rate is greater than 5.3^oC/s, and Cu content increases rapidly when the quenching rate is lower than 5.3^oC/s. The quench-induced changes in the morphology and chemical composition of precipitates at GB/SGB are the main reasons for the SCC susceptibility to increase first and then decrease with the decrease in the quenching rate.

Key words: aluminum alloy stress corrosion cracking quenching rate microstructure

Received: 29 January 2021

ZTFLH:

TG146.2

Fund: National Key Research and Development Program of China(2016YFB0300901);Scientific Research Project of Inner Mongolia Colleges and Universities(NJZY21092)

About author: LIU Shengdan, professor, Tel: (0731)88830265, E-mail: lsd_csu@csu.edu.cn

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https://www.ams.org.cn/EN/10.11900/0412.1961.2021.00053 OR https://www.ams.org.cn/EN/Y2022/V58/I9/1118

Fig.1 Schematic of end quenching and drawing of slow strain rate tensile sample (unit: mm; ED—extruded direction, ND—normal direction, TD—transverse direction; blue indicates the part in the water, gray indicates the part in the air)

Fig.2 Cooling curves and average quenching rates at different positions of the 7136 aluminum alloy sample
(a) time-temperature curve
(b) average quenching rate

Fig.3 Stress-strain curves of the 7136 aluminum alloy samples under quenching rates of 263.0, 5.3, and 1.8^oC/s

Fig.4 The strength, elongation, and fracture time of 7136 aluminum alloy samples under different quenching rates, and corresponding reduction rate
(a) strength (R_mOil is the strength in silicone oil, and R_mNaCl is the strength in NaCl solution) and strength reduction rate (K_R)
(b) elongation (A_Oil is the elongation in silicone oil, and A_NaCl is the elongation in NaCl solution) and elongation reduction rate (K_A)
(c) fracture time (t_Oil is the fracture time in silicone oil, and t_NaCl is the fracture time in NaCl solution) and fracture time reduction rate (K_t)

Fig.5 Stress corrosion sensitivity index (I_SSRT) of 7136 aluminum alloy samples under different quenching rates

Fig.6 Low (a-c) and locally high (a1-c1) magnified fracture SEM images of 7136 aluminum alloy samples in NaCl solution under quenching rates of 263.0^oC/s (a, a1), 5.3^oC/s (b, b1), and 1.8^oC/s (c, c1)

Fig.7 SEM images near the fracture surface of 7136 aluminum alloy in NaCl solution under quenching rates of 263.0^oC/s (a, a1, a2), 5.3^oC/s (b, b1, b2), and 1.8^oC/s (c, c1, c2)
(a-c) low magnification
(a1-c1) locally magnified images of box regions in Figs.7a-c, respectively
(a2-c2) locally magnified images of box regions in Figs.7a1-c1, respectively (GB—grain boundary, GBP—precipitate at grain boundary, SGB—sub-grain boundary)

Fig.8 Grain orientation maps of 7136 aluminum alloy samples under quenching rates of 263.0^oC/s (a) and 1.8^oC/s (b)

Fig.9 SEM images of 7136 aluminum alloy samples under quenching rates of 263.0^oC/s (a) and 1.8^oC/s (b)

Fig.10 STEM-HAADF images of 7136 aluminum alloy samples under quenching rates of 263.0^oC/s (a), 5.3^oC/s (b), and 1.8^oC/s (c) (SGBP—precipitate at sub-grain boundary, PFZ—precipitate free zone)

Fig.11 Parameters of GBP, SGBP, and PFZ in 7136 aluminum alloy samples under different quenching rates

Fig.12 STEM-HAADF image (a) and EDS element maps of Al (b), Zn (c), Mg (d), and Cu (e) for 7136 aluminum alloy sample under quenching rate of 263.0^oC/s

Fig.13 Contents of Zn, Mg, and Cu in GBPs of 7136 aluminum alloy samples under different quenching rates

Fig.14 Schematics of crack propagation of stress corrosion cracking (SCC) in 7136 aluminum alloy under quenching rates of 263.0^oC/s (a), 5.3^oC/s (b), and 1.8^oC/s (c)

Fig.15 Proportions of grain boundaries and sub-grain boundaries with different PFZ widths in 7136 aluminum alloy samples under quenching rates of 5.3 and 1.8^oC/s

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