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

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 degrees C/s with the I-SSRT value of approximately 0.049. With the decrease in the quenching rate, the crack propagation changes from a transgranular to a intergranular mode; the 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 degrees C/s, and Cu content increases rapidly when the quenching rate is lower than 5.3 degrees C/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.