Critical quenching rate for high hardness and good exfoliation corrosion resistance of Al-Zn-Mg-Cu alloy plate

Abstract

By means of the end-quenching technique, we investigated the relationship between quenching rate and hardness as well as exfoliation corrosion rating for Al-2.21 Zn-3.59 Mg-0.45 Cu-0.038 Zr (at%) alloy plate. In order to achieve an exfoliation corrosion rating of P or EA, the quenching rate must be greater than approximately 460°C/min and 300°C/min, respectively, and the drop degree in hardness should simultaneously be lower than approximately 2.0% and 3.5%, respectively. The results of microstructural and microchemical examination using a scanning transmission electron microscope indicate that a lower quenching rate leads to a higher content of Zn, Mg, and Cu in the grain-boundary particles and a greater width of precipitate-free zones near grain boundaries; therefore, grain-boundary particles with Zn and Mg contents less than approximately 13.39% and 10.23% (at%), respectively, and precipitate-free zones near grain boundaries with widths less than about 107 nm can contribute to an exfoliation corrosion rating better than EA. The amount of quench-induced η-phase particles, which lead to lower hardness, increases with decreasing quenching rate, and the area fraction of these particles is approximately 2.9% at a quenching rate of 300°C/min.

Investigation of the Quenching Sensitivity of the Mechanical and Corrosion Properties of 7475 Aluminum Alloy

“…Song et al [15] observed an increase in the exfoliation corrosion sensitivity of the AA7050 alloy with a decreasing quenching rate. Li Dongfeng et al [16] discovered that as the quenching rate decreased from 2160 • C/min to 100 • C/min, the exfoliation corrosion grade of Al-5Zn-3Mg-1Cu alloy sheets gradually shifted from P grade to ED grade. Liu et al [17] revealed that the maximum exfoliation corrosion depth of AA7055 alloy gradually increases as the quenching rate decreases, indicating an ascending trend in exfoliation corrosion sensitivity.…”

Response of Microstructure in High Strength Aluminium Alloy to Graff Sargent Etchant

“…This etchant was also used to reveal the grain structure of high strength 7000 series ingot metallurgy alloys. For instance, it could give good results of recrystallized grains and even subgrains in some 7000 series aluminum alloys [2][3][4], and it was likely because grain boundaries (GBs) and subgrain boundaries (SGBs) in these alloys could be preferentially attacked during immersion in this etchant. In high strength 7000 series aluminum alloys, the microstructure is often very complex due to partical recrystallization and intermetallic particles (e.g.…”