The effect of quench transfer time on microstructure and localized corrosion behavior of 7050‐T6 Al alloy

Abstract

The microstructure and localized corrosion behavior of the 7050‐T6 Al alloys treated with different quench transfer time were investigated. Optical microscope observations show that the volume fraction of the recrystallized grains increases slightly with prolonging quench transfer time. Scanning electron microscope observations reveal that the stable η (MgZn2) phase nucleates and precipitates on grain boundaries in the process of transferring to quench. Further observations, using transmission electron microscope, found that the size, nearest neighbor distance, and copper content of the grain‐boundary precipitates increase with quench delay. As a result, the open‐circuit potentials and charge transfer resistance (Rt) of the alloys reduce with increasing transfer time, while the susceptibility to intergranular corrosion (IGC) and strength loss after exfoliation corrosion tests increase sharply. In addition, the IGC network appearance changes from large network to fine network structure, due to the different temperature range where very rapid η phase precipitation takes place between on grain boundaries and on sub‐grain boundaries.

Mechanism of low exfoliation corrosion resistance due to slow quenching in high strength aluminium alloy

“…Statistically for Zn, the content tends to increase gradually with the decrease of quench rate, and this trend is different from previous results. In Chen's work [31], the Zn content was reported to decrease with the decrease of quench rate; in Knight's [32] and Song's [33] work, the Zn content was not affected by quench rate. For Mg, the content increases significantly with quench rate decreasing from 1250°C/min to 630°C/min, then remains almost unchanged and exhibits a further slight increase with quench rate further decreasing to 138°C/min.…”

“…For an AlZn-Mg-Cu alloy thick plate, the content of Cu in GBPs was also found to decrease with the decrease of quench rate, which possibly contributes to lower corrosion resistance [30]. For a 7050 aluminium alloy sheet, however, the Cu content in GBPs was found to increase with the increase of quench delayed time, which was supposed to be the reason for the improvement of corrosion resistance [33]. For a 76 mm plate of 7079 aluminium alloy, the content of Cu in GBPs was higher in the mid-thickness position than the T/6 position because of a lower quench rate, and this was believed to be the main reason for higher resistance to stress corrosion cracking [24].…”

Pitting corrosion of naturally aged AA 7075 aluminum alloys with bimodal grain size

“…A concurrent decrease in zinc, magnesium content and increase in copper content would make η-phase less anodic. Therefore, the increase in copper content shifts the potential of η-phase towards noble direction, while the presence of zinc and magnesium would decrease the potential of η-phase and make it more active [9,10,40,43,66,76,[81][82][83][84][85][86][87]. In this study, the η-phase in coarse grains has bigger size and higher content of magnesium, zinc and copper compared to that in fine grains.…”

“…For example, Al-Zn-Mg-Cu series alloys are widely used in aerospace industry because of their low density and high strength, but they are very susceptible to localized corrosion, such as pitting, intergranular corrosion (IGC) and exfoliation corrosion (EXCO). Both the anodic particles (usually containing Al, Zn and Mg) and the cathodic particles (usually containing Al, Fe, Cu and Mn) can induce pitting corrosion, because they exhibit different electrochemical activity and passivation ability compared to the surrounding matrix [7][8][9][10].…”

Effects of Solution Treatment on Microstructure and High-Cycle Fatigue Properties of 7075 Aluminum Alloy

“…The recrystallized grain and the remaining large second phase significantly influenced the fracture toughness properties of the aluminum alloy. After the solution treatment, the second phase of the aluminum alloy, the recrystallization fraction, and the change of the grain morphology also have a great influence on the material's fatigue property [11][12][13][14][15]. The dissolution of the large second phase is beneficial to the increase of the supersaturation degree of a supersaturated solid solution before ageing, thus reducing the source of micro-cracks and promoting the fatigue performance of the alloy [16].…”