Study On The Mechanical Properties Of 7055 Aluminum Alloy After Local Corrosion

Al-Zn-Mg-Cu alloys have been widely used as structural materials of aircraft due to its high strength and low density. During service process, local corrosion often occurs inevitably for these materials. So it is desirable to study not only the corrosion resistance but also the mechanical properties after local corrosion, which is of great importance for evaluation of its reliability and life.In this paper, the accelerated corrosion method has been used to study the influence of intergranular corrosion （IGC） and exfoliation corrosion （EXCO） on the ambient mechanical properties of 7055 aluminum alloy sheet with different microstructure. The mechanism was discussed based on the examination of optical microscopy, scanning electron microscopy, energy dispersed spectrum and transmission electron microscopy. The main conclusions are as follows:（1） After intergranular corrosion immersion, the strength and elongation are decreased for T6, T73 and RRA temper 7055 aluminum alloy sheet. The elongation exhibits a significantly higher drop percentage than the strength. The drop percentage of both strength and elongation is the highest for the T6 temper sheet, intermediate for the RRA temper sheet, and the lowest for the T73 temper one. Significant intergranular corrosion （IGC） was observed for T6 and RRA temper sheet. The largest corrosion depth is 0.170mm for the former temper with IGC rank of four, and only 0.093mm for the latter one with IGC rank of three. Pitting occurs for the T73 temper sheet.（2） After exfoliation corrosion, brittle fracture occurs for T6 temper sheet, its ultimate strength is decreased by 30%. For RRA temper sheet, the drop percentage in strength is about 18%, but that in the elongation is significantly higher to about 67%. For T73 temper sheet, the drop percentage is the lowest for both the strength and the elongation, which is about 14% and 40%, respectively. The EXCO rating is EB, EA and PC for the T6, RRA and T73 temper sheet , respectively.（3） After intergranular corrosion and exfoliation corrosion, a number of corrosion notches and intergranular cracks are present on the sheet surface. During tensile testing, stress concentration can occur at these sites, which increases the number of crack source. Meanwhile, the hydrogen due to the corrosion along grain boundary can decrease the strength of grain boundaries. Consequently, the ratio of intergranular fracture is increased for the corroded sheet after tensile testing, thus the strength and especially the elongation are decreased significantly. This effect is related to the grain structure as well as the second phase particles at grain boundaries in the sheet. In the T6 temper sheet, the second phase particles are distributed continuously at grain boundaries, which is favorable for the rapid growth of corrosion and crack along boundaries, thus, the highest drop percentage in the mechanical properties is observed. In the RRA and T73 temper sheet, however, the second phase particles coarsen and become more spaced at grain boundaries, which can retard propagation of corrosion and crack along boundaries, thus, lower drop percentage in the properties is observed. Elongated grains are more susceptible to EXCO than equiaxed grains, therefore, the RRA temper sheet with elongated grains exhibits lower tensile properties than that with equiaxed grains after exfoliation corrosion, and the drop in both the strength and the elongation is significantly higher as well.（4） With the exfoliation corrosion immersion time increasing, both the strength and the elongation exhibit a rapid decline in the initial 6h and subsequently a slower decline. After 24h, the tensile strength is lower than 500MPa, and the elongation is only about 1%. The elongation exhibits significantly higher drop than the strength after corrosion. The relationship between the drop percentage in the tensile properties and the corrosion time （t） can be expressed by the following equations.Δσ₍0.2(（t）=6.17×t ^0.356Δσ b（t）=4.86×t^0.440 Δδ（t）=84.7×(1-e^-0.576t) With the immersion time increasing, the corrosion depth is increased, more intergranular cracks due to corrosion are present, and the notch depth is larger, thus, a gradual drop in the tensile properties occurs. After mechanical removing the corrosion damage zone in the surface layer in the sheet, the tensile properties are not recovered to that before corrosion, which indicates occurrence of hydrogen embrittlement due to hydrogen produced by corrosion.