Effect of Aging Heat Treatment on Corrosion Fatigue Crack Propagation of 7075 Aluminum Alloy

Abstract

A study has been made of corrosion fatigue crack growth for 7075 aluminum alloy in 3.5% NaCl solutions under both open circuit and cathodic charging, a pure water, an air and a dry argon at ambient temperature. The effect of aging treatment at 150°C on the corrosion fatigue crack growth rate (da/dN) as a function of ΔK was examined and the fracture surfaces were observed. The crack propagation curves (da/dN vs. ΔK) for 7075 alloy solution-treated underaged, peakaged, overaged and as-received were obtained in above different environments. The value of da/dN for solution-treated, underaged and overaged alloys was arranged in the following order of the environments: open circuit>catholic charging>pure water>air>dry argon. In peakaged 7075 alloy, the crack growth rate under cathodic charging was greater than that in open circuit, and this acceleration of crack growth was explained by the influence of hydrogen embrittlement. The relative contribution of the corrosive environment to the fatigue crack propagation was well evaluated by comparison of (da/dN)_{c. f.} in each environment with (da/dN)_ref. in a reference dry argon environment at a certain ΔK. The additional effect of stress corrosion cracking (SCC) to corrosion fatigue crack propagation was discussed on the basis of the value of K_ISCC and from both sides of the active path corrosion and the hydrogen embrittlement types for SCC mechanism in 7000 series aluminum alloys -3.5% NaCl solution system.