STRESS-CORROSION CRACKING OF SUPERPLASTICALLY FORMED 7475-ALUMINUM-ALLOY

The effects of biaxial superplastic deformation and postforming heat t reatment upon the stress corrosion cracking (SCC) of a fine-grained 74 75Al alloy plate have been investigated. For all postforming tempered conditions, increasing the extent of superplastic deformation, which c reated more cavitations, would decrease the mechanical properties, the SCC resistance, and the corrosion resistance. The influence of cavita tion on the decay of elongation of the superplastically formed workpie ces is larger than that on the decay of its strength. Post-forming tem pered by retrogression and reaging (RRA) treatment could effectively i mprove the SCC resistance of workpieces in postforming T6 temper while not sacrificing the strength. However, the benefit of improving the S CC resistance by means of the postforming RRA temper was decreased wit h increasing the extent of superplastic deformation, because the SCC s usceptibility increased as the extent of superplastic deformation incr eased for each postforming tempered condition. The cavitation led to m ore anodic corrosion potential and pitting potential and to an increas e in both corrosion current density and passive current density, which would increase the SCC susceptibility.