Microstructure and properties of superplastically formed aluminium alloy 7475 pans

Abstract

The effect of back pressure and forming pressure on thickness distribution, cavitation, grain growth, surface topography, and corrosion properties was studied in superplastically formed aluminium alloy 7475 pans. Deformation did not significantly affect grain size. Cavitation and surface roughness increased with increasing strain. Moreover, thickness distribution was more uniform and cavitation was minimised when the parts were formed under optimum superplastic conditions and back pressure was more than half of the flow stress. Large cavities were formed at intersection sites of shear surfaces along which groups of grains slid as blocks. The operation of such shear surfaces also resulted in the formation of both steps at the surface of the deformed sheets and threadlike fibres at sliding grain boundaries. Exfoliation corrosion of superplastically formed 7475 heat treated to T5 was comparable to that of aluminium alloy 7075-T6. Exposure of the superplastically formed material to an aggressive salt spray environment significantly deteriorated its ductility, but had much less of an effect on yield and ultimate tensile strength.