AbstractSuperplastic forming of AA 7475 aluminium alloy sheets of grain size ~15μm has been studied by imposing hydrostatic back pressures of between 0·1 and 2·86 MPa. Qualitative and quantitative studies of the effect of hydrostatic pressure on the cavitation of the alloy were carried out on formed domes. On the diametral section of a dome, cavities were mainly found in the centre of sheet thickness, where the grains were coarser than those near the surface. Dome height at failure was significantly increased by imposing hydrostatic back pressures during superplastic forming. For domes that were formed into top hat shapes, a maximum height/base radius ratio of 2:1 was obtained under a hydrostatic pressure of 2·86 MPa. Flat tensile testpieces were machined from top hats to evaluate the room temperature mechanical properties. It was found that the postformed ductility deteriorated with superplastic strain more rapidly than did the ultimate tensile strength. The effect of surface grooves, which were observed in formed domes and top hats, on postformed properties was also examined.MST/1576