A study of post-form static and fatigue properties of superplastic 7475-SPF and 5083-SPF aluminium alloys

The intention of this work was to derive conditions for superplastic forming of aircraft skin components to eliminate or minimise the deleterious effects of cavitation, so to meet design criteria, and thus to qualify superplastic forming as a forming technique for the manufacture of aircraft structural components. Two aluminium alloys were chosen for investigation, 7475-SPF (Formall 700) and 5083-SPF (Formall 545), supplied by Alusuisse Swiss Aluminium, in sheets of 1.6 and 2 mm thickness, respectively. These were chosen as representative of a high strength precipitation hardening alloy liable to cavitation and a solid solution alloy chosen for formability. A press was built permitting manual control of temperature, forming pressure and back-pressure. To assist selection of the forming conditions for the chosen component, the frictional behaviour of the two target alloys was measured and the forming process was simulated by the finite element method (FEM) using the implicit version of the ABAQUS™ [ABAQUS, Hibbitt, Karlsson and Sorenson Inc., RI, USA] FE package. Post-form properties of the superplastically formed component—thickness distribution, cavitation, static strength and fatigue strength—were studied to qualify the component for use in aircraft structures. It was shown that a maximum of 46% reduction in thickness occurred in the trial component. Cavitation was suppressed by using a back-pressure of 2 MPa, approximately 10 times the forming pressure. This compares with flow stresses of 3 and 5 MPa for 7475-SPF and 5083-SPF, respectively, under such conditions. Because the maximum equivalent plastic strain was limited to approximately to 0.8, relatively few cavities (1–2 vol.%) were formed. These reduced the tensile strength by only about 5%, but the effect on ductility was substantially greater, about 20%. Annealing after forming had limited effect on the post-forming properties of the alloys because it caused only some of the small cavities to sinter. However, in the T6 condition, the fatigue strength of 7475-SPF alloy formed under a back-pressure of 2 MPa was retained.