OPTIMISATION OF STRAIN RATE SENSITIVITY DURING SUPERPLASTIC DEFORMATION OF Al-Li-ALLOY LITAL A

Abstract
A study has been made of the superplastic behaviour of Al-Li alloy (Lital A) sheet of 3 mm thickness deformed in uni-axial tension under constant strain rate and constant cross-head velocity conditions. Significantly higher strains to failure were observed for constant velocity straining than for constant strain rate testing. Optical metallographic studies and measurements of the strain rate sensitivity of flow stress, m, as a function of strain showed that during superplastic deformation grain growth led to a displacement of the strain rate for maximum m, the optimum strain rate, to progressively lower levels. Hence, if the strain rate during a superplastic forming (SPF) process is correspondingly reduced, as it would be during a constant velocity uni-axial test, it should be possible to maintain a high m value during forming and to minimise non-uniform thinning and premature fracture (and maximise elongation to failure in a uni-axial test). A progressive fall in strain rate could also minimise the rise in flow stress due to grain growth and this could be beneficial if SPF is being carried out with an imposed hydrostatic pressure.