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A new phenomenological model, designed to capture the sigmoidal nature of stress dependency on strain rate for superplastic deformation, is presented. The model is developed for the Ti-6Al-2Sn-4Zr-2Mo alloy using data obtained under controlled strain-rate tensile tests spanning a range of strain rates and temperatures, from 930 to 980 8C. The sigmoidal model performance is compared with that of a more conventional double-power law, strain, and strain-rate hardening model using time-dependent finite element and theoretical analyses. The primary intended application of the sigmoidal model is for more accurate simulation of the effects of strain-rate variation within test specimens and sheet during superplastic deformation. Analysis of this variation within two designs of tensile test specimens is presented to illustrate this aspect.

This paper reports on recent work in the development of a finite element (FE) based forging optimization methodology. It utilizes the commercial FE package ABAQUS and the optimization code VisualDOC. Taking into account the effect of die and press elastic deflections and thermal distortion in cooling, a direct compensation approach and optimized weighting factor method are used to achieve the optimized die shape for improved dimensional accuracy of forged aerofoil blades. The significant predicted reduction in aerofoil shape errors using a single 2D section from a forged nickel-based blade demonstrates the efficiency and potential for applying this approach to more complex, realistic and industrially relevant 3D forging problems.

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