A superplastic forming limit diagram concept for Ti-6Al-4V

The current paper is concerned with the development of a simplified method for predicting failure due to plastic instability during the superplastic forming (SPF) of titanium alloys. The rationale is that a key factor in the process of reliable failure prediction is the incorporation of a mechanisms-based model, which includes microstructural effects, such as static and dynamic grain growth and associated hardening, and which is also independent of the forming strainrate. Existing methods for predicting plastic instability during conventional metal-forming are discussed along with previous attempts at predicting failure during SPF. It is shown that no easy-to-interpret method, such as the forming limit diagram (FLD) in conventional forming, exists for SPF. Consequently, an SPFLD concept in a major strain (ε1), minor strain (ε3), and equivalent strain-rate space (εeq) is presented on the basis of uniaxial SP ductilities across a range of strainrates along with the Hill-Swift instability criteria and using finite element-predicted ε1-ε3-εeq paths for key points on the forming blank to predict failure. The predicted results are validated against measured data for Ti-6Al-4V at different strain-rates.