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 strain-rate. 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 (ε_eq) is presented on the basis of uniaxial SP ductilities across a range of strain-rates along with the Hill-Swift instability criteria and using finite element-predicted ε₁-ε₃- ε_eq 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.