Metallurgical Characterization of Superplastic Forming

A systematic characterization of the superplasticity for regular- grade Ti-6Al-4V, ELI-grade Ti-6Al-4V, simulated coil-rolled Ti-6Al-4V, Ti-3Al-2. 5V, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1Mo-1V, and Ti-15V-3Cr-3Sn-3Al sheets with several different microstructures and texture was performed. The effects of alloy chemistry, grain size, volume fraction of constituent phases, and anomalous microstructures on the superplastic parameters were determined by incremental strain-rate, constant stress, constant strain rate and biaxial constant stress cone-forming tests. The strain-rate time or equivalently, strain dependences of flow stress and strain-rate sensitivity were identified as the most important superplasticity parameters, with the continuous changes in alloy microstructures during superplastic deformation requiring proper consideration. In the alpha-beta and near-alpha titanium alloys, the flow stress decreases and necking resistance increase with decreasing grain size at 850-950 F 1562 - 1742 F with increasing strain and time the flow stress at constant strainrate increases and the strain rate at constant applied stress decreases as a consequence of increasing grain size. Ti-6Al-4V alloys with elongated-alpha have significantly higher flow stress than equiaxed regular grade Ti-6Al-4V. The flow stress at a constant strain rate of different alpha-beta alloys is uniquely related to grain size, beta transus temperature, and volume fractions of constituent phases at the test temperature.