Hot Compression Behaviour of Two-Phase Ti-6A1-4V: Experiments and State-Variable Modelling

Mulyadi (2007). Hot Compression Behaviour of Two-Phase Ti-6A1-4V: Experiments and State-Variable Modelling. PhD thesis The Open University.



The flow stress behaviour of two-phase Ti-6A1-4V and its individual α and β phases has been characterised during isothermal forging at temperatures of 850-1050°C and strain rates of 0.3-0.003/s. The influence of initial pre-form microstructure has also been investigated by heat-treating the as-received globular microstructure to produce an acicular β-transformed microstructure. It is found that flow stress behaviour exhibits a strong dependence on working temperature and imposed strain rate for the two-phase globular, two-phase acicular and single α-phase materials. However, for the single β-phase, steady state stress is found to be relatively constant with increasing temperature for the range of 925-975°C and strain rates of 0.3-0.003/s.

A pronounced discontinuous yielding phenomenon has been observed for globular Ti-6A1-4V at high temperature (≥900°C) and high strain rates (≥0.01/s). On the other hand, for acicular Ti-6A1-4V, flow stress curves reveal a broad peak stress level at low strains followed by moderate to extensive flow softening until a steady-state stress is reached. For the individual phases in Ti-6A1-4V, flow stresses of the single β-phase is found to be much lower than for the α-phase and most of flow stress curves of the β-phase are of a steady-state type with negligible flow softening. There is an indication of initial work hardening behaviour after the onset of plastic deformation at low strains for deformation of single α-phase at 925-950°C and 0.3/s, whilst a pronounced flow softening is exhibited particularly at 975°C and strain rates of 0.03-0.3/s.

A semi-empirical, history-dependent constitutive model for the prediction of the flow behaviour of Ti-6A1-4V which incorporates the temperature-dependent volume fraction and flow properties of the individual α and β phases is presented. It is found that a modified iso-strain approach can be employed in order to better predict the entire flow stress curve of acicular Ti-6Al-4V including the post-peak softening behaviour. This is achieved by introducing structural variable to represent the strain accumulation resulting from gross interaction mechanisms between the α and β phases during deformation.

The structural interaction variable has been linked with microstructure information from deformed cylindrical work-pieces, allowing derivation of a set of microstructure relationships. To provide the basis for more general process simulations, the modified iso-strain model has been implemented as a user-routine in DEFORM-2D finite element software and validated for an isothermal forging of a complex-shape double-truncated cone specimen. Excellent agreement was observed between the predicted forging load and the measured load-displacement data and trends in the evolved microstructure were also well-predicted.

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