Abstract:The paper focuses on developing constitutive models for superplastic deformation behaviour of near-α titanium alloy (Ti-2.5Al-1.8Mn) at elevated temperatures in a range from 840 to 890 • C and in a strain rate range from 2 × 10 −4 to 8 × 10 −4 s −1 . Stress-strain experimental tensile tests data were used to develop the mathematical models. Both, hyperbolic sine Arrhenius-type constitutive model and artificial neural-network model were constructed. A comparative study on the competence of the developed models to predict the superplastic deformation behaviour of this alloy was made. The fitting results suggest that the artificial neural-network model has higher accuracy and is more efficient in fitting the superplastic deformation flow behaviour of near-α Titanium alloy (Ti-2.5Al-1.8Mn) at superplastic forming than the Arrhenius-type constitutive model. However, the tested results revealed that the error for the artificial neural-network is higher than the case of Arrhenius-type constitutive model for predicting the unmodelled conditions.

The study presents an integrated approach for superplastic forming of Ti-6%Al-4%V titanium alloy. The flow behavior of the studied alloy was investigated using uniaxial constant strain rate tensile tests in a temperature range of 800-900 °C and a strain rate range of 3×10−4-3×10-3s-1. The obtained flow behavior was modeled using the simple Johnson-Cook (S J-C), modified Johnson-Cook (M J-C) and artificial neural network (ANN) models. An assessment study between the constructed models was performed in order to evaluate the predictability of each model. Standard statistical comparative quantities such as correlation coefficient (R), mean absolute relative error (AARE) and the root mean square error (RMSE) were used to ascertain the model viability. The S J-C model proved ineffectual in predicting the flow behavior of Ti-6%Al-4%V alloy. The M J-C and ANN models are able to successfully describe the flow behavior of the alloy. The validity of the model used for the simulation was ascertained by testing the predicted data with the constructed models at a temperature of 875 °C and a strain rate of 2×10-3s-1 using DEFORM 3D finite element simulation (FES). The obtained results from the FES were verified with the experimental results after superplastic forming process. The FES results show the possibility of using uniaxial tensile test data to simulate superplastic forming process of the Ti-6%Al-4%V titanium sheets.

Superplastic deformation behaviour of conventional sheets of a near-α titanium alloy (Ti-2.5Al-1.8Mn) was studied by a step-by-step decrease of the strain rate and constant strain rate tests in a temperature range of 790-915 °C. The research found that superplastic deformation is possible in a temperature range of 815-890 °С and a constant strain rate range of 2 × 10 −4 to 1 × 10 −3 s −1 with elongation above 300% and m-index above 0.4. Also, the research identified the optimum superplastic temperature range of 815-850 °C and constant strain rate of 4 × 10−4 s−1 which provide a maximum elongation of 600-650%. Strain hardening is accelerated by dynamic grain growth at high temperatures of 865 and 890 °С. High dislocation activity is observed at superplastic flow in α-phase. Constitutive modelling of superplastic deformation behaviour is performed, and possible deformation mechanisms are discussed. It is suggested that grain boundary sliding between the α-grains is accommodated by a dislocation slip/creep mechanism.

Determining a desirable strain rate-temperature range for superplasticity and elongation-to-failure are critical concerns during the prediction of superplastic forming processes in α + β titanium-based alloys. This paper studies the superplastic deformation behaviour and related microstructural evolution of conventionally processed sheets of Ti-6Al-4V alloy in a strain rate range of 10–5–10–2 s–1 and a temperature range of 750–900 °C. Thermo-Calc calculation and microstructural analysis of the as-annealed samples were done in order to determine the α/β ratio and the grain size of the phases prior to the superplastic deformation. The strain rate ranges, which corresponds to the superplastic behaviour with strain rate sensitivity index m ˃ 0.3, are identified by step-by-step decreasing strain rate tests for various temperatures. Results of the uniaxial isothermal tensile tests at a constant strain rate range of 3 × 10−4–3 × 10−3 s−1 and a temperature range of 800–900 °C are presented and discussed. The experimental stress-strain data are utilized to construct constitutive models, with the purpose of predicting the flow stress behaviour of this alloy. The cross-validation approach is used to examine the predictability of the constructed models. The models exhibit excellent approximation and predictability of the flow behaviour of the studied alloy. Strain-induced changes in the grain structure are investigated by scanning electron microscopy and electron backscattered diffraction. Particular attention is paid to the comparison between the deformation behaviour and the microstructural evolution at 825 °C and 875 °C. Maximum elongation-to-failure of 635% and low residual cavitation were observed after a strain of 1.8 at 1 × 10−3 s−1 and 825 °C. This temperature provides 23 ± 4% β phase and a highly stable grain structure of both phases. The optimum deformation temperature obtained for the studied alloy is 825 °C, which is considered a comparatively low deformation temperature for the studied Ti-6Al-4V alloy.

This paper studies the superplasticity of conventional sheets of Ti-1V-4Al-3Mo (α+β) alloy. The flow behavior was investigated in a temperature range of 775 °C-900 °C and a constant strain rate range of 2×10 −4-5×10 −3 s −1 via uniaxial tensile tests. The microstructure evolution during the superplastic deformation was analyzed. The results revealed that, the flow behavior of Ti-1V-4Al-3Mo (α+β) alloy is characterized by strain softening phenomena. The experimental stress-strain data were used to build a power law constitutive model. A processing map, which shows the safe and unsafe regions of deformation, was also constructed for the studied alloy. The optimal deformation regime was attained at a temperature of 875 °C and strain rate of 1×10 −3 s −1 which provided a β phase fraction of 52%. Equiaxed fine-grained α and β structure with size of 2-3 μm as well as dislocation activity inside the α-grains were identified in the optimum deformation regime.