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Numerical control (NC) warm bending is a proven strategy to form the large diameter thin-walled (LDTW) Ti–6Al–4V tubes, which are typical light-weight and high-performance structural components urgently required in many industries. In virtue of unveiling the thermo-mechanical coupled deformation behaviors, uniaxial tensile tests were conducted on Ti–6Al–4V tube within wide ranges of temperatures (25–600 °C) and strain rates (0.00067–0.1 s−1). Moreover, a modified Johnson–Cook (JC) model is proposed with a consideration of nonlinear strain rate hardening and the interaction between strain hardening and thermal softening. Resultantly, the present model gives more accurate predictions for flow stress over the entire deformation ranges and the maximum error decreases by about 90%. By employing proposed model to NC warm bending, preferable precision is obtained in predicting forming defects including fracture, wrinkling and over thinning. The present work lays foundation for the forming limit prediction and process optimization in NC warm bending of LDTW Ti–6Al–4V tubes.

To control the tri-modal microstructure and performance, a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed. The staged isothermal local loading experiment on TA15 alloy indicates that there exist four important microstructure evolution phenomena in the development of tri-modal microstructure, i.e., the generation of lamellar α, content variation of equiaxed α, spatial orientation change of lamellar α and globularization of lamellar α. Considering the laws of these microstructure phenomena, the microstructure model was established to correlate the parameters of tri-modal microstructure and processing conditions. Then, the developed microstructure model was integrated with finite element (FE) model to predict the tri-modal microstructure in the isothermal local loading forming. Its reliability and accuracy were verified by the microstructure observation at different locations of sample. Good agreements between the predicted and experimental results suggest that the developed microstructure model and its combination with FE model are effective in the prediction of tri-modal microstructure in the isothermal local loading forming of TA15 alloy.

To study deformation banding in β working of TA15 titanium alloy, hot simulation compression experiments were carried out on a Gleeble 3500 thermal simulator, and the microstructure was investigated by optical microscopy (OM) and electron back-scattered diffraction (EBSD). It is found that in β working of TA15 titanium alloy, deformation banding is still an important grain refinement mechanism up to temperature as high as 0.7Tm (Tm is the melting temperature). Boundaries of deformation bands (DBBs) may be sharp or diffusive. Sharp DBBs retard discontinuous dynamic recrystallization (DDRX) by prohibiting nucleation, while the diffusive ones are sources of continuous dynamic recrystallization (CDRX). Deformation banding is more significant at high strain rate and large initial grain size. The average width of grain subdivisions is sensitive to strain rate but less affected by temperature and initial grain size. Multi-directional forging which produces crossing DDBs is potential to refine microstructure of small-size forgings.

Microstructural development in hot working of TA15 titanium alloy with primary strip α structure was investigated with the aim to globularize α strips. Results show that the mechanisms of morphology transformation are the same to the spheroidization mechanisms of lamellar structure. Boundary splitting and termination migration are more important than coarsening due to the large size of strip α . The α strips are stable in annealing due to the unfavorable geometrical orientation of intra- α boundaries, the large thickness of strip and the geometrical stability of α particles. Predeformation and low speed deformation accelerate globularization of α strips in the following ways: direct changing of particle shape, promotion of boundary splitting and termination migration by increasing high angle grain boundaries and interfacial area, promotion of coarsening by forming dislocation structures. Large predeformation combined with high temperature annealing is a feasible way to globularize strip α .

The role of subtransus hot working on microstructure morphology of TA15 titanium alloy plate with elongated α phases was studied by quantitative metallography on different sections. The results show that the microstructure morphology is mainly affected by loading direction. When the sample is compressed along normal direction, microstructure on the section vertical to normal direction has equiaxed primary α phase but microstructure on the section vertical to rolling direction has strip primary α phase with long axis along tangential direction. When the sample is compressed along rolling direction, microstructure on the section vertical to normal direction has strip primary α phase elongated along tangential direction but microstructure on the section vertical to rolling direction consists of strip and irregular broad-band primary α phase. The strip primary α phase aspect ratio is smaller at lower temperature due to the dynamic break-down of α phase. The difference on primary α phase aspect ratio between different sections decreases after compression along distinct directions in two loading passes, suggesting the improvement of equiaxity of primary α phase.

A new method for separating complex touching equiaxed and lamellar alpha phases in the optical micrograph of titanium alloy was proposed for quantitative characterization. This new method involves three steps. First, concave points of the microstructural feature are identified with a threshold of the concaveness of the corner points which are extracted from the binarized image. Secondly, concave points pairs are selected from the concave points group established by means of marker circle or distance. Third, whether a candidate separation line which connects two concave points within a pair can be accepted or not is determined by the proposed four rules. The obtained results show that this method is effective on separating complex touching microstructural features.

In order to study influences of geometric parameters on the T-shaped components local loading process, a new mathematical model considering the fillet radius and draft angle was established by using the slab method. The results obtained by the mathematical model agree with the data form experiment and numerical simulation, and the results are closer to the experimental and simulation results. The influence of draft angle may be neglected under the forming conditions used. The influence of fillet radius is notable, especially in the case that the ratio of fillet radius to rib width is less than 0.75.

The dynamic globularization kinetics of TA15 (Ti-6Al-2Zr-1Mo-1V) titanium alloy with a colony α microstructure during deformation at temperature range of 860-940 °C and strain rate range of 0.01-10 s−1 was quantitatively studied through isothermal compression tests. It is found that the dynamic globularization kinetics and the kinetics rate of TA15 are sensitive to deformation parameters. The dynamic globularized fraction increases with increasing strain, temperature but decreasing strain rate. The variation of globularized fraction with strain approximately follows an Avrami type equation. Using the Avrami type equation, the initiation and completion strains for dynamic globularization of TA15 were predicted to be 0.34-0.59 and 3.40-6.80. The kinetics rate of dynamic globularization increases with strain at first, then decreases. The peak value of kinetics rate, which corresponds to 20%-33% globularization fraction, increases with increasing temperature and decreasing strain rate.