Xiaoguang Fan

Dynamic globularization kinetics during hot working of TA15 titanium alloy with colony microstructure

Abstract 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.

A New FE Modeling Method for Isothermal Local Loading Process of Large-scale Complex Titanium Alloy Components Based on DEFORM-3D

Isothermal local loading process provides a new way to form large‐scale complex titanium alloy components. The forming process is characterized by an extreme size (large scale in global and compared small size in regional), multi‐parameter effects, and complicated loading path. To establish a reasonable finite element model is one of the key problems urgently to be solved in the research and development of isothermal local loading forming process of large‐scale complex titanium alloy components. In this paper, a new finite element model of the isothermal local loading process is developed under the DEFORM‐3D environment based on the solution of some key techniques. The modeling method has the following features: (1) different meshing techniques are used in different loading areas and the number of meshed elements is determined according to the deformation characteristic in different local loading steps in order to improve computational efficiency; (2) the accurate magnitude of the friction factor under titanium alloy hot forming (isothermal forming) condition is adopted instead of the typical value for lubricated hot forming processes; (3) different FEM solvers are chosen at different stages according to the loading characteristic and the contact state; (4) in contrast to the local component model, a full 3D component is modeled to simulate the process. The 3D‐FE model is validated by experimental data of a large‐scale bulkhead forming under isothermal local loading. The model can describe the quantitative relationships between the forming conditions and the forming results. The results of the present study may provide a basis for studying the local deformation mechanism, selecting the reasonable parameters, optimizing the die design and the process control in isothermal local loading process of large‐scale complex titanium alloy components.

Influences of fillet radius and draft angle on local loading process of titanium alloy T-shaped components

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.

Shape Optimization of Initial Billet for TA15 Ti-Alloy Complex Components Preforming

Abstract A finite element (FE) model of preforming process of TA15 Ti-alloy complex components has been established based on DEFORM-3D platform. Combined with the experiments, the preforming process and typical forming defects have been analyzed. An excellent preform with good macroscopical forming quality, uniform and fine microstructure has been fabricated using an optimized billet with the proper initial shape, which is acquired by a systematic stepwise reverse optimization method, effectively avoiding the defects of underfilling and folding in the preforming process. The preforming experiment was carried out based on the optimized initial billet and the result of optimization has been verified which is reasonable and reliable.

Morphology development of elongated α phases in hot working of large-scale titanium alloy plate

Abstract 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.

Prediction of tri-modal microstructure under complex thermomechanical processing history in isothermal local loading forming of titanium alloy

Abstract 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.

Morphology transformation of primary strip α phase in hot working of two-phase titanium alloy

Abstract 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 .

A new method for separating complex touching equiaxed and lamellar alpha phases in microstructure of titanium alloy

Abstract 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.

Deformation banding in β working of two-phase TA15 titanium alloy

Abstract 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.

Phase fraction evolution in hot working of a two-phase titanium alloy: experiment and modeling

In the present work, the coupled effects of initial structure and processing parameters on microstructure of a two-phase titanium alloy were investigated to predict the microstructural evolution in multiple hot working. It is found that microstructure with different constituent phases can be obtained by regulating the initial structure and hot working conditions. The variation of deformation degree and cooling rate can change the morphology of the constituent phases, but do not alter the phase fraction. The phase transformation during heating and holding determines the phase fraction for a certain initial structure. β–α–β transformation occurs during heating and holding. β to α transformation leads to a significant increase in content and size of lamellar α. The α to β transformation occurs simultaneously in equiaxed α and lamellar α. The thickness of lamellar α increases with temperature, which is caused by the vanishing of fine α lamellae due to phase transformation and coarsening by termination migration. By assuming a quasi-equilibrium phase transformation in heating and holding, a modeling approach is proposed for predicting microstructural evolution. The three stages of phase transformation are modeled separately and combined to predict the variation of phase fraction with temperature. Model predictions agree well with the experimental results.