Deformation Behavior Of Titanium Alloy Research Articles

Metamodelling the hot deformation behaviour of titanium alloys using a mean-field approach

During the thermomechanical processing of titanium alloys in the β-domain, the β-phase undergoes restoration phenomena. This work describes them by a mean-field physical model that correlates the flow stress with the microstructural evolution. To reduce the computational time of process simulations, metamodels are developed for specific outputs of the mean-field physical model using Artificial Neural Network (ANN) and Decision Tree Regression (DTR). The performance of the obtained metamodels is evaluated in terms of the coefficient of determination (R²), the root-mean-square error (RMSE), and the mean relative error (MRE). No significant difference was observed between R2training and R2testing, meaning that all the metamodels correctly generalise the overall behaviour of the outputs for a wide range of inputs. The evolution of the metamodel outputs is compared with the model predictions in two different situations: 1) at a constant strain rate and temperature, and 2) during Finite Element (FE) simulations of the hot deformation of a hat-shaped sample, where temperature and effective strain rate vary at each element during deformation. The evolution of the outputs at constant and non-constant strain rates and temperature demonstrated the robustness of the metamodels in predicting the heterogeneous deformation within a workpiece. The computational time required by the metamodels to calculate selected outputs can be more than 100 times less than that of the model itself at a constant strain rate using MATLAB® and up to 19% less when coupled with FE simulations. The simulation results combined with microstructural analysis are used to visualise the different restoration mechanisms occurring in different regions of the hat-shaped sample as a function of the local thermomechanical history. The changes in strain rate and temperature during deformation influence the evolution of the wall dislocation density and the immobilisation rate of mobile dislocations at subgrain boundaries, leading to different kinetics of microstructure evolution.

Prediction of anisotropic deformation behavior of TA32 titanium alloy sheet during hot tension by crystal plasticity finite element model

An in-depth understanding of the anisotropic deformation behavior of titanium alloy sheet during hot deformation has important significance to fully exert the forming performance of the materials. In this work, a crystal plasticity finite element model (CPFEM) was established to accurately predict the anisotropic deformation behavior of TA32 titanium alloy rolled sheet during hot tension. An additional constitutive relationship was introduced into CPFEM to define the grain boundary sliding (GBS) mechanism. Meanwhile, the cellular automata (CA) model was embedded into CPFEM to simulate the dynamic recrystallization (DRX) behavior which has a significant effect on anisotropy during hot deformation. The results showed that the proposed model accurately predicts the stress-strain responses, r-values and DRX-acted microstructure evolution of TA32 alloy during hot tension along roll direction (RD) and transverse direction (TD). The transverse texture of the initial sheet results in the r-value of TA32 alloy for TD tension is higher than that for RD tension. Additionally, the higher dislocation density, especially geometrically necessary dislocation (GND) density, caused by more inhomogeneous plastic deformation during TD tension is the main reason for the larger flow stress of the material. Meanwhile, more dislocation density makes the volume fraction of DRX for TD tension higher than that for RD tension. The occurrence of DRX weakens the deformation anisotropy of TA32 alloy, which is mainly due to the fact that DRX enhances the effect of GBS and promotes the compatible deformation between grains, and the generation of substantial DRX grains with random orientation reduces the intensity of texture.

Effects of hydrogen as a solid solution element on the deformation behavior of a near-alpha titanium alloy

The effects of hydrogen on the room-temperature mechanical properties and the deformation behavior of near-alpha alloy Ti–6Al–3Nb–2Zr–1Mo were systematically investigated. It was found that hydrogen addition increased the strength and plasticity of titanium alloy, while the impact toughness firstly increased for the sample with 65 wppm hydrogen and then decreased at higher hydrogen content. The strength was increased due to the pinning effect and the solid-solution strengthening effect of hydrogen, while the plasticity was improved by the increase of dislocation mobility and the breakage of beta laths. The increase of impact toughness at 65wppm hydrogen resulted from the enhancement of dislocation motion and multiplication, but further increasing hydrogen addition would reduce impact toughness by decreasing the cohesion strength at interfaces and grain boundaries and the breakage of beta laths. In addition, deformation behaviors of titanium alloys with different hydrogen content were discussed. Hydrogen inhibited twinning and enhanced dislocation slip to some extent. Cross-slip was hindered and planar slip was favored due to the decrease of stacking fault energy after hydrogenation. Further, basal slip and pyramidal slip were activated while prismatic slip was suppressed by hydrogen addition.

Plastic deformation behavior of titanium alloy by warm laser shock peening: Microstructure evolution and mechanical properties

The plastic deformation behavior of Ti6Al4V titanium alloy subjected to 300 °C-warm laser shock peening (WLSP) was inferred from its microstructure evolution and mechanical properties. Through dynamic strain aging (DSA), WLSP achieved higher dense dislocations than room temperature laser shock peening (LSP). In addition, significant 101—2 deformation twinning activity was observed after WLSP. Twinning activity at such an ultrahigh strain rate and high temperature is a first-time observation in titanium alloy, and was attributed to an enhanced dislocation dissociation mechanism during WLSP. Moreover, an amorphization layer was generated on the top surface of the WLSP-processed sample, but not on the LSP-processed sample. This amorphization was effected by the increased free energy provided by the multiplying dislocations and deformation twins, combined with the reduced energy barrier of the crystal-to-amorphous transformation at high temperature. Relative to LSP, WLSP increased the width and depth of the compressive residual stress in the titanium alloy by 36.2% and 21.8% respectively, and improved the surface microhardness by 5%. The latter enhancement was conferred by the increased density of dislocations and deformation twins. Overall, WLSP improved the mechanical properties of the titanium alloy.

Modelling approach for predicting the superplastic deformation behaviour of titanium alloys with strain hardening/softening characterizations

This paper introduces an approach for modelling the flow behaviour of different titanium alloys (VT6, OT4-1 and VT14 alloys by Russian specifications) in superplastic deformation temperature and strain rate ranges. The initial microstructure parameters () before starting the deformation test were included in the constructed model for each alloy. The investigated alloys have different initial microstructures and flow behaviour characteristics. The isothermal uniaxial tensile deformation tests were performed at the superplastic deformation temperature and strain rate ranges of each alloy. The VT6, OT4-1 alloys were characterized by strain hardening effect during the deformation test, while VT14 alloy was characterized by strain softening effect. A comparison study between the experimental and modelled data was performed. The general equation of the constructed models was affected by the flow behaviour of the investigated alloys. The comparison results proved the good capability of the constructed models to predict the flow behaviour of the studied alloys. The correlation coefficient R was 0.98, 0.95 and 0.97 for VT6, OT4-1 and VT14, respectively. The predictability of the constructed model was assessed by the cross-validation technique, which ascertained the quality of the constructed model.

Microstructural evolution during hot forming of Ti-6Al-4V alloy with complex initial microstructure

In this study, the microstructural evolution that occurred during hot forming of Ti-6Al-4V with an initial microstructure of elongated and equiaxed hcp α grains and a network of intergranular bcc β-phase was investigated. Isothermal compression tests were carried out at different deformation temperatures (800–950 °C) and strain rates (0.01, 1 and 10s−1) using Gleeble 3500 thermomechanical simulator. The microstructures of the deformed samples were examined using optical and scanning electron microscopy. The images were analysed using ImageJ 1.48V software. The analysis showed that hot forming of the alloy was generally influenced by dynamic globularisation and dynamic recovery which are known mechanisms that control the deformation behaviour of titanium alloys. However, unique phenomena such as deformation-induced phase transformation and abnormal grain growth that are rarely reported in Ti-6Al-4V alloy with conventional starting microstructures were observed under certain deformation conditions for the Ti-6Al-4V with complex initial microstructure. Abnormal grain growth occurred at lower deformation temperatures and the higher strain rates of 1 and 10 s−1. Deformation-induced phase transformation reduced the β transus temperature of the alloy by 45 °C at the low strain rate of 0.01 s−1.

A Study on the Cellphone Cover-Forming Process of Titanium Alloys Using the Finite Element Method

In recent years, the style of 3C products demanded thin and small results due to gradual trends, though most of the industry is forming a continuous manner of stamping. However, the material thickness cannot be changed in products. This paper employs the rigid-plastic finite element (FE) DEFORMTM 3D software to investigate the plastic deformation behavior of titanium alloy (Ti-6Al-4V) workpiece for the forming processes of cellphone covers. In addition, this study utilizes the Solid Work 2010 3D graphics-rendering software for modeling, which is simulation software used to import various forming process conditions. The software analyzes the effective strain, the effective stress, critical damage value, and the die radius load distribution of the work-piece. Furthermore, this study used simulative software to analyze its forming processes for changes in grain size of the microstructure. The analytical results confirm the suitability of the current finite element software for forming processes of cellphone covers.