Behavior modeling and microstructural evolutions of Ti–6Al–4V alloy under hot forming conditions

Abstract

Ti-6Al-4V alloy is widely used in superplastic forming process. The conventional conditions require high forming temperatures (T ≥ 900 • C) and low strain rates (ε ≤ 10 −3 s −1). In order to reduce the costs of the industrial process, recent investigations focus on the micro-structural refinement of the material. It allows an improvement of the forming conditions which makes lower forming temperatures and higher strain rates eligible whereas low strain rates (ε ≤ 10 −3 s −1) and high temperatures (T ≥ 800 • C) are particularly suitable for conventional superplastic forming conditions. However, the mechanical response of the Titanium alloy strongly depends on the starting micro-structure considered and on its evolution with the temperature and the deformation. The objective of the present investigation is to observe the micro-structural evolutions of Ti-6Al-4V alloy under thermal and mechanical loadings from different starting micro-structures. Hence, an internal grain growth variable is identified by the use of these observations. Then, it is introduced into the behavior model and its influence on the mechanical response of the material is analysed. The final constitutive equations are able to take into account viscosity, strain hardening and grain size evolution for a wide range of strain rates and forming temperatures (10 −4 s −1 ≤ε ≤ 10 −2 s −1 ; 650 • C ≤ T ≤ 870 • C). Moreover, the model is able to consider several starting microstructures (different initial grain sizes) and to predict their influence on the viscous flow and the strain hardening. At last, some model verifications are conducted to check the validity of the non-isothermal model formulation. Some predictions are also performed by considering several starting microstructures.

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

“…The both VT6 (figures 3(a)-(c)) and OT4-1 (figures 3(d)-(f)) alloys are characterized predominantly by a strain hardening, while the VT14 alloy (figures 3(g)-(h)) is mostly characterized by a strain softening. A similar characterization was noted for VT6 and OT4-1 alloys in [37,39,40], and for VT14 alloy in [19,41].…”

Superplasticity in Fine Grain Ti-6Al-4V Alloy: Mechanical Behavior and Microstructural Evolution

“…2a, 2b and 2c shows the stress-strain curves obtained for the SM at respectively 750°C, 870°C and 920°C. At 750°C and 870°C, experimental data are compared with computed data (lines) obtained using the mechanical model from Velay [7]. As detailed in Ref.…”

“…As detailed in Ref. 7, an internal variable related to grain growth is introduced into the mechanical model through the viscous flow and the hardening. So by taking into account the grain growth (as revealed by SEM observations on Fig.3), computed and experimental data are in good agreement.…”

“…Tensile tests at high temperature. Tensile tests, along the rolling direction, with different temperatures and different strain rate were conducted, respectively in the range of 750°C-920°C and 10 -2 s -1 and 10 -4 s -1 , on dog bone specimen [6] according to the procedure presented by Velay [7]. Before each tests, the samples were held at temperature for about 20 min prior tensile loading.…”

The Effect of Initial Annealing Microstructures on the Forming Characteristics of Ti–4Al–2V Titanium Alloy

“…Titanium alloy is usually deformed at high temperature, such as 900 • C, in order to avoid low ductility and spring back. The mechanical properties of titanium alloys are closely dependent on the microstructure, such as volume fraction, morphology, and grain size of α and β phases [7,8]. Furthermore, flow instability is caused by flow softening, which may result in deformation defects.…”