Search

Abstract: The deformation behavior in isothermal compression of Ti–6Al–4V alloy is investigated in the deformation temperatures ranging from 1093K to 1303K, the strain rates ranging from 0.001s−1 to 10.0s−1 at an interval of an order magnitude and the height reductions ranging from 20% to 60% at an interval of 10%. Based on the experimental results in isothermal compression of Ti–6Al–4V alloy, the effect of processing parameters and grain size of primary α phase on the strain rate sensitivity exponent m and the strain hardening exponent n is in depth analyzed. The strain rate sensitivity exponent m at a strain of 0.7 and strain rate of 0.001s−1 firstly tends to increase with the increasing of deformation temperature, and maximum m value is obtained at deformation temperature close to the beta-transus temperature, while at higher deformation temperature it drops to the smaller values. Moreover, the strain rate sensitivity exponent m decreases with the increasing of strain rate at the deformation temperatures below 1253K, but the m values become maximal at a strain rate of 0.01s−1 and the deformation temperature above 1253K. The strain rate affects the variation of strain rate sensitivity exponent with strain. Those phenomena can be explained reasonably based on the microstructural evolution. On the other hand, the strain hardening exponent n depends strongly on the strain rate at the strains of 0.5 and 0.7. The strain affects significantly the strain hardening exponent n due to the variation of grain size of primary α phase with strain, and the competition between thermal softening and work hardening. [Copyright &y& Elsevier]

Abstract: The internal state variable approach nowadays is more and more used to describe the deformation behavior in all of the metallic materials. In this paper, firstly the dislocation density rate and the grain growth rate varying with the processing parameters (deformation temperature, strain rate and strain) are established using the dislocation density rate as an internal state variable. Secondly the flow stress model in high temperature deformation process is analyzed for each phase of titanium alloys, in which the flow stress contains a thermal stress and an athermal stress. A Kock–Mecking model is adopted to describe the thermally activated stress, and an athermal stress model is established using two-parameter internal state variables. Finally, a constitutive model coupling the grain size, volume fraction and dislocation density is developed based on the microstructure and crystal plasticity models. And, the material constants in present model may be identified by a genetic algorithm (GA)-based objective optimization technique. Applying the constitutive model to the isothermal compression of Ti–6Al–4V titanium alloy in the deformation temperature ranging from 1093 to1303K and the strain rate ranging from 0.001 to 10.0s−1, the 20 material constants in those models are identified with the help of experimental flow stress and grain size of prior α phase in the isothermal compression of Ti–6Al–4V titanium alloy. The relative difference between the predicted and experimental flow stress is 6.13%, and those of the sampled and the non-sampled grain size are 6.19% and 7.94%, respectively. It can be concluded that the constitutive model with high prediction precision can be used to describe the high temperature deformation behavior of titanium alloys. [Copyright &y& Elsevier]

Abstract: The deformation behavior of isothermally compressed Ti–6Al–4V alloy in the deformation temperature range from 1093K to 1303K, the strain rate range from 0.001s−1 to 10.0s−1 and the height reduction range from 20% to 60% has been investigated in depth. Effect of the strain on the flow stress, the grain size and the apparent activation energy for deformation of isothermally compressed Ti–6Al–4V alloy is analyzed. The results show that the apparent activation energy for deformation and the grain size of isothermally compressed Ti–6Al–4V alloy in the α+β two-phase region vary slightly with strain: the activation energy increases with strain after an early drop as the primary α grain size varies with strain. On the other hand, the apparent activation energy for deformation of isothermally compressed Ti–6Al–4V alloy in the β single-phase region increases firstly with the increasing of strain and then does that slightly in the strain range from 0.3 to 0.7. A processing map of the isothermally compressed Ti–6Al–4V alloy is constructed at a strain of 0.6. The processing map of isothermally compressed Ti–6Al–4V alloy exhibits that the optimal processing parameters are the deformation temperature of 1143K and the strain rate of 0.001s−1. Meanwhile, two instability domains in the processing map of isothermally compressed Ti–6Al–4V alloy are as follows: one is in the deformation temperature range from 1093K to 1183K and the strain rate range from 0.011s−1 to 2.02s−1, and another is in the deformation temperature range from 1243K to 1303K and the strain rate range from 1.0s−1 to 10.0s−1. [Copyright &y& Elsevier]

Abstract: Effect of the strain on the grain size, the efficiency of power dissipation and the instability parameter are investigated in the isothermal compression of Ti–6Al–4V alloy. And, the processing maps in the isothermal compression of Ti–6Al–4V alloy, Ti–5.6Al–4.8Sn–2.0Zr alloy and Ti–10V–4.5Fe–1.5Al alloy are constructed at a series of strains. Those results show that the grain size of primary α phase of Ti–6Al–4V alloy varies significantly with strain. The effect of strain on the efficiency of power dissipation is significant at strain rates of 0.001s−1 and 10.0s−1. However, the efficiency of power dissipation varies significantly with strain at a strain rate of 0.1s−1 in the α+β two-phase region, but that does slightly in the β single-phase region. And, the instability parameter of Ti–6Al–4V alloy varies significantly with strain at each strain rate and deformation temperature. Effect of the strain on the unstable regions in the processing maps of isothermally compressed Ti–6Al–4V alloy, Ti–5.6Al–4.8Sn–2.0Zr alloy and Ti–10V–4.5Fe–1.5Al alloy are obtained. Compared with the processing maps of different types titanium alloys, it can be found that the effect of strain on the processing map of Ti–10V–4.5Fe–1.5Al alloy (a kind of β type titanium alloy) is most significant, that of Ti–6Al–4V alloy (a kind of α+β type titanium alloy) is secondary and that of Ti–5.6Al–4.8Sn–2.0Zr alloy (a kind of near α type titanium alloy) is less. [Copyright &y& Elsevier]

Abstract: The modeling of constitutive relationships and microstructural variables of the Ti–6.62Al–5.14Sn–1.82Zr alloy during high temperature deformation by using a fuzzy set and artificial neural network (FNN) technique with a back-propagation learning algorithm is the basis of this research. To obtain experimental results for the modeling, the isothermal compression of the titanium alloy in different deformation scenarios was conducted and quantitative metallography was thus obtained. The predicted results of flow stress and microstructural variables, including grain size and volume fraction of the α phase, are compared with the experimental data and the difference is less than 15%. The predicted results are consistent with the experimental data. Furthermore, the comparison between the predicted results of flow stress based on the FNN approach and those by using the regression method has illustrated that the FNN approach is efficient in predicting the flow stress of the alloy. [Copyright &y& Elsevier]

Abstract: Isothermal compression of the Ti–6Al–4V alloy was conducted at a 2500ton isothermal hydrostatic press, and the mechanical properties including ultimate tensile strength, yield strength, elongation and area reduction of the post-forged Ti–6Al–4V alloy were measured at a ZWICK/Z150 testing machine. A fuzzy neural network (FNN) was applied to acquire the relationships between the mechanical properties and the processing parameters of post-forged Ti–6Al–4V alloy. In establishing those relationships, the forging temperature, strain and strain rate were taken as the inputs, whilst the ultimate tensile strength, yield strength, elongation and area reduction were taken as the output respectively. The predicted results using the present FNN model is in a good agreement with the experimental data of the post-forged Ti–6Al–4V alloy, and the optimum processing parameters can be quickly and conveniently selected to achieve the desired mechanical properties by means of the prediction based on the fuzzy neural network model. [Copyright &y& Elsevier]

Abstract: Isothermal compression of Ti–5.6Al–4.8Sn–2.0Zr alloy was carried out on a Thermecmaster-Z simulator at deformation temperatures between 960 and 1060°C, constant strain rate between 0.001 and 10.0s−1, and a maximum height reduction of 70%. The high temperature deformation behavior of Ti–5.6Al–4.8Sn–2.0Zr was characterized based on an analysis of the stress–strain behavior, kinetics and processing map. The activation energy for deformation obtained during high temperature compression of Ti–5.6Al–4.8Sn–2.0Zr alloy are 221.7kJ/mol in the β phase region and 1082.4kJ/mol in the α+β phase region. The constitutive equation proposed to describe the flow stress as a function of strain rate, strain and deformation temperature during high temperature compression of Ti–5.6Al–4.8Sn–2.0Zr alloy enables a close agreement between the predicted and experimental stress–strain curves. [Copyright &y& Elsevier]