Service properties of ultrafine-grained Ti–6Al–4V alloy at elevated temperature

This study deals with investigation of mechanical properties and fatigue behavior of the ultra-fine grained (UFG) alloy Ti–6Al–4V at elevated temperatures. UFG samples were produced by means of combination of equal-channel angular pressing and thermomechanical treatments. Studies of the temperature dependence of mechanical properties of the UFG alloy demonstrated their thermal stability upto 175–350 °C. It was revealed that 100-hour creep rupture strength at 300 °C increased from 750 MPa in the conventional state to 890 MPa in the UFG state. The alloy demonstrates stability of the UFG structure at 300 and 370 °C in the conditions of long-term tests. The fatigue tests were conducted with axial loading applied on a sample at 175 °C, the asymmetry factor of the cycle was 0.1. The fatigue endurance limit of the UFG alloy was almost 50 % higher than that of the CG alloy.     

Constitutive modeling and the effects of strain-rate and temperature on the formability of Ti–6Al–4V alloy sheet

The constitutive model considering the strain-rate and temperature effects was presented by fitting the true stress–strain curves of Ti–6Al–4V alloy over a wide range of strain-rates (0.0005–0.05 s⁻¹) and temperatures (923–1023 K). The Forming Limit Curve (FLC) of Ti–6Al–4V alloy at 973 K was measured by conducting the hemispherical dome test with specimens of different widths. The forming limit prediction model of Ti–6Al–4V alloy, which takes strain-rate and temperature sensitivity into account, was predicted based on Marciniak and Kuczynski (M–K) theory along with Von Mises yield criterion. The comparison shows that the limit strain decreases with temperature lowering but strain-rate increasing. The comparison between theoretical analysis and experiment of FLC verifies the accuracy and reliability of the proposed methodology, which considers the strain-rate and temperature effects, to predict limit strains in the positive minor strain region of Forming Limit Diagram (FLD).     

Prediction of flow stress in isothermal compression of Ti–6Al–4V alloy using fuzzy neural network

Isothermal compression of Ti–6Al–4V alloy at the deformation temperatures ranging from 1093 K to 1303 K with an interval 20 K, the strain rates ranging from 0.001 s⁻¹ to 10.0 s⁻¹ and the height reductions ranging from 20% to 60% with an interval 10% were carried out on a Thermecmaster-Z simulator. Based on the experimental results, a model for the flow stress in isothermal compression of Ti–6Al–4V alloy was established in terms of the fuzzy neural network (FNN) with a back-propagation learning algorithm using strain, strain rate and deformation temperature as inputs. The maximum difference and the average difference between the predicted and the experimental flow stress are 18.7% and 4.76%, respectively. The comparison between the predicted results based on the FNN model for flow stress and those using the regression method has illustrated that the FNN model is more efficient in predicting the flow stress of Ti–6Al–4V alloy.     

Microindentation study of Ti–6Al–4V alloy

In order to study the micromechanical behavior of Ti–6Al–4V alloy, microindentation experiments were performed with five different maximum loads of 100, 150, 200, 250 and 300 mN, and with three loading speeds of 6.4560, 7.7473 and 9.6841 mN/s respectively. The experimental results revealed that loading speed has little influence on microhardness and Young’s modulus. Microindentation hardness experiments showed strong indentation size effects, i.e. increase of indentation hardness with the decrease of indentation load or depth. Then microindentation constitutive equation that described the stress as a function of the strain was proposed through dimensional analysis. And the finite element simulation results showed that the predicted computational indentation data from developed constitutive equation can track the microindentation experimental data of Ti–6Al–4V alloy.     

Workability of Ti–6Al–4V alloy at high temperatures and strain rates

Hot workability of Ti–6Al–4V has been investigated by means of hot compression tests carried out in the 880–950 °C temperature range and 1–50 s⁻¹ strain rate range. The effect of microstructural characteristics of the deformed specimens have been studied and correlated with the test temperature, total strain and strain rate. A constitutive equation for the flow stress has been defined and the test conditions for a homogeneous deformation evaluated. The machine employed for testing allowed to reach very high strain rates by means of a uniform compression for long strains (until 0.9), whereas data extracted from the scientific literature are significantly limited in comparison. In this way, a higher accuracy could be obtained in material behaviour modelling for forging process simulation.     

HCF strength estimation of notched Ti–6Al–4V specimens considering the critical distance size effect

Generally not only the material but also the notch geometry affects the HCF strength. So, when evaluating the fatigue limit of notched Ti–6Al–4V specimens using the conventional theory of critical distance (TCD), a possible critical distance size effect was identified. In this paper, in order to overcome the critical distance size effect, Kt was introduced. And based on an assumption that the product of critical distance and Kt was constant, the TCD was modified to evaluate the HCF strength of notched Ti–6Al–4V specimens. The prediction results fell into an error interval of about ±15% using the modified TCD.     

Microstructure study and constitutive modeling of Ti–6Al–4V alloy at elevated temperatures

A reliable and accurate prediction of flow behavior of metals in industrial forming process considering the coupled effects of strain, strain rate and temperature is crucial in understanding the workability of the metal and optimizing parameters for hot forming process. In this study, the tensile fracture behavior of the Ti–6Al–4V alloy is examined with scanning electron microscope (SEM) over the range of magnifications. SEM study revealed that microvoids and shallow dimples are observed at the fracture surface which indicates the fracture is predominately ductile in nature. Also, an investigation on flow behavior of Ti–6Al–4V alloy is done using constitutive models. Four constitutive models; modified Johnson-Cook (m-JC), modified Arrhenius type equations (m-Arr), modified Zerilli–Armstrong (m-ZA) and Rusinek–Klepaczko (RK) models are developed to predict the flow stress. The predictions of these constitutive models are compared with each other using statistical measures like correlation coefficient, average absolute error and its standard deviation. Comparing the statistical measures, m-Arr model is a better model for predicting the flow stress, but considering the fact that m-ZA model is a physical based model, m-ZA model is preferred over the m-Arr model.     

Plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy

Abstract

The titanium affinity for oxygen is one of the main factors that limit the application of its alloys as structural materials at high temperatures. The objective of this work was to estimate the influence of the plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy, focusing on the determination of the experimental parameters related to the creep stages. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was air plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on the Ti–6Al–4V alloy in air for coated and uncoated samples and in a nitrogen atmosphere for uncoated samples at 600°C to evaluate the oxidation protection on creep of the Ti–6Al–4V alloy. The steady-state creep rate of the coated alloy is smaller than that of the uncoated alloy in air and nitrogen atmosphere. Results about the activation energies and the stress exponent values indicate that the primary and stationary creep, for all test conditions, was probably controlled by dislocation climb. The plasma-sprayed coatings increased the time to rupture and the strain at rupture is smaller than for uncoated samples tested in air.     

Neural network modelling of flow stress in Ti–6Al–4V alloy with equiaxed and Widmanstätten microstructures

Abstract

In the present study, artificial neural networks (ANNs) were used to model flow stress in Ti–6Al–4V alloy with equiaxed and Widmanstätten microstructures as initial microstructures. Continuous compression tests were performed on a Gleeble 3500 thermomechanical simulator over a wide range of temperatures (700–1100°C) with strain rates of 0˙001–100 s^–1 and true strains of 0˙1–0˙6. These tests have been focused on obtaining flow stress data under varying conditions of strain, strain rate, temperature, and initial microstructure to train ANN model. The feed forward neural network consisted of two hidden layers with a sigmoid activation function and backpropagation training algorithm was used. The architecture of the network includes four input parameters: strain rate ?, Temperature T, true strain ? and initial microstructure and one output parameter: the flow stress. The initial microstructure was considered qualitatively. The ANN model was successfully trained across (α+β) to β phase regimes and across different deformation domains for both of the microstructures. Results show that the ANN model can correctly reproduce the flow stress in the sampled data and it can predict well with the nonsampled data. A graphical user interface was designed for easy use of the model.     

Photocatalytic activity of low temperature oxidized Ti–6Al–4V

Numerous advanced surface modification techniques exist to improve bone integration and antibacterial properties of titanium based implants and prostheses. A simple and straightforward method of obtaining uniform and controlled TiO₂ coatings of devices with complex shapes is H₂O₂-oxidation and hot water aging. Based on the photoactivated bactericidal properties of TiO₂, this study was aimed at optimizing the treatment to achieve high photocatalytic activity. Ti–6Al–4V samples were H₂O₂-oxidized and hot water aged for up to 24 and 72 h, respectively. Degradation measurements of rhodamine B during UV-A illumination of samples showed a near linear relationship between photocatalytic activity and total treatment time, and a nanoporous coating was observed by scanning electron microscopy. Grazing incidence X-ray diffraction showed a gradual decrease in crystallinity of the surface layer, suggesting that the increase in surface area rather than anatase formation was responsible for the increase in photocatalytic activity.     

Post-forming tensile properties of superplastic Ti–6Al–4V alloy

Abstract

A study has been made of the influence of uniaxial superplastic deformation on the ambient temperature tensile properties of Ti–6Al–4V sheet. Material was deformed to various strains up to 200% at temperatures from 850 to 970°C at strain rates in the range 1·1?18 × 10^{;amp;#x2212;4}s^?1 (0·7?11% min^?1). Tests were also performed on statically annealed material to separate the effects of high temperature exposure and superplastic deformation. Mechanical property changes were complex and depended on the relative contributions from the strengthening and softening mechanisms occurring during either superplastic deformation or heat cycling. Structural features influencing mechanical properties were phase size and morphology, dislocation density, and crystallographic texture. The strength after superplastic deformation was always less than that of as-received material but a significant reduction in strength was attributable to heat cycling. In some cases, the strength of the superplastically deformed material was greater than that after heat cycling.

MST/593     

Effect of strain rate and temperature on strain hardening behavior of a dissimilar joint between Ti–6Al–4V and Ti17 alloys

The aim of this study was to evaluate the influence of strain rate and temperature on the tensile properties, strain hardening behavior, strain rate sensitivity, and fracture characteristics of electron beam welded (EBWed) dissimilar joints between Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys. The welding led to significant microstructural changes across the joint, with hexagonal close-packed martensite (α′) and orthorhombic martensite (α″) in the fusion zone (FZ), α′ in the heat-affected zone (HAZ) on the Ti–6Al–4V side, and coarse β in the HAZ on the Ti17 side. A distinctive asymmetrical hardness profile across the dissimilar joint was observed with the highest hardness in the FZ and a lower hardness on the Ti–6Al–4V side than on the Ti17 side, where a soft zone was present. Despite a slight reduction in ductility, the yield strength (YS) and ultimate tensile strength (UTS) of the joints lay in-between the two base metals (BMs) of Ti–6Al–4V and Ti17, with the Ti17 alloy having a higher strength. While the YS, UTS, and Voce stress of the joints increased, both hardening capacity and strain hardening exponent decreased with increasing strain rate or decreasing temperature. Stage III hardening occurred in the joints after yielding. The hardening rate was strongly dependent on the strain rate and temperature. As the strain rate increased or temperature decreased, the strain hardening rate increased at a given true stress. The strain rate sensitivity evaluated via both common approach and Lindholm approach was observed to decrease with increasing true strain. The welded joints basically failed in the Ti–6Al–4V BM near the HAZ, and the fracture surfaces exhibited dimple fracture characteristics at different temperatures.     

Deformation of titanium alloy Ti–6Al–4V under dynamic compression

Deformation localisation is the main reason for material failure in cold forging of titanium alloys and is thus closely related to the production yield of cold forging. Recent research has revealed that the width of shear band of titanium alloys after dynamic compression is related with their static and dynamic mechanical properties and processing parameters. To explore the influences of these factors on titanium alloys in dynamic compression, the distributions of stress, strain, strain rate and temperature of the specimens over the macro and microscales have been systematically studied. This work can be beneficial to process parameter optimisation and material designing for cold forging. In the study of the influence of process parameters on dynamic compression, considering material constitutive behaviour, physical parameters and process parameters, a numerical dynamic compression model for titanium alloys has been constructed. The entire dynamic compression process is simulated and a good agreement with experiments is observed. By extracting and comparing the stress, strain and temperature distribution under prescribed conditions, the effects of friction and compression velocity on the macrostate and distribution of strain and stress of compression samples are studied. Friction and compression rate are important factors influencing the spread and the stress state of deformation localisation zone. When friction is reduced to a certain level, deformation localisation can be effectively alleviated. The increase of friction and compression rate can lead to early appearance of tension stress in the deformation localisation zone, which may explain the experimental finding that crack tendency increases with higher compression rate and poorer lubrication. By adjusting the process parameters, the severity of strain localisation and stress state in the localised zone can be controlled thus enhancing the compression performance of titanium alloys.     

Corrosion behavior of Ti–8Al–1Mo–1V alloy compared to Ti–6Al–4V

The corrosion behavior of Ti–8Al–1Mo–1V alloy was investigated in 3.5% NaCl and 5% HCl solutions. Corrosion properties of Ti–6Al–4V alloy were also evaluated under the same conditions for comparison. It was found that both Ti–8Al–1Mo–1V and Ti–6Al–4V alloys exhibited spontaneous passivity and low corrosion current densities in 3.5% NaCl solution. The potentiodynamic polarization curves obtained in 5% HCl solution revealed an active–passive transition behavior and similar corrosion rates for the examined alloys. However, the results of the weight loss experiments under accelerated immersion conditions (5 M HCl at 35 °C) indicated that Ti–8Al–1Mo–1V alloy exhibited inferior corrosion behavior compared to Ti–6Al–4V alloy. These results were confirmed by scanning electron microscopy (SEM) analysis of the samples after immersion tests which revealed that the β phase was corroded preferentially for both alloys, but to a larger extent in the case of Ti–8Al–1Mo–1V alloy.     

Deformation characteristics of Ti–6Al–4V

Abstract

The deformation characteristics of Ti–6Al–4V have been established by torsion testing in the temperature range 800–1150°C. Constitutive equations are proposed for both the β-region and the α+β-region which, it is suggested, may have some practical applications. Extensive optical and electron microscopy have established that dynamic recovery is the operative deformation mode in the β-region, while dynamic recrystallisation predominates in the α+β-region.

MST/806     

Prediction of the mechanical properties of the post-forged Ti–6Al–4V alloy using fuzzy neural network

Isothermal compression of the Ti–6Al–4V alloy was conducted at a 2500 ton 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.     

Prediction of flow stress in a wide temperature range involving phase transformation for as-cast Ti–6Al–2Zr–1Mo–1V alloy by artificial neural network

The isothermal compressions of as-cast Ti–6Al–2Zr–1Mo–1V titanium alloy in a wide temperature range of 1073–1323 K and strain rate range of 0.01–10 s⁻¹ with a reduction of 60% were conducted on a Gleeble-1500 thermo-mechanical simulator. The flow stress shows a complex non-linear intrinsic relationship with strain, strain rate and temperature, meanwhile the strain-softening behavior articulates dynamic recrystallization mechanism in α phase, dynamic recovery mechanism in β phase and their comprehensive function during phase transformation (α + β). Based on the experimental data, an artificial neural network (ANN) was trained with standard back-propagation learning algorithm to generalize the complex deformation behavior characteristics. In the present ANN model, strain and temperature were taken as inputs, and flow stress as output. A comparative study has been made on ANN model and improved Arrhenius-type constitutive model, and their predictability has been evaluated in terms of correlation coefficient (R) and average absolute relative error (ARRE). During α, α + β and β phase regime, R-value and ARRE-value for the improved Arrhenius-type model are 0.9824% and 6.02%, 0.9644% and 21.02%, and 0.9627% and 12.38%, respectively, while the R-value and ARRE-value for the ANN model are 0.9992% and 0.91%, 0.9996% and 1.47%, and 0.9975% and 2.17%, respectively. The predicted strain–stress curves outside of experimental conditions articulate the similar intrinsic relationships with experimental strain–stress curves. The results show that the feed-forward back-propagation ANN model can accurately tracks the experimental data in a wide temperature range and strain rate range associated with interconnecting metallurgical phenomena, and in further it has a good capacity to model complex hot deformation behavior of titanium alloy outside of experimental conditions.     

Constitutive analysis of compressive deformation behavior of ELI-grade Ti–6Al–4V with different microstructures

In this study, a constitutive analysis of the flow responses of Ti–6Al–4V under various strain rates [(e)\dot] \dot{\varepsilon } was conducted by separately quantifying the hardening and softening effects of microstructure, interstitial solute and deformation heating on the total stress. For this purpose, a series of compression tests on an extra-low interstitial grade alloy with equiaxed, lamellar, or bimodal microstructures was performed at 10 ^{- 3} ￡ [(e)\dot] ￡ 10  \texts ^{- 1} 10^{ - 3} \le \dot{\varepsilon } \le 10\;{\text{s}}^{ - 1} until the metal fractured, and the results were compared to those of the commercial grade alloy. The thermal stress σ^* increased with an increasing interstitial solute concentration; the athermal stress increased in the order of equiaxed, lamellar, and bimodal microstructures. Load–unload–reload tests revealed that the flow softening at a relatively high [(e)\dot] \dot{\varepsilon } was likely caused by deformation heating rather than by microstructure change; thus flow softening was attributed to a decrease in σ^*. Finally, a mechanical threshold stress model was extended to capture those observations; the modified model can provide a reasonable prediction of flow stress in Ti–6Al–4V with different microstructures and interstitial solute concentrations.     

The effect of post-weld heat treatment on the notched tensile fracture of Ti–6Al–4V to Ti–6Al–6V–2Sn dissimilar laser welds

Dissimilar welding of the Ti–6Al–4V (Ti-6-4) to Ti–6A1–6V–2Sn (Ti-6-6-2) alloys was performed by CO₂ laser in this work. The effect of post-weld heat treatment (PWHT) on the notched tensile strength (NTS) of the dissimilar weld was evaluated. Moreover, the results were also compared with the homogeneous laser welds with the same PWHT. Similar to the Ti-6-4 welds, the NTS of the FZ for dissimilar welds was less sensitive to PWHT conditions; the NTS of the FZ for distinct dissimilar welds fell within the range of 1060–1180 MPa. The results indicated a minor rise in the Mo equivalent of the titanium alloy promoted the formation of fine α + β microstructures in the form of basket weave in the welds, which resulted in high hardness accompanied with low NTS of the welds.