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.     

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     

Constitutive equations for elevated temperature flow stress of Ti–6Al–4V alloy considering the effect of strain

In order to study the workability of Ti–6Al–4V alloy, the experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (800–1050 °C) and strain rates (0.0005–1 s⁻¹), were used to develop the constitutive equation of different phase regimes (α + β and β phase). The effects of temperature and strain rate on deformation behaviors a represented by Zener–Holloman parameter in an exponent-type equation. The influence of strain was incorporated in constitutive analysis by considering the effect of strain on material constants. Correlation coefficient (R) and average absolute relative error (AARE) were introduced to verify the validity of the constitutive equation. The values of R and AARE were 0.997% and 9.057% respectively, which indicated that the developed constitutive equation (considering the compensation of strain) could predict flow stress of Ti–6Al–4V alloy with good correlation and generalization.     

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.     

Effects of intense cooling on microstructure and properties of friction-stir-welded Ti–6Al–4V alloy

Friction stir welding (FSW) was used to join Ti–6Al–4V alloy in air and under intense cooling conditions. The results show that the application of liquid nitrogen is beneficial in decreasing the peak temperature and in reducing the extent of the high-temperature region during welding, leading to a smaller stir zone (SZ). Intense cooling can lead to refined and homogeneous grains in the SZ, resulting in increased microhardness. The FSW joint produced with intense cooling had a tensile strength of 1020?MPa, which is nearly equivalent to that of the base material and is up to 2.6% higher than for the air-cooled joint. The fractographs for both types of joint were characterised by dimples, indicating that the fractures were ductile.     

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.     

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.     

Modeling the constitutive relationship of powder metallurgy Al–W alloy at elevated temperature

The deformation behavior of Al–W alloy was researched with isothermal compression tests at various deformation temperatures and strain rates to evaluate the deformation activation energy and to develop the constitutive relationship equation, which is in pursuit of revealing the dependence of the flow stress on the strain, strain rate and deformation temperature. The compression tests were conducted in the temperature range between 420 and 570 °C and at strain rates between 0.001 and 5.0 s⁻¹. With the help of determination of related material constants (such as A, β and α) and activation energy Q (451.15 kJ mol⁻¹), the Arrhenius-type constitutive relationship equation of Al–W alloy is developed. It was found that the correlation coefficient R and the AARE is 0.997% and 4.08%, respectively. The results show that the Arrhenius-type model, which considers the combined influence of strain rate and deformation temperature, is able to provide the accurate prediction of high temperature flow stress for the researched alloy.     

Microstructural characteristics and mechanical properties of friction stir welded joints of Ti–6Al–4V titanium alloy

The α + β titanium alloy, Ti–6Al–4V, was friction stir welded at a constant tool rotation speed of 400 rpm. Defect-free welds were successfully obtained with welding speeds ranging from 25 to 100 mm/min. The base material was mill annealed with an initial microstructure composed of elongated primary α and transformed β. A bimodal microstructure was developed in the stir zone during friction stir welding, while microstructure in the heat affected zone was almost not changed compared with that in the base material. An increase in welding speed increased the size of primary α in the stir zone. The weld exhibited lower hardness than the base material and the lowest hardness was found in the stir zone. Results of transverse tensile test indicated that all the joints had lower strength and elongation than the base material, and all the joints were fractured in the stir zone.     

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.     

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     

Fatigue crack propagation properties of Ti–6Al–4V in vacuum environments

To determine the effects of vacuum environment on fatigue crack propagations in a Ti–6Al–4V alloy, K-decreasing tests were conducted in air and vacuum. The fatigue crack propagation rate became slower and threshold stress intensity factor range became larger with decreasing vacuum pressure. The tendency cannot be fully explained by the crack closure. Based on fracture surface observations, granular region of a few micrometer size asperities was observed on the fracture surface only in high vacuum and ultra high vacuum. The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of adsorbed gas on fracture surfaces relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between vacuum and air environments. A new mechanism for the formation of the granular region was proposed, and that is one of the phenomena which can explain the reduction of crack propagation rate in vacuum.     

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).     

Antimicrobial effects of oxygen plasma modified medical grade Ti–6Al–4V alloy

Biomedical titanium alloy (Ti–6Al–4V) has good mechanical properties and cytocompatibility but post-operative implant-related bacterial infection is a big concern. Therefore, it is very important to suppress adhesion of bacteria on the implants or even kill the bacteria by proper surface modification. In this study, oxygen plasma immersion ion implantation (O-PIII) is employed to produce antibacterial effects on medical grade titanium alloy. The correlation between the surface chemistry and topography on antibacterial behavior is systematically investigated. Colony forming unit (CFU) counting is carried out to evaluate the adhesion of bacteria on the surface and enhanced green fluorescent protein (EGFP) mouse osteoblastic cells are used to study the cytocompatibility after the plasma treatment. Our results suggest that the nanostructured TiO₂ layer produced by O-PIII can significantly suppress bacterial adhesion while the original cytocompatibility can be retained. The nanoscale TiO₂ layer is promising in the prevention of implant-related bacterial infection on orthopedic implants.     

Narrow-gap tungsten inert gas welding of 78-mm-thick Ti–6Al–4V alloy

Narrow-gap tungsten inert gas welding with in situ protective system is applied to 78-mm-thick Ti–6Al–4V alloy. The martensite basketweave microstructure is formed in fusion zone (FZ), and heat-affected zone (HAZ) near FZ consists of acicular martensite and a small portion of primary alpha phase. The HAZ near base metal (BM) consists of primary alpha and transformed beta phase. Microhardness values of FZ and HAZ are relatively higher, and peak values appear at HAZ near FZ. The mechanical heterogeneity is dependent on variation of beta phase and martensite, which in turn depend on specific thermal cycle(s). The average tensile strength reaches 783.3?MPa and is 91.2% of BM, which appears in the middle of the weld with orthogonal martensites.     

Low cycle fatigue life of Ti–6Al–4V alloy under non-proportional loading

Multiaxial low cycle fatigue life of Ti–6Al–4V under non-proportional loading was studied. Strain-controlled multiaxial fatigue tests at room temperature were carried out using tubular specimens. The strain paths employed were push–pull loading, reversed torsion loading, and two kinds of 90° out-of-phase loadings. The former two loadings are proportional loading tests where the principal directions of stress and strain are fixed in the cycle. The latter two are non-proportional loading tests where there is a 90° phase difference between axial and shear loadings, and the principal directions are cyclically rotated continuously. Failure lives are reduced obviously by non-proportional loadings in comparison with those in proportional loading tests. This paper focuses on determining a suitable fatigue model for evaluating the failure lives of Ti–6Al–4V under multiaxial loading.     

Development of Al–Mn–Mg 3004 alloy for applications at elevated temperature via dispersoid strengthening

In this study, the potential applications of Al–Mn–Mg 3004 alloy at elevated temperature have been evaluated through the systematic study of the precipitation behavior of α-Al(MnFe)Si dispersoids and their effect on material properties during precipitation treatment and long-term thermal holding. The results demonstrate a significant dispersion strengthening effect caused by the precipitation of fine uniformly distributed dispersoids during precipitation treatment. The peak compression yield strength (YS) at 300 °C of the experimental 3004 alloy can reach as high as 78 MPa due to a large volume fraction (~ 2.95 vol.%) of α-Al(MnFe)Si dispersoids. The dispersoids are found to be thermally stable at 300 °C for up to 1000 h of holding, leading to consistently high mechanical performance and creep resistance. The superior and stable YS and creep resistance at 300 °C enables the 3004 alloy to be applied to weight-sensitive applications at elevated temperatures.     

Microplasticity in HCF of Ti–6Al–4V

Previous research has shown that Ti–6Al–4V exhibits pronounced stress ratio effects under high cycle fatigue (HCF) loading. At high stress ratios (R>0.7), a transition of failure mode occurs from traditional surface fatigue crack initiation and growth to bulk-dominated damage initiation and coalescence of multiple microcracks consistent with a ductile tensile test. At these high stress ratios, ratchetting was shown to occur (Int. J. Fatigue 21 (1999) 679; Mech. Time-Dependent Mater. 2 (1999) 195), leading to progressive strain accumulation until final failure. This study explores the microstructural origins of this stress ratio transition in HCF using computational micromechanics. The material being studied is a two-phase Ti–6Al–4V plate forging, consisting of a duplex microstructure with a hexagonal close-packed (hcp) α-phase and lamellar grains with layers of body-centered cubic (bcc) β-phase and secondary hcp α-phase. Crystallographic slip is the dominant mode of plastic deformation in this material. A 2-D crystal plasticity model that incorporates nonlinear kinematic and isotropic hardening at the slip system level is implemented into the finite element method to simulate the cyclic plasticity behavior. The finite element model is used to qualitatively understand the distribution of microplasticity in this alloy under various loading conditions. For typical HCF stress amplitudes, it is shown that microstructure scale ratchetting becomes dominant at R=0.8, but is insignificant at R=0.1 and 0.5. Reversed cyclic microplasticity is insignificant at all three stress ratios. The effects of phase morphology and orientation distribution are shown to affect the microscale plastic strain distribution in terms of the location and magnitudes of the plastic shear bands that form within clusters or chains of primary α grains. The results of the finite element modeling are also considered in light of previous experimental results.