Fatigue Resistance of Powder Metallurgy Ti–6Al–4V Alloy

Fatigue-resistance characteristics of Ti–6Al–4V alloy synthesized by the simplest powder metallurgy method involving the processes of pressing and sintering of blended elemental titanium hydride-based powders were studied. Powder materials have a relatively fine-grain -phase, which despite the presence of residual pores, makes for quite a high fatigue limit (500 MPa) comparable to that of the corresponding cast alloys. Fatigue cracks in the powder alloys are initiated from such stress raisers as major pores open to the surface of the specimen gauge length. Along with a significant decrease in the production costs of titanium alloys and articles of them, the use of this method provides obtaining materials with satisfactory static and dynamic mechanical characteristics suitable for practical applications.     

Ultrasonic Assisted Electrical Discharge Machining of Ti–6Al–4V Alloy

In this research, an attempt was made to investigate the influence of copper tool vibration with ultrasonic frequency on output parameters in the electrical discharge machining of Ti–6Al–4V. The selected input parameters for the experiment comprise of ultrasonic vibrations of tool, current and pulse duration and the outputs are tool wear ratio (TWR), material removal rate (MRR), and stability of machining process and surface integrity of a workpiece, including surface roughness, thickness of recast layer, and formation of micro cracks. Scanning electron microscope and X-Ray diffraction were employed to examine the surface integrity of the workpiece. The results revealed that tool vibration with ultrasonic frequency enhances MRR via increasing normal discharges and decreasing arc discharges and open circuit pulses. Also, by using ultrasonic vibrations in finishing regimes, the density of cracks and TWR decrease while in roughing regimes, the thickness of recast layer, density of cracks, and TWR increase.     

Cryogenic Drilling of Ti–6Al–4V Alloy Under Liquid Nitrogen Cooling

This article is focused on experimental study of the effects of cryogenic liquid nitrogen (LN₂) coolant during drilling of Ti–6Al–4V alloy material with three different levels of cutting speed (V_c) and feed rate (f) at a constant depth. Cutting temperature (T), thrust force (F_z), torque (M_z), surface roughness (R_a), and hole quality are the output responses investigated by using cryogenic LN₂ coolant compared with a wet coolant. Tool wear and chip morphology were examined with the changes in cryogenic LN₂ coolant. It is found that cryogenic LN₂ coolant results in lowering cutting zone temperature which helps more removal of heat from the cutting zone. Lower thrust forces and surface roughness were observed due to less friction and better chip breaking in cryogenic LN₂ condition. Also better chipping results in improvement in hole quality, viz., circularity and cylindricity in cryogenic LN₂ condition. Less serration and uniform segmentation results in better chip morphology and no damage to the cutting inserts resulted in improved tool life in cryogenic LN₂ condition. The main application of cryogenic LN₂ coolant in the cutting zone provides better lubrication and is more effective than wet coolant. The effects of this investigation show that cryogenic LN₂ coolant is an alternative approach for a wet coolant in the drilling process.     

Tribological Performance of Titanium Alloy Ti–6Al–4V via CVD–diamond Coatings

In the present study, HFCVD nanocrystalline, microcrystalline and boron-doped nanocrystalline diamond coatings have been deposited on titanium alloy. The effect of boron doping on coefficient of friction and residual stresses of diamond coatings have been studied. The tribological characteristics of the aforementioned three coatings on Ti–6Al–4V substrates were studied using ball on disc micro-tribometer, the thickness of the coatings being 3 μm. The coated Ti–6Al–4V discs were slid against alumina (Al₂O₃) balls with normal load ranging from 1 to 10 N. The boron-doped NCD coated sample disc was found to possess the lowest average coefficient of friction ~ 0.0804 while the undoped NCD and MCD coated sample discs were found to possess the average coefficients of friction of ~ 0.143 and ~ 0.283, respectively. Raman spectroscopy studies revealed that the residual stresses in boron-doped nanocrystalline coatings were tensile in nature, while the residual stresses in undoped NCD and MCD were found to be of compressive nature.     

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     

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.     

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.     

Tensile properties of friction stir welded and friction stir welded-superplastically formed Ti–6Al–4V butt joints

A University and Industry collaborative research project was undertaken to evaluate the performance of as friction stir welded (FSW) and friction stir welded-superplastically formed Ti–6Al–4V alloy sheets. The purpose of this particular effort was to evaluate the tensile properties of friction stir welded and superplastically formed friction stir welded Ti–6Al–4V. Welds were produced out of both standard grain and fine grained titanium and tested in the as welded, stress relieved (SR) and superplastically formed (SPF) conditions. The preliminary results of the FSW and post FSW–SPF joint were found to be close to that of as received titanium with respect to strength, but elongations were decreased.     

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 Cryogenic Cooling of Tool Electrode on Machining Titanium Alloy (Ti–6Al–4V) during EDM

The discharge characteristics and discharge gap of machining Ti–6Al–4V titanium alloy by cryogenically cooled tool electrode electrical discharge machining (EDM) in distilled water were investigated in this study using the monopulse discharge method. The influence of the cryogenically cooled tool electrode on the discharge gap and the initial maintaining voltage between the electrode and workpiece were analyzed under various temperatures. Test results showed the initial maintaining voltage of the cryogenically cooled tool electrode EDM was lower than that of conventional EDM. The discharge gap of the cryogenically cooled tool electrode EDM was also smaller than that of conventional EDM, which improved the copying accuracy of die-sinking EDM. A comparative experiment of machining Ti–6Al–4V titanium alloy was carried out by using cryogenically cooled tool electrode EDM and conventional EDM, lower electrode wear, higher material removal ratio, and higher corner size machining accuracy was obtained by using cryogenically cooled tool electrode EDM.     

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.     

Thermomechanical Behavior of Ti-6Al-4V Alloy

Ti-6Al-4V, among the Ti alloys, is the most widely used. In the present work, the behavior of Ti-6Ak-4V alloy has beeninvestigated by the uniaxial hot isothermal compression tests and a series of dilatometric experiments were also carried out todetermine the transformation temperatures at different cooling rates. Specimens for hot compression tests were homogenizedat 1050℃ for 10 min and then quickly cooled to different straining temperatures from 1050 to 850℃. Cooling rates were chosenfast enough to prevent high temperature transformation during cooling. Compression tests were conducted at temperaturesfrom 1050 to 850℃ in steps of 50℃ at constant true strain rates of 10~(-3) or 10~(-2) s~(-1). The apparent activation energy forcompression in two-phase region was calculated 420 kJ·mol~(-1). Partial globularization of cr phase was observed in the specimendeformed at low strain rates and at temperatures near the transformation zone and annealed after deformation.     

Transient liquid phase diffusion bonding of continuous SiC fibre reinforced Ti–6AI–4V composite to Ti–6AI–4V alloy

Abstract

A continuous SiC fibre reinforced Ti–6Al–4V composite was diffusion bonded in transient liquid phase to Ti–6Al–4V alloy plate using Ti–Cu–Zr amorphous filler metal. Joint strength increased with bonding time up to 1·8 ks and reached the maximum value of 850 MN m^?2 which corresponded to 90% of the tensile strength of Ti–6Al–4V. The extent of deformation of Ti–6Al–4V in the vicinity of the bonding interface was small compared with that of solid diffusion bonding because of the low bonding pressure. The bonding layer had an acicular microstructure which was composed of Ti₂Cu and α titanium with dissolved zirconium. Brittle products such as (Ti, Zr )₅ Si₃ or (Ti, Zr )₅ Si₄ were formed at the interface between the SiC fibres and the filler metal. These products existed only at the end of fibres, in very small amounts, therefore joint strength was not significantly affected by the products.

MST/1989     

Microstructure and Tensile Properties of Spray-deposited IC6 Alloy

The microstructure and mechanical properties of a Ni3Al based Ni-Al-Mo superalloy-IC6 which possesses low impurity content and high density by spray deposition technique have been studied.The results indicate that the IC6 alloy has good tensile strength and elongation both at room and intermediate temperatures     

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.     

Machine Learning Based Predictive Modeling of Machining Induced Microhardness and Grain Size in Ti–6Al–4V Alloy

Titanium and its alloys are today used in many industries including aerospace, automotive, and medical device and among those Ti–6Al–4 V alloy is the most suitable because of favorable properties such as high strength-to-weight ratio, toughness, superb corrosion resistance, and bio-compatibility. Machining induced surface integrity and microstructure alterations size play a critical role in product fatigue life and reliability. Cutting tool geometry, coating type, and cutting conditions can affect surface and subsurface hardness as well as grain size. In this paper, predictions of machining induced microhardness and grain size are performed by using 3D finite element (FE) simulations of machining and machine learning models. Microhardness and microstructure of machined surfaces of Ti–6Al–4 V are investigated. Hardness measurements are conducted at elevated temperatures to develop a predictive model by utilizing FE-based temperature fields for hardness profile. Measured hardness, grain size, and fractions are utilized in developing predictive models. Predicted microhardness profiles and grain sizes are then utilized in understanding the effect of machining parameters such as cutting speed, tool coating, and edge radius on the surface integrity. Optimization using genetic algorithms is performed to identify most favorable tool edge radius and cutting conditions.     

Effect of Vacuum Heat Treatment on the Microstructure and Tribological Property of Ti–6Al–4V Alloy with Cu Coating

The effect of vacuum heat treatment on the interface microstructure and tribological property of Cu-coated Ti – 6Al – 4V alloy is investigated herein. After the vacuum heat treatment process, a diffusion layer is formed at the interface between the Cu coating and the Ti – 6Al – 4V substrate. The formed intermetallic compounds at the interface between the Ti – 6Al – 4V substrate and Cu coating are CuTi₂, CuTi, Cu₄Ti₃, and β-Cu₄Ti. The activation energy of intermetallic compound growth in the diffusion zone of Cu-coated Ti – 6Al – 4V is 126.0 kJ mol⁻¹, and the pre-exponential factor is 0.1 m² s⁻¹. The tribological properties of the Cu-coated Ti – 6Al – 4V alloy are best when subjected to diffusion treatment at 700 °C for 300 min, with weight loss reduced by 58.2% compared to the Ti – 6Al – 4V alloy. The wear resistance of the Ti – 6Al – 4V alloy can be enhanced by Cu coating and vacuum diffusion heat treatment, and the formation of the Cu – Ti intermetallic compound contributes to this improvement. These findings offer new insights for further advancements in the tribological properties of titanium alloys.     

Longitudinal Compression Behavior of Functionally Graded Ti–6Al–4V Meshes

The compressive deformation behavior in the longitudinal direction of graded Ti–6Al–4V meshes fabricated by electron beam melting was investigated using experiments and finite element methods(FEM).The results indicate that the overall strain along the longitudinal direction is the sum of the net strain carried by each uniform mesh constituent and the deformation behavior fits the Reuss model well. The layer thickness and the sectional area have no effect on the elastic modulus, whereas the strength increases with the sectional area due to the edge effect of each uniform mesh constituent. By optimizing3 D graded/gradient design, meshes with balanced superior properties, such as high strength, energy absorption and low elastic modulus, can be fabricated by electron beam melting.