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.     

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.     

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.     

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.     

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.     

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     

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.     

Passivation Behavior of Water-Quenched and Heat-Treated Ti–6Al–4V in Hank's Solution

Ti–6Al–4V is a prevalent material utilized in various industrial applications, and its microstructure modification commences with quenching, followed by diverse heat treatments. Although many works have concentrated on the mechanical properties of Ti–6Al–4V with tailored microstructures resulting from heat treatments, their corresponding corrosion behavior still lacks attention. In this study, the corrosion behavior of water-quenched Ti–6Al–4V that undergoes heat treatment between 700 and 850 °C in Hank's solution is investigated. Various electrochemical methods, such as open-circuit potential tests, potentiodynamic/potentiostatic polarization, electrochemical impedance spectroscopy, and Mott–Schottky tests, are jointly employed. The water-quenched Ti–6Al–4V displays a quick-cooling microstructure with a plentiful amount of martensite α/α′ phase. Heat treatment at 700 °C significantly alters the microstructures of the samples. Due to competitive factors, heat treatment at low temperatures results in uneven alloy composition, leading to poor uniformity of the passive film. At this phase, negative effects dominate, and the corrosion resistance of the samples deteriorates. When the heat-treatment temperature increases to 850 °C, the content of β phase, which possesses better corrosion resistance, increases and becomes dominant. Consequently, the corrosion resistance of the samples improves in Hank's solution.     

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.     

Investigation of joining Al–C–Ti cermets and Ti6Al4V by combustion synthesis

The paper has addressed a route for the welding of titanium alloy (Ti6Al4V) and Al–C–Ti powders by the combustion synthesis (CS) method. Al–C–Ti powders were compressed in the titanium alloy pipes with relative densities of 65%, and then the powder compact was sintered by two reaction mode at the same time as the annulus of titanium alloy and the synthesized product were joined. The paper has studied the effects of reaction mode and Al content in starting powders on the structure and property of the welded joints. And it has also discussed the microstructure of welded joints by laser-induced combustion synthesis (LCS). The mechanical properties of the welding seam have been also tested. The results show that LCS welding has realized fusion welding and the welding seam has good mechanical properties. Furthermore, SEM analysis has indicated that nano-size grains of TiC were formed in the joint layer.     

Microstructures and mechanical properties of electron beam-rapid manufactured Ti–6Al–4V biomedical prototypes compared to wrought Ti–6Al–4V

This study represents an exploratory characterization and comparison of electron-beam melted (EBM) or rapid manufacturing (RM) of Ti–6Al–4V components (from nominal 30 μm diameter powder) with wrought products. Acicular α and associated β microstructures observed by optical metallography and electron microscopy (SEM and TEM) are compared along with corresponding tensile test and hardness data; including the initial powder particles where the Vickers microindentation hardness averaged 5.0 GPa in comparison with the fully dense, EB manufactured product with an average microindentation hardness ranging from 3.6 to 3.9 GPa. This compared with wrought products where the Vickers microindentation hardness averaged 4.0 GPa. Values of UTS for the EBM samples averaged 1.18 GPa for elongations ranging from 16 to 25%. Biomaterials/biomedical applications of EBM prototypes in direct prosthesis or implant manufacturing from CT or MRI data are discussed in the context of this work, especially prospects for tailoring physical properties through EB control to achieve customized and optimized implant and prosthetic products direct from CT-scans.     

Comparison of residual stresses in Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo linear friction welds

Abstract

In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α–β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d ₀ variation across the weld has been evaluated using the biaxial sin²Ψ technique with laboratory X-rays. It was found that failure to account for the d ₀ variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (～750 MPa) than for Ti-64 (～650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+β titanium alloy.     

Time dependence of microstructure and hardness in plasma carbonized Ti–6Al–4V alloys

Ti–6Al–4V alloy was treated by plasma carburizing process at 950 °C for different durations of 1 h, 2 h, 3 h, and 4 h. Plasma carburizing was performed in pure Ar gas under 32 ± 2 Pa. Graphite rod was employed as carbon supplier. Optical microscope (OM) observations showed a carburizing layer formed after carburizing. Further FESEM examinations and XRD analysis confirmed that the carburizing layer consists of a TiC/α-Ti mixed layer and a thin compound (TiC) layer on the mixed layer. With increasing the carbonizing time, the thickness of the carburizing layer increased and the specimen treated at 950 °C for 3 h obtains maximum values of the hardness.     

Low Temperature Thermal Conductivity of Ti6Al4V Alloy

The CUORE detector, to be installed in 2010 at LNGS, is made of 988 TeO₂ crystals to be cooled to 10 mK. It consists of a large cryogen-free cryostat cooled by five pulse tubes and one high-power specially designed dilution refrigerator (R. Ardito et al. in , [2005]). The cryostat is ∼ 3 m high and has a diameter of ∼ 1.6 m. About 5 tons of lead shielding are to be cooled to below 1 K and a mass of 1.5 ton must be cooled to 10 mK. Some tie-rods sustain the different parts of the experiment. One end of each rod is at low temperature (10 mK for the detector frame, 50 mK for the coldest radiation shield, 700 mK for the shield linked to the still) with the other end usually at room temperature. A thermalization of the rods at the temperature of the first stage of the pulse tubes will be realized. Hence the value of the thermal conductivity of the material up to room temperature is important. At the lowest temperatures, the thermal conductivity has great influence in establishing the thermal load on the dilution refrigerator. The thermal conductivity of the structural material candidates for such tie-rods is usually known down to 4.2 K. Here we present data of thermal conductivity for the Ti6Al4V alloy below its superconductive transition temperature (4.38 K). A comparison over the full temperature range of operation is also done with other materials, such as 316 stainless steel and Torlon, candidates for the realization of the tie-rods.      

Failure and formability studies in warm deep drawing of Ti–6Al–4V alloy

Deep drawing experiments have been performed in order to study formability of Ti–6Al–4V alloy sheet at temperature ranging from room temperature to 400 °C. It is found that below 150 °C, formability of the material is very poor and above 150 °C till 400 °C, limiting drawing ratio (LDR) is found to be 1.8 which is substantially lesser than other structural alloys. For better understanding of failures in failed cups, failure regions have been identified in neck and wall which are validated using finite element (FE) simulations. Fractured surface has been examined with scanning electron microscope (SEM) which reveals different types of shallow dimples indicating predominantly ductile failure. Additionally, in the properly drawn cups, thickness distribution has been studied over a temperature range of 150–400 °C and blank diameter 50–54 mm. In order to optimize blank diameter and temperature to obtain uniform thickness distribution of drawn cups, artificial neural network (ANN) and genetic algorithm (GA) have been employed. Thickness distribution for optimized parameters is validated using FE simulation.     

Performance Analysis of Machining Ti–6Al–4V Under Cryogenic CO2 Using PVD-TiN Coated Tool

Journal of Failure Analysis and Prevention - Demand for titanium alloys in different sectors is increasing in recent years due to their outstanding strength-to-weight ratio, ability to retain...     

Corrosion and corrosion-fatigue behavior of cp-Ti and Ti–6Al–4V laser-marked biomaterials

The aim of this work was to determine the influence of laser surface modification treatments on mechanical and electrochemical behavior in Ti and Ti–6Al–4V implants. For each metal, different samples were laser modified simulating the markings according to the international requirements. (It is necessary in each metallic biomaterial to mark the serial, batch and company numbers.) Microstructural changes produced by this treatment were observed: (a) the melting zone with small grain sizes and martensitic structures in above-mentioned metals and (b) the heat-affected zone (HAZ) with alpha phase in cp-Titanium with bigger grain sizes and Widmanstatten structure in Ti–6Al–4V. Positive tensile residual stress was determined by means X-ray analysis in the zones marked by laser. Furthermore, corrosion behavior was studied in a simulated body fluid at 37°C. Pitting was observed in different zones near the HAZ and the results showed a decrease of the corrosion resistance in the laser treated samples. Residual stresses and the martensitic microstructures favoured the decrease of the corrosion-fatigue life around 20% of both metals under physiological conditions.     

Laser welding of Ti–6Al–4V to Nitinol

Formation of brittle intermetallic phases in addition to different thermal expansion coefficients associated with dissimilar welding leads to the formation of transverse cracks in weld metal and eventually restricts widespread applications of dissimilar joints. Therefore, joining technology should be expanded in field of dissimilar welding in order to solve its difficulties. In the present study, an experimental work with pulsed Nd:YAG laser was performed for dissimilar welding of Ti–6Al–4V and Nitinol. Autogenous welding of these two alloys resulted in joints with poor strength and ductility due to the formation of transverse cracks in the weld metal. Therefore, the chemical composition of the weld metal has to be modified in order to reduce the formation of brittle phases and eliminate subsequent cracking. In this work, this was done by insertion of a copper interlayer with a thickness of 75 μm between the base metals. The results indicated that insertion of copper interlayer has a great influence on the reduction of the amount of Ti₂Ni brittle intermetallic phase, elimination of transverse cracks through the weld metal and eventually improvement of mechanical properties of the joints. Insertion of copper interlayer was very useful since it altered the cracked autogenous joint to a joint which could withstand a tensile stress of 300 MPa.     

Application of ultrasonic vibration assisted MQL in grinding of Ti–6Al–4V

This work presents experimental investigations performed to evaluate the improvement in grinding performance of Ti–6Al–4V alloy using ultrasonic vibration assisted minimum quantity lubrication (UMQL) technique. The grinding experiments have been performed using an indigenously designed and fabricated UMQL setup. In UMQL, the ultrasonic vibration of the horn has been used to atomise the cutting fluid into ultra-fine droplets of uniform size. Sunflower oil in 1, 5 and 10% of the volume have been added in water to prepare the biodegradable emulsions and used as grinding medium. The grinding performance during UMQL has been evaluated by comparing the grinding forces, surface roughness and the ground surface topography obtained during dry and conventional MQL (CMQL) techniques. Surface quality and chip morphology have been studied using microscopic imaging techniques. The UMQL grinding results in smaller grinding forces, and improved surface quality as compared to CMQL grinding. The experimental findings demonstrate that the UMQL has a strong potential to enhance the grindability of Ti–6Al–4V. The present work is also a step forward in finding a sustainable grinding technique for high strength materials using vegetable oil as a coolant.