Ti 6A1 4V Research Articles

Deformation and Microstructure of Titanium Chips and Workpiece

Abstract In order to improve the machinability of titanium alloys, a better understanding of the chip formation process is needed. Microstructur alanalysis of chips produced at high cutting speeds can lead to an estimate of temperatures and the deformation state in the chips if structural changes can be observed. Two-dimensional cross-sectional analyses are performed by means of optical microscopy (LM) and transmission electron microscopy (TEM). To study the deformation state of the finished workpiece, a scratch-grid method in combination with a scanning electron microscope (SEM) is used. The results of the different analyses for Ti 6A14V are presented in this paper and are compared with a finite element simulation of an orthogonal cutting process.

Developments on Electron Beam Melting (EBM) of Ti–6Al–4V: A Review

In producing parts using 3D printing technology, electron beam melting (EBM) has been presented as an additive manufacturing (AM) process. In producing parts using the EBM process, powder melting enabled in a high vacuum atmosphere happens simultaneously at multiple points without compromising on surface finish, precision or build speed. Heat treatments on the unique microstructure and microstructure evolution, mechanical properties of produced layers such as ultimate tensile strength (UTS) by the production method, precision mechanical systems for reducing waste materials, fatigue properties in EBM methods, study of produced parts for analyzing the corrosion resistance, analysis of Surface roughness of produced parts using EBM, applications of EBM to biomaterial products and effect of EBM parameters and porous structure on mechanical properties are considered in order to develop the EBM process. EBM is a high technology increasingly employed in different industries including bioengineering, aerospace and marine not only because of its increasing efficiency in the process of part production but also because of its time and cost-efficient accurate production results. Moreover, time and cost of accurate production can be decreased as a result of increasing efficiency in process of part production. However, despite the quantity of research in this area, there are few papers reviewing the achievements. Therefore, this paper reports on recent achievements in production of EBM processed Ti–6A1–4V parts.

Globularisation of α Lamellae in Titanium Alloy: Effect of Strain, Strain Path and Starting Microstructure

To gain in-depth understating on the globularisation of α lamellae in titanium alloys, a systematic study has been carried out to evaluate the effect of strain and strain path on the microstructural evolution of titanium alloy Ti–6Al–4V with two different starting microstructures viz., water quenched martensitic and air cooled lamellar microstructure. Ti–6A1–4V was subjected to similar amounts of strain using uniaxial (monotonic) and multi-axial (non-monotonic) deformation. A vis-a-vis comparison of the microstructures obtained after deformation revealed that the material with fine α lamellae exhibits better globularisation kinetics irrespective of the mode of deformation. Further alteration of the strain path (multi-axial or non-monotonic) in the material before achieving critical percentage of deformation results in reduced globularisation. Beyond this critical amount of deformation, the results obtained for both monotonic and non-monotonic deformation are comparable. However, multi-axial or non-monotonic deformation results in better homogeneity in terms of microstructural evolution.

Kinetic features of wear-resistant coating growth by plasma electrolytic oxidation

The kinetic features of coating growth on titanium alloy Ti–6A1–4V by plasma electrolytic oxidation (PEO) are investigated. It was observed that the coating growth rate decreases significantly with the increasing of the PEO time treatment. At the same time, large longitudinal pores (cracks) were formed; thus, the major part of the coating is separated from the substrate. Coating growth is realized by the two mechanisms: (a) migration-diffusional and (b) thermochemical treatment of the deposited aluminum hydroxide (electrolysis) and oxidation of the substrate at the bottom of the coating pores. It presumably occurs due to the saturation of the electrolyte in the pores with titanium hydroxide and the relatively low intensity of the microdischarges, which is insufficient to eject the substance from the channels for ‘pancake’ formation. This increases the amount of the aluminum titanate (TiAl2O5) in the coating. The latter presumably forms due to a eutectic reaction in the coating or due to the exothermic reactions of titanium dioxide formation. The TiAl2O5-based coating containing at least 10% aluminum oxide α-Al2O3 (more than 20% in the outer layer) and 11% titanium dioxide (TiO2) increases the wear resistance of the titanium alloy at least six times.

A nanocrystalline zirconium carbide coating as a functional corrosion-resistant barrier for polymer electrolyte membrane fuel cell application

A ZrC nanocrystalline coating is engineered onto a Ti–6Al–4V substrate using a double cathode glow discharge technique in order to improve the corrosion resistance and long-term stability of this alloy. The new coating exhibits an extremely dense, homogeneous microstructure composed of equiaxed grains with an average grain size of ∼12 nm and is well adhered on the surface of the substrate. The corrosion behaviour of the coating is systematically investigated using various electrochemical methods, including potentiodynamic, potentiostatic polarizations and electrochemical impedance spectroscopy (EIS), in a simulated polymer electrolyte membrane fuel cell (PEMFC) operating circumstances under different temperatures. The results show that with rising temperature, the corrosion potential (Ecorr) decreases and the corrosion current density (icorr) of the ZrC coated specimen increases, indicating that the corrosion resistance decreased with increasing temperature. However, at a given temperature, the ZrC-coated Ti–6Al–4V alloy has a higher Ecorr and lower icorr as compared to the bare substrate. The results of EIS measurements show that the values of the resistance for the ZrC coated Ti–6Al–4V alloy are three orders of magnitude larger than those of Ti–6A1–4V in the simulated PEMFC environment.

Evaluation of the mechanical and corrosion protection performance of electrodeposited hydroxyapatite on the high energy electron beam treated titanium alloy

In our present study, the Ti–6Al–4V alloy surface was modified by irradiating with the high energy low current DC electron beam (HELCDEB) using 700keV DC accelerator. Following this, the HELCDEB treated surface was coated with hydroxyapatite by adopting electrodeposition method. The microstructure and hardness of HELCDEB treated Ti–6A1–4V alloy with and without electrodeposited hydroxyapatite were investigated. Also, the electrochemical corrosion characteristics of the samples in simulated body fluid (SBF) was studied by potentiodynamic polarisation and electrochemical impedence techniques (EIS) which showed an enhanced corrosion resistance and revealed an improved life time for the hydroxyapatite coating developed on the HELCDEB treated Ti–6A1–4V alloy than the untreated sample.

Oxidation resistance of Mo(Si1−xAlx)2 nanocrystalline films and characterization of their oxide scales by electrochemical impedance spectroscopy

To explore the influence of Al alloying on the oxidation resistance of MoSi2, four Mo(Si1−xAlx)2 nanocrystalline films were fabricated on Ti–6A1–4V substrates and their cyclic oxidation behavior was characterized.

Role of Al additions in wear control of nanocrystalline Mo(Si1−xAlx)2 coatings prepared by double cathode glow discharge technique

Four kinds of nanocrystalline Mo(Si1−xAlx)2 coatings with differing Al contents are prepared onto a Ti–6Al–4V substrates by a double cathode glow discharge apparatus. The microstructural features of the deposited coatings were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These coatings are composed of the equiaxed C40–MoSi2 grains with the average grain size of ∼5 nm. Nano-indentation measurements indicated that the hardness H, elastic modulus E and the H/E or H3/E2 ratio of the nanocrystalline Mo(Si1−xAlx)2 coatings slightly increase with the increase in Al content. The tribological behaviour of the nanocrystalline Mo(Si1−xAlx)2 coating sliding against a ZrO2 ceramic ball at room temperature has been compared using a ball-on-disc type tribometer under unlubricated conditions. Experimental results showed that the specific wear rates of the nanocrystalline Mo(Si1−xAlx)2 coatings decrease with increasing Al content and are dramatically reduced by more than 1–2 orders of magnitude over the uncoated Ti–6A1–4V. The enhancement of wear resistance of the nanocrystalline Mo(Si1−xAlx)2 coatings by Al additions is correlated with the increased mechanical properties and the forming oxide layer by tribochemical reactions.

Biomechanical effects of the distance from the fracture zone to the interlocking fixation screw of intramedullary nail

The goal of any internal fixation produce is to increase the stability of fractures, to transfer load across the fracture site, and to maintain anatomic alignment to induce bony union. While there is general agreement on the usefulness of intramedullary fixation techniques, the method has been criticized because of technical difficulties and mechanical failures. Intramedullary nails are load-sharing devices, allowing the bone to transmit compressive forces while maintaining axial alignment. The function of an intramedullary nail to facilitate healing is secured by its stiffness, which is usually expressed according to load directions as bending or torsional stiffness. Nail stiffness has been determined in a large number of studies. Nail design influences both bending and torsional stiffness. Important parameters are nail diameter, nail material, cross-section (open vs. closed) and design of the screw/nail interface. Nail strength is influenced by material, design and manufacturing properties. The video extensometer markers were placed on distal and proximal side of the fracture. The displacements between the markeres were measured with two non-contact CCD camera extensometers (Non-contact Video Extensometer DVE-101/201, Shimadzu, Japan). The axial compression was applied to all materials with the loading speed of 5 mm/min (max. 800 N). Biomechanical test, axial compression, was performed on materials. In present study, standard intramedullary nails with 11 mm diameter and 400 mm length are used. The intramedulary nails are Ti–6A1–4V alloys. Fracture gap of the models was 10mm. In three different groups, nine tests in each one were carried out. The intramedullary nail distance of the screws was 210 mm in the first group, 110 mm (between two distal gaps) in the second group and six screw fixed in the third groups. Polyethylene materials with 210 mm length are fixed to each intramedullary nails. Results of compression tests of all groups are evaluated and compared statistically. According to statistical assessment there is significant change between each three groups. In a recent, more detailed comparative study it was shown, that significant variations in axial rigidity exist between the proximal and the distal segment of certain nails, and that more screw do not always lead to increased stiffness. If the interface is insufficient, the load on the bone will be taken mostly by the screw holes through the screws instead of by the nail body; however, the screw holes are the weakest part of the nail. Local stress peaks in the region of the interlocked screws can lead to a material failure of the bone, which can result in complications such as nail or screw brake-down and bone refracture.

Titanium Alloy Lattice Structures with Millimeter Scale Cell Sizes

AbstractTitanium sandwich panels with cellular cores of a uniform 1–5 mm diameter open cell size are well suited for impact energy absorption and cross flow heat exchange applications. Periodic cellular structures (lattices) made from high specific strength, high temperature alloys are preferred for these multifunctional uses. A diffusion bonding method has been applied here to make cellular lattice structures from a Ti–6A1–4V alloy. To illustrate the approach, lattice structures with both square and diamond collinear topologies, a 2 mm open cell size, and a relative density of 15% were made from 254 µm diameter titanium alloy wires. These structures were found to have a compressive strength of 40 ± 5 MPa that was controlled by plastic yield followed by buckling of the struts. The cellular structures have been brazed to titanium alloy face sheets to create sandwich panel structures that appear well suited for multifunctional applications up to 420 °C.

Salt spray corrosion test of micro-plasma oxidation ceramic coatings on Ti alloy

Ceramic coatings were prepared on Ti-6A1–4V alloy in NaAlO 2 solution by micro-plasma oxidation (MPO). The salt spray tests of the coated samples and the substrates were carried out in a salt spray test machine. The phase composition and surface morphology of the coatings were investigated by XRD and SEM. Severe corrosion occurred on the substrate surface, while there were no obvious corrosion phenomena on the coated samples. The coatings were composed of Al 2TiO 5 and a little α-Al 2O 3 and rutile TiO 2, and the salt spray test did not change the composition of the coatings. The weight loss rate of the coatings decreased with increasing MPO time because of the increase in density and thickness of the coatings. The surface morphology of the coatings was influenced by salt spray corrosion test. Among the coated samples, the coating prepared for 2 h has the best corrosion resistance under salt spray test.

Sliding wear characteristics of the diamond-like carbon films on alloy substrates

This study used an un-balanced magnetron sputtering system to deposit different thickness of diamond-like carbon (DLC) film on Co–Cr–Mo and Ti–6A1–4V alloy substrate. Wear test, Raman spectroscopy analysis, nano-indentation test and morphology of contact surface were conducted to evaluate the tribological performance of these films for bio-medical applications. The ring-on-disk wear testing shows that the thicker film leads to the lower friction coefficient and higher wear resistance. In the testing process, the fluctuating friction coefficient of DLC coated Co–Cr–Mo substrate was less than that of DLC coated Ti–6Al–4V substrate. Raman spectroscopy analysis and nano-indentation test provide the underlying mechanism for the enhancement of the wear resistance. The DLC coated Co–Cr–Mo substrate provides better wear resistance than the DLC coated Ti–6Al–4V substrate owing to better adhesion of DLC film to the Co–Cr–Mo substrate.

Finite element modelling of fretting wear surface evolution: Application to a Ti–6A1–4V contact

Fretting is defined as a small displacement oscillatory motion between two solids in contact. Depending on the loading conditions (displacement amplitude, normal force, etc.) fretting damage can be induced by surface adhesion, abrasion, and fatigue with debris formation and ejection being the by products of wear. For the studied titanium alloy Ti–6A1–4V, fretting conditions were reproduced through a simple cylinder on plane contact. By introducing an energy wear concept, the relative impact of pressure sliding amplitude and number of fretting cycles can be rationalized through a single parameter. Predicting wear kinetics as well as geometrical changes of the wear scar under fretting conditions appears to be of great interest for industrial applications, so a specific numerical method to model the progressive evolution of the wear scar is developed. This method, based on a finite element analysis combined with global experimental wear kinetics, allows the modification of nodal coordinates to simulate material removal. This approach is then validated by comparing numerical and experimental results. Finally, these results are discussed and further investigations are outlined.

Residual stress measurement with focused acoustic waves and direct comparison with X-ray diffraction stress measurements

The technique of measuring small changes in acoustic wave velocity due to external or internal stress has been used for quantitative determination of residual stress in materials during the last decade. Application of similar methodology with focused acoustic waves leads to residual stress measurement with spatial resolution of a few millimeters to a few microns. The high spatial resolution residual stress measurement required development of new methodologies in both the design of acoustic lenses and the instrumentation for acoustic wave velocity determination. This paper presents two new methodologies developed for the measurement of residual stress with spatial resolution of a few millimeters. The design of new type of acoustic lens for achieving higher spatial resolution in residual stress measurement is introduced. Development of instrumentation for high precision local surface wave velocity measurement will be presented. Residual stresses measured around a crack tip in a sample of Ti–6A1–4V using a focused beam will be compared with X-ray diffraction measurements performed on the same region of the sample. Results of residual stress measurements along a direction perpendicular to the electron beam weld in a sample of Ti–6A1–4V, determined using focused acoustic waves and X-ray diffraction technique, are also presented. The spatial resolution and penetration depth of X-rays and focused acoustic beams with reference to residual stress measurements are discussed.

Machinability of titanium alloy (Ti’6Al’4V) by abrasive waterjets

Titanium alloy is known as one of the difficult-to-machine materials using conventional machining processes, although it has superior formability. In the present study, a widely used aircraft structural titanium alloy (Ti—6A1—4V) was machined with an abrasive waterjet (AWJ) to investigate its machinability under varying cutting conditions. Machinability was evaluated in terms of kerf geometry, cut surface quality and microstructural integrity. Quality of the machined surface and microstructure features were examined using surface profilometry, scanning electron microscopy and energy dispersive X-ray (EDX) analysis. The surface roughness was ranged several micrometres near the jet entrance region and was observed to increase at the jet exit of the workpiece. Scanning electron microscopy (SEM) analysis of the surface microstructure revealed that the mechanism of material removal was a combination of scooping induced ductile shear and ploughing actions of the abrasive particles. EDX analysis showed garnet particle embedment in titanium throughout the cutting depth, and the particle size was estimated to be several tens of micrometres.

Computational modelling of shot-peening effects on crack propagation under fretting fatigue

Recent experimental studies have demonstrated fretting fatigue life enhancement of titanium alloy Ti-6A1–4V specimens after treatment by shot-peening. Because of complexities in tracking crack growth under fretting conditions experimentally, the present work describes computational modelling for crack propagation behaviour in specimens with and without shot-peening. A crack growth model is combined with a finite element submodelling technique to assess the crack trajectory and crack propagation life in the specimens under fretting fatigue. A parametric numerical analysis has been performed to investigate crack trajectories and stress intensity factors along the crack path under different loading conditions. Obtained results revealed the features of the crack growth trajectory and stress intensity factors in the presence of residual stresses from shotpeening. These results also demonstrated a significant (2–3 times) increase in the crack propagation life of shot-peened specimens relative to virgin specimens (i.e. without shot-peening), which is in agreement with experimental observations.

Microstructural evolution of a Ti–6Al–4V alloy during thermomechanical processing

Variation of microstructure of a Ti–6A1–4V alloy during thermomechanical processing at a range of hot working conditions (temperature: 850–1050 °C, strain rate: 0–1.0 s −1 has been investigated for further understanding and control of the processing–structure–property relationships. The influences of hot working parameters on deformation localization, width of α platelets, (α→β) phase transformation, and (meta)dynamic recrystallization were systematically examined by optical microscopy and scanning electron microscopy. The experimental results showed that the lamellar morphology of the α phase can be sub-divided into five different categories for samples processed in the (α+β) phase field, and these represent three stages of the (α→β) phase transformation. It was also noted that a certain degree of the (α→β) phase transformation occurred concurrently with the mechanical deformation in the (α+β) phase field. The experimental evidence suggests that dynamic and/or metadynamic recrystallization has occurred in the β phase field, but was not involved in the (α+β) phase field under the experimental conditions.

Superplastic behaviour of ultrafine-grained Ti–6A1–4V alloys

Superplastic behavior of the ultrafine-grained (UFG) Ti–6A1–4V alloy produced by high pressure torsion (HPT) has been studied. High elongations (more than 500%) have been observed in this alloy during tensile tests at relatively low temperatures and high strain rates. At the same time, the superplastic behavior of this alloy has several specific features such as the relatively low values of strain rate sensitivity of flow stress and significant strain hardening. Moreover, it was shown that the alloy, processed by HPT, demonstrates an outstanding room temperature strength about 1500 MPa after superplastic deformation.

Characterization of immersed hydroxyapatite-bioactive glass coatings in Hank’s solution

Monolithic HA and hydroxyapatite-bioactive glass (HA–BG) coatings have been plasma-sprayed on Ti–6A1–4V substrate using sinter-granulated HA and HA–BG powders. The immersion behavior of these coatings in Hank’s physiological solution was investigated. The results showed that sinter-granulated monolithic HA powder was irregularly-shaped and appeared quite dense. When 5 wt.% or more BG was added in HA, the powder became rough and porous. X-ray diffraction patterns showed that the presence of BG enhanced the decomposition of HA structure during fabrication of the powder. The plasma spray process itself also enhanced the decomposition of apatite. Surface morphology of the present HA–BG coatings was not much different from that of monolithic HA coating, that comprised patches of smooth and shiny glassy film and irregularly-shaped particles on its surface. Reasonably high bond strengths were obtained from all as-plasma-sprayed coatings. When immersed in Hank’s solution, these bond strengths largely declined.

Microstructure and tensile properties of mechanically alloyed Ti–6A1–4V with boron additions

Particulate titanium metal matrix composites (MMCs) offer advantages for a wide range of structural applications with increases in stiffness and possibly wear resistance. A powder metallurgy/mechanical alloy route was used successfully to give a fine, uniform distribution of boron in titanium alloy powder. This powder was hot isostatically pressed and during the process TiB reinforcement was formed by an in-situ reaction. Significant tensile ductility in MA titanium alloy (without boron), equivalent to wrought material was achieved, along with good tensile strength. In materials with high TiB volume fractions it is thought that the fine boride size and high strength matrix caused embrittlement. This suggests that the use of a lower strength matrix, such as commercial purity titanium, and shorter milling times would provide a better balance of strength, stiffness and ductility for this type of composite.