Ti 6Al 4V Extra Low Interstitial Research Articles

An interlaboratory comparison of X-ray computed tomography measurement for texture and dimensional characterisation of additively manufactured parts

This paper presents the results of the CT-STARR (CT-Surface Texture for Additive Round Robin) Stage 1 interlaboratory comparison. The study compared the results obtained for the extraction of areal surface texture data per ISO 25178-2 from five X-ray computed tomography (CT) volume measurements from each of four laboratories. Two Ti6Al4V ELI (extra-low interstitial) components were included in each of the CT acquisitions. The first component was an additively manufactured (AM) cube manufactured using an Arcam Q10 electron beam melting (EBM) machine. Surface texture data was extracted from CT scans of this part. The values of selected parameters per ISO 25178-2 are reported, including Sa, the arithmetic mean height, for which the values from the Nikon MCT 225 metrology CT measurements were all within 0.5% of the mean reference focus variation measurement. CT resolution requirements are discussed. The second component was a machined dimensional test artefact designed to facilitate independent analysis of CT global voxel scaling errors and surface determination errors. The results of mathematical global scaling and surface determination correction of the dimensional artefact data is reported. The dimensional test artefact errors for the XT H 225 commercial CT for length, outside diameter and inside diameter reduced from -0.27%, -0.83% and -0.54% respectively to less than 0.02% after performing mathematical correction. This work will assist the development of surface texture correction protocols, help define surface-from-CT measurement envelope limits and provide valuable information for an expanded Stage 2 interlaboratory comparison, which will include a more diverse range of CT systems and technologies, further expanding the surface-from-CT knowledge base.

Effects of Superplastic Forming on Modification of Surface Properties of Ti Alloys for Biomedical Applications

In the present work, both the surface chemical contamination and the mechanical alteration of Ti–6Al–4V (Ti64) and Ti–6Al–4V extra low interstitial (Ti64ELI) titanium alloys subjected to superplastic forming (SPF) for the manufacturing of highly customized biomedical prostheses have been investigated. As case study, a cranial implant was considered. The design of the manufacturing process was assisted by a numerical model calibrated on free inflation experimental tests. Glow discharge optical emission spectrometry (GDOES) analyses, nanoindentation tests, and metallographic analyses allowed to relate the mechanical alteration to the oxygen enrichment due to the environmental exposition during processing. While similar diffusion kinetics were found, different oxidation rates were measured in the two investigated alloys. The hardness variation was strictly related to the oxygen content. In order to verify the material biocompatibility, cytotoxicity tests were conducted on the most oxidized part. Results highlighted that the oxygen enrichment due to the manufacturing process did not significantly affect the cells viability.

Qualification of Ti6Al4V ELI Alloy Produced by Laser Powder Bed Fusion for Biomedical Applications

Rectangular Ti6Al4V extralow interstitials (ELI) samples were manufactured by laser powder bed fusion (LPBF) in vertical and horizontal orientations relative to the build platform and subjected to various heat treatments. Detailed analyses of porosity, microstructure, residual stress, tensile properties, fatigue, and fracture surfaces were performed based on x-ray micro-computed tomography, scanning electron microscopy, and x-ray diffraction methods. The types of fracture and the tensile fracture mechanisms of the LPBF Ti6Al4V ELI alloy were also studied. Detailed analysis of the microstructure and the corresponding mechanical properties were compared against standard specifications for conventional Ti6Al4V alloy for use in surgical implant applications. Conclusions regarding the mechanical properties and heat treatment of LPBF Ti6Al4V ELI for biomedical applications are made.

Formation of Hydroxyapatite Layer on Ti–6Al–4V ELI Alloy by Fine Particle Peening

Fine particle peening (FPP) using hydroxyapatite (HAp) shot particles can form a HAp layer on room-temperature substrates by the transfer and microstructural modification of the shot particles. In this study, FPP with HAp shot particles was applied to form a HAp surface layer and improve the fatigue properties of Ti–6Al–4V extra-low interstitial (ELI) for use in bio-implants. The surface microstructures of the FPP-treated specimens were characterized by micro-Vickers hardness testing, scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. FPP with HAp shot particles successfully formed a HAp layer on the surface of Ti–6Al–4V ELI in a relatively short period by shot particle transfer at room temperature; however, the thickness and elemental composition of the HAp layer were independent of the FPP treatment time. The original HAp crystal structure remained in the surface-modified layer formed on Ti–6Al–4V ELI after FPP. Furthermore, FPP increased the surface hardness and generated compressive residual stresses at the treated surface of Ti–6Al–4V ELI. Four-point bending fatigue tests were performed at stress ratios of 0.1 and 0.5 to examine the effect of FPP with HAp shot particles on the fatigue properties of Ti–6Al–4V ELI. The fatigue life of the FPP-treated specimen was longer than that of the un-peened specimen because of the formation of a work-hardened layer with compressive residual stress. However, no clear improvement in the fatigue limit of Ti–6Al–4V ELI occurred after FPP with HAp shot particles because of subsurface failures from characteristic facets.

Characterization and analysis of surface notches on Ti-alloy plates fabricated by additive manufacturing techniques

Rectangular plates of Ti–6Al–4V with extra low interstitial (ELI) were fabricated by layer-by-layer deposition techniques that included electron beam melting (EBM) and laser beam melting (LBM). The surface conditions of these plates were characterized using x-ray micro-computed tomography. The depth and radius of surface notch-like features on the LBM and EBM plates were measured from sectional images of individual virtual slices of the rectangular plates. The stress concentration factors of individual surface notches were computed and analyzed statistically to determine the appropriate distributions for the notch depth, notch radius, and stress concentration factor. These results were correlated with the fatigue life of the Ti–6Al–4V ELI alloys from an earlier investigation. A surface notch analysis was performed to assess the debit in the fatigue strength due to the surface notches. The assessment revealed that the fatigue lives of the additively manufactured plates with rough surface topographies and notch-like features are dominated by the fatigue crack growth of large cracks for both the LBM and EBM materials. The fatigue strength reduction due to the surface notches can be as large as 60%–75%. It is concluded that for better fatigue performance, the surface notches on EBM and LBM materials need to be removed by machining and the surface roughness be improved to a surface finish of about 1 μm.

Corrosion behavior of Ti–Nb–Ta–Zr–Fe alloy for biomedical applications in Ringer's solution

The corrosion resistance of Ti–25Nb–10Ta–1Zr–0.2Fe (mass fraction, %) (TNTZF) alloy in Ringer's solution at 37 °C was investigated by potentiodynamic polarization measurement. Ti–6Al–4V ELI (Extra low interstitial) alloy was also investigated to make a comparison. The results show that TNTZF alloy has higher corrosion potential, lower corrosion current density, more stable passive current density and wider passive region compared with Ti–6Al–4V ELI alloy, which indicates that TNTZF alloy has better corrosion resistance. In addition, pitting corrosion is observed on the surface passive film of Ti–6Al–4V ELI alloy but is not found on that of TNTZF alloy. The XPS analysis results reveal that the passive film formed on TNTZF alloy is composed of Nb2O5, NbO2, Ta2O5, ZrO2, TiO and Ti2O3 oxides in the matrix of TiO2, which makes the passive film more stable and protective than that formed on Ti–6Al–4V ELI alloy and contributes much to its superior corrosion resistance.

Shock-induced mechanical response and spall fracture behavior of an extra-low interstitial grade Ti–6Al–4V alloy

The mechanical response and spall fracture behavior of an extra-low interstitial (ELI) grade Ti–6Al–4V alloy are systemically investigated during one-dimensional shock loading. The effects of oxygen content on the shock response and dynamic failure characteristic of Ti–6Al–4V are also shown through the comparison of the obtained results with those for commercial Ti–6Al–4V. The measured Hugoniot elastic limit (HEL) of Ti–6Al–4V ELI is lower than that of commercial Ti–6Al–4V. While the fitted shock parameters and the measured Hugoniot in the stress-particle velocity space of Ti–6Al–4V ELI are found to be almost identical to those of commercial Ti–6Al–4V. These results indicate that the oxygen content can significantly affect the HEL of Ti–6Al–4V, but has little or no influence on the shock response of this alloy beyond the HEL. The postshock Ti–6Al–4V ELI does not display shock-induced strengthening during quasistatic and dynamic compression tests. Transmission electron microscopy (TEM) analyses reveal that the lack of high density dislocations or dislocation cells limits the shock-induced strengthening effect, although dislocation multiplication and tangles lead to increased yield strength and strain hardening rate of the reloaded material. Finally, Ti–6Al–4V ELI is demonstrated to spall in a ductile manner, and has similar spall strengths to those of commercial Ti–6Al–4V under different shock loading conditions. The oxygen content exerts no effect on the spall fracture manner of Ti–6Al–4V, although reducing the oxygen content enables this alloy to endure more micro-damages.

Influence of boron and oxygen on the microstructure and mechanical properties of high-strength Ti66Nb13Cu8Ni6.8Al6.2 alloys

The influence of boron additions and different oxygen contamination levels on the microstructure and the mechanical properties in the Ti66−xNb13Cu8Ni6.8Al6.2Bx (0⩽x⩽1) system were investigated. The alloys were prepared by levitation copper mould casting as rods with a diameter of 5mm using different grades of starting elements. The alloy without boron exhibits a maximum compressive stress of more than 2500MPa, associated with a compressive strain of more than 30%. The ultimate tensile stress is ∼1075MPa with a maximum elongation of 1.6%. Increased oxygen content leads to a rise of yield strength due to solid solution hardening. Boron additions promote grain refinement and reinforce the interdendritic phase compound by forming needle-like TiB precipitates. This change in microstructure increases the yield stress and the Young’s modulus and lowers the plastic strain. The microstructure was analysed in terms of the boron content by means of scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy. The presented mechanical properties are compared with the compression and tensile properties of the commercially available Ti6Al4V ELI (ELI=extra low interstitial) alloy.

Evaluation of titanium alloy fabricated using electron beam melting system for dental applications

In recently advanced rapid prototyping and manufacturing methods, one additional process is to use an electron beam to fabricate metal objects by the layer by layer sintering and/or melting metal powder. This method is often called electron beam melting (EBM). This study examined the mechanical properties, the grindability and corrosion resistance of Ti–6Al–4V ELI (extra low interstitial) specimens which were fabricated by the electron beam melting (EBM) process. Dumbbell-shaped specimens and two kinds of plate specimens were prepared using the Ti–6Al–4V ELI powder in the EBM system. The yield strength, tensile strength, modulus of elasticity and percent elongation at a crosshead speed of 0.25 mm/min were tested. The Vickers hardness in interior structures was determined. Grindability was evaluated as volume loss (mm 3) when the specimen was abraded using a SiC wheel at 1250 m/min for 1 min. Lastly, corrosion behavior was examined using the dynamic potentiostatic polarization technique in an artificial saliva at 37 °C. As controls, cast and commercial wrought alloys of Ti–6Al–4V ELI and commercially pure titanium (CP Ti) were evaluated. Cast specimens were prepared in a centrifugal casting machine using a MgO based mold. For the cast specimens, all the mechanical properties, grindability and corrosion characteristics were tested. On the other hand, for wrought specimens, only grindability and corrosion properties were tested. The yield and tensile strength of the as-fabricated Ti–6Al–4V ELI specimens without any additional metallurgical treatments were found to be 735 MPa and 775 MPa, respectively. The elongation was 2.3%. These values are well within many of precious and non-precious dental casting alloys.

Evaluation of corrosion behavior of surface modified Ti–6Al–4V ELI alloy in hanks solution

The passive film behavior of Ti–6Al–4V ELI (Extra Low Interstitial) alloy after a chemical treatment followed by a thermal treatment was morphologically and electrochemically characterized. The surface morphological studies were carried out using scanning electron microscopy (SEM), electron dispersive X-ray analysis (EDAX), and Fourier Transformed-Infra Red (FT-IR) analysis to investigate the nucleation and growth of apatite on the chemically and thermally treated Ti–6Al–4V ELI alloy immersed for different durations in Hanks solution. Polarization and impedance spectroscopic (EIS) measurements were carried out in Hanks solution and the electrical parameters were obtained for the passive film using a non-linear least square fitting (NLLS) method.

Effect of heat treatment on mechanical properties of Ti–6Al–4V ELI alloy

Ti–6Al–4V alloy of extra low interstitial (ELI) grade has been used in the biomedical applications because of its high strength-to-weight ratio and excellent biocompatibility. However, its relatively poor wear resistance leading to excessive wear and implant loosening requires proper surface hardening. The body implants are also subjected to a variety of loads at varying strain rates. The objective of this study was to evaluate the hardening behavior, strain rate sensitivity and fracture mechanisms with or without hardened surface layer. It was observed that the hardened surface layer had only a small effect on the strength but reduced the ductility. The yield strength (YS) and ultimate tensile strength (UTS) were higher in the water quenching and aging condition while the ductility was lower, when compared to the as-received condition and air cooling and aging condition. As the strain rate increased both YS and UTS increased and ductility decreased. The strain hardening exponent decreased with increasing strain rate. The strain rate sensitivity evaluated via both the common approach and Lindholm approach decreased as the true strain increased. Fractal dimension of fracture surfaces was observed to be associated with the heat treatment condition, ductility, fracture surface characteristics and roughness. The fractal dimension increased with increasing roughness of fracture surfaces.

Fatigue behavior of ultrafine-grained Ti–6Al–4V ‘ELI’ alloy for medical applications

In this study the cyclic deformation behavior and fatigue lives of Ti–6Al–4V ELI (extra low interstitials) alloy with conventional grain sized and ultrafine-grained (UFG) microstructure are investigated. The UFG condition was obtained by a combination of equal channel angular pressing followed by thermomechanical treatment. Fatigue experiments were carried out under strain as well as under stress control using plastic strain amplitudes ranging from 3 × 10 −4 to 5 × 10 −3 and stress amplitudes ranging from 550 to 670 MPa, respectively. Based on the results obtained, it can be concluded that the UFG microstructure results in a significant enhancement of the fatigue properties of the alloy.

A study on diffusion bonding of superplastic Ti–6Al–4V ELI grade

Abstract Ti–6Al–4V ELI (extra low interstitials) grade alloy provides improved ductility and fracture toughness comparing to grade 5 Ti–6Al–4V alloy. In order to find the optimum superplastic forming and diffusion bonding (SPF/DB) condition, a series of tensile tests was carried out at the strain rate range of 10 −4 to 10 −2 s −1 and temperature range of 1073–1223 K. The maximum elongation of 1898% was obtained at the strain rate of 10 −3 s −1 at 850 °C. It was shown that the ELI grade alloy performs better than the grade 5 alloy in terms of the optimum superplastic condition for Ti–6Al–4V. Based on this result, diffusion bonding process of superplastic Ti–6Al–4V ELI sheet metals was developed. Bonding was completed by means of inert gas pressure applied in a bonding tool at high temperature. The microstructure of the bonding area was investigated and the bonding interface was microscopically undetectable. The evidence of nucleation of new grains and migration of grain boundaries at the interface proves the diffusion bonding process is successful. It is shown that the superplastic forming and diffusion bonding of Ti–6Al–4V ELI grade is possible at the temperature lower than those of conventional Ti–6Al–4V.

Corrosion behaviour of Ti–6Al–7Nb and Ti–6Al–4V ELI alloys in the simulated body fluid solution by electrochemical impedance spectroscopy

The corrosion behaviour of Ti–6Al–7Nb and Ti–6Al–4V ELI (extra low interstitial) was investigated as a function of immersion hours in simulated body fluid (SBF) condition, utilizing potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. Polarisation experiments were conducted after 0, 120, 240 and 360 h of immersion in SBF solution. From the polarisation curves, very low current densities were obtained for Ti–6Al–7Nb alloy compared to Ti–6Al–4V ELI, indicating a formation of stable passive layer. Impedance spectra were represented in the form of Bode plots and it was fitted using a non-linear least square (NLLS) fitting procedure, in which it exhibited a two time constant system suggesting the formation of two layers. The surface morphology of the titanium alloys have been characterized by SEM and EDAX measurements.

Corrosion resistance and biocompatibility of Ti–Ta alloys for biomedical applications

Corrosion resistance, wear resistance and biocompatibility of the studied Ti–Ta alloys with Ta contents of 10, 30 and 70 mass% together with the currently used metallic biomaterials pure titanium (Ti) and Ti–6Al–4V extra low interstitial (ELI) alloy were investigated for biomedical applications. Corrosion resistance was measured by an anodic polarization test using an automatic potentiostat in 5% HCl solution at 310K. Wear resistance was evaluated using a pin-on-disk type friction wear test system with a load of 4.9N at 310K in a simulated body fluid (Ringer's solution), and biocompatibility was judged by evaluating the cyto-toxicity through MTT assay. The passive behaviors are observed for all the studied Ti–Ta alloys, and the TiO2 passive films strengthened by the more stable Ta2O5 passive films result in improved corrosion resistance of the studied Ti–Ta alloys with increasing Ta content. All the studied Ti–Ta alloys are non-cytotoxic like pure Ti. The crystal structure shows little influence on the corrosion resistance and cyto-toxicity of the studied Ti–Ta alloys. The experimental results conform the expected excellent corrosion resistance and biocompatibility of the studied Ti–Ta alloys, which are better than or similar to those of pure Ti or Ti–6Al–4V ELI alloy used as standard biomaterials, suggesting their promising potential for biomedical applications.