Extra-low Interstitial Grade Research Articles

Carbon emissions, techno-economic and machinability assessments to achieve sustainability in drilling Ti6Al4V ELI for medical industry applications

In the last decade, cryogenic machining has made a significant impact as a sustainable machining technique, demonstrating machinability improvements in terms of tool life, surface quality of machine parts, low environmental effects and reduced cost of overall machining operation. A near-to-dry machining technique, Electrostatic Minimum Quantity Lubrication (EMQL) has also emerged as a sustainable machining technique, which is an environmentally-friendly alternative to the flood machining process. Although these coolant/lubrication strategies have been studied extensively, there is no direct comparison of both these machining techniques in the aspects of ecology, economy and machining performances. In this novel study, cryogenic liquid carbon dioxide (LCO2) and EMQL techniques are used for high-speed drilling of Ti6Al4V ELI (extra low interstitial) grade, and the sustainability and machinability assessment of these techniques are evaluated which can help medical implant manufacturing industries to implement these techniques as an economic and sustainable replacement of the conventional cooling/lubrication techniques. Ti6Al4V ELI is widely used in the medical industry for Hip and Knee joint endoprosthesis, bone screws, collar bone and dental implants. The sustainability aspects were evaluated in terms of carbon emission and cost analysis of the processes. It was found that the EMQL cutting condition exhibited 27.69% higher carbon emission and 67.60% higher cost of machining operation compared to cryogenic LCO2 cutting condition. Moreover, the tool life experienced when machining using cryogenic LCO2 cutting condition was at least 4 times higher than tool life found when using the EMQL cutting condition. The surface roughness of the workpiece material was higher (56.40%) when using the EMQL coolant. These results were attributed to adhesion wear and chip clogging phenomena which were found to be the main factors for reduced tool life and higher surface roughness in EMQL cutting condition. Therefore, from this study, it can be concluded that cryogenic LCO2 coolant performs significantly better compared to EMQL coolant, providing improved machinability of the titanium workpiece and superior sustainability of the cutting operation.

Electrochemical characteristics of Sr/Si-doped hydroxyapatite coating on the Ti alloy surface via plasma electrolytic oxidation

The purpose of this study is to investigate the electrochemical characteristics of Sr/Si-doped hydroxyapatite coating on the Ti-6Al-4V alloy surface via plasma electrolytic oxidation(PEO).Ti-6Al-4V alloy of extra low interstitial grade disk was used as the anode and the carbon rod was used as the cathode. PEO treatment was carried out using pulsed DC power at 280 V for 3 min. The electrolyte used for PEO was prepared by mixing Ca(CH3COO)2•H2O, C3H7NaCaO6P, Sr(CH3COO)2•0.5H2O, and Na2SiO3•9H2O. The potentiodynamic polarization test were performed by potentiostat in a 0.9% NaCl solution at 36.5 ± 1°C. Electrochemical impedance tests were carried out using potentiostat at the open circuit potential in a 0.9% NaCl solution at 36.5 ± 1°C. The PEO-treated surface and corrosion morphology were investigated by field mission scanning microscopy and energy dispersive X-ray spectroscopy.The number of pores increase and average size of the pores decreased as Sr increase, and area occupied by pore was decreased. In the case of Si and Sr addition to electrolyte, corrosion potential was apparently increased compared to specimen of Sr addition to electrolyte. Polarization resistances of PEO-treated Sr and Sr/Si specimens showed the higher than those of Bulk and CaP specimens from data of electrochemical impedance test.

Reuse of Grade 23 Ti6Al4V Powder during the Laser-Based Powder Bed Fusion Process

Titanium alloy powder used for laser-based powder bed fusion (L-PBF) process is costly. One of the solutions is the inclusion of a powder recycling strategy, allowing unused or exposed powder particles to be recuperated post manufacture, replenished and used for future builds. However, during a L-PBF process, powder particles are exposed to high levels of concentrated energy from the laser. Particularly those in close proximity to the melt pool, leading to the formation of spatter and agglomerated particles. These particles can settle onto the powder bed, which can then influence the particle size distribution and layer uniformity. This study analysed extra-low interstitial (ELI) Ti6Al4V (Grade 23) powder when subjected to nine recycle iterations, tracking powder property variation across the successive recycling stages. Characterisation included chemical composition focusing upon O, N, and H content, particle size distribution, morphology and tapped and bulk densities. On review of the compositional analysis, the oxygen content exceeded the 0.13% limit for the ELI grade after 8 recycles, resulting in the degradation from Grade 23 level.

Submicrocristalline Structure and Dynamic Recovery of Cold Flowformed ELI Grade Ti-6Al-4V

Flowforming is a means to produce seamless tubes by plastic deformation at room temperature. It consists in reducing the thickness of a tubular part mounted on a mandrel by deforming it using several rollers translating along the tube axis, while the tube is rotating along its axis. Thanks to the high compressive stresses, and to the incremental nature of the deformation process, flowforming can lead to a high thickness reduction and thus to high elongation of the deformed tubes. Ti-6Al-4V (Extra Low Interstitial grade) tubes have been deformed by cold flowforming, with a thickness reduction ratio higher than 60%, and their microstructures have been investigated using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Based on EBSD data, a post-processing analysis has been performed in order to study the texture of the flowformed parts. Optical Microscopy showed that the material could be deformed without displaying flow instability such as adiabatic shear banding, despite the fact that it has been processes out of the stable processing maps (high strain rate and low temperature). It also evidenced a major deformation along the tube axis accompanied with a slight twist due to torsion stress. EBSD analysis indicated the occurrence of continuous dynamic recrystallization, which is rarely reported in the α-β domain of such alloys. The recovery/ recrystallization effects resulted in a submicrocrystalline equiaxed structure, which is consistent with that previously reported for Ti-6Al-4V subjected to severe plastic deformation (SPD). The texture of the hexagonal α-phase appeared to be similar to that obtained on extruded Ti-6Al-4V, with a basal component perpendicular to the tube axis.

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.

Constitutive analysis of compressive deformation behavior of ELI-grade Ti–6Al–4V with different microstructures

In this study, a constitutive analysis of the flow responses of Ti–6Al–4V under various strain rates \( \dot{\varepsilon } \) was conducted by separately quantifying the hardening and softening effects of microstructure, interstitial solute and deformation heating on the total stress. For this purpose, a series of compression tests on an extra-low interstitial grade alloy with equiaxed, lamellar, or bimodal microstructures was performed at \( 10^{ - 3} \le \dot{\varepsilon } \le 10\;{\text{s}}^{ - 1} \) until the metal fractured, and the results were compared to those of the commercial grade alloy. The thermal stress σ* increased with an increasing interstitial solute concentration; the athermal stress increased in the order of equiaxed, lamellar, and bimodal microstructures. Load–unload–reload tests revealed that the flow softening at a relatively high \( \dot{\varepsilon } \) was likely caused by deformation heating rather than by microstructure change; thus flow softening was attributed to a decrease in σ*. Finally, a mechanical threshold stress model was extended to capture those observations; the modified model can provide a reasonable prediction of flow stress in Ti–6Al–4V with different microstructures and interstitial solute concentrations.

Mechanical behaviour and superplastic forming capabilities of extra-low interstitial grade Ti-6Al-4V wire alloy with numerical verification

In this paper, the behaviour of extra-low interstitial (ELI) grade Ti-6Al-4V wire alloy has been extensively studied at varying strain rates in the range of 10-5-102 s-1 at temperatures ranging between 750 °C and 1050 °C using processing maps and experimental data to determine the material's mechanical and superplastic forming capabilities. From the slope of a log plot of strain rate versus flow stress, the strain-rate sensitivity index, m, was determined. A plot of m versus the log of strain rate was produced in order to identify the key strain rates in which ELI grade Ti-6Al-4V exhibits its superplastic regime. It was determined that this alloy is most sensitive to superplastic forming within a narrow strain-rate band of 10-4-10-3 s-1 and has a maximum strain-rate sensitivity index, m = 0.45. At strain rates below and above this range, the material exhibited negligible levels of superplasticity. The key temperature for superplastic forming was determined to be 950 °C. The effect of temperature on flow stress and flow stability was analysed using the Zener-Holloman parameter. The experimentally determined results showed excellent agreement with Guo and Ridley's activation mathematical model.

Effect of welding on microstructure and tensile properties of flowformed Ti-6Al-4V tubes

In this study, extra-low interstitial grade Ti-6Al-4V tubes manufactured by the flowforming process were joined by plasma arc welding. Welded tubes were subjected to a stress-relief heat treatment prior to the removal of specimens for microstructural examination, and tensile and fatigue testing. Welded tubes exhibited tensile and yield strength values of 892 and 790 MPa, respectively, and elongation and reduction of area values of 7 and 30%, respectively, compared to tensile and yield strength of 1217 and 1149 MPa, respectively, and elongation and reduction of area values of 13 and 41%, respectively, of base metal. The fatigue strength of welded tubes was also 10–20% lower than that of the base metal. The significant difference in mechanical properties was attributed to microstructural changes in flowformed tubes upon welding.

Strain-Induced Porosity during Cogging of Extra-Low Interstitial Grade Ti-6Al-4V

The phenomenon of strain-induced porosity (SIP) in extra-low interstitial (ELI) grade Ti-6Al-4V with a transformed β starting microstructure is investigated to understand its origin during α-β cogging. For this purpose, the constitutive behavior of the material is coupled with finite-element method (FEM) simulations of the cogging process. Two distinct types of SIP relevant to cogging speeds and temperatures, viz., shear cracking and void nucleation, are identified. While the former occurs at the prior β grain boundaries below 825 °C, the latter occurs at the prior colony boundaries when the deformation temperature is close to the β transus. The FEM simulations have shown that deformation conditions existing in the midregion of the billet are favorable for void nucleation. The mechanism of void growth in the presence of tensile residual stress and temperature during resoaking is modeled using the Cocks-Ashby coupled growth model. Repeated cogging and resoaking steps cause multiplication of void population in large numbers. To avoid both types of defects in any region of the billet, a practical solution has been developed by introducing a differential temperature profile from the surface to the center, and the validity of the proposed scheme is verified with FEM heat-transfer simulations.

A Study of Beta Processing of Ti-6Al-4V: Is it Trivial?

Deformation of Ti-6Al-4V alloy in the β phase field is generally considered to be trivial since the material is highly workable at these temperatures and does not normally pose problems during processing. In view of this, studies on the hot deformation behavior of β are scanty compared to those on the α−β deformation. This paper is focussed on understanding the β deformation characteristics in Ti-6Al-4V with a view to examine whether such studies help in optimizing the process design and achieving microstructural control. The emphasis has been on the two industrial grades, viz. commercial purity (CP) versus extra-low interstitial (ELI), and also on the effect of starting microstructure (transformed β versus equiaxed α+β). The stress-strain curves obtained in compression exhibited steady-state behavior at strain rates lower than 1 s−1 and oscillatory/softening behavior at higher strain rates. Kinetic analysis of the flow stress data obtained at different temperatures and strain rates has shown that the stress exponent is about 3.6-3.8 and the apparent activation energy is in the range 150-287 kJ/mole, which is comparable to that of self-diffusion in β phase (150 kJ/mol). Dynamic recrystallization (DRX) of β is identified as the microstructural mechanism in all the cases except in ELI grade with transformed β starting structure. The prior β grain size in the DRX region exhibits a good correlation with the Zener-Hollomon parameter. In case of ELI grade, a mechanism of large grained superplasticity involving sliding of prior colony boundaries has been identified. However, deformation close to the transus in ELI grade causes nucleation of voids which may grow during soaking at the deformation temperature under the influence of tensile residual stress. The results clearly show that a study of β deformation mechanisms holds the key during hot working of Ti-6-4. More importantly, the processing schedule used for CP grade Ti-6-4 should not be used for ELI grade to achieve microstructural control and avoid defects.

Influence of oxygen content on the forging response of equiaxed (α+β) preform of Ti–6Al–4V: commercial vs. ELI grade

The hot deformation characteristics of equiaxed (α+β) preform of Ti–6Al–4V have been evaluated in two oxygen grades viz., commercial grade and extra-low interstitial (ELI) grade. Constant strain rate hot compression tests were conducted on cylindrical specimens in the temperature range 750–1100°C and strain rate range 10 −3–100 s −1 up to a true strain of about 0.5. The shapes of stress–strain curves in the α–β region and β-region, the kinetic parameters, and the processing maps obtained on the two grades have been compared with a view to evaluate the effect of oxygen on the processing of Ti–6Al–4V with equiaxed (α+β) preform. The shapes of the stress–strain curves are similar in the two oxygen grades although the flow stress of the commercial grade is slightly higher than that of the ELI grade. While the values of the stress exponent are similar in both grades, the apparent activation energy for hot deformation of commercial grade is higher than that of the ELI grade in both α–β and β ranges. The processing maps revealed that superplasticity occurs in the α–β range while dynamic recrystallization (DRX) occurs in the β range. In the commercial grade, the superplasticity domain is spread over a wider temperature range and occurs at lower strain rates than that in the ELI grade. On the other hand, the ductility peak in the ELI grade is narrow and may require closer temperature control during processing. The DRX of β occurs at 1100°C in both grades but at a higher strain rate in the commercial grade than in the ELI grade. Unlike the commercial grade, deformation of the ELI grade close to the β transus has to be avoided since a possibility of void nucleation or wedge crack formation exists at slow strain rates. Both grades exhibit flow instabilities manifested as adiabatic shear bands and flow localization when deformed at strain rates higher than about 1 s −1.

Hot deformation mechanisms in Ti–6Al–4V with transformed β starting microstructure: commercial v. extra low interstitial grade

The hot deformation behaviour of commercial and extra low interstitial (ELI)grades of Ti–6Al–4V (Ti-6-4) alloy with a transformed β starting microstructure has been studied in the temperature range 750–1100°C and strain rate range 0.001–10 s-1. On the basis of the flow stress data as a function of temperature and strain rate, processing maps have been developed for these two grades and compared in order to bring out the differences, if any. While the stress–strain behaviour has not varied appreciably with the grade of Ti-6-4, significant differences have been observed in the processing maps as well as the tensile ductility variation with temperature. At lower strain rates in the α–β range (<0.01 s-1), both the grades exhibit globularisation of the lamellar structure, the optimum temperature being higher for the commercial grade than the ELI grade. The apparent activation energy for globularisation is higher in the commercial grade (455 kJ mol-1)than that of the ELI grade (370 kJ mol-1). At temperatures lower than about 900°C and strain rates less than about 0.1 s-1, a regime of strain induced porosity (SIP)at the prior β grain boundaries has been observed and the SIP regime is narrower in the ELI grade than the commercial grade. Strain induced porosity cracks are nucleated as a result of the stress concentrations produced by the sliding of prior β grain boundaries which is promoted by the lower strain rates. The mechanism of hot deformation in the β range is sensitive to the grade of Ti-6-4. In the ELI grade, the β phase deforms by large grained superplasticity, but deformation close to the transus nucleates voids within the prior β grains resulting in a drop in the tensile ductility. On the other hand, the commercial grade exhibits dynamic recrystallisation of β phase. The apparent activation energy for β deformation is lower in the commercial grade (173 kJ mol-1) than the ELI grade (287 kJ mol-1), although both the values are comparable to that for self­diffusion in β. The flow instability regime, as predicted by the continuum instability criterion, is not significantly different in the two grades of Ti-6-4 even though the domain of cracking along the adiabatic shear bands is wider in the commercial grade than the ELI grade.

Effect of preform microstructure on the hot working mechanisms in ELI grade Ti–6Al–4V: transformed β v. equiaxed (α + β)

Processing of Ti–6Al–4V includes hot working above and below the β transus and the various stages of manufacture involve preforms with either transformed β (Widmanstätten colony type) or equiaxed (α + β ) microstructure. For achieving defect free products during manufacture, it is important to understand the response of these two preform microstructures to the imposed hot working conditions. In this paper, the influence of the preform microstructure on the hot working mechanisms of extra low interstitial (ELI) grade Ti–6Al–4V has been studied with the help of hot compression experiments conducted in the temperature range 750–1100°C and strain rate range 0·001–100 s-1. The data have been analysed using the standard kinetic approach as well as the more recent approach of processing maps. In the α–β range, the stress–strain behaviour of transformed β preform is marked by a higher flow stress and a continuous flow softening while the equiaxed (α +β ) preform exhibits steady state flow at lower strain rates. By deforming in the α–β range, the transformed β microstructure is converted into an equiaxed one by a process of globularisation. On the other hand, the equiaxed (α + β ) preform deforms superplastically with an associated minor change in its microstructure. In the β range, the transformed β deforms by a process of large grained superplasticity involving the sliding of prior colony (Widmanstätten) boundaries with an associated diffusion accommodated flow. However, dynamic recrystallisation of β occurs in the equiaxed preform. Deformation near the transus for both the preforms is associated with local minima in the tensile ductility indicating the possibility of void nucleation. At strain rates higher than about 0·1 s-1, both the preforms exhibit flow instabilities manifested in the form of flow localisation due to adiabatic shear band formation which is severe in the case of transformed β preform.

Comparative study on impact toughness of gas tungsten arc and electron beam weld metals of Ti-6Al-4V

As welded metal toughness of gas tungsten arc welds has been found to be higher than that of their electron beam welded counterparts. In general, air cooling from 930°C for 1 h resulted in the best possible toughness for both base and weld metals. Prolonged exposure at 930°C or slow cooling from 930°C decreased the toughness values of weld metals. Heat treatment above the β transus resulted in poor toughness values. After heat treatment, the toughness values of gas tungsten arc weld metals were either equal to or better than their electron beam weld counterparts. The effect of prior heat input was found to be nullified after post-weld heat treatment above the β transus. Commercial grade weld metals showed relatively poor toughness values compared with their extra low interstitial grade counterparts.

Comparative study on impact toughness of gas tungsten arc and electron beam weld metals of Ti-6Al-4V

As welded metal toughness of gas tungsten arc welds has been found to be higher than that of their electron beam welded counterparts. In general, air cooling from 930°C for 1 h resulted in the best possible toughness for both base and weld metals. Prolonged exposure at 930°C or slow cooling from 930°C decreased the toughness values of weld metals. Heat treatment above the β transus resulted in poor toughness values. After heat treatment, the toughness values of gas tungsten arc weld metals were either equal to or better than their electron beam weld counterparts. The effect of prior heat input was found to be nullified after post-weld heat treatment above the β transus. Commercial grade weld metals showed relatively poor toughness values compared with their extra low interstitial grade counterparts.