A comparative study of surface integrity of Ti–6Al–4V alloy machined by EDM and AECG

The electrical discharge machining (EDM) process produces the recast layer with or without cracks on the surface that requires a remedial post-treatment in the manufacture of critical or highly stressed surfaces. One of the frequently used post-treatment processes is also the abrasive electrochemical grinding (AECG) and it has been widely used in the precision machining of difficult-to-cut materials due to an enhanced surface integrity and productivity. The aim of this study is to investigate improvability of surface integrity in terms of machining voltage, electrolyte flow rate and table feed rate parameters of AECG in EDMed Ti6Al4V alloy. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrograph (EDS) and surface roughness measurement were performed to study the surface characteristics of the machined samples. Experimental results indicate that the AECG process effectively improves the surface roughness and eliminates the EDM damages completely by setting suitable grinding parameters.     

Laser treatment and PVD TiN coating of Ti–6Al–4V alloy

Laser gas assisted processing can be used to modify the surface properties of Ti–6Al–4V alloy through the use of gaseous interaction with the laser melted surface. Laser surface melting of titanium and its alloys in nitrogen to form a layer of TiN embedded in a metallic matrix which is enriched in alloying elements has attracted considerable interest. The surface roughness of the laser-treated surface is poor, therefore, a secondary processing becomes essential. In the present study, duplex treatment of Ti–6Al–4V alloy was carried out. The alloy surface was melted initially under a controlled nitrogen atmosphere, which in turn resulted in a laser-induced nitrided surface. The resulting workpiece surface, then, was PVD TiN coated. In order to assess the wear properties of the resulting surface, friction tests were carried out. SEM, XRD and microhardness were carried out for microstructural analysis and material characterization. It was found that the adhesion of the TiN coating to the base alloy improved considerably in the case of laser-treated workpieces and smooth transition in plastic shearing resistance between the TiN coating and the base alloy enhanced the wear properties of the laser-treated surface.     

High cycle fatigue behaviour of Ti–6Al–4V alloy at elevated temperatures

High cycle fatigue behaviour of Ti–6Al–4V alloy was studied at 623 K and 723 K. Fatigue strength decreased at elevated temperatures compared with at ambient temperature. In the short life regime, fatigue strength was lower at 723 K than at 623 K, but in the long life regime it was nearly the same at both temperatures. At elevated temperatures, cracks were generated earlier at applied stresses below the fatigue limit at ambient temperature, indicating lowered crack initiation resistance. Small cracks grew faster at elevated temperatures than at ambient temperature, which became more noticeable with increasing temperature. After allowing for the elastic modulus, small cracks still grew faster at elevated temperatures.     

Thermodynamic analysis of alloys Ti–Al, Ti–V, Al–V and Ti–Al–V

Thermodynamic analysis of three binary Ti-based alloys: Ti–Al, Ti–V, and Al–V, as well as ternary alloy Ti–Al–V, is shown in this paper. Thermodynamic analysis involved thermodynamic determination of activities, coefficient of activities, partial and integral values for enthalpies and Gibbs energies of mixing and excess energies at four different temperatures: 2000, 2073, 2200 and 2273 K, as well as calculated phase diagrams for the investigated binary and ternary systems. The FactSage is used for all thermodynamic calculations.     

Observations on chip formation and acoustic emission in machining Ti–6Al–4V alloy

Orthogonal cutting tests were undertaken to investigate the mechanisms of chip formation for a Ti–6Al–4V alloy and to assess the influences of such on acoustic emission (AE). Within the range of conditions employed (cutting speed, v_c=0.25–3.0 m/s, feed, f=20–100 μm), saw-tooth chips were produced. A transition from aperiodic to periodic saw-tooth chip formation occurring with increases in cutting speed and/or feed. Examination of chips formed shortly after the instant of tool engagement, where the undeformed chip thickness is slightly greater than the minimum undeformed chip thickness, revealed a continuous chip characterised by the presence of fine lamellae on its free surface. In agreement with the consensus that shear localisation in machining Ti and its alloys is due to the occurrence of a thermo-plastic instability, the underside of saw-tooth segments formed at relatively high cutting speeds, exhibiting evidence of ductile fracture. Chips formed at lower cutting speeds suggest that cleavage is the mechanism of catastrophic failure, at least within the upper region of the primary shear zone. An additional characteristic of machining Ti–6Al–4V alloy at high cutting speeds is the occurrence of welding between the chip and the tool. Fracture of such welds appears to be the dominant source of AE. The results are discussed with reference to the machining of hardened steels, another class of materials from which saw-tooth chips are produced.     

Production of submicrocrystalline structure in large-scale Ti–6Al–4V billet by warm severe deformation processing

The “abc” deformation method for production of large-scale billets with submicrocrystalline structure was developed. A large billet of Ti–6Al–4V alloy (150-mm diameter × 200-mm length) with a homogeneous submicrocrystalline structure was produced. The refined structure with a grain/subgrain size of about 0.4 μm leads to a substantial mechanical properties improvement.     

Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy

The paper discusses processing and property aspects of oxide films formed on a Ti–6Al–4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations. Structure, composition, mechanical tribological and corrosion resistant characteristics of the films formed are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing. It is found that the films produced from the aluminate–phosphate electrolyte are dense and uniform and are composed mainly of Al₂TiO₅ and TiO₂ phases of the rutile form. The films possess a beneficial combination of 50–60 μm thickness, 575 kg/mm² hardness and high adhesion and provide a low wear rate (3.4×10⁻⁸ mm³/Nm) but a relatively high friction coefficient of μ=0.6–0.7 against steel, caused by material transfer from the counterface. A minimum friction coefficient of μ=0.18 is recorded during the testing of softer rutile–anatase films, 7 μm thick, produced from a phosphate electrolyte. Both of these types of film show good corrosion resistance in NaCl and physiological solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate. SiO₂/TiO₂-based films with 70–90 μm thickness and high bulk porosity produced from silicate and silicate–aluminate electrolytes demonstrate better corrosion behaviour in H₂SO₄ solution, due to the greater chemical stability of the film phase components in this environment.     

Friction and cutting forces in cryogenic machining of Ti–6Al–4V

Conventional cutting fluid serves both as a coolant and lubricant. In cryogenic machining, liquid nitrogen (LN2) is recognized as an effective coolant due to its low temperature; however, its lubrication properties are not well known. The focus of this study was to investigate how the friction between the chip and the tool is affected by focused jetting LN2 to the cutting point in machining Ti–6Al–4V. Results of cutting force measurements indicated that the cold strengthening of titanium material increased the cutting force in cryogenic machining, but lower friction reduced the feed force. A mathematical model was developed to convert the measured 3D forces in oblique cutting into the normal and frictional force components on the tool rake face, and then to calculate the effective friction coefficient. It was found that the friction coefficient on the tool–chip interface was considerably reduced in cryogenic machining. Increased shear angle and decreased thickness of the secondary deformation zone, findings from a chip microstructure study, offer further evidence that friction is reduced.     

Oxidation behavior of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation

Compound ceramic coatings prepared on Ti–6Al–4V alloy by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO₂ solution were oxidized under different temperature in air. The phase composition and surface morphology of the coatings before and after oxidation were investigated by X-ray diffractometry and scanning electron microscopy, respectively. Meantime, the weight gains and the high temperature oxidation characteristics of the coated samples were investigated. The results show that the coatings prepared by MPO were composed of a large amount of Al₂TiO₅ and a little -Al₂O₃ and rutile TiO₂. And the oxidation process of the coated samples included the decomposition of the Al₂TiO₅ in the coating, the oxidation of the substrate and the changes of the coating structure. After high temperature oxidation, the increase of -Al₂O₃ in the coating was due to the decomposition of Al₂TiO₅, whereas the increase of rutile TiO₂ in the coating was attributable to both the decomposition of Al₂TiO₅ and the oxidation of the Ti substrate. The main crystalline of the coatings became rutile TiO₂ after the oxidation of 1000 °C for 1 h. The decomposition of Al₂TiO₅ in the coating occurred at 900 and 1000 °C, and its half decomposition time was less than 1 h at 1000 °C. Increasing oxidation temperature or extending oxidation time, the weight gains of coated samples was increased to different extent. However, the weigh gains of the coated samples was much lower than that of the substrate, so the ceramic coatings improved the oxidation resistance of Ti alloy greatly under the experimental conditions.     

Corrosion behaviour of laser gas nitrided Ti–6Al–4V in HCl solution

Laser surface nitriding of Ti–6Al–4V alloy was carried out with a Nd:YAG pulsed laser. The microstructure and corrosion behaviour of the nitrided samples were examined, using SEM, XRD, XPS, and anodic polarization tests in 2 M HCl solution. Laser nitriding produced a thin continuous TiN layer followed by TiN dendrites and TiN_0.3 needles. The laser nitrided specimen exhibited less corrosion current density, passivated more readily and also, maintained a lower current density over the duration of the experiment. This was correlated with the formation of very thin, continuous TiN_xO_y film, which could retard chloride ions ingress into the substrate.     

Corrosion inhibition of Ti‐6Al‐4V alloy in sulfuric and hydrochloric acid solutions using inorganic passivators

The effect of iodate (IO₃^?), metavanadata (VO₃^?) and molybdate (MoO₄^2?) anions on the passivation of Ti‐6Al‐4V alloy in sulfuric and hydrochloric acid solutions was studied using open‐circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS). The alloy surface was examined by scanning electron microscopy (SEM). It was found that an optimum concentration of the passivator is essential for the corrosion inhibition process. Above this concentration the rate of alloy corrosion decreases as the concentration of the passivating ion increases. Scanning electron micrographs have shown that the flawed regions present in the alloy surface were repaired in the presence of the passivator anion. The optimum concentration of each anion and its corrosion inhibition efficiency for titanium and Ti‐6Al‐4V alloy have been determined. It turned out that the corrosion inhibition efficiencies of IO₃^?, VO₃^? and MoO₄^2? anions for the corrosion of Ti and Ti‐6Al‐4V in both hydrochloric and sulphuric acid solutions exceed 98%.     

Deposition of Ti–6Al–4V using a high power diode laser and wire, Part I: Investigation on the process characteristics

In this paper the deposition of Ti–6Al–4V wire with High Power Diode Laser was investigated by producing single tracks. The effect of the wire feeding direction and angle was firstly studied. The influence of laser power, traverse speed and wire feed rate on the weight and dimension of the deposited single tracks was then investigated. The microstructure and hardness of the single tracks were examined. Deposition with diode laser and wire was proved to provide a high deposition rate with good quality. Columnar grains were found in the deposits. Wire feeding orientation, laser power, traverse speed and wire feed rate were verified as factors which influenced the quality of the deposit. With similar energy level, different power/traverse speed produced deposits with different hardness value. Hardness values tended to increase from the deposit, via the re-melted zone till to the heat affected zone, and then decrease again when the measurements were taken in the unaffected base material.     

X-ray line-broadening investigation of deformation during hot rolling of Ti–6Al–4V with a colony-alpha microstructure

X-ray line-broadening techniques that were previously developed and applied to quantify deformation behavior during the hot rolling of commercial-purity titanium were applied to Ti–6Al–4V plate with a colony-alpha preform microstructure. The work quantified the challenges in using line-broadening techniques for two-phase titanium alloys which undergo a phase transformation during cooling following hot working.     

Influence of sintering on the corrosion behavior of a Ti‐6Al‐4V alloy

This work studies the influence of the sintering conditions of a Ti‐6Al‐4V alloy on its corrosion performance. The alloy was vacuum sintered in different conditions of time and temperature. The density and microstructure (designating phase distribution) are evaluated. Corrosion resistance through electrochemical techniques (EIS) in 2 N and 6 N hydrochloric acid solutions, and oxidation resistance at 900, 1000 and 1100°C are appraised, and corrosion is studied by microstructural and X‐ray diffraction.     

A new material model for 2D numerical simulation of serrated chip formation when machining titanium alloy Ti–6Al–4V

A new material constitutive law is implemented in a 2D finite element model to analyse the chip formation and shear localisation when machining titanium alloys. The numerical simulations use a commercial finite element software (FORGE 2005^®) able to solve complex thermo-mechanical problems. One of the main machining characteristics of titanium alloys is to produce segmented chips for a wide range of cutting speeds and feeds. The present study assumes that the chip segmentation is only induced by adiabatic shear banding, without material failure in the primary shear zone. The new developed model takes into account the influence of strain, strain rate and temperature on the flow stress and also introduces a strain softening effect. The tool chip friction is managed by a combined Coulomb–Tresca friction law. The influence of two different strain softening levels and machining parameters on the cutting forces and chip morphology has been studied. Chip morphology, cutting and feed forces predicted by numerical simulations are compared with experimental results.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.     

A study of the microstructure and hardness of two titanium alloys: Commercially pure and Ti–6Al–4V

In this paper, the microstructure and hardness of two titanium alloys was determined and the results are presented and briefly discussed. Samples of the alloy for microstructural examination were prepared from the as-provided stock using standard metallographic procedures and then examined in a low magnification light optical microscope. Both microhardness and macrohardness measurements were made across the polished surfaces of the two titanium materials. Both the microhardness and macrohardness of the Ti–6Al–4V alloy was noticeably higher than the commercially pure counterpart. The intrinsic influence of alloy composition and secondary processing, i.e., annealing, on microstructural development is presented and hardness of the two alloys is highlighted. The role of microstructure in governing the hardness of the two titanium materials is discussed.