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.     

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.     

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.     

Influence of heating and solidification conditions on the structure and surface quality of electron-beam melted Ti–6Al–4V ingots

Electron-beam cold-hearth melting is an emerging process used to eliminate high- and low-density inclusions during melt processing and to reduce the number of remelting steps for high quality titanium alloys. In the present work, the effect of ingot heating conditions on the evolution of ingot macrostructure and surface quality during solidification following electron-beam melting of Ti–6Al–4V was established via prototype production trials. Macrostructure observations correlated well with temperature gradients and solidification rates estimated from solidification calculations. These calculations also provided insight into the effect of melting conditions on ingot surface quality and hence associated product yield.     

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.     

Production of Al–Ti–B master alloys from Ti sponge and KBF4

It is of great interest to replace the K₂TiF₆ salt so as to reduce the volume of fluoride-bearing particulate material to be added to the aluminium melt in the popular “halide salt” process. Ti sponge was used in the present work as the source of Ti in the production of an Al–5Ti–1B grain refiner. Addition of Ti granules into molten aluminium, either premixed with or before KBF₄ salt, has produced Al–5Ti–1B alloys where the boride particles were relatively few and predominantly of the AlB₂ type. The grain refining efficiency of these alloys were far from satisfactory. TiB₂ was the dominant boride phase with sufficient number of blocky aluminide particles when Ti, in excess of the TiB₂ stoichiometry was supplied before hand and the balance was reserved for co-addition with KBF₄. Al₃Ti particles were generated soon after the Ti solubility limit was exceeded in the first step while the boride particles were subsequently produced by the reaction between molten aluminium, KBF₄ and K₂TiF₆. The Al–5Ti–1B master alloy thus produced provided an adequate grain refining performance while the amount of particulate material to be added to the aluminium melt was reduced by nearly 30%.     

Fatigue crack initiation and multiplication of unnotched titanium matrix composites

Fatigue crack initiation and multiplication of the unnotched SCS-6 silicon carbide fiber-reinforced titanium matrix composites with different matrix and interfacial properties have been investigated experimentally and analytically. Ti–15V–3Al, Ti–6Al–4V, and Ti–22Al–23Nb were chosen as matrix materials. The initiation and propagation of each individual matrix crack as a function of fatigue cycles and applied stress levels were carefully monitored. The statistical distribution of crack growth rates in each composite has been constructed and analyzed. The evolution of normalized matrix crack density and stiffness reduction of these composites under fatigue loading also has been characterized. A modified shear-lag model, coupled with the strain-life equation and a fiber bridging model were used to predict the fatigue crack initiation life, matrix crack growth rate, normalized matrix crack density, and residual stiffness of the composites. The predicted fatigue properties correlated well with experimental results.     

Hydrogen evolution from cathodically charged titanium aluminide alloy Ti–24Al–11Nb

A Ti₃Al-based titanium aluminide alloy, Ti–24Al–11Nb, was cathodically charged with hydrogen in a 5% H₂SO₄ aqueous solution for various charging times, and the formation and dissociation of the hydride, the hydrogen evolution behavior and the total hydrogen uptake were investigated mainly by means of X-ray diffractometry and thermal desorption spectroscopy (TDS). The same kind of hydride phase as observed previously in Ti–25Al alloy (hexagonal hydride) was presumably formed in the Ti–24Al–11Nb alloy after cathodic charging. No damage, such as cracks, was induced by hydrogen charging. Two kinds of TDS peaks, one probably corresponding to hydride dissociation and the other to hydrogen dissolution in the normal lattice site, were found after longer hydrogen charging. It is suggested that niobium addition to Ti₃Al-based titanium aluminide alloy may reduce hydrogen susceptibility during cathodic charging.     

Production of Al–Ti–B grain refining master alloys from Na2B4O7 and K2TiF6

It is very desirable to replace the KBF₄ salt in the popular “halide salt” process to reduce the volume of fluoride salts to be added to molten aluminium in the production of Al–Ti–B grain refiners. Being over 2 times richer in B, Na₂B₄O₇ is a promising replacement for KBF₄, and is used in the present work to produce Al–Ti–B grain refiner master alloys. A fraction of the aluminide particles were entrapped in the spent salt giving a relatively lower Ti recovery when KBF₄ was replaced by Na₂B₄O₇. The grain refining performance of the Al–Ti–B grain refiner alloy thus produced was nevertheless acceptable. The spent salt became too viscous with the oxides, aluminides and borides to be removed by decanting when Na₂B₄O₇·5H₂O was used to supply boron. The viscous spent salt, entrained in the grain refiner alloy, did not only impair its performance, but also hurt the fluidity of the molten alloy and made pouring difficult.     

The performance of Al–Ti–C grain refiners in twin-roll casting of aluminium foilstock

The master alloys based on the Al–Ti–B system have been used extensively for refining the grain structure of aluminum alloys. The quality-related problems linked with the TiB₂ particles, however, have generated an interest in the Al–Ti–C grain refiners as an acceptable replacement for Al–Ti–B master alloys. TiC particles are smaller than the TiB₂ particles and are less prone to agglomeration. Al–3Ti–0.15C grain refiners have been in use for some time in several alloy systems. Much of the work reported on this alloy, however, has been from DC casting while performance data in strip casting is not available. In the present work, a commercial Al–3Ti–0.15C grain refiner was employed in the twin-roll casting of AA8111 foilstock. Its grain refining efficiency was compared with that of the Al–5Ti–0.2B master alloy, the standard grain refiner in aluminium industry for the manufacture of aluminium foil products.     

An improved practice to manufacture Al–Ti–B master alloys by reacting halide salts with molten aluminium

The well-established “halide salt” route was employed in the present work to produce Al–Ti–B grain refiner alloys with consistent, good properties. The holding step in the production cycle was revised, however, to avoid oxidation of the molten alloy which is believed to be responsible for the relatively low Ti recoveries and thus for the inadequate and inconsistent grain refining efficiency. Stirring during holding was found to degrade the grain refining properties when molten potassium aluminium fluride salt was left on the molten alloy to avoid excessive oxidation. Likewise, holding temperatures higher than 800 °C and holding times longer than 30 min both had an undesirable effect on the grain refining performance. The experimental Al–5Ti–1B grain refiner alloy produced according to the present method provided consistent and better overall grain refining performance.     

Creep behavior of Ti–6Al–2Sn–4Zr–2Mo: I. The effect of nickel on creep deformation and microstructure

The effect of trace levels of Ni on the intermediate temperature creep behavior of the alloy Ti–6Al–2Sn–4Zr–2Mo (wt%) has been investigated. Creep experiments were performed in tension over the temperature range 510–565 °C at stress range 138–413 MPa. Two heats of commercial grade Ti–6Al–2Sn–4Zr–2Mo with Ni levels of 0.006 and 0.035 wt% were studied. The high Ni material uniformly exhibited higher primary creep strains and minimum strain rates than the lower Ni material. Stress exponents in the range 5–7 and 4–6 were obtained for the high Ni and low Ni material respectively. At 565 °C a transition to a low stress region with a stress exponent equal 1 is found for both materials. At all stress levels, the apparent activation energy was lower for the high Ni material. The apparent activation energy is in excellent agreement with those reported for lattice self-diffusion in -titanium in the presence of fast diffusing impurities. The results also suggest that creep in the higher stress regime is controlled by dislocation motion within the -phase. We suggest that trace levels of Ni in the -phase accelerate self-diffusion therefore increasing the rate of dislocation climb leading to the higher creep rates observed in the high Ni material. In Part II, direct evidence in support of dislocation-based creep being important in both low and high stress regimes is presented.     

Dual fatigue failure modes in Ti–6Al–2Sn–4Zr–6Mo and consequences on probabilistic life prediction

The variability in fatigue life of the Ti–6Al–2Sn–4Zr–6Mo (Ti-6-2-4-6) alloy was investigated. Cumulative life distribution plots were found to be composed of two failure mechanisms. The data could be closely represented by a cumulative distribution function (CDF) resulting from the superposition of the CDFs of the individual mechanisms. An approach for life prediction based on the data due to the worst-case mechanism is suggested.     

Effect of the salt addition practice on the grain refining efficiency of Al–Ti–B master alloys

The impact of the salt addition practice on the microstructure and grain refining efficiency of Al–Ti–B alloys produced by the “halide salt” route was investigated. The grain refining performance of an experimental Al–5Ti–1B master alloy was optimized when the halide salts were pre-mixed before addition to aluminium melt at 800 °C during the production of the grain refiner. The stirring action provided during salt addition was found to degrade, while a high rate of addition was found to improve, the grain refining efficiency. In view of the above, an improved salt addition practice to ensure an exceptional grain refining performance is claimed to comprise the following steps: melting commercial purity aluminium ingot; addition of pre-mixed salts to molten aluminium at 800 °C, at once to facilitate a rapid salt reaction, gently mixing the salts with the aluminium melt without introducing any stirring. The grain refiner master alloy thus produced gives an average grain size of 102 μm 2 min after inoculation.     

Analysis of diamond-like carbon and Ti/MoS2 coatings on Ti–6Al–4V substrates for applicability to turbine engine applications

Ti–6Al–4V substrates have been coated by diamond-like carbon (DLC) films, with no surface pretreatment, and have been coated by Ti/MoS₂ films, with a simple surface pre-cleaning. The DLC films were deposited by planar coil r.f. inductively-coupled plasma-enhanced chemical vapor deposition (r.f. ICPECVD); the Ti/MoS₂ films were deposited by magnetron sputtering. Both the DLC and Ti/MoS₂ films were characterized by pull tests, hardness tests, scanning electron microscopy (SEM), and wear tests (pin-on-disk and block-on-ring) to compare their adhesion, hardness, surface topology, and wear properties to plasma-sprayed Cu–Ni–In coating currently used for turbine engine applications. The DLC films were easily characterized by their optical properties because they were highly transparent. We used variable-angle spectroscopic ellipsometry (VASE) to characterize thickness and to unequivocally extract real and complex index of refraction, providing a rapid assessment of film quality. Thicker coatings yielded the largest hardness values. The DLC coatings did not require abrasive pretreatment or the formation of bond-layers to ensure good adhesion to the substrate. Simple surface pre-cleaning was also adequate to form well-adhered Ti/MoS₂ on Ti–6Al–4V. The results show that the DLC and Ti/MoS₂ coatings are both much better fretting- and wear-resistant coatings than plasma-sprayed Cu–Ni–In. Both show excellent adhesion to the substrates, less surface roughness, harder surfaces, and more wear resistance than the Cu–Ni–In films.     

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.     

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.     

Interface effects during consolidation in titanium alloy components locally reinforced with matrix-coated fibre composite

An investigation has been made of diffusion bonding at the interface between a local reinforcing metal matrix composite and a monolithic engineering material. Diffusion bonding occurs during the consolidation of the composite during component manufacture. In this study, the composite is made up from Ti–6Al–4V titanium alloy coated SiC fibres, and the monolithic engineering material is also Ti–6Al–4V, but with a different microstructure.

An interface model is presented which takes account of diffusion bonding and which is able to describe the deformation behaviour at the interface between composite and monolithic material during composite consolidation. The model is developed from an existing diffusion bonding theory, and is implemented into finite element software.

The finite element simulations, and results of experiments, show that diffusion bonding can lead to localised deformation, the inhibition of consolidation, and a resulting inhomogeneous distribution of consolidated and unconsolidated regions during component manufacture. A further effect of the diffusion bonding is to increase the level of component distortion which results from the constraint imposed on the consolidating composite.

The interface model presented enables the simulation of practical forming processes so that process variables such as temperature and pressure can be chosen to ensure appropriate finished component properties.     

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.     

Phase identification and effect of W on the microstructure and micro-hardness of Ti2AlNb-based intermetallic alloys

The O-phase (orthorhombic Ti₂AlNb phase) intermetallic alloy has been considered one of the strongest materials for high temperature application. The primary compositions of the test alloys in this paper are Ti–22Al–27Nb (at.%) and Ti–22Al–20Nb–2W (at.%). By in-situ X-ray diffraction (XRD) tests, each phase is identified and the lattice parameters of each phase are calculated for different temperatures. XRD tests show that a Ti₂AlNb-β phase, partially ordered b.c.c. phase, exists besides O, ₂ and B2 that are usually expected in the Ti–Al–Nb alloy system. In this paper, the effects of tungsten on the microstructure and micro-hardness of a Ti₂AlNb-based (O+b.c.c.) alloy are investigated. The effects of W on Ti₂AlNb-based intermetallic alloys are widely known to cause a refinement of microstructure and improvement of mechanical strength, however, the reason has not yet been reported. In this paper, the role of W in the improvement of mechanical strength is clarified. The main effect of W addition is not the formation of a solid solution or the formation of W-precipitates but the refinement of the Widmanstätten structure.