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.     

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.     

Thermodynamic assessment of the Ti–B system

A consistent thermodynamic data set for the Ti–B system is obtained by means of CALPHAD technology. The sublattice model is used to describe the solid solution phases: (Ti%)₁(B, Va%)_0.5 and (Ti%)₁(B, Va%)₃ for the terminal solution (Ti) and (βTi), and Ti₁(B%, Ti)₁ and (B, Ti%)₁(B%, Ti)₂ for the compound solution TiB and TiB₂, respectively. The intermetallic compound Ti₃B₄ is treated as a stoichiometric compound. The liquid solution phase is assumed to be a substitutional solution with Redlich–Kister formula for the expression of its excess Gibbs energy. The complete T–x phase diagram for the Ti–B binary system is given. The calculation results agree well with experiments.     

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.     

The impact characteristics of Ti–6Al–4V plates hardfacing by laser alloying NiAl+ZrO2 powder

This investigation considers the alloying of NiAl powders, with 0, 10, 20, 30, and 40 wt.% of ZrO₂ added, by the CO₂ laser upon Ti–6Al–4V base metals. Trial experiments are performed to obtain the optimum thickness of the powder, 0.1 mm, and the transverse speed, 1 mm/s, upon which the hardfacing process was based. The microstructures of the alloying layers were analyzed by OM, X-ray spectroscopy and SEM/EDS. The mechanical properties of the alloying layers were analyzed by micro-hardness and impact tests. The results indicated that the microstructure of the hardfacing layer was finer and its micro-hardness was higher than those of the base material. During the hardfacing process, NiAl and ZrO₂ powder were dissolved in a molten pool, reacted with other elements, and new phases were then formed. Impact tests revealed that the absorption of the vibration increased as the ZrO₂ added.     

A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys

Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB₂ and Al₃Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B₂ or AlB₂ particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al₃Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB₂ and the soluble Al₃Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min.     

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.     

Microstructural characterization of NiCr1BSiC laser clad layer on titanium alloy substrate

Laser cladding of NiCrBSiC powders on Ti–6Al–4V alloy substrate was carried out, and the microstructure of the laser clad layer was characterized by TEM and SEM. Results show that the phases of TiC, TiB₂, CrB and M₂₃C₆ were formed in situ in the clad layer. The TiC phase exists in the form of dendrites with two types of interface morphology including the non-faceted and the faceted one. The TiB₂ phase nucleates on the facets of TiC dendrites, and can grow to form a special morphology of microstructure in which the TiC dendrite is encased by the TiB₂ phase. The CrB and M₂₃C₆ phases exist in the form of rod-shaped morphology, inside which stacking faults could be observed. The clad layer matrix consists of primary γ_p-Ni dendrites and lamellar eutectics of γ_e-Ni+Ni₃B. The formation mechanism of the microstructure of the clad layer was discussed.     

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.     

Fabrication and dry sliding wear behavior of in situ Al–K2ZrF6–KBF4 composites reinforced by Al3Zr and ZrB2 particles

Aluminum matrix composites reinforced by Al₃Zr and ZrB₂ particles were fabricated from Al–x wt.%(K₂ZrF₆–KBF₄) (x = 5, 10, 15, 20, 25) systems via magnetochemistry in situ reaction and the dry sliding wear properties and behavior of the composites were investigated. XRD and SEM analysis show that ZrB₂ and Al₃Zr reinforcement phases have been obtained and been distributed uniformly in the aluminum matrix. Wear test results show that the values of wear weight loss of the composites decrease with the increase of x under all identical wear conditions, and that of the relative wear resistance R_relat. increases under the applied load of 100 N. Especially, when x = 25, the wear weight loss (under a sliding time of 120 min and an applied load of 100 N), which is 0.245 to that of the A356 alloy, and the R_relat. (under the intermediate wear-sliding stage and an applied load of 100 N) is 4.772, which is 1.513 to that of the primary stage, respectively. Two modes of the wear mechanisms of the as-prepared composites were identified.     

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.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Activity measurements of Al and Cu in Si–Al–Cu melt at 1273 and 1373 K by the equilibration with molten Pb

For the effective control of Al introduction to solidified Si during the solidification refining of Si with the Si–Al-based melt for the solar cell material or the LPE Si film growth processes from the Si–Cu–Al solvent, thermodynamic properties of the Si–Al–Cu melt were investigated at 1273 and 1373 K. Activities of Al and Cu in the Si–Al–Cu melt were measured by the equilibration with molten Pb. Also, the excess Gibbs energy of the melt was studied by the ternary regular solution model.

The evaluated thermodynamic properties of the Si–Al–Cu melt indicated that Cu addition to the Si–Al melt brings the smaller activity coefficient of Al and is effective for reducing the Al content of solidified Si from the melt more effectively than its dilution effect for Al.     

A fundamental study on Ti–6Al–4V's thermal and electrical properties and their relation to EDM productivity

There are strong needs for productive/quality machining strategies of notoriously “difficult-to-machine” aerospace materials. The current means of machining these materials is dominated by mechanical cutting methods, which are costly due to high tooling costs, poor surface quality and limitations in the workpiece features and operations that can be machined. The newest EDM technology may be able to circumvent problems encountered in mechanical machining methods. In this paper, the EDM technology has been used to machine titanium alloy Ti–6Al–4V to investigate the effect of Ti–6Al–4V's thermal and electrical properties on the EDM productivity. In the study, temperature measurements have been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes of difficulty in machining titanium alloys and observe the optimal duty factor in terms of productivity and quality.     

Thermomechanical processing of a twin-roll cast Al–1Fe–0.2Si alloy

A sound homogenization practice is identified for a twin-roll cast Al–1Fe–0.2Si (AA8079) alloy, a popular foil stock with a higher Fe/Si ratio than most other AlFeSi commercial alloys. Homogenization below 793 K produces a very fine dispersion of _c particles which in turn yields a very coarse and heterogeneous grain structure upon interannealing. When the Al–1Fe–0.2Si strip is homogenized above 833 K, _c particles are replaced by relatively coarser Al₃Fe particles with a favorable effect on the recrystallized grain size. It is thus concluded that a homogenization temperature of at least 833 K must be employed to obtain a coarse particle dispersion which in turn would produce a fine grain size after interannealing.     

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.     

The role of Fe and Ti additions in the microstructure of Nb–18Si–5Sn silicide-based alloys

The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)_ss, (Nb,Ti)₃Sn, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si, FeNb₄Si, and Fe₂Nb₃ and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)₃Si, (Nb,Ti)₃Sn and the Ti rich oxide. In NV8 the formation of Nb₅Si₃ was destabilised, the stability of Nb₃Si was enhanced and the eutectic between Nb₅Si₃ and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb₄Si and Fe₂Nb₃ phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)₃Sn, β(Ti,Nb)_ss, (Ti,Nb)₃Si and (Ti,Nb)₅Si₃ phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)_ss + (Ti,Nb)₅Si₃ → (Ti,Nb)₃Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb₃Sn, Ti₃Sn, and Nb₃Si, Ti₃Si, and Ti and Nb base 5-3 silicides are discussed.