Microstructure and grain refining performance of a new Al–Ti–C–B master alloy

A kind of Al–Ti–C–B master alloy with a uniform microstructure is prepared using a melt reaction method. It is found that the average grain size of α-Al can be reduced from 3500 to 170 μm by the addition of 0.2 wt.% of the prepared Al–5Ti–0.3C–0.2B and the refining efficiency does not fade obviously within 60 min. It is considered that the TiC_xB_y and TiB_2−mC_n particles found at the grain center are the effective and stable nucleating substrates for α-Al during solidification, which accounts for the good 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.     

Al–Ti–B grain refiners via powder metallurgy processing of Al/K2TiF6/KBF4 powder blends

The response to thermal exposure of ball-milled Al/K₂TiF₆/KBF₄ powder blends was investigated to explore the potential of PM processing for the manufacture of Al–Ti–B alloys. K₂TiF₆ starts to be reduced by aluminium as early as 220 °C when ball-milled Al/K₂TiF₆/KBF₄ powder blends are heated. The reaction of KBF₄ with aluminium follows soon after. The Ti and B thus produced are both solutionized in aluminium before precipitating out as Al₃Ti and TiB₂. All these reactions take place below the melting point of aluminium. The ball-milled Al/K₂TiF₆/KBF₄ powder blends heat treated at approximately 525 °C can be compacted to produce Al–Ti–B pellets with in situ formed Al₃Ti and TiB₂ particles. These pellets are shown to be adequate grain refiners for aluminium alloys.     

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.     

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%.     

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.     

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.     

Dry drilling of alloy Ti–6Al–4V

This work is focused on the combined study of the evolution of tool wear, quality of machined holes and surface integrity of work-piece, in the dry drilling of alloy Ti–6Al–4V. Tool wear was studied with optical microscope and SEM–EDS techniques. The quality of machined holes was estimated in terms of geometrical accuracy and burr formation. Surface integrity involves the study of surface roughness, metallurgical alterations and microhardness tests. The end of tool life was reached because of catastrophic failure of the drill, but no significant progressive wear in cutting zone was observed previously. High hole quality was observed even near tool catastrophic failure, evaluated from the point of view of dimensions, surface roughness and burr height. However, microhardness measurements and SEM–EDS analysis of work-piece showed important microstructural changes related with a loss of mechanical properties. Depending on the application of the machined component, the state of the work-piece could be more restrictive than the tool wear, and the end of tool life should be established from the point of view of controlled damage in a work-piece.     

A novel fading-resistant Al-3Ti-3B grain refiner for Al-Si alloys

Al-Ti-B refiners with excess-Ti perform adequately for wrought aluminum alloys but inefficiently in the case of foundry alloys. The high content of silicon in the latter, which forms silicides with Ti and severely impairs the refining potency of the nuclei, is known to be responsible for the poor performance. Hence, new grain refiners, such as Al-3B and Al-3Ti-3B master alloys with excess-B have been developed with well documented advantages for Al-Si alloys. It is very desirable to involve TiAl₃ particles in the Al-3Ti-3B master alloy to maximize its grain refining efficiency. However, fading phenomenon is a key drawback for application of the TiAl₃-containing refiners in aluminum foundry. In the present work, new Al-3Ti-3B grain refiners, containing TiB₂, AlB₁₂ and TiAl₃ particles were developed with an aim to prolong the acting time after inoculation. The results showed that inoculation of Al-7Si alloy with thus meliorated Al-3Ti-3B grain refiner has produced a fine grain structure which was approximately maintained up to 30 min.     

Corrosion behavior of Ti–6Al–4V alloy weldment in hydrochloric acid

The corrosion behavior of a thin sheet of Ti–6Al–4V alloy was investigated in hydrochloric acid solution after welding by gas tungsten arc welding. The resulting microstructure of the weld metal (WM) consisted of coarse prior β grains containing fine acicular α platelets. It was found that both base metal (BM) and WM exhibited active–passive behavior after surface activation in open circuit potential experiments. The corrosion resistance of the BM and WM was found to decrease with increasing the temperature and acid concentration. However, the WM exhibited higher corrosion rate than the BM in all examined conditions. These results were also corroborated by electrochemical impedance spectroscopy measurements. Furthermore, it was revealed that the activation energy of the corrosion process for the WM was lower as compared to the BM, which confirmed the inferior corrosion behavior of the WM.     

Thermo-optical modulation for improved ultrasonic fatigue crack detection in Ti–6Al–4V

Pulsed infrared laser irradiation was used to positively identify small fatigue cracks on the surface of fatigue damaged Ti–6Al–4V specimens. The resulting transient thermoelastic deformation perceptibly changes the opening of partially closed surface cracks without affecting other scatterers, such as surface grooves, corrosion pits, coarse grains, etc. that might hide the fatigue crack from ultrasonic detection. We found that this method, which was previously shown to be very effective in 2024 aluminum alloy, must be modified in order to successfully adapt it to Ti–6Al–4V titanium alloy, where significant thermo-optical modulation was found even from straight corners or open notches. This spurious modulation is caused by direct thermal modulation of the sound velocity in the intact material rather than thermal stresses via crack closure. Different methods have been developed to distinguish direct thermal modulation from crack-closure modulation due to thermoelastic stresses. It was found that the modified thermo-optical modulation method can increase the detectability of hidden fatigue cracks in Ti–6Al–4V specimens by approximately one order of magnitude.     

Influence of regulated modification of nitride layer by oxygen on the electrochemical behavior of Ti–6Al–4V alloy in the Ringer's solution

The corrosion behavior of oxynitrided Ti–6Al–4V alloy was investigated in the Ringer's solution (simulated body fluid) at a temperature of 37°C. The oxynitriding of the alloy was carried out by leaking controlled oxygen‐containing medium into the reaction chamber at the final stage of the nitride formation. It was determined that oxynitriding improved corrosion resistance of Ti–6Al–4V alloy as it provided lower corrosion current density by 1.3–1.5 times and higher corrosion potential. In this paper, we analyzed that the resistance of the double layer had increased with the increase of the oxygen content in titanium oxynitride. Its value was higher compared with untreated alloy, indicating higher corrosion resistance of the oxynitrided one.     

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.     

Development of ultrafine grained Al–Mg–Si alloy with enhanced strength and ductility

The microstructure and mechanical properties of a precipitation hardenable Al–Mg–Si alloy subjected to cryorolling (CR), short annealing and ageing treatments are reported in this present work. The pre-cryorolled solid solution treatment combined with post-CR short annealing (155 °C for 5 min) and then ageing treatment (125 °C for 12 h) has been found to be the optimum processing condition to obtain the ultrafine grained microstructure with substantial improvement of tensile strength (286 MPa) and good tensile ductility (14%) in the Al–Mg–Si alloy. The significant improvement of the mechanical properties of the cryorolled and peak aged 6063 Al alloys have been observed as compared to its bulk alloys in the peak-aged condition (T6).     

Al–Ti–C–Sr master alloy—A melt inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys

Present article is focused on the microstructural features of Al–Ti–C–Sr master alloy, an inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys. This master alloy is basically a metal matrix composite consisting of TiC and Al₄Sr phases formed in situ in the Al-matrix. TiC particles initiate the refinement of primary α-Al through heterogeneous nucleation in molten hypoeutectic Al–Si alloy, while Al₄Sr phase dissolves in molten Al–7Si alloy enriching the melt with Sr, which eventually leads to modification of eutectic silicon during solidification of the Al–7Si alloy casting. Thus present master alloy serves in both ways, as a grain refiner and a modifier for hypoeutectic Al–Si alloys.     

Corrosion behavior of newly developed Ti–Ag–Fe dental alloys in neutral saline solution

The corrosion behavior of Ti–5Ag–xFe alloys (x = 1, 2.5, 5 wt%) in neutral saline solution was investigated by the open‐circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and potentiostatic tests. The microstructural observation indicated that β‐Ti phase was retained by the addition of Fe into Ti–Ag alloys. Compared with commercially pure (CP) Ti, Ti–5Ag–xFe alloys exhibited higher corrosion potentials, lower current densities, and larger impedance, these suggested that Ti–5Ag–xFe alloys have nobler electrochemical corrosion behavior when compared with CP Ti in neutral saline solution.     

Electrochemical aspects of Ti–Ta alloys in HBSS

Corrosion behaviour of the studied Ti–Ta alloys with Ta contents of 30, 40 and 50 wt% together with the currently used metallic biomaterials commercial pure titanium (Cp–Ti) was investigated for biomedical applications. All the samples were tested by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in Hank's balanced salt solution (HBSS) with and without albumin protein at 25 °C. Very low corrosion current densities and passive current densities (in the order of 10^?6 A/cm²) were obtained from the polarization curves, indicating a typical passive behaviour for all the samples in HBSS with and without albumin. The EIS technique was applied to study the nature of the passive film formed on all the samples at various imposed potentials ?500 mV (SCE), 0 mV (SCE), 500 mV (SCE) and 1000 mV (SCE). The equivalent circuit (EC) used successfully to describe the behaviour of the samples suggests a single passive film present on the metals' surface in HBSS with and without albumin. The results showed that the presence of albumin in HBSS had an influence on the zero current potential (ZCP), polarization resistance (R_p) and capacitance (C). The presence of albumin protein in HBSS improved slightly the corrosion resistance of the entire sample. The experimental results confirm that the electrochemical behaviour of the studied Ti–Ta alloys is better to that of Cp–Ti, suggesting their promising potential for biomedical applications.     

Electrochemical corrosion behavior of biomedical Ti–22Nb and Ti–22Nb–6Zr alloys in saline medium

The aims of this study were to investigate the effects of Zr addition and potentiodynamic polarization on the microstructure and corrosion resistance of Ti–22Nb and Ti–22Nb–6Zr alloy samples. The corrosion tests were carried out in 0.9% NaCl at 37 °C and neutral pH value, utilizing the OCP, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. The results of XRD and optical microscopy indicated that the addition of Zr stabilized the β phase, which plays a crucial role in the corrosion resistance improvement of the Ti–22Nb–6Zr alloy. From the polarization curves, it can be seen that the alloys exhibited a wide passive region without the breakdown of the passive films and also low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with the addition of 6 at% Zr into the Ti–22Nb alloy. The EIS results of these two alloy samples after 1‐h immersion in 0.9% NaCl solution, and being fitted by R_S(Q_PR_P) model, suggested that the corrosion resistance of the passive films improved with the addition of Zr and only a single passive film formed on the surfaces. However, two time constants were observed for the Ti–22Nb and Ti–22Nb–6Zr alloy samples after potentiodynamic polarization, the spectra of which can be fitted using the R_s(Q_o(R_o(Q_bR_b))) model. In addition, the corrosion resistance of the two alloy samples was reinforced significantly because of polarization when compared to the immersed samples. All these observations suggested a nobler electrochemical behavior of the titanium alloys with the addition of Zr element and after polarization.