A New Method for Preparing Titanium Aluminium Alloy Powder

Abstract

Due to TiAl alloys’ excellent properties, TiAl alloys have received widespread attention from researchers. However, the high energy consumption and lengthy process of traditional preparation methods have always limited the large-scale application of TiAl alloys. This article develops a new method for preparing TiAl-based alloy powder via the magnesium thermal reduction of TiO₂ in AlCl₃-KCl molten salt. In this study, the proportion of AlCl₃&KCl molten salts was determined. We conducted phase analysis on the final product by studying the changes in temperature and time. It was found that the TiAl₃ alloy powder could be obtained by being kept at 750 °C for 2 h, with an oxygen content of 3.91 wt%. The reaction process for the entire experiment was determined through thermodynamic calculations and experimental analysis, and the principles of the reduction process are discussed.

1. Introduction

Metal compounds offer both the plasticity of metals, and the high-temperature strength of ceramics in specific compositions, due to the metal compound ordered arrangement of their atoms, and the coexistence of inter-atomic metal bonds and covalent bonds. TiAl-based intermetallic compounds were highly valued in the aerospace industry and other fields, due to their excellent mechanical properties and low density, in the 1950s [1,2]. Nowadays, the TiAl-based alloy is still recognized as a high-end material in the world [3,4]. Due to its high specific strength, high temperature resistance [5,6], corrosion resistance, oxidation resistance [7,8], and excellent biocompatibility, the TiAl-based alloy is widely used in the field of high-end materials in modern life [9,10]. For example, large aeroplanes, submarines, aerospace technology, and artificial bones [11,12,13]. This high-end material should also be popularized in ordinary daily life but, due to its high cost, it has not been applied, and can only be applied in high- and middle-value fields. The reason for this dilemma is the long cycle and high cost of preparing the metal Ti. Since the discovery of metallic titanium, only the Kroll method has produced sponge titanium on a large scale [14,15,16,17]. Therefore, most methods for preparing TiAl-based alloys are element approaches [18,19,20,21], prepared by adding proportional amounts of the elements Ti and Al in a high-temperature melting furnace. Adding Ti separately also causes production costs for TiAl-based alloys.

Many researchers have adopted different methods to find an efficient method for producing TiAl-based alloys. The most traditional way is to prepare TiAl-based alloys via casting [22,23,24,25] and cast alloys with different compositions using a vacuum induction melting furnace, centrifuge, hot press, and other equipment. Among them, J Lapin [26] et al. prepared the Ti-42.6Al-8.7Nb-0.3Ta-2.0C and Ti-41.0Al-8.7Nb-0.3Ta-3.6C (in at.%) TiAl alloys via the casting method, and studied the effect of adding 2.0C and 3.6C on the properties of the TiAl alloys. They also studied the solid-state phase transformation grain refinement of the as-cast peritectic TiAl-based alloys. Powder metallurgy is also the preparation method for most TiAl-based alloys; it can overcome the defects generated through traditional manufacturing methods, and obtain uniform and fine microstructures, significantly improving the mechanical properties of the alloys. Heike Gabrisch et al. [27] added 0.5–1.0 at.% C into Ti-45Al-5Nb alloy via powder metallurgy, and used a transmission electron microscope and high-energy XRD to study the influence of solid-solution carbon and carbide precipitation on the hardness of the TiAl alloy. In addition to the above two methods, emerging additive manufacturing technologies [28,29,30] can also prepare designed alloys, by stacking powders layer by layer. The powder preparation methods mentioned above all require titanium powder as a raw material, which is expensive, and increases production costs. Therefore, there is a need for a method that can directly prepare alloys from TiO₂, to improve the existing technology, reduce the process flow, and increase the popularity of TiAl-based alloys [31,32].

Researchers have now prepared TiAl-based alloys using other methods. Zhao et al. [33,34] proposed a two-step thermal reduction method for preparing TiAl-based alloys. In this method, firstly, Na₂TiF₆ is reduced by TiAl-based alloy powder, which is the first reduction stage. After vacuum distillation, the second-stage reduction of Al is carried out, to obtain TiAl-based alloys. The TiAl-based alloy powder collected in the purification process can also be used for the first-stage reduction. This method successfully realizes the overall round-robin preparation of TiAl-based alloys through the Al thermal reduction of Na₂TiF₆. Dou, Song, and Zhang [35,36] successfully prepared 20 kg TiAl-based alloy ingots with an oxygen content of approximately 1.09 wt% through multi-stage profound reduction. This method successfully achieved the one-step preparation of TiAl-based alloys through adding KClO₃ as a heating agent, and self-propagating aluminium heat.

However, the two-step aluminothermic reduction of Na₂TiF₆, and the high-temperature self-propagation method, require experiments at high temperatures, resulting in energy consumption. However, due to the limited reduction effect of metal aluminium on TiO₂ at low temperatures, it is impossible to prepare TiAl-based alloys by reducing TiO₂ at low temperatures. Moreover, TiAl-based alloy ingots exhibit significant room temperature brittleness, with a derivation rate of less than 1%, and fracture during stretching, resulting in a poor machinability, and difficulties in their application. Due to the significant difference in melting points between Ti and Al, casting high-quality alloy ingots is difficult and costly. This article proposes a new method of preparing TiAl-based alloy powder by reducing TiO₂ in AlCl₃&KCl molten salt through a magnesiothermic reduction. This method can only be carried out at low temperatures, with a simple production method, and a low level of environmental pollution. This work provides new ideas for the future large-scale application of TiAl-based alloy powder, and to solve the problem of the long preparation process of existing TiAl-based alloys.

2. Materials and Method

2.1. Materials

Anhydrous AlCl₃, KCl, HCl, TiO_2, and binder were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). The particle size of anhydrous AlCl₃, KCl, and TiO₂ is below 74 μm. The binder used in this paper is ethyl cellulose, a polymer compound with the chemical formula (C₁₂H₂₂O₅)_n. Ethyl cellulose is a white powder at room temperature. The primary function of this binder is to aggregate the raw materials, and increase the contact area of raw materials. Under high-temperature conditions, ethyl cellulose will be decomposed into organic compounds that do not impact the experiment. As a reducing agent, Mg powder is obtained from Sinopharm Chemical Reagent Co., Ltd. (Ningbo, China). The Mg powder particle size is below 200 μm.

2.2. Introduction to AlCl₃ and Selection of AlCl₃-KCl Molten Salt Ratio

AlCl₃ is a white crystalline powder with a solid hydrochloric acid odour, and a light yellow industrial product, and is easily soluble in water, alcohol, chloroform, etc. Its melting point is 194 °C, but it is prone to sublimation and deliquescence at 178 °C. Moreover, due to the exothermic hydration reaction, it may explode when encountering water. Therefore, the AlCl₃ must be sealed and stored in a dry environment.

(1)

Physical properties of AlCl₃

AlCl₃ was first prepared by Biltz. W [37] in 1923, via the reaction of 99.5% pure Al powder with dry HCl gas. Subsequently, Baker Analytics and Grothe studied the basic properties of AlCl₃, etc. [37]. The density and viscosity of AlCl₃ are key physical properties during the thermal reduction process. Based on previous research data, the density of AlCl₃ varies with the temperature, as shown in Figure 1. It can be seen that the density of AlCl₃ shows a steady downward trend with the temperature rising within 460–560 °C. The following formula is the density formula, fitted according to the data (estimated standard error: 0.15%).

$$\mathsf{\rho }=2.19-5.40\times {10}^{-4}\mathrm{T}-1.14\times {10}^{-5}{\mathrm{T}}^2+3.04\times {10}^{-8}{\mathrm{T}}^3$$

(1)

$\mathsf{\rho }$ indicates the density of AlCl₃ (g·cm⁻³); T indicates the temperature (°C).

Figure 2 shows the viscosity data of AlCl₃. According to these data, the viscosity formula of AlCl₃ is fitted, and the estimated standard error is 1.05%.

$$\mathsf{\eta }=1.71·\exp (4943.8/\mathrm{R}\mathrm{T})$$

(2)

η indicates the viscosity of AlCl₃ (Pa·s), and R indicates a constant of 8.314 (kJ/mol); T indicates the temperature (°C).

From the graph showing the viscosity and density changes with temperature, it can be seen that the density and viscosity of AlCl₃ show an overall decreasing trend with an increasing temperature. The decrease in density and viscosity is conducive to the complete contact of reactants during the reduction reaction process, increasing the reaction efficiency, which is a favourable factor in magnesium thermal reduction.

(2)

Basic Physical Properties of AlCl₃-KCl Mixed Molten Salt

Due to the low melting point and easy sublimation of AlCl₃, to prevent the loss of raw materials due to the large amount of sublimation and volatilization of AlCl₃, a mixed molten salt AlCl₃-KCl is configured, which can effectively suppress the volatilization of AlCl₃. As shown in Figure 3, when the AlCl₃ content accounts for 80 wt% and above, the binary phase diagram of AlCl₃-KCl shows that a large amount of AlCl₃ gas is generated. As the KCl increases, the blue part in the figure shows the liquid phase zone of the AlCl₃-KCl eutectic salt without gas generation. Therefore, this part can be selected as the area used in the raw material ratio in this experiment, where the mass ratio of AlCl₃/(AlCl₃-KCl) is 0.65.

The density of the AlCl₃-KCl is based on Carter and Morrey’s [37] work to obtain the following AlCl₃-KCl density data, as shown in

Table 1. The density of the AlCl₃-KCl varies with the KCl content.

$\mathsf{\rho }=\mathbf{a}+\mathbf{b}\mathbf{T}$
Mol % KCl	a	b·10−3	Standard Error
20.00	2.0252	−1.0038	0.11%
33.33	1.9889	−0.7901	0.14%
50.03	1.9556	−0.6622	0.05%
66.66	1.9734	−0.6101	0.02%

. The temperature dependence of AlCl₃-KCl under different KCl contents was plotted using Table 1, as shown in Figure 4. It can be seen that as the KCl content increases, the overall density of the AlCl₃-KCl shows an upward trend. As the temperature increases, the density of the AlCl₃-KCl decreases. When the molar ratio of KCl exceeds 50%, the overall density of the AlCl₃-KCl does not change significantly with temperature. The incremented density is due to the melting point of KCl being higher. After the KCl content increases, the overall melting point of the AlCl₃-KCl increases. An increasing KCl content can inhibit AlCl₃ volatilization. This result is consistent with the calculation results in Figure 4.

2.3. Methods

The raw materials are mixed with the binder in a mortar. After that, the mixed raw materials are pressed into a round green mass. For the reduction, we place the green mass into the molybdenum crucible and the tube furnace (KF1100 Nanjing Boyuntong (Nanjing, China), as shown in Figure 5). After reaching the specified temperature, this furnace can be loaded into the reaction furnace tube. The heating rate is 15 °C/min, and the insulation is maintained after reaching the specified temperature. According to the analysis of the physical properties and phase diagram of AlCl₃&KCl in the previous section, to suppress the volatilization of AlCl₃, the molten salt is weighed based on the mass ratio of AlCl₃/(AlCl₃+KCl) of 0.65. Therefore, this experiment’s molten salt mass ratio of AlCl₃: KCl is 1.8. Considering that AlCl₃ has the characteristic of volatilizing at low temperatures, it is necessary to increase the content of AlCl₃ appropriately during the raw material configuration process. The amount of TiO₂ used in each experiment is 5 g, the proportion of molten salt is four times the total mass of the TiO₂ added, and the amount of magnesium added is the molar mass of the AlCl₃ completely reacted.

The experimental process is shown in Figure 6. The experiment was conducted in a tubular box mixing furnace. The raw materials were mixed evenly, loaded into a molybdenum crucible, and placed in a reactor. Inert gas was first introduced into the reactor to exhaust the air, with a flow rate of 300 mL/min. After the high-temperature furnace rose to the set temperature, the reactor was loaded into the furnace, without increasing the temperature of the furnace (to reduce the evaporation of the molten salt). This experiment explored the influence of time on the reaction products’ phase and oxygen content. The overall reaction time starts from the installation of the reaction furnace tube into the furnace, and a specific insulation time is set. After the reaction, argon gas is continuously introduced and cooled in the furnace, to ensure that the sample does not come into contact with oxygen. The final reactant is extracted for subsequent acid leaching and flotation, to obtain the final alloy product for analysis. Among the experiments, 5 wt% dilute HCl is used for acid leaching. The acid-leaching process is carried out in a water bath at 80 °C for 2 h. The acid-leaching process is mainly used to remove Mg and MgO. The specific reactions are shown in the formulae below. The reaction byproduct Al₂O₃ is removed via ball milling. The mass ratio of the ball-milling process as ball:material:H₂O during ball milling is 1.6:1:3. The ball mill rotates at a speed of 350 r/min for 15 min each time. Each experiment requires ball milling at least three times.

Mg + 2HCl = MgCl₂ + H₂

(3)

MgO + 2HCl = MgCl₂ + H₂O

(4)

2.4. Analysis

X-ray powder diffraction (XRD, Cu Kα radiation, PANalytical X’Pert Powder, Malvern PANalytical B.V., Almelo, The Netherlands), with a scanning range of 10–90° and a scanning step of 5 deg./min, was carried out, to confirm the phase composition of sample powder three times. Scanning electron microscopy (SEM, TESCAN VEGA 3 LMH system, TESCAN, Brno, Czech Republic) was also performed, to evaluate the microanalysis and surface morphology of the reaction product. Energy-dispersive spectroscopy (EDS) microanalysis was conducted. The accelerating voltage used for the EDS analysis was 10 kV. The thermodynamic software FactSage8.0 calculated the feasibility of the reaction. The oxygen content of the product was analyzed using the JXA82 electron probe from JEOL Corporation, and the particle size analyzer from NanoBrook Omni of Brookhaven Instruments, Holtsville, NY, USA.

3. Results and Discussion

3.1. Phase Analysis of Reactants at Different Temperatures

The XRD phase diagrams and SEM of the products at different reaction temperatures are shown in Figure 7 and Figure 8. Comparing the XRD analyses of the products at reaction temperatures of 750~950 °C, it can be found that the reaction products are TiAl-based alloy powders. Compared with traditional magnesiothermic reduction, TiO₂ is not directly reduced; other substances participate in the reaction. The Gibbs free energy diagram of the reaction between Mg and AlCl₃, TiO_2, was drawn using the thermodynamic software FactSage8.0, as shown in Figure 9. At temperatures ranging from 400 °C to 950 °C, the Gibbs free energy of the reaction between Mg and AlCl₃, TiO₂ can be observed. At the same magnesium content, Mg preferentially reacts with AlCl₃, and the entire process uses aluminium as a reducing agent in reducing the TiO₂. Moreover, magnesium-reducing AlCl₃ releases a large amount of heat, to drive the aluminothermic reduction of TiO₂. Therefore, the entire reaction process is as follows:

3Mg + 2AlCl₃ → 2Al + 3MgCl₂

(5)

Al + TiO₂ → Ti + Al₂O₃

(6)

6Ti + 6Al → 4Ti + 2TiAl₃

(7)

4Ti + 2TiAl₃ → Ti₃Al + TiAl + 2TiAl₂

(8)

Ti₃Al + 2TiAl₂ + TiAl → 6TiAl

(9)

According to literature reports [38], the alloy reaction between Al and Ti at low temperatures is mainly liquid–solid. In this experiment, the main product phase is the TiAl₃ alloy after 2 h of reaction at 750 °C. As the reaction temperature increases, from 800 °C to 950 °C, the peak of TiAl₃ gradually decreases, while the peak of TiAl₂ gradually increases. The peak of TiAl₂ gradually increases because external Al reacts before Ti to generate TiAl₃ during the reaction process, and then gradually forms TiAl₂, as shown in Equation (8). The increase in temperature is more conducive to the formation of the TiAl alloy. From the energy spectrum detected via SEM surface scanning, it can be seen that the obtained TiAl alloy has a uniform distribution of components, with an oxygen content of 4.23 wt%. The reason for such a high oxygen content is that the reduction effect of Al is limited, and TiO₂ cannot be reduced to a lower oxygen content state. However, due to the presence of Mg, some O is absorbed, to some extent. As the reaction temperature increases, it is found that the oxygen content in the reaction product is low, at 750 °C.

3.2. Effect of Reduction Temperature

In the previous section, the study of different reaction temperatures in the magnesiothermic reduction of TiO₂ in the AlCl₃&KCl molten salt system found that the oxygen content of the TiAl₃-based alloys generated at 750 °C was relatively low and stable. Therefore, 750 °C was chosen as the optimal reaction temperature. This section takes the reaction time as a single variable, and experimentally studies the effect of different reaction times on the preparation of TiAl-based alloys at 750 °C. We set the reaction time at 750 °C for 1, 2, and 4 h, respectively, to study the effect of the reaction time on the reduction effect during the reaction process. Figure 10 and Figure 11 show that XRD and SEM characterized the reaction products. After reacting at 750 °C for 1 h, the main phases of the reaction products are TiAl₃ and metallic Al. The TiAl₃ alloy is caused by the short reaction time between Al and Ti, and some of the reduced Al has not yet formed an alloy with Ti. Therefore, a large amount of metal Al has not reacted in the product after one hour of reaction. With the extension of the reaction time, when the reaction temperature is 2 h, the main product of the reaction is TiAl₃. After further prolonging the reaction time to 4 h, the main reaction products are the TiAl alloy, and a small amount of Al₂O₃. The occurrence of Al₂O₃ is because, with the extension of the reaction time, some Al forms Al₂O₃ powder through solid-state diffusion with the alloy, after reducing the TiO₂. This Al₂O₃ cannot be removed from the surface of the alloy through acid leaching and flotation. The attached Al₂O₃ can be seen in the SEM image in Figure 12; a small amount of matte and rough surface substance is attached to the metal surface, which is the Al₂O₃ generated through the extended reaction time. As mentioned above, a stable TiAl₃ alloy powder can be obtained by reacting TiO₂ with AlCl₃&KCl molten magnesium salt for 2 h at 750 °C.

As the reaction time increases, the oxygen content in the reaction product gradually increases. At the same time, this is also because the solid-phase-to-solid-phase diffusion reaction between the Al₂O₃ and TiAl-based alloys occurs at high temperatures, which finally causes part of the Al₂O₃ to enter the TiAl alloy phase, as shown in Figure 12. The TiAl alloy powder is a coarse powder bonded to the surface of the TiAl alloy, with a high oxygen content and no Ti element, which can be determined as Al₂O₃ powder. Based on the above analysis, the optimal reaction temperature for the thermal reduction of AlCl₃&KCl mixed molten salt magnesium is 750 °C, and the main product generated by the reaction at 750 °C is TiAl₃. The oxygen content of the product was analyzed using the JXA82 electric probe. The effect of the reaction time on the oxygen content of the product is shown in Figure 13, with the lowest oxygen content of 3.91 wt% after two hours of reaction.

3.3. Analysis of the AlCl₃&KCl-Molten-Salt-Assisted Magnesium Thermal Reduction Process

The above experiments indicate that the AlCl₃&KCl molten salt system serves as a medium for the magnesium thermal reduction of TiO₂, to prepare TiAl₃ alloy powder. The schematic diagram of the entire reaction process is shown in Figure 14. At the beginning of the reaction, Mg is ionized in the molten salt to form Mg²⁺, which reacts with Cl⁻ in the molten salt to form MgCl₂, while Al³⁺ is reduced to metallic aluminium. At this time, the metallic aluminium is not covered by a surface oxide film, and the generated liquid metal Al reacts with TiO₂ to generate TiAl-based alloys. The melting temperature of Al is relatively low. At 750 °C, a liquid–solid reaction occurs between the Al liquid and the reduced Ti solid, generating TiAl₃ on the surface of the titanium. Afterwards, the internal metal Ti reacts with TiAl₃ to generate TiAl₂. With the prolongation of the holding time, various TiAl-based alloys react, to generate TiAl alloy powder. The entire TiAl alloy formation process is shown in Equations (5)–(9). This experiment can prepare TiAl₃ alloy powder stably at 750 °C for 2 h, demonstrating a new method for preparing TiAl alloy at low temperatures. Moreover, the TiAl₃ alloy powder has a low density, high modulus, and strong oxidation resistance, making it an excellent high-temperature structural and layer material.

3.4. Product Analysis and Comparison

The elements of the TiAl₃ alloy powder product prepared in this experiment are shown in

Table 2. Composition of the TiAl₃ alloy (wt%).

Ti	Al	O
28.41	67.68	3.91

, and the particle size is shown in Figure 15. The Ti content is approximately 28.41 wt%, and the Al content is approximately 67.68 wt%. The TiAl₃ alloy prepared in this experiment has a larger particle size. The final products D10, D50, and D90 were 135 μm, 453 μm, and 912 μm, respectively. From the discovery of metallic titanium to the current preparation of titanium alloys, in addition to traditional high-temperature melting and mechanical alloying, many low-cost methods have also been used for preparing TiAl-based alloys. The TiAl-based alloy shows an excellent performance. Developing low-cost and straightforward equipment for new processes, to meet increasing performance requirements, is a hot research topic. These new processes all have the advantages of continuity and low cost. Among emerging technologies, high-temperature self-propagation is the most representative method, but the process controllability of this method is too poor, and there are certain risks. The multi-level deep reduction method is also a relatively novel method that has recently emerged. Still, it also has the problem of a high reaction temperature, which leads to a high energy consumption.

Table 3. Comparison of new methods for titanium alloy preparation.

Method	Features	Advantages	Disadvantages
Perform reduction process (PRP) [39]	Ca as a reducing agent	Complete reaction with a high recovery rate	Unable to apply on a large scale
SHS process [40]	Spontaneous reaction through the heating agent	Efficiency, high energy consumption, and low consumption	Uncontrollable reaction process
Two-stage thermite process [41]	Reduction using Na2TiF6 as a raw material	Short process and low energy consumption	Multiple reactions
AlCl3&KCl-molten-salt assisted magnesium thermal reduction	Low temperature, AlCl3&KCl as the reaction medium	Low energy consumption, controllable reaction	AlCl3 volatilization

compares new methods for preparing TiAl-based alloys in recent years, and this experiment. These preparation processes for preparing metallic titanium alloys are currently at the laboratory stage, and there is still a long way to go from the laboratory to industrial production.

4. Conclusions

This article proposes a new process for preparing TiAl₃ alloy powder, mainly discussing how AlCl₃&KCl molten salt can prepare TiAl₃ alloy powder via the magnesium thermal reduction of TiO₂. At the same time, the influence of different experimental factors on the reduction effect is studied, mainly the influence of the reduction temperature and time on the product phase and oxygen content. Based on the experimental research, the following main conclusions have been drawn:

(1)

Through the study of the basic physical properties of the AlCl₃ and AlCl₃&KCl, it was found that the density of the AlCl₃ decreased from 1.3 g·cm⁻³ to 1 g·cm⁻³ in the temperature range of 460–560 °C, and the viscosity also decreased from 0.34 Pa·s to 0.27 Pa·s with the temperature. For AlCl₃&KCl eutectic salts, when the KCl content is 20 wt% and 33.33 wt%, the density of the AlCl₃&KCl decreases from 1.65 g·cm⁻³ to below 1.5 g·cm⁻³ within the range of 350–850 °C. However, when the KCl content is 50 wt% and 66.66 wt%, the temperature has little effect on the density change of the AlCl₃&KCl, with a density of approximately 2 g·cm⁻³. To reduce the volatilization of the AlCl₃, the experiment selected a mass ratio of 0.65 for the AlCl₃/(AlCl₃+KCl) as the selected eutectic salt ratio.

(2)

Using TiO₂ as the raw material, magnesium thermal reduction experiments were conducted in a AlCl₃&KCl molten salt medium, to clarify the reaction action and sequence of AlCl₃ during the reaction process. During the magnesium thermal reduction process, Mg preferentially reacts with AlCl₃ at 400–1000 °C to generate metal Al, which reacts with TiO₂, and generates TiAl₃ alloy powder at 750–950 °C.

(3)

When the experimental temperature is within the range of 750–950 °C, the products of the AlCl₃&KCl molten-salt-assisted magnesium thermal reduction gradually form a TiAl alloy from the TiAl₃ alloy at 750 °C to 950 °C. The oxygen content also increases from 4.23 wt% at 750 °C, to 11.23 wt% at 950 °C, and the final powder oxygen content also increases with the extension of the reduction time. After 4 h of reaction at 750 °C, the oxygen content reaches approximately 8.5 wt%, and the reaction temperature is at 950 °C; after 4 h of reaction time, it will cause an increase in, and adhesion of, the Al₂O₃ in the product. The optimal reaction time for this process is 2 h, and the reaction temperature is 750 °C. TiAl₃ alloy powders with Ti, Al, and O contents of 28.41 wt%, 67.68 wt%, and 3.91 wt% can be obtained, respectively. The powder particle size is concentrated at around 450 μm.