Effect of Preparation Process on the Microstructure and Characteristics of TiAl Pre-Alloyed Powder Fabricated by Plasma Rotating Electrode Process

Abstract

TiAl pre-alloyed powder is the foundation for additive manufacturing of TiAl alloys. In this work, TiAl pre-alloyed powder was prepared using a plasma rotating electrode process (PREP). The effects of electrode rotating speeds and current intensity on the microstructure and characteristics of TiAl pre-alloyed powder have been investigated in detail. The results show that the electrode rotating speeds mainly affected the average particle size of the powder (D50). As the electrode rotating speed increased, the D50 of the powder decreased. The current intensity mainly affected the particle size distribution of the powder. As the current intensity increased, the particle size distribution of the powder became narrower, which was concentrated at 45~105 μm. In addition, the current intensity had a significant effect on the sphericity degree of the powder with the particle size > 105 μm, but it had little effect on that <105 μm powder. TiAl pre-alloyed powder with a particle size > 45 μm demonstrated a dendritic + cellular structure, and the <45 μm powder had a microcrystalline structure. The powder was mainly composed of the α₂ phase and γ phase. There were two kinds of phase structure inside the powder, namely the α₂ + γ lamellar microstructure (particle size < 45 µm) and the α₂ + γ network microstructure (particle size > 45 µm). The phase structure of the powder was related to the solidification path and cooling rate of molten droplets in the PREP. The average thickness of the α₂ + γ lamellar was about 200 nm, in which the lamellar γ phases were arranged in an orderly manner in the α₂ phase matrix with a thickness of about 20 nm. The network phase structure was corrugated, and the morphology of the γ phase was not obvious.

1. Introduction

TiAl alloy is a type of alloy that combines the properties of titanium and aluminum. It typically has excellent mechanical properties, including high strength and hardness [1]. This alloy offers good resistance to corrosion and oxidation, which makes it suitable for use in various demanding environments [2]. It also has relatively low density, which is beneficial for applications where being lightweight is crucial [3]. In addition, TiAl alloy has good thermal stability and can withstand high temperatures [4]. Due to these characteristics, it finds extensive applications in the aerospace industry, automotive sector, and in the manufacturing of high-performance components and parts [5]. It is often used for engine components, turbine blades, and other critical structures where strength, durability, and a lightweight nature are highly desired [6].

The main preparation methods of TiAl alloy include ingot metallurgy technology, rapid solidification technology, composite material technology, and powder metallurgy technology [7,8]. However, the TiAl alloy prepared by these traditional processes has problems, such as composition segregation and an uneven structure, which seriously restrict the performance of the alloy. Furthermore, the low plasticity at room temperature of the TiAl alloy makes some traditional processing approaches, like rolling, forging, and turning, extremely difficult [9]. In comparison with traditional manufacturing technology, the greatest advantage of additive manufacturing (AM) technology lies in the fact that it can achieve the near-net shaping of complex parts, and it has no demands regarding the geometrical complexity of parts and the subsequent machining process [10,11].

TiAl pre-alloyed powder for additive manufacturing must meet the requirements of having a fine particle size, narrow particle size distribution, and high sphericity degree [12]. Gas atomization (GA) and the plasma rotating electrode process (PREP) are the main effective techniques for the preparation of TiAl pre-alloyed powder [13]. The shape of TiAl pre-alloyed powder prepared by GA is spherical. However, closed pores frequently form inside the powder particles (hollow powder particles) because of the impact of high-speed Ar gas on the molten TiAl alloy droplets [14]. These hollow powder particles contain a certain amount of Ar gas, which is insoluble in metal. Therefore, Ar gas cannot easily be excluded from the powder metallurgy process [15]. In the subsequent heat treatment, these pores are prone to heat-induced growth, resulting in a decrease in the relative density of the TiAl alloy, thereby reducing the performance of the material [16]. Compared with GA technology, the PREP does not directly disperse the molten TiAl alloy droplets with high-speed inert gas flow for atomization, which can avoid the formation of hollow powder particles [17]. In this work, a high-speed PREP technique was employed to produce TiAl pre-alloyed powder for additive manufacturing. The impacts of the PREP on the microstructure and features of TiAl pre-alloyed powder were investigated, which offered the raw material foundation for the subsequent AM of the TiAl alloy.

2. Materials and Methods

2.1. Raw Material

TiAl alloy ingots with the composition of Ti-48Al-2Nb-2Cr (at. %) were prepared by a vacuum arc remelting (VAR). The ingots were machined to round rods required for the PREP with dimensions of Φ30 mm × 150 mm, as shown in Figure 1. The composition of the alloy is exhibited in

Table 1. Composition of the TiAl alloy (at. %).

Element	Ti	Al	Nb	Cr	Total
Content	47.84	48.16	2.04	1.95	100

.

The phase composition of the TiAl alloy is depicted in Figure 2. The phase structure of the TiAl alloy consisted of the γ phase and the α₂ phase, as presented in Figure 2a. In Figure 2b, it can be observed that the TiAl alloy was mainly made up of lamellar γ + α₂ double phases and equiaxed γ phases.

2.2. Preparation Process

The powder was fabricated using SL-ZFDW high-speed PREP machinery (as shown in Figure 3). The atomization chamber of the PREP equipment was vacuumized twice, and the furnace was washed with Ar gas. Finally, the high-purity (99.99%) Ar gas was poured into the furnace. The TiAl alloy electrode rod was sent into the atomization chamber and the motor rotated at a high speed. One end of the alloy electrode was arced and melted by the plasma torch. The molten alloy liquid film was immediately thrown out under the action of a high-speed rotating centrifugal force. The molten alloy liquid film rubbed with Ar gas in the atomization chamber and was further broken into fine droplets under the action of shear stress. These molten droplets were rapidly solidified to form pre-alloyed powder particles. The main processing parameters of the PREP technique are shown in

Table 2. Main processing parameters of the PREP technique.

Pressure (MPa)	Current Intensity (A)	Rotating Speed (r/min)
0.02	650~700	32,000~42,000

.

2.3. Analysis and Characterization

The powder was sieved using the AMC Powder WQS vibrating screen with the following sieve sizes: <25 µm, 25~45 µm, 45~75 µm, 75–105 µm, 105–150 µm, and >150 µm. The particle size distribution and morphology of the powder were characterized using the Malven M4 particle size and shape analyzer. The powder’s microstructure was examined utilizing the Carl Zeiss-Axio Vert. A1 metallographic microscope, as well as the S4700 and FEI-Sirion scanning electron microscopes (SEM). The transmission electron microscope (TEM) thin foils were cut with a Tescan Amber FIB-SEM and observed with the JEM-2100 TEM. The phase composition of the powder was analyzed by employing the X Pert3 Powder X-ray diffractometer, utilizing Cu Kα radiation with a scanning rate of 3°/min and a scanning range spanning from 30° to 100°.

3. Results and Discussion

3.1. Effects of PREP Parameters on Characteristics of TiAl Pre-Alloyed Powder

3.1.1. Effect of the Electrode Rotating Speeds on the Particle Size of the Powder

Figure 4 shows the particle size distribution of the TiAl pre-alloy powder prepared at different electrode rotating speeds after sieving. It can be observed from the diagram that the particle size of the powder presented a normal distribution, which was mainly concentrated at 45~105 μm. The mass percentage of the powder with the particle size < 150 μm was more than 95%, and the mass percentage of the fine powder (particle size < 25 μm) did not exceed 5%. For the powder with the particle size < 150 μm, as the electrode rotating speed increased, the mass percentage of the powder increased.

In order to intuitively analyze the change in the particle size, the variations in D10, D50, and D90 with the electrode rotating speeds are exhibited in Figure 5. It can be observed from the figure that with the increase in the electrode rotating speed, the particle size of the powder presented a downward tendency. As the speed of the electrode increased, the amount of fine powder increased, and the D50 of the powder became smaller (as shown in Figure 5a–c). However, for the powder sieved using a 45 µm sieve (as shown in Figure 5d), the particle size did not change significantly, because the powder was too fine.

The formation process of the powder prepared using the PREP was as follows [18,19,20,21]. Firstly, the molten liquid film on the terminal face of the TiAl electrode rod flows towards the edge of the rod under the effect of centrifugal force. However, the liquid film cannot be immediately broken and thrown out because of the surface tension. Therefore, a quasi-circular “crown” is formed. As the rod terminal face is continuously melted, the liquid films constantly enter the liquid crown under the centrifugal action, and finally the micro-droplets are formed. During the further melting process, the mass of the liquid crown keeps on increasing. When the centrifugal force surpasses the surface tension of the liquid crown, the micro-droplets are ejected from the liquid crown, and small droplets are created. Due to the effect of surface tension, the molten droplets become spheroidized and are rapidly cooled in an Ar gas atmosphere, and eventually solidify into spherical powder particles. Hence, the crucial condition for powder formation in the PREP is that the centrifugal force is equivalent to the surface tension, as in the following equation:

$$\frac{m{\omega }^{2}d}{2}=\sigma \pi d^{\prime }$$

(1)

where m represents the mass of the droplet, ω stands for the angular velocity of the TiAl electrode rod, d indicates the diameter of the rod, σ is the surface tension coefficient, and d’ refers to the diameter of the micro-droplet. Equation (2) is as follows:

$$m=\rho \pi D^3/6$$

(2)

where ρ and D, respectively, stand for the density and diameter of the droplet. Equation (3) is as follows:

$$\omega =2\pi n/60$$

(3)

where n is the rotating speed of the TiAl electrode rod. Equation (4) is as follows:

$$d^{\prime }=\eta D$$

(4)

where η is 0.8. Bringing Equations (2)–(4) into Equation (1), the diameter of the droplet can be calculated as follows:

$$D=\frac{29.59}{n}\sqrt{\frac{\sigma }{d\rho }}$$

(5)

where d = 30 mm = 0.03 m, so Equation (6) is as follows:

$$D=\frac{170.84}{n}\sqrt{\frac{\sigma }{\rho }}$$

(6)

The calculating formula for the particle size of the powder fabricated by the PREP is, as such, attained. It should be noted that the value computed by Equation (6) is the average particle size D50 of the powder. It can be perceived from Equation (6) that, when the physical quantity of the powder is fixed, the particle size of the powder is inversely related to the rotating speed of the electrode rod, namely that the higher the rotating speed, the smaller the particle size of the powder, as shown in Figure 4 and Figure 5.

3.1.2. Effect of the Current Intensity on the Particle Size of the Powder

The addictive manufacturing technology requires the particle size of the powder at the range of 45–150 μm [22]. As shown in Figure 4, when the electrode rotating speed increased, the mass percentage of the powder with the particle size of 45~150 μm increased. Therefore, the electrode rotating speed was set to 42,000 r/min, and the current intensities of the plasma generator were chosen as 650 A, 670 A, and 700 A. The influence of the current intensity on the particle size distribution of the powder is presented in Figure 6.

As can be seen from Figure 6, the particle size of the powder exhibited a normal distribution. When the current intensity increased from 650 A to 700 A, the mass percentage of the powder with a particle size of 45~75 μm increased from 37.5% to 48.1%, and the percentage of 75~105 μm powder increased from 21.9% to 26.3%. Meanwhile, the particle size distribution of the powder became narrower, and it was concentrated from 45 μm to 105 μm.

Figure 7 depicts the relationship between the particle size and the current intensity. It can be observed in Figure 7 that as the current intensity increased, the particle size of the powder presented a progressively increasing tendency. At the same time, the current intensity merely altered the width of the particle size distribution peak, without changing the D50 value.

The particle size and its distribution are mainly related to the melting speed and the thickness of the liquid film [23]. The thickness of the liquid film can be represented through Equation (7), as follows [24]:

$$\delta ={\left(\frac{6\mu Q}{\pi d^2\rho {\omega }^2}\right)}^{\frac{1}{3}}$$

(7)

where δ is the thickness of the liquid film, and the melting speed Q can be expressed by Equation (8) as follows:

$$Q=\frac{\alpha IU}{\rho \Delta H}$$

(8)

where α is thermal efficiency of the plasma gun (α = 0.35), I is the current intensity, U is the voltage (U = 55 V), and ΔH is the amount of heat required to heat a unit mass of an electrode rod from room temperature to melting point. From Equations (7) and (8), we can find that as the current intensity increases, the thickness of the liquid film increases. When the electrode rotating speed is fixed, the greater the current intensity, the thicker the film and the larger the powder particle size.

3.1.3. Effect of Electrode Rotating Speeds on the Sphericity Degree of the Powder

Figure 8, Figure 9 and Figure 10 are optical projection photos of the TiAl pre-alloyed powder at different rotating speeds. We can perceive from the figures that the powder with a particle size ranging from 45 µm to 150 µm was completely spherical (as shown in Figure 8b,c, Figure 9b,c, and Figure 10b,c). However, for the powder with the particle size > 150 µm, there were non-spherical particles due to the presence of an irregularly shaped liquid films, which were thrown off by centrifugal force at the initial stage of metal melting process (as shown in Figure 8a, Figure 9a, and Figure 10a).

The impact of electrode rotating speeds on the sphericity degree of TiAl pre-alloyed powder is presented in Figure 11. As can be seen from the figure, when the electrode rotating speed increased from 32,000 r/min to 42,000 r/min, the sphericity degree of the powder changed only slightly. In previous studies, electrode rotating speed was the most important process parameter, which affected the sphericity degree of the titanium pre-alloyed powder prepared by the PREP [25]. With the increase in the electrode rotating speed, the centrifugal force experienced by the liquid metal increased, and the droplets were more likely to form a spherical shape during the flight cooling process [26]. Therefore, the sphericity degree of the powder was directly proportional to the electrode rotating speed. At the same time, when the electrode rotating speed was identical, the smaller the particle size, the higher the degree of sphericity [27,28,29]. However, the sphericity degree of the powder prepared in this work was above 88%, and the electrode rotating speed had a negligible impact on the degree of sphericity of the powder. This is mainly associated with the low density of the TiAl alloy. Although increasing the electrode rotating speed could reduce the liquid film thickness of the molten electrode rod, the particle size of the powder was large (as shown in Figure 4). As such, the influence of the electrode rotating speed on the degree of sphericity was not distinct. As shown in Figure 11, the sphericity degree of the powder with a particle size from 45 µm to 150 µm could reach more than 98%.

3.1.4. Effect of Current Intensity on the Sphericity Degree of the Powder

Figure 12, Figure 13 and Figure 14 are optical projection photos of the TiAl pre-alloyed powder prepared with different current intensities. It can be observed from the figures that as the current intensity increased, the shape of the powder did not alter significantly, as most powders were uniformly spherical. However, irregularly shaped particles existed in the powder with a particle size > 150 µm (as shown in Figure 13a and Figure 14a). This is because in the early stage of the preparation, the electrode rotating speed was low, resulting in the small centrifugal force of the liquid films.

The influence of current intensity on the degree of sphericity of TiAl pre-alloy powder is presented in Figure 15. It can be perceived from the figure that the degree of sphericity of the powder with the particle sizes >150 µm and 105 µm to 150 µm increased significantly as the current intensity rose. Nevertheless, the degree of sphericity of the powder with the particle sizes of 45 µm to 105 µm and <45 µm was not notably affected by the current intensity. This is because, for the powder with a larger particle size (>105 µm), the higher the current intensity, the higher the temperature of the liquid metal, and the more thorough the melting process. At the same time, as the current intensity increased, the fluidity of the liquid metal increased, and the viscosity decreased. Under the combined effect of centrifugal force and surface tension, it more easily solidified into spherical particles, thereby enhancing the degree of sphericity of the powder. For the powder with the particle size < 45 µm, due to the small particle size of the powder, the mass of the initial metal droplets was small, so the centrifugal force on the droplets was small. Hence, the degree of sphericity of the powder was mainly determined by the surface tension of the liquid metal, which was not greatly affected by the current intensity. It can be seen from the figure that the degree of sphericity of the powder with the particle size of 45 µm to 150 µm was larger than 90%, which could meet the requirements for the subsequent AM of TiAl alloy.

3.2. Effects of PREP Parameters on the Microstructure of TiAl Pre-Alloyed Powder

3.2.1. Phase Constitution

Figure 16 presents the XRD diffraction patterns of the TiAl pre-alloyed powder prepared under different PREP conditions. It can be noticed in the figure that the phase configuration of the powder consisted of the γ phase and the α₂ phase. When sieving conditions were the same, the diffraction peaks of the powder prepared at different electrode rotating speeds and current intensity were basically the same.

3.2.2. Microstructure

Figure 17 reflects the effect of electrode rotating speeds on the surface morphology of the TiAl pre-alloyed powder. It can be observed from the figure that the particles show a completely spherical shape. The irregular particles and satellite-shaped particles that frequently occur in the GA process were hardly seen. At the same time, no porosity within the inner particles was detected. Moreover, it can be found that the greater the rotating speed, the smaller the powder particle size, and the more uniform the powder particle size. However, with the increase in the rotating speed, the surface morphology changed only slightly. As the rotating speed increased, the particle size decreased, and the surface morphology of the powder changed greatly, from coarse to smooth.

Figure 18 shows the effect of current intensity on the surface morphology of the TiAl pre-alloyed powder. It can be seen from the figure that as the current intensity increased, the particle size of the powder became smaller, and the surface morphology of the powder became smoother.

Figure 19 and Figure 20 show the effect of the electrode rotating speeds and current intensity on the cross-section microstructure of TiAl pre-alloyed powder, respectively. We can see from the figures that when the particle size was the same, the effect of rotating speeds and current intensity on the internal microstructure of the particles was not obvious. Two kinds of internal microstructures were observed: the dendritic + cellular structure and the microcrystalline structure. In addition, it seems that the internal microstructure of the particles was related to the particle size of the powder. For particles with the size of 45~150 μm, the internal microstructure was a dendritic + cellular structure (as shown in Figure 19a–c and Figure 20a–c). However, for particles with the size < 45 μm, the internal microstructure was a microcrystalline structure (as shown in Figure 19d–f and Figure 20d–f).

Figure 17, Figure 18, Figure 19 and Figure 20 reflect that the surface morphology and internal microstructure of the particles are mainly related to the particle size of the powder. Consequently, it is necessary to explore the connection between the particle size and the surface morphology of the powder. Figure 21 presents the morphologies of the powder with diverse particle sizes (42,000 r/min, 670 A). Two sorts of surface morphologies were witnessed in Figure 21b,e,h: the coarse configuration (Figure 21b,e) and the smooth configuration (Figure 21h). By comparing Figure 21b,e,h, it can also be discovered that the variance in the surface morphologies of the powder was related to the particle size: particles with the size > 45 µm generally showed a coarse structure, while the finer particles (<45 µm) presented a smooth structure. Additionally, there were notable differences in the internal microstructures of the particles, which was also associated with the particle size of the powder (as depicted in Figure 21c,f,i). For particles with sizes ranging from 45 µm to 150 µm, the internal microstructure was a dendritic + cellular structure (as shown in Figure 21c,f). However, for particles with the size < 45 µm, the internal microstructure was a microcrystalline structure (as shown in Figure 21i).

The surface morphology and internal microstructure of TiAl pre-alloyed particles are governed by the cooling rate when the droplets solidify [30]. For particles with the size of 105~150 μm, the cooling rate was slower, resulting in a smaller undercooling, and a lower nucleation rate, which allowed the grains to have enough time to grow and formed the dendritic + cellular structures (as shown in Figure 21c). As the cooling rate increased, the undercooling became larger, and the nucleation rate was higher. There was not sufficient time for the growth of grains. The number of dendritic structures decreased, while the number of cellular structures increased (as shown in Figure 21f). As the cooling rate further increased, the undercooling and nucleation rate also increased, which caused the grains to solidify before growth. Therefore, the independently dispersed microcrystalline structure inside the particles was formed (as shown in Figure 21i).

Figure 22 shows the effect of electrode rotating speeds on the element distribution of the TiAl pre-alloyed powder. As shown in the figure, when the rotating speed was low (32,000 r/min), there was segregation of the elements (as shown in Figure 22a,b). As the rotating speed increased (39,000 r/min and 42,000 r/min), the alloy elements were uniformly distributed within the material without any segregation, and Al element segregation could not be observed (as shown in Figure 22c–f). The element distribution is related to the cooling rate during the solidification of the droplets. When the rotating speed was low, the cooling rate was slow, which gave the alloy elements sufficient time for diffusion. However, due to the different solute diffusion rates of each element, microscopic segregation occurred after solidification, resulting in the uneven distribution of alloy elements (as shown in Figure 22a,b). With the increase in the rotating speed, the cooling rate increased, as well as the composition undercooling. The growth rate of the solid–liquid interface was significantly greater than the solute diffusion rate. The solute elements were partially intercepted to form a solid solution with a large solid solubility and a high lattice distortion. Therefore, the degree of element segregation was also low, and it was difficult to observe (as shown in Figure 22c–f).

Table 3. Effect of electrode rotating speeds on the element content of TiAl pre-alloyed powder (at.%).

Element	32,000 r/min	39,000 r/min	42,000 r/min
Ti	47.84	48.07	47.93
Al	48.16	47.98	48.08
Cr	1.96	1.94	1.97
Nb	2.04	2.01	2.02
Total	100.00	100.00	100.00

presents the influence of electrode rotating speeds on the element contents of the TiAl pre-alloyed powder. It can be observed from Table 3 that the electrode rotating speed had a negligible impact on the content of alloy elements in the powder. At the same time, there was no loss of alloy element contents of the powder during the PREP, which met the Chinese national standard for the spherical titanium–aluminum powder (YS/T 1296-2019) [31]. The stable contents of the alloy elements in the powder are the premise of the subsequent AM, which ensures the excellent performance of the final formed parts.

Figure 23 shows the effect of current intensity on the element distribution of the TiAl pre-alloyed powder. It can be perceived from the figure that as the current intensity increased, the distribution of various alloying elements was uniform, and no element segregation occurred.

Table 4. Effect of current intensity on the element content of TiAl pre-alloyed powder (at.%).

Element	650 A	670 A	700 A
Ti	48.26	47.68	48.09
Al	47.8	48.32	47.87
Cr	2.01	1.99	1.94
Nb	1.93	2.01	2.10
Total	100.00	100.00	100.00

indicates the impact of current intensity on the element contents of the TiAl pre-alloyed powder. It can be noted from Table 4 that the current intensity had little effect on the content of alloy elements of the powder. In addition, there was no loss of alloy element content in the powder during the PREP, which met the Chinese national standard for the spherical titanium–aluminum powder (YS/T 1296-2019) [31].

The TEM photographs of the powder are shown in Figure 24. As shown in Figure 24a,c, there were two kinds of microstructure inside the powder: a lamellar microstructure in dendritic + cellular particles (particle size > 45 µm) and a network microstructure in microcrystalline particles (particle size < 45 µm), which was in line with the outcomes of the SEM images (as presented in Figure 21). The average thickness of the α₂ + γ lamellar inside the powder was about 200 nm. Some fine lamellar γ phases were arranged in an orderly fashion in the α₂ phase matrix with a thickness of about 20 nm (Figure 24a). The HRTEM photograph of the γ/α₂ interface is shown in Figure 24b. The network microstructure within the powder was wavy, and the morphology of the γ phase was not distinct (Figure 24c). Furthermore, a great quantity of dislocations in phase α₂ were formed because of the anisotropy on the crystal growth velocities that was also observed inside the dendritic + cellular particles (Figure 24d).

It can be seen from the above research results that the microstructure of the TiAl pre-alloyed powder prepared by the PREP is mainly related to the particle size, whose essence is thermodynamics and the kinetics of solidification. The solidification path and microstructure of the TiAl pre-alloyed powder are mainly reliant on the content of the Al element (as presented in the Ti-Al binary phase diagram) and the cooling rate.

The kinetics of solidification are mainly reflected by the cooling rate (ʋ). The relationship between average cooling rate (ʋ) and particle size (d) can be calculated by the following Equation (9) [32,33]:

$$\upsilon =6h\left(\frac{T_m-T_a}{\rho dC}\right)$$

(9)

where T_m and T_a represent the material temperature and ambient temperature, respectively. ρ is the material density, d is the particle size, C is the specific heat capacity of the material, and h is the heat transfer coefficient. h can be calculated by using the following Equation (10):

$$h=\frac{2K}{d}+0.6{\left(\frac{K^4{\rho }_g^3{C}_g}{y}\right)}^{\frac{1}{6}}\sqrt{\frac{u}{d}}$$

(10)

where K is the thermal conductivity of Ar gas, ρ_g is the density of Ar gas, C_g is the specific heat capacity of Ar gas, y is the viscosity of Ar gas, and u is the linear velocity of the electrode rod. The physical parameters of Ar gas are listed in

Table 5. Physical parameters of Ar gas [34].

Physical Parameter	Numerical Value
K/J·(cm·K·s)−1	3.55 × 10−4
ρg/g·cm−3	9.7 × 10−4
Cg/J·(g·K)−1	0.521
y/g·(cm·s)−1	4.62 × 10−4

.

In this work, the diameter of the electrode rod was 30 mm, and the rotating speeds were 32,000 r/min, 37,000 r/min, and 42,000 r/min, respectively. Therefore, u is calculated to be 5.03 × 10³, 5.81 × 10³, and 6.60 × 10³, respectively. Putting the above values of u into Equations (10) allows us to obtain the following Equation (11):

$$\{\begin{array}{l}h=7.1\times {10}^{-4}{d}^{-1}+192.34\times {10}^{-4}{d}^{-\frac{1}{2}}(\mathrm{32,000} \mathrm{r}/\min )\\ h=7.1\times {10}^{-4}{d}^{-1}+206.54\times {10}^{-4}{d}^{-\frac{1}{2}}(\mathrm{37,000} \mathrm{r}/\min )\\ h=7.1\times {10}^{-4}{d}^{-1}+220.32\times {10}^{-4}{d}^{-\frac{1}{2}}(\mathrm{42,000} \mathrm{r}/\min )\end{array}$$

(11)

The material density ρ is 4.2 g/cm³, and the specific heat capacity of the material C is 0.61 J/(g·K)⁻¹ [35]. Putting the above values and Equation (11) into Equation (9) results in the following Equation (12):

$$\{\begin{array}{l}\upsilon =2.4d^{-2}+65.09d^{-\frac{2}{3}}(\mathrm{32,000} \mathrm{r}/\min )\\ \upsilon =2.4d^{-2}+69.89d^{-\frac{2}{3}}(\mathrm{37,000} \mathrm{r}/\min )\\ \upsilon =2.4d^{-2}+74.56d^{-\frac{2}{3}}(\mathrm{42,000} \mathrm{r}/\min )\end{array}$$

(12)

It is apparent that there is a definite relationship between ν and d: the cooling rate is inversely proportional to the powder particle size. In this work, the particle size varies from 20 μm to 250 μm, and, thus, the cooling rate in the current condition is within the range of 3.84 × 10⁴ K·s⁻¹~1.20 × 10⁶ K·s⁻¹.

It can be observed from the Ti-Al binary phase diagram that there are two mechanisms for the formation of TiAl intermetallic compounds with about 48 at. % Al: (1) For the powder with a larger particle size, the cooling rate is lower, the composition undercooling is smaller, and the solidification rate is slower. The phase transition path is $\alpha \to \alpha +\gamma \to {\alpha }_2+\gamma$ [36]. (2) For the powder with a smaller particle size, the cooling rate is higher, and the solidification rate is faster. The high temperature phase α is directly ordered to become α₂ phase, accompanied by a small amount of γ phase precipitation. The phase transition path is $\alpha \to {\alpha }_2\to {\alpha }_2+\gamma$. In this work, when the cooling rate was 1.19 × 10⁵ K·s⁻¹ (particle size > 45 μm), the molten droplets were inclined to form coarse dendritic + cellular particles with an α₂ + γ lamellar microstructure, while some droplets tended to form smooth microcrystalline particles with an α₂ + γ network microstructure when the cooling rate was >1.19 × 10⁵ K·s⁻¹ (particle size < 45 μm).

4. Conclusions

(1)

The electrode rotating speed mainly affected the D50 of the powder. As the electrode rotating speed increased, the D50 of the powder decreased. The current intensity mainly affected the particle size distribution of the powder. As the current intensity decreased, the particle size distribution of the powder became concentrated, but the D50 changed only slightly.

(2)

The electrode rotating speed had little effect on the sphericity degree of the powder. The current intensity had a significant effect on the sphericity degree of the powder with a particle size > 105 μm, but it had little effect on the powder with a particle size < 105 μm.

(3)

There are two types of surface morphologies of TiAl powder fabricated by the PREP, which were decided by the solidification rate of molten droplets during the PREP. When the cooling rate was less than 1.19 × 105 K·s⁻¹ (particle size > 45 μm), the molten droplets were apt to form coarse dendritic + cellular particles, while some droplets tended to take the form of smooth microcrystalline particles when the cooling rate was greater than 1.19 × 105 K·s⁻¹ (particle size < 45 μm).

(4)

The powder was chiefly constituted by the α₂ phase and the γ phase. There were two kinds of microstructure inside the TiAl pre-powder, which were the α₂ + γ lamellar microstructure (particle size > 45 µm), and the α₂ + γ network microstructure (particle size < 45 µm). The average thickness of the α₂ + γ lamellar was approximately 200 nm. The lamellar γ phases were arranged in an orderly fashion in the α₂ phase matrix with a thickness of about 20 nm. The network microstructure inside the powder was corrugated, and the morphology of the γ phase was not obvious. Furthermore, a large quantity of dislocations which formed because of the anisotropy in the crystal growth velocities were also noticed within the powder.