Spherical TiAl Alloy Powders by PREP: Preparation, Microstructure and Properties

TiAl alloy powders were prepared by plasma rotate electrode pulverization (PREP) with high speed. The effect of preparation process on the microstructure and properties of powders was studied. The results showed that the titanium aluminum powder prepared by PREP method was solid and spherical without satellite powder. The powders had two morphologies: cracked surface and smooth surface. The relationship between particle size and surface morphology was as follows: the smaller the particle size, the smoother the surface. Ti, Nb and Gr elements were uniformly distributed, while Al elements were segregated, but no alloy element loss occurred. The powderswere mainly composed of γ-TiAl and α2-Ti3Al phase and contained a small amount of NbCr2 phase. The average thickness of laminates of α 2+γ was about 0.2μm. Thelamellar thickness of γ phase was about 20nm. The long axis of NbCr2 phase was 200nm, and the short axis was 50 ~ 100nm. The higher the rotating speed of the electrode, the smaller the particle size and the higher the sphericity of the powders. With the increase of plasma generator current, the particle size decreased and the sphericity increased. The yield of - 100 mesh ~ + 325 mesh powders for EBSM molding could reach more than 80.68 %, when42000r / min and 670A process parameters were adopted.

In this paper, high-speed PREP technology was used to prepare high-quality titanium aluminum alloy powders for additive manufacturing.The effects of preparation process on the microstructure and properties of TiAl powders were studied.Subsequently, the prepared powders were used for EBSM.

Materials
The titanium-aluminum alloy prepared by vacuum arc melting (VAR) method was processed into a raw material electrode with a diameter of 30mm and a length of 150mm by machining method.The microstructure of the raw material electrode is shown in Figure1.It is a near lamellar structure composed of α2-Ti3Al phase and a small amount of γ-TiAl phase, and contains a small amount of pore defects.The element content is shown in Table 1.
TiAl alloy powders were prepared by SL-ZFDW high-speed PREP equipment.The rotating speed of the electrode was 32000 ~50000r/min, and the current intensity of the plasma generator was 650 700A.The prepared alloy powders were sieved by a vibrating screen under vacuum, and the mesh numbers were 150,200 and 325 meshes, respectively.

Experimental methods
The phase structure of the powder was analyzed by X Pert3 Powder X-ray diffractometer.The powder morphology was observed by SU5000 field emission SEM.The powder samples were prepared by TESCAN AMBER FIB-SEM, and the microstructure of the powders were observed by JEM-2100 TEM.The particle size distribution and sphericity of the powders were analyzed by Morphologi 4 automatic particle size analyzer.The apparent density and fluidity of powders were measured by Scott volumeterand Hall flowmeter.

Effect of PREP process on particle size distribution and yield of powders
Titanium aluminum alloy powders were prepared by high-speed PREP process.When the current was 650A, the effect of electrode rotation speed on the average particle size of the alloy powderswas shown in Figure 2a.It can be seen from the figure that with the increase of electrode rotation speed, the particle size of the powders shows a downward trend.Figure 2b is the effect of electrode rotation speed on the yield of powders with different screening grades.From the diagram, it can be seen that with the increase of electrode rotation speed, the powder yield of-150 ~+ 325 mesh and-100 ~+ 150 mesh powders increase, and the relative + 100 mesh coarse powder gradually decreases.However, the electrode rotation speed has little effect on the yield of-325 mesh fine powders.In this study, the highspeed PREP method was used to prepare titanium-aluminum alloy powder, which was mainly used for electron beam printing.The particle size of the powder was -100 mesh to + 325 mesh.It can be seen that the powder yield of the milling process used in this study is more than 62.50 %.When the milling speed is 42000r /min, the yield of -100 ~+ 325 mesh powder is more than 80.68 %.Figure3 is the change of powder particle size and powder yield with current when the electrode rotation speed is 42000r / min.From Figure 3a, it can be seen that with the increase of current intensity, the particle size of the powder shows a gradual upward trend.However, it can also be found that the current intensity only changes the width of the particle size distribution peak, but almost does not change the D50 value.It can be seen from Figure 3b that with the increase of current intensity, the yield of-325 mesh fine powder and + 100 mesh coarse powder increased.The other two screening grade powder yield has little effect.In the process of PREP, the higher the current intensity, the greater the electrode melting speed, the greater the proportion of small droplets and large droplets formed by the fracture of the liquid line, resulting in the increase of the proportion of fine powder and coarse powders.Figure5 is the morphologies of Ti-Al alloy powders prepared by PREP method.It can be seen from the figure that the powders are solid spherical, the surface is smooth, and there is basically no satellite ball, indicating that the powder will have good fluidity.It can be seen from Figure 5a, b and c that the higher the rotation speed, the more uniform the particles size of the powders, and the higher sphericity of the single powder.It can be seen from Figure 6c, 6f and 6i that the higher the current intensity, the smaller the powder diameter, and the smoother the surface.However, increasing the rotation speed has little effect on the surface morphology of the powders.The surface of the larger size powder particles graded by -100 ~+ 150 mesh sieve is dominated by finer dendrites (Figure 5a), with obvious secondary crystal axis.The sur-face of the medium-sized powder particles of -150 ~+ 325 mesh is equiaxed cellular crystal, and the interior of each cellular crystal is uniform and fine microcrystalline structure (Figure 6d).When the particle size is further reduced to-325 mesh sieve, the cellular crystal disappears, all of which are uniform microcrystalline structures, and spherical particles with smooth surface will be formed under the action of surface tension (Figure g).As the size of the powder particles decreases, the solidification structure of the powder particles gradually changes from dendrites to grown cellular crystal structure (Figure 5e), and finally to microcrystalline structure (Figure 5h).The microstructure is obviously refined, and the multi-point nucleation characteristics inside the powder particles can be found.Figure6 is the EDS analysis results of TiAl powder prepared by PREP method.It can be seen from the diagram that Ti, Nb and Gr in the powders are relatively uniform, while Al element has segregation phenomenon.According to the distribution of Al element, the alloy phase composition at different positions in the powder can be determined, as shown in Figure 6b.Table 2 is the chemical composition of titanium aluminum alloy powders prepared by different processes.Comparing the composition of the raw alloy rods in Table 1, it can be seen that no volatilization loss of alloying elements occurred under the current intensity used in this study.The results of TEM inside the powders are shown in Figure7.It can be seen from the figure that the average thickness of the α2 + γ lamellaes inside TiAl powders is about 0.2μm.The size of the γlamellar structure is very small, about 20nm, as shown in Figure 7a.Figure7c is the TEM of the network structure inside the powder, which is irregular.The selected area diffraction proves that there are more γ phases in the alloy structure in the observation area, but the morphology of the γ phase is not obvious.At the same time, the diffraction spots show that there is a twin structure of the γ phase.
In addition, there are a large number of linear dislocations inside the powders, and the dislocations are entangled and form a dislocation wall, as shown in Figure 6d. Figure 8 shows the morphology and structure analysis of the precipitated phase inside TiAl powders.It can be seen from Figure 8a that the precipitated phase is irregularly shaped inside the powder, with a long axis of 200nm and a short axis of 50-100nm.The interface is stepped and well bonded with the matrix alloy .The energy spectrum analysis shows that the precipitated phase mainly contains three elements of Ti, Cr and Nb.Further electron diffraction analysis showed that the precipitated phase in the titanium aluminum alloy powder was Laves phase NbCr2.

Effect of PREP process on powder properties
Figure9 analyzes the influence of PREP preparation process on the apparent density of powders.It can be seen from the figure that as the electrode speed and current intensity increase, the apparent density of the powders increases.The higher the rotational speed, the stronger the current, and the smaller the powder particle size, the higher the apparent density.

Conclusions
(1) TiAl powders prepared by PREP method was solid spherical without satellite powder.The powder had two morphologies of cracked surface and smooth surface.The smaller the particle size of the powders, the smoother the surfaces.Ti, Nb and Gr elements were evenly distributed, while Al element segregated, but no alloy element loss occurred.
(2) The powderswere mainly composed of γ-TiAl and α2-Ti3Al phases, and contained a small amount of NbCr2 phase.The average thickness of α2 + γ lamellae was about 0.2 μm.The thickness of γ layer was about 20 nm.The long axis of NbCr2 phase was 200 nm, and the short axis was 50 ~100 nm.
(3) The higher the rotation speed of the electrode, the smaller the particle size of the powders and the higher the sphericity.With the increase ofcurrent intensity of the plasma generator, the particle size of the powders decreased and the sphericity increased.The yield of -100 mesh ~+ 325 mesh powders for EBSM molding could reach more than 80.68 %, when42000r / min and 670A process parameters were adopted.

Figure 2 .
Figure 2. Effect of electrode speed on powder particle size and productivities (a) particle Size, (b) productivities.

Figure 3 .
Figure 3.Effect of current intensity on powder particle size and productivities (a)particle size, (b) productivities3.2.The effect of PREP process on the microstructure of the powdersFigure4 is the XRD analysis results of powders prepared by different processes.From the diagram, it can be seen that the diffraction peaks of TiAl powders prepared by PREP method with different electrode rotation speeds and different plasma currents are almost the same when the powder

Figure 9 .
Figure 9.The effect of process parameters on powder bulk density (a)electrode speed，(b)current intensity.

Figure 10reflects the influence
Figure10reflectsthe influence of electrode rotation speed and plasma generator current intensity on fluidity of TiAl powders by PREP method.It can be seen from the figure that as the rotation speed of the electrode rod increases, the sphericity of the powders increases, resulting in better fluidity.At the same time, the smaller the particle size of the powders, the worse the fluidity.However, it can also be found that the increase of the current intensity of the plasma generator has little effect on improving the powder fluidity.

Figure 10 .
Figure 10.The effect of process parameters on powder fluidity.(a)electrode speed，(b)current intensity

Table 2 .
The Effect of current tensity on thechemical composition of TiAl alloy powder