Facile synthesis of nano-particles attached spherical Ti-6Al-4V powder based on plasma spheroidization
Release time:
2025-06-06
1. Introduction
Spherical powders were preferred for three dimensional (3D) printing [1–5], thermo spraying [6,7] and spark plasma sintering [8–10] due to its better flow ability and higher apparent density than irregular powders. Nevertheless, there are only a limited number of alloys available for 3D printing even with spherical powders [11,12]. Majority of alloys used today cannot be printed because the intolerable microstructures are formed during printing [13,14]. All these limitations hinder further development of 3D printing.
Recently, introducing secondary particles into spherical powder has been a promising way for 3D printing to get out of the dilemma [15–20]. Originally, nano-particles attached spherical alloy powders were synthesized with prefabricated nano-particles and spherical powders by ball-milling [18–20]. However, it is difficult to achieve uniform istribution of nano-particles throughout the powder surface due to the tendency of nano-particles to agglomerate. This resulted in microstructural inhomogeneity and mechanical performances decline of printed parts [21–23]. Electroless plating has been proved effective to improve the distribution uniformity of nano-particles [24–26]. Nevertheless, lectroless plating is costly and environmentally unfriendly because noble catalysts are indispensable for efficient reactions [27,28]. Subsequently, in order to solve the above problems, Zhang et al. [29,30] prepared core-shell structured powder by fluidized bed chemical vapor deposition (FBCVD). However, defluidization caused by large particle aggregates was the major barrier to deposit composite powders with high nanoparticles content [31]. Besides, the coating processes of all above methods are separate from spheroidization process, which makes them time- and energy-consuming. It is thus of technical and synthesis of nano-partjcles attached spherical alloy powders.
Plasma spheroidization is an efficient method to prepare spherical powders with narrow size distribution, high-purity and good flow ability [32-36]. Many years ago, it was noted that a large number of small grains would form on the surface of tungsten powder after plasma pheroidization [37], which showed the potential of plasma spheroidization to produce nano-particles attached spherical powders. However, only tungsten nano-particles attached spherical powder has been achieved so far [38]. Moreover, its synthesis mainly depended on manipulating feedstock and plasma gas composition. Therefore, a facile method for synthesis of nano particles attached spherical alloy powders was established based on plasma spheroidization in present study. With this method, nano-particles attached spherical powders can be achieved by just controlling plasma gas composition. Ti-6Al-4V powder which has been widely used in additive manufacturing was taken as an example in present work. Microstructure and phase composition of nano-particles attached spherical Ti-6Al-4V powder were also investigated.
2. Materials, samples preparation and characterization
Commercial hydride–dehydride (HDH)Ti-6Al-4V powder(Western Baode Technologies Co:,Ltd.,China)were used as feedstock, and the chemicalcomposition ofHDHTi-6Al–4V powdeΓ are listed in Table 1.Equipment used for synthesis of nano-particles attached spherical Ti-6Al4V powdeΓis shown in Fig.1a…The synthesis pΓocess is similar to the spheroidization of metal powders which has been reported indetail elsewhere [39]. The key points for this Frocess aΓe adding a certainamount ofhydrogen (puIity≥99.999%,YuanzhengTechnology)
into carrier gas argon (purity≥99.999%, Yumeng Technology) and using a high-temperature area extended tube. Moreover, nitrogen (purity≥99.999%, Yumeng Technology) was used as quenching gas to accelerate the cooling process. Detailed process parameters for synthesis of nano-particles attached spherical Ti-6Al-4V powder are shown in Table 2.
In order to clarify the struct In order to clarify the structure of nano-particles attached spherical Ti-6Al-4V powder, the nano-particles and spherical powder were separated. The separation process is shown in Fig. 1b. First, the nano-particles attached spherical Ti-6Al-4V powder was dispersed in ethanol, assisted by ultrasonic treatment for 15 min. Then, the suspension was centrifuged at 3000 rpm for 10 min, thus spherical Ti-6Al-4V powder sank to the bottom of centrifuge tube while the nano-particles suspended in the upper layer. Finally, the spherical Ti-6Al-4V powder and nano-particles were collected and dried in a vacuum oven at 50 °C for 12 h, respectively. Morphologies of the HDH Ti-6Al-4V powder, spheroidized Ti-6Al-4V powder, spherical Ti-6Al-4V powder and separated nanoparticles were observed with a field emission scanning electron microscope (FESEM, Quanta FEG 250, FEI) and a transmission electron microscope (TEM, Tecnai G2 F20, FEI). Phase compositions of all these powders were investigated via an X-ray diffractometer (XRD,PANalytical, Empyrea) with Cu Kα radiation at 45 kV and 40 mA. Particle size distribution of the spherical Ti-6Al-4V powder and separated nano-particles were evaluated with a laser particle size analyzer (Winner 2308, Ji’nan Winner Particle Instruments) and a laser particle size & zeta potential analyzer (Zetasizer Nano ZS90, Malvern Instruments), respectively
3. Results and discussions
Fig. 2a and b show morphologies of the HDH Ti-6Al-4V powder and spheroidized Ti-6Al-4V powder, respectively. It is seen that the angular Ti-6Al-4V particles transformed into spherical with a continuous layer on its surface after plasma spheroidization. Fig. 2c and d show high magnification images of nano-particles attached spherical Ti-6Al-4V powder, it can be observed that the layer is composed of numerous of fine particles. In order to clarify the structure of fine particles, they were separated from the spheroidized Ti-6Al-4V powder. Morphologies and particle size distribution of the separated fine particles are shown in Fig. 2e and i, respectively. It can be seen that the fine particles are mainly composed of spherical nano-particles with uniform particle size. Moreover, average particle size of the nano-particles is 85.45 nm. Morphology of a separated spherical Ti-6Al-4V powder is shown in Fig. 2f. It is obvious that the surface of separated spherical Ti-6Al-4V powder is smoother than the nano-particles attached spherical Ti-6Al-4V powder. Therefore, the separation process could also be used to remove the fine particles generated during plasma spheroidization. Particle size distribution of the spherical Ti-6Al-4V powder is shown in Fig. 2i. Comparing with the angular HDH Ti-6Al-4V powder (Fig. 2g), its particle size is much smaller and the distribution is also narrower. The average particle size of angular HDH Ti-6Al-4V powder and spherical Ti-6Al-4V powder are 111.26 µm and 70.14 µm, respectively. Rapid reduction in particle size indicates severe vaporization of molten droplets during plasma spheroidization. The severe vaporization could provide enough materials to deposit on the spherical powder during subsequent cooling process.
XRD patterns of the HDH Ti-6Al-4V powder, spheroidized Ti-6Al-4V powder, spherical Ti-6Al-4V powder and separated nanoparticles are shown in Fig. 3. It is seen that diffraction peaks of the HDH Ti-6Al-4V powder and spheroidized Ti-6Al-4V powder are similar, which indicates that phase composition of the major powders has no significant change before and after plasma spheroidization. However, the full width at half maximum (FWHM) of the diffraction peaks of spheroidized Ti-6Al-4V powder is larger, which indicates that the crystallinity of Ti-6Al-4V powder decreases after plasma spheroidization. Therefore, it can be deduced that the molten part of Ti-6Al-4V powder became amorphous due to the rapid cooling process during plasma spheroidization. It also can be seen that the separated nano-particles are composed of AlTi, AlTi3, AlTi2N and TiN, which is significantly different from the spherical Ti-6Al-4V powder in composition. It can be deduced that part of the metal vapor condensed into AlTi and AlTi3, while the other part reacted with nitrogen and formed AlTi2N and TiN in quenching chamber during cooling process.
Fig. 4 shows TEM images of the separated nano-particles. It is seen that the separated nano-particle is also spherical. Moreover, the particle size of separated nano-particles observed with TEM is consistent with that evaluated with the laser particle size analyzer. Fig. 4b shows a high magnification image of a single separated nanoparticle. This nano-particle is identified to be AlTi3 according to its high resolution TEM (HRTEM) image and corresponding selected area electron diffraction (SAED) pattern (Fig. 4c). Fig. 4b shows another separated nano-particle with a high crystallinity core (hexagonal close-packed structure) and an amorphous shell. It can be seen from Fig. 4e that thickness of the amorphous shell is about 7 nm. This particle is identified to be AlTi2N according to its HRTEM image and corresponding SAED pattern (Fig. 4f).
Synthesis of the nano-particles attached spherical alloy powders in this work could be explained as follows: After being fed into plasma spheroidization system, the angular HDH Ti-6Al-4V powde particles melted rapidly under high temperature and transformed into spherical droplets due to surface tension. When the droplets fell out of high temperature area, they solidified into spherical Ti-6Al-4V particles immediately due to extremely high cooling rate. At the same time of melting, powder particles partially vaporized on the surface, and a ertain amount of metal vapor was produced. When metal vapor contacted the quenching gas nitrogen bellow, they reacted and formed metal nitride. Under the effect of quenching gas, the supersaturation of metal nitride and rest metal vapor was high enough, leading to the formation of nano-particles through homogeneous nucleation. Subsequently, these nano-particles attached on the surface of spherical Ti-6Al-4V powder by electrostatic attraction.
According to the XRD patterns (Fig. 3), phase composition of the nano-particles is completely different from the feedstock powder and spherical Ti-6Al-4V powder, which indicates that plasma spheroidization has led to the formation of new phase. Results of XRD and TEM show that metal nitride nanoparticles were formed. This is attributed to the reaction of metal vapor and nitrogen at high temperature, as well as subsequent rapid nucleation and condensation. Fig. S1 (available as Supplementary Material) shows the variation in ibbs free energy changes of reactions for formation of metal nitride. It can be seen that the minimum Gibbs free energy change is for formation of titanium nitride among metal nitride. This is preferred for formation of titanium nitride, which is consistent to the result of XRD. However, titanium hydride was not detected although a certain amount of hydrogen was added into the carrier gas. As shown in Fig. S1, the Gibbs free energy change for formation of titanium hydride is greater than zero when the temperature is above 1000 K. That means the reaction would not happen spontaneously.
On the other hand, the Gibbs free energy change for formation of titanium hydride is greater than that of metal nitride, which means titanium hydride is not preferred to form in the presence of nitrogen. In addition, the absence of titanium hydride is also consistent with the predominance diagram for Ti-H-N system (Fig. S2, available as Supplementary Material). As shown in Fig. S2, titanium hydride would be formed at hydrogen partial pressure of 25 kPa (equal to the chamber pressure), as well as nitrogen partial pressure of 1 × 10−4 Pa.
However, it is almost impossible to realize due to the extreme conditions. As shown in Fig. S1, the reaction for formation of titanium oxide has the highest priority due to its minimum Gibbs free energy change. Nevertheless, the diffraction peak of titanium oxide has not been detected by XRD, which means almost no titanium oxide was formed. It attributes to the strict control of oxygen content in raw materials. On the other hand, hydrogen added in the carrier gas forms highly active radicals in plasma region, providing a reducing atmosphere for the system. In addition, the added nitrogen competes with oxygen which reduced the formation of titanium oxide. There are also titanium and intermetallic in the nano-particles. This is mainly due to the rapid decrease of temperature and resulted slow reaction rate when metal vapor contacted with nitrogen in quenching chamber. Therefore, a certain amount of metal retained and condensed into metal nanoparticles. Metal nitride anoparticles are expected to improve the properties of 3D printing, thermal spraying and spark plasma sintering materials, but there are still many problems to be solved. For example, the size, dispersive state, composition and yield of nano-particles need to be controlled for the large-scale application of nanoparticles attached spherical Ti-6Al-4V powder. Based on the above analysis, the size, dispersive state, composition and yield of nano-particles could be tune by adjusting the key process parameters. The size and dispersive state of nano-particles depend on the interaction between plasma and powder particles as well as the interaction between nano-particles and spherical powders, such as heat transfer and collision probability. In detail, the size of nano-particles is positively correlated with the total vaporization of raw powder, and negatively correlated with the cooling rate. Thus, the size of nano-particles could be reduced by decreasing the powder feed rate, plasma power, hydrogen content of carrier gas, or increasing the quenching gas flow rate, and vice versa. For the dispersion situation of nano-particles, the higher powder vaporization ratio and collision probability between nano-particles and spherical powder could increase the thickness and uniformity of the nanoparticles layer. Therefore, the carrier gas flow rate should be lower, while the plasma power, hydrogen content of carrier gas and quenching gas flow rate should be higher for better dispersive situation of nano-particles. In addition, the yield of metal nitride could be changed by adjusting the way and amount of nitrogen addition. For example, the nitrogen will convert into excited molecular/atom state and possess high reactivity in the plasma region. Therefore, the yield of metal nitride could be increased by ejecting nitrogen into the reactor chamber as carrier gas.
4. Conclusions
To conclude, nano-particles attached spherical Ti-6Al-4V powder was in situ synthesized via a plasma spheroidization system by manipulating plasma gas composition. The nano-particles are uniformly distributed throughout the surface of spherical Ti-6Al-4V powder, which is ascribed to the vaporization and condensation of raw powder undergoing a steep variation of the temperature. The nano-particles are composed of AlTi, AlTi3, AlTi2N, and TiN with an average size of 85.45 nm. Further, the nano-particles attached spherical Ti-6Al-4V powder was synthesized by one-step which is time- and energy-saving compared with conventional synthesis processes. The present study demonstrated that plasma spheroidization is a simple and reliable method to synthesize nano-particles attached spherical alloy powder.
Paper citation information
Joumal of Alloys and Compounds 858 (2021)158313
About Stardust Technology
Stardust Technology (Guangdong) Co., Ltd. is a national high-tech enterprise specializing in the research, development, production and sales of high-end spherical powder materials for 3D printing, powder metallurgy, surface engineering and other fields. The company insists on taking radio frequency plasma spheroidization powder making technology as the core, and provides internationally advanced powder products and application solutions.
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