Spheroidization of a granulated molybdenum powder by radio frequency inductively coupled plasma
Release time:
2025-09-23
1. Introduction
Molybdenum is a refractory metal with many excellent properties,such as high hardness and strength, high melting point, good wear resistance and thermal shock resistance, which make it an attractive material for high temperature applications [1–3]. Spherical molybdenum powders have been widely used in thermal spray [4–6], liquid metal infiltration [7], powder injection molding [8] and additive manufacturing [9] due to its good flow properties and high apparent density.
Preparation of spherical powders has received increasing attention due to the rapid development of additive manufacturing because raw powder materials can greatly affect the as-built parts in additive manufacturing [10]. Gas atomization and the plasma rotating electrode process (PREP) are often used to fabricate spherical metal and alloy powders [11–14]. However, these methods have some shortcomings that are difficult to avoid, such as impurities and hollow particles in powders prepared by gas atomization and wide size distribution of powders prepared by PREP.
Radio frequency (RF) inductively coupled plasma spheroidization is a sufficient method to fabricate spherical powders with uniform composition, narrow size distribution, high degree of purity and good flowability [15]. Recently, it has been utilized to fabricate spherical metal, alloy and ceramic powders [16–19]. RF inductively coupled plasma is quite suitable for spheroidization of refractory metals due to its high temperature and high enthalpy [20–22]. Spherical molybdenum powders have been successfully prepared from irregular particles in some researches [7,23,24]. However, these researches mainly focused on influences of powder feed rate and flow rate of carrier gas on spheroidization percentage. Effects of plasma power and composition of plasma gases on the spheroidization process have been never reported. Another issue is that mass loss during the spheroidization process is up to approximately 20% due to evaporation and sticking on the chamber wall [23]. Moreover, the microstructure evolution and spheroidization mechanism of the powders during the spheroidization process are not yet clear.
Therefore, in this research, granulated molybdenum powders were used as raw materials to reduce mass loss and increase spheroidization efficiency during the spheroidization process. Spherical molybdenum powders were prepared at different plasma power. Moreover, hydrogen was added into the sheath gas to increase spheroidization efficiency and control particle size. Furthermore, typical morphologies during the spheroidization process were obtained. The microstructure evolution of molybdenum powder particles during spheroidization was further studied by adjusting the residence time of the powder in the plasma. A spheroidization mechanism of the granulated particle was also proposed.
2. Materials and methods
2.1. Preparation of granulated molybdenum powders
A granulated molybdenum powder was used as the raw material for the spheroidization process. The granulated molybdenum powder was prepared by spray granulation from commercially available reduction Mo powder(purity > 99.9%; Jinduicheng Molybdenum Co., LTD., China) with an average diameter of 5 μm. The reduction Mo powder was added into a 3 wt% polyvinyl alcohol solution under constant mechanical stirring to form a slurry. The slurry containg 55 wt% Mo was used to produce granulated Mo powder by a spray drier(YC-015,Shanghai Pilotech Instrument & Equipment Co., Ltd., China). Inlet and outlet temperature during the spray drying process are 210 °C and 115 °C, respectively. Then the granulated powder was heated up to 850 °C in hydrogen to remove the polyvinyl alcohol binder and other impurities. Finally, the granulated powder was sintered at 1300 °C to enhance the combination between the particles. Apparent density and Hall flow time of the granulated powder are 2.6 g/cm3 and 32 s/50 g,respectively. SEM images and particle size distribution of the granulated molybdenum powder are shown in Fig. 1. It can be seen from Fig. 1(a) and (b) that the granulated particles were agglomerates of original particles. Pores caused by evaporation of water and polyvinyl alcohol binder can be also observed. The D50 and Dav of the granulated powders are 33.4 μm and 36.9 μm, respectively.
2.2. RF spheroidization of granulated molybdenum powders
The granulated molybdenum powder was fed into a self-designed radio frequency inductively coupled plasma powder spheroidization system which is shown in Fig. 2. The powder spheroidization system is composed of a plasma torch, a power supply unit, a powder feeding system, a gas delivery system, a water-cooled quenching chamber and a powder collector. Powders were carried into the reaction chamber by Ar gas at a controlled speed. Powders would undergo a rapid melting process in the plasma torch then a rapid solidification process in the quenching chamber. Finally, the spheroidized molybdenum powders were collected from the powder collector at the bottom of the system.Mass loss percentage Rm caused by vaporization and sticking on the chamber wall during the spheroidization process can be calculated by the following equation:
Where mr is the mass of granulated powder fed into spheroidization system and mc is mass of the spheroidized powder collected from the powder collector. Detailed parameters for spheroidization process are listed in Table 1.
A specially designed quartz tube was used to inject powders into the induction coil. The powder feeding tube is water cooled to guarantee a low temperature in it. Therefore, the initial position of the particle in the high temperature zone of plasma can be adjusted by adjusting the insert depth of powder feeding tube into the induction coil. The flight path length was then controlled by adjusting the injection point into the induction coil. The residence time of granulated molybdenum powders in the high temperature zone of plasma was therefore adjusted by changing the flight path length while other parameters are kept constant. Spheroidization process was further studied by investigation of powder particles spheroidized with different residence time.
2.3. Characterization
Morphologies of spheroidized powders were observed by a field emission scanning electron microscope (FESEM, Quanta 250 FEG, FEI).The spheroidization percentage of the spherical powder Rs is calculated from SEM images by the following equation:
Where A is the total number of molybdenum powder particles counted in several SEM images and B is the number of spherical molybdenum powder particles counted in those SEM images. Particle size distribution of powders before and after spheroidization was analyzed by a wet laser particle size analyzer. The average particle size Dav can be calculated from the detected particle size distribution data by the following equation:
where di is detected particle diameter and ni is the percentage of the particles with the diameter of di. Flowability of the spheroidized powders was investigated by a Hall flowmeter (GB/T 1482–2010) and apparent density was measured by a Scott volumeter (GB/T1479.1–2011).
3. Results and discussion
3.1. Characterization of granulated molybdenum powders spheroidized with different plasma power
Fig. 3 shows SEM images of molybdenum powders spheroidized with different plasma power. Spherical particles and unmelted particles can be recognized in Fig. 3(a), (b) and (c). However, unmelted particles can be hardly found in Fig. 3(d). Furthermore. it can be seen that the number of unmelted molybdenum powder particles decreases while the number of spherical molybdenum powder particles increases with the increase of plasma power. The heat required for complete melting of a certain powder particle is constant, and higher plasma powder can generate more heat. Therefore, more powder particles melt when plasma power of the spheroidization process is higher.
The spheroidization process is a melting and then a solidification process; more molten particles can generate more spherical particles.Therefore, the spheroidization percentage increases with the increase of the plasma power during the spheroidization process as shown in Fig. 4(a). Furthermore, it can be seen from Fig. 4(a) that the spheroidization percentage of the spheroidized molybdenum powders shows a linear increase when plasma power is between 19 and 35 kW. Then the spheroidization percentage reaches nearly 100% when the plasma power is 43 kW.
The mass loss during the spheroidization process is shown in Fig. 4(b). It is obvious that mass loss increases with the increase of the plasma power, which can be explained by the higher vaporization rate with higher input plasma power. The heat Q required for complete melting of a particle can be calculated by the following equation [22,23]:
Where d is the particle diameter, ρ is theoretical density, cp is specific heat, Tm is melting point, T0 is room temperature, and Hm is latent heat of fusion. It can be seen that the heat required for complete melting of a particle is proportional to d [3], which indicates that more energy for melting of bigger particles would lead to vaporization of smaller particles. Therefore mass loss and spheroidization percentage increase basically linearly when plasma power increases from 19 to 35 kW because more particles completely melt. When plasma power increases to 43 kW, spheroidization percentage shows a slight increase and the mass loss increases rapidly to 6.94% because the additional heat mainly gives rise to the evaporation of more small particles.However, this mass loss is much lower than previous research [23]. It can be deduced that the granulated powders composed of original particles with narrower particles distribution can be spheroidized with less evaporation of small particles. Therefore, granulated powders can reduce mass loss during the pheroidization process.
Fig. 5 shows particle size distributions of molybdenum powders spheroidized with different plasma power. It can be observed in Fig. 5(a) that the D50 and D90 of the powder pheroidized with plasma power of 19 kW are larger than the granulated powder, which indicates that particles coalesce during spheroidization. Another reason is that complete vaporization of small particles also leads to an increase of D50 and D90. However, the average particle size of powders spheroidized with the plasma power of 19 kW is smaller than the granulated powder,which indicates that densification is the dominant process.
The average particle size of the spheroidized molybdenum powders increases from 35.5 μm to 56.6 μm when plasma power increases from 19 kW to 35 kW. It can be deduced that the coalescence behavior and evaporation of small particles become stronger when the plasma power.increases from 19 kW to 35 kW. Therefore, large particles can be observed in Fig. 3(c). However, the coalescence behavior is restricted by rapid melting and evaporation on surface of the particles when the plasma power is 43 kW. Therefore, the average particle size shows a rapid decrease when plasma power increases from 35 kW to 43 kW. And the molybdenum powders spheroidized with the plasma power of 43 kW have the smallest average particles size and narrowest particle size distribution. This result can be also observed from Fig. 3(d).
Fig. 6 shows the effects of plasma power on the Hall flow time and apparent density of molybdenum powders after spheroidization.Flowability, which is inversely related to Hall flow time, and apparent density of the molybdenum powders increase with the increase of the plasma power during the spheroidization process as shown in Fig. 6.The apparent density of the spheroidized molybdenum powders increases linearly when plasma power is between 19 and 35 kW, then increases slightly and reaches the best value when the plasma power is43 kW. It can be seen that the increase of apparent density has the same trend as the increase of spheroidization percentage, which indicates that apparent density was mainly influenced by the spheroidization percentage of the spheroidized powder. However, the Hall flow time decreases linearly when plasma power increases from 19 to 43 kW. This result indicates that the improvement of flowability may not only benefit from the higher spheroidization percentage but also from the smoother surface when the plasma power is higher. The best Hall flow time and apparent density of the spheroidized powder are 12.5 s/50 gand 5.52 g/cm3, respectively. It can be concluded that higher plasma power can lead to improvement of flowability and apparent density due to the decrease of the contact area between particles and elimination of defects in particles by the spheroidization process.
3.2. Characterization of granulated molybdenum powders spheroidized with hydrogen in the sheath gas
Fig. 7 shows the morphologies and particle size distribution of the spheroidized molybdenum powder with H2 in the sheath gas at 35 kW.Compared with powders spheroidized without hydrogen in the sheath gas(Fig. 3(c) and Fig. 5(c)), the spheroidization percentage is higher and the particle size is much smaller. The particle size distribution is also narrower. Mass loss, apparent density and Hall flow time were measured to be 6.23%, 5.51 g/cm3 and 12.79 s/50 g, respectively. The addition of hydrogen in the sheath gas has a significant influence on spheroidization, especially on the particle size. This result can be explained by hydrogen's higher thermal conductivity and enthalpy than argon. Li et al. [25] and Pfender [26] have reported the phenomenon that the gas temperature and high-temperature area increase with the increase of the hydrogen content when hydrogen was added in the gas flow. The phenomenon is related to the high enthalpy and thermal conductivity of hydrogen according to their investigation. The higher temperature of plasma and higher thermal conductivity of hydrogen can enhance the energy transfer from plasma to injected particles [27,28]. Therefore, the melting rate and evaporation rate increase with the improvement of the energy transfer. In this research, the improvement is even greater due to the higher specific surface area of the porous structure of the granulated powder. Eventually, the spheroidization percentage and mass loss increase with hydrogen in the sheath gas.
3.3. Microstructure evolution of molybdenum powders during the spheroidization process
To fully understand the spheroidization process, the granulated molybdenum powders were spheroidized with different times then rapidly cooled. The residence time τf of a particle in the high-temperature zone of the plasma is determined by the following equations [29]:
where R and ρ are radius and density of the particles, respectively, Vg and ηg are velocity and viscosity of the plasma gas, respectively, and S is the length of flight path of the powder particles in high temperature zone of plasma. Granulated molybdenum powders spheroidized with different times can be obtained by changing the flight path length while other parameters are kept constant, and it was further investigated under SEM.
Fig. 8 shows typical morphologies of molybdenum powder particles in order of spheroidization. The residence times in the high temperature zone of the plasma were calculated assuming that the particles have the same density and radius. Velocity, density and radius changes during the spheroidization process were also neglected. The residence times for each granule shown in Fig. 8 were calculated to be 7.9 ms, 9.0 ms,10.0 ms, 10.9 ms, 11.8 ms and 12.5 ms, respectively. It can be seen from Fig. 8(a) that major original molybdenum granules have a large unmelted region, which indicates that only edge region of the original molybdenum granules was melted and spread in this stage. Then, from Fig. 8(b) to Fig. 8(d), the unmelted regions of the original molybdenum granules become smaller and the surfaces of the spheroidized particle become smoother. Moreover, some original particles were sintered together and interfaces between them were eliminated during this process. And then, in Fig. 8(e), it can be seen that the particle is almost completely spheroidized, and its surface is smooth with only some gaps between the resolidified particles. Unmelted regions of the original molybdenum granules cannot be found. Finally, in Fig. 8(f), the granulated particle is completely spheroidized and original particles cannot be recognized. Large particles solidified from the melting original molybdenum powder particles can be observed and interfaces between them can be recognized.
Fig. 9 shows typical morphologies of coalescence behavior between two granules during spheroidization. The spheroidization process the two coalescing granules is similar to a single granule's spheroidization process shown in Fig. 8. The difference is that a sintering neck forms between the two granules. Therefore, material is also transported to the sintering neck during the spheroidization process. As a result, the two granulates were sintered together and formed an ellipsoidal particle.
4. Conclusions
Spheroidization of granulated molybdenum powders prepared by spray granulation can be achieved by radio frequency plasma.
Spheroidization percentage, apparent density and mass loss of the spheroidized powders increase with increasing plasma power while Hall flow time decreases. The best spheroidization percentage, apparent density, flowability and acceptable mass loss of spheroidized powder can be obtained when the plasma power is 43 kW. Spheroidization percentage, Hall flow time and apparent density of powders spheroidized at 43 kW are 99.8%, 12.5 s/50 g, 5.52 g/cm3 and 6.94%, respectively. Powders spheroidized with addition of hydrogen in the sheath gas have better spheroidization and narrower particle size distribution. Typical morphologies of incompletely spheroidized powder particles indicate that the spheroidization of granulated powder is a melting, sintering and solidification process. Spheroidization of two coalescing particles is a combination of spheroidization of individual granules and sintering between two granules.
Paper citation information
International Journal of Refractory Metals & Hard Materials 82 (2019) 15-22
Stardust uses a radio frequency plasma spheroidization process to produce spherical molybdenum powder. This powder boasts high purity, low oxygen content, high sphericity, a smooth surface, no satellites, uniform particle size distribution, and excellent flow properties. It is widely used in contact materials, high-temperature aerospace components, and targets.Spherical molybdenum powder is suitable for processes such as laser/electron beam additive manufacturing, laser direct deposition, hot isostatic pressing, injection molding, and laser cladding.Customized particle sizes are available to meet the diverse needs of our customers.http://en.stardusttech.cn/products/37.html
For more details, please contact Vicky Zhang +(86)13318326185
News
Stardust Technology (Guangdong) Co., Ltd.
101, Building 1, Liandong Youzhi Zone, Senshuji Road, Nansha Community, Danzao Town, Nanhai District, Foshan City,Guangdong Pro.,China
Tel：13318326187; 13318326185；
QR code