Research Progress and Prospect of Metal Powder Preparation by Aerosolization

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As a key raw material for additive manufacturing, the quality of metal powder largely determines the final quality of the product. With the rapid development of additive manufacturing technology and its process specificity, the quality requirements for metal powder are getting higher and higher, such as high sphericity, good fluidity, low gas and impurity content and other requirements. At the same time, with the continuous expansion of additive manufacturing applications, the types of metal powders required are also increasing. At present, the methods of preparing metal powder mainly include atomization method, mechanical pulverization method, rotating electrode method, electrochemical corrosion method, reduction method and so on. Among them, only the gas atomization method (GA) and the plasma rotating electrode method (PREP) can directly produce spherical powders, while the other methods require additional processing to obtain near-spherical powders.The powder prepared by PREP has a high degree of sphericity, but due to the limitation of the process principle, the fine powder yield is low, and it is mainly used in the preparation of powder for powder feeding additive manufacturing. The powder prepared by aerosolized powder technology has the advantages of high sphericity, good fluidity, low O, N, H content, etc., and a large adjustable range of powder size distribution, which has become the main method for producing high-performance spherical metal powders.

This paper reviews the basic principles and characteristics of aerosolized powder making technology, summarizes the research progress in recent years on the types of nozzle structure for aerosolized powder making, the structure and simulation of the gas flow field, the regulation of powder quality and the control of process parameters, etc., and looks forward to the prospects for the development of aerosolized powder making technology.

1 Research status of aerosolized powder making 1.1 Principle of aerosolized powder making

The basic principle of aerosolization is to impact the metal melt with a high-speed gas stream, through the collision of the kinetic energy of the gas into the surface energy of the metal melt, so that the molten metal stream is broken into tiny droplets, and then in the atmosphere of the gas stream quickly cooled and solidified to form powder.

Master alloy raw materials in the aerosolized powder production process undergoes three main processes: melting, atomization and solidification. At present, the mainstream atomization process is carried out in a vacuum or inert gas environment, in order to reduce the oxygen content and impurity content in the powder, improve the purity of the powder. Some studies have shown that the oxygen in the powder is basically brought in during the melting process. Therefore, no matter in the master alloy preparation or atomization process to maintain a vacuum or inert gas environment. After the master alloy is melted, it is broken and dispersed into small droplets by the high pressure and high speed gas flow (inert gas), and the small droplets lose their heat rapidly during the falling process, and solidify into spherical powder quickly under the action of surface tension.

Atomization equipment, atomizing gas and metal liquid flow parameters determine the final effect of the aerosolization process, as shown in Figure 1. Atomization equipment parameters mainly refers to the nozzle structure, the structure of the guide tube and its position, but whether to use the guide tube according to the equipment and working conditions to decide; atomization gas adjustable parameters are the nature of the gas, inlet pressure, air velocity, etc., which, the air velocity is usually determined by other parameters; the nature of the metal liquid stream (surface tension, viscosity, etc.) by the alloy composition of the molten liquid and the degree of superheat, etc., factors such as the decision, the liquid stream diameter by the guide pipe Diameter of the liquid stream is determined by the inner diameter of the guide tube. Aerosolized powder process, through the adjustment and optimization of the parameters and matching to adjust the powder particle size, particle size distribution and microstructure.




1.2 Numerical simulation and study of atomization flow field

Atomization of powder is a complex physical process with multi-phase flow coupling, and the structure of the gas flow field affects the stability of the atomization process as well as the powder particle size and size distribution.Anderson et al. used numerical simulation and experimental methods to study the changes of the gas flow field under different atomization pressures, and the simulation results found that the characteristics of the flow field under the conventional atomization pressure include a strong circulation zone and a mixed shear layer, and the shock wave zone occurs when high-pressure atomization is carried out. Shock wave region occurs when the gas is atomized at high pressure. When the pressure is higher, a Mach disk is formed on the central axis of the flow field, which closes the flow field region and is in high agreement with the experimental results.Ting et al. simulated the flow field characteristics of a tightly coupled aerosol nozzle under pure gas flow conditions, and found that the flow field is an inverted cone structure, with the gas at the top of the cone having the highest pressure and the velocity tends to be close to zero, which is known as the gas lagging point. After coming out of the nozzle, the supersonic gas flow drops to subsonic speed at the lag point and enters the inverted cone flow field from the bottom up. After encountering obstacles, it flows radially along the guide tube, and then flows downward under the pressure of the turbulent boundary layer at the edge of the flow field, separating from the supersonic flow, as shown in Figure 2.



Ting and Mates et al. found that two flow patterns exist in the lower part of the atomizing nozzle at different inlet pressures. Under high-pressure conditions, the supersonic flow expands after the lagging point to form a Mach disk, which forms a closed region of the flow field called the closed vortex; under low-pressure conditions, the Mach disk cannot be formed and is in the open vortex state. From open vortex to closed vortex transition critical pressure is called the wake closure pressure, as shown in Figure 3.



Tong et al. used a contracting-expanding ring-seam nozzle with numerical simulation and experiment, and found that the simulated flow field image was more similar to the flow field characteristics observed in the experiment, but the trailing closure pressure obtained in the experiment was slightly higher than that in the numerical simulation.

1.3 Mechanism and process study of aerosolized powder preparation Lubanska proposed the Lubanska powder particle size equation, which is still widely used, by studying the technology of preparing iron powder by aerosolization, which reveals the relationship between the median mass particle size (d50) and the parameters of the aerosolization process, as shown in the following equation:




Where: D is the inner diameter of the guide tube (liquid flow diameter); K is a constant depending on the atomization conditions, taking the value of 40 ~ 50; vL, vg for the molten liquid, atomized gas kinematic viscosity, respectively, m2 / s; We for the Weber number; M, A, respectively, for the melt, the mass flow rate of the gas, kg / s; ρL, γ, respectively, for the density of the liquid metal, surface tension; V for the gas impact on the melt speed, m / s. The gas is the melt, the velocity of the melt. m/s.

Dombrowski et al. conducted experiments and numerical analysis, and found that the metal melt first breaks from the liquid column into a liquid film, but the liquid film of the melt can not be stabilized for a long time, and then extends into a wave shape, and then tears into a band at the edge of the wave, and then further breaks, as shown in Figure 4. The diameter of the ribbon melt is related to the thickness and wavelength of the liquid metal film, as shown in the following equation, where λ is the wavelength of the wave and S is the thickness of the liquid metal film:


Fig. 4 Dombrowski's fluctuating fragmentation model


Mansour et al. investigated the atomization mechanism and pointed out that there are three fracture modes of the molten metal during atomization: edge fracture, wave fracture, and hole fracture, as shown in Fig. 5.




The nature of the metal melt and the atomizing pressure affect the coarseness and particle size distribution of the powder. Metal melts with low viscosity, low surface tension and high density can produce finer powders. When the flow rate of the metal melt increases, the particle size of the powder increases, and when the ratio of gas flow rate to melt flow rate remains constant, continuing to increase the atomization pressure hardly affects the particle size distribution. Increased pressure of the atomizing medium will lead to smaller particle size, the size of the negative pressure at the end of the deflector tube is related to the atomization pressure, the lower the negative pressure, the smaller the particle size of the powder, the greater the degree of energy exchange between the gas and the melt.

The metal melt must have a certain degree of superheat before it can be atomized stably, the higher the temperature of the metal melt, the lower the viscosity, so increase the degree of superheat of the melt can prepare finer metal powder, but the degree of superheat increases to a certain extent, it will have an impact on the performance of the powder, because too high a temperature of the melt will lead to an increase in the solidification time, droplets are easy to adhere to each other in flight fusion, but also to increase the emergence of satellite powder probability, which is not conducive to powder molding.

Different atomizing gases will also affect the coarseness and size distribution of the powder. Some scholars have studied the effect of different atomizing media on the particle size of aluminum powder. The results found that the helium atomization of the finest powder, nitrogen in the middle, argon atomization of the largest particle size of the powder. This is because helium has the highest surface heat transfer coefficient and the greatest degree of energy exchange with the metal liquid stream. The traditional vacuum induction melting-gas atomization system (VIGA) is highly susceptible to contamination of the master alloy due to the use of materials such as crucibles and deflector tubes, which are usually mitigated by the use of highly heat-stable deflector tubes or inner-wall coating processes. Zhao Shaoyang et al. coated Y2O3 coating on the inner wall of the graphite guide tube, and experiments have proved that the carbonization reaction between the titanium alloy melt and the graphite guide tube can be effectively prevented under high temperature conditions, so as to control the carbon content of the atomized powder.

Aerosolized prepared metal powder particle size is lognormal distribution, by reducing the width of the powder particle size distribution can increase the powder yield, powder yield increase can effectively reduce the cost of powder preparation. Another way to reduce the cost of metal powder preparation is to use the atomization gas circulation system, Liu Xuehui et al. used crucible-free induction heating Ar gas atomization to produce titanium and titanium alloy powders, and used the Ar gas purification and recycling system to reduce the cost. It was found that: with the increase of Ar gas recycling time, the N, O content in Ar gas is basically unchanged; and because Ti and Ti alloys are a dehydrogenation process when melting spray at high temperature, the removed H enters into Ar gas, which makes the H content in it increase linearly, and then leads to the increase of H content in the powders, so the addition of the hydrogen absorbing device in the recirculation system can increase the number of times of recycling of Ar gas.

Powder making process once the use of different composition of raw materials or nozzle structure, the atomization parameters should be adjusted; the production of different particle size of the powder should also be redesigned process parameters. Aerosolized powder is a complex multiphase flow coupling process, so far there is no complete theoretical explanation of the entire aerosolized powder process, but also can not form a unified design specifications and standards for atomization parameters, which is also a key direction for future research.

1.4 Rapid solidification of powder organization

In the process of aerosolization, the high-speed inert gas impacts the melt, and the metal liquid stream is broken into tiny droplets. Mainly through convection and conduction mode of heat transfer, the heat energy of the melt is rapidly dissipated, the cooling rate of the molten metal can reach 104 ~ 105 ℃ / s, the small droplets solidify quickly to form powder.

Aerosolized metal powder internal fast solidification organization not only reflects the final solidification state of the metal powder, but also reflects the metal powder in its solidification process of nucleation and growth. The surface organization of aerosolized powder is mainly dendrites and cellular crystals. And as the particle size decreases, the number of dendrites decreases and the number of cytosolic crystals increases. During the droplet cooling process, the cooling rate of large particles is slow, which is conducive to the full growth of dendritic crystals, while the cooling rate of small particles is fast, and the grains can not be grown into dendrites before they cool and solidify to form cytosolic crystals. The internal microstructure of the powder particles shows the same pattern as the surface organization, as shown in Figure 6.




Aerosolized powder manufacturing process in the metal liquid stream quickly solidified into powder, for additive manufacturing, raw material powder in the heat source input rapid melting and solidification, also belongs to the rapid solidification process. In this process, the microstructure of the raw material powder will be "inherited" into the printed part, so the quality of the 3D printed part is affected by the quality of the powder. As the composition of conventional alloys are for the conventional casting or forging process characteristics, combined with the requirements of the alloy performance after a long period of research and determined, so they may not be fully suitable for the characteristics of rapid solidification technology. At the same time, rapid solidification of the microstructure of the alloy microstructure improvement is also a breakthrough in the original alloy composition limitations, the development of new alloys to provide the possibility. In the original alloy composition on the basis of some appropriate improvements is relatively simple and reliable method, and can provide the basis for the development of new alloys with new components.

2 atomization nozzle classification nozzle design is critical, directly affecting the finished powder morphology, particle size, purity and stability of production. In the 1830s, the formation of free-fall nozzles and restriction nozzles represented by the atomization nozzle, as shown in Figure 7.



The free-fall nozzle has a simple structure, the distance between the metal liquid stream and the atomized gas stream is far, the nozzle is easy to design, and the clogging frequency in the atomization process is low, but the energy conversion efficiency of this structure is low, the gas consumption is large, and the atomization efficiency is low. On the other hand, the structure of restriction nozzle is compact, the distance between the melt and the gas stream is reduced, and the atomization efficiency is significantly improved, but there are some problems with the structure of this nozzle, such as complex design, unstable atomization process, and processing difficulties, etc. Grant developed the ultrasonic aerosol powder making technology by connecting the Hartman vibrating tube and the Laval nozzle in series.The Hartman vibrating tube can produce high-frequency Ultrasonic waves can be generated in the Hartman vibrating tube, which increases the kinetic energy of the atomized gas and thus improves the cooling rate of the molten metal. Experiments have proved that when the atomizing gas pressure in 2 ~ 2.5 MPa, ultrasonic frequency of 80 ~ 100 kHz, at this time the cooling rate of the metal melt up to 104 ~ 105 ℃ / s. When the atomizing gas pressure increased to 8.3 MPa, the average particle size of the prepared metal aluminum powder 22 μm, the powder has a high degree of sphericality, the surface is bright and clean. The closer the distance between the atomizing gas stream and the metal liquid stream, the higher the efficiency of converting the kinetic energy of the gas stream into the surface energy of the liquid stream, and the higher the atomization efficiency. Based on this feature, Miller et al. designed a tightly coupled atomization nozzle, the nozzle of the airflow outlet to the metal liquid stream distance is very short, as shown in Figure 8. The powder produced by tightly coupled atomization technology has the advantages of fine particle size, narrow particle size distribution range, high cooling rate. At present, tightly coupled atomization has become the mainstream aerosolized powder technology for additive manufacturing powder.


Figure 8 Schematic diagram of tightly coupled aerosolized nozzle and device


Gerking invented the laminar flow ultrasonic atomization nozzle, and the nozzle structure is shown in Figure 9. In laminar flow ultrasonic atomization, the angle between the airflow and the melt is very small and almost parallel to each other. During the atomization process, the melt is broken and deformed under the combined effect of shear force and squeezing pressure, showing a fibrous layered shape. When the fibrous layered liquid stream leaves the nozzle, the pressure difference between the inside and outside of the airflow will break the liquid stream into droplets, and the metal droplets solidify into powder. Compared with other atomization processes, laminar flow atomization consumes less gas and can produce metal powder with narrow particle size distribution and fast cooling speed.


Figure 9 Laminar flow ultrasonic atomizing nozzle


According to the gas equation of state: PV=nRT, under the same gas pressure, increasing the gas temperature will cause the gas volume to expand, which in turn will increase the gas exit velocity.Strauss proposed the concept of thermal gas atomization based on tightly coupled gas atomization. Studies have shown that under the same gas pressure and gas consumption, increasing the temperature of the atomizing medium can significantly increase the kinetic energy of the gas, thus improving the aerosolization efficiency and effectively reducing the average particle size of the powder.

3 Powder performance regulation 3.1 Hollow powder formation mechanism and control methods

Hollow powder is a common class of defects in aerosolized powders, holes in the powder generally exists in two forms: one is the atomized gas is wrapped in the powder inside the formation of closed pores, the size of which is generally 10% to 90% of the powder, generally in the thicker particle size (>70 μm) of the most common in the powder; the other is the dendritic crystal condensation contraction of the formation of the pores, the size of which is generally less than 5% of the size of the powder in the The size is generally less than 5% of the size of the powder, in the powder internal and surface are distributed. Generally with the increase in powder particle size, the number of pores in the powder, size, gas content will increase accordingly.

The formation of hollow powder is related to the droplet crushing mechanism in the atomization process. In the process of aerosolization, according to the different energies of the interaction between the atomized gas and the molten metal, there are many different types of droplet crushing mechanisms occurring at the same time. When one of the most energetic mechanisms, bag crushing, occurs, large droplets form a bag-like sheet under the action of the gas flow, spreading in a direction perpendicular to the gas flow. When the liquid viscosity is small, the outside of the liquid film is broken to form fine droplets; however, the droplets are cooled very quickly during the aerosolization process, and the viscosity rises sharply as the droplet temperature drops rapidly. When the droplet viscosity is high enough, the crushing of the bagging film is suppressed, and the ports on both sides of the liquid film combine to form a hollow droplet wrapped with atomized gas, as shown in Figure 10. Therefore, to inhibit the generation of hollow powder, the energy of the crushing process must be reduced to avoid the occurrence of bag crushing, but this is difficult to do without precise control of the atomization process.



3.2 Satellite powder formation mechanism and control methods

Satellite powder refers to small-size powder adhering to the surface of large-size powder, forming a satellite-like powder structure, as shown in Figure 11. Satellite powder reduces the sphericity, fluidity and bulk density of the powder, which is another common defect in aerosolized powder making. There are two different theories to explain the appearance of satellite powder. One classical theory attributes the appearance of satellite powder to the collision adhesion of fine and coarse powders during their downward flight in the atomization chamber. It has been shown that during atomization, the fine droplets solidify before the larger droplets

cool and solidify, accelerate in the high-speed airflow, and eventually collide and weld onto the larger droplets, resulting in the formation of satellite powders.Ozbilen's study found that the chance of satellite powders became greater when the atomized powder size distribution was wider and the surface of the larger particles was rougher.



Anderson et al. observed in their atomization experiments that a vertically upward stream of fine powders could be seen along the walls of the atomization chamber, and that the airflow sent these fine powders into the flow field below the nozzle, and thus proposed an alternative theory: it is believed that the already solidified fine powders are sucked up by the cyclonic airflow into the jet zone below the nozzle, and that they collide with the droplets that are not yet solidified completely, and ultimately form satellite powders. This led to the development of a 30 cm diameter atomization chamber, which has been experimentally proven to reduce the probability of satellite powder. However, this method leads to premature collision of droplets with the wall of the atomization chamber, which reduces the powder yield.

In summary, the current study mainly reduces the appearance of satellite powder by two methods. First, by controlling the atomization process and melt properties, the particle size distribution width of the atomized powder is reduced, i.e., the particle size difference of the powder is reduced, which can, in principle, reduce the difference in the state of motion between the powders, thus reducing the collision frequency between the powders and the liquid droplets; second, by introducing auxiliary airflow into the atomization chamber or optimizing the structure of the atomization chamber, the airflow gyration inside the chamber is inhibited, thus decreasing the collision chances between the powders and the liquid droplets The second is to reduce the chance of collision between powder and liquid droplets by introducing auxiliary airflow into the atomization chamber or optimizing the structure of the atomization chamber.

3.3 Powder size distribution control method

The particle size distribution of metal powder prepared by aerosolization is lognormal, as shown in Figure 12 for the powder particle size distribution schematic, after pre-screening, the larger particle size of the powder is eliminated. The powder yield can be increased by reducing the width of the powder size distribution, and the increase in the powder yield can effectively reduce the cost of making the powder. In addition, the reduction of particle size distribution width can also inhibit the formation of satellite powder, which is important for improving the quality and performance of the powder.




The research on controlling the powder size distribution is mainly carried out in three aspects: optimizing the nozzle structure design, and regulating the properties of the atomizing medium and the metal melt.Schwenck et al. designed a shrink-expand annular slit nozzle with a diameter of 0.8 mm (at the narrowest part of the throat), which was compared with the conventional shrink-expand annular slit nozzles with a diameters of 0.8 mm and 0.4 mm. The results showed that the median particle size of the powders prepared by the shrink-expand ring-seam nozzle was smaller than that of the conventional nozzle, and the geometric standard deviation of the powder size was reduced from 2.14 to 1.87, which indicated a narrowing of the powder size distribution and an increase in the powder yield. In addition, he also studied the effect of hot gas atomization, and found that when the gas was heated to 320 ℃, the powder particle size and size distribution were further reduced, which could effectively improve the powder yield.

4 Outlook

Aerosolized powder technology is one of the main methods for producing high-performance metal and alloy powders, but people's understanding of the atomization mechanism is still insufficient, the control of process parameters and coordination of different materials has not formed a complete set of theories, which restricts the industrialization of powder technology. The key technologies that need to be improved for future aerosolized powder making include:

(1) the need for aerosolized powder technology to carry out a large number of atomization mechanism and basic process research, the formation of a unified design specifications and standards for atomization parameters;.

(2) Need to further optimize the design of the atomization nozzle and atomization device to regulate the width of the powder particle size distribution, improve the powder yield and powder quality;.

(3) In order to effectively reduce the production cost of metal powder, the need to carry out in-depth research on the recycling of powder, improve the utilization rate of powder, and the need to take effective means to promote the technology to the rapid transition to industrialized production.

(4) With the continuous development of additive manufacturing technology, we need to further study the microstructural properties of raw material powder and the impact on the quality and performance of 3D printed parts, and develop a series of special alloy compositions according to the technical characteristics of additive manufacturing.

It is believed that with the in-depth study of the basic process and the mechanism of aerosolized powder making, the aerosolized powder making technology will continue to be developed and improved.

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