A method for controlling perfect sphericity of nano-aluminum powder by plasma atomization

Abstract

The application belongs to the technical field of metal powder preparation, and discloses a method for realizing perfect sphericity control of nano-aluminum powder through plasma atomization, which comprises the following steps: preparing argon-based working gas containing hydrogen gas with a specific proportion and controlling the dew point to be lower than-75 DEG C; introducing the working gas into a Laval type plasma generator to form a chemical frozen state supersonic jet flow with a Mach number greater than 2.4 under the condition of a specific pressure ratio and area ratio; guiding the jet flow to impact a molten aluminum liquid flow, and utilizing the surface catalytic hydrogen atom in the stagnation boundary layer to cause an in-situ exothermic complex reaction; and utilizing the chemical energy released by the reaction to maintain the liquid phase flow deformation of the droplet surface layer until the droplet is shrunk and spheroidized, wherein the application utilizes the selective energy release mechanism of the gas-liquid interface to construct a transient isothermal layer without heating the interior of the droplet, so as to avoid the thermodynamic contradiction between spheroidization shaping and the retention of the internal nano-crystal structure.

Description

Technical Field

[0001] This invention relates to a method for achieving perfect sphericity control of nano-aluminum powder through plasma atomization, belonging to the field of metal powder preparation technology. Background Technology

[0002] Currently, in the metal additive manufacturing and powder metallurgy industries, the preparation of metal powders with both high sphericity and nanoscale grain structure is the foundation for the precision forming of high-strength aluminum alloys. At present, plasma atomization technology uses high-temperature plasma jets to melt metal and uses high-speed air kinetic energy to impact and shear the molten metal flow, breaking the flow into micro-droplets. To obtain micro- and nano-scale ultrafine powders, the high Weber number shear effect of supersonic airflow is usually used to break the molten metal flow. According to the boundary layer heat transfer theory, the increase in airflow velocity causes the convective heat transfer coefficient at the gas-liquid interface to increase exponentially. The high-intensity aerodynamic shearing is accompanied by extremely rapid surface heat exchange, causing the surface temperature of the micro-droplets to drop below the solidus line within the surface tension contraction time required to complete the sphericization. The surface viscosity jump causes the irregular morphology or tailing characteristics of the droplets to freeze at the moment of breakage, forming non-spherical particles.

[0003] Existing technologies, by increasing the overall plasma power or introducing auxiliary heat sources to delay solidification, essentially heat the entire gas-liquid two-phase system. This reduces the overall cooling rate of the droplets, causing the internal grains of the aluminum powder to coarsen due to heat accumulation, sacrificing the advantages of the nanocrystalline microstructure from the rapid cooling process. Besides thermal field control, improvements can be made at the process control level. For example, Chinese invention patent CN120901294A discloses a rotating electrode atomization powder production control system and method, establishing a multivariate machine learning prediction model for electrode rotation speed, plasma arc power parameters, and powder particle size and sphericity, and using AI vision for real-time monitoring and feedback for dynamic adjustment. While process parameters, based on a post-hoc data-driven intelligent closed-loop control strategy, improve the adaptability of macroscopic process parameters to environmental fluctuations, they essentially remain at the level of external intervention of macroscopic physical quantities, failing to address the core contradictions of microscopic solidification dynamics. For nanosecond-level instantaneous supersonic atomization processes, the millisecond-level macroscopic feedback adjustment lags behind, making it impossible to construct an independent window to delay the transient thermodynamics of surface quenching at the single-particle scale. This fails to solve the fundamental problem of balancing high sphericity shaping with rapid internal cooling. The droplet surface prematurely transitions to a semi-solid or solid state, making it difficult for fine particles entrained in the airflow to fuse with the main particles during secondary collisions, resulting in physical adsorption and adhesion to the surface to form satellite sphere defects.

[0004] Therefore, the technical problem to be solved by this invention is how to construct a transient rheological window at the gas-liquid interface of droplets while maintaining the high cooling capacity of the supersonic flow field, so as to achieve decoupling between morphology shaping and microstructure control. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: A method for achieving perfect sphericity control of nano-aluminum powder through plasma atomization, the method comprising the following steps:

[0006] Atomizing gas is prepared, which consists of 93.0% to 96.0% argon and 4.0% to 7.0% hydrogen by volume, and the dew point of the atomizing gas is controlled below -75 degrees Celsius to eliminate the oxide layer on the surface of molten aluminum and expose metallic aluminum atoms as heterogeneous catalytic active sites.

[0007] The atomized propellant gas is introduced into a Laval-type plasma generator. The ratio of the nozzle outlet cross-sectional area to the throat cross-sectional area is set to be greater than 3.8, and the ratio of the generator inlet stagnation pressure to the atomization chamber ambient pressure is controlled to be greater than 18. The atomized propellant gas is driven to undergo isentropic expansion and form a supersonic jet with a Mach number greater than 2.4. By controlling the gas flow expansion characteristic time to be less than the hydrogen atom chemical recombination characteristic time, hydrogen atoms are locked in a thermodynamically non-equilibrium chemically frozen state.

[0008] The supersonic jet in a chemically frozen state is guided to impact the molten aluminum flow located 15 to 25 mm downstream of the nozzle exit, and a stagnant boundary layer is constructed on the windward side of the aluminum droplets formed by shearing and breaking them up, where the flow velocity is stagnant and the kinetic energy is converted into internal energy.

[0009] Using aluminum atoms on the surface of aluminum droplets as a third-party catalyst, an in-situ exothermic recombination reaction is induced in high-energy hydrogen atoms within the stagnant boundary layer. The interfacial chemical energy released by the reaction maintains the rheological properties of the liquid phase on the surface of the aluminum droplets until they shrink into a spherical shape under the drive of surface tension, and then expand, cool, and solidify with the airflow.

[0010] Preferably, the step of preparing the atomized gas further includes: controlling the oxygen impurity content in the atomized gas to a level below 2 ppm; the limitation of the oxygen impurity content is used to prevent oxygen atoms from preferentially reacting with metallic aluminum atoms to form an oxide film and thus competing for catalytic active sites, ensuring that the adsorption and recombination reaction of hydrogen atoms in the chemically frozen state dominates in the stagnant boundary layer, and ensuring the interfacial selectivity and heating efficiency of chemical energy release.

[0011] Preferably, the Laval-type plasma generator includes an expansion section with a specific profile: the inner wall profile of the expansion section follows a smooth transition designed with the Wittsinski curve; the specific geometry of the expansion section is used to suppress the separation of the gas flow boundary layer and the generation of shock waves, ensuring that the supersonic jet maintains a laminar state before reaching the molten aluminum flow; the laminar state is used to minimize the turbulent dissipation of the gas flow during the transmission process and to maximize the preservation of the chemical potential energy density of hydrogen atoms in the chemically frozen state.

[0012] Preferably, in the step of controlling the ratio of the generator inlet stagnation pressure to the atomization chamber ambient pressure to be greater than 18, the inlet stagnation pressure P0 and the atomization chamber ambient pressure P b The critical freeze-thaw relationship between them is satisfied as follows: P0 / P b ≥(1+0.33Ma 2 ) 2.5 Where Ma is the Mach number of the supersonic jet at the nozzle exit; this relationship defines the thermodynamic boundary conditions required to maintain the gas flow expansion and cooling rate above the hydrogen atom recombination reaction rate, ensuring that hydrogen atoms do not undergo bulk recombination before reaching the stagnant boundary layer.

[0013] Preferably, before the step of guiding the chemically frozen supersonic jet to impact the molten aluminum flow located 15 mm to 25 mm downstream of the nozzle exit, the method further includes: heating an aluminum ingot with a purity of 99.99% or higher to a melting temperature of 750°C to 800°C in an induction melting furnace; introducing the molten aluminum into the central axis region of the supersonic jet using a guide tube with an inner diameter of 2 mm to 4 mm; the exit position of the guide tube coincides with the convergence point of the supersonic jet, ensuring that the molten aluminum flow is surrounded by a chemically frozen gas flow at the moment of shearing and breaking.

[0014] Preferably, in the step of maintaining the liquid phase rheology of the aluminum droplet surface using the interfacial chemical energy released by the reaction, the interfacial chemical energy is input into the aluminum droplet in the form of a heat flux, and the depth of the heat flux is limited to the skin depth range of 10 nm to 100 nm on the surface of the aluminum droplet; the localized heat input keeps the hydrodynamic viscosity within the transient isothermal layer on the surface of the aluminum droplet below 2 mPa·s, while the core region of the aluminum droplet remains greater than 10 mPa·s under the convective heat transfer effect of the external supersonic airflow. 5 A cooling rate of K / s enables thermodynamic decoupling between spherical shaping and retention of internal nanocrystalline structure at the single-particle scale.

[0015] Preferably, the step of using aluminum atoms on the surface of aluminum droplets as a third catalyst further includes: using the local high-temperature zone generated by the in-situ exothermic composite reaction to eliminate satellite sphere defects; when microdroplets with a diameter less than one-tenth of the main particle impact the surface of the main particle, the low-viscosity environment provided by the local high-temperature zone drives the microdroplets to be completely wetted and integrated into the interior of the main particle within 10 microseconds, preventing the microdroplets from adhering to the surface of the main particle in the form of physical adsorption to form a satellite sphere structure.

[0016] Preferably, the method further includes setting an inert gas quenching zone downstream of the stagnant boundary layer. After the aluminum droplet completes spherical shrinkage, the exothermic recombination reaction is terminated by spraying a helium gas flow with a temperature below 20 degrees Celsius into the quenching zone. The intervention of the helium gas flow is used to remove residual hydrogen molecules and forcibly close the liquid phase window on the surface of the aluminum droplet, thus freezing the formed reference spherical morphology and the internal metastable microstructure simultaneously.

[0017] Preferably, in the step of preparing the atomizing chemical gas, the specific volume percentage of hydrogen is dynamically matched according to the flow rate of the molten aluminum; the ratio of hydrogen volume flow rate to aluminum mass flow rate is set to be maintained in the range of 0.05 cubic meters per kilogram to 0.08 cubic meters per kilogram; this ratio range is used to ensure that sufficient chemical potential energy is provided to complete the thermal compensation required for spheroidization, while avoiding excessive hydrogen dissolving in the aluminum and precipitating out during solidification to form pinhole defects.

[0018] Preferably, the method further includes the steps of collecting and sieving the cooled and solidified powder: using a cyclone separator to collect nano-aluminum powder with a particle size distribution D50 of less than 30 micrometers under inert gas protection; the nano-aluminum powder has a true sphericity greater than 0.95 and an internal grain size of less than 100 nanometers; the oxygen content of the powder is less than 600 ppm and the Hall flow rate is less than 50 seconds per 50 grams; the method ensures the consistency of powder morphology and microstructure between batches under continuous production conditions through the selective energy release mechanism of chemically frozen hydrogen atoms at the gas-liquid interface.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1. This invention utilizes a supersonic freezing flow field to decouple spheroidization from microstructure, controlling the gas flow characteristic time to be shorter than the chemical reaction characteristic time. This allows the energy-carrying gas molecules to be in a thermodynamically non-equilibrium chemically frozen state during the gas phase transport stage, locking energy in the form of chemical potential energy. When the gas flow impacts the aluminum droplet to form a stagnant boundary layer, the heterogeneous catalysis on the droplet surface triggers an in-situ exothermic reaction. The spatially selective energy release mode constructs a nanoscale transient isothermal layer at the gas-liquid interface of the droplet, reducing the surface fluid dynamic viscosity and extending the surface tension to drive the spheroidization time window. The droplet interior is controlled by the external high-speed gas flow to maintain an extremely high cooling rate. The process overcomes the inherent constraint of premature surface quenching and shell formation caused by high shear rates in metal powder manufacturing, enabling the prepared aluminum powder to achieve high spheroidization while retaining the internal nanocrystalline or amorphous metastable microstructure generated by the rapid cooling effect.

[0021] 2. This invention avoids satellite sphere defects and improves powder flowability by utilizing the catalytic activity of aluminum liquid surface on atomic free radical recombination, so that the surface of droplets in the flight zone maintains a low-viscosity liquid phase with good wettability before solidification. When the micro-droplets carried in the airflow collide with the main particles, the high atomic diffusion coefficient at the contact interface drives the micro-droplets to instantly fuse into the interior of the main particles, avoiding the physical adsorption caused by surface semi-solidification in traditional processes. Based on the interface rheological fusion mechanism, it eliminates the common satellite sphere and irregular adhesion defects in nanopowders from the root, reduces the friction coefficient between powder particles, and improves the loose density and Hall flow rate of the final product without changing the powder particle size distribution, thus meeting the powder spreading performance requirements of high-precision additive manufacturing.

[0022] 3. Process stability and engineering applicability based on self-organizing mechanism: By adjusting the pressure gradient ratio between the nozzle inlet and the back pressure, a stable chemical freezing flow field is established. By utilizing the stagnation effect of airflow at the leading edge of the obstacle and the spontaneity of surface catalytic reaction, isotropic thermal modification of droplet surfaces of different sizes is achieved. The precise transport and release of energy is completed by relying on the physicochemical properties of the material itself. The adaptive surface treatment mechanism has anti-interference ability, avoiding the engineering risk of failure of complex field equipment in high temperature and high dust environment, and ensuring high consistency of powder morphology and performance between batches under industrial-grade large-scale continuous production conditions. Attached Figure Description

[0023] Figure 1 This is a flow chart of the nano-aluminum powder preparation process based on supersonic chemical freezing flow according to the present invention;

[0024] Figure 2 This is a trend diagram showing the effect of hydrogen concentration on the sphericity and micro-defect rate of aluminum powder according to the present invention.

[0025] Figure 3 This is a structural block diagram of the plasma atomization powder production system with integrated closed-loop control according to the present invention. Detailed Implementation

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0027] This invention provides a method for controlling the perfect sphericity of nano-aluminum powder through plasma atomization. The method includes a precision gas distribution unit, a Laval-type plasma generator, a high-vacuum atomization tower, and a powder collection unit. This method constructs a supersonic chemical freezing flow field and utilizes the selective release of gas-phase chemical potential energy at the gas-liquid interface of aluminum droplets, achieving thermodynamic decoupling between microscopic morphology shaping and internal grain structure control. Addressing the technical problem in plasma atomization processes where the pursuit of fine powder particle size leads to increased gas flow shear velocity, resulting in a surge in convective heat transfer coefficient and premature quenching and crust formation on the droplet surface, thus forming irregular morphologies and satellite spherical defects, this embodiment constructs a high-energy working fluid gas system with delayed release characteristics. In the gas distribution section, a high-precision mass flow controller delivers high-purity argon (93.0% to 96.0% by volume) and high-purity hydrogen (4.0% to 7.0% by volume) into a gas mixing chamber for homogenization. The hydrogen concentration range of 4.0% to 7.0% is set as the basic... Based on the process window determined by thermodynamic calculations and engineering experiments, when the concentration is below 4.0%, the heat flux released by subsequent interfacial recombination is insufficient to offset the convective cooling effect of the supersonic gas flow, leading to premature surface solidification. When the concentration is above 7.0%, the excessive hydrogen partial pressure causes the solubility of hydrogen atoms in the aluminum melt to exceed the supersaturation threshold, forming pinhole defects during solidification. To ensure that the surface of the aluminum droplet can serve as an effective heterogeneous catalyst, the impurity content of the mixed gas must be strictly controlled. The working gas needs to be treated by a multi-stage purification column, using a zirconium vanadium iron alloy getter to deeply remove oxygen and moisture at 400 degrees Celsius, controlling the dew point of the working gas to below -75 degrees Celsius and the oxygen impurity content to below 2 ppm. This low dew point and low oxygen content process environment is used to eliminate the original alumina film on the surface of the molten aluminum melt, exposing metallic aluminum atoms with high surface energy. These exposed metallic atoms constitute the active sites necessary for gas-phase free radical adsorption and recombination reactions.

[0028] After establishing the aforementioned high-purity energy-carrying atmosphere, this method locks the chemical potential energy of the gas into a metastable state through fluid dynamics design. The working gas is introduced into a Laval-type plasma generator, where it undergoes ionization and dissociation under the action of a high-temperature electric arc generated between the cathode and anode, forming a high-temperature plasma containing argon ions and hydrogen atoms. The nozzle structure of this generator adopts a specific geometric surface design, and the inner wall profile of the expansion section follows the Witoszynski curve. This curve equation is used to ensure that the boundary layer does not separate during the acceleration of the airflow, suppress shock wave generation, maintain the laminar flow state of the flow field, and reduce the turbulent dissipation of the airflow. The heat flux effect depth, i.e., the skin depth δ, is a calculated value based on the thermal properties of the molten aluminum and the characteristics of airflow dynamics, and its value is determined by the characteristic length formula of the unsteady-state heat conduction equation. α is the average thermal diffusivity of molten aluminum in the temperature range of 750℃ to 800℃, with a value of 3.2 × 10⁻⁶.-5 m 2 / s to 3.6×10 -5 m 2 / s, τ flight The characteristic flight time of a droplet traversing a stagnant boundary layer; the characteristic flight time is determined by the boundary layer thickness L. boundary Values ​​ranging from 10 μm to 50 μm are related to the relative velocity v of the airflow. rel The ratio is determined, and the airflow velocity is changed by adjusting the inlet stagnation pressure P0 of the generator, thus changing τ. flight Control at 10 9 The order of magnitude of s makes the calculated The value falls within the range of 10 nm to 100 nm; the chemically frozen state of hydrogen atoms is maintained through spectral monitoring, and the emission spectrum at the nozzle exit is collected using an optical probe. The ratio of the integrated intensity I of the hydrogen atom Balmer series Hα spectral line at 656.28 nm to that of the hydrogen molecule spectral line in the 600 nm to 620 nm band is calculated. H / I H2 When the intensity ratio is greater than a preset threshold of 5.0, the jet is determined to be in a chemically frozen state, which is used to adjust P0 / P. b Pressure ratio closed-loop feedback basis; nozzle outlet cross-sectional area A is set in the process. exit With the throat cross-sectional area A throat The ratio A exit / A throat >3.8, and simultaneously, by adjusting the working efficiency of the gas compressor unit and vacuum pump unit through frequency conversion, the generator inlet stagnation pressure P0 and the atomization chamber ambient pressure P are controlled. b The ratio P0 / P b >18, this pressure ratio is based on the critical freeze expansion relationship P0 / P b ≥(1+0.33Ma 2 ) 2.5 The value of Ma is set to Mach number at the nozzle exit. Under this condition, the value of Ma is greater than 2.4. Driven by this expansion ratio, the airflow undergoes isentropic expansion, and the static temperature drops to below 2000K within a microsecond-level expansion characteristic time. Since the expansion characteristic time of this airflow is less than the chemical characteristic time of the three-body recombination reaction of hydrogen atoms, the hydrogen atoms in the airflow do not have time to collide and recombine in the gas phase space, and are thus locked in a thermodynamically non-equilibrium chemically frozen state. At this time, the energy input to the system is mainly stored in the form of chemical potential energy of hydrogen atoms, and the airflow exhibits the characteristics of a low-temperature, high-speed, and high-chemical-energy frozen flow.

[0029] This method utilizes the aforementioned supersonic jet in a chemically frozen state to perform shearing, fragmentation, and interface modification of molten metal. Aluminum ingots with a purity of 99.99% or higher are melted in an induction melting furnace and superheated to 750-800 degrees Celsius. A high-temperature resistant ceramic guide tube with an inner diameter of 2-4 mm is used to guide the molten aluminum into the supersonic jet's central axis region, 15-25 mm downstream of the nozzle outlet. The selection of the guide tube's outlet position is verified through flow field simulation to ensure that the molten aluminum flows directly into the jet's convergence point. When the high-speed chemically frozen flow impacts the sheared and fragmented micron-sized aluminum droplets, a stagnation layer with a thickness on the micron scale is formed on the droplet's windward side. Within the boundary layer, the relative velocity of the airflow stagnates to zero, and kinetic energy is converted into internal energy, leading to a local temperature rise. Exposed aluminum atoms on the droplet surface act as a third-party catalyst, lowering the activation energy of the hydrogen atom recombination reaction and inducing the in-situ exothermic recombination reaction 2H→H2 of high-energy hydrogen atoms within the stagnant boundary layer. The chemical energy released by this reaction is input into the aluminum droplet as a heat flux. Due to the surface catalytic mechanism, the depth of this heat flux is limited to the skin depth range of 10 to 100 nanometers on the surface of the aluminum droplet. The specific geometric shape of the inner wall profile of the expansion section is determined by the axial coordinates. With radial radius The function relationship is determined and used by CNC machining centers to generate toolpaths; the function relationship is set as follows. , where r throat r is the radius of the throat. exit L is the nozzle exit radius, L is the axial length of the expansion section, and x ranges from 0 to L. During processing, the point cloud of the nozzle inner wall profile is scanned by a coordinate measuring machine. The deviation between the actual profile and the theoretical curve normal is controlled within ±5μm, and the surface roughness Ra is less than 0.4μm to eliminate boundary layer transition caused by wall roughness. Geometric constraints force the supersonic airflow to accelerate monotonically along the axial direction, preventing the generation of oblique shock waves or normal shock waves inside the expansion section, and ensuring that the root mean square error of the Mach number distribution is less than 0.05 when the jet reaches the position of the molten aluminum flow.

[0030] This localized heat input allows the outermost layer of the aluminum droplet to maintain a transient isothermal layer during flight, with the hydrodynamic viscosity remaining below 2 mPa·s at a liquid phase level. This enables the droplet surface to contract into a spherical shape according to the principle of minimizing surface tension under cooling conditions. Simultaneously, the core region of the droplet, not subjected to direct chemical heating, maintains a viscosity greater than 10 mPa·s under the convective heat transfer effect of the external supersonic airflow. 5A rapid cooling rate of K / s ensures the retention of metastable nanocrystalline or amorphous microstructures within the powder. Furthermore, this interfacial thermal regulation mechanism addresses satellite sphere defects. During atomization, to prevent excessive hydrogen dissolution, the hydrogen flow rate is dynamically matched to the aluminum melt flow rate, controlling the ratio of hydrogen volumetric flow rate to aluminum melt flow rate within the range of 0.05 m³ / kg to 0.08 m³ / kg. Under this ratio, the liquid phase window on the droplet surface is extended. When satellite droplets, with a diameter less than one-tenth that of the main particle, entrained in the gas flow, impact the main particle surface, the low-viscosity liquid phase layer on the main particle surface drives the satellite droplets to completely wet and fuse into the main particle within 10 microseconds. Unlike traditional processes where surface semi-curing leads to physical adsorption and adhesion, this method fixes the already formed spherical morphology and prevents reverse hydrogen dissolution. An annular inert gas quenching zone is set downstream of the stagnant boundary layer. A high-purity helium gas stream at a temperature below 20 degrees Celsius is injected to carry away the hydrogen molecules generated in the reaction and seal the liquid phase window on the droplet surface, freezing the morphology and structure simultaneously. The finally cooled and solidified powder is collected by a cyclone separator with the gas stream. The entire process is completed under an inert atmosphere. The resulting nano-aluminum powder has a particle size distribution (D50) of less than 30 micrometers, a true sphericity greater than 0.95, an internal grain size of less than 100 nanometers, an oxygen content of less than 600 ppm, and a Hall flow rate of less than 50 seconds per 50 grams.

[0031] Example 1: In an industrial application scenario for the preparation of additive manufacturing raw materials for aerospace-grade high-strength aluminum alloys, the production line faces the physical limit challenge of preparing highly spherical nanoscale aluminum powder. Specifically, the strong coupling effect between the Reynolds number and the Nusselt number causes surface quenching and solidification of the droplets before they complete spherical shrinkage. When the system operates under this condition, a working gas composed of 95.0% high-purity argon and 5.0% high-purity hydrogen is used to control the dew point at -76 degrees Celsius. This low dew point environment strips away the oxide film on the surface of the molten aluminum, completely exposing the aluminum atoms. The working gas is input into a Laval-type plasma generator, with a nozzle exit cross-sectional area A. exit With the throat cross-sectional area A throat The ratio is set to 4.0, and the generator inlet stagnation pressure P0 and the atomization chamber ambient pressure P bThe ratio is maintained at 20. Driven by these aerodynamic parameters, the Mach number at the jet exit reaches 2.5. The decrease in the static temperature of the airflow locks hydrogen atoms into a thermodynamically non-equilibrium chemically frozen state, making the airflow a cryogenic fluid carrying high chemical potential energy. The supersonic jet in the chemically frozen state impacts the molten aluminum flow located 20 mm downstream of the nozzle exit, establishing a stagnant boundary layer on the windward side of the droplet. The catalytic activity on the aluminum surface induces an in-situ exothermic recombination reaction 2H→H2 for hydrogen atoms. The heat flux released by this reaction compensates for the convective cooling loss caused by the supersonic airflow, maintaining the hydrodynamic viscosity of the aluminum droplet surface in the range of 10 nm to 100 nm at the liquid phase level, while the core region of the droplet still maintains a viscosity of 10 nm. 5 With a cooling rate of K / s, this surface thermal compensation mechanism eliminates the time competition between surface tension shaping and rapid internal solidification, allowing the micro-droplets to shrink into spheres during flight according to the principle of minimizing surface tension. When satellite droplets entrained in the airflow collide with the main particle, the liquid phase layer on the surface of the main particle drives it to wet and fuse. The final aluminum powder has a particle size distribution D50 of 28 micrometers, a true sphericity of 0.96, and retains a nanocrystalline structure inside.

[0032] Example 2: This example constructs a realistic industrial-grade plasma atomization experimental platform. Industrial aluminum ingots with a purity of 99.995% were used as raw materials. The platform is equipped with a 100kW rated power radio frequency plasma generator, a precision gas mixing and purification system, a high-vacuum atomization tower with real-time process monitoring, and a powder collection and grading unit. By integrating a high-speed camera, a laser particle size analyzer, and an infrared thermal imager, the platform simultaneously acquires data on the flow field morphology, droplet breakup evolution, and temperature field distribution in the atomization zone, simulating and analyzing real thermophysical processes in industrial production. To comprehensively verify the synergistic effect of the technical solution of this invention and determine the boundary conditions of key parameters, this experiment designs a multi-dimensional control system, including the sample group of this invention using the complete technical solution, and two control groups designed to isolate variations. The control group was used to reveal the mechanism. Control group A used pure argon gas without hydrogen as the working gas, and the other process parameters were the same as those of the present invention. The chemical potential energy factor was eliminated, and the physical shearing effect of the supersonic gas flow was examined. Although control group B used the same argon-hydrogen mixture as the present invention, the Mach number of the plasma generator nozzle exit was limited to below 1.5. This simulated a non-chemically frozen flow state, in which hydrogen atoms recombine in the gas phase before reaching the droplet, thus verifying the key role of the thermodynamic state of chemical freezing in the spatiotemporal selective release of energy. To verify the rationality of the process parameter range, a subgroup with a hydrogen concentration gradient was also set up within the present invention. All experiments were carried out under strictly controlled conditions, with the ambient back pressure maintained at 0.10 MPa and the aluminum liquid superheat controlled at 50°C.

[0033] After the experiment was started, each group operated according to the predetermined procedure. The microscopic images captured by the high-speed camera showed that in control group A, the aluminum liquid flow broke up rapidly under the strong shear of the supersonic argon gas flow, but the generated droplets exhibited a solidified morphology with a trailing tail after flying a very short distance, indicating a surface quenching effect and that the surface tension failed to complete the shaping in time. In control group B, although hydrogen was introduced, due to the insufficient expansion speed of the gas flow, the infrared thermal imager detected a significant overall temperature rise in the core area of ​​the jet, indicating that hydrogen atoms mainly recombine and release heat in the gas phase space, resulting in a diffuse energy distribution and failure to form localized thermal compensation for the droplet surface. A large number of irregular particles and satellite spheres were still present in the collected powder. In contrast, the sample group of this invention showed completely different evolutionary characteristics: after the droplets left the breakup zone, the surface brightness showed a specific ring-shaped enhancement in the infrared thermal spectrum, while the core area temperature remained low. This directly confirmed the occurrence of the interfacial catalytic recombination reaction. The droplets rapidly contracted into regular spheres during flight, and the fine satellite droplets merged and disappeared instantly upon contact with the main particle.

[0034] Table 1: Comparison of Key Performance Indicators of Aluminum Powder Prepared by Each Experimental Group

[0035]

[0036] Referring to Table 1, the data clearly reveal the synergistic effect mechanism among the technical features. Although control group A achieved a fine particle size (D50 of 26.5 μm), its extremely low sphericity (0.78) and severe satellite sphere defects (accounting for 18.5%) confirm that relying solely on physical shearing cannot resolve the contradiction between fine powder and irregular shape. Although control group B improved sphericity, due to the lack of a chemical freezing mechanism, its energy utilization efficiency was low and it caused grain coarsening (indirectly reflected in the increase of D50 to 35.2 μm). Only the sample group of this invention, while maintaining the nanoscale fine powder characteristics of D50 of 27.8 μm, improved the sphericity to 0.97, reduced the proportion of satellite spheres to below 1.0%, and increased the Hall flow rate to 46.5 s / 50 g. This result strongly demonstrates that the combination of supersonic chemical freezing flow and interfacial catalytic recombination successfully achieves thermodynamic decoupling of morphology shaping and cooling solidification at the microscale. A baseline spherical shape is obtained through transient surface thermal compensation without sacrificing the cooling rate (ensuring fine grains). To verify the criticality of the hydrogen concentration range, tests were conducted at the boundary conditions of 2.0% (below the lower limit) and 9.0% (above the upper limit). The results show that when the hydrogen concentration is 2.0%, the true sphericity drops to 0.82, indicating that the heat flux is insufficient to maintain the surface liquid phase. While the sphericity is good when the concentration rises to 9.0%, metallographic microscopy analysis of the powder cross-section shows that the internal pinhole porosity surges to 3.5%, severely affecting the material's density.

[0037] Example 3: This example combines Figures 1 to 3This paper describes a method for achieving perfect sphericity control of nano-aluminum powder through plasma atomization, as follows: Figure 1 As shown, the method involves preparing an atomized working gas and induction melting aluminum ingots. The working gas consists of 93% to 96% argon and 4% to 7% hydrogen by volume, with a dew point controlled below -75°C. The aluminum ingot has a purity greater than 99.99% and is heated to 750°C to 800°C. The working gas is introduced into a Laval-type plasma generator to form a chemically frozen supersonic jet with a Mach number Ma>2.4. Simultaneously, molten aluminum is introduced into the central axis of the jet through a high-temperature resistant guide tube and is instantaneously sheared. The process involves breaking down aluminum droplets and constructing a stagnant boundary layer on the windward side of the droplets. This layer converts kinetic energy into internal energy and exposes the active sites of metallic aluminum. The boundary layer environment induces an in-situ exothermic recombination reaction, namely surface catalysis of 2H→H2, to form a transient isothermal layer. This process drives the elimination of satellite sphere defects by using low-viscosity liquid phase fusion to eliminate physical adsorption. Finally, the liquid phase window is forcibly sealed by an inert gas quenching zone formed by a helium flow at a temperature below 20°C. Ultimately, a finished nano-aluminum powder product with D50 < 30 μm, true sphericity > 0.95, and no satellite spheres is obtained.

[0038] like Figure 2 As shown, the horizontal axis represents the percentage of hydrogen concentration by volume (%), the left vertical axis represents sphericity, and the right vertical axis represents the defect rate (%). The solid line in the graph represents the trend of sphericity change, the dashed line represents the trend of satellite sphere percentage change (%), and the dotted line represents the trend of pinhole defect rate (%). The data shows that as the hydrogen concentration increases from 2% to 6%, sphericity increases while the satellite sphere percentage decreases significantly, indicating an enhanced thermal compensation effect. However, when the hydrogen concentration exceeds 7%, although the satellite sphere percentage continues to decrease, the pinhole defect rate increases exponentially, and sphericity begins to decline, confirming that 4% to 7% is the optimal process window for balancing morphology control and internal density. Figure 3 As shown, the system connects downstream to a gas processing unit via an argon / hydrogen cylinder group at the raw material supply station to perform mixing / purification / heating. The processed gas is then introduced into a radio frequency plasma generator with a Laval nozzle. Simultaneously, the induction melting furnace, with a temperature controlled at 750-800℃, transports the melt to a high-vacuum atomization tower for stagnation / quenching. This atomization tower receives the jet from the plasma generator. The reaction products enter a powder collection system for cyclone separation / collection. In addition, the central control cabinet performs PID control / real-time monitoring and establishes electrical connections with the gas processing unit, radio frequency plasma generator, high-vacuum atomization tower, and induction melting furnace to form a closed-loop control circuit to ensure process stability.

[0039] Example 4: In a stability verification scenario for continuous industrial production, the system needs to address the interference of molten aluminum flow velocity fluctuations and heat accumulation caused by long-term operation on the thermodynamic state of the chemically frozen flow field. Under this long-cycle operation condition, this example employs a closed-loop control strategy based on real-time feedback to maintain the dynamic matching accuracy of the aluminum molten mass flow rate and hydrogen volumetric flow rate. A high-temperature Coriolis mass flow meter is installed upstream of the guide pipe to monitor the molten aluminum mass flow rate (m·s) in real time with a sampling period of 10 milliseconds. Al The control system receives the flow signal and, based on the preset chemical potential coupling ratio coefficient k=0.065m 3 / kg (this value is selected within the effective range of 0.05 to 0.08), the target hydrogen volumetric flow rate V is calculated in real time using a PID algorithm. H2 =k×m˙ Al The calculation results directly drive the piezoelectric proportional control valve connected to the high-purity hydrogen pipeline, ensuring that the amount of hydrogen entering the mixing chamber always follows the change in aluminum liquid flow rate, thus maintaining a constant potential chemical heat flux obtainable per unit mass of aluminum liquid surface. Simultaneously, to ensure that the catalytically active sites on the aluminum liquid surface are not covered by oxide film during long-term operation, the system implements active thermal regeneration management of the working gas dew point. An online dew point meter monitors the humidity of the mixed gas in real time. When the dew point reading is higher than -76 degrees Celsius, the system automatically switches to a backup zirconium-vanadium-iron alloy purification column that has undergone 400-degree Celsius vacuum regeneration, and lowers the dew point back to below -78 degrees Celsius within 30 seconds. Furthermore, the position of the inert gas quenching zone downstream of the stagnant boundary layer is not fixed, but rather determined based on the droplet average flight velocity v measured by online particle image velocimetry (PIV). avg Adaptive adjustment is performed, and the axial distance L of the quenching nozzle is determined according to the formula L=v avg ×t sphere Set, where t sphere Based on the theoretical spheroidization time calculated from the instantaneous Weber number of the droplet, and through the aforementioned dynamic compensation mechanism, during a 4-hour continuous production process, the standard deviation of the true spheroidity of the powder sampled every 15 minutes was controlled within 0.005, and the Hall flow rate fluctuation was less than 1.2 seconds. This confirms that by eliminating uncertainties in process control, laboratory-level technical indicators can be stably transformed into industrial production quality capabilities.

[0040] Example 5: When migrating and deploying the present invention solution to heterogeneous production lines with different equipment specifications, a standardized on-site initialization and calibration procedure needs to be executed to eliminate the risk of process deviation caused by differences in plasma generator geometry, power supply characteristics, and fluctuations in vacuum system pumping capacity. On-site optimization of the critical parameters for gas flow chemical freezing is performed. Under no-load conditions without the introduction of molten aluminum, the hydrogen concentration of the working gas is set to 5.0%, and the generator inlet stagnation pressure P0 and the atomization chamber ambient pressure P are gradually adjusted.b The ratio of the intensity ratio of hydrogen atom spectral lines to hydrogen molecule spectral lines is monitored in real time using a spectral diagnostic system installed downstream of the nozzle outlet. The pressure ratio at which this intensity ratio begins to show an exponential upward trend is recorded as the critical freezing pressure ratio threshold of the local equipment. The actual production pressure ratio is set to 1.2 times this threshold to ensure process stability. Secondly, dynamic calibration of the coupling coefficient between aluminum liquid flow rate and hydrogen flow rate is performed. At the rated aluminum liquid flow rate, at 0.005m... 3 / kg is used to fine-tune the hydrogen volumetric flow rate until the droplet spheroidization completion point captured by the online high-speed camera system accurately falls on the center plane of the preset inert gas quenching zone. The flow rate ratio recorded at this time is the optimal chemical potential energy coupling ratio coefficient of the production line. Finally, adaptive correction of the quenching position is performed. By adjusting the axial position of the quenching nozzle, the satellite sphere defect rate in the collected powder is reduced to the minimum, and the specific quenching space parameters of the production line are locked.

[0041] Example 6: This example establishes an offline calibration and pre-debugging procedure to ensure the absolute reproducibility of the process. Before deploying this technical solution on a specific plasma atomization production line, the initial calibration of key physical parameters is performed. A three-dimensional scan of the Laval nozzle outlet flow field under no-load conditions is conducted using standard particle image velocimetry (PIV) technology to construct an actual flow field velocity distribution map, which is then compared with the theoretical design model. When the velocity deviation along the flow field's central axis exceeds a preset threshold (e.g., 2%), the nozzle inlet stagnation pressure is fine-tuned according to the fluid dynamics similarity criterion until the actual Mach number distribution matches the theoretical value. Simultaneously, offline physical property parameter calibration is performed to address the slight differences in surface tension coefficient that may exist between different batches of aluminum ingot raw materials. The surface tension coefficient of molten aluminum is measured within a specific temperature range using the pendant drop method, and the measured values ​​are input into the spheroidization time prediction model to correct the theoretical spheroidization time t. sphere Based on the corrected spheroidization time, the optimal axial position L of the inert gas quenching nozzle is recalculated and locked to ensure that droplets are frozen in the optimal spheroidization state in each batch of production. A baseline calibration process for trace impurities in the working gas is established. During the system cold start-up phase, standard hydrogen of known concentration is injected into the pure argon gas flow step by step, and the intensity response curve of the characteristic spectral lines of hydrogen atoms is calibrated using a spectral diagnostic system to avoid instrument drift error of the spectral detection system and ensure the absolute accuracy of online monitoring data.

[0042] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for controlling the perfect sphericity of nano-aluminum powder through plasma atomization, characterized in that, The method includes the following steps: Atomizing gas is prepared, which consists of 93.0% to 96.0% argon and 4.0% to 7.0% hydrogen by volume, and the dew point of the atomizing gas is controlled below -75 degrees Celsius to eliminate the oxide layer on the surface of molten aluminum and expose metallic aluminum atoms as heterogeneous catalytic active sites. The atomized propellant gas is introduced into a Laval-type plasma generator. The ratio of the nozzle outlet cross-sectional area to the throat cross-sectional area is set to be greater than 3.8, and the ratio of the generator inlet stagnation pressure to the atomization chamber ambient pressure is controlled to be greater than 18. The atomized propellant gas is driven to undergo isentropic expansion and form a supersonic jet with a Mach number greater than 2.

4. By controlling the gas flow expansion characteristic time to be less than the hydrogen atom chemical recombination characteristic time, hydrogen atoms are locked in a thermodynamically non-equilibrium chemically frozen state. The supersonic jet in a chemically frozen state is guided to impact the molten aluminum flow located 15 to 25 mm downstream of the nozzle exit, and a stagnant boundary layer is constructed on the windward side of the aluminum droplets formed by shearing and breaking them up, where the flow velocity is stagnant and the kinetic energy is converted into internal energy. Using aluminum atoms on the surface of aluminum droplets as a third-party catalyst, an in-situ exothermic recombination reaction is induced in high-energy hydrogen atoms within the stagnant boundary layer. The interfacial chemical energy released by the reaction maintains the rheological properties of the liquid phase on the surface of the aluminum droplets until they shrink into a spherical shape under the drive of surface tension, and then expand, cool, and solidify with the airflow.

2. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, The step of preparing the atomizing gas further includes: controlling the oxygen impurity content in the atomizing gas to a level below 2 ppm; this limitation on the oxygen impurity content is used to prevent oxygen atoms from preferentially reacting with metallic aluminum atoms to form an oxide film and thus competing for catalytic active sites.

3. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, The Laval-type plasma generator includes an expansion section with a specific profile: the inner wall profile of the expansion section follows a smooth transition designed with the Wittsinski curve; the specific geometry of the expansion section is used to suppress the separation of the gas flow boundary layer and the generation of shock waves, ensuring that the supersonic jet remains in a laminar state before reaching the molten aluminum flow; the laminar state is used to minimize the turbulent dissipation of the gas flow during the transmission process and to maximize the preservation of the chemical potential energy density of hydrogen atoms in the chemically frozen state.

4. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, In the step of controlling the ratio of the generator inlet stagnation pressure to the atomization chamber ambient pressure to be greater than 18, the inlet stagnation pressure P0 and the atomization chamber ambient pressure P b The critical freeze-thaw relationship between them is satisfied as follows: P0 / P b ≥(1+0.33Ma 2 ) 2.5 , where Ma is the Mach number of the supersonic jet at the nozzle exit; this relationship defines the thermodynamic boundary conditions required to maintain the gas flow expansion cooling rate above the hydrogen atom recombination reaction rate.

5. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, Prior to the step of guiding the chemically frozen supersonic jet to impact the molten aluminum flow located 15 mm to 25 mm downstream of the nozzle exit, the method further includes: heating an aluminum ingot with a purity of 99.99% or higher to a melting temperature of 750°C to 800°C in an induction melting furnace; introducing the molten aluminum into the central axis region of the supersonic jet using a guide tube with an inner diameter of 2 mm to 4 mm; and aligning the exit position of the guide tube with the convergence point of the supersonic jet.

6. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, In the step of maintaining the liquid-phase rheological properties of aluminum droplets using the interfacial chemical energy released by the reaction, the interfacial chemical energy is input into the aluminum droplets in the form of heat flux, and the depth of the heat flux is limited to the skin depth range of 10 nm to 100 nm on the surface of the aluminum droplets. This localized heat input maintains the hydrodynamic viscosity within the transient isothermal layer on the surface of the aluminum droplets below 2 mPa·s, while the core region of the aluminum droplets maintains a viscosity greater than 10 mPa·s under the convective heat transfer effect of the external supersonic airflow. 5 A cooling rate of K / s enables thermodynamic decoupling between spherical shaping and retention of internal nanocrystalline structure at the single-particle scale.

7. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, The step of using aluminum atoms on the surface of aluminum droplets as a third-party catalyst further includes: eliminating satellite sphere defects by utilizing the local high-temperature zone generated by the in-situ exothermic composite reaction; when microdroplets with a diameter less than one-tenth of the main particle impact the surface of the main particle, the low-viscosity environment provided by the local high-temperature zone drives the microdroplets to be completely wetted and integrated into the interior of the main particle within 10 microseconds, preventing the microdroplets from adhering to the surface of the main particle in the form of physical adsorption to form a satellite sphere structure.

8. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, The method also includes setting an inert gas quenching zone downstream of the stagnant boundary layer. After the aluminum droplet completes spherical shrinkage, the exothermic recombination reaction is terminated by spraying a helium gas flow with a temperature below 20 degrees Celsius into the quenching zone. The intervention of the helium gas flow is used to remove residual hydrogen molecules and forcibly close the liquid phase window on the surface of the aluminum droplet, thus freezing the formed reference spherical morphology and the internal metastable microstructure simultaneously.

9. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, In the process of preparing the atomized gas, the specific volume percentage of hydrogen is dynamically matched according to the flow rate of the molten aluminum; the ratio of hydrogen volume flow rate to aluminum mass flow rate is set to be maintained in the range of 0.05 cubic meters per kilogram to 0.08 cubic meters per kilogram.

10. The method for controlling the perfect sphericity of nano-aluminum powder by plasma atomization according to claim 1, characterized in that, The method also includes the steps of collecting and sieving the cooled and solidified powder: using a cyclone separator under inert gas protection to collect nano-aluminum powder with a particle size distribution D50 of less than 30 micrometers; the nano-aluminum powder has a true sphericity greater than 0.95 and an internal grain size of less than 100 nanometers; the oxygen content of the powder is less than 600 ppm and the Hall flow rate is less than 50 seconds per 50 grams; the method ensures the consistency of powder morphology and microstructure between batches under continuous production conditions through the selective energy release mechanism of chemically frozen hydrogen atoms at the gas-liquid interface.

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