Method and apparatus for removing oxide layer from metallic powders

Abstract

An apparatus is proposed for removing the oxide layer from metallic powders produced by a twin-wire arc plasma atomization process. A sonification system breaks the brittle oxide layer on the particle surface by creating high-pressure waves within the solution due to the cavitation phenomenon. After a certain exposure time, the ultrasonic process stops, and the powder is transferred to a vessel. The suspended oxide particles are removed from the drain of the vessel by means of a peristaltic pump. The wet powder cake is subsequently rinsed within the vessel to remove any residual solvent. Following the rinsing process, the wet powder cake is subjected to vacuum drying. This drying process ensures the powder is rendered moisture-free. Utilizing a vacuum accelerates the drying time and mitigates the risk of excessive re-oxidation by eliminating oxygen from the process. Finally, the fully dried powder is transferred into a canister.

Description

METHOD AND APPARATUS FOR REMOVING OXIDE LAYER FROMMETALLIC POWDERSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims priority on U.S. Provisional Application No. 63 / 532,670, now pending, filed on August 14, 2023, which is herein incorporated by reference.FIELD

[0002] The present subject matter relates to advanced materials and, more particularly, to the post-processing of metal powders produced by atomization for diverse applications, such as additive manufacturing for the aerospace and medical industries.BACKGROUND

[0003] 3D printing is a relatively new technique for production of parts, for example metallic components, with reduced loss of material and the ability to fabricate more intricate shapes than conventional methods (for example, machining and die casting).

[0004] In a 3D printer for metallic parts, successive layers of metallic powder are laid and fused together by lasers or an electron beam.

[0005] To maximize the quality of the printed part and ensure good fusion of the metallic particles, critical powder quality properties must meet strict requirements, namely density, particle size distribution, and oxygen content. Excessive oxygen content detrimentally affects the cohesion of the printed part.

[0006] Several methods exist to produce metallic powders for 3D printing, such as water jet atomization, gas atomization, electrode inert gas atomization (EIGA), and, most notably, plasma atomization.

[0007] Plasma atomization produces arguably the finest, most spherical, and densest powders with high yield in the desirable 0-106-micron range, while minimizing gas entrapment which affects powder density and printing quality.

[0008] Plasma atomization typically uses a wire as a feedstock and a source of plasma generated by one or several plasma torches as atomizing agent to simultaneously melt and break up the particles. Wire is preferable to a powder or liquid feedstream because the pressure of the plasma plume causes these latter materials to bounce back. Using a wire provides the strength and rigidity required to ensure the narrow plasma jets are aimed properly, to melt the wire and atomize it in a single step.

[0009] However, the main disadvantage of this technology is its relatively low production rate in comparison to water and gas atomization, since plasma atomization is an energetically inefficient process. Reported production rates for plasma atomization are between 2.7 and 13 kg / h. To improve the production rate, a twin wire arc technology has been proposed according to US patent application Publication No. US 2021 / 0229170 A1. This technology essentially consists of a high current passing through two wires to generate an electrical arc. The high energy intensity near the wire tips provides a desirable condition for rapid melting and atomization, resulting in higher production rates. However, this high energy intensity can cause excessive vaporization and condensation of the metal, whether pure or in alloy form, during the process and result in final powder oxygen concentrations above the specified maximum required for certain additive manufacturing applications. Similar excessive vaporization and subsequent deposition have been observed in other processes, including earlier iterations of plasma atomization technologies (U.S. Patent No. 5,707,419) and plasma spheroidization using an inductively coupled plasma torch (U.S. Patent No. 7,572,315 B2).

[0010] Fine particles generated from vaporization oxidize more easily due to their relatively high reactive surface area and thus condense on the surface of larger size particles as oxide crusts. For instance, Ti-6AI-4V alloy forms an AI2O3 layer on the surface of atomized powder. The presence of this oxide crust, an accumulation of fine dust, increases the overall oxygen content of the powder.

[0011] It would therefore be desirable to remove this oxide crust to reduce oxygen content of the powder and improve its performance in the 3D printing process and, ultimately, the quality of the parts.

[0012] Several methods have been proposed for the removal of this fine dust from powder.

[0013] For example, ultrasonic solution cleaning is known to remove contamination from surfaces by imposing high frequency energy, induced by a probe in a solution. The high-frequency movement of the probe creates cavitation within the solution, generating high-intensity shear stress on the exposed surface and, in turn, removing contaminants from the original particle surface. A minimum intensity and duration are normally required in this process to achieve successful cleaning. A similar approach to resolve the oxide layer problem is proposed in aforementioned U.S. Patent No. US 7,572,315 B2, wherein the oxide layer is removed by applying ultrasonic vibration in sonification medium. However, the process is dependent on the volume of the sonification medium and its powder loading, limiting process capacity.

[0014] Another common way of cleaning surfaces is to use acidic or caustic solution using chemical reactions to remove the impurities from the surface. One of the most common solutions for cleaning surfaces is acetic acid due to its low price and availability compared to other chemical solutions. Various solvents, including water or other chemical solutions, could be utilized as cleaning agents.

[0015] The liquid separation technique is another important factor to be considered since it removes the suspended oxide particles from the target material. In aforementioned U.S. Patent No. US 7,572,315 B2, it is suggested that the liquid shall be removed through a sieve by gravitational force and differentialsedimentation; however, this stage could be a time-consuming step in a powder production line.

[0016] It is also known that extended exposure to oxygen or moisture results in oxidation of metallic surfaces, especially at elevated temperature. Hence, it is necessary to eliminate moisture from the targeted surface with minimum exposure to heat and oxygen.

[0017] U.S. patent application Publication No. US 2008 / 0173594 A1 describes an apparatus to combine liquid separation and drying into one single unit. However, this apparatus could potentially re-introduce suspended oxide powders into the targeted material once the liquid passes through powder bed and sieve.

[0018] Therefore, it would be desirable to devise an apparatus that extracts the suspended oxide particles from the mother liquid prior to final separation.SUMMARY

[0019] It would thus be desirable to provide a novel apparatus and method to reduce oxygen content from metallic powders.

[0020] The embodiments described herein provide in one aspect an apparatus for removing an oxide layer from metallic powders, for instance produced by a twin-wire arc atomization process, comprising an ultrasonic system applied to a mixture of chemical solution and metallic powder.

[0021] Also, the embodiments described herein provide in another aspect wherein an intensity of a sonification power of the ultrasonic system is adjusted for effective oxide layer removal as well as the duration of the cleaning process.

[0022] Furthermore, the embodiments described herein provide in another aspect wherein the ultrasonic system includes one of a volumetric control system and a flow-through system.

[0023] Furthermore, the embodiments described herein provide in anotheraspect

[0024] Furthermore, the embodiments described herein provide in another aspect wherein a mechanical agitator is provided for producing a homogeneous mixture required for effective cleaning.

[0025] Furthermore, the embodiments described herein provide in another aspect wherein the cleaning solution comprises acetic acid.

[0026] Furthermore, the embodiments described herein provide in another aspect a device for pre-drying a washed powder produced by the apparatus of any one of Claims 1 to 5, wherein liquid is first separated from the powder by means of a peristaltic pump from a bottom followed by vacuum drying.

[0027] Furthermore, the embodiments described herein provide in another aspect wherein a cleaning vessel is under inert atmosphere (e.g., argon) prior to an introduction of powder in the apparatus.

[0028] Furthermore, the embodiments described herein provide in another aspect a device for liquid separation and pre-drying powder from the apparatus of any one of Claims 1 to 5, comprising a filter-dryer system where the filter is selected according to a smallest particle size, a vessel being adapted to separate the liquid from a bottom of the vessel by way of a peristaltic pump.

[0029] Furthermore, the embodiments described herein provide in another aspect wherein remaining moisture is extracted by heating the vessel under vacuum, and the vacuum is connected at a top and at a bottom of the vessel.

[0030] Furthermore, the embodiments described herein provide in another aspect wherein walls of the vessel are heated by a removable electric heated jacket.

[0031] Furthermore, the embodiments described herein provide in another aspect wherein walls of the vessel are heated by an embedded liquid heated jacket followed by a cooling stage prior to powder collection.

[0032] Furthermore, the embodiments described herein provide in another aspect wherein a temperature control unit is adapted to control a temperature of the liquid circulating through channels inside the walls of the vessel.

[0033] Furthermore, the embodiments described herein provide in another aspect wherein a mechanical agitator is provided to mix powder with rinsing water in the vessel, as well as to ensure continuous powder movement during the vacuum drying stage to enhance drying.

[0034] Furthermore, the embodiments described herein provide in another aspect wherein a knockout drum and a cold trap are provided to avoid moisture transfer toward a vacuum pump that is connected to the vessel.BRIEF DESCRIPTION OF THE DRAWINGS

[0035] For a better understanding of the embodiments described herein and to show more clearly how they may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, which show at least one exemplary embodiment, and in which:

[0036] Fig. 1 illustrates a process flow diagram starting from plasma atomized powder containing oxide layers and ending with the surface cleaned powder, in accordance with an exemplary embodiment;

[0037] Figs. 2 and 3 are two different schematics for the sonification of metallic powders in a chemical solution, using the ISP-3600 industrial scale processor equipped with a Half-wave Barbell Homs® sonification probe for volumetric and flow-through methods respectively, in accordance with exemplary embodiments;

[0038] Fig. 4 shows a schematic of an apparatus for rinsing and filtration of powders from the liquid and drying the filtered powders, in accordance with an exemplary embodiment;

[0039] Fig. 5 illustrates Scanning Electron Microscope (SEM) images and Energy-Dispersive X-ray Spectroscopy (EDS) results of Ti-6AI-4V Grade 23 before and after washing, in accordance with an exemplary embodiment; and

[0040] Fig. 6 is a graph plotting powder oxygen content reduction as a function of number of sonification cycles in flow-through mode, in accordance with an exemplary embodiment.DESCRIPTION OF VARIOUS EMBODIMENTS

[0041] The present subject matter disclosed herein provides methods and apparatuses for reducing oxygen content of atomized powder.

[0042] Fig. 1 illustrates a method for washing and drying powders that involves a sequence of steps, beginning with step 101 where oxide-covered powder is introduced into a specialized vessel. In step 102, the powder is thoroughly mixed with an aqueous solution to initiate the cleaning process. The mixture is then circulated in a closed loop by a pump as depicted in step 103. A portion of this circulating flow is directed in front of an ultrasonic probe; the use of ultrasonic cleaning in this stage effectively detaches and removes surface contaminants from the powder, creating a slurry mixture. Following the cleaning process, in step 104, the slurry mixture undergoes solid-liquid separation and powder rinsing. This is achieved through various mechanisms, one example being a Nutsche filter. A Nutsche filter performs multiple tasks, including filtration, displacement washing, reslurry washing, and vacuum or convection drying. This device separates the cleaned powder from the liquid. Once separated and rinsed, the wet powder proceeds to the drying phase, depicted in step 105. The end result of this comprehensive process is a dry, thoroughly cleaned powder with an oxide- and contaminant-free surface, as shown in step 106.

[0043] More particularly, Fig. 2 illustrates an apparatus for processing metallic powders in a volumetric control mode, wherein cleaning solvent (example: acetic acid) 201 and deionized water 202 are metered respectively via feed lines 203 and 204 into a vessel 210 to obtain a certain concentration. Thevessel 210 is then purged with argon inlet 205 and outlet 207 prior to introducing powder into the system via feed port 206, to avoid the risk of reaction between powder and trapped oxygen inside the vessel 210. An ultrasonic horn 208 attached to a transducer 209 is then placed into the processing volume to provide the power required for cleaning the powder in the mixture.

[0044] A favorable setup of the washing step is particularly shown in Fig. 3 for processing metallic powders in a flow-through mode, wherein additional elements are added to the process: a mechanical agitator 305, a peristaltic pump 309 and a return line 315 to the vessel 317. The supplementary features provide homogeneity and a circulation aspect to the slurry solution that allow the flexibility to scale up this process compared to the volumetric mode. In this configuration, the cleaning solvent and deionized water tanks 301 and 302, solution feed lines 303 and 304, purge argon inlet 306 and argon outlet 307, and a powder feed 308 remain as described hereinabove as to Fig. 2. The sonification takes place in a sonication chamber 311. Inside this sonication chamber 311 , a sonication horn 312 is mounted on a transducer 316 that is water-cooled by a chiller unit 313 to prevent excessive heat generation that might damage the transducer 316. The transducer 316 is connected to a control unit 314, which provides the power required for the ultrasonic cleaning process. A discharge port 310 is also provided for slurry transfer from the washing unit into the filtration, rinsing and drying stage shown in Fig. 4.

[0045] In the next stage, Fig. 4 shows an apparatus for separating metallic powders from a slurry mixture, rinsing the powder, and drying it. This process is conducted under argon inlet 402 and outlet 405, to minimize impurities contributing to oxygen content.

[0046] In this filtration step, while agitator 404 is rotating and valve 411 is closed, an inlet port 401 , which is connected upstream to the washing system, introduces the slurry mixture into a vessel 421. A sieve 410, placed at the bottom of the vessel 421 , separates the metallic powder from the slurry mixture accordingto the minimum particle size. The liquid is then transferred by a peristaltic pump 408 through a valve 407 into a barrel 409.

[0047] In the rinsing step, while the agitator 404 is operating, a port 403 introduces deionized water to rinse the powder, removing any residual acetic acid, with the liquid again being transferred into the “wastewater” barrel 409.

[0048] In the vacuum-drying step, while the valve 407 is closed, the vessel 421 is subjected to vacuum from the top 406 and bottom 411 , via a vacuum pump 415. A knockout drum 412, a cold trap 413, and a particle filter 414 are included in the setup to protect the vacuum pump 415 from moisture and solid particles. The vessel 421 is heated by a liquid jacket 416, covering the bottom plate and connected to a thermal control unit 418, and an electrically heated jacket 417 covering the vessel walls. The powder is agitated to break agglomerations and ensure sufficient surface exposure for effective drying.

[0049] Once the powder is dried, the electrical jacket 417 is turned off, and the temperature setpoint on the thermal control unit 418 is decreased. The cooled powder is then collected from the powder discharge port 419 into a canister 420.

[0050] The construction material of the described apparatuses is primarily stainless steel for corrosion resistance. However, the sonic horn is made of Ti- 6AI-4V for superior erosion resistance compared to SS 316, making it more suitable for applications involving high-velocity fluids, abrasive particles, or cavitation.

[0051] According to the described washing process of Fig. 3, the process functionality depends on the specific particle concentration in the solution. The flow-through mode washing process requires proper flowability of the slurry solution without experiencing separation of the solid and fluid phases. Excessive phase separation may cause interruption in operation or insufficient oxide removal because of the high packing density of the powder.

[0052] It is noted that the erosion rate of the sonification horn 312 also depends on the power intensity and powder concentration. The erosion rate increases with power intensity and powder concentration.

[0053] In each of the washing systems shown in Figs. 2 and 3, the duration of each process is another key factor to acquire sufficient sonification exposure as required for optimum cleaning. In the system of Fig. 3, the exposure time is defined as the number of cycles that the entire powder passes through the sonification chamber during circulation. The number of cycles can be estimated from the total volume of transfer liquid in the system, pump rate, and total duration Vxt of the process according to the following equation: N = — wherein N is the number of cycles, V is the pump rate, t is the total duration of the sonification process, and V is the total volume of transfer fluid.EXAMPLE RESULTS

[0054] Results from Ti-6AI-4V powder washing experiments demonstrate its effectiveness in improving powder surface quality and reducing oxygen content, as summarized in Figs. 5 and 6 and in below Table 1 .

[0055] The Scanning Electron Microscope (SEM) image and Energy- Dispersive X-ray Spectroscopy (EDS) results presented in Fig. 5 highlight the change in particle surface before and after washing. Post-washing SEM images clearly depict a smoother particle surface free of satellites (mostly Al-rich, highly oxidized mixture), indicating the successful removal of surface contaminants. Corresponding EDS data provide a semi-quantitative analysis of the chemical content of the particles. While EDS does not provide sufficiently precise compositional data, a notable decrease in oxygen content after the washing process is evident.

[0056] Fig. 6 further elucidates this decrease in oxygen content by mapping it as a function of the number of sonification cycles in the flow-through mode (as described hereinabove and illustrated in Fig. 3). The graph indicates a steep decline in oxygen content within the first few cycles, which stabilizes afterapproximately five cycles, suggesting that subsequent cycles do not contribute significantly to oxygen reduction.

[0057] Fig. 6 indicates a stepwise decrease of oxygen content obtained from powder analysis, corresponding with an increasing number of cycles calculated according to the above formula.

[0058] Table 1 provided hereinbelow compares powder oxygen content reduction based on production rate for volumetric and flow-through modes. The flow-through mode is evidently preferred due to its higher capacity (150 g / min) compared to the volumetric mode (2.78 g / min). In 50 minutes, the flow-through mode can process 7500 g to reach an average oxygen content of 888 ppm. Conversely, the volumetric mode takes a longer duration (90 minutes) to reach an oxygen content slightly above 1139 ppm and can only process 250 g at a time.Table 1 - Oxygen reduction comparison based on production rate for volumetric and flow-through modes

[0059] In summary, the flow-through mode demonstrates greater efficiency, processing a larger quantity of powder in a shorter time while achieving a lower oxygen content. This is significant since for certain applications, the oxygen content needs to be systematically below 1000 ppm. Therefore, for large- scale implementations, only the flow-through mode can meet these stringent requirements.

[0060] Therefore, the subject matter described hereinabove provides an apparatus for oxide removal of metallic powders from the particle surfaces, which comprises an ultrasonic apparatus in an aqueous solution as part of a process adapted with respect to the intensity of ultrasonic power generation, the acid concentration, and the process duration to obtain targeted oxygen content.

[0061] Also, the subject matter described hereinabove provides an apparatus for separating and removing moisture from metallic powders, which comprises a vessel with a drain from the bottom that removes liquid from the solid and a subsequent drying process under vacuum by applying a suction from the top and bottom to a heated vessel. The described solid-liquid separation apparatus is known as the “Agitated Nutsche Filter Dryer”.

[0062] While the above description provides examples of the embodiments, it will be appreciated that some features and / or functions of the described embodiments are susceptible to modification without departing from the spirit and principles of operation of the described embodiments. Accordingly, what has been described above has been intended to be illustrative of the embodiments and non-limiting, and it will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the embodiments as defined in the claims appended hereto.REFERENCES[1] Francois Proulx, Christopher Alex Dorval Dion, and Pierre Carabin, “Method and Apparatus for Producing High Purity Spherical Metallic Powders at High Production Rates from One or Two Wires”, U.S. Patent Application Publication No. 2021 / 0229170 A1 , July 29, 2021.[2] Maher I. Boulos, Christine Nessim, Christian Normand, and Jerzy Jurewicz, “Process for the Synthesis, Separation and Purification of Powder Materials”, U.S. Patent No. 7,572,315 B2, August 11 , 2009.[3] Alexey S. Peshkovsky, “Ultrasonic Horn with A Large High-Amplitude Output Surface”, U.S. Patent Application Publication No. 2020 / 0129953 A1 , April 30, 2020.[4] Robert Gerard Stoerzer, “Method and Device for Removal of Residual Products”, U.S. Patent Application Publication No. 2008 / 0173594 A1 , July 24, 2008.

Claims

CLAIMS1 . An apparatus for removing an oxide layer from metallic powders, for instance produced by a twin-wire arc atomization process, comprising an ultrasonic system applied to a mixture of chemical solution and metallic powder.

2. The apparatus of Claim 1 , wherein an intensity of a sonification power of the ultrasonic system is adjusted for effective oxide layer removal as well as the duration of the cleaning process.

3. The apparatus of any one of Claims 1 and 2, wherein the ultrasonic system includes one of a volumetric control system and a flow-through system.

4. The apparatus of any one of Claims 1 to 3, wherein a mechanical agitator is provided for producing a homogeneous mixture required for effective cleaning.

5. The apparatus of any one of Claims 1 to 4, wherein the cleaning solution comprises acetic acid.

6. A device for pre-drying a washed powder produced by the apparatus of any one of Claims 1 to 5, wherein liquid is first separated from the powder by means of a peristaltic pump from a bottom followed by vacuum drying.

7. The apparatus of any one of Claims 1 to 6, wherein a cleaning vessel is under inert atmosphere (e.g., argon) prior to an introduction of powder in the apparatus.

8. A device for liquid separation and pre-drying powder from the apparatus of any one of Claims 1 to 5, comprising a filter-dryer system where thefilter is selected according to a smallest particle size, a vessel being adapted to separate the liquid from a bottom of the vessel by way of a peristaltic pump.

9. The device of Claim 8, wherein remaining moisture is extracted by heating the vessel under vacuum, and the vacuum is connected at a top and at a bottom of the vessel.

10. The device of any one of Claims 8 to 9, wherein walls of the vessel are heated by a removable electric heated jacket.11 . The device of any one of Claims 8 to 10, wherein walls of the vessel are heated by an embedded liquid heated jacket followed by a cooling stage prior to powder collection.

12. The device of Claim 11 , wherein a temperature control unit is adapted to control a temperature of the liquid circulating through channels inside the walls of the vessel.

13. The device of any one of Claims 8 to 11 , wherein a mechanical agitator is provided to mix powder with rinsing water in the vessel, as well as to ensure continuous powder movement during the vacuum drying stage to enhance drying.

14. The device of any one of Claims 8 to 11 , wherein a knockout drum and a cold trap are provided to avoid moisture transfer toward a vacuum pump that is connected to the vessel.

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