ALONG WITH METHODS OF THEIR FORMATION

PRIORITY INFORMATION

[0001] The present application claims priority to U.S. Provisional Patent Application Serial No. 62/551,981 titled "High Quality Spherical Powders for Additive Manufacturing Processes Along with Methods of Their Formation" filed on Aug. 30, 2017, which is incorporated by reference herein.

FIELD

[0002] The present invention generally relates to systems and methods for forming high quality spherical powders from a metallic powder feedstock. The high quality spherical powders are particularly suitable for additively manufacturing an object or part.

BACKGROUND

[0003] Additive manufacturing processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though "additive manufacturing" is an industry standard term, additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of additive manufacturing terms, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model.

[0004] A particular type of additive manufacturing process uses an energy beam, for example, an electron beam or electromagnetic radiation such as a laser beam, to sinter or melt a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. Different material systems, for example, engineering plastics, thermoplastic elastomers, metals, and ceramics are in use. Laser sintering or melting is also a notable additive manufacturing process for rapid fabrication of functional prototypes and tools. Applications include patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of additive manufacturing processes.

[0005] Laser sintering is a common industry term used to refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. More accurately, sintering entails fusing (agglomerating) particles of a powder at a temperature below the melting point of the powder material, whereas melting entails fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material.

[0006] In this process, the physical and chemical characteristics of the powder material can impact the quality of the resulting object. That is, the properties of a component built through additive manufacturing depends on the metal powder itself, with higher quality powders (e.g., denser, cleaner, and more spherical) behaving more predictably and thus results in better parts. As such, high quality powder material is required for components formed from Additive Manufacturing techniques, particularly when used to manufacture components for gas turbine machinery and/or medical implant or devices applications.

[0007] Powder making methods from a metal source mainly (as there are other techniques like hydride/dihydride, ball milling, rotating electrode, plasma atomization etc.) include gas atomization and water atomization. Generally, gas atomization techniques result in particles with a more spherical and consistent shape, while water atomization techniques result in particles with an irregular shape. Additionally, due to the presence of oxygen in water, an oxidized layer may form on the outside of the particles formed by water atomization techniques. Currently, powders from gas atomization techniques are preferred for additive manufacturing over powders formed from water atomization techniques, since powders formed from gas atomization techniques are more regular in shape (e.g., more spherical) and have a limited oxidized layer thereon.

[0008] However, powders formed from gas atomization are much more expensive to produce than water atomization powders. Thus, the cost of the resulting component formed from a gas atomized powder is high. As such, a need exists for reducing the cost of high quality powders for additive manufacturing for higher adoption, while retaining control of the physical and chemical characteristics of the powder material.

BRIEF DESCRIPTION

[0009] Aspects and advantages will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

[0010] Methods are generally provided for forming a high-quality powder from a feedstock powder of feedstock particles having irregular shapes. In one embodiment, the method includes exposing the feedstock powder to a plasma field to form a treated powder of treated particles having a more spherical shape than the feedstock particles. Prior to the plasma field exposure, the feedstock particles have an oxidized layer thereon as a result from previous exposure to water. After exposure to the plasma field, the treated particles are substantially free from an oxidized layer.

[0011] In one embodiment, the feedstock powder may be formed from water atomization, mechanical crushing or grinding, gas atomization, and/or plasma atomization. For example, the oxidized layer on the feedstock particles may be a result of exposure to water during a water atomization process that formed the feedstock particles, or from exposure to water vapor in the air during mechanical grinding.

[0012] To expose the feedstock powder to the plasma field, the method may include introducing the feedstock powder into the plasma field such that the surface of the feedstock particles melts and/or evaporates to form the more spherical shape.

[0013] In particular embodiments, the plasma field includes a reducing component that reacts with the oxidized layer on the feedstock particles, such as hydrogen, carbon monoxide, or a mixture thereof.

[0014] Through such a method, the treated particles may have an average particle size that is less than an average particle size of the feedstock particles. For instance, the treated particles may have an average particle size that is about 10% to about 90% of the average particle size of the feedstock particles.

[0015] The feedstock particles may be formed from a metal material, such as a pure metal, an iron alloy, an aluminum alloy, a nickel alloy, a chrome alloy, a nickel-based superalloy, an iron- based superalloy, a cobalt-based superalloy, or a mixture thereof. In one embodiment, particles an alloying element, such as carbon, may be mixed with the feedstock particles within the plasma field.

[0016] In one embodiment, the method of forming a high-quality powder may include: forming a feedstock powder via water atomization such that the feedstock powder includes feedstock particles having irregular shapes and have an oxidized layer thereon; and thereafter, exposing the feedstock powder to a plasma field to melt and/or evaporate the surface of the feedstock particles such that a treated powder of treated particles is formed having a more spherical shape than the feedstock particles. The plasma field may include a reducing component (e.g., hydrogen, carbon monoxide, or a mixture thereof) that reacts with the oxidized layer on the feedstock particles such that the treated particles are substantially free from an oxidized layer. In one particular embodiment, the treated particles have an average particle size that is less than an average particle size of the feedstock particles.

[0017] The resulting treated powders comprising the treated particles are also generally provided herein, along with methods of additively manufacturing a component from such treated powders. [0018] These and other features, aspects and advantages will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain certain principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figs., in which:

[0020] FIG. 1 shows an exemplary apparatus for plasma spheroidization of a powder material improving the properties of a powder material such that the improved powder material may be more suitable for additive manufacturing techniques;

[0021] FIG. 2A is a scanning electron microscope (SEM) image of an exemplary feedstock powder according to Example;

[0022] FIG. 2B is a magnified SEM image of the exemplary feedstock powder of FIG. 2 A;

[0023] FIG. 3 A is a SEM image of an exemplary spheroidized powder formed from the feedstock powder shown in FIGs. 2A and 2B prior to washing according to Example;

[0024] FIG. 3B is a magnified SEM image of the exemplary spheroidized powder of FIG. 3 A;

[0025] FIG. 4A is a SEM image of the exemplary spheroidized powder shown in FIGs. 3 A and 3B after washing according to the Example; and

[0026] FIG. 4B is a magnified SEM image of the exemplary washed, spheroidized powder of FIG. 4A.

[0027] Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0028] Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. [0029] As used herein, the terms "first", "second", and "third" may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components.

[0030] The terms "upstream" and "downstream" refer to the relative direction with respect to fluid flow in a fluid pathway. For example, "upstream" refers to the direction from which the fluid flows, and "downstream" refers to the direction to which the fluid flows.

[0031] Methods are generally provided for creating higher quality powder materials (i.e., a treated powder) from a lower-quality powder source (i.e., a feedstock powder), along with apparatus to perform such methods and the resulting particles. In one embodiment, a powder formed from water atomization techniques and having irregular shapes (such as formed from water atomization techniques) is transformed into a higher quality powder. In one embodiment, treated particles of the treated powder may have a more spherical shape than the feedstock particles of the feedstock powder, which may be irregular, non-spherical in shape. Additionally, any oxidation layer present on the feedstock powder may be removed (e.g., through chemical reduction). In one embodiment, the treated powder may be substantially free from any oxidation layer on its surface. As used herein, the term "substantially free" means no more than an insignificant trace amount present and encompasses completely free (e.g., 0 molar % up to 0.01 molar %).

[0032] In one embodiment, the treated powder is subjected to (e.g., exposed to) plasma spheroidization to produce the high quality powder. Referring to Fig. 1, a diagram of a plasma spheroidization apparatus 10 is generally shown. The feedstock powder 12 (composed of a plurality of feedstock particles 13) is generally introduced into a plasma chamber 14, along with a working gas 16 (also referred to as the plasma gas, no matter its state of matter). A plasma field 18 may be formed within the plasma chamber 14 through heating to a temperature sufficient to convert the plasma gas 16 from its gaseous state into its plasma state. For example, heating elements 20 may be included within the plasma chamber 14, such as an induction coil.

[0033] As stated above, the feedstock particles 13 may have an irregular shape (e.g., non- spherical) when introduced into the plasma chamber 14. In certain embodiments, the feedstock particles 13 have a maximum size of about 150 micrometers (μπι). For example, the feedstock particles 13 may have an average size of about 10 μπι to about 150 μπι (e.g., about 50 μπι to about 100 μιη).

[0034] Generally, the feedstock powder 12 may be any metal material. In one embodiment, the metal material may include, but is not limited to, pure metals, iron alloys, aluminum alloys, nickel alloys, chrome alloys, nickel-based superalloys, cobalt-based superalloys, iron-based superalloys, or mixtures thereof. In particular embodiments, alloying elements may be mixed with the feedstock powder 12 prior to or during exposure to the plasma gas 16. As such, the chemical composition of the resulting treated powder may be controlled. For example, in one particular embodiment, carbon particles may be mixed with the feedstock particles within the plasma field.

[0035] As the feedstock powder 12 is passed through the plasma field 18 that includes the plasma gas 16 in its plasma state, the surface of the feedstock particles 13 melts or evaporates within a melting zone 22 that includes the plasma field 18. However, without wishing to be bound by any particular theory, it is believed that the feedstock particles 13 do not entirely melt and/or evaporate, but rather that the surfaces of the feedstock particles 13 are melted/softened so as to reshape into a more regular shape (e.g., more spherical) while having a smaller size. Thus, at least a portion of the surface of the feedstock particles 13 are melted/softened within the melting zone 22.

[0036] In one embodiment, the working gas 16 (i.e., the plasma gas) includes a reducing gas, such as hydrogen, carbon monoxide, or a mixture thereof. The reducing gas may react with any oxide layer on the surface of the feedstock particles 13, which may be in the form of chromium oxide, iron oxide, etc. Such a reducing gas may react with the oxide to remove it from the surface such that the resulting treated powder 24 (in the form of a plurality of the resulting treated particles 25) are substantially free from any oxide layer thereon. Thus, in one particular embodiment, the reducing component reduces any oxide layer on the surface of the feedstock particles such that the resulting treated particles are substantially free from any oxide layer thereon.

[0037] Through this plasma spheroidization process, the size of the feedstock particles 13 may be decreased such that the resulting treated particles 25 have an average particle size that is less than an average particle size of the feedstock particles 13. In one embodiment, the resulting treated particles 25 have an average particle size that is about 10% to about 90% of the average particle size of the feedstock particles 13. In certain embodiments, the treated particles 25 have a maximum size of about 150 μπι (e.g., an average size of about 10 μπι to about 150 μπι). In particular embodiments, the treated particles 25 have a maximum size of about 50 μπι (e.g., an average size of about 10 μπι to about 50 μπι).

[0038] Such a technique can be used to recondition powders as well.

[0039] As stated, the plasma spheroidization of the feedstock powder 12 improves the properties of the feedstock powders 12 such that the improved powder material (i.e., the treated powder 24) may be more suitable for additive manufacturing techniques. As used herein, the terms "additively manufactured" or "additive manufacturing techniques or processes" refer generally to manufacturing processes wherein successive layers of material(s) are provided on each other to "build-up," layer- by-layer, a three-dimensional component. The successive layers generally fuse together to form a monolithic component which may have a variety of integral sub-components. Although additive manufacturing technology is described herein as enabling fabrication of complex objects by building objects point-by-point, layer-by-layer, typically in a vertical direction, other methods of fabrication are possible and within the scope of the present subject matter. For example, although the discussion herein refers to the addition of material to form successive layers, one skilled in the art will appreciate that the methods and structures disclosed herein may be practiced with any additive manufacturing technique or manufacturing technology. For example, embodiments of the present invention may use layer-additive processes, layer-subtractive processes, or hybrid processes.

[0040] Suitable additive manufacturing techniques in accordance with the present disclosure include, for example, Fused Deposition Modeling (FDM), Selective Laser Sintering (SLS), 3D printing such as by inkjets, laser jets, and binder jets, Sterolithography (SLA), Direct Selective Laser Sintering (DSLS), Electron Beam Sintering (EBS), Electron Beam Melting (EBM), Laser

Engineered Net Shaping (LENS), Laser Net Shape Manufacturing (LNSM), Direct Metal Deposition (DMD), Digital Light Processing (DLP), Direct Selective Laser Melting (DSLM), Selective Laser Melting (SLM), Direct Metal Laser Melting (DMLM), and other known processes.

[0041] The additive manufacturing processes described herein may be used for forming components using any suitable material. For example, the material may be plastic, metal, concrete, ceramic, polymer, epoxy, photopolymer resin, or any other suitable material that may be in solid, liquid, powder, sheet material, wire, or any other suitable form or combinations thereof. More specifically, according to exemplary embodiments of the present subject matter, the additively manufactured components described herein may be formed in part, in whole, or in some combination of materials including but not limited to pure metals, iron alloys, aluminum alloys, nickel alloys, chrome alloys, and nickel -based, iron-based, or cobalt-based superalloys (e.g., those available under the name Inconel® available from Special Metals Corporation). These materials are examples of materials suitable for use in the additive manufacturing processes described herein, and may be generally referred to as "additive materials."

[0042] In addition, one skilled in the art will appreciate that a variety of materials and methods for bonding those materials may be used and are contemplated as within the scope of the present disclosure. As used herein, references to "fusing" may refer to any suitable process for creating a bonded layer of any of the above materials. For example, if an object is made from polymer, fusing may refer to creating a thermoset bond between polymer materials. If the object is epoxy, the bond may be formed by a crosslinking process. If the material is ceramic, the bond may be formed by a sintering process. If the material is powdered metal, the bond may be formed by a melting or sintering process. One skilled in the art will appreciate that other methods of fusing materials to make a component by additive manufacturing are possible, and the presently disclosed subject matter may be practiced with those methods.

[0043] In addition, the additive manufacturing process disclosed herein allows a single component to be formed from multiple materials. Thus, the components described herein may be formed from any suitable mixtures of the above materials. For example, a component may include multiple layers, segments, or parts that are formed using different materials, processes, and/or on different additive manufacturing machines. In this manner, components may be constructed which have different materials and material properties for meeting the demands of any particular application. In addition, although the components described herein are constructed entirely by additive manufacturing processes, it should be appreciated that in alternate embodiments, all or a portion of these components may be formed via casting, machining, and/or any other suitable manufacturing process. Indeed, any suitable combination of materials and manufacturing methods may be used to form these components.

[0044] An exemplary additive manufacturing process will now be described. Additive manufacturing processes fabricate components using three-dimensional (3D) information, for example a three-dimensional computer model, of the component. Accordingly, a three-dimensional design model of the component may be defined prior to manufacturing. In this regard, a model or prototype of the component may be scanned to determine the three-dimensional information of the component. As another example, a model of the component may be constructed using a suitable computer aided design (CAD) program to define the three-dimensional design model of the component.

[0045] The design model may include 3D numeric coordinates of the entire configuration of the component including both external and internal surfaces of the component. For example, the design model may define the body, the surface, and/or internal passageways such as openings, support structures, etc. In one exemplary embodiment, the three-dimensional design model is converted into a plurality of slices or segments, e.g., along a central (e.g., vertical) axis of the component or any other suitable axis. Each slice may define a thin cross section of the component for a predetermined height of the slice. The successive cross-sectional slices together form the 3D component. The component is then "built-up" slice-by-slice, or layer-by-layer, until finished.

[0046] In this manner, the components described herein may be fabricated using the additive process, or more specifically each layer is successively formed, e.g., by fusing or polymerizing a plastic using laser energy or heat or by sintering or melting metal powder. For example, a particular type of additive manufacturing process may use an energy beam, for example, an electron beam or electromagnetic radiation such as a laser beam, to sinter or melt a powder material. Any suitable laser and laser parameters may be used, including considerations with respect to power, laser beam spot size, and scanning velocity. The build material may be formed by any suitable powder or material selected for enhanced strength, durability, and useful life, particularly at high temperatures.

[0047] Each successive layer may be, for example, between about 10 μιη and 200 μπι, although the thickness may be selected based on any number of parameters and may be any suitable size according to alternative embodiments. Therefore, utilizing the additive formation methods described above, the components described herein may have cross sections as thin as one thickness of an associated powder layer, e.g., 10 μπι, utilized during the additive formation process.

[0048] In addition, utilizing an additive process, the surface finish and features of the

components may vary as need depending on the application. For example, the surface finish may be adjusted (e.g., made smoother or rougher) by selecting appropriate laser scan parameters (e.g., laser power, scan speed, laser focal spot size, etc.) during the additive process, especially in the periphery of a cross-sectional layer which corresponds to the part surface. For example, a rougher finish may be achieved by increasing laser scan speed or decreasing the size of the melt pool formed, and a smoother finish may be achieved by decreasing laser scan speed or increasing the size of the melt pool formed. The scanning pattern and/or laser power can also be changed to change the surface finish in a selected area.

[0049] Notably, in exemplary embodiments, several features of the components described herein were previously not possible due to manufacturing restraints. However, the present inventors have advantageously utilized current advances in additive manufacturing techniques to develop exemplary embodiments of such components generally in accordance with the present disclosure. While the present disclosure is not limited to the use of additive manufacturing to form these components generally, additive manufacturing does provide a variety of manufacturing advantages, including ease of manufacturing, reduced cost, greater accuracy, etc.

[0050] In this regard, utilizing additive manufacturing methods, even multi-part components may be formed as a single piece of continuous metal, and may thus include fewer sub-components and/or joints compared to prior designs. The integral formation of these multi-part components through additive manufacturing may advantageously improve the overall assembly process. For example, the integral formation reduces the number of separate parts that must be assembled, thus reducing associated time and overall assembly costs. Additionally, existing issues with, for example, leakage, joint quality between separate parts, and overall performance may advantageously be reduced.

[0051] Also, the additive manufacturing methods described above enable much more complex and intricate shapes and contours of the components described herein. For example, such components may include thin additively manufactured layers and unique fluid passageways with integral mounting features. In addition, the additive manufacturing process enables the manufacture of a single component having different materials such that different portions of the component may exhibit different performance characteristics. The successive, additive nature of the manufacturing process enables the construction of these novel features. As a result, the components described herein may exhibit improved functionality and reliability.

EXAMPLES

[0052] As an example, water atomized powder was purchased from under their designation 316 powder, which had the sizing of -325 mesh/15 microns. This water atomized powder is an iron-based alloy. The water atomized powder was found to have an apparent density of 2.75 (g/cm ³ ) with an oxygen content of 0.164% (by wt), nitrogen content of 0.047% (wt %), and hydrogen content of 0.001%) (by wt. %). The water atomized powder was found to have the particle size distribution shown in Table 1 prior to any treatment performed.

Table 1 : Volume Statistics (Arithmetic)

Calculations from 0.375 μιη to 2000 μιη

Volume: 100%

Mean: 35.65 μιη SD: 15.95 μιη

Median: 34.62 μιη Variance: 254.4 μιη ²

Mean/Median Ratio: 1.030 C.V.: 44.7%

Mode: 37.97 μιη Skewness: 0.588 right skewed

Kurtosis: 0.773 Leptokurtic

[0053] FIGs. 2A and 2B show SEM images of the water atomized powder prior to any treatment performed. As shown, the water atomized powder includes particles of varying size and shape.

[0054] Then, the water atomized powder was spheroidized using argon as a primary gas, with hydrogen as a secondary gas. Other experiments were also performed using helium and nitrogen as a secondary gas, with argon being the primary gas. It was found that the spheroidization resulted in a more uniform size and shape of the particles in the powder.

[0055] FIGs. 3 A and 3B shown images of the spheroidized powder after spheroidized using argon as a primary gas and hydrogen as a secondary gas. [0056] Then, the spheroidized powder was washed using an industrial washing unit. FIGs. 4A and 4B show images of the spheroidized powder. As seen, relatively clean and uniform particles make up the powder following this spheroidization and washing process.

[0057] The spheroidized powder was found to have an oxygen content of 0.057% (wt %), nitrogen content of 0.009% (wt %), and hydrogen content of 0.0007% (wt %). Thus, the

spheroidized powder had significantly reduced contents of oxygen, nitrogen, and hydrogen.

[0058] Table 2 shows the particle size distribution after spheroidization and washing.

Table 2: Volume Statistics (Arithmetic)

Calculations from 0.375 μπι to 2000 μπι

Volume: 100%

Mean: 30.30 μιη SD: 8.014 μιη

Median: 30.48 μιη Variance: 64.23 μιη ²

Mean/Median Ratio: 0.994 C.V.: 26.4%

Mode: 31.51 μιη Skewness: -0.085 left skewed

Kurtosis: -0.347 Platykurtic

[0059] In conclusion, spheroidization of water atomized powder was successful and overcame both of the major issues of irregular shape and high oxygen content.

[0060] This written description uses exemplary embodiments to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.