Method for producing spherical metal particles

Abstract

The present invention provides a method for producing spherical metal particles, the method comprising the steps of providing a particulate metal precursor comprising flake-like particles having a maximum dimension greater than 250 microns; grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material comprising non-spherical ground particles; classifying the ground material to separate a fraction of the non-spherical ground particles from larger and / or smaller particles, wherein the classification occurs during and / or after grinding; and spheroidizing the separated non-spherical ground particles by dry milling in the impact chamber or in a separate impact chamber, thereby producing spherical metal particles, wherein each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact surfaces that strike the particles in the impact chamber as the rotor rotates during dry milling.

Description

[Technical Field]

[0001] Priority Cross-Reference [1] This application claims priority to Australian Provisional Patent Application No. 2022903913, filed on 20 December 2022, the contents of which are to be deemed incorporated herein by reference.

[0002] [2] The present invention relates to a method for producing spherical metal particles, the method comprising the steps of: grinding a particulate metal precursor comprising flake particles by dry milling to produce a ground material comprising non-spherical ground particles; classifying the ground material to separate a fraction of the non-spherical ground particles from larger and / or smaller particles; and spheroidizing the separated non-spherical ground particles by dry milling to produce spherical metal particles. The present invention further relates to metal powders for additive manufacturing or powder metallurgy, and methods for producing cold-compactable metal powders. [Background technology]

[0003] [3] Metal components are commonly produced by subtractive manufacturing techniques, such as turning, milling, drilling, sawing, threading, and gear cutting, which start from preforms produced by molten metallurgy. However, a significant fraction of the metal in the preform is typically removed by machining during the manufacturing process, forming swarf (machining debris) as low-value waste.

[0004] [4] This is of particular concern in manufacturing processes that utilize expensive metal compositions. For example, titanium and titanium alloy components are favored in many industrial and commercial applications due to their attractive properties, including high specific strength, low density, excellent corrosion resistance, and outstanding biocompatibility. Ti-6Al-4V (Ti64) alloy is one of the most commonly used alloys in the aerospace, robotics, chemical, energy, marine, and biomedical industries. Large amounts of Ti64 shavings are generated daily in commercial operations, and in some manufacturing processes in the aerospace industry, up to 95% of expensive titanium alloy forgings is removed by machining.

[0005] [5] Traditional recycling methods for titanium alloy and other metal waste particles involve remelting and recasting the metal composition. However, such processing techniques are very energy intensive, considering that a large fraction of the metal composition goes through multiple melt-reworking cycles. Furthermore, recycling titanium waste into high-performance products is difficult due to titanium's strong affinity for embrittlement interstitial elements. Titanium alloys such as Ti64 are particularly susceptible to increasing oxygen content over multiple machining-reworking cycles, which renders the reworked material out of specification for many high-value applications.

[0006] [6] Additive manufacturing (AM) and powder metallurgy (PM) technologies are becoming increasingly attractive as alternatives to subtractive manufacturing methods. These techniques are near-net-shape technologies, meaning that only a relatively small amount of the metal powder feedstock is lost in the subsequent processing of the metal components. However, metal powders with morphologies suitable for AM and PM manufacturing are high-technology materials that require specialist manufacturing, and are therefore generally expensive and available in only limited compositions.

[0007] [7] Despite their particulate morphology, metal shavings are not suitable as a raw material for AM or PM manufacturing. Cold forming of metal shavings, as required for some PM techniques, has proven extremely difficult due to poor morphology. Titanium alloy shavings are flaky and irregularly sized, and generally have high yield strength, high hardness, low ductility, and a lack of interlocking between flake particles, making them considered impossible to consolidate into dense, non-brittle compacts at room temperature.

[0008] [8] Therefore, the development of new technologies for converting metal shavings, such as titanium and titanium alloy shavings, into high-value AM and / or PM feedstocks would offer significant economic and environmental benefits. While the above discussion has focused specifically on titanium-based shavings, it is understood that similar considerations apply to a range of other metal shaving compositions, and indeed, other sources of low-value flaked metal particulates.

[0009] [9] Therefore, there is a continuing need for new methods of improving particulate metal feedstocks, including flake particles, that at least partially address one or more of the above-mentioned shortcomings or provide a useful alternative.

[0010]

[10] Any reference herein to a patent document or other publication given as prior art should not be construed as an admission that the document or publication was publicly known or that the information it contains was part of the general public knowledge at the priority date of any of the claims. Summary of the Invention [Means for solving the problem]

[0011]

[11] The present inventors have now discovered that particulate metal precursors, including flake particles such as titanium alloy turnings, can be refined to produce smaller sized spherical metal particles by a method comprising two dry milling processes: a first grinding step or stage in which fine particles are broken down primarily by milling action to form smaller, but still non-spherical, ground particles, and a second spheroidization step or stage in which a classified fraction of the non-spherical ground particles are spheroidized primarily by milling action. Between the separate grinding and spheroidization process steps, or in the initial stage of a continuous dry milling process having sequential grinding and spheroidization steps, the non-spheroidized ground particles are classified to isolate a selected size fraction of the particles for spheroidization.

[0012]

[12] A classification step prior to spheronization has been found to be important for controlling the particle size distribution and morphology of the resulting product particles, especially when small particles (e.g., <100 microns) are the desired primary target (as spheronized particles) or by-product. Spheronization of particles with a selected, typically narrow, size range can prevent smaller particles from reattaching to larger particles, allowing spheronization conditions and / or equipment to be tailored to the powder feed. Furthermore, because the initial flake-like morphology of precursor metals typically limits the amount of precursor that can be milled in a given volume of impact chamber, a classification step allows for more efficient use of milling equipment. After initial milling and classification, the target size fraction of particles can be spheronized in the impact chamber at much higher loading concentrations and for longer periods than typically required for this process.

[0013]

[13] Either dry milling process may be carried out in an impact chamber, which includes a rotor rotating at high speed within a cylindrical stator, causing the rotor's impact blades to strike particles circulating within the impact chamber. While similar equipment has previously been used to spheroidize small, irregularly shaped particles, it is considered surprising that dry milling in such equipment can effectively reduce particles larger than 250 microns, up to 3 mm or larger, to a size range in which spheroidization becomes the dominant particle modification process. Without wishing to be bound by any theory, it is proposed that the flaky particle morphology, in the case of shavings, enhanced by work hardening and by defects (cracks, jagged edges, perforations) caused by the machining action that generates the particles, allows the particulate metal precursor to be effectively broken down by the particle impact action during dry milling.

[0014]

[14] An additional advantage of performing the entire particle modification process by dry milling in an impact chamber is that it provides the opportunity to rigorously exclude oxygen and other contaminants that can degrade metal compositions, especially those that are sensitive to contamination, such as titanium alloys.

[0015]

[15] In contrast, the inventors have found that high-shear wet milling techniques, in which particle modification is primarily caused by shear in the narrow gap between the rotor and stator, are inadequate for processing large (>500 micron) flaked particles of high yield strength and / or contamination-sensitive metal compositions, such as titanium alloy turnings. Wet milling equipment was susceptible to wear and failure due to jamming of particulate material in the gap between the rotor and stator, the equipment failed to satisfactorily achieve both comminution and spheroidization, and the process was prone to contaminating sensitive metal compositions by contact with the liquid medium.

[0016]

[16] Thus, according to a first aspect, the present invention provides a method for producing spherical metal particles. The method includes providing a particulate metal precursor. The particulate metal precursor may comprise flake particles. The flake particles may have a maximum dimension greater than 250 microns. The method includes grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material. The ground material may comprise non-spherical ground particles. The method includes classifying the ground material to separate a fraction of the ground particles from larger and / or smaller particles. Classification may occur during and / or after grinding. The method includes spheroidizing the separated ground particles by dry milling in the same impact chamber or in a separate impact chamber, thereby producing spherical metal particles. Each impact chamber may include a rotor configured to rotate within the impact chamber, the rotor including multiple impact surfaces that strike the particles within the impact chamber as the rotor rotates during dry milling.

[0017]

[17] In a first embodiment, a method for producing spherical metal particles includes providing a particulate metal precursor comprising flake particles having a maximum dimension greater than 250 microns; grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material comprising non-spherical ground particles; classifying the ground material to separate a fraction of the non-spherical ground particles from larger and / or smaller particles, wherein the classification occurs during and / or after grinding; and spheroidizing the separated non-spherical ground particles by dry milling in the impact chamber or in separate impact chambers, thereby producing spherical metal particles, each impact chamber including a rotor configured to rotate therein, the rotor including a plurality of impact surfaces that strike the particles in the impact chamber as the rotor rotates during dry milling.

[0018]

[18] In some embodiments, the particulate metal precursor comprises flake particles having a maximum dimension greater than 500 microns, or greater than 1 mm, or greater than 2 mm, such as greater than 3 mm, for example greater than 4 mm.

[0019]

[19] In some embodiments, the particulate metal precursor is shavings.

[0020]

[20] In some embodiments, classifying the ground material comprises separating a fraction of non-spherical ground particles from larger particles, the separated non-spherical ground particles having a maximum dimension D1 of less than 250 microns, or less than 200 microns, or less than 150 microns, e.g., less than 100 microns, e.g., less than 75 microns. The method may further comprise, after separation, grinding the larger particles by dry milling in the impact chamber or in a separate impact chamber, thereby producing additional non-spherical ground particles having a maximum dimension less than D1 for spheronization.

[0021]

[21] In some embodiments, classifying the ground material comprises separating a fraction of non-spherical ground particles from smaller particles, the smaller particles having a maximum dimension D2 of less than 100 microns, or less than 75 microns, e.g., less than 50 microns, e.g., less than 30 microns. In some embodiments, the method further comprises, after separation, spheronizing the smaller particles by dry milling.

[0022]

[22] In some embodiments, the ground material is classified by a method selected from sieving, air classification, and hydrocyclone classification.

[0023]

[23] In some embodiments, the method further includes combining at least a portion of the smaller particles with at least a portion of the spherical metal particles, and subjecting the combination to an impact blending process to adhere the smaller particles to the spherical metal particles, thereby producing a cold-compactable metal powder comprising non-spherical particles that include one spherical metal particle as a core and multiple smaller metal particles as protrusions from the core.

[0024]

[24] In some embodiments, the particulate metal precursor comprises or consists of a metal composition having a yield stress of at least 400 MPa, or at least 600 MPa, such as at least 1000 MPa.

[0025]

[25] In some embodiments, the particulate metal precursor comprises or consists of a metal composition having an elongation to break of at least 1%, e.g., at least 3%, at least 5%, or at least 10%. In some embodiments, the particulate metal precursor comprises or consists of a metal composition having an elongation to break in the range of 5% to 30%, or 10% to 30%, or 10% to 20%. Elongation to break herein refers to the elongation to break of the metal composition in mill-annealed form, measured according to ASTM E8 / E8M-13.

[0026]

[26] In some embodiments, the particulate metal precursor comprises or consists of a metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, alloys thereof, metal matrix composites thereof, low ductility alloys of iron, low ductility alloys of zinc, low ductility alloys of magnesium, and low ductility alloys comprising both aluminum and copper.

[0027]

[27] In some embodiments, the particulate metal precursor comprises or consists of a metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, alloys thereof, and metal matrix composites thereof.

[0028]

[28] In some embodiments, the particulate metal precursor comprises or consists of a metal composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof.

[0029]

[29] In some embodiments, the particulate metal precursor comprises or consists of a titanium alloy, such as TiAl6V4.

[0030]

[30] In some embodiments, each impact chamber includes a cylindrical stator defining an outer cylindrical wall of the impact chamber, and a recirculation conduit connecting an inlet port located in the outer cylindrical wall to an outlet port directed toward a central portion of the impact chamber, wherein particles are continuously recirculated through the recirculation conduit during dry milling.

[0031]

[31] In some embodiments, the rotor of each impulse chamber includes a plurality of impulse blades defining an impulse face, and the outer edge of each impulse blade at the periphery of the rotor is spaced from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm, for example in the range of 2 mm to 4 mm.

[0032]

[32] In some embodiments, the rotor of each impact chamber includes a plurality of impact blades defining an impact surface, and the rotor is rotated during comminution such that the outer edge of each impact blade at the periphery of the rotor has a peripheral speed of greater than 50 m / s, e.g., greater than 55 m / s, e.g., about 60 m / s or greater.

[0033]

[33] In some embodiments, the particulate metal precursor is milled for a period of time between 1 second and 1 minute, such as for a period of time between 10 seconds and 40 seconds.

[0034]

[34] In some embodiments, the separated non-spherical ground particles are spheronized for a period of 1 minute to 30 minutes, such as for a period of 5 minutes to 15 minutes.

[0035]

[35] In some embodiments, dry milling during grinding and spheronization is carried out in a dry inert gas atmosphere.

[0036]

[36] In some embodiments, the particulate metal precursor has an average sphericity of less than 0.5, such as less than 0.4.

[0037]

[37] In some embodiments, the isolated non-spherical ground particles have an average sphericity of less than 0.6 prior to spheronization.

[0038]

[38] In some embodiments, the spherical metal particles have an average sphericity greater than 0.8.

[0039]

[39] In a second aspect, the present invention provides a metal powder for additive manufacturing, comprising spherical metal particles produced according to any embodiment of the first aspect.

[0040]

[40] In a third aspect, the present invention provides the use of a metal powder comprising spherical metal particles produced according to any embodiment of the first aspect for additive manufacturing.

[0041]

[41] In a fourth aspect, the present invention provides a method for producing a cold-compactable metal powder, comprising the steps of: providing a particulate metal precursor comprising flake-shaped particles having a maximum dimension greater than 250 microns; grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material comprising non-spherical ground particles; classifying the ground material to separate a fraction of the non-spherical ground particles from smaller particles, wherein the classification occurs during and / or after grinding; spheroidizing the separated non-spherical ground particles by dry milling within the impact chamber or in a separate impact chamber, thereby producing spherical metal particles; The method includes the steps of combining at least a portion of the small particles with at least a portion of the spherical metal particles, and subjecting the combination of particles to impact blending in the impact chamber or in a separate impact chamber to cause the smaller particles to adhere to the spherical metal particles, thereby producing a cold-formable metal powder comprising non-spherical particles comprising one spherical metal particle as a core and multiple smaller metal particles as protrusions from the core, wherein each impact chamber includes a rotor configured to rotate within the impact chamber, the rotor including multiple impact surfaces that strike the particles in the impact chamber as the rotor rotates during dry milling or impact blending.

[0042]

[42] It will be appreciated that the method according to the fourth aspect may incorporate one or more features associated with the particulate metal precursor and milling, classification and spheronization steps disclosed herein in the context of the first embodiment.

[0043]

[43] In some embodiments of the fourth aspect, at least a portion of the smaller metal particles attached to the spherical metal particles are metallurgically bonded to the spherical metal particles.

[0044]

[44] In some embodiments, each impact chamber includes a cylindrical stator defining an outer cylindrical wall of the impact chamber, and a recirculation conduit connecting an inlet port located in the outer cylindrical wall with an outlet port directed toward a central portion of the impact chamber, wherein particles are continuously recirculated through the recirculation conduit during dry milling or impact blending.

[0045]

[45] In some embodiments, the rotor of each impulse chamber includes a plurality of impulse blades defining an impulse face, and the outer edge of each impulse blade at the periphery of the rotor is spaced from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm, for example in the range of 2 mm to 4 mm.

[0046]

[46] In some embodiments, the rotor of each impact chamber includes a plurality of impact blades defining an impact surface, and the rotor is rotated during impact blending such that the outer edge of each impact blade at the periphery of the rotor has a peripheral speed of less than 60 m / s, e.g., less than 55 m / s, e.g., less than 50 m / s. In some such embodiments, the outer edge of each impact blade at the periphery of the rotor has a peripheral speed of more than 35 m / s, e.g., more than 40 m / s, during impact blending.

[0047]

[47] In some embodiments, the particle combination is impact blended for a time period of 1 second to 10 minutes, such as for a time period of 5 seconds to 5 minutes, such as for a time period of 10 seconds to 1 minute.

[0048]

[48] ​​In some embodiments, impact blending is carried out in a dry inert gas atmosphere.

[0049]

[49] In a fifth aspect, the present invention provides a cold-compactable metal powder comprising non-spherical particles produced by a method according to any embodiment of the fourth aspect.

[0050]

[50] In a sixth aspect, the present invention provides the use of a cold-formable metal powder comprising non-spherical particles produced by a method according to any embodiment of the fourth aspect, for producing a porous metal article by cold forming.

[0051]

[51] When used in this specification (including the claims), the terms "comprise", "comprises" and "comprising" are to be construed as specifying stated features, integers, steps or components but not excluding the presence of one or more other features, integers, steps or components or groups thereof.

[0052]

[52] As used herein, the terms “first,” “second,” “third,” etc., in connection with various features of the disclosed devices, methods, systems, etc., are assigned arbitrarily and are intended merely to distinguish between two or more such features that the devices, methods, systems, etc. may incorporate in various embodiments. The terms themselves do not indicate any particular direction or order. Furthermore, it should be understood that the presence of a “first” feature does not indicate the presence of a “second” feature, the presence of a “second” feature does not indicate the presence of a “first” feature, etc.

[0053]

[53] Further aspects of the present invention appear below in the detailed description of the invention.

[0054]

[54] Embodiments of the present invention are herein described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0055] [Figure 1]

[55] FIG. 1 is a schematic diagram of a dry milling apparatus for use in methods according to some embodiments of the present invention. [Figure 2]

[56] Figure 1 is an isometric view of the impact chamber of the rotary impact blending / dry milling apparatus (Nara Hybridization System, NHS-0) used in the examples. [Figure 3]

[57] FIG. 1 illustrates the impact chamber and rotor of a rotary impact blending / dry milling apparatus for use in methods according to some embodiments of the present invention. [Figure 4]

[58] FIG. 1 is a block flow diagram that schematically illustrates a method for producing spherical metal particles according to some embodiments of the present invention. [Figure 5]

[59] FIG. 1 is a block flow diagram that schematically illustrates a method for producing cold-formable metal powders according to some embodiments of the present invention. [Figure 6]

[60] Figure 6 shows a schematic of an impact blending process for converting a combination of large spheroidized particles and small particles into non-spherical particles with one large metal particle as a core and multiple small metal particles as protrusions from the core. [Figure 7]

[61] Figure 6A is a schematic illustration of a proposed interlocking pattern that may occur between adjacent non-spherical particles produced in accordance with an embodiment of the present invention when compressed to form a porous metal article. [Figure 8]

[62] FIG. 11 is a scanning electron microscope (SEM) image of Ti-6Al-4V alloy turnings used in the examples, showing the curved, flake-like morphology of the particles. [Figure 9]

[63] Another SEM image of Ti-6Al-4V alloy shavings showing the jagged edges and cracks in the shaving particles. [Figure 10] 10 is another SEM image of Ti-6Al-4V alloy shavings showing the jagged edges and cracks of the shaving particles. [Figure 11]

[64] Figure 10 shows particle size distribution of Ti-6Al-4V alloy turnings determined by sieve analysis. [Figure 12]

[65] Figure 10 is an SEM image of the classified 75-250 μm fraction of the milled Ti-6Al-4V alloy turnings produced in Example 1. [Figure 13]

[66] Figure 10 is an SEM image of the classified 500-1000 μm fraction of the milled Ti-6Al-4V alloy turnings produced in Example 1. [Figure 14]

[67] Figure 10 is a mass flow diagram of Ti-6Al-4V alloy turnings subjected to five milling and classification runs in Example 2. [Figure 15]

[68] Figure 10 is an SEM image of the classified <75 μm fraction of the milled Ti-6Al-4V alloy turnings produced in Example 2. [Figure 16]

[69] Figure 10 is an SEM image of spheroidized metal particles produced by spheroidizing the classified <75 μm fraction of crushed Ti-6Al-4V alloy turnings in Example 3. [Figure 17]

[70] Figure 10 is an SEM image of the classified 75-150 μm fraction of the milled Ti-6Al-4V alloy turnings produced in Example 2. [Figure 18]

[71] SEM image of spheroidized metal particles produced by spheroidizing the classified 75-150 μm fraction of crushed Ti-6Al-4V alloy turnings in Example 3. [Figure 19]

[72] Figure 10 is an SEM image of the classified 150-250 μm fraction of the milled Ti-6Al-4V alloy turnings produced in Example 2. [Figure 20]

[73] Figure 10 is an SEM image of spheroidized metal particles produced by spheroidizing the classified 150-250 μm fraction of crushed Ti-6Al-4V alloy turnings in Example 3. [Figure 21]

[74] SEM image of non-spherical metal particles comprising a large metal particle as a core and multiple small metal particles as protrusions from the core, produced in Example 4 by impact blending a combination of spheroidized Ti-6Al-4V alloy particles (produced by spheroidization in Example 3) and small Ti-6Al-4V alloy particles at 8,000 rpm for 30 seconds. [Figure 22]

[75] SEM image of non-spherical metal particles comprising a large metal particle as a core and multiple small metal particles as protrusions from the core, produced in Example 4 by impact blending a combination of spheroidized Ti-6Al-4V alloy particles (produced by spheroidization in Example 3) and small Ti-6Al-4V alloy particles at 9,000 rpm for 30 seconds. DETAILED DESCRIPTION OF THE INVENTION

[0056] Method for producing spherical metal particles

[76] The present invention relates to a method for producing spherical metal particles from a particulate metal precursor. The particulate metal precursor may include flake particles. The flake particles may have a maximum dimension greater than 250 microns. The method includes grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material containing non-spherical ground particles. The ground material is classified during and / or after grinding to separate a fraction of the non-spherical ground particles from larger and / or smaller particles. The separated non-spherical ground particles are spheroidized by dry milling in the impact chamber or in a separate impact chamber, thereby producing spherical metal particles. Each impact chamber used in the method may include a rotor configured to rotate within the impact chamber, the rotor including multiple impact surfaces that strike particles within the impact chamber as the rotor rotates during dry milling.

[0057] Particulate metal precursors

[77] In some embodiments, the methods disclosed herein use as a feedstock a particulate metal precursor comprising flaked particles having a maximum dimension greater than 250 microns. In some embodiments, the particulate metal precursor comprises flaked particles having a maximum dimension greater than 500 microns, or greater than 1 mm, or greater than 2 mm, or greater than 3 mm, e.g., greater than 4 mm.

[0058]

[78] As used herein, the maximum dimension of a particle, alternatively referred to as particle size, refers to the maximum distance across the particle from surface to surface. The maximum dimension of a sphere is its diameter, while for elongated particles, the maximum dimension is the distance across the particle in the direction of elongation. One way to determine the maximum dimension of a particle is by sieve analysis, preferably according to ASTM B214-22: Standard test method for sieve analysis of metal powders. Thus, at least a fraction of the particulate metal precursor does not pass through a 250 micron sieve. Alternatively, the maximum dimension of a particle can be determined by scanning electron microscopy or other known methods in materials science.

[0059]

[79] As used herein, a flaked particle refers to a particle having a sheet-like or wafer-like morphology in which the thickness of the particle is substantially less than the length and width of the flaked particle across its major surfaces. In some embodiments, the flaked particle has a thickness of less than 100 microns, e.g., less than about 80 microns, e.g., in the range of 30 to 100 microns. The flaked particle need not be planar, and in fact, the flaked particle, such as a swarf particle, may be significantly distorted, e.g., curved or rounded, by the forces applied during machining.

[0060]

[80] Due to the presence, and typically predominance, of flake-shaped particles, the particulate metal precursor is highly non-spherical. In some embodiments, the particles of the particulate metal precursor have an average sphericity of less than 0.5, for example, less than 0.4.

[0061]

[81] Average sphericity is a measure of the degree to which particles in a powder approximate the shape of a sphere. Sphericity is defined as the ratio of the radius of the inscribed circle to the radius of the circumscribed circle, with respect to a cross section through the particle (where the inscribed circle is the largest circle within the particle cross section, and the circumscribed circle is the smallest circle outside the particle cross section, and both circles are centered at the particle's center of mass). A perfectly spherical object has a sphericity of 1. Particle sphericity can be determined from image analysis of imaged particle cross sections obtained with a scanning electron microscope (SEM), usually using image analysis software such as "Image J." The average sphericity of particles in a powder is determined by calculating the individual sphericities of several representative particles and taking the average.

[0062]

[82] It is not necessary for all particles in the precursor to be flaky or have a maximum dimension greater than 250 microns. The metal particles of a particulate metal precursor typically exist in a variety of particle sizes. The particle size distribution (PSD) of a particulate composition may be characterized by the d10, d50, and d90 particle sizes, defined such that 10% by volume of the composition is present in particles having a size (maximum dimension) less than the d10 particle size, 50% by volume of the composition is present in particles having a size (maximum dimension) less than the d50 particle size, and 90% by volume of the composition is present in particles having a size (maximum dimension) less than the d90 particle size. The d10, d50, and d90 particle sizes may be measured or estimated by routine methods in materials science, such as sieve analysis (ASTM B214-22) or laser diffraction techniques for particles less than about 1000 microns (ASTM B822-20).

[0063]

[83] In some embodiments, the particles of the particulate metal precursor have a d50 particle size greater than 250 microns, or greater than 500 microns, or greater than 1 mm, or greater than 2 mm.

[0064]

[84] The methods disclosed herein are useful for modifying particulates of a wide range of metal compositions, particularly non-ferrous metals. Thus, in some embodiments, the particulate metal precursor comprises or consists of a non-ferrous metal composition.

[0065]

[85] The methods disclosed herein are particularly useful for improving particulates of high yield strength metal compositions. In some embodiments, the particulate metal precursor comprises or consists of a metal composition having a yield stress or 0.2% yield strength of at least 400 MPa, or at least 600 MPa, or at least 700 MPa, or at least 800 MPa, e.g., at least 1000 MPa. As used herein, yield stress is a bulk material property of the metal composition and can be measured by ASTM E8 / E8M-13a. As reported in the ASM Materials Property Handbook: Titanium Alloys, mill-annealed Ti-6Al-4V has a 0.2% yield strength of greater than 750 MPa.

[0066]

[86] Without wishing to be limited by any theory, the ductility of the metal composition of the particulate metal precursor is also believed to be a relevant consideration. A minimum level of ductility may be necessary to avoid particle fracture into excessively small particles under dry milling conditions and to allow plastic deformation of the milled particles during the spheroidization process. On the other hand, an overly ductile metal composition may tend to foul, agglomerate, or coat the dry milling equipment, thereby preventing satisfactory completion of the milling and / or spheroidization processes.

[0067] 87 The actual ductility of particulate metal precursors is expected to be affected by factors such as microstructure and processing history, the latter being particularly relevant in the case of swarf precursors, as discussed below. However, the ductility of metal compositions of particulate metal precursors can be usefully characterized as a composition-specific parameter using the elongation to break, determined according to ASTM E8 / E8M-13, based on the mill-annealed form of the composition. As those skilled in the art will appreciate, mill annealing refers to the process of heating a rolled, extruded, or milled material to a temperature sufficient to relieve stresses introduced into the material by its processing. Elongation to break (% elongation) is reported for the mill-annealed form of many metal compositions in standard texts, such as the ASM Materials Property Handbook—Titanium Alloys. For example, mill-annealed Ti-6Al-4V has a % elongation of 15% (Grade 23) or 14% (Grade 5). Mill-annealed CP titanium has a % elongation of 28% (Grade 2).

[0068]

[88] In some embodiments, the particulate metal precursor comprises or consists of a metal composition having an elongation at break of at least 1%, e.g., at least 3%, preferably at least 5%, or at least 10%, e.g., in the range of 5% to 30%, or 10% to 30%, or 10% to 20% (all based on the mill-annealed composition, measured according to ASTM E8 / E8M-13). Metal compositions having ductility below these ranges may be less preferred for the methods disclosed herein because particulate metal precursors comprising such metal compositions tend to be overly crushed (fractured) and poorly spheroidized. The inventors have found that CP titanium (Grade 2) powder can be processed by dry milling. However, it is believed that materials with substantially higher ductility than CP titanium may be less preferred for the methods disclosed herein than materials with lower ductility due to significant sticking to the dry milling equipment.

[0069] 89 Comminution of particulate titanium materials, including alloys such as CP titanium and Ti-6Al-4V, is generally considered difficult due to their ductility ("gummy"). Therefore, such materials are often milled by a hydrogenation-dehydrogenation approach, resulting in a less ductile hydrogenated intermediate. Without being limited by theory, it is believed that the flaky morphology of particulate metal precursors, enhanced by defects (cracks, jagged edges, perforations) in the chips, allows for effective comminution of particulate metal precursors by particle impact action during dry milling, even when using relatively ductile materials (e.g., greater than 5% or 10% elongation to break) such as titanium materials.

[0070]

[90] Non-limiting examples of metal compositions having yield strength and ductility properties suitable for the methods disclosed herein include titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, alloys thereof, metal matrix composites thereof, low-ductility alloys of iron, zinc, or magnesium, and low-ductility alloys containing both aluminum and copper. As used herein, low-ductility alloys have elongation to failure of less than 30%, preferably less than 20%. In contrast, very soft, ductile metal compositions, such as commercially pure copper or aluminum, may be less suitable because they deform very easily and / or stick to the surfaces of the milling equipment. Many metal compositions containing substantial amounts of metals such as copper, lead, zinc, tin, and iron are also expected to be soft and therefore susceptible to undesired deformation and / or functionalization by small particles via surface embedding (rather than metallurgical bonding). Metal compositions containing such metal elements are generally only suitable if their yield stress is high, such as at least 400 MPa. For example, certain ferritic and martensitic steels have adequate yield strength, while pure iron and austenitic steels are expected to be very soft. In some embodiments, the particulate metal precursor comprises or consists of a metal composition that contains copper, lead, zinc, tin, and iron only as alloying elements in trace amounts (<20 wt.%, preferably <10 wt.%) in alloys of other metals, or that is substantially free of these metals.

[0071]

[91] The metal composition of the particulate metal precursor may also have suitable fracture properties to avoid or minimize particle fracture during the spheroidization process, instead of the desired deformation that would occur during spheroidization. Thus, in some embodiments, the particulate metal precursor may have a melting point of 10 to 150 MPa m 1 / 2 range, e.g., 40 to 150 MPa m 1 / 2 Fracture toughness (K 1c ) and / or consisting of a metal composition having K 1c is a bulk material property of the metal composition and can be measured by ASTM E1820-18.

[0072]

[92] The methods of the present disclosure are also particularly useful for improving particulates of oxygen- and / or carbon-sensitive metal compositions. Thus, in some embodiments, the particulate metal precursor comprises or consists of an oxygen- and / or carbon-sensitive metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, alloys of these metals, and metal matrix composites.

[0073]

[93] In some embodiments, the particulate metal precursor comprises or consists of a metal composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof. In some embodiments, the particulate metal precursor comprises or consists of an alloy of titanium, tantalum, or niobium. In some embodiments, the particulate metal precursor comprises or consists of a titanium alloy. Non-limiting examples of suitable titanium alloys include Ti-6Al-4V (i.e., titanium alloyed with 6 wt.% Al, 4 wt.% V) and Ti-10V-2Fe-3Al (i.e., titanium alloyed with 10 wt.% V, 2 wt.% Fe, 3 wt.% Al).

[0074]

[94] As used herein, an "alloy of metal X" refers to an alloy in which metal X is the most abundant metallic element by atomic %. Thus, for example, an alloy of titanium (or titanium alloy) refers to a metal alloy in which titanium is the element present at the highest atomic %. In some embodiments, metal X is present in an alloy of metal X in an amount of at least 50 atomic %. Alloys are typically fully reacted homogeneous mixtures; therefore, the solid solutions and intermetallic phases formed are specific to the alloy, appropriate to the thermodynamic and kinetic conditions associated with their reactions, and distinct from elemental metal mixtures. Thus, as used herein, a metal alloy may include intermetallic compounds.

[0075]

[95] As used herein, a "metal matrix composite of metal X" refers to a composition comprising a matrix of commercially pure metal X or an alloy of metal X and a non-metallic particulate phase dispersed in the matrix. Exemplary non-metallic particulate phases include ceramics added as reinforcing phases.

[0076]

[96] In some embodiments, the particulate metal precursor is shavings, and thus is a product (typically a by-product) of a machining process that removes flaked particles from a metal substrate. Without wishing to be bound by any theory, it is believed that work hardening caused by the machining process, as well as defects such as cracks, jagged edges, and perforations present in certain metal shaving particles (also referred to as machining shavings), make the shaving material amenable to particle manipulation by the methods disclosed herein. In particular, the inherent flaky morphology, surface defects, and / or work hardening of the shavings may facilitate effective comminution of the shavings precursor when in an annealed form, even if the ductility of the shavings precursor composition is too high. Thus, the methods disclosed herein utilize properties of the shavings that would otherwise render them a low-value product to refine the shavings into a useful form. In some embodiments, the particulate metal precursor is titanium alloy shavings, such as Ti64 shavings.

[0077]

[97] The relevance of intrinsic chip properties to the results of the methods disclosed herein is demonstrated by comparative experiments in which Ti-6Al-4V chips were subjected to beta annealing prior to dry milling. In contrast to the results using unmodified chips, the annealed chips tended to agglomerate when subjected to dry milling due to the gumminess (ductility) of the fully annealed metal composition.

[0078]

[98] In some embodiments, the swarf includes curved flake swarf, such as C-shaped swarf and / or six-shaped swarf, in addition to any flat swarf. Spiral or string-shaped swarf particles are considered less suitable and, therefore, in some embodiments, are excluded from the swarf. While most machining operations are equipped with chip breakers to avoid spiral and string-type swarf morphologies, if such particles remain in the swarf, they may need to be removed prior to machining to avoid clogging in the impact chamber.

[0079]

[99] The swarf may be the product of a machining process carried out on a metal substrate formed from a metal composition having (i) a yield stress or 0.2% yield strength of at least 400 MPa, or at least 600 MPa, or at least 700 MPa, or at least 800 MPa, such as at least 1000 MPa, and / or an elongation to break of at least 1%, such as at least 3%, preferably at least 5%, or at least 10%, for example in the range of 5% to 30%, or 10% to 30%, or 10% to 20% (measured according to ASTM E8 / E8M-13).

[0080]

[0100] As a result of the machining process, the shavings may be work-hardened relative to (i) the metal substrate from which they were formed, and / or (ii) the metal substrate having the same metal composition in the shavings and / or mill-annealed form after being subjected to an annealing process. For example, the shavings may have an average Vickers hardness number, as measured by ASTM E92-82(2003), that is at least 30 Hv units, or at least 50 Hv units, greater than either or each of (i), (ii), and (iii).

[0081]

[0101] The particulate metal precursor does not need to be heat-treated, and in fact, this may be undesirable, as discussed above for some swarf precursors. Thus, in some embodiments, the swarf is not annealed prior to processing by the methods disclosed herein. Also, the particulate metal precursor does not need to be hydrogenated, and in some embodiments, the particulate metal precursor is not a hydrogenated material. Thus, when ground, the particulate metal precursor may comprise a metal composition containing less than 0.1 wt.% hydrogen, preferably less than 0.2 wt.% hydrogen, and most preferably substantially no hydrogen. Thus, the ground material does not need to be dehydrogenated, and in some embodiments, neither the ground material nor the separated non-spherical ground particles are subjected to dehydrogenation before or during spheroidization.

[0082] Dry milling in an impact chamber

[0102] In some embodiments of the disclosed method, the particulate metal precursor is modified by dry milling to crush and then spheroidize the precursor particles. As used herein, dry milling refers to a milling process conducted in a gas medium and thus can be distinguished from a wet milling process in which the material is dispersed in a liquid medium during milling.

[0083]

[0103] Both steps or stages of dry milling are carried out in impact chambers. The crushing process and the spheronization process may suitably be carried out in the same impact chamber or in different impact chambers. Each impact chamber includes a rotor configured to rotate within the impact chamber, the rotor including a plurality of impact surfaces that strike particles in the impact chamber as the rotor rotates during dry milling.

[0084]

[0104] In some embodiments, each impact chamber includes a cylindrical stator defining the cylindrical outer wall of the impact chamber and a recirculation conduit connecting an inlet port located on the cylindrical outer wall with an outlet port directed toward the central portion of the impact chamber. During dry milling, particles are continuously recirculated through the recirculation conduit for reintroduction into the center of the impact chamber, thus ensuring that the particles are repeatedly struck by the rotating impact surface. The rotor may include multiple impact elements, such as impact blades, that define the impact surface for striking the particles. The outer edge of each impact blade at the rotor's periphery may be spaced from the cylindrical stator by a gap distance ranging from 1 mm to 5 mm, e.g., 2 mm to 4 mm.

[0085]

[0105] As seen in FIGS. 1, 2, and 3, the dry milling process may be carried out in an apparatus 100 including an impact chamber 108, a cylindrical outer wall defined by a stator 110, and a rotor 112 rotating within the impact chamber. The impact chamber 108 is surrounded by a removable front panel 113 and a rear wall 117. The generally disc-shaped rotor includes a plurality of radially oriented impact blades 114 having an impact face 116 and an outer edge 118 at the rotor's periphery and spaced from the cylindrical stator by a small gap 120. The rotor also includes a rear radial rib 115 spaced from the rear wall 117 of the impact chamber by a narrow gap 119. The apparatus includes a recirculation conduit 122 extending between an inlet port 124 located on the cylindrical outer wall of the impact chamber and an outlet port 126 located on the front panel 113 toward the center of the impact chamber. Particulate is added to the impingement chamber through inlet 128 and discharged through powder exit port 133, also located on front panel 113, through discharge valve 137 and into powder outlet 135. Cooling water circulates within the stator through coolant ports 130, 132.

[0086]

[0106] In use, the metal material to be dry-milled is delivered from a sealed vessel to the impact chamber by a high-pressure stream of inert gas (e.g., argon). The rotor is rotated so that the impact blades sweep through the impact blending chamber. Thus, metal particles of the precursor material are repeatedly struck at high speed by the impact surfaces of the impact blades. The spinning rotor also generates vortices that accelerate the particles toward the outer periphery gap by centripetal force, creating a powerful recirculating gas flow through the recirculation conduit by a fan effect, so that the particles circulate continuously through the recirculation conduit during dry milling. In addition to being struck by the impact blades, the accelerated particles collide with each other and strike the stator, undergoing shearing in the gap between the outer edge of the impact blades and the stator. Particles that move to the rear of the rotor are pushed back toward the periphery by the rotating ribs 115.

[0087]

[0107] An example of such an apparatus, called the Nara Hybridization System (NHS-0), is commercially available from Nara Machinery Manufacturing Co., Ltd. Other suitable impact blending apparatus include the Mechanofusion system and the Cyclomix from Hosokawa Micron Corporation.

[0088] Milling of particulate metal precursors

[0108] In an embodiment of the disclosed method, a particulate metal precursor initially comprising flake particles having a maximum dimension greater than 250 microns is pulverized by dry milling in an impact chamber to produce a pulverized material comprising non-spherical pulverized particles. The pulverized material will have a reduced average particle size compared to the precursor, although it will be understood that the actual resulting particle size distribution will depend on the particle size, morphology, and composition of the particulate metal precursor, as well as the process conditions.

[0089]

[0109] Several equipment and process parameters may be particularly important to the operability and / or efficiency of the dry milling process for comminution, as well as to the resulting particle morphology. These include blade design, particularly the gap between the rotating blades and the stator (gap 120). The inventors have found that a gap distance of 3.5 mm is suitable for modifying Ti-6Al-4V shavings having a d50 particle size of approximately 1.2 mm, including flaked particles with a maximum dimension of up to 5 mm. In contrast, attempts to comminut this shavings by wet milling in a high-shear mill with a rotor-stator gap distance of less than 1 mm have been found to be unsuccessful. Thus, in some embodiments, the outer edge of each impact blade at the rotor periphery is spaced from the cylindrical stator by a gap distance ranging from 1 mm to 5 mm, e.g., 2 mm to 4 mm.

[0090]

[0110] Another important parameter is the rotor rotational speed. Without wishing to be limited by any theory, it is proposed that the rotational speed of the impact blades near the rotor periphery must be sufficient so that the impact of the strikes on the particulate metal precursor shears and breaks the flake particles. The inventors have found that a rotor peripheral speed of 61.8 m / s (equivalent to 10,000 rpm in the equipment used) is suitable for milling Ti-6Al-4V shavings, but it is understood that lower or higher speeds may be preferable for different precursor compositions and particle sizes. Higher speeds advantageously reduce processing time, but excessive speeds should be avoided as they may reduce powder yield and / or unacceptably increase the temperature of the particulate material. In some embodiments, the rotor is rotated so that the outer edge of each impact blade has a peripheral speed of greater than 50 m / s during milling.

[0091]

[0111] Another important parameter is the dry-milling time during the milling step or stage, which should be long enough to break down the particulate metal precursor into smaller particles, but not so long that the modified particles substantially spheroidize or recombine before classification. If the dry-milling to grind and spheroidize the precursor occurs in a single, undifferentiated process step, the opportunity to control the particle size distribution of the process may be lost. In particular, in situations where small particles, e.g., less than 100 microns or less than 75 microns, are the desired primary or by-product of the overall process, a short milling time may be desirable to avoid the finely milled particles generated early in the milling process from re-adhering to larger particles.

[0092]

[0112] In some embodiments, the particulate metal precursor is milled for a period of 1 second to 1 minute, e.g., 10 seconds to 40 seconds. We have found that Ti-6Al-4V shavings, initially present in particles of which about 85 wt.% have a maximum dimension greater than 1000 microns, can be effectively milled into non-spherical particles, all or most (>95 wt.%) of which are smaller than 1000 microns, within such a period. However, it is notable that as milling time increases from 30 seconds to 60 seconds, 120 seconds, and 180 seconds, the fraction of fine particles (<75 microns, not present in the initial shavings) remaining in the milled product decreases. To maximize the amount of such particles recovered in the classification step, very short milling times, such as less than 30 seconds or even less than 15 seconds, may be preferred.

[0093]

[0113] As used herein, dry milling time refers to the milling time after the rotor reaches the set speed.

[0094]

[0114] It will be understood that the preferred equipment configuration, milling speed and milling time for milling in any particular implementation will depend on the composition and initial morphology of the particulate metal precursor.

[0095]

[0115] The particulate metal precursor may be milled in a dry inert gas atmosphere in an impact chamber, which advantageously limits oxygen uptake or other contamination of the metal composition in the process, despite the potential vulnerability of the freshly milled particles to contamination from exposed raw metal at the newly formed surface.

[0096]

[0116] The ground particles produced in the initial dry-milling process are generally smoothed and may have more regular contours than the flaked particulate metal precursor. Nevertheless, when subjected to the classification process, they remain primarily non-spherical. In some embodiments, the non-spherical ground particles have an average sphericity of less than 0.6, or even less than 0.5. For example, in the case of Ti-6Al-4V shavings particles, which have cracks, jagged edges, perforations, and other defects as precursors, a short initial dry-milling step was found to not only reduce particle size but also smooth and crease the edges and surfaces of the particles. Nevertheless, the particles retained a flat or "blocky" morphology that clearly corresponds to the initial flaked morphology of the precursor.

[0097] Classification of crushed materials

[0117] In some embodiments of the disclosed methods, the ground material produced in the initial dry milling step is classified during and / or after milling to separate the fraction of non-spherical ground particles from larger and / or smaller particles.

[0098]

[0118] In some embodiments, the ground material is classified after completing the grinding process. Thus, the ground material may be removed from the impact chamber and subjected to a subsequent classification process before spheronizing the resulting separated fraction. However, it is contemplated that classification may alternatively be performed during the grinding process. For example, particles continuously (or intermittently) withdrawn from the impact chamber during dry milling may be classified online, for example, using a hydrocyclone classifier, to remove fine particles, and larger particles may be continuously (or intermittently) returned to the impact chamber for further grinding and spheronization. In this way, particulate metal precursors may be converted into spherical metal particles in a single, long-duration dry milling process in which particle classification is performed online during the initial grinding stage to control the particle size distribution in the subsequent spheronization stage.

[0099]

[0119] The classification process does not need to quantitatively separate larger and / or smaller particles from the fraction to be spheronized. In other words, it is acceptable for the fraction to be spheronized to retain some particles having sizes outside the target range and / or to lose some particles having sizes within the target range. However, classification should generally be sufficiently selective to significantly change the particle size distribution of the particles to be spheronized and / or to produce a significant by-product fraction of small or large particles.

[0100]

[0120] The ground material may be classified by any technique capable of removing larger or smaller particles from the spheroidized fraction in accordance with the principles disclosed herein. For example, the ground material may be classified by sieving. In some embodiments, the ground material is classified in an air classifier. Air classifiers are particularly suitable for classifying metal powders and may be performed in an inert gas atmosphere if the ground material is sensitive to oxygen contamination. Air classification is generally based on the centrifugal counterflow principle in a high-speed deflector wheel classifier. In other embodiments, the ground material is classified in a hydrocyclone. Hydrocyclones are particularly effective at removing high-value undersize material.

[0101]

[0121] In some embodiments, classification removes large particles from a target fraction of smaller particles that are subjected to spheronization, which can advantageously increase the mass fraction of small spheronized particles produced throughout the process, as small particles would otherwise reattach to larger particles during spheronization.

[0102]

[0122] Thus, in some embodiments, classifying the ground material involves separating the fraction of non-spherical ground particles having a maximum dimension D1 of less than 250 microns, or less than 200 microns, or less than 150 microns, or less than 100 microns, or less than 75 microns from larger particles (i.e., particles having a maximum dimension greater than D1). In such embodiments, the non-spherical ground particles having only or primarily a particle size less than D1 are then spheronized.

[0103]

[0123] Optionally, the larger particles rejected by classification may be subjected to further grinding by dry milling, thereby producing additional non-spherical ground particles for spheroidization having a maximum dimension less than D1. For example, the larger particles may be reused for grinding together with previously unprocessed particulate metal precursor. Alternatively, the larger particles rejected by classification (having only or mainly a particle size greater than D1) may be subjected to a separate spheroidization process. Advantageously, therefore, the larger particles can be spheroidized without adsorbing the fine fraction of the smaller particles.

[0104]

[0124] In some embodiments, classification removes small particles from a target fraction of larger particles that are subjected to spheronization. Removal of the fine fraction produced by milling can prevent undesired redeposition of these small particles onto the target particles during spheronization.

[0105]

[0125] Thus, in some embodiments, classifying the ground material involves separating the fraction of non-spherical ground particles from smaller particles having a maximum dimension D2 of less than 100 microns, or less than 75 microns, or less than 50 microns, or less than 30 microns. In such embodiments, the non-spherical ground particles having only or primarily a particle size greater than D2 are then spheronized.

[0106]

[0126] Optionally, the smaller particles rejected by classification (having only or mainly a particle size less than D2) may be subjected to a separate spheronization process. Advantageously, this allows these smaller particles to be spheronized without being adsorbed by the larger particles. Alternatively, the smaller particles rejected by classification may be used in a subsequent process step to produce cold-formable particles, as described in more detail below.

[0107]

[0127] In some embodiments, classification separates a target fraction of particles to be spheronized from both larger and smaller particles. Advantageously, therefore, the particles to be spheronized fall within a well-defined, potentially narrow, size range. Thus, in some embodiments, classification of the ground material involves separating a fraction of non-spherical ground particles having a maximum dimension between D2 and D1 from the larger and smaller particles, where D1 and D2 are as defined above and D1>D2.

[0108]

[0128] In some embodiments, the particulate metal precursor is subjected to repeated cycles of grinding and classification. Thus, the ground material produced in each cycle is classified into two or more size fractions, one or more fractions are subjected to spheroidization, one or more size fractions (typically, larger fractions composed of particles having a maximum dimension greater than D1 as defined herein) are subjected to repeated grinding (e.g., by recycling for grinding with previously unprocessed particulate metal precursor), and optionally one or more size fractions (typically, smaller fractions composed of particles having a maximum dimension less than D2 as defined herein) are rejected.

[0109]

[0129] In some embodiments, non-spherical ground particles that are separated from larger and / or smaller particles for subsequent spheroidization have an average sphericity of less than 0.6, or less than 0.5, or less than 0.45, e.g., less than 0.4. For ground Ti-6Al-4V shavings particles, the sphericity of the ground particles depends on the particle size, with particles less than 75 microns and in the 150-250 micron range both found to have an average sphericity of about 0.4 after classification.

[0110] Spheroidization of crushed particles

[0130] In the method of the present disclosure, the non-spherical ground particles of the fraction separated in the classification step are spheroidized by dry milling in an impact chamber to produce spherical metal particles.

[0111]

[0131] It is understood that spherical metal particles are typically not perfectly spherical. Nevertheless, spheroidization has the effect of significantly modifying particle morphology to increase particle sphericity. In some embodiments, the spherical metal particles have an average sphericity greater than 0.65, or greater than 0.7, or greater than 0.75, or greater than 0.8, or greater than 0.85. For example, spheroidization of milled Ti-6Al-4V shavings classified into the ranges of less than 75 microns, 75-150 microns, and 150-250 microns has been found to produce spheroidized Ti-6Al-4V particles with average sphericities of approximately 0.70, 0.88, and 0.89, respectively.

[0112]

[0132] The particle size distribution of ground material can be affected by the spheroidization process. Apart from the inherent effect of particle reshaping on the observed particle size, it is possible that some larger particles may be crushed and / or some smaller particles may be adsorbed by larger particles during dry milling. Nevertheless, these processes can be advantageously minimized or at least controlled by appropriate selection of size fractions for spheroidization in the classification step. Thus, for example, when the fractions of ground Ti-6Al-4V chips in the ranges of less than 75 microns, 75-150 microns, and 150-250 microns were spheroidized over an extended period of time, only relatively minor changes to the particle size distribution were observed.

[0113]

[0133] The spheroidization process typically requires a longer dry milling period than the milling process. Therefore, in some embodiments, the separated non-spherical ground particles are dry milled for a longer period than the particulate metal precursor during the milling process. In some embodiments, the separated non-spherical ground particles are spheroidized for a period of 1 minute to 30 minutes, e.g., 5 minutes to 15 minutes. For example, the fractions of milled Ti-6Al-4V shavings in the ranges of less than 75 microns, 75-150 microns, and 150-250 microns were each spheroidized for 14 minutes to achieve the sphericity values ​​disclosed above.

[0114]

[0134] The non-spherical milled particles may have a substantially higher bulk density, or tap density, compared to the flaked particulate metal precursor. This advantageously allows the spheroidization process to be carried out at a higher loading concentration of metal particulate in the impact chamber than is possible with a milling process. This allows for more efficient use of the milling equipment, especially since spheroidization is typically a much longer process than milling.

[0115]

[0135] The rotational speed of the impact blades should be sufficient to adequately spheronize the particles. The inventors have found that a rotor peripheral speed of 61.8 m / s (equivalent to 10,000 rpm in the equipment used) is suitable for spheronizing various size fractions of milled Ti-6Al-4V shavings, although it will be understood that lower or higher speeds may be preferable for different precursor compositions and particle sizes. In some embodiments, the rotor is rotated so that the outer edge of each impact blade has a peripheral speed of greater than 50 m / s during spheronization.

[0116]

[0136] The spheroidization process may be carried out in the same milling equipment as the grinding process. In fact, it has been found that the same dry milling equipment is suitable for grinding Ti-6Al-4V shavings and for spheroidizing various size fractions of the ground shavings, provided that a longer spheroidization time is used. However, in commercial processes, it may be preferable to carry out spheroidization in different milling equipment sized and configured to accommodate different process objectives and size ranges of particle fractions to be spheroidized.

[0117]

[0137] In some embodiments of the apparatus used for spheronization, the outer edge of each impact blade on the periphery of the rotor is spaced from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm, for example in the range of 2 mm to 4 mm.

[0118]

[0138] It is understood that the preferred equipment configuration, milling speed, and milling time for spheronization in any particular implementation will depend on factors such as the composition, morphology, and initial size range of the fraction of non-spherical ground particles to be spheronized. In particular, it is expected that smaller particles may require more vigorous dry milling conditions than larger particles to achieve the desired average sphericity.

[0119]

[0139] The crushed particles may be spheronized in an impact chamber in a dry inert gas atmosphere to avoid oxygen contamination or other contamination.

[0120] Embodiment

[0140] Various embodiments of the present invention will now be described with reference to Figure 4. A method 200 for producing spherical metal particles includes providing 202 a particulate metal precursor 204 comprising flaked particles having a maximum dimension greater than 250 microns. Optionally, the precursor 204 comprises flaked particles having a maximum dimension greater than 500 microns, or greater than 1 mm, or greater than 2 mm, or greater than 3 mm, or greater than 4 mm. For example, the precursor 204 may be shavings, such as titanium alloy shavings.

[0121]

[0141] In the milling step 206, the precursor 204 is milled by dry milling to produce a milled material 208 comprising non-spherical milled particles. The dry milling occurs in an impact chamber containing a rotor. The rotor includes multiple impact surfaces that strike and therefore mill particles within the impact chamber as the rotor rotates during dry milling.

[0122]

[0142] In classification step 210, the ground material 208 is classified to separate a fraction of non-spherical ground particles 212 from a fraction of larger particles 214 and / or a fraction of smaller particles 216. For example, the ground material 208 may be classified into fractions 212 and 214, or into fractions 212 and 216, or into fractions 212, 214 and 216 by sieving.

[0123]

[0143] In the spheroidization step 218, all or a portion of the separated fraction 212 of non-spherical ground particles is spheroidized by dry milling in an impact chamber to produce spherical metal particles 220. The dry milling is performed in an impact chamber containing a rotor. The rotor includes multiple impact surfaces that strike and thus spheroidize particles within the impact chamber as the rotor rotates during dry milling. Rejecting larger and / or smaller particles during spheroidization advantageously prevents smaller particles from reattaching to larger particles during spheroidization, thereby achieving a desirable particle size distribution and properties of the spheroidized metal particles 220.

[0124]

[0144] In some embodiments of method 200, the ground material 208 is classified to separate a fraction 212 of non-spherical ground particles from a fraction 214 of larger particles. The particles of fraction 212 may have a maximum dimension D1 less than 250 microns, or less than 200 microns, or less than 150 microns. In some embodiments, the particles of fraction 212 may have a maximum dimension D1 less than 100 microns, or less than 75 microns. Thus, the particles of fraction 212 that are subjected to spheronization in spheronization step 218 have only or primarily a maximum dimension less than D1.

[0125]

[0145] At least a portion of the larger, non-spherical ground particles of fraction 214 may optionally be recycled to milling step 206 via recirculation 222 for milling with (or separately from) particulate metal precursor 204. Thus, the larger particles are further milled to produce additional non-spherical ground particles having a maximum dimension less than D1 for classification into fraction 212 and spheroidization in step 218. Alternatively (or in addition), at least a portion of the larger, non-spherical ground particles of fraction 214 may be spheroidized in spheroidization step 224 by dry milling in an impact chamber to produce spherical metal particles 226. Spheroidizing this fraction separately from fraction 212 advantageously avoids redeposition of smaller particles.

[0126]

[0146] In some embodiments of method 200, the ground material 208 is classified to separate a fraction 212 of non-spherical ground particles from a fraction 216 of smaller particles. The smaller particles of fraction 216 may have a maximum dimension D2 less than 100 microns, or less than 75 microns, or less than 50 microns, or less than 30 microns. Thus, the particles of fraction 212 that are subjected to spheronization in spheronization step 218 have only or primarily a maximum dimension greater than D2.

[0127]

[0147] Optionally, at least a portion of the smaller, non-spherical ground particles of fraction 216 may be spheroidized in a spheroidization step 228 by dry milling in an impact chamber to produce spherical metal particles 230. Alternatively (or in addition), at least a portion of the smaller, non-spherical ground particles of fraction 216 may be used in a subsequent process step to produce a cold-compactable metal powder.

[0128]

[0148] In some embodiments of method 200, the ground material 208 is classified to separate a fraction 212 of non-spherical ground particles from a fraction 214 of larger particles and a fraction 216 of smaller particles. Thus, the particles of fraction 212 that are subjected to spheronization in spheronization step 218 have only or primarily a maximum dimension within the range between D2 and D1, as defined above.

[0129] Method for producing cold-compactable metal powders

[0149] The present invention further relates to a method of producing a cold-formable metal powder from a particulate metal precursor, which may comprise flake-like particles, the flake-like particles having a maximum dimension greater than 250 microns.

[0130]

[0150] The method includes grinding a particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material comprising non-spherical ground particles. The ground material is classified during and / or after grinding to separate a fraction of the non-spherical ground particles from smaller particles. The separated non-spherical ground particles are spheroidized by dry milling in the impact chamber or in a separate impact chamber, thereby producing spherical metal particles.

[0131]

[0151] At least a portion of the separated smaller particles are then combined with at least a portion of the spherical metal particles, and the particle combination is subjected to impact blending in the impact chamber or in a separate impact chamber to adhere the smaller particles to the spherical metal particles. This results in a cold-formable metal powder comprising non-spherical particles, each of which comprises a spherical metal particle as a core and a plurality of smaller metal particles as protrusions from the core.

[0132]

[0152] Each impact chamber used in the present methods, i.e., milling, spheronizing, and impact blending, includes a rotor configured to rotate within the impact chamber, the rotor including a plurality of impact surfaces that strike particles within the impact chamber as the rotor rotates during dry milling and impact blending.

[0133]

[0153] The dry milling equipment and the grinding, classification and spheronization steps are generally as disclosed herein in the context of the method for producing spherical metal particles.

[0134]

[0154] In some embodiments, the smaller particles separated in the classification process have a maximum dimension D2 of less than 100 microns, or less than 75 microns, or less than 50 microns, or less than 30 microns. In such embodiments, the non-spherical ground particles subjected to spheronization have only or primarily particle sizes greater than D2. All of such non-spherical ground particles may then be spheronized, or only a narrower size fraction of the non-spherical ground particles, for example, particles in the 150-250 micron range, may be spheronized.

[0135]

[0155] In some embodiments, the ground material is classified into (i) a first fraction of small particles having a maximum dimension D2 of less than 100 microns, or less than 75 microns, or less than 50 microns, or less than 30 microns, and (ii) a second fraction of large particles having a maximum dimension D3 of at least 100 microns, or at least 150 microns, e.g., in the range of 150 to 250 microns. Some or all of the second fraction of particles is then subjected to a spheronization process.

[0136]

[0156] In some embodiments, the small particles of the first fraction have a maximum dimension D2 of less than 75 microns, preferably less than 50 microns, and the large particles of the second fraction have a maximum dimension D3 of at least 150 microns, for example in the range of 150 to 250 microns.

[0137]

[0157] Optionally, the smaller particles separated in the classification process may be spheroidized, but it is not believed that these particles need to be substantially spheroidized to produce a cold-compactable metal powder, and the limited amount of particle smoothing that occurs in the milling and / or impact blending steps is likely to be sufficient to produce good results.

[0138] Particle Combination and Impact Blending

[0158] Following spheroidization of the target fraction of non-spherical ground particles, some or all of the resulting spheroidized metal particles are combined with some or all of the smaller particles. For example, the first and second fractions disclosed above can be combined. The combined particles are then subjected to impact blending in an impact chamber to cause the smaller particles to adhere to the spherical metal particles, thereby producing a cold-compactable metal powder containing non-spherical particles comprising one spherical metal particle as a core and multiple smaller metal particles as protrusions from the core.

[0139]

[0159] As used herein, impact blending refers to a dry powder blending process that involves high-velocity impact between particles to cause smaller metal particles to adhere, preferably metallurgically bond, to larger particles with sufficient strength. Impact blending is carried out in an impact chamber as disclosed herein. For example, impact blending may be carried out in the apparatus disclosed herein with reference to Figures 1-3.

[0140]

[0160] Several equipment and process parameters of the impact blending process can be particularly important to the resulting powder morphology. These include blade design, particularly the gap between the rotating blades and the stator (gap 120). We have found that a gap distance of 3.5 mm is suitable for adhering small Ti-6Al-4V particles (5-25 microns) to spheroidized Ti-6Al-4V particles having a particle size between 150-250 microns, and we expect similar gap distances to be suitable for impact blending a wide range of large and small particle sizes. Thus, in some embodiments of the impact blending apparatus, the outer edge of each impact blade is spaced from the cylindrical outer wall by a gap distance ranging from 1 mm to 5 mm, e.g., 2 mm to 4 mm.

[0141]

[0161] Another important parameter is the rotor rotational speed. Without wishing to be limited by any theory, it is proposed that the rotational speed of the impact blades near the rotor periphery must be sufficient to generate the high-intensity collisions between powder particles necessary for robust particle adhesion. The inventors have found that rotor peripheral speeds in the range of approximately 37.1 m / s to 55.6 m / s (corresponding to 6,000 rpm to 9,000 rpm for the impact blender used by the inventors) are suitable for modifying large, spheroidized Ti-6Al-4V particles having particle sizes in the range of 150 to 250 μm, although it is understood that lower or higher speeds may be preferable for different powder compositions and particle sizes. In some embodiments, the rotor is rotated so that the outer edge of each impact blade has a peripheral speed greater than 35 m / s, e.g., greater than 40 m / s, during impact blending.

[0142]

[0162] However, the intensity of the impact blending should not be so high that the desired morphology of the impact blended particles is lost due to the adhered small particles coating or forming a film on the surface of the larger particles. Thus, in some embodiments, the rotor is rotated during impact blending such that the outer edge of each impact blade has a peripheral speed of less than 60 m / s, or less than 55 m / s, for example less than 50 m / s, during impact blending.

[0143]

[0163] Another important parameter is the impact blending time, which should be long enough to attach the small metal particles to the large core particles, but not so long that the modified particles become spheroidized by coating. In some embodiments, the particle combination is impact blended for a time period of 1 second to 10 minutes, such as 5 seconds to 5 minutes, or 10 seconds to 1 minute. It will be appreciated that the time required to produce the desired particle morphology may be inversely related to the intensity of the impact blending conditions.

[0144]

[0164] The particle combination may be impact blended in a dry inert gas atmosphere, which advantageously limits oxygen contamination of the metal powder during impact blending. Without being limited by theory, it is proposed that the newly exposed metal surfaces of particles formed during impact blending in an inert atmosphere are not immediately sealed by an oxide layer, but instead maintain the ability to adhere to and intermix with the metal surfaces of other particles.

[0145]

[0165] Impact blending of particle combinations under appropriate conditions causes the small metal particles to adhere to the larger particles, resulting in non-spherical particles that contain one large metal particle as a core and multiple small metal particles as protrusions from the core. This morphology is also referred to herein as a "core-corona" morphology. Impact blended powders containing non-spherical core-corona particles have been found to have desirable cold-forming properties that cannot be attributed solely to the metal composition of the modified powder and therefore result from the modified particle morphology in the powder.

[0146]

[0166] 6 shows a schematic of an impact blending process 400 for converting a combination 410 including large particles 412 (spheroidized metal particles) and small particles 414 (separated crushed particles) into a non-spherical particle 416 having a core-corona morphology. The particle 416 includes one of the large metal particles 412 as a core 418 and multiple small metal particles 414 as protrusions 420 from the core. Some of the protrusions (420a) include a single small particle 414, while other protrusions (420b) include clusters of the small particles 414.

[0147]

[0167] Without wishing to be limited by any theory, the inventors propose, based on scanning electron microscope (SEM) analysis of core-corona particle cross sections, that the small metal particles are metallurgically bonded to the large particles along at least a portion of the interparticle interface. This mode of chemical bonding is distinct from simple mechanical embedding of the type that occurs when soft, non-metallic core particles are surface-modified with hard guest particles. Thus, the large and small metal particles are effectively integrated into a single non-spherical metal particle as the core and protrusions. The mechanical integrity of the resulting non-spherical particles is believed to be important for desirable cold-forming properties, because the attachment of the protrusions to the core must withstand severe compressive forces when the particles are consolidated under pressure into a compact.

[0148]

[0168] Non-spherical core-corona particles typically contain many protrusions distributed around the periphery of the core particle. Each protrusion may contain a single small particle or a cluster of small particles. The small particles may be deformed by the impact that forms a bond with the core particle or by subsequent high-velocity impacts of non-spherical particles during the impact blending process. The degree of deformation may depend on the yield strength and ductility of the metal composition, as well as the impact blending conditions and time. Some deformation and spheroidization are acceptable, provided the particles retain their core-corona morphology. However, if spheroidization continues to the extent that small particles attached to the core are knocked out into a substantially uniform shell around the core, the cold-forming properties of the powder may be adversely affected.

[0149]

[0169] Particle morphology can be quantified by its convexity index, which is defined as the ratio of the perimeter of the particle's convex hull to the perimeter of the object itself (both measured relative to the particle's cross section). In some embodiments, the convexity index of non-spherical (core-corona) particles is less than 0.8, e.g., in the range of 0.4 to 0.8.

[0150]

[0170] It is not necessary for all particles in an impact-blended metal powder to exhibit a core-corona morphology, and good cold-forming properties may be obtained when only a fraction of the particles have this morphology. Thus, in some embodiments, the cold-formable metal powder comprises at least 20 wt.%, e.g., at least 50 wt.%, or at least 60 wt.%, of non-spherical particles comprising one large metal particle as the core and multiple smaller metal particles as protrusions from the core.

[0151] Embodiment

[0171] Various embodiments of the present invention will now be described with reference to Figure 5. A method 300 for producing a cold-formable metal powder includes providing 302 a particulate metal precursor 304 comprising flaked particles having a maximum dimension greater than 250 microns. Optionally, precursor 304 comprises flaked particles having a maximum dimension greater than 500 microns, or greater than 1 mm, or greater than 2 mm, or greater than 3 mm, or greater than 4 mm. For example, precursor 304 may be shavings, such as titanium alloy shavings.

[0152]

[0172] In the milling step 306, the precursor 304 is milled by dry milling to produce a milled material 308 comprising non-spherical milled particles. The dry milling occurs in an impact chamber containing a rotor. The rotor includes multiple impact surfaces that strike and mill particles within the impact chamber as the rotor rotates during dry milling.

[0153]

[0173] In classification step 310, the ground material 308 is classified to separate a fraction 312 of non-spherical ground particles from a fraction 316 of smaller particles. For example, the ground material 308 may be separated into fractions 312 and 316 by sieving. Fractions 312 and 316 need not be adjacent fractions. In other words, a further fraction (not shown) of particles having a size intermediate between fractions 312 and 316 may be separated in the classification.

[0154]

[0174] For example, the ground material 308 is classified into (i) a fraction 316 of smaller particles having a maximum dimension D2 of less than 75 microns, or less than 50 microns, or less than 30 microns, and (ii) a fraction 312 of larger particles having a maximum dimension D3 of at least 100 microns, or at least 150 microns, e.g., in the range of 150-250 microns.

[0155]

[0175] In a spheronization step 318, all or a portion of the separated fraction 312 of non-spherical ground particles is spheronized by dry milling in an impact chamber to produce spherical metal particles 320. Dry milling occurs in an impact chamber that includes a rotor. The rotor includes multiple impact surfaces that strike and spheronize the particles within the impact chamber as the rotor rotates during dry milling.

[0156]

[0176] In a combining step 332 , some or all of the smaller particle fraction 316 is combined with some or all of the spherical metal particles 320 to produce a combination 334 .

[0157]

[0177] In an impact blending step 336, the particle combination 334 is then subjected to impact blending in an impact chamber to cause the smaller particles to adhere to the spherical metal particles, thereby producing a cold-formable metal powder 338 containing non-spherical particles comprising one spherical metal particle as a core and multiple smaller metal particles as protrusions from the core. Impact blending occurs in an impact chamber containing a rotor. The rotor includes multiple impact surfaces that strike and accelerate the particles within the impact chamber as the rotor rotates during dry milling, thereby causing the smaller particles to adhere to the larger, spheroidized particles.

[0158] Metal powders for additive manufacturing or powder metallurgy

[0178] The methods disclosed herein are believed to be useful for producing metal powders for additive manufacturing or powder metallurgy.

[0159]

[0179] Spherical metal particles can be useful as a feedstock for additive manufacturing. For example, spherical metal particle powders can be a suitable feedstock for powder bed additive manufacturing techniques. Laser powder bed fusion and binder jet printing techniques require controlled powder dispersion and particle sizes typically in the 15-63 micron range. Electron beam powder bed techniques (sometimes called selective electron beam powder beds) require particle sizes in the 40-106 micron range because finer powders would destroy the electron beam filaments. In all powder bed applications, the powder must be uniformly dispersed within the layer, so spherical particles are required to provide the necessary flow control and good packing density. The finer the powder, the greater the geometric fidelity of the resulting part features. In other situations, spherical metal particle powders can be a suitable feedstock for directed energy deposition (DED) additive manufacturing, such as sprayed powder DED techniques, which typically require particle sizes in the 30-150 micron range.

[0160]

[0180] Non-spherical (core-corona) particles that include one large metal particle as a core and multiple smaller metal particles as protrusions from the core can be useful in powder metallurgy, particularly manufacturing techniques in which the powder is first cold-compacted to form a porous metal article.

[0161]

[0181] For example, a cold forming process may include any cold forming method in which a metal powder, with or without a binder, is consolidated under pressure at a temperature below the sintering temperature to produce a porous metal body (compact) that has sufficient structural integrity to withstand further processing, e.g., via sintering or other metal working techniques, to produce a final metal product. Suitable cold forming techniques may include cold isostatic pressing, cold die pressing, direct powder rolling, and metal injection molding.

[0162]

[0182] Without wishing to be bound by any theory, it is proposed that protrusions on non-spherical particles promote particle interlocking during compaction, increasing the contact area at the interfaces between adjacent particles in the compact. Thus, interparticle adhesion is increased, and the resulting compact has improved mechanical properties. Figure 7 schematically illustrates several proposed interlocking modes that may occur between adjacent non-spherical particles 516 in a porous metal compact. At some interparticle interfaces, such as interface 510ab between particles 516a and 516b, the particles engage through protrusions 520a and 520b on both particles. At other interparticle interfaces, such as interface 510bc between particles 516b and 516c, particle engagement is assisted by protrusion 520c on only one particle.

[0163]

[0183] The cold compaction process consolidates a cold-formable metal powder containing core-corona particles to produce a porous metal article. Thus, voids exist between the formed metal particles in the metal structure, and the voids are emptied after the binderless compaction process. The porosity of the metal article depends on the particle morphology, the particle's deformability under compaction pressure (which may be low if the particle core is formed from a high-yield-strength composition), and the compaction pressure. Thus, in some embodiments, the porous metal article has a density of at least 70% of the theoretical density. However, it will be understood that the porosity of a suitably robust compact may vary in other implementations depending on the factors discussed above. Furthermore, the density of the porous compact can be increased by adding small particles to the cold-formable metal powder, where the small particles are sized to occupy a portion of the voids between the interlocking non-spherical particles after compaction.

[0164]

[0184] Cold-formable metal powders containing core-corona particles may be compacted at any pressure sufficient to consolidate the cold-formable metal powder and thus form a porous metal article. The metal powder can advantageously be compacted at significantly lower pressures than those required for rounded (e.g., spherical) particles lacking protrusions but having a similar metal composition. The inventors have found that non-spherical particles having a core (and Ti-6Al-4V protrusions) derived from Ti-6Al-4V shavings can be compacted to form robust compacts at pressures of only 380 MPa. In contrast, pre-alloyed Ti-6Al-4V spherical powder (gas-atomized powder) is not cold-formable even at 413 MPa, and it is expected that pressures in excess of 1000 MPa may be required to compact these spherical particles. Thus, in some embodiments, the cold-formable metal powder may be consolidated by cold forming at a pressure of less than 450 MPa, or less than 400 MPa, for example less than 350 MPa.

[0165]

[0185] The porous metal article produced directly by cold forming is typically not a final metal product, but instead will be further processed. In some embodiments, the porous metal article is sintered to produce a sintered metal structure that can be further processed by conventional metal processing techniques. In other embodiments, the porous metal article may be a feedstock for metal manufacturing techniques such as extrusion. In one exemplary application, the porous metal article is a cylindrical rod-shaped titanium alloy compact, suitable for extrusion to produce titanium alloy wire, as disclosed in U.S. Patent No. 9,468,960, having dimensions of, for example, 12.5 mm in diameter and 400 mm in length. [Example]

[0166]

[0186] The invention will now be described with reference to the following examples, which should be understood as illustrative of the invention described herein and not limiting.

[0167] material

[0187] Titanium alloy shavings generated as a by-product of aerospace manufacturing processes were characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The chemical composition, shown in Table 1, confirmed that the shavings were Ti-6Al-4V alloy and contained approximately 0.19% oxygen, which is within the Grade 5 specification for this alloy.

[0168] [Table 1]

[0169]

[0188] The shavings were characterized by scanning electron microscopy (SEM, ZEISS Merlin™ FE-SEM) to investigate their morphology. As seen in Figure 8, the shavings are flake-like with a slight curvature relative to the structure typical of materials generated by machining processes. As seen in Figures 9 and 10, the shavings surface also contains numerous defects, such as cracks, jagged edges, and perforations. The flaky shavings particles have thicknesses ranging from 30 to 100 μm and grain sizes (maximum dimension) up to 5 mm in length.

[0170]

[0189] The shavings were subjected to sieve analysis according to ASTM B214-22 to determine the particle size distribution shown in Figure 11. The d10, d50 and d90 particle sizes of the shavings were estimated to be 0.64 mm, 1.25 mm and 2.2 mm, respectively.

[0171]

[0190] The apparent density (ASTM B417-22; Standard Test Method for Apparent Density of Non-Free-Flow Metal Powders Using the Carney Funnel) and tap density (ASTM B527-22; Standard Test Method for Determination of Tap Density of Metallic Powders and Compounds) of the shavings were 0.616 g / cm, respectively. 3 and 0.751 g / cm 3The theoretical density of Ti-6Al-4V alloy was measured as 4.429 g / cm. 3 The very low apparent density values ​​compared to the curved flake particles are consistent with the low packing efficiency of the curved flake particles.

[0172]

[0191] Highly spherical gas-atomized titanium alloy Ti-6Al-4V (Ti64) powder was received from a commercial manufacturer. The material was the undersize (small) fraction (5–25 μm, hereafter Ti64-S1) produced as a low-value by-product in the gas atomization process.

[0173] Dry milling apparatus and method

[0192] The Nara Hybridization System (NHS-0), available from Nara Machinery Manufacturing Co., Ltd. and shown schematically in Figure 1, was used as the particle modification device. This system includes an impact chamber defined by a rotor and a cylindrical stator with a recirculation duct. During dry milling, particles can exit the impact blending chamber through the stator outlet and are recirculated back to the center of the chamber via the recirculation duct. The chamber is surrounded by a jacket through which a coolant circulates to maintain the internal processing temperature below 100 °C, typically below 50 °C. The NHS-0 was operated under a high-purity argon (3 ppm O2) atmosphere to keep oxygen levels as low as possible and thus reduce the chance of oxygen contamination of the titanium powder material.

[0174]

[0193] Schematics of the NHS-0 impulse chamber are shown in Figures 1, 2, and 3. The outer wall of the impulse chamber is defined by a stator 110. A 118 mm diameter rotor 112 includes six radially oriented impulse blades 114 with an impulse face 116 and an outer edge 118 around the rotor's periphery. The impulse blades have a radial length of 20 mm, a thickness of 5 mm, and flattened edges. The gap 120 between the blade's outer edge 118 and the stator 110 was 3.5 mm. The rotor also includes a rear radial rib 115 spaced from the rear wall 117 of the impulse chamber by a 0.9 mm gap 119.

[0175]

[0194] The NHS-0 can operate at rotor speeds up to 16,000 rpm. Generally, speeds between 6,000 rpm and 10,000 rpm were considered most suitable. The corresponding conversion from rotational speed to peripheral speed (speed at the outer edge 118 of the blade) is shown in Table 3. The device took some time to reach the set rotational speed (23, 30, and 38 seconds to reach 6,000, 8,000, and 10,000 rpm, respectively). Impact blend times referred to in subsequent examples refer to the time after the set rotational speed was reached.

[0176] [Table 2]

[0177]

[0195] The maximum batch size for the NHS-0 Hybridizer is approximately 50 g. However, due to the very low apparent density of the as-received shavings, a batch size of 10 g was used when dry milling this material to ensure good milling action and particle circulation in the impact chamber and recirculation duct.

[0178] Cold isostatic pressing apparatus and method

[0196] A cold isostatic press (CIP) with a maximum pressure of 413 MPa (60 ksi) was used for cold compaction testing. A 9 mm inner diameter (ID) cylindrical latex bag was used as a flexible module for the CIP experiments, which produced "mini-rods" to demonstrate powder compactibility. Each mini-rod measured approximately 8.5 mm in diameter, 30-50 mm in length, and weighed approximately 6.0 grams.

[0179]

[0197] The following general procedure was used for the cold isostatic pressure experiments: Put the powder into a flexible bag and frequently tap the bag to make the powder filling as uniform and dense as possible. Insert a stopper into the opening of the bag and fasten it with a rubber band to ensure that the bag is tightly sealed and the compressed liquid cannot come into contact with the compact. The bag is secured to a long, narrow metal support (an L-shaped metal angle) to keep the bag upright during cold isostatic pressing and ensure uniform pressure on the outside of the bag. The secured bag is placed in a basket configured to be placed in the pressure chamber of the CIP. · Operate the CIP and isostatically press the compact to a preset pressure. After pressing, remove the green rod from the flexible bag.

[0180] Example 1

[0198] The as-received turnings were subjected to dry milling in an NHS-0 system at 10,000 rpm for times ranging from 30 to 180 seconds. As seen in Table 2, the temperature of the material measured in the recirculation duct increased from 63°C to 95°C between 30 and 180 seconds of milling. The yield of recovered ground material also decreased during this period, which was attributed to increased deposition of metallic material on the rotor blades or other surfaces of the equipment at higher temperatures. The recovered ground powder was then subjected to sieve analysis to determine particle size distribution. The results are also shown in Table 2.

[0181] [Table 3]

[0182]

[0199] As seen in Table 2, the particle size of the shavings generally decreased with increasing dry milling time. However, the percentage of very fine particles, i.e., less than 75 μm, decreased with processing time, indicating that very fine particles generated early in the dry milling process may reattach to the coarse particles. Therefore, a short processing time is preferred to obtain a larger amount of fine particles (~75 μm). Also, a short processing time for milling is desirable to prevent excessive temperatures and the resulting loss of yield.

[0183]

[0200] Classified fractions of shavings subjected to dry milling at 10,000 rpm for 30 seconds were characterized by scanning electron microscopy (SEM, ZEISS Merlin™ FE-SEM) to examine morphology. Compared to the as-received shavings, particles in each fraction had smooth surfaces with jagged edges removed, rounded facets, and folded edges. However, particles maintained a flaky, "blocky" morphology and therefore remained nonspherical. Figures 12 and 13 show representative SEM images of the 75-250 μm and 500-1000 μm fractions.

[0184] Example 2

[0201] The as-received shavings were subjected to five consecutive dry milling runs for 30 seconds on an NHS-0 system at 10,000 rpm. In the first run, 100 g of shavings were processed in 10 batches. Due to the low apparent density of the shavings, they were limited to 10 g per batch. After the first run, 10 batches of ground material were sieved into three size ranges: ~250 microns (less than 250 microns), 250-500 microns, and +500 microns (greater than 500 microns). For subsequent dry milling runs (using up to 50 g of material per batch), the two larger fractions were dry milled separately and classified into the same three size fractions. For the next run, the newly formed ~250 micron fraction was combined with the previously formed ~250 micron fraction, the two newly formed 250-500 micron fractions were combined, and the two newly formed +500 micron fractions were combined.

[0202] Figure 14 shows the mass flow diagram for five milling passes. The particle size decreased with each pass; thus, after five milling passes, 65% of the initial turnings were converted to particle sizes less than 250 μm, 30% of the particles were in the 250-500 μm range, and only 3% of the particles were larger than 500 μm. Advantageously, the yield of recovered material after five milling passes was nearly 97%, compared to a yield of approximately 80% when milling continuously for a similar total time (see Table 2). This may be due to the improved temperature control obtained with the stepwise approach.

[0185] Example 3

[0203] Although particle size reduction was achieved by grinding the shavings in Examples 1 and 2, the particles still exhibited a flaky, non-spherical morphology. Such particles are unsuitable for applications such as additive manufacturing, which require a spherical particle morphology. Furthermore, since there is no interlocking mechanism between the particles, they are unsuitable for powder metallurgy techniques using cold compaction. Therefore, it is desirable to spheroidize the ground particles to facilitate their use as raw materials in various manufacturing techniques or to facilitate further morphological manipulation of the spheroidized particles for cold compactability.

[0186]

[0204] The ~250 micron fraction of particles produced after five milling passes in Example 2 was further classified by sieving into ~75 μm (<75 μm), 75-150 μm, and 150-250 μm fractions. The amount of the ~75 μm fraction was 8.1 wt.% of the total initial turnings. Each size range of particles was then separately subjected to dry milling at 10,000 rpm for 14 minutes. The amount of material processed in each batch was limited only by the amount of available material and varied from 10 g to 25 g, thus demonstrating that the spheronization process can be performed at a higher loading concentration in the impact chamber than the grinding process. The particle morphologies before and after spheronization are shown in Figures 15 and 16 (~75 μm, before and after spheronization), Figures 17 and 18 (75-150 μm, before and after spheronization), and Figures 19 and 20 (150-250 μm, before and after spheronization).

[0187]

[0205] Spheroidization and surface smoothing of milled shaving particles occurred in all three particle size ranges, although the degree of spheroidization varied slightly. Under the same dry milling conditions, the most effective spheroidization occurred in the 150–250 μm particle size range, the least effective for particles smaller than 75 μm, and intermediate for particles between 75 and 150 μm. It appears that spheroidization of ~75 μm particles could be improved by more intensive dry milling conditions (longer milling time and / or higher speed).

[0188]

[0206] Based on the SEM images, it is clear that only a slight reduction in particle size occurred during spheroidization in all three particle size ranges, with particles less than 75 μm experiencing the least size reduction. The particle size reduction was primarily due to changes in particle shape (spheroidization) and, to a much lesser extent, to the generation of fines. Without wishing to be limited by any theory, spheroidization has advantages over grinding compared to milling of as-received turnings due to both the smaller initial particle size and the smoothing of particles and removal of defects introduced in the grinding process.

[0189]

[0207] To quantify the effect of the dry milling process on particle morphology, the sphericity of the powders was characterized using image analysis of SEM images. Sphericity is a measure of the degree to which particles in a powder approximate the shape of a sphere. Sphericity is measured based on the cross-section of an imaged particle using image analysis software, in this case "Image J." Sphericity is defined as the ratio of the radius of the inscribed circle to the radius of the circumscribed circle with respect to the cross-section of the imaged particle (where the inscribed circle is the largest circle inside the particle cross-section, and the circumscribed circle is the smallest circle outside the particle cross-section, and both circles are centered at the center of mass of the particle). A perfectly spherical object has a sphericity of 1.

[0190]

[0208] The sphericity values ​​of the as-received shavings, the input powder to spheronization (the milled material of Example 2, classified into fractions of ∼75 μm, 75–150 μm, and 150–250 μm), and the resulting spheronized powder (still in the fractions of ∼75 μm, 75–150 μm, and 150–250 μm) are shown in Table 3. The average sphericity is the average of 10 particles. The as-received shavings are very irregular, and sphericity remains low for all fractions after milling (performed in Example 2). After the spheronization step, sphericity is significantly improved, especially for the two larger fractions. It is expected that longer dry-milling times and / or more intense dry-milling conditions may result in further improvements in sphericity.

[0191] [Table 4]

[0192] Example 4

[0209] The three fractions of spheroidized particles isolated in Example 3 were recombined (~250 μm) and mixed with undersized gas-atomized Ti-6Al-4V alloy powder (Ti64-S1; 5-25 μm) in an 80:20 weight ratio. The combined powders were then subjected to rotary impact blending (15 g per batch) for 30 seconds in an NHS-0 system at various rotor rotation speeds (6,000-9,000 rpm). The impact-blended powders were then collected and analyzed by SEM to investigate the resulting morphology.

[0193]

[0210] Under low-impact milling conditions (6000 rpm for 30 seconds), only a small number of small Ti64-S1 particles adhered to the larger, spheroidized particles. Most Ti64-S1 particles remained untouched by the milling. Under slightly more aggressive milling conditions (7000 rpm for 30 seconds), many more small particles adhered.

[0194]

[0211] After dry milling at medium intensity (8000 rpm for 30 seconds), a substantial fraction of the small Ti64-S1 particles adhered to the larger, spheroidized particles. The resulting powder contained non-spheroidal particles with a large metal particle (originating from the previously spheroidized particle) as the core and multiple small metal particles (originating from Ti64-S1) as protrusions from the core. A representative SEM image is shown in Figure 21.

[0195]

[0212] At even higher intensity conditions (9000 rpm for 30 seconds), most of the small Ti64-S1 particles adhered to the larger spheroidized particles, and some flattening of the resulting protrusions was also observed. A representative SEM image is shown in Figure 22.

[0196]

[0213] The morphology of impact-blended non-spherical particles, which includes a large metal particle as a core and multiple small metal particles as protrusions from the core, is believed to provide improved cold formability properties compared to spherical precursor particles. One way to quantify the difference in morphology is by the convexity of the particle. Convexity is the relative amount by which an object differs from a convex object. In this case, the convex object is the particle measured in cross section. The convexity measurement is obtained by forming the ratio of the perimeter of the convex hull of the particle to the perimeter of the object itself, according to the following formula: A convex hull is a polygon that encloses the cross section of the particle, with no point of the polygon curving inward.

[0197]

number

[0198]

[0214] If the particle cross-section is a convex object (e.g., a circular particle or an ellipse with a smooth surface), the convex hull and the perimeter of the object are the same, so the convexity is 1. If the object has an irregular boundary, the value is less than 1. For impact-blended particles, if the convexity value is closer to 1, then its shape is closer to the original core particle before impact blending.

[0199]

[0215] Convexity measurements of the as-received shavings, the spheroidized powder (size range ∼250 μm, three recombined portions from Example 3), and the impact blended powder (8000 rpm, as produced in Example 4 from the spheroidized powder) are shown in Table 4. Measurements were obtained from SEM images processed using the image processing software Image J. At least 10 measurements were made for each particle composition.

[0200] [Table 5]

[0201]

[0216] The convexity value of the Ti64-S1 gas-atomized small particles is very close to 1, consistent with nearly perfectly circular particles with smooth surfaces. The increase in convexity of the spheroidized shavings particles compared to the as-received shavings indicates that the milling and spheroidization processes transformed the shavings into nearly circular or elliptical particles with relatively smooth surfaces, consistent with the SEM results. After impact blending the spheroidized shavings particles with the Ti64-S1 gas-atomized small particles, the convexity value decreased again due to the formation of a "core-corona" morphology.

[0202] Example 5

[0217] The powder produced in Example 4, which contained non-spherical particles with a large metal particle as a core and multiple smaller metal particles as protrusions from the core, was subjected to cold isostatic pressing (CIP) at a pressure of 380 MPa (55 ksi) to determine whether the powder was cold-formable. The powder was compacted to form "mini-rods" approximately 8.5 mm in diameter and 30-50 mm in length; a powder was considered cold-formable if it emerged from the CIP mold intact and non-brittle, solid rods (brittleness means that the compact easily reverts to powder during handling).

[0203]

[0218] Although minirods formed from powders produced by low-intensity milling (6000 rpm, 30 seconds) were weaker than the other rods, each of the powders produced in Example 4 was found to be cold-formable. Clearly, there is a wide process window for modifying spheroidized particles to a "core-corona" morphology for improved cold formability, and even smaller amounts of "small" particles (e.g., a 90:10 weight ratio of large to small particles) can be expected to be sufficient to render the spheroidized particles cold-formable. Furthermore, similar results can be expected if the spheroidized particles are modified with fine particles (e.g., a fines fraction less than 75 μm) from the shavings milling process instead of spherical Ti64-S1 particles.

[0204]

[0219] In contrast, metal powders with spheroidized particles of ~250 μm (i.e., before impact blending) were found not to be compactible at 380 MPa. The results indicate that spheroidized particles produced by milling, classification, and spheroidization of shavings can be improved for powder metallurgy applications involving cold compaction by the impact blending methodology.

[0205] Example 6

[0220] The powder produced in Example 4 (impact blended at 8,000 rpm for 30 seconds), which contained non-spherical particles with a large metal particle as the core and multiple small metal particles as protrusions from the core, was used to investigate the bonding mode between the core (derived from the crushed and spheroidized Ti64 shavings) and the protrusions (derived from the small Ti64-S1 particles). The impact-blended powder particles were cold-mounted and polished to 1200 grit using SiC paper, followed by a final polish using an OP-S suspension. To identify and examine the interface between the large core particle and the small Ti64-S1 particle, the polished samples were etched using Kroll's reagent. The impact-blended powder was then characterized using a high-magnification SEM (ZEISS Merlin™ FE-SEM).

[0206]

[0221] The SEM images supported the following conclusions. The shavings particles were spheroidized by folding and spreading, consistent with a ductile spheroidization mechanism. · The outer layer of the shavings showed a laminated microstructure, suggesting that there was some deformation of the shavings during the impact blending process. · The fine particles formed a crown or "corona" on top of the core particle. Many of the protrusions from the core consisted of clusters of multiple particles. Despite some small voids observed at the interfaces, the integrity of the interfaces formed between connected particles (both core-small particle and small particle-small particle interfaces) is consistent with metallurgical bonding not only between corona particles and core particles, but also between corona particles within clusters.

[0207]

[0222] To investigate how the impact-blended powder produced in Example 4 (impact blended at 8,000 rpm for 30 seconds) responded to cold compaction, the green compacts produced by cold isostatic pressing in Example 5 were characterized by high-magnification SEM (ZEISS Merlin™ FE-SEM). Cross sections of the green compacts were cold-mounted and polished to 1200 grit using SiC paper, followed by a final polish using an OP-S suspension. To identify and examine the interfaces between particles in the compacts, the polished samples were etched with Kroll's reagent prior to SEM analysis.

[0208]

[0223] SEM images showed that the large core particles were not significantly deformed by compaction, and many of the interfaces between adjacent large particles contained protrusions from one or both particles. Metallurgical bonding between the shaving-derived core particles was unlikely because the cold compaction pressure was insufficient to induce such bonding. It was concluded that interparticle interlocking, promoted by corona morphology, was important for the cold compatibility of the powder.

[0209] Example 7

[0224] A portion of the as-received shavings was subjected to a beta-annealing process, i.e., temperatures above the beta transus temperature. Thus, the shavings were gradually heated to 1100 °C in a vacuum furnace and, after a dwell time of 30 minutes, slowly cooled to room temperature in the furnace. Scanning electron microscopy examination revealed that the microstructure changed from a dual structure (spheroidal / recrystallized alpha and transformed beta) in the as-received shavings to a fully lamellar structure with large lath thickness and potentially increased colony size under the beta-annealing + slow cooling conditions.

[0210]

[0225] The as-received and annealed shavings were then evaluated using Vickers microhardness testing (ASTM E92-82(2003)). The Vickers hardness number (Hv) of the as-received shavings was found to be 330±15, while the Hv of the annealed shavings was 254±68. The results demonstrate that the annealing process relaxes the metal composition of the shavings, reversing the work hardening that occurs during the machining process, thereby increasing the ductility of the composition.

[0211]

[0226] The annealed shavings were then subjected to dry milling for 60 seconds in an NHS-0 system at 10,000 rpm. The shavings particles agglomerated in the impact chamber due to their increased ductility and were not well crushed or spheroidized under the test conditions.

[0212] Example 8 (Comparative Example)

[0227] Multiple attempts were made to crush and spheronize the as-received shavings in the high shear liquid milling process disclosed in WO 2015 / 192166 using distilled water as the milling media and a rotor-stator gap size of 0.45 mm. The process in WO 2015 / 192166 is useful for crushing soft materials such as titanium sponge. However, this process was ineffective in processing the shavings because the shaving particles jammed in the mill, causing excessive heat buildup, high noise and vibration, and resulting in equipment damage.

[0213]

[0228] Turnings could only be milled in a high-shear mill by performing an initial ring-milling process step to reduce particle size to the 38-150 micron range. Furthermore, the milled particles remained substantially non-spherical even after 30 minutes of high-shear liquid milling. Furthermore, ring milling was found to significantly degrade the titanium alloy composition through oxygen contamination, and further contamination is expected to occur during the subsequent high-shear liquid milling process.

[0214]

[0229] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, and it is to be understood that the invention includes all such variations and modifications within the spirit and scope of the invention.

Claims

1. 1. A method for producing spherical metal particles, comprising: providing a particulate metal precursor comprising flake-like particles having a maximum dimension greater than 250 microns; grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material comprising non-spherical ground particles; Classifying the ground material to separate the fraction of non-spherical ground particles from larger and / or smaller particles, wherein the classification is carried out during and / or after grinding; and spheronizing the separated non-spherical ground particles by dry milling them in the impact chamber or in a separate impact chamber, thereby producing spherical metal particles; The method, wherein each impact chamber includes a rotor configured to rotate within the impact chamber, the rotor including a plurality of impact surfaces that strike particles within the impact chamber as the rotor rotates during dry milling.

2. The method of claim 1 , wherein the particulate metal precursor comprises flake-like particles having a maximum dimension greater than 1 mm.

3. 3. The method of claim 1 or claim 2, wherein the particulate metal precursor is shavings.

4. 4. The method of claim 1, wherein the step of classifying the ground material comprises separating a fraction of non-spherical ground particles from larger particles, the separated non-spherical ground particles having a maximum dimension D1 of less than 250 microns.

5. 5. The method of claim 4, wherein the separated non-spherical ground particles have a maximum dimension D1 of less than 100 microns.

6. 6. The method of claim 4 or claim 5, further comprising the step of grinding larger particles by dry milling in the impact chamber or in a separate impact chamber after separation, thereby producing additional non-spherical ground particles for spheronization having a maximum dimension less than D1.

7. 7. The method of claim 1, wherein the step of classifying the ground material comprises separating a fraction of non-spherical ground particles from smaller particles, the smaller particles having a maximum dimension D2 of less than 100 microns.

8. 8. The method of claim 7, further comprising, after separation, spheronizing the smaller particles by dry milling.

9. 9. The method of claim 7 or claim 8, further comprising the steps of combining at least a portion of the smaller particles with at least a portion of the spherical metal particles, and subjecting the combined particles to an impact blending process to cause the smaller particles to adhere to the spherical metal particles, thereby producing a cold-compactable metal powder comprising non-spherical particles comprising one spherical metal particle as a core and a plurality of smaller metal particles as protrusions from the core.

10. 10. The method of any one of claims 1 to 9, wherein the particulate metal precursor comprises or consists of a metal composition having a yield stress of at least 600 MPa.

11. 11. A method according to any one of claims 1 to 10, wherein the particulate metal precursor comprises or consists of a metal composition having an elongation at break of at least 1%, preferably at least 3%.

12. 11. The method of any one of claims 1 to 10, wherein the particulate metal precursor comprises or consists of a metal composition having an elongation to break in the range of 5% to 30%.

13. 13. The method of claim 1, wherein the particulate metal precursor comprises a metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, alloys thereof, metal matrix composites thereof, low-ductility alloys of iron, low-ductility alloys of zinc, low-ductility alloys of magnesium, and low-ductility alloys comprising both aluminum and copper.

14. 14. The method according to claim 1, wherein the particulate metal precursor comprises a metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, alloys thereof, and metal matrix composites thereof.

15. 15. The method of any one of claims 1 to 14, wherein the particulate metal precursor comprises a metal composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof.

16. 16. The method of any one of claims 1 to 15, wherein the particulate metal precursor comprises or consists of a titanium alloy.

17. 17. The method of any one of claims 1 to 16, wherein each impact chamber includes a cylindrical stator defining an outer cylindrical wall of the impact chamber, and a recirculation conduit connecting an inlet port located in the outer cylindrical wall with a discharge port directed toward a central portion of the impact chamber, wherein particles are continuously recirculated through the recirculation conduit during dry milling.

18. 18. A method according to any one of claims 1 to 17, wherein the rotor of each impulse chamber includes a plurality of impulse blades defining an impulse face, and wherein the outer edge of each impulse blade at the periphery of the rotor is spaced from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm.

19. 19. A method according to any one of claims 1 to 18, wherein the rotor of each impact chamber comprises a plurality of impact blades defining an impact surface, and wherein the rotor rotates during comminution such that the outer edge of each impact blade at the periphery of the rotor has a peripheral speed of more than 50 m / s.

20. 20. The method of any one of claims 1 to 19, wherein the particulate metal precursor is milled for a time period of from 1 second to 1 minute.

21. 21. The method of any one of claims 1 to 20, wherein the separated non-spherical ground particles are spheronized for a period of time between 1 minute and 30 minutes.

22. 22. The method of any one of claims 1 to 21, wherein the dry milling during grinding and spheronization is carried out in a dry inert gas atmosphere.

23. 23. The method of any one of claims 1 to 22, wherein the particulate metal precursor has an average sphericity of less than 0.5, preferably less than 0.

4.

24. 24. The method of any one of claims 1 to 23, wherein the separated non-spherical ground particles have an average sphericity of less than 0.6 prior to spheronization.

25. 25. The method of any one of claims 1 to 24, wherein the spherical metal particles have an average sphericity of greater than 0.

8.

26. 26. A metal powder for additive manufacturing comprising spherical metal particles produced according to any one of claims 1 to 25.

27. 26. Use of a metal powder comprising spherical metal particles produced according to any one of claims 1 to 25 for additive manufacturing.

28. 1. A method for producing a cold-formable metal powder, comprising: providing a particulate metal precursor comprising flake-like particles having a maximum dimension greater than 250 microns; grinding the particulate metal precursor by dry milling in an impact chamber, thereby producing a ground material comprising non-spherical ground particles; classifying the ground material to separate the fraction of non-spherical ground particles from smaller particles, the classification being carried out during and / or after grinding; spheronizing the separated non-spherical ground particles by dry milling in said impact chamber or in a separate impact chamber, thereby producing spherical metal particles; combining at least a portion of the smaller particles with at least a portion of the spherical metal particles; and subjecting the combination of particles to impact blending in said impact chamber or in a separate impact chamber to cause smaller particles to adhere to spherical metal particles, thereby producing a cold-compactable metal powder comprising non-spherical particles comprising one spherical metal particle as a core and a plurality of smaller metal particles as protrusions from the core; The method, wherein each impact chamber includes a rotor configured to rotate within the impact chamber, the rotor including a plurality of impact surfaces that strike particles within the impact chamber as the rotor rotates during dry milling or impact blending.

29. 30. A cold-compactable metal powder comprising non-spherical particles produced by the method of claim 28.

30. 30. Use of a cold-formable metal powder comprising non-spherical particles produced by the method of claim 29 to produce a porous metal article by cold forming.

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