EP4638041A1

EP4638041A1 - A method of producing spheroidal metallic particles

Info

Publication number: EP4638041A1
Application number: EP23904859.8A
Authority: EP
Inventors: Robert Simon WILSON; Shiqin Yan; Ling Chen; Kun Yang
Original assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Current assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Priority date: 2022-12-20
Filing date: 2023-12-20
Publication date: 2025-10-29
Also published as: WO2024130318A1; JP2026501295A; AU2023407190A1; KR20250137131A

Abstract

The invention provides a method of producing spheroidal metallic particles, the method comprising: providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron; comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non-spheroidal comminuted particles; classifying the comminuted material to separate a fraction of the non-spheroidal comminuted particles from larger and/or smaller particles, wherein the classifying is conducted during and/or after the comminuting; and spheroidizing the separated non-spheroidal comminuted particles by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles, wherein each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling.

Description

A method of producing spheroidal metallic particles

Priority cross-reference

[1 ] The present application claims priority from Australian provisional patent application No. 2022903913 filed on 20 December 2022, the contents of which should be considered to be incorporated into this specification by this reference.

Technical Field

[2] The invention relates to a method of producing spheroidal metallic particles. The method comprises comminuting a particulate metallic precursor comprising flakeshaped particles by dry milling to produce comminuted material comprising non- spheroidal comminuted particles, classifying the comminuted material to separate a fraction of the non-spheroidal comminuted particles from larger and/or smaller particles, and spheroidizing the separated non-spheroidal comminuted particles by dry milling to produce spheroidal metallic particles. The invention further relates to a metallic powder for additive manufacturing or powder metallurgy, and to a method of producing a cold compactible metallic powder.

Background of Invention

[3] Metal components are commonly produced by subtractive manufacturing techniques, such as lathe-turning, milling, drilling, sawing, thread-cutting and gearcutting, which start with a preform produced by melt metallurgy. However, a substantial fraction of the metal in the preform is typically machined away during the manufacturing process, forming swarf (machining chips) as a low value waste product.

[4] This is of particular concern in manufacturing processes which utilize high value metallic compositions. For example, titanium and titanium alloy components are favoured in many industrial and commercial applications because of attractive characteristics including high specific strength, low density, excellent corrosion resistance and exceptional biocompatibility. Ti-6AI-4V (Ti64) alloy is one of the most commonly used alloys in the aerospace, robotics, chemical, energy, marine and biomedical industries. A large amount of Ti64 swarf is produced every day in commercial operations, with some manufacturing processes in the aerospace industry seeing up to 95% of the expensive titanium alloy forging machined away.

[5] Conventional recycling methods for titanium alloy and other metallic waste particulates involve re-melting and re-casting the metallic composition. However, such processing techniques are highly energy intensive given that a large fraction of the metallic composition passes through multiple melt reprocessing cycles. Moreover, recycling of titanium waste to high-performance products is challenging due to the strong affinity of titanium to embrittling interstitial elements. Titanium alloys such as Ti64 are particularly susceptible to increasing oxygen content over multiple machiningreprocessing cycles which puts the reprocessed material out of specification for many high value applications.

[6] Additive manufacturing (AM) and powder metallurgy (PM) technologies are increasingly attractive alternatives to subtractive manufacturing methods. Since these techniques are net near shape technologies, only a relatively small amount of the metal powder feedstock is lost in post-processing of the metal component. However, metal powders with suitable morphology for AM and PM manufacturing are high technology materials requiring specialist manufacture, and are thus typically expensive and available only in limited compositions.

[7] Despite its particulate form, metallic swarf is unsuitable as feedstock for AM or PM manufacturing. Cold compaction of metallic swarf, as required for some PM techniques, has proven extremely challenging due to the poor morphology. Titanium alloy swarf is flake shaped, irregularly sized and generally considered impossible to consolidate into a high density, non-friable compact at room temperature due to the high yield strength, high hardness, low ductility and the lack of interlocking between flake particulates.

[8] The development of new technology for converting metallic swarf, for example titanium and titanium alloy swarf, to high value AM and/or PM feedstocks would therefore provide significant economic and environmental benefits. While the above discussion has focused on titanium-based swarf in particular, it will be appreciated that similar considerations apply to a range of other metallic swarf compositions and indeed to other sources of low value flake-shaped metallic particulates.

[9] There is therefore an ongoing need for new methods of upgrading particulate metallic feedstocks comprising flake-shaped particles which at least partially address one or more of the above-mentioned short-comings, or provide a useful alternative.

[10] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention

[1 1 ] The inventors have now discovered that a particulate metallic precursor comprising flake-shaped particles, such as titanium alloy swarf, may be upgraded to produce smaller-sized spheroidal metallic particles by a method which includes two dry milling processes: a first comminution step or phase in which the particulates are primarily comminuted by the milling action to form smaller but still non-spheroidal comminuted particles, and a second spheroidizing step or phase in which a classified fraction of the non-spheroidal comminuted particles are primarily spheroidized by the milling action. Between the discrete comminution and spheroidizing process steps, or during the initial stages of a continuous dry milling process having sequential comminution and spheroidization phases, the non-spheroidal comminuted particles are classified to separate a selected size fraction of the particles for spheroidization.

[12] The classification step prior to spheroidization has been found important to control the resultant particle size distribution and particle morphology of the product particles, particularly when small particles (e.g. < 100 micron) are a desirable primary target (as the spheroidized particles) or co-product. Spheroidization of particles having a selected, and typically narrow, size range can prevent reattachment of smaller particles to larger particles and allows the spheroidization conditions and/or apparatus to be appropriately tailored to the powder feed. Moreover, the classification step allows more efficient use of the milling apparatus, since the initial flake-shaped morphology of the precursor metallic precursor typically limits the amount of precursor that can be comminuted in an impact chamber of a given volume. After the initial comminution and classification, the target size fraction of particles may then be spheroidized, over the longer durations typically required for this process, in much higher loading concentrations in the impact chamber.

[13] Both dry milling processes may be conducted in an impact chamber comprising a rotor which rotates at high speed within the cylindrical stator, causing the impact blades of the rotor to strike the particles circulating in the impact chamber. While similar apparatus has previously been used to spheroidize small, irregularly shaped particles, it is considered surprising that dry milling in such an apparatus can effectively comminute particles having a particle size greater than 250 micron, and up to 3mm or even higher, into a size range where spheroidization becomes the dominant particle modification process. Without wishing to be bound by any theory, it is proposed that the flake-shaped particle morphology, augmented in the case of swarf by work hardening and defects (cracks, jagged edges, perforations) caused by the machining action which produces the particles, allows the particulate metallic precursor to be effectively comminuted by the particle striking action during dry milling.

[14] A further advantage of conducting 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 which may degrade the metallic composition, particularly contamination-susceptible compositions such as titanium alloys.

[15] In contrast the inventors have found that high shear wet milling techniques, where particle modification is primarily caused by shearing in a narrow gap between rotor and stator, are unsatisfactory for processing large (> 500 micron) flake-shaped particles of high yield strength and/or contamination-sensitive metallic compositions, such as titanium alloy swarf. The wet milling equipment was susceptible to wear and failure due to jamming of the particulate material in the gap between rotor and stator, the equipment could not satisfactorily achieve both comminution and spheroidization, and the process was liable to contaminate sensitive metallic compositions due to contact with the liquid medium.

[16] According to a first aspect, the invention thus provides a method of producing spheroidal metallic particles. The method comprises providing a particulate metallic precursor. The particulate metallic precursor may comprise flake-shaped particles. The flake-shaped particles may have a maximum dimension of greater than 250 micron. The method comprises comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing a comminuted material. The comminuted material may comprise non-spheroidal comminuted particles. The method comprises classifying the comminuted material to separate a fraction of the comminuted particles from larger and/or smaller particles. The classifying may be conducted during and/or after the comminuting. The method comprises spheroidizing the separated comminuted particles by dry milling in the same impact chamber or another impact chamber, thereby producing spheroidal metallic particles. Each impact chamber may comprise a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling.

[17] In a first set of embodiments, a method of producing spheroidal metallic particles comprises: providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron; comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non-spheroidal comminuted particles; classifying the comminuted material to separate a fraction of the non-spheroidal comminuted particles from larger and/or smaller particles, wherein the classifying is conducted during and/or after the comminuting; and spheroidizing the separated non-spheroidal comminuted particles by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles, wherein each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling.

[18] In some embodiments, the particulate metallic precursor comprises flakeshaped particles with a maximum dimension of greater than 500 micron, or greater than 1 mm, or greater than 2 mm, such as greater than 3 mm, for example greater than 4 mm.

[19] In some embodiments, the particulate metallic precursor is a swarf. [20] In some embodiments, classifying the comminuted material comprises separating the fraction of the non-spheroidal comminuted particles from larger particles, wherein the separated non-spheroidal comminuted particles have a maximum dimension, D1 , of less than 250 micron, or less than 200 micron, or less than 150 micron, such as of less than 100 micron, for example less than 75 micron. The method may further comprise comminuting the larger particles, after separation, by dry milling in the impact chamber or another impact chamber, thereby producing further non- spheroidal comminuted particles having a maximum dimension of less than D1 for spheroidizing.

[21 ] In some embodiments, classifying the comminuted material comprises separating the fraction of the non-spheroidal comminuted particles from smaller particles, wherein the smaller particles have a maximum dimension, D2, of less than 100 micron, or less than 75 micron, such as less than 50 micron, for example less than 30 micron. In some embodiments, the method further comprises spheroidizing the smaller particles, after separation, by dry milling.

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

[23] In some embodiments, the method further comprises combining at least a portion of the smaller particles with at least a portion of the spheroidal metallic particles, and subjecting the combination to an impact blending process to adhere the smaller particles to the spheroidal metallic particles, thereby producing a cold compactible metallic powder comprising non-spherical particles comprising one of the spheroidal metallic particles as a core and a plurality of the smaller metal particles as protrusions from the core.

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

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

[26] In some embodiments, the particulate metallic precursor comprises, or consists of, a metallic 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 aluminium and copper.

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

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

[29] In some embodiments, the particulate metallic precursor comprises, or consists of, titanium alloy, for example TiAI6V4.

[30] In some embodiments, each impact chamber comprises a cylindrical stator which defines a cylindrical outer wall of the impact chamber and a recirculation conduit which connects an entry port located in the cylindrical outer wall and an exit port directed to a central portion of the impact chamber, wherein particles continuously recirculate through the recirculation conduit during the dry milling.

[31] In some embodiments, the rotor of each impact chamber comprises a plurality of impact blades which define the impact faces and the outer edge of each impact blade at the periphery of the rotor is spaced apart 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.

[32] In some embodiments, the rotor of each impact chamber comprises a plurality of impact blades which define the impact faces, and the rotor is rotated during the comminuting such that the outer edge of each impact blade at the periphery of the rotor has a peripheral velocity of above 50 m/s, such as above 55 m/s, for example about 60 m/s or higher.

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

[34] In some embodiments, the separated non-spheroidal comminuted particles are spheroidized for a time between 1 minute and 30 minutes, such as between 5 minutes and 15 minutes.

[35] In some embodiments, dry milling during the comminuting and the spheroidizing is conducted in a dry inert gas atmosphere.

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

[37] In some embodiments, the separated non-spheroidal comminuted particles have an average sphericity of less than 0.6 before the spheroidizing.

[38] In some embodiments, the spheroidal metallic particles have an average sphericity of greater than 0.8.

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

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

[41 ] In a fourth aspect, the invention provides a method of producing a cold compactible metallic powder, the method comprising: providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron; comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non-spheroidal comminuted particles; classifying the comminuted material to separate a fraction of the non-spheroidal comminuted particles from smaller particles, wherein the classifying is conducted during and/or after the comminuting; spheroidizing the separated non- spheroidal comminuted particles by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles; combining at least a portion of the smaller particles with at least a portion of the spheroidal metallic particles; and subjecting the combination of particles to impact blending in the impact chamber or another impact chamber to adhere the smaller particles to the spheroidal metallic particles, thereby producing a cold compactible metallic powder comprising non- spherical particles comprising one of the spheroidal metallic particles as a core and a plurality of the smaller metal particles as protrusions from the core, wherein each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling or impact blending.

[42] It will be appreciated that methods according to the fourth aspect may embody one or more features relating to the particulate metallic precursor and the comminuting, classifying and spheroidizing steps as disclosed herein in the context of the first set of embodiments.

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

[44] In some embodiments, each impact chamber comprises a cylindrical stator which defines a cylindrical outer wall of the impact chamber and a recirculation conduit which connects an entry port located in the cylindrical outer wall and an exit port directed to a central portion of the impact chamber, wherein particles continuously recirculate through the recirculation conduit during the dry milling or impact blending.

[45] In some embodiments, the rotor of each impact chamber comprises a plurality of impact blades which define the impact faces and the outer edge of each impact blade at the periphery of the rotor is spaced apart 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. [46] In some embodiments, the rotor of each impact chamber comprises a plurality of impact blades which define the impact faces, and the rotor is rotated during the impact blending such that the outer edge of each impact blade at the periphery of the rotor has a peripheral velocity of below 60 m/s, such as below 55 m/s, for example below 50 m/s. In some such embodiments, the outer edge of each impact blade at the periphery of the rotor has a peripheral velocity of above 35 m/s, such as above 40 m/s, during the impact blending.

[47] In some embodiments, the combination of particles is impact blended for a time between 1 second and 10 minutes, such as between 5 seconds and 5 minutes, for example between 10 second and 1 minute.

[48] In some embodiments, the impact blending is conducted in a dry inert gas atmosphere.

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

[50] In a sixth aspect, the invention provides use of a cold compactible metallic powder comprising non-spherical particles produced by a method according to any embodiment of the fourth aspect to produce a porous metallic article by cold compaction.

[51 ] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[52] As used herein, the terms “first”, “second”, “third” etc in relation to various features of the disclosed devices, methods, systems etc are arbitrarily assigned and are merely intended to differentiate between two or more such features that the device, methods, systems etc may incorporate in various embodiments. The terms do not of themselves indicate any particular orientation or sequence. Moreover, it is to be understood that the presence of a “first” feature does not imply that a “second” feature is present, the presence of a “second” feature does not imply that a “first” feature is present, etc.

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

Brief Description of Drawings

[54] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

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

[56] Figure 2 is an isometric drawing of the impact chamber of rotational impact blending I dry milling apparatus (Nara Hybridization System, NHS-0), as used in the Examples.

[57] Figure 3 depicts the impact chamber and rotor of a rotational impact blending I dry milling apparatus for use in methods according to some embodiments of the invention.

[58] Figure 4 is a block flow diagram which schematically depicts a method of producing spheroidal metallic particles according to some embodiments of the invention.

[59] Figure 5 is a block flow diagram which schematically depicts a method of producing a cold compactible metallic powder according to some embodiments of the invention.

[60] Figure 6 schematically depicts an impact blending process for converting a combination of large spheroidized particles and small particles into non-spherical particles comprising one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.

[61] Figure 7 schematically depicts proposed interlocking modes that may occur between adjacent non-spherical particles, as produced according to embodiments of the invention, when compressed to form a porous metal article. [62] Figure 8 is a scanning electron microscopy (SEM) image of a Ti-6AI-4V alloy swarf, as used in the Examples, which shows the curved flake-shaped morphology of the particles.

[63] Figures 9 and 10 are other SEM images of the Ti-6AI-4V alloy swarf, which show the jagged edges and cracks in the swarf particles.

[64] Figure 11 show the particle size distribution of the Ti-6AI-4V alloy swarf, as determined by sieve analysis.

[65] Figure 12 is a SEM image of a classified 75-250 pm fraction of the comminuted Ti-6AI-4V alloy swarf, as produced in Example 1 .

[66] Figure 13 is a SEM image of a classified 500-1000 pm fraction of the comminuted Ti-6AI-4V alloy swarf, as produced in Example 1 .

[67] Figure 14 is a mass flow diagram of the Ti-6AI-4V alloy swarf as it is subjected to 5 rounds of milling and classification in Example 2.

[68] Figure 15 is a SEM image of a classified -75 pm fraction of the comminuted Ti-6AI-4V alloy swarf produced in Example 2.

[69] Figure 16 is a SEM image of spheroidized metallic particles produced by spheroidizing the classified -75 pm fraction of comminuted Ti-6AI-4V alloy swarf in Example 3.

[70] Figure 17 is a SEM image of a classified 75-150 pm fraction of the comminuted Ti-6AI-4V alloy swarf produced in Example 2.

[71] Figure 18 is a SEM image of spheroidized metallic particles produced by spheroidizing the classified 75-150 pm fraction of comminuted Ti-6AI-4V alloy swarf in Example 3.

[72] Figure 19 is a SEM image of a classified 150-250 pm fraction of the comminuted Ti-6AI-4V alloy swarf produced in Example 2. [73] Figure 20 is a SEM image of spheroidized metallic particles produced by spheroidizing the classified 150-250 pm fraction of comminuted Ti-6AI-4V alloy swarf in Example 3.

[74] Figure 21 is a SEM image of non-spheroidal metallic particles comprising a large metal particle as the core and a plurality of small metal particles as protrusions from the core, as produced in Example 4 by impact blending a combination of spheroidized Ti-6AI-4V alloy particles (produced by spheroidizing in Example 3) and small Ti-6AI-4V alloy particles at 8,000 rpm for 30 seconds.

[75] Figure 22 is a SEM image of non-spheroidal metallic particles comprising a large metal particle as the core and a plurality of small metal particles as protrusions from the core, as produced in Example 4 by impact blending a combination of spheroidized Ti-6AI-4V alloy particles (produced by spheroidizing in Example 3) and small Ti-6AI-4V alloy particles at 9,000 rpm for 30 seconds.

Detailed Description

Method of producing spheroidal metallic particles

[76] The present invention relates to a method of producing spheroidal metallic particles from a particulate metallic precursor. The particulate metallic precursor may comprise flake-shaped particles. The flake-shaped particles may have a maximum dimension of greater than 250 micron. The method comprises comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non-spheroidal comminuted particles. The comminuted material is classified, during and/or after the comminuting, to separate a fraction of the non-spheroidal comminuted particles from larger and/or smaller particles. The separated non-spheroidal comminuted particles are spheroidized by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles. Each impact chamber used in the method may comprise a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strikes particles in the impact chamber as the rotor rotates during dry milling.

Particulate metallic precursor [77] In some embodiments, the methods disclosed herein use a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron as feedstock. In some embodiments, the particulate metallic precursor comprises flake-shaped particles with a maximum dimension of greater than 500 micron, or greater than 1 mm, or greater than 2 mm, or greater than 3 mm, for example greater than 4 mm.

[78] As used herein, the maximum dimension of a particle, alternatively referred to as the particle size, means the maximum distance across the particle from surface to surface. The maximum dimension of a sphere is its diameter, but for elongated particles the maximum dimension is the distance across the particle in the direction of elongation. One approach to determine the maximum dimension of particles is via 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 metallic precursor will not pass through a 250 micron sieve. Alternatively, the maximum dimension of particles may be determined by scanning electron microscopy or other known methods in materials science.

[79] As used herein, a flake-shaped particle refers to a particle having a sheet- or wafer-like morphology where the particle thickness is substantially smaller than the length and width of the flake-shaped particle across its primary surfaces. In some embodiments, the flake-shaped particles have a thickness of less than 100 micron, such as less than about 80 micron, for example in the range of 30 to 100 micron. The flake-shaped particles need not be planar, and indeed flake-shaped particles such as swarf particles may be significantly distorted, for example curved or even curled up, due to the forces applied during machining.

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

[81 ] Average sphericity is a measure of the degree to which the particles in a powder approach the shape of a sphere. Sphericity is defined, with respect to a crosssection through a particle, as the ratio of the radius of the inscribed circle to the radius of the circumscribed circle (where the inscribed circle is the largest circle inside the particle cross-section, the circumscribed circle is the smallest circle outside the particle cross-section, and both circles are centred on the particles’ centre of mass). The sphericity of a perfectly spherical object is 1. The sphericity of particles can be determined from image analysis, typically using image analysis software such as “Image J”, of the imaged cross-sections of particles obtained with scanning electron microscopy (SEM). The average sphericity of particles in a powder is determined by calculating the individual sphericities of a multiple representative particles and taking an average.

[82] It is not necessarily required that all of the particles in the precursor are flakeshaped or have a maximum dimension of greater than 250 micron. The metal particles of the particulate metallic precursor are typically present in a range of particle sizes. The particle size distribution (PSD) of particulate compositions may be characterised by d10, d50 and d90 particle sizes, defined such that 10 volume % of the composition is present in particles having a size (maximum dimension) less than the d10 particle size, 50 volume % of the composition is present in particles having a size (maximum dimension) less than the d50 particle size and 90 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 (ASTM B822-20) for particles below about 1000 micron.

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

[84] The methods disclosed herein are useful for upgrading particulates of a wide range of metallic compositions, and particularly non-ferrous metals. In some embodiments, therefore, the particulate metallic precursor comprises, or consists of, a non-ferrous metallic composition.

[85] The methods disclosed herein are particularly useful for upgrading particulates of high yield strength metallic compositions. In some embodiments, the particulate metallic precursor comprises, or consists of, a metallic composition having a yield stress, or a 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. As used herein, yield stress is a bulk material property of a metallic composition, and can be measured by ASTM E8/E8M-13a. As reported in the ASM Materials Property Handbook Titanium Alloys, mill annealed Ti-6AI-4V has a 0.2% yield strength of more than 750 MPa.

[86] Without wishing to be limited by any theory, the ductility of the metallic composition of the particulate metallic precursor is also believed to be a relevant consideration. A minimum level of ductility may be required to avoid shattering of the particles under dry milling conditions to excessively small particulates, and to allow plastic deformation of the comminuted particles during the spheroidization step. On the other hand, excessively ductile metallic compositions will tend to smear, agglomerate or coat the dry milling apparatus, so that the comminution and/or spheroidizing processes are not satisfactorily achieved.

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

[88] In some embodiments, the particulate metallic precursor comprises, or consists of, a metallic composition having an elongation at 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 in the range of 10% to 30%, or 10% to 20% (all based on mill annealed compositions and as measured according to ASTM E8/E8M-13). Metallic compositions with ductility below these ranges may be less preferred for the methods disclosed herein, as particulate metallic precursors comprising such metallic compositions may be excessively predisposed to comminution (shattering) and insufficiently predisposed to spheroidization. The inventors have found that CP titanium (grade 2) powder can be processed by dry milling, albeit with significant sticking to the dry milling apparatus, and thus consider that materials with substantially higher ductility than CP titanium may be less preferred for the methods disclosed herein than lower ductility materials.

[89] Comminution of particulate titanium materials, including CP titanium and alloys such as Ti-6AI-4V, is generally considered challenging due to the ductility (“gumminess”). Such materials are thus often comminuted by a hydrogenationdehydrogenation approach, with the low-ductility hydrogenated intermediate subjected to the comminution. Without limitation by theory, it is believed that the flake-shaped morphology of the particulate metallic precursor, augmented in the case of swarf by defects (cracks, jagged edges, perforations), allows the particulate metallic precursor to be effectively comminuted by the particle striking action during dry milling even when using comparatively ductile materials (e.g. above 5% or above 10% elongation at break) such as titanium materials.

[90] Non-limiting examples of metallic compositions with yield strength and ductility properties suitable for the methods disclosed herein may 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 comprising both aluminium and copper. As used herein, low ductility alloys have an elongation at break of less than 30%, preferably less than 20%. By contrast, very soft and ductile metallic compositions, such as commercially pure copper or aluminium, may be less suitable because they deform too easily and/or stick to the surfaces of the milling apparatus. Many metallic compositions including substantial amounts of metals such as copper, lead, zinc, tin and iron are also expected to be soft, and thus susceptible to undesirable deformation and/or to functionalisation by the small particles via surface embedment (rather than metallic bonding). Metallic compositions comprising such metal elements will generally be suitable only if the yield stress is high, such as at least 400 MPa. For example, certain ferritic and martensitic steels have appropriate yield strength, whereas pure iron and austenitic steels are expected to be too soft. In some embodiments, the particulate metallic precursor comprises, or consists of, a metallic composition which includes copper, lead, zinc, tin and iron only as minor (<20 wt.%, preferably < 10 wt.%) alloying elements in alloys of other metals or which is substantially free of these metals.

[91 ] The metallic composition of the particulate metallic precursor may also have fracture properties suitable to avoid or minimise shattering of the particles during the spheroidizing step instead of the desired deformation believed to occur during spheroidizing. Thus, in some embodiments, the particulate metallic precursor comprises, or consists of, a metallic composition having a fracture toughness (Kic) in the range of 10 to 150 MPa.m^1/2, such as in the range of 40 to 150 MPa.m^1/2. Kic is a bulk material property of a metallic composition, and can be measured by ASTM E1820-18.

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

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

[94] As used herein, an “alloy of metal X” refers to an alloy in which metal X is the most abundant metal element by atomic %. Thus, for example, an alloy of titanium (alternatively a titanium alloy) refers to a metallic alloy in which titanium is the element present in the highest atomic %. In some embodiments, metal X is present in an amount of at least 50 atomic % in an alloy of metal X. An alloy is typically a fully reacted and homogenous mixture such that the solid solutions and any intermetallic phases that form are unique to the alloy, befitting the thermodynamic and kinetic circumstances related to their reaction, and different from the elemental metal mixtures. As used herein, metal alloys thus may include intermetallic compounds.

[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, with a non- metallic particulate phase dispersed in the matrix. Exemplary non-metal particulate phases include ceramics added as a strengthening phase.

[96] In some embodiments, the particulate metallic precursor is a swarf, and thus a product (typically a by-product) of a machining process which removes flake-shaped particles from a metallic substrate. Without wishing to be bound by any theory, the work hardening and defects such as cracks, jagged edges and perforations present in certain metallic swarf particles (also called machine chips), caused by the machining process, is believed to predispose swarf materials to particle manipulation by the methods disclosed herein. In particular, the flake-shaped morphology, surface defects and/or work-hardening inherent to swarf may facilitate effective comminution of a swarf precursor even when its composition is too ductile when in an annealed form for effective comminution. The methods disclosed herein thus take advantage of properties of swarf, that otherwise render it a low-value product, to upgrade it to a useful form. In some embodiments, the particulate metallic precursor is a titanium alloy swarf, such as a Ti64 swarf.

[97] The relevance of the inherent swarf properties to the outcome of the methods disclosed herein has been demonstrated by comparative experiments where a Ti-6AI-4V swarf was subjected to beta annealing prior to dry milling. In contrast to the results with unmodified swarf, the annealed swarf tended to agglomerate when subjected to dry milling due to the gumminess (ductility) of the fully annealed metallic composition.

[98] In some embodiments, the swarf comprises curved flake-shaped chips such as C-shaped chips and/or 6-shaped chips in addition to any planar chips. Spiralshaped or string-like swarf particles are considered less suitable, and in some embodiments are thus excluded from the swarf. Most machining operations have chip breakers to avoid spiral- and string-type swarf morphologies, but if such particles remain present in the swarf they may need to be removed prior to processing to avoid jamming in the impact chamber.

[99] The swarf may be a product of a machining process conducted on a metallic substrate formed of a metallic composition having (i) a yield stress, or a 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 at 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 in the range of 10% to 30%, or 10% to 20% (as measured according to ASTM E8/E8M-13).

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

[101] There is no need to heat-treat the particulate metallic precursor, and this may in fact be undesirable, as discussed above, for some swarf precursors. In some embodiments, therefore, the swarf is not annealed prior to processing according to the methods disclosed herein. There is also no need to hydrogenate the particulate metallic precursor, and in some embodiments the particulate metallic precursor is not a hydrogenated material. The particulate metallic precursor may thus comprise a metallic composition containing less than 0.1 wt.% hydrogen, preferably less than 0.2 wt.% hydrogen, most preferably substantially no hydrogen, when comminuted. Accordingly, there is no need to dehydrogenate the comminuted material, and in some embodiments neither the comminuted material nor the separated non-spheroidal comminuted particles are subjected to dehydrogenation before or during their spheroidizing.

Dry milling in an impact chamber

[102] In some embodiments of the disclosed methods, the particulate metallic precursor is modified by dry milling to comminute and then spheroidize the precursor particles. As used herein, dry milling refers to a milling process conducted in a gas medium, and may thus be distinguished from wet milling processes where the material is dispersed in a liquid medium during milling.

[103] Both steps, or phases, of dry milling are conducted in an impact chamber. The comminution and spheroidizing processes may suitably be conducted in the same impact chamber or different impact chambers. Each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling.

[104] In some embodiments, each impact chamber comprises a cylindrical stator which defines a cylindrical outer wall of the impact chamber and a recirculation conduit which connects an entry port located in the cylindrical outer wall and an exit port directed to a central portion of the impact chamber. During the dry milling, the particles continuously recirculate through the recirculation conduit for reintroduction into the centre of the impact chamber, thus ensuring that the particles are repeatedly struck by the rotating impact faces. The rotor may comprise a plurality of impact elements, such as impact blades, which define the impact faces for striking the particles. The outer edge of each impact blade at the periphery of the rotor may be spaced apart from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm, such as in the range of 2 mm to 4 mm.

[105] As seen in Figures 1 , 2 and 3, the dry milling processes may be performed in an apparatus 100 that includes an impact chamber 108, the cylindrical outer wall of which is defined by a stator 1 10, and a rotor 112 which rotates in the impact chamber. Impact chamber 108 is enclosed by removable front panel 1 13 and rear wall 1 17. The generally disc-shaped rotor includes a plurality of radially oriented impact blades 1 14 having an impact face 1 16 and an outer edge 118 at the periphery of the rotor and spaced apart from the cylindrical stator by a small gap 120. The rotor also includes radial ribs 115 on the reverse side which are spaced apart from the rear wall 1 17 of the impact chamber by a narrow gap 119. The apparatus includes a recirculation conduit 122 extending between an entry port 124 located in the cylindrical outer wall of the impact chamber and an exit port 126 located in front panel 113 which is directed to the centre of the impact chamber. Particulates are added to the impact chamber via inlet 128 and discharged through powder outlet port 133, also located in front panel 1 13, via discharge valve 137 to powder outlet 135. Cooling water is circulated through the stator via coolant ports 130, 132.

[106] In use, the metallic material to be dry milled is fed from a sealed vessel into the impact chamber via a high-pressure stream of inert gas (e.g. argon). The rotor is rotated such that the impact blades sweep through the impact blending chamber. The metal particles of the precursor material are thus repeatedly struck at high velocity by the impact face of the impact blades. The spinning rotor also creates a vortex that accelerates the particles by centripetal forces to the peripheral gap and causes a strong recirculating gas flow through the recirculation conduit by a fan effect, so that the particles continuously circulate through the recirculation conduit during the dry milling. In addition to striking of the particles by the impact blades, the accelerated particles collide with each other, strike the stator and are subjected to shearing in the gap between the impact blade outer edges and the stator. Particles that migrate to the back of the rotor are forced back to the periphery by the rotating ribs 115.

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

Comminuting the particulate metallic precursor

[108] In embodiments of the methods of this disclosure the particulate metallic precursor, which initially comprises flake-shaped particles with a maximum dimension of greater than 250 micron, is comminuted by dry milling in an impact chamber to produce comminuted material comprising non-spheroidal comminuted particles. The average particle size of the comminuted material is reduced compared to the precursor, although it will be appreciated that the actual resultant particle size distribution will depend on the particle size, morphology and composition of the particulate metallic precursor as well as the process conditions.

[109] A few apparatus and process parameters may be particularly significant to the operability and/or efficiency of the dry milling process for comminuting and to the resultant particle morphology. These include the blade design, and in particular the gap (gap 120) between the rotating blades and the stator. The inventors have found that a gap distance of 3.5 mm was suitable for modifying a Ti-6AI-4V swarf with a d50 particle size of about 1 .2 mm which included flake-shaped particles with a maximum dimension of up to 5 mm. In contrast, attempts to comminute this swarf by wet milling in a high shear mill with a rotor-stator gap distance of less than 1 mm were found unsuccessful. Thus, in some embodiments, the outer edge of each impact blade at the periphery of the rotor is spaced apart from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm, such as in the range of 2 mm to 4 mm.

[1 10] Another significant parameter is the rotation speed of the rotor. Without wishing to be limited by any theory, it is proposed that the rotational velocity of the impact blades near the rotor periphery must be sufficient that the striking impact on particulate metallic precursor shears and breaks the flake-shaped particles. The inventors have found that peripheral rotor velocities of 61.8 m/s (corresponding to 10,000 rpm in the apparatus used) were suitable for comminuting the Ti-6AI-4V swarf, but it will be appreciated that lower or higher speeds may be preferred for different precursor compositions and particle sizes. Higher speeds will advantageously reduce the processing time, but excessive speeds should be avoided as this may reduce powder yield and/or unacceptably raise the temperature of the particulate material. In some embodiments, the rotor is rotated such that the outer edge of each impact blade has a peripheral velocity of above 50 m/s during the comminuting.

[1 1 1] A further significant parameter is the dry milling time during the comminuting step or phase, which should be sufficient to comminute the particulate metallic precursor into smaller particles but not so long that the modified particles substantially spheroidize or recombine prior to classification. The opportunity to control the particle size distribution of the process may be lost if dry milling to comminute and spheroidize the precursor is conducted in a single undifferentiated process step. Particularly in scenarios where small particles, for example less than 100 micron or less than 75 micron, are a desired primary product or co-product of the overall process, short milling times may be desirable to avoid reattachment of fine comminuted particles generated early in the comminution process to larger particles.

[1 12] In some embodiments, the particulate metallic precursor is comminuted for a time between 1 second and 1 minute, such as between 10 seconds and 40 seconds. The inventors have found that the Ti-6AI-4V swarf, of which about 85 wt.% was initially present in particles having a maximum dimension greater than 1000 micron, can be effectively comminuted to non-spheroidal particles which are entirely or mostly (>95 wt.%) smaller than 1000 micron within such periods. Notably however, the fraction of fine particles (< 75 micron, none present in the initial swarf) remaining in the comminuted product decreased as milling time increased from 30 to 60, 120 and 180 seconds. To maximise 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.

[1 13] As used herein, dry milling times refers to the milling time once the rotor reaches its set speed.

[1 14] It will be appreciated that the preferred apparatus configuration, milling speed and milling time for comminution in any particular implementation will depend on the composition and initial morphology of the particulate metallic precursor.

[1 15] The particulate metallic precursor may be comminuted in a dry inert gas atmosphere in the impact chamber. This advantageously limits oxygen incorporation or other contamination of the metal composition in the process, despite the potential vulnerability of the freshly comminuted particles to contamination due to exposure of pristine metal at the newly formed surfaces.

[1 16] The comminuted particles produced in the initial dry milling process are typically smoothed and may have a more regular contour than the flake-shaped particulate metallic precursor. Nevertheless, they remain predominantly non- spheroidal when subjected to the classification step. In some embodiments, the non- spheroidal comminuted particles have an average sphericity of less than 0.6, or less than 0.5. For example, in the case of a Ti-6AI-4V swarf particles having cracks, jagged edges, perforations and other defects as the precursor, a short initial dry milling step was found not only to reduce the particle size but also to smooth and fold the particle edges and surfaces. Nevertheless, the particles retained a flat or “blocky” morphology recognisably corresponding to the initial flake-shaped morphology of the precursor.

Classifying the comminuted material [1 17] In some embodiments of the methods of this disclosure the comminuted material produced in the initial dry milling process is classified during and/or after the comminuting to separate a fraction of the non-spheroidal comminuted particles from larger and/or smaller particles.

[1 18] In some embodiments, the comminuted material is classified after completing the comminuting step. Thus, the comminuted material may be removed from the impact chamber and subjected to a subsequent classification process before spheroidizing the resultant separated fraction. However, it is contemplated that the classification may alternatively be conducted during the comminution process. For example, particles continuously (or intermittently) withdrawn from the impact chamber during the dry milling may be classified online, e.g. with a hydrocyclone classifier, to remove fine particles, with the larger particles being continuously (or intermittently) returned to the impact chamber for further comminution and spheroidizing. In this way, the particulate metallic precursor may be converted to spheroidal metallic particles in a single extended dry milling process where particle classification is conducted online during the initial comminution phase to control the size distribution of the particles in the later spheroidization phase.

[1 19] The classification process need not separate the larger and/or smaller particles from the fraction to be spheroidized with quantitative selectivity. In other words, it is permissible that the fraction to be spheroidized retains some particles having a size outside of the target range and/or loses some particles sized within the target range. Nevertheless, the classification should generally be sufficiently selective to significantly alter the particle size distribution of the particles subjected to spheroidization and/or to produce a significant co-product fraction of small or large particles.

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

[121] In some embodiments, the classification removes large particles from a target fraction of smaller particles to be subjected to spheroidization. This may advantageously increase the mass fraction of spheroidized small particles produced in the overall process, since the small particles would otherwise reattach to the large particles during spheroidization.

[122] Thus, in some embodiments, classifying the comminuted material comprises separating a fraction of non-spheroidal comminuted particles having a maximum dimension, D1 , of less than 250 micron, or less than 200 micron, or less than 150 micron, or less than 100 micron, or less than 75 micron from larger particles (i.e. particles having a maximum dimension greater than D1 ). In such embodiments, non- spheroidal comminuted particles having only, or predominantly, a particle size of less than D1 are then spheroidized.

[123] Optionally, the larger particles excluded by the classification may be subjected to further comminution by dry milling, thereby producing further non- spheroidal comminuted particles having a maximum dimension of less than D1 for spheroidizing. For example, the larger particles may be recycled for comminution together with previously unprocessed particulate metallic precursor. Alternatively, the larger particles excluded by the classification (having only, or predominantly, a particle size of greater than D1 ) may be subjected to a separate spheroidization process. Advantageously, the larger particles can thus be spheroidized without absorbing a fines fraction of smaller particles.

[124] In some embodiments, the classification removes small particles from a target fraction of larger particles to be subjected to spheroidization. The removal of a fines fraction produced in comminution may prevent undesirable reattachment of these small particles to the target particles during spheroidization.

[125] Thus, in some embodiments, classifying the comminuted material comprises separating a fraction of non-spheroidal comminuted particles from smaller particles having a maximum dimension, D2, of less than 100 micron, or less than 75 micron, or less than 50 micron, or less than 30 micron. In such embodiments, non-spheroidal comminuted particles having only, or predominantly, a particle size of greater than D2 are then spheroidized.

[126] Optionally, the smaller particles excluded by the classification (having only, or predominantly, a particle size of less than D2) may be subjected to a separate spheroidization process. Advantageously, these smaller particles can thus be spheroidized without being absorbed by larger particles. Alternatively, the smaller particles excluded by the classification may be used in a subsequent process step to produce cold compactible particles, as will be explained in greater detail hereafter.

[127] In some embodiments, the classification separates the target fraction of particles to be spheroidized from both larger and smaller particles. Advantageously, the particles subjected to spheroidization will thus fall within a well-defined and potentially narrow size range. Thus, in some embodiments, classifying the comminuted material comprises separating a fraction of non-spheroidal comminuted particles having a maximum dimension of between D2 and D1 from larger and smaller particles, where D1 and D2 are as defined above and D1 > D2.

[128] In some embodiments, the particulate metallic precursor is subjected to repeat cycles of comminution and classification. Thus, the comminuted material produced in each cycle is classified into two or more size fractions, with one or more fractions being directed to spheroidization, one or more size fractions (typically a larger fraction, for example composed of particles with a maximum dimension greater than D1 as defined herein) being subjected to repeat comminution (for example by recycling for comminution with previously unprocessed particulate metallic precursor), and optionally one or more size fractions (typically a smaller fraction, for example composed of particles with a maximum dimension smaller than D2 as defined herein) being excluded.

[129] In some embodiments, the non-spheroidal comminuted particles which 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, such as less than 0.4. For the case of comminuted Ti-6AI-4V swarf particles, it has been found that the sphericity of the comminuted particles depends on the particle size, with particles in the -75 micron and 150-250 micron ranges both having average sphericities of about 0.4 after classification.

Spheroidizing the comminuted particles

[130] In the methods of this disclosure the non-spheroidal comminuted particles of the fraction separated in the classification step are spheroidized by dry milling in an impact chamber to produce spheroidal metallic particles.

[131] It will be appreciated that the spheroidal metallic particles will typically not be perfectly spherical. Nevertheless, the spheroidizing has the effect of significantly modifying the particle morphology to increase the sphericity of the particles. In some embodiments, the spheroidal metallic particles have an average sphericity of 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 comminuted Ti-6AI-4V swarf classified into -75 micron, 75-150 micron and 150-250 micron ranges was found to produced spheroidized Ti-6AI-4V particles having average sphericities of about 0.70, 0.88 and 0.89 respectively.

[132] The particle size distribution of the comminuted material may be affected by the spheroidization process. Apart from the inherent effect on observed particle size due to particle reshaping, it is likely that some larger particles may be comminuted and/or some smaller particles absorbed by larger particles during the dry milling. Nevertheless, these processes may advantageously be minimised or at least controlled by suitable selection of the size fraction for spheroidization in the classification step. Thus, for example, only relatively minor changes to the particle size distribution were observed when spheroidizing -75 micron, 75-150 micron and 150-250 micron range fractions of comminuted Ti-6AI-4V swarf over extended periods.

[133] The spheroidization process typically requires an extended period of dry milling in comparison to the comminution process. In some embodiments, therefore, the separated non-spheroidal comminuted particles are subjected to dry milling for a longer time period than the particulate metallic precursor during the comminution step. In some embodiments, the separated non-spheroidal comminuted particles are spheroidized for a time between 1 minute and 30 minutes, such as between 5 minutes and 15 minutes. For example, the -75 micron, 75-150 micron and 150-250 micron range fractions of comminuted Ti-6AI-4V swarf were each spheroidized for 14 minutes to achieve the sphericities disclosed above.

[134] The non-spheroidal comminuted particles may have a substantially higher bulk density, or tap density, compared to the flake-shaped particulate metallic precursor. This may advantageously allow the spheroidization process to be conducted with a higher loading concentration of metallic particulate in the impact chamber than was the case for the comminution process. This makes more efficient use of the milling apparatus, particularly since spheroidization is typically a much longer process than comminution.

[135] The rotational velocity of the impact blades should be sufficient to adequately spheroidize the particles. The inventors have found that peripheral rotor velocities of 61.8 m/s (corresponding to 10,000 rpm in the apparatus used) were suitable for spheroidizing variously sized classified fractions of the comminuted Ti-6AI-4V swarf, but it will be appreciated that lower or higher speeds may be preferred for different precursor compositions and particle sizes. In some embodiments, the rotor is rotated such that the outer edge of each impact blade has a peripheral velocity of above 50 m/s during the spheroidizing.

[136] The spheroidizing process may take place in the same milling apparatus as the comminution process. Indeed, it was found that the same dry milling apparatus was suitable for comminuting Ti-6AI-4V swarf and for spheroidizing variously sized classified fractions of the comminuted swarf, provided that a longer time is used for the spheroidizing. However, in a commercial process it may be preferable for the spheroidizing to take place in a different milling apparatus which is sized and configured to account for the different process intent and for the size range of the particle fraction to be spheroidized.

[137] In some embodiments of the apparatus used for spheroidization, the outer edge of each impact blade at the periphery of the rotor is spaced apart from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm, such as in the range of 2 mm to 4 mm.

[138] It will be appreciated that the preferred apparatus configuration, milling speed and milling time for spheroidizing in any particular implementation will depend on factors such as the composition, morphology and initial size range of the fraction of non-spheroidal comminuted particles to be spheroidized. In particular, it is expected that smaller particles may need more intense dry milling conditions than larger particles to achieve a desired average sphericity.

[139] The comminuted particles may be spheroidized in a dry inert gas atmosphere in the impact chamber to avoid oxygen incorporation or other contamination.

Embodiments

[140] Various embodiments of the invention will now be described with reference to Figure 4. Method 200 of producing spheroidal metallic particles comprises step 202 of providing a particulate metallic precursor 204 comprising flake-shaped particles with a maximum dimension of greater than 250 micron . Optionally, precursor 204 comprises flake-shaped particles with a maximum dimension of greater than 500 micron, or greater than 1 mm, or greater than 2 mm, or greater than 3 mm, or greater than 4 mm. For example, precursor 204 may be a swarf such as a titanium alloy swarf.

[141] In comminution step 206, precursor 204 is comminuted by dry milling to produce comminuted material 208 which comprises non-spheroidal comminuted particles. The dry milling is conducted in an impact chamber comprising a rotor. The rotor comprises a plurality of impact faces which strike and thus comminute particles in the impact chamber as the rotor rotates during the dry milling.

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

[143] In spheroidization step 218, all or part of the separated fraction 212 of non- spheroidal comminuted particles is spheroidized by dry milling in an impact chamber to produce spheroidal metallic particles 220. The dry milling is conducted in an impact chamber comprising a rotor. The rotor comprises a plurality of impact faces which strike and thus spheroidize particles in the impact chamber as the rotor rotates during the dry milling. The exclusion of the larger and/or smaller particles during the spheroidizing may advantageously avoid reattachment of smaller particles to larger particles during spheroidization and thus allow preferred particle size distributions and properties of spheroidized metallic particles 220 to be achieved.

[144] In some embodiments of method 200, comminuted material 208 is classified to separate fraction 212 of non-spheroidal comminuted particles from fraction 214 of larger particles. The particles of fraction 212 may have a maximum dimension, D1 , of less than 250 micron, or less than 200 micron, or less than 150 micron. In some embodiments, the particles of fraction 212 may have a maximum dimension, D1 , of less than 100 micron, or less than 75 micron. Thus, the particles of fraction 212 subjected to spheroidization in spheroidization step 218 have only, or predominantly, a maximum dimension of less than D1 .

[145] At least a portion of the larger non-spheroidal comminuted particles of fraction 214 may optionally be recycled via recycle 222 to comminution step 206 for comminution together with (or separately from) particulate metallic precursor 204. The large particles are thus further comminuted to produce further non-spheroidal comminuted particles having a maximum dimension of less than D1 for classification into fraction 212 and spheroidizing in step 218. Alternatively (or in addition) at least a portion of the larger non-spheroidal comminuted particles of fraction 214 may spheroidized in spheroidization step 224 by dry milling in an impact chamber to produce spheroidal metallic particles 226. The spheroidization of this fraction separately from fraction 212 advantageously avoids reattachment of smaller particles.

[146] In some embodiments of method 200, comminuted material 208 is classified to separate fraction 212 of non-spheroidal comminuted particles from fraction 216 of smaller particles. The smaller particles of fraction 216 may have a maximum dimension, D2, of less than 100 micron, or less than 75 micron, or less than 50 micron, or less than 30 micron. Thus, the particles of fraction 212 subjected to spheroidization in spheroidization step 218 have only, or predominantly, a maximum dimension of greater than D2.

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

[148] In some embodiments of method 200, comminuted material 208 is classified to separate fraction 212 of the non-spheroidal comminuted particles from fraction 214 of larger particles and from fraction 216 of smaller particles. Thus, the particles of fraction 212 subjected to spheroidization in spheroidization step 218 have only, or predominantly, a maximum dimension in the range between D2 and D1 , as defined above.

Method of producing a cold compactible metallic powder

[149] The present invention further relates to a method of producing a cold compactible metallic powder from a particulate metallic precursor. The particulate metallic precursor may comprise flake-shaped particles. The flake-shaped particles may have a maximum dimension of greater than 250 micron.

[150] The method comprises comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non- spheroidal comminuted particles. The comminuted material is classified, during and/or after the comminuting, to separate a fraction of the non-spheroidal comminuted particles from smaller particles. The separated non-spheroidal comminuted particles are spheroidized by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles.

[151] At least a portion of the separated smaller particles is then combined with at least a portion of the spheroidal metallic particles, and the combination of particles is subjected to impact blending in the impact chamber or another impact chamber to adhere the smaller particles to the spheroidal metallic particles. This results in the formation of a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the spheroidal metallic particles as a core and a plurality of the smaller metal particles as protrusions from the core. [152] Each impact chamber used in the method, i.e. for comminuting, spheroidizing and impact blending, comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during the dry milling and impact blending.

[153] The dry milling apparatus as well as the comminution, classification and spheroidization steps are generally as disclosed herein the context of the methods of producing spheroidal metallic particles.

[154] In some embodiments, the smaller particles separated in the classifying process have a maximum dimension, D2, of less than 100 micron, or less than 75 micron, or less than 50 micron, or less than 30 micron. In such embodiments, the non- spheroidal comminuted particles subjected to spheroidizing have only, or predominantly, a particle size of greater than D2. All of these non-spheroidal comminuted particles may then be spheroidized, or only a narrower size fraction thereof, for example particles in the range of 150 to 250 micron.

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

[156] In some embodiments, the small particles of the first fraction have a maximum dimension, D2, or less than 75 micron, preferably less than 50 micron, and the large particles of the second fraction have a maximum dimension, D3, of at least 150 micron, such as in the range of 150 to 250 micron.

[157] Optionally, the smaller particles separated in the classifying process may be spheroidized. However, it is not considered necessary that these particles should be substantially spheroidized to produce a cold compactible metal powder, and the limited amount of particle smoothing occurring in the comminution step and/or impact blending steps is likely sufficient to obtain a satisfactory result.

Particle combination and impact blending [158] Following spheroidization of the target fraction of non-spheroidal comminuted particles, some or all of the resultant spheroidized metallic particles are combined with some or all of the smaller particles. For example, the first and second fractions, as disclosed above, may be combined. The combination of particles is then subjected to impact blending in an impact chamber to adhere the smaller particles to the spheroidal metallic particles, thereby producing a cold compactible metallic powder comprising non-spherical particles comprising one of the spheroidal metallic particles as a core and a plurality of the smaller metal particles as protrusions from the core.

[159] As used herein, an impact blending process refers to a dry powder blending process which causes high velocity impacts between the particles with sufficient intensity to adhere, and preferably metallurgically bond, the smaller metal particles to the large particles. The impact blending process is conducted in an impact chamber as disclosed herein. For example, the impact blending may be conducted in an apparatus as disclosed herein with reference to Figures 1 -3.

[160] A few apparatus and process parameters of the impact blending process may be particularly significant to the resultant powder morphology. These include the blade design, and in particular the gap (gap 120) between the rotating blades and the stator. The inventors have found that a gap distance of 3.5 mm was suitable for adhering small Ti-6AI-4V particles (5 to 25 micron) to spheroidized Ti-6AI-4V particles with a particle size of between 150 and 250 micron, and it is expected that a similar gap distance will be suited for impact blending a wide range of large and small particle sizes. Thus, in some embodiments of the apparatus for impact blending, the outer edge of each impact blade is spaced apart from the cylindrical outer wall by a gap distance in the range of 1 mm to 5 mm, such as in the range of 2 mm to 4 mm.

[161] Another significant parameter is the rotation speed of the rotor. Without wishing to be limited by any theory, it is proposed that the rotational velocity of the impact blades near the rotor periphery must be sufficient to generate the high intensity collisions between the powder particles needed for robust particle adhesion. The inventors have found that peripheral rotor velocities in the range of about 37.1 m/s to 55.6 m/s (corresponding to 6,000 rpm to 9,000 rpm in the impact blender used by the present inventors) were suitable for modifying spheroidized Ti-6AI-4V large particles with a particle size in the range of 150-250 pm, but it will be appreciated that lower or higher speeds may be preferred for different powder compositions and particle sizes. In some embodiments, the rotor is rotated such that the outer edge of each impact blade has a peripheral velocity of above 35 m/s, such as above 40 m/s, during the impact blending.

[162] However, the intensity of impact blending should not be so high that the desired morphology of the impact blended particles is lost due to smearing or filming of the adhered small particles over the surface of the larger particles. In some embodiments, therefore, the rotor is rotated during the impact blending such that the outer edge of each impact blade has a peripheral velocity of below 60 m/s, or below 55 m/s, such as below 50 m/s, during the impact blending.

[163] A further significant parameter is the impact blending time, which should be sufficient to adhere the small metal particles to the large core particles but not so long that the modified particles become spheroidized due to filming. In some embodiments, the combination of particles is impact blended for a time between 1 second and 10 minutes, such as between 5 seconds and 5 minutes, or between 10 second and 1 minute. It will be appreciated that the time required to create a desirable particle morphology may be inversely correlated with the intensity of the impact blending conditions.

[164] The combination of particles may be impact blended in a dry inert gas atmosphere. This advantageously limits oxygen contamination of the metal powders during the impact blending. Without limitation by theory, it is proposed that freshly exposed metal surfaces of the particles, as formed during impact blending under inert atmosphere, are not immediately sealed by an oxidic layer and thus remain capable of adhering and intermixing with the metallic surfaces of other particles.

[165] Impact blending of the combination of particles under appropriate conditions causes adhesion of the small metal particles to the large particles and thus produces non-spherical particles comprising one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core. This type of morphology is also referred to herein as a “core-corona” morphology. Impact blended powders comprising non-spherical core-corona particles have been found to have desirable cold compaction properties which cannot be attributed solely to the metallic composition of the modified powder and thus derives from the modified particle morphology in the powder.

[166] Figure 6 schematically depicts an impact blending process 400 which converts a combination 410 comprising large particles 412 (spheroidized metallic particles) and small particles 414 (separated comminuted particles) into non-spherical particles 416 having a core-corona morphology. Particles 416 comprise one of the large metal particles 412 as core 418 and a plurality of the small metal particles 414 as protrusions 420 from the core. Some of the protrusions (420a) comprise a single small particle 414 while other protrusions (420b) comprise a cluster of small particles 414.

[167] Without wishing to be limited by any theory, the inventors propose, on the basis of scanning electron microscopy (SEM) analysis of cross-sectioned core-corona particles, that the small metal particles are metallurgically bonded to the large particles along at least a portion of the inter-particle interfaces. This mode of chemical bonding is distinguished from mere mechanical embedding of the type produced when surfacemodifying soft non-metallic core particles with hard guest particles. The large and small metal particles are thus effectively integrated into a single non-spherical metallic particle as core and protrusions. The resultant mechanical integrity of the non-spherical particles is considered to be important for desirable cold compaction properties, since adhesion of the protrusions to the core must withstand the severe compressive forces when the particles are consolidated into a compact under pressure.

[168] The non-spherical core-corona particles typically comprise many protrusions distributed around the periphery of the core particle. Each protrusion may comprise a single small particle or a cluster of small particles. The small particles may be deformed by the bond-forming impact with the core particle, or by subsequent high velocity impacts of the non-spherical particle during the impact blending process. The extent of deformation may depend on the yield strength and ductility of the metallic composition, as well as the impact blending conditions and time. A degree of deformation and spheroidization can be tolerated, provided that the particles retain a core-corona morphology. However, if spheroidization continues to the extent that the small particles adhered to the core are beaten out into a substantially uniform shell around the core, the cold compaction properties of the powder may be adversely affected. [169] The morphology of particles may be quantified with a convexity number, defined as the ratio of the perimeter of a particle’s convex hull to the perimeter of the object itself (both measured with respect to a cross-section of the particles). In some embodiments, the convexity of the non-spherical (core-corona) particles is below 0.8, such as in the range of 0.4 to 0.8.

[170] It is not required that all particles in the impact blended metallic powder exhibit the core-corona morphology; satisfactory cold compaction properties may be obtained when only a fraction of the particles have this morphology. In some embodiments, the cold compactible metallic powder thus comprises at least 20 wt.%, such as at least 50 wt.%, or at least 60 wt.%, of the non-spherical particles comprising one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.

Embodiments

[171] Various embodiments of the invention will now be described with reference to Figure 5. Method 300 of producing a cold compatible metal powder comprises step 302 of providing a particulate metallic precursor 304 comprising flake-shaped particles with a maximum dimension of greater than 250 micron. Optionally, precursor 304 comprises flake-shaped particles with a maximum dimension of greater than 500 micron, 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 a swarf such as a titanium alloy swarf.

[172] In comminution step 306, precursor 304 is comminuted by dry milling to produce comminuted material 308 which comprises non-spheroidal comminuted particles. The dry milling is conducted in an impact chamber comprising a rotor. The rotor comprises a plurality of impact faces which strike and thus comminute particles in the impact chamber as the rotor rotates during the dry milling.

[173] In classification step 310, the comminuted material 308 is classified to separate fraction 312 of the non-spheroidal comminuted particles from fraction 316 of smaller particles. For example, comminuted material 308 may be classified by sieving into fractions 312 and 316. It is not required that fractions 312 and 316 are adjacent fractions; in other words a further fraction (not shown) of particles with sizes intermediate fractions 312 and 316 may be separated in the classification. [174] For example, comminuted material 308 is classified into (i) fraction 316 of smaller particles having a maximum dimension, D2, of less than 75 micron, or less than 50 micron, or less than 30 micron and (ii) fraction 312 of large particles having a maximum dimension, D3, of at least 100 micron, or at least 150 micron, such as in the range of 150 to 250 micron.

[175] In spheroidization step 318, all or part of the separated fraction 312 of non- spheroidal comminuted particles is spheroidized by dry milling in an impact chamber to produce spheroidal metallic particles 320. The dry milling is conducted in an impact chamber comprising a rotor. The rotor comprises a plurality of impact faces which strike and thus spheroidize particles in the impact chamber as the rotor rotates during the dry milling.

[176] In combination step 332, some or all of fraction 316 of smaller particles is combined with some or all of spheroidal metallic particles 320 to produce combination 334.

[177] In impact blending step 336, the combination of particles 334 is then subjected to impact blending in an impact chamber to adhere the smaller particles to the spheroidal metallic particles, thereby producing a cold compactible metallic powder 338 comprising non-spherical particles comprising one of the spheroidal metallic particles as a core and a plurality of the smaller metal particles as protrusions from the core. The impact blending is conducted in an impact chamber comprising a rotor. The rotor comprises a plurality of impact faces which strike and accelerate the particles in the impact chamber as the rotor rotates during the dry milling, thus causing the small particles to adhere to the large spheroidized particles.

Metallic powder for additive manufacturing or powder metallurgy

[178] The methods disclosed herein are considered useful for producing metallic powders for additive manufacturing or powder metallurgy.

[179] The spheroidal metallic particles may be useful as a feedstock for additive manufacturing. For example, powders of the spheroidal metallic particles may be suitable feedstocks for powder bed additive manufacturing techniques. Laser-powder bed fusion techniques and binderjet printer techniques require controlled powder spreading and typically particle sizes in the range of 15 to 63 micron. Electron beam powder bed techniques (sometimes called selective electron beam powder bed) require particle sizes in the range of 40 to 106 micron, as finer powers destroy the e-beam filament. In all powder bed applications, the powder needs to be evenly spread in a layer, so spheroidal particles are required to provide the necessary flow control and good packing density. The finer the powders can be, the greater the geometric fidelity of the resultant parts’ features. In other scenarios, powders of the spheroidal metallic particles may be suitable feedstocks for directed energy deposition (DED) additive manufacturing, such as blown powder DED techniques, which typically require particle sizes in the range of 30 to 150 micron.

[180] The non-spherical (core-corona) particles comprising one large metal particles as a core and a plurality of small metal particles as protrusions from the core may be useful for powder metallurgy, and in particular manufacturing techniques wherein the powder is initially cold compacted to form a porous metallic article.

[181] For example, the cold compaction process may involve any cold compaction method wherein a metallic powder is consolidated under pressure, with or without a binder, at temperatures below the sintering temperature, to produce a porous metal object (a compact) with sufficient structural integrity to withstand further processing, e.g. via sintering or other metalworking techniques, to produce a final metal product. Suitable cold compaction techniques may include cold isostatic pressing, cold die pressing, direct powder rolling and metal injection moulding.

[182] Without wishing to be bound by any theory, it is proposed that the protrusions of the non-spherical particles facilitate particle interlocking during compaction and increase the contact area at the interface between adjacent particles in the compact. Thus, the adhesion between particles is increased and the resultant compact has improved mechanical properties. Figure 7 schematically depicts some proposed interlocking modes that may occur between adjacent non-spherical particles 516 in the porous metal compact. At some interparticle interfaces, such as interface 510ab between particles 516a and 516b, the particles engage via protrusions 520a and 520b of both particles. At other interparticle interfaces, such as interface 510bc between particles 516b and 516c, the engagement of the particles is assisted by protrusion 520c of one particle only. [183] The cold compaction process consolidates the cold compactible metallic powder comprising the core-corona particles to produce a porous metallic article. There are thus voids between the compacted metal particles in the metallic structure, which will be empty following a binderless compaction process. The porosity of the metallic article will depend on the morphology of the particles, the deformability of the particles under the compaction pressure (which may be low if the particle cores are formed of a high yield strength composition) and the compaction pressure. In some embodiments, therefore, the porous metallic article has a density of at least 70 % of theoretical density. However, it will be appreciated that the porosity of suitably robust compacts may vary in other implementations, depending on the factors mentioned above. Furthermore, it is envisaged that the density of the porous compact may be increased by adding small particles to the cold compactible metallic powder, the small particles sized to occupy a portion of the voids between the interlocked non-spherical particles after compaction.

[184] The cold compactible metallic powder comprising core-corona particles may be compacted at any pressure sufficient to consolidate the cold compactible metallic powder and thus form a porous metallic article. The metallic powders may advantageously be consolidated at significantly lower pressures than required for rounded (e.g. spherical) particles which lack protrusions but have a similar metallic composition. The inventors have found that compaction of non-spherical particles with cores derived from Ti-6AI-4V swarf (and also Ti-6AI-4V protrusions) may be consolidated to form robust compacts at pressures of only 380 MPa. By contrast, prealloyed Ti-6AI-4V spherical powders (gas-atomised powders) are not cold compactible even at 413 MPa and it is expected that pressures in excess of 1000 MPa may be needed to consolidate these spherical particles. In some embodiments, therefore, the cold compactible metallic powder may be consolidated by cold compaction at a pressure below 450 MPa, or below 400 MPa, such as below 350 MPa.

[185] The porous metallic articles produced directly by cold compaction are typically not final metal products but will instead be further processed. In some embodiments, the porous metal article is sintered to produce a sintered metal structure which may be further processed by conventional metalworking techniques. In other embodiments, the porous metallic articles may be a feedstock for a metal manufacturing technique such as extrusion. In one exemplary application, the porous metallic article is a cylindrical rod titanium alloy compact, for example with dimensions of 12.5mm diameter and 400mm length, suitable for extrusion to make titanium alloy wire as disclosed in US patent 9,468,960.

EXAMPLES

[186] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

Materials

[187] A titanium alloy swarf, produced as a by-product of a manufacturing process in the aerospace industry, was characterised by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The chemical composition as shown in Table 1 confirms that the swarf is Ti-6AI-4V alloy and contains about 0.19% oxygen, which is within Grade 5 specification for this alloy.

Table 1.

[188] The swarf was characterised by scanning electron microscopy (SEM, ZEISS Merlin™ FE-SEM) to investigate its morphology. As seen in Figure 8, the swarf is flakeshaped with some curvature to the structure typical of material generated through machining processes. As seen in Figures 9 and 10, there are also a large number of various defects, such as cracks, jagged edges and perforations on the swarf surfaces. The flake shaped swarf particles have a thickness ranging from 30-100 pm, and a particle size (maximum dimension) of up to 5 mm in length.

[189] The swarf was subjected to sieve analysis according to ASTM B214-22 to determine the particle size distribution as shown in Figure 1 1 . It was estimated that the d10, d50 and d90 particle sizes for the swarf were 0.64 mm, 1.25 mm and 2.2 mm respectively. [190] 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 swarf were measured as 0.616 g/cm³ and 0.751 g/cm³ respectively. The very low apparent density values, compared to a theoretical density of 4.429 g/cm³ for Ti-6AI-4V alloy, is consistent with the low packing efficiency of the curved, flake shaped particles.

[191] Highly spherical, gas-atomized titanium alloy Ti-6AI-4V (Ti64) powder was received from a commercial manufacturer. The material was an undersized (small) fraction (5-25 pm, hereafter Ti64-S1 ) produced as a low value by-product in a gas atomisation process.

Dry milling apparatus and methods

[192] A Nara Hybridization System (NHS-0), available from Nara Machinery Co., Ltd. and schematically depicted in Figure 1 , was used as the particle modification apparatus. The system is equipped with an impact chamber, defined by a cylindrical stator, with a rotor and a recirculation duct. During dry milling the particles can leave the impact blending chamber via an outlet in the stator and are re-fed into the chamber centre via the recirculation duct. The chamber is surrounded with a jacket in which coolant is circulated to keep the inside treatment temperature under 100°C, typically under 50°C. The NHS-0 was operated under a high purity argon (3ppm O2) atmosphere in order to keep oxygen levels as low as possible and thus reduce the opportunity for oxygen contamination of the titanium powder materials.

[193] Schematic drawings of the impact chamber of the NHS-0 are shown in Figures 1 , 2 and 3. The outer wall of the impact chamber is defined by stator 110. Rotor 112, with a diameter of 118 mm, includes six radially-oriented impact blades 114 having an impact face 116 and an outer edge 118 at the periphery of the rotor. The impact blades have a length in the radial direction of 20 mm, a thickness of 5 mm and flattened edges. The gap 120 between outer edge 118 of the blades and stator 110 was 3.5 mm. The rotor also includes radial ribs 115 on the reverse side which are spaced apart from the rear wall 117 of the impact chamber by gap 119, which was 0.9 mm. [194] The NHS-0 is operable at rotor rotation speeds of up to 16,000rpm. Generally, speeds of 6,000rpm to 10,000rpm were considered most suitable. The corresponding conversion of rotational velocity to peripheral velocity (i.e. the speed of outer edge 118 of the blades) is shown in Table 3. The apparatus took some time to reach the set rotation speed (23, 30 and 38 seconds to reach 6,000, 8,000 and 10,000 rpm respectively). The impact blending times referred to in the subsequent examples refer to the time once the set rotation speed was reached.

Table 3.

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

Cold isostatic pressing apparatus and methods

[196] A Cold Isostatic Press (CIP) with a maximum pressure of 413 MP (60 ksi) was used for cold compaction studies. Elongate, cylindrical latex bags with an internal diameter (ID) of 9 mm were used as flexible moulds for the cold isostatic pressing experiments, thus producing “mini rods” to demonstrate the powder compactability. Each mini rod had dimensions of about 8.5mm diameter and a length of 30 to 50 mm, and weighed about 6.0 grams.

[197] The following general procedure was used for the cold isostatic pressure experiments:

• Fill the flexible bag with the powders, frequently tapping the bag to make the powder packing as uniform and as dense as possible,

• Insert a stopper into the mouth of the bag and tighten it with a rubber band to ensure the bag is sealed tightly and the compressing fluid cannot contact the compacts, • Secure the bag to an elongate metal support (an L-shaped metal angle) to hold the bag straight during cold isostatic pressing while still allowing uniform pressure to be applied to the exterior of the bag,

• Place the secured bag into the basket configured for placement in the pressure chamber of the CIP.

• Run the CIP to press the compacts isostatically to the pre-set pressure.

• After pressing, remove the green rod(s) from the flexible bags.

Example 1.

[198] The as-received swarf was subjected to dry milling in the NHS-0 system at 10,000 rpm for periods of time ranging from 30 seconds to 180 seconds. As seen in Table 2, the temperature of the material, as measured in the recirculation duct, rose from 63°C to 95°C between 30 seconds and 180 seconds of milling. The yield of recovered comminuted material decreased over this period as well, which was attributed to increasing deposition of the metallic material on the rotor blades or other surfaces of the apparatus at higher temperatures. The recovered comminuted powders were then subjected to sieve analysis to determine the particle size distribution, with the results also shown in Table 2.

Table 2 [199] As seen in Table 2, the swarf particle size generally decreased with increasing time of dry milling. However, the percentage of the very fine particles, i.e. less than 75 pm, decreased with processing time, indicating that very fine particles produced early in the dry milling process may re-attach to the coarse particles. In order to obtain a greater quantity of fine particles (-75pm), a short processing time is thus preferred. A short processing time for comminution is also desirable to prevent excessive temperatures and resultant loss of yield.

[200] The classified fractions of the swarf subjected to dry milling for 30 seconds at 10,000 rpm were characterised by scanning electron microscopy (SEM, ZEISS Merlin™ FE-SEM) to investigate the morphology. Compared to the as-received swarf, the particles in each fraction had smoothed surfaces, with jagged edges removed, facets rounded and edges folded over. However, the particles retained a flake shaped “blocky” morphology and thus remained non-spheroidal. Figures 12 and 13 show representative SEM images of the 75-250 pm and 500-1000 pm fractions.

Example 2.

[201] The as-received swarf was subjected to five sequential rounds of dry milling in the NHS-0 system at 10,000 rpm for 30 seconds. In the first round of milling, 100 g of swarf was processed in 10 batches, with the limitation of 10 g per batch due to the low apparent density of the swarf. After the first round of milling, the comminuted material from the 10 batches was sieved into three size ranges: -250 micron, 250-500 micron and +500 micron. In the second and subsequent rounds of dry milling (using up to 50 g material per batch), the two larger fractions were separately dry milled and classified into the same three size fractions. The newly formed -250 micron fraction was combined with the prior-formed -250 micron fraction, the two newly formed 250- 500 micron fractions were combined and the two newly formed +500 micron fractions were combined for the next round of milling.

[202] Figure 14 shows a mass flow diagram for the five rounds of milling. The particle sizes decreased with each round, so that after five rounds of milling, 65% of the initial swarf was converted to particle sizes less than 250 pm, 30% of the particles were in the range of 250-500 pm and only 3% of particles were larger than 500 pm. Advantageously, the yield of material recovered after the five rounds of milling was nearly 97%, in contrast to yields of about 80% when milling continuously for similar total times (c.f. Table 2). This may be attributed to the improved temperature control obtained in the step-wise approach.

Example 3.

[203] Despite the particle size reduction achieved by comminuting the swarf in Examples 1 and 2, the particles still exhibited a flake-like, non-spheroidal morphology. Such particles are not suited to applications such as additive manufacturing which require a spheroidal particle morphology. Moreover, they are not suited for powder metallurgy techniques using cold compaction due to the lack of any interlocking mechanism between particles. It is therefore desirable to spheroidize the comminuted particles to facilitate their use as feedstocks for various manufacturing technologies, or to facilitate further morphological manipulation of the spheroidized particles for cold compactability.

[204] The -250 micron fraction of particles, as produced after five rounds of milling in Example 2, was further classified by sieving into -75 pm, 75-150 pm and 150- 250 pm fractions. The amount of the -75 pm fraction was 8.1 wt.% of the total initial swarf. The particles in each size range were then separately subjected to dry milling at 10000 rpm for 14 minutes. The amount of material processed in each batch varied from 10g to 25g, limited only by the amount of material available, thus demonstrating that the spheroidization step can be conducted at higher loading concentrations in the impact chamber than the comminution step. The particle morphologies prior to and post spheroidizing are shown in Figures 15 and 16 (-75 pm pre- and post), Figures 17 and 18 (75-150 pm pre- and post) and Figures 19 and 20 (150-250 pm pre- and post).

[205] Spheroidizing and surface-smoothing of the milled swarf particles occurred in all three particle size ranges, although the extents of sphering were slightly different. Under the same dry milling conditions, the most effective spheroidizing occurred in the particle size range of 150-250 pm, the least effective for the particles smaller than 75 pm and in the middle for the particles between 75-150 pm. It is believed that spheroidization of the -75 pm particles could be improved with more intense dry milling conditions (longer milling times and/or higher speeds). [206] Based on SEM images, only a small reduction in particle sizes occurred during spheroidizing for all three particle size ranges, with the least size reduction evident for the particles below 75 pm. The particle size reduction was attributed primarily to the particle shape change (spheroidization), and to a far lesser extent to the generation of fine particles. Without wishing to be limited by any theory, spheroidization is favoured over comminution, in contrast to milling of the as-received swarf, both by the smaller initial particle sizes and by the particle smoothing and defect elimination that occurred during the comminution step.

[207] To quantify the effect of the dry milling steps on the particle morphology, the sphericity of the powders was characterised using image analysis of SEM images. Sphericity is a measure of the degree to which the particles in a powder approach the shape of a sphere. It is measured based on the imaged cross-sections of the particles using image analysis software, in this case, “Image J”. Sphericity is defined, with respect to the imaged cross-section of the particles, as the ratio of the radius of the inscribed circle to the radius of the circumscribed circle (where the inscribed circle is the largest circle inside the particle cross-section, the circumscribed circle is the smallest circle outside the particle cross-section, and both circles are centred on the particles’ centre of mass). The sphericity of a perfectly spherical object is 1 .

[208] The sphericity values for the as-received swarf, the input powders to spheroidization (Example 2 comminuted material, classified into -75 pm, 75-150 pm and 150-250 pm fractions) and resultant spheroidized powders (still in the -75 pm, 75- 150 pm and 150-250 pm fractions) are shown in Table 3, where the average sphericity is the average of 10 particles. The as-received swarf is highly irregular, and the sphericity remains low for all fractions after comminution (as performed in Example 2). After the spheroidizing step, the sphericities are greatly improved, particularly for the two large fractions. It is expected that further improvements in sphericity could be obtained via longer dry milling times and/or higher intensity dry milling conditions.

Table 3

Example 4.

[209] The three portions of spheroidized particles separated in Example 3 were re-combined (-250pm) and mixed with the undersized gas-atomized Ti-6AI-4V alloy powder (Ti64-S1 ; 5-25 pm) in a weight ratio of 80:20. The combined powders were then subjected to rotational impact blending (15g per batch) in the NHS-0 system at various rotor rotational speeds (6,000 to 9,000 rpm) for 30 seconds. Afterwards, the impact blended powders were recovered and analysed by SEM to investigate the resultant morphology.

[210] Under low impact milling conditions (6000 rpm for 30 s), only a few of the small Ti64-S1 particles had attached to the larger spheroidized particles. Most Ti64- S1 particles remained unaffected by the milling. At slightly more intense milling conditions (7000 rpm for 30 s), more small particles had attached.

[211] After dry milling at medium intensity conditions (8000 rpm for 30 s), a substantial fraction of the small Ti64-S1 particles had attached to the larger spheroidized particles. The resultant powder thus comprised non-spherical particles comprising a large metal particle (derived from the previously spheroidized particles) as the core and a plurality of the small metal particles (derived from Ti64-S1 ) as protrusions from the core. A representative SEM image is shown in Figure 21 .

[212] At still higher intensity conditions (9000 rpm for 30 s) most of the small Ti64- S1 particles had attached to the larger spheroidized particles, with some flattening of the resultant protrusions also evident. A representative SEM image is shown in Figure 22.

[213] The morphology of the impact blended non-spherical particles, comprising a large metal particle as the core and a plurality of the small metal particles as protrusions from the core, is believed to provide improved cold compactability properties compared to the spheroidal precursor particles. One way to quantify the difference in morphology is via the convexity of the particles. Convexity is the relative amount that an object differs from a convex object. In this case, the convex object is the particle as measured in cross section. A measurement of convexity is obtained by forming the ratio of the perimeter of a particle’s convex hull to the perimeter of the object itself, according to the equation below. The convex hull is a polygon that encloses the particle cross section with no point of the polygon bending inwards.

„ particle convex perimeter

Convexity = - particle perimeter

[214] If the particle cross section is a convex object (e.g. round particle or ellipse with a smooth surface), the convexity will be 1 , as the perimeters of the convex hull and the object are the same. The value will be less than 1 if the object has an irregular boundary. For the impact blended particles, if their convexity values are closer to 1 , their shapes are closer to the original core particles before impact blending.

[215] The convexity measurements of the as-received swarf, the spheroidized powder (size range of -250 pm, re-combined three portions from Example 3) and the impact blended powder (8000 rpm, as produced in Example 4 from that spheroidized powder) are shown in Table 4. The measurements were obtained from SEM images processed using Image J image-processing software. At least 10 measurements were done for each particle composition. Table 4

[216] The TI64-S1 gas-atomised small particles have a convexity value of very close to 1 , consistent with a near-perfect round particle with a smooth surface. The increase in convexity of the spheroidized swarf particles compared to the as-received swarf indicate that the comminution and spheroidizing process transformed the swarf to near round or ellipse-shaped particles with relatively smooth surfaces, consistent with the SEM results. After impact blending the spheroidized swarf particles with the Ti64-S1 gas-atomised small particles, the convexity value reduced again due to the formation of the “core-corona” morphology.

Example 5.

[217] The powders produced in Example 4, comprising non-spherical particles comprising a large metal particle as the core and a plurality of the small metal particles as protrusions from the core, were then subjected to cold isostatic pressing (Cl Ping) at pressures of 380 MPa (55ksi) to determine if the powders were cold compactible. The powders were compacted to form “mini rods” with dimensions of about 8.5mm diameter and a length of 30 to 50 mm, with the powders considered cold compactable if intact and non-friable solid rods were retrieved from the CIPing mould (friable means that the compact is easily reduced back to powder during handling).

[218] Each of the powders produced in Example 4 was found to be cold compactible, although the mini rod formed from powder produced at low intensity milling (6000 rpm, 30s) had a lower strength than the other rods. Evidently, there is a wide process window for modifying the spheroidized particles into “core-corona” morphology for cold compactability enhancement, and it can be expected that smaller amounts of “small” particles (e.g. large and small particles in a weight ratio of 90:10) would also be sufficient to render the spheroidized particles cold compactable. Moreover, similar outcomes can be expected if the spheroidized particles were modified with fine particles derived from the swarf comminution step (e.g. the fines fraction below 75 pm), instead of the spherical Ti64-S1 particles.

[219] In contrast, the metallic powder of the -250 pm spheroidized particles (i.e. pre-impact blending) was found to be non-compactible at 380 MPa. The results show that spheroidized particles produced by comminution, classification and spheroidizing of swarf can be upgraded for powder metallurgy applications involving cold compaction by an impact blending methodology.

Example 6.

[220] The powder produced in Example 4 (8,000 rpm, 30s impact blending), comprising non-spherical particles comprising a large metal particle as the core and a plurality of the small metal particles as protrusions from the core, was used to study the mode of bonding between the core (derived from the comminuted and spheroidized Ti64 swarf particles) and the protrusions (derived from the small Ti64-S1 particles). The impact blended powder particles were cold mounted and ground to 1200 grit using SiC papers and final polished using OP-S suspension. To identify and examine the interface between the large core particles and the small Ti64-S1 particles, the polished samples were etched by Kroll’s reagent. The impact blended powder was then characterized by SEM (ZEISS Merlin™ FE-SEM) at high magnification.

[221] The SEM images supported the following conclusions.

• The swarf particles were spheroidized by being folded and smeared - consistent with a ductile spheroidization mechanism.

• The outside layer of the swarf showed a laminated microstructure, suggesting there was some deformation of the swarf during the impact blending process.

• The fine particles formed a crown or “corona” over the core particles.

• Many of the protrusions from the core consisted of clusters of multiple fine particles.

• Despite some small voids evident at the interfaces, the integrity of the interfaces formed between connected particles (both core-small particle interfaces and small particle-small particle interfaces) is consistent with metallurgical bonding between not only the corona fine particles and core particles, but also among the corona fine particles in the clusters.

[222] To investigate how the impact blended powder produced in Example 4 (8,000 rpm, 30s impact blending) responded to the cold compaction, the green compact produced by cold isostatic pressing in Example 5 was characterized by SEM (ZEISS Merlin™ FE-SEM) at high magnification. A cross-section of the green compact was cold mounted and ground to 1200 grit using SiC papers and final polished using OP-S suspension. To identify and examine the interface between the particles in the compact, the polished samples were etched by Kroll’s reagent before the SEM analysis.

[223] The SEM images showed that the large core particles are not significantly deformed by the compaction and that protrusions from one or both particles are present at many of the interfaces between adjacent large particles. It is unlikely there is metallurgical bonding between the swarf-derived core particles, since the cold compaction pressure is insufficient to induce such bonding. It was concluded that interlocking between the particles, facilitated by the corona morphology, is key to the cold compatibility of the powders.

Example 7.

[224] A portion of the as-received swarf was subjected to a beta annealing process, i.e. above the beta-transus temperature. The swarf was thus heated gradually to 1 100°C in a vacuum furnace, with dwell time of 30 minutes and slow subsequent cooling to room temperature in the furnace. Scanning electron microscopy investigations revealed that the microstructure changed from being duplex (globular/recrystallized alpha and transformed beta) for the as received swarf to fully lamellar with large lath thickness and potentially increased colony sizes in the beta annealed + slow cooled condition.

[225] The as-received swarf and annealed swarf were then evaluated with a Vickers microhardness test (ASTM E92-82(2003)). The as-received swarf was found to have a Vickers hardness number (Hv) of 330 ± 15, whereas the annealed swarf had Hv of 254 ± 68. The results demonstrate that the annealing process relaxes the metallic composition of the swarf, reversing the work-hardening that occurs in the machining process and thus increasing the ductility of the composition. [226] The annealed swarf was then subjected to dry milling in the NHS-0 system at 10,000 rpm for 60 seconds. The swarf particles agglomerated in the impact chamber due to the enhanced ductility, and were thus not successfully comminuted or spheroidized at the test conditions.

Example 8. (Comparative)

[227] Several attempts were made to comminute and spheroidize the as-received swarf in a high shear liquid milling process as disclosed in WO2015/192166, using distilled water as the milling medium and a rotor-stator gap size of 0.45 mm. The process of WO2015/192166 is useful to comminute soft materials such as titanium sponge. However, the process was ineffective for processing the swarf due to jamming of swarf particles in the mill, causing excessive heat build-up, high noise and vibration leading to equipment damage.

[228] The swarf could only be comminuted in the high shear mill by conducting an initial ring milling process step to reduce the particle size into the range of 38 to 150 micron. Furthermore, the milled particles remained substantially non-spheroidal even after 30 minutes of high shear liquid milling. Moreover, the ring milling was found to significantly degrade the titanium alloy composition due to oxygen contamination, and it is expected that further contamination occurred in the subsequent high shear liquid milling process.

[229] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

1 . A method of producing spheroidal metallic particles, the method comprising: providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron; comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non-spheroidal comminuted particles; classifying the comminuted material to separate a fraction of the non- spheroidal comminuted particles from larger and/or smaller particles, wherein the classifying is conducted during and/or after the comminuting; and spheroidizing the separated non-spheroidal comminuted particles by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles, wherein each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling.

2. The method according to claim 1 , wherein the particulate metallic precursor comprises flake-shaped particles with a maximum dimension of greater than 1 mm.

3. The method according to claim 1 or claim 2, wherein the particulate metallic precursor is a swarf.

4. The method according to any one of claims 1 to 3, wherein classifying the comminuted material comprises separating the fraction of the non-spheroidal comminuted particles from larger particles, wherein the separated non-spheroidal comminuted particles have a maximum dimension, D1 , of less than 250 micron.

5. The method according to claim 4, wherein the separated non-spheroidal comminuted particles have a maximum dimension, D1 , of less than 100 micron.

6. The method according to claim 4 or claim 5, further comprising comminuting the larger particles, after separation, by dry milling in the impact chamber or another impact chamber, thereby producing further non-spheroidal comminuted particles having a maximum dimension of less than D1 for spheroidizing.

7. The method according to any one of claims 1 to 6, wherein classifying the comminuted material comprises separating the fraction of the non-spheroidal comminuted particles from smaller particles, wherein the smaller particles have a maximum dimension, D2, of less than 100 micron.

8. The method according to claim 7, further comprising spheroidizing the smaller particles, after separation, by dry milling.

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

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

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

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

13. The method according to one of claims 1 to 12, wherein the particulate metallic precursor comprises a metallic 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 aluminium and copper.

14. The method according to one of claims 1 to 13, wherein the particulate metallic precursor comprises a metallic composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, alloys thereof, and metal matrix composites thereof.

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

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

17. The method according to any one of claims 1 to 16, wherein each impact chamber comprises a cylindrical stator which defines a cylindrical outer wall of the impact chamber and a recirculation conduit which connects an entry port located in the cylindrical outer wall and an exit port directed to a central portion of the impact chamber, wherein particles continuously recirculate through the recirculation conduit during the dry milling.

18. The method according to any one of claims 1 to 17, wherein the rotor of each impact chamber comprises a plurality of impact blades which define the impact faces and the outer edge of each impact blade at the periphery of the rotor is spaced apart from the cylindrical stator by a gap distance in the range of 1 mm to 5 mm.

19. The method according to any one of claims 1 to 18, wherein the rotor of each impact chamber comprises a plurality of impact blades which define the impact faces, and wherein the rotor is rotated during the comminuting such that the outer edge of each impact blade at the periphery of the rotor has a peripheral velocity of above 50 m/s.

20. The method according to any one of claims 1 to 19, wherein the particulate metallic precursor is comminuted for a time between 1 second and 1 minute.

21 .The method according to any one of claims 1 to 20, wherein the separated non- spheroidal comminuted particles are spheroidized for a time between 1 minute and 30 minutes.

22. The method according to any one of claims 1 to 21 , wherein dry milling during the comminuting and the spheroidizing is conducted in a dry inert gas atmosphere.

23. The method according to any one of claims 1 to 22, wherein the particulate metallic precursor has an average sphericity of less than 0.5, preferably less than 0.4.

24. The method according to any one of claims 1 to 23, wherein the separated non- spheroidal comminuted particles have an average sphericity of less than 0.6 before the spheroidizing.

25. The method according to any one of claims 1 to 24, wherein the spheroidal metallic particles have an average sphericity of greater than 0.8.

26. A metallic powder for additive manufacturing, comprising spheroidal metallic particles produced according to any one of claims 1 to 25.

27. Use of a metallic powder comprising spheroidal metallic particles produced according to any one of claims 1 to 25 for additive manufacturing.

28. A method of producing a cold compactible metallic powder, the method comprising: providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron; comminuting the particulate metallic precursor by dry milling in an impact chamber, thereby producing comminuted material comprising non-spheroidal comminuted particles; classifying the comminuted material to separate a fraction of the non- spheroidal comminuted particles from smaller particles, wherein the classifying is conducted during and/or after the comminuting; spheroidizing the separated non-spheroidal comminuted particles by dry milling in the impact chamber or another impact chamber, thereby producing spheroidal metallic particles; combining at least a portion of the smaller particles with at least a portion of the spheroidal metallic particles; and subjecting the combination of particles to impact blending in the impact chamber or another impact chamber to adhere the smaller particles to the spheroidal metallic particles, thereby producing a cold compactible metallic powder comprising non-spherical particles comprising one of the spheroidal metallic particles as a core and a plurality of the smaller metal particles as protrusions from the core, wherein each impact chamber comprises a rotor configured to rotate within the impact chamber, the rotor comprising a plurality of impact faces which strike particles in the impact chamber as the rotor rotates during dry milling or impact blending.

29. A cold compactible metallic powder, comprising non-spherical particles produced by a method according to claim 28.

30. Use of a cold compactible metallic powder comprising non-spherical particles produced by a method according to claim 29 to produce a porous metallic article by cold compaction.