Method of producing a doped metallic powder

Abstract

The invention provides a method of producing a cold compactible, doped metallic powder, the method comprising: providing (i) a first metallic powder comprising large metal particles, (ii) a second metallic powder comprising small metal particles and (iii) a dopant powder comprising small particles of a dopant composition; and subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles having a convexity number of less than 0.8, wherein the non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions.

Description

[0001] Method of producing a doped metallic powder

[0002] Technical Field

[0003] [1] The invention relates to a method of producing a cold compactible, doped metallic powder by subjecting a first metallic powder comprising large metal particles, a second metallic powder comprising small metal particles, and a dopant powder comprising small particles of a dopant composition to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles. The non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions. The invention also relates to a method of producing a porous doped metallic article by subjecting the cold compactible metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible doped metallic powder, thereby producing a porous doped metallic article, such as a porous metallic billet. The invention also relates to method of producing a doped metallic extrudate by extruding the porous metallic billet to form a doped metallic extrudate, such as a doped metallic wire.

[0004] Background of Invention

[0005] [2] Cold compaction of metallic powders, including techniques such as direct powder rolling, cold die pressing, cold isostatic pressing and metal injection moulding, provides significant benefits over traditional hot working techniques in various metallurgical manufacturing processes. Such cold compaction methods have in common that the metallic powder is consolidated under pressure at temperatures below the sintering temperature to produce a porous metal object (a compact) with sufficient structural integrity to withstand further processing via sintering or other techniques to produce a final metal product.

[0006] [3] In one application of interest, a porous billet produced by cold compaction of metallic powders may be extruded via a continuous rotary extrusion process to produce an extrudate such as a metal wire. For example, US patent 9,468,960 discloses a continuous rotary extrusion process suitable for extrusion of high temperature formable and optionally oxygen-sensitive metals such as titanium, tantalum and niobium metals and alloys. A range of other high yield strength, low ductility and / or refractory metallic compositions can in principle also be processed by such methods. Extrusion of a porous billet precursor can provide significant benefits compared to powder feeds, including better control of pre-heating, avoidance of powder separation issues, reduced wear on the extrusion tooling, reduced contamination of the extruded metal (e.g. with oxygen) and / or reduced porosity in the extrudate. [4] The resultant wires may be useful feedstocks for additive manufacturing techniques, such as wire arc additive manufacturing (WAAM), and other advanced metallurgical methods involving melting of the wire composition. The manufacture of wire feedstock by extrusion in the solid state provides several advantages over conventional melt metallurgy wire-forming methods, such as avoiding undesirable reactions like oxygen contamination and delaying reactions or physical processes within the metallic composition which may be desirable but only during subsequent melt processing of the wire.

[0007] [5] Compacts produced from metallic powders may also be useful precursors for other metallurgical processes such as various melt metallurgy (fusion) methods, hot working processes such as hot rolling, sintering and solid-state forming methods such as friction stir processing.

[0008] [6] Cold compaction generally requires the application of significant pressures to bond the metallic particles to each other and to increase the density of the compact to an acceptable level (typically greater than 70% of theoretical density). Very high pressures can be required, particularly for powders of high yield strength and low ductility metals and / or relatively spheroidal particle morphologies. The use of binders can reduce the pressure necessary for consolidation, but this may be undesirable because of the potential to introduce contaminants to the metallic composition. Successful powder consolidation may not be possible without binders at practically attainable compaction pressures with some commercially available metallic powder feedstocks, for example atomized titanium pre-alloyed powders and similar spheroidal, high yield strength metallic powders. These issues undesirably restrict the range of feedstocks that can be used in metallurgical manufacturing processes involving a cold compaction process step.

[0009] [7] Furthermore, a significant issue with melt metallurgy processing of compacts or extrudates thereof is the poor control of metal morphology in the resultant metal object. In the context of additive manufacturing techniques such as WAAM and other directed energy deposition (DED) methods where the metal feedstock is melted and selectively deposited onto a workpiece, epitaxial grain growth in the heat flow direction (build direction) across build layers can generate undesirable non-isotropic properties in the resultant additively manufactured metallic article.

[0010] [8] This issue could in principle be addressed by combining grain-refining dopants, such as an inoculating ceramic dopant, with the metal composition. The ceramic reacts with the molten metal to form a new phase which is believed to nucleate re-crystallisation of the metal as it cools. Other grain-refining dopants are believed to similarly nucleate recrystallisation via non-reactive mechanisms. During additive manufacture, grain-refining dopants thus inhibit the formation of larger epitaxial grains in favour of nucleated equiaxed grains, improving the isotropic properties of the manufactured metallic article.

[0011] [9] However, it remains challenging to accurately control doping during additive manufacturing and other metallurgical manufacturing methods, particularly when the doping action relies on reaction or irreversible physical processes taking place in the melt or recrystallisation on the workpiece. In such cases, the dopant typically cannot be pre-dispersed in the metallic feedstock by melt processing. A powder feed containing a physical mixture of metal and dopant particles may avoid premature reaction but is unsatisfactory due to the risk of particle segregation and inhomogeneous dopant distribution.

[0012]

[0010] Doped metal wires suitable for WAAM and similar applications have previously been produced by rotational extrusion of a physical mixture of metal and dopant particles, with the dopant thus dispersed in the metal without melting. However, this approach is restricted to relatively ductile metals such as aluminium and copper which can be directly extruded as powder, i.e. without pre-forming a compact. Moreover, there remains a risk of inhomogeneous doping in the wire due to segregation of the metal and dopant particles in the powder feed to the extrusion process.

[0013]

[0011] There is therefore an ongoing need for methods capable of upgrading metallic powders, via cold compaction and typically also further processing step(s), into doped feedstocks for additive manufacturing or other advanced metallurgical manufacturing methods. Such processes should preferably be compatible with a wide range of metallic powder feedstocks, provide accurate control of dopant amount and distribution and / or avoid melting the metallic composition.

[0014]

[0012] 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.

[0015] Summary of Invention

[0016]

[0013] In the methods disclosed herein, a dopant powder is combined and impact blended with two metallic powders having large and small metal particle sizes to produce a cold compactible doped metallic powder having a non-spherical particle morphology and with dopant composition adhered to the non-spherical particles, typically by embedment.

[0017]

[0014] The non-spherical particle morphology may facilitate consolidation of the powder in a cold compaction process to form compacts with desirable mechanical properties (e.g. high strength), even with high yield strength metallic compositions.

[0015] Advantageously, the approach disclosed herein allows both the amount and distribution of the dopant composition to be controlled in the porous compact and in metallic articles produced by subsequent metallurgical processing of the porous compact, e.g. by solid-state forming, hot-working or melt-processing (fusion) techniques. The pre-adhesion of dopant to the non-spherical metal particles may avoid or at least mitigate the dopant inhomogeneity issues that can occur when processing a physical mixture of metal and dopant particles. Moreover, dopant incorporation into the cold compactible doped metallic powder and porous compact does not rely on melt-processing, thus avoiding premature interactions of the dopant composition with the molten phase of the metallic composition and / or contamination of the metallic composition in the melt.

[0018]

[0016] The dopant composition may comprise any reactive or non-reactive materials capable of beneficially modifying the properties of metallic articles produced via metallurgic processing. Non-limiting examples include grain-refining dopant compositions capable of modifying the grain structure of the metallic composition in subsequent melt-processing steps, and wettable particles destined to become the discontinuous phase in a metal matrix composite in subsequent solid-state or melt-processing steps.

[0019]

[0017] In one exemplary application, a cold compactible doped metallic powder comprising a grain-refining dopant composition is cold compacted to produce a porous metallic billet (compact), which is then subjected to extrusion to produce a doped metallic extrudate such as a doped wire. The extrudate containing grain-refining dopant composition may then be used as feedstock to an additive manufacturing process, such as WAAM, where the feedstock is melted and selectively deposited onto the workpiece in successive build layers. The dopant composition may improve isotropic properties in the resultant additively manufactured metal article.

[0020]

[0018] In accordance with a first aspect, disclosed herein is a method of producing a cold compactible, doped metallic powder, the method comprising: providing (i) a first metallic powder comprising large metal particles, (ii) a second metallic powder comprising small metal particles and (iii) a dopant powder comprising small particles of a dopant composition; and subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions.

[0021]

[0019] The non-spherical particles may be characterised by a convexity number of below 0.85, such as below 0.8. Impact blended metallic powders comprising such particles have been found suitably cold compactible, in contrast to metallic powders of similar composition comprising more convex particles.

[0022]

[0020] In some embodiments, the dopant composition is a thermally stable solid at least up to the melting point of the metallic composition of the large metal particles. In some embodiments, the dopant composition has a melting point higher than the metallic composition of the large metal particles of the first metallic powder.

[0023]

[0021] In some embodiments, the dopant composition is selected from the group consisting of a ceramic comprising a non-metal element, an intermetallic compound, and a metal or alloy with melting point higher than the first metallic powder.

[0024]

[0022] In some embodiments, the dopant composition comprises at least one non-metal element, optionally selected from boron, carbon, oxygen, nitrogen, silicon and combinations thereof, for example selected from boron, nitrogen and carbon.

[0025]

[0023] In some embodiments, the dopant composition comprises at least one metal element, wherein the dopant composition and the metallic composition of the large metal particles have at least one metal element in common.

[0026]

[0024] In some embodiments, the dopant composition is a ceramic comprising at least one metal element and at least one non-metal element selected from boron, carbon, oxygen, nitrogen, silicon and combinations thereof. The ceramic and the metallic composition of the large metal particles optionally comprise the same metal element.

[0027]

[0025] In some embodiments, the dopant composition comprises a boride, such as a metal boride wherein the metal boride and the metallic composition of the large metal particles comprise the same metal element.

[0028]

[0026] In some embodiments, the dopant composition is a grain-refining dopant composition for the metallic composition of the first metallic powder.

[0029]

[0027] In some embodiments, the dopant composition is an inoculating dopant composition capable of reacting with the metallic composition of the large metal particles when the metallic composition melts and resolidifies. Preferably, the inoculating dopant composition modifies the grain structure of the metallic composition when resolidified.

[0030]

[0028] In some embodiments, the dopant composition is a low-reactivity dopant composition which remains at least partially unreacted, and typically predominantly unreacted, when the metallic composition of the large metal particles melts and resolidifies.

[0031]

[0029] In some embodiments, the low-reactivity dopant composition modifies the grain structure of the metallic composition when resolidified.

[0030] In some embodiments, the metallic composition wets and encapsulates dispersed solid particles of the dopant composition when melted, thereby forming a metal matrix composite when the metallic composition resolidifies.

[0032]

[0031] In some embodiments, the cold compactible metallic powder comprises the dopant composition in an amount of less than 20 vol. %, typically less than 10 vol.%, or less than 5 vol.%, optionally between 2 vol.% and 7 vol.%.

[0033]

[0032] In some embodiments, the cold compactible metallic powder comprises the dopant composition in an amount of less than 2 vol.%, optionally less than 1.5 vol.%, such as between 0.2 vol.% and 1.5 vol.%.

[0034]

[0033] In some embodiments, the non-spherical particles comprise a plurality of the small particles of the dopant composition embedded in the surface of at least the core.

[0035]

[0034] In some embodiments, the method comprises: combining the first metallic powder, the second metallic powder and the dopant powder; and subjecting the combined powders to an impact blending process step to produce the cold compactible metallic powder.

[0036]

[0035] In some embodiments, the method comprises: subjecting the first metallic powder and the dopant powder to a first impact blending process step to produce a modified powder wherein the dopant composition is adhered to the large metal particles, optionally wherein a plurality of the small particles of the dopant composition are embedded in the surface of the large metal particles; combining the modified powder with the second metallic powder; and subjecting the modified powder and the second metallic powder to a second impact blending process step to produce the cold compactible metallic powder.

[0037]

[0036] In some embodiments, the d50 particle size of the first metallic powder is greater than 5 pm, or greater than 10 pm, or greater than 20 pm, such as greater than 50 pm. In some embodiments, the d50 particle size of the first metallic powder is between 20 pm and 500 pm, or between 80 pm and 500 pm, or between 100 pm and 250 pm, such as between 110 pm and 180 pm.

[0038]

[0037] In some embodiments, the d50 particle size of the second metallic powder is less than 40%, such as less than 20%, of the d50 particle size of the first metallic powder.

[0039]

[0038] In some embodiments, the d50 particle size of the dopant powder is less than 20%, such as less than 10%, for example less than 5%, of the d50 particle size of the first metallic powder.

[0040]

[0039] In some embodiments, the d50 particle size of the dopant powder is between 0.5 and 5 pm.

[0040] In some embodiments, the large metal particles and the small metal particles have substantially the same metallic composition.

[0041]

[0041] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition having a yield stress of at least 600 MPa.

[0042]

[0042] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition having an elongation at break of at least 1%, such as at least 3%, for example in the range of 5% to 30%.

[0043]

[0043] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition selected from the group consisting of titanium, tantalum, rhenium, 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.

[0044]

[0044] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof.

[0045]

[0045] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a titanium alloy, optionally Ti-6AI-4V.

[0046]

[0046] In some embodiments, at least one, or each, impact blending process step comprises impact blending particles with a rotor-stator impact blending apparatus. In some such embodiments, the rotor comprises a plurality of impact blades each having an impact face configured for particle striking when the rotor is rotated. In some embodiments, the rotor-stator impact blending apparatus comprises a cylindrical stator and a rotor operable to rotate within the stator, and the rotor comprises a plurality of impact blades having an impact face and an outer edge at the periphery of the rotor.

[0047]

[0047] In some embodiments, each impact blending process step comprises impact blending particles in an impact blending chamber of an apparatus comprising (i) a stator which defines a cylindrical outer wall of the impact blending chamber, and (ii) a rotor operable to rotate in the impact blending chamber, the rotor comprising a plurality of impact blades having an impact face and an outer edge at the periphery of the rotor.

[0048]

[0048] In some embodiments, the apparatus further comprises a recirculation conduit between an entry port located in the cylindrical outer wall of the impact blending chamber and an exit port directed to a central portion of the impact blending chamber, wherein the particles continuously recirculate through the recirculation conduit during the impact blending.

[0049] In some embodiments, 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.

[0049]

[0050] In some embodiments, the method further comprises producing the first metallic powder by: providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron; comminuting and spheroidizing the particulate metallic precursor in one or more impact blending process steps. In some embodiments, the particulate metallic precursor is a swarf.

[0050]

[0051] In accordance with a second aspect, disclosed herein is a cold compactible, doped metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise a large metal particle as a core, a plurality of small metal particles as protrusions from the core and a dopant composition adhered to at least one of the core and the protrusions.

[0051]

[0052] The non-spherical particles may be characterised by a convexity number of below 0.85, such as below 0.8.

[0052]

[0053] In some embodiments, the dopant composition is selected from the group consisting of a ceramic comprising a non-metal element, an intermetallic compound, and a metal or alloy with melting point higher than the first metallic powder.

[0053]

[0054] In some embodiments, the dopant composition comprises at least one non-metal element, optionally selected from boron, carbon, oxygen, nitrogen, silicon and combinations thereof, for example selected from boron, nitrogen and carbon.

[0054]

[0055] In some embodiments, the dopant composition comprises at least one metal element, wherein the dopant composition and the metallic composition of the large metal particles have at least one metal element in common.

[0055]

[0056] In some embodiments, the dopant composition is a ceramic comprising at least one metal element and at least one non-metal element selected from boron, carbon, oxygen, nitrogen, silicon and combinations thereof. Optionally, the ceramic and the metallic composition of the large metal particles comprise the same metal element.

[0056]

[0057] In some embodiments, the dopant composition comprises a boride, optionally a metal boride wherein the metal boride and the metallic composition of the large metal particles comprise the same metal element.

[0057]

[0058] In some embodiments, the dopant composition is a grain-refining dopant composition for the metallic composition of the first metallic powder.

[0058]

[0059] In some embodiments, the dopant composition is an inoculating dopant composition capable of reacting with the metallic composition of the large metal particles when the metallic composition melts and resolidifies. In some embodiments, the inoculating dopant composition modifies the grain structure of the metallic composition when resolidified.

[0059]

[0060] In some embodiments, the dopant composition is a low-reactivity dopant composition which remains at least partially unreacted when the metallic composition of the large metal particles melts and resolidifies.

[0060]

[0061] In some embodiments, the low-reactivity dopant composition modifies the grain structure of the metallic composition when resolidified.

[0061]

[0062] In some embodiments, the metallic composition wets and encapsulates dispersed solid particles of the low-reactivity dopant composition when melted, thereby forming a metal matrix composite when the metallic composition resolidifies.

[0062]

[0063] In some embodiments, the cold compactible metallic powder comprises the dopant composition in an amount of less than 20 vol. %, typically less than 10 vol.%, or less than 5 vol.%, optionally between 2 vol.% and 7 vol.%.

[0063]

[0064] In some embodiments, the cold compactible metallic powder comprises the dopant composition in an amount of less than 2 vol.%, optionally less than 1.5 vol.%, such as between 0.2 vol.% and 1.5 vol.%.

[0064]

[0065] In some embodiments, the non-spherical particles comprise a plurality of the small particles of the dopant composition embedded in the surface of at least the core.

[0065]

[0066] In some embodiments, the large metal particles and the small metal particles have substantially the same metallic composition.

[0066]

[0067] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition having a yield stress of at least 600 MPa.

[0067]

[0068] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition having an elongation at break of at least 1%, optionally at least 3%, such as in the range of 5% to 30%.

[0068]

[0069] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition selected from the group consisting of titanium, tantalum, rhenium, 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.

[0070] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a metallic composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof.

[0069]

[0071] In some embodiments, the large metal particles, and optionally also the small metal particles, comprise a titanium alloy, for example Ti-6AI-4V.

[0070]

[0072] In accordance with a third aspect, disclosed herein is a method of producing a porous doped metallic article, the method comprising subjecting a cold compactible, doped metallic powder according to any embodiment of the second aspect, or a cold compactible, doped metallic powder produced by a method according to any embodiment of the first aspect, to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous doped metallic article.

[0071]

[0073] In some embodiments, the porous doped metallic article has a density of at least 70 % of theoretical density.

[0072]

[0074] In some embodiments, the pressure is below 450 MPa.

[0073]

[0075] In some embodiments, the cold compactible metallic powder is subjected to the cold compaction process in the absence of a binder.

[0074]

[0076] In some embodiments, the cold compaction process is selected from cold isostatic pressing, cold die pressing and direct powder rolling.

[0075]

[0077] In some embodiments, the porous doped metallic article is a rod-shaped billet.

[0076]

[0078] In some embodiments, the method further comprises sintering the porous doped metallic article to produce a sintered preform for use in a solid-state metallurgical forming process.

[0077]

[0079] In accordance with a fourth aspect, disclosed herein is a method of producing a doped metallic extrudate, the method comprising: subjecting a cold compactible, doped metallic powder according to any embodiment of the second aspect, or a cold compactible, doped metallic powder produced by a method according to any embodiment of the first aspect, to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous metallic billet; and extruding the porous metallic billet to form a doped metallic extrudate.

[0078]

[0080] In some embodiments, the method comprises heating the porous metallic billet, and extruding the heated billet through a continuous rotary extruder to form the doped metallic extrudate.

[0079]

[0081] In some embodiments, the doped metallic extrudate is a doped metallic wire.

[0082] In accordance with a fifth aspect, disclosed herein is a doped metallic extrudate, produced by a method according to any embodiment of the fourth aspect.

[0080]

[0083] In accordance with a sixth aspect, disclosed herein is a method of additive manufacturing, comprising melting a doped metallic extrudate, produced by a method according to any embodiment of the fourth aspect, to produce a molten composition, and selectively depositing the molten composition on a workpiece to fabricate or repair a metallic article.

[0081]

[0084] In some embodiments, the method of additive manufacturing is selected from wire arc additive manufacturing, laser wire additive manufacturing, and laser cladding.

[0082]

[0085] In accordance with a seventh aspect, disclosed herein is use of a doped metallic extrudate, produced by a method according to any embodiment of the fourth aspect, as a feedstock in a fusion metallurgical process.

[0083]

[0086] In some embodiments, the fusion metallurgical process is an additive manufacturing process, such as a directed energy deposition additive manufacturing process. In some embodiments, the fusion metallurgical process is a powder manufacturing process.

[0084]

[0087] In accordance with a seventh aspect, disclosed herein is a porous doped metallic article, produced by a method according to any embodiment of the third aspect.

[0085]

[0088] In accordance with a seventh aspect, disclosed herein is use of a porous doped metallic article, produced by a method according to any embodiment of the third aspect, as a feedstock or preform for a fusion, hot working or solid-state forming process.

[0086]

[0089] In some embodiments, the fusion, hot working or solid-state forming process is a process selected from rolling, forging and friction stir processing.

[0087]

[0090] 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.

[0088]

[0091] 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.

[0092] Further aspects of the invention appear below in the detailed description of the invention.

[0089] Brief Description of Drawings

[0090]

[0093] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0091]

[0094] Figure 1 is a schematic depiction of a rotational impact blending apparatus for use in methods according to some embodiments of the invention.

[0092]

[0095] Figure 2 is an isometric drawing of the impact blending chamber of rotational impact blending apparatus (Nara Hybridization System, NHS-0), as used in the Examples.

[0093]

[0096] Figure 3 depicts the impact blending chamber and rotor of a rotational impact blending apparatus for use in methods according to some embodiments of the invention.

[0094]

[0097] Figure 4 schematically depicts a single-step impact blending process for converting a first metallic powder comprising large metal particles, a second metallic powder comprising small metal particles and a dopant powder comprising small particles of a dopant composition into non-spherical particles comprising one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and a plurality of small particles of dopant composition embedded in the core and protrusions.

[0095]

[0098] Figure 5 schematically depicts a two-step impact blending process for converting a first metallic powder comprising large metal particles, a second metallic powder comprising small metal particles and a dopant powder comprising small particles of a dopant composition into non-spherical particles comprising one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and a plurality of small particles of dopant composition embedded in the core.

[0096]

[0099] Figure 6 schematically depicts proposed interlocking modes that may occur between adjacent non-spherical particles, as produced according to embodiments of the disclosure, when compressed to form a porous metal article.

[0097]

[0100] Figure 7 is a SEM image of large gas-atomized titanium alloy (Ti-6AI-4V) particles (>140# mesh, 105-250 pm), as used in the examples.

[0098]

[0101] Figure 8 is a SEM image of small gas-atomized titanium alloy (Ti-6AI-4V) particles (5-25 pm), as used in the examples.

[0099]

[0102] Figure 9 is a SEM image of small TiB2 dopant particles, as used in the examples.

[0100]

[0103] Figure 10 is a SEM image of non-spherical particles comprising a large pre-alloyed Ti-6AI-4V particle as the core, a plurality of small pre-alloyed Ti-6AI-4V particles as protrusions from the core, and small THB2 particles embedded mainly in the core but also in the protrusions, as produced in Example 1.

[0101]

[0104] Figure 11 is a SEM image of spheroidal particles comprising a large pre-alloyed Ti-6AI-4V particle and small TiB2 particles embedded in the Ti-6AI-4V particle, as produced in Example 2.

[0102]

[0105] Figure 12 is a SEM image of non-spherical particles comprising a large pre-alloyed Ti-6AI-4V particle as the core, a plurality of small pre-alloyed Ti-6AI-4V particles as protrusions from the core, and small TiB2 particles embedded in the core, as produced in Example 2.

[0103]

[0106] Figure 13 is a SEM image of spheroidized large metal particles produced by comminution and spheroidizing of Ti-6AI-4V swarf by impact blending in Example 3.

[0104]

[0107] Figure 14 is a SEM image of non-spherical particles comprising a spheroidized large Ti-6AI-4V particle as the core, a plurality of small pre-alloyed Ti-6AI-4V particles as protrusions from the core, and small TiB2 particles embedded mainly in the core, as produced in Example 4.

[0105]

[0108] Figure 15 is a SEM image of a spheroidal particle (in cross-section) comprising a large stainless steel 410L particle and small B4C particles embedded in a composite surface layer, as produced in Example 8.

[0106]

[0109] Figure 16 is a SEM image of a spheroidal particle (in cross-section) comprising a large Inconel 718 particle with AI2O3 adhered in a composite surface layer, as produced in Example 9.

[0107]

[0110] Figure 17 is a SEM image of a spheroidal particle (in cross-section) comprising a large H13 steel particle with AI2O3 adhered in a composite surface layer, as produced in Example 10.

[0108]

[0111] Figure 18 is a SEM image of spheroidal particles comprising a large stainless steel 410L particles and small SiC particles embedded in a composite surface layer, as produced in Example 11.

[0109] Detailed Description

[0110] Method of producing a cold compactible, doped metallic powder

[0111]

[0112] Disclosed herein is a method of producing a cold compactible, doped metallic powder. The method comprises providing large metal particles, small metal particles and small particles of a dopant composition. The method may thus comprise providing (i) a first metallic powder comprising the large metal particles, (ii) a second metallic powder comprising the small metal particles and (iii) a dopant powder comprising the small particles of a dopant composition. The method may further comprise subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles. The non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to, typically by embedment, at least one of the core and the protrusions. The non-spherical particles may have a convexity number of below 0.85, such as below 0.8.

[0112] First metallic powder

[0113]

[0113] The methods of the disclosure may be used to modify both the morphology and composition of a first metallic powder comprising large metal particles with a second metallic powder comprising small metal particles and a dopant powder.

[0114]

[0114] The metal particles in the first and second metallic powders, and the small particles of the dopant powder, are typically present in a range of particle sizes. The particle size distribution (PSD) of such powders may be characterised by d10, d50 and d90 particle sizes, defined such that 10 volume % of the powder is present in particles having a size less than the d10 particle size, 50 volume % of the powder is present in particles having a size less than the d50 particle size and 90 volume % of the powder is present in particles having a size less than the d90 particle size. The d 10, d50 and d90 particle sizes may be measured by routine methods in materials science such as laser diffraction techniques. Suitable instruments for measuring PSD include the Mastersizer range of laser diffraction particle sizers, available from Malvern Panalytical. Particle sizes, including d10, d50 and d90 sizes, may be measured in accordance with ASTM B822 (Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering).

[0115]

[0115] Metallic powders with a wide range of d50 particle sizes may be modified, provided that the particles of the second metallic powder and the dopant powder are correspondingly smaller. The first metallic powder may thus in some embodiments have a d50 particle size of between 1 pm and 500 pm. In some embodiments, the d50 particle size is greater than 5 pm, or greater than 10 pm, for example greater than 20 pm, or greater than 50 pm. In some embodiments, the d50 particle size is between 20 pm and 500 pm, or between 80 pm and 500 pm, or between 100 pm and 250 pm, such as between 110 pm and 180 pm. Optionally, the first metallic powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution and / or to exclude undesirably under-sized or oversized particles. In some embodiments, the large metal particles of the first metallic powder have a particle size of greater than 5 pm, or greater than 10 pm, for example greater than 20 pm, or greater than 50 pm. In some embodiments, the large metal particles of the first metallic powder have a particle size of between 20 m and 500 pm, or between 80 pm and 500 pm, such as between 100 pm and 250 pm.

[0116]

[0116] The particles of the first metallic powder may comprise any type of metallic composition, including commercially pure metals, metal alloys and metal matrix composites. In some embodiments, the metallic composition is a non-ferrous metallic composition.

[0117]

[0117] The methods of the disclosure are particularly useful for upgrading and doping metallic powders of high yield strength metallic compositions, which may not otherwise be cold compactible. Thus, in some embodiments, the particles of the first metallic powder comprise 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.

[0118]

[0118] Without wishing to be limited by any theory, the ductility of the metallic composition of the first metallic powder is also believed to be a relevant consideration. A minimum level of ductility may be required to avoid shattering of the particles under impact blending conditions. On the other hand, excessively ductile metallic compositions will tend to smear, agglomerate or coat the impact blending apparatus.

[0119]

[0119] The actual ductility of metallic compositions may be influenced by factors such as the microstructure and work history. However, the ductility of the metallic composition of the first metallic powder 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).

[0120]

[0120] In some embodiments, the large metal particles of the first metallic powder comprise, or consist 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 due to shattering and / or unsatisfactory modification by the small metal particles or dopant powder. Metallic compositions with ductility above these ranges may be susceptible to smearing, and may also not require modification as disclosed herein to become cold-compactible.

[0121]

[0121] Non-limiting examples of high yield strength and suitably ductile metallic compositions include titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel and cobalt, alloys and metal matrix composites of these metals, low ductility alloys of iron, zinc and 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%.

[0122]

[0122] 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 impact blender surfaces. 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 metal 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 some austenitic steels are expected to be too soft. In some embodiments, the particles of the first metallic powder include copper, lead, zinc, tin and iron only as minor (<20 wt.%, preferably < 10 wt.%) alloying elements in alloys of other metals or are substantially free of these metals.

[0123]

[0123] The particles of the first metallic powder may also comprise a metallic composition having fracture properties suitable to avoid or acceptably minimise shattering of the particles during impact blending instead of the desired co-yielding and intermixing believed to occur during formation of the desirable core-corona morphology. Thus, in some embodiments, the particles of the first metallic powder comprise a metallic composition having a fracture toughness (Kic) in the range of 10 to 150 MPa.m1 / 2, such as in the range of 40 to 150 MPa.m1 / 2. Kic is a bulk material property of a metallic composition, and can be measured by ASTM E1820.

[0124]

[0124] The methods of the disclosure are also particularly useful for upgrading and doping powders of oxygen- and / or carbon-sensitive metallic compositions. Compaction of such powders is preferably done without binders to avoid contamination of the metal composition, and methods for improving binderless cold compactibility are thus needed. In some embodiments, therefore, the metal particles of the first metallic powder comprise 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.

[0125]

[0125] In some embodiments, the metal particles of the first metallic powder comprise, or consist of, a metallic composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof. In some embodiments, the metal particles comprise, or consist of, alloys of titanium, tantalum, or niobium. In some embodiments, the metal particles of the first metallic powder comprise, or consist 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).

[0126]

[0126] In some embodiments, the metal particles of the first metallic powder comprise, or consist of, a superalloy, for example a nickel-based superalloy. Suitable nickel-based superalloys may comprise nickel in an amount of at least 50 wt.%, chromium and other alloying elements. In some embodiments, the metallic composition is an Inconel, for example Inconel 718.

[0127]

[0127] In some embodiments, the metal particles of the first metallic powder comprise, or consist of, a steel. Suitable steels may include tool steels, for example H13 grade steel, and stainless steels, for example stainless steel 410L or stainless steel 316.

[0128]

[0128] 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.

[0129]

[0129] 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.

[0130]

[0130] The methods of the disclosure are particularly useful for upgrading and doping powders with relatively spherical morphologies for cold compaction applications, since such materials (without modification) can be hard to consolidate due to the small interparticle interfaces. Thus, in some embodiments, the large metal particles in the first metallic powder are substantially spherical. As used herein, “substantially spherical” particles are spherical or near-spherical in shape, in contrast to the morphologies of non-spherical powders composed of irregular or flake particles. The morphology of particles may be quantified with a sphericity number, defined with respect to the 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.) In some embodiments, the sphericity of the large metal particles is greater than 0.7, or greater than 0.75.

[0131]

[0131] Certain metallic powders produced by gas atomization, for example, have substantially spherical particle morphologies. Commercial gas atomization processes generally produce a range of particle sizes, of which only a fraction may be commercially valuable (e.g. as powders for additive manufacturing). The out-of-specification particle sizes, including both oversized and undersized fractions, may thus be a low cost by-product, particularly considering that re-processing these fractions through gas atomization would require remelting the metallic composition. In some embodiments, therefore the metal particles of the first metallic powder are produced by gas atomization. The inventors have demonstrated that an oversized fraction (105-250 pm) of pre-alloyed Ti-6AI-4V particles produced by gas atomization can be upgraded by the present methods for cold compaction applications, despite the highly spherical morphology of the unmodified material.

[0132]

[0132] In other embodiments, providing the first metallic powder comprises comminuting and / or spheroidizing precursor metal particles to form the large particles of the first metallic powder, typically prior to combination with the second metallic powder. This particle modification step may include milling the precursor metal particles to reduce the particle size in suitable milling equipment. Spheroidization of the precursor particles may be conducted in any suitable equipment, for example impact blending apparatus which may be the same or different to the apparatus subsequently used to impact blend the combined first and second metallic powders and dopant powder. Impact blending, for example in a Hybridizer apparatus, may be used to spheroidize irregular-shaped particles to more spheroidal form, and such modified particles may be suitable as core particles for further modification according to the principles of the present disclosure. As a further advantage, it is expected that spheroidization by impact blending will harden the larger particles and that this may be helpful to compatibilize these particles with the small particles so that a desirable core-corona morphology is produced in the subsequent impact blending process step. Exemplary methods for comminuting and spheroidizing precursor metal particles are disclosed in the internation al patent application published as WO2024 / 130318A1. Second metallic powder

[0133]

[0133] The methods of the disclosure modify the morphology of the first metallic powder using a second metallic powder comprising small metal particles, and which thus has a d50 particle size less than that of the first metallic powder. In some embodiments, the d50 particle size of the second metallic powder is no more than 40%, or no more than 30%, or no more than 25% of the d50 particle size of the first metallic powder. In some embodiments, the d50 particle size of the second metallic powder is at least 4% of the d50 particle size of the first metallic powder. In some embodiments, the d50 particle size of the second metallic powder in the range of 10% to 25% of the d50 particle size of the first metallic powder.

[0134]

[0134] The second metallic powder may in some embodiments have a d50 particle size of between 0.1 pm and 100 pm, or between 5 pm and 100 pm, such as between 5 pm and 60 pm, for example between 10 pm and 40 pm. Optionally, the second metallic powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution.

[0135]

[0135] It is not essential that all particles of the second metallic powder are smaller than all particles of the first metallic powder. Nevertheless, the particle size distributions are typically sufficiently differentiated that a combination of the first and second metallic powders will have a multimodal particle size distribution comprising at least a first modal peak corresponding to the first metallic powder and a second modal peak corresponding to the second metallic powder. In some embodiments, the d50 particle size of the second metallic powder is less than the d10 particle size of the first metallic powder. In some embodiments, the d90 particle size of the second metallic powder is less than the d10 particle size of the first metallic powder.

[0136]

[0136] The particles of the second metallic powder may comprise any type of metallic composition, including commercially pure metals, metal alloys and metal matrix composites. In some embodiments, the metallic composition is a non-ferrous metallic composition. The small metal particles of the second metallic powder and the large metal particles of the first metallic powder may have a different metallic composition or substantially the same metallic composition. Substantially the same metallic composition means the same metal or alloy composition with variations only due to permissible manufacturing variability and / or minor contamination.

[0137]

[0137] In embodiments where the large particles of the first metallic powder are produced by comminution and spheroidizing of an irregular metallic precursor, the small particles of the second metallic powder may comprise a fines fraction produced in the comminuting.

[0138]

[0138] Alternatively, a second metallic powder with a different metallic composition may be preferred, to provide a cold compactible powder with a composition which differs from that of the first metallic powder.

[0139] Whereas the first metallic powder may in some embodiments advantageously be composed of a high yield strength metallic composition, the smaller particles of the second metallic powder may be formed of either a high yield strength material, for example as disclosed herein for the first metallic powder, or a lower strength, more ductile metallic material. In some embodiments, therefore, the metallic composition of the large metal particles has a yield stress that is greater than or about equal to that of the metallic composition of the small metal particles. For example, large pre-alloyed Ti-6AI-4V spherical particles may be modified with either high yield strength pre-alloyed Ti-6AI-4V small particles or relatively soft commercially pure titanium small particles.

[0139]

[0140] Without wishing to be limited by any theory, it is proposed that the mechanical properties of the large and small particles should preferably be matched to allow some mutual yielding and seizing between their surfaces during impact, so that a core-corona morphology with metallurgical bonding between core and protrusions can be produced. In principle, only one of the surfaces must yield to allow joining of the particles. However, if the small particles are substantially harder than the larger particles, they are less likely to co-yield and more likely to penetrate and embed (i.e. via mechanical rather than metallurgical bonding). In some embodiments, therefore, the large particles of the first metallic powder are harder, or of equivalent hardness and of greater or equivalent yield strength, than the smaller particles of the second metallic powder. However, it is contemplated that the small particles may still deform sufficiently to form a desirable core-corona morphology, despite having a greater intrinsic yield strength, if the resolved shear stress of the small particles is higher during impact. This is possible because the impact surface area of the small particles may be less than the surface it impacts on the larger particle. Thus, in a collision between a large and a small particle, the smaller particle will see a larger stress for at least part of the impact than the larger particle.

[0140]

[0141] The small particles of the second metallic powder may have any suitable particle shape. In some embodiments, they may be substantially spherical particles, for example as produced by gas atomization. The inventors have demonstrated that an undersized fraction (5-25 pm) of pre-alloyed Ti-6AI-4V particles produced by gas atomization can be used to form the protrusions of non-spherical particles. In other embodiments, the small particles may have non-spherical, such as irregular, blocky or angular, morphologies.

[0141] Dopant powder

[0142]

[0142] The methods of the disclosure combine and impact blend (in one or more impact blending steps) a dopant powder with metallic powders to produce cold compactible doped metallic powders having a non-spherical particle morphology, which facilitates consolidation during cold compaction, and with a dopant composition adhered to the non-spherical particles.

[0143]

[0143] Advantageously, this approach allows both the amount and distribution of the dopant composition to be controlled in the metallic articles resulting from cold compaction of the cold compactible doped metallic powder, subsequent metallurgical processing of the porous compact, or direct metallurgical processing of the cold compactible doped metallic powder, i.e. without an intermediate cold compaction step. As will be discussed in greater detail hereafter, metallurgical processing of the porous compact or cold compactible doped metallic powder may involve one or more selected from extrusion, friction stir processing, other solid state forming techniques such as forging and rolling, sintering, additive manufacturing via directed energy deposition (DED) such as WAAM, Laser Wire-Feed Metal Additive Manufacturing (LWAM), fusion-based processes, and other metallurgical techniques involving melt-processing (fusion) of metallic feedstock.

[0144]

[0144] The dopant composition may thus comprise any of a wide range of reactive or non-reactive materials capable of beneficially modifying the properties of metallic articles produced via such methods. Non-limiting examples of various classes of such dopants will now be disclosed.

[0145]

[0145] In some embodiments, the dopant composition is a grain-refining dopant composition, meaning that it nucleates or otherwise modulates the crystallisation of metallic compositions from the melt to produce smaller and / or more isotropic grain morphologies. Specifically, it may be a grain-refining dopant composition for the metallic composition of the first metallic powder. Thus, when that metallic composition, which may remain substantially unchanged in compacts or other solid-state processed articles made from the cold compactible doped metallic powder, is melt processed, the dopant composition modulates the grain structure of the resultant resolidified metallic composition.

[0146]

[0146] In some embodiments, the dopant composition is an inoculating dopant composition, also known as an inoculant, specifically an inoculating dopant composition for the metallic composition of the first metallic powder. Inoculating dopants are capable of reacting with the metallic composition of a chosen feedstock to form a new phase, such as a ceramic or intermetallic compound, when the metallic composition melts and resolidifies. Inoculation mechanisms are not always understood and are alloy specific. However, without wishing be limited by any theory, in some embodiments, the inoculating dopant composition reacts via a peritectic reaction with the molten metallic composition forming a liquid and solid phase field with the solid nuclei facilitating heterogenous nucleation of the remaining melt to form the new metallurgical phase as a solid. In other embodiments, the inoculating dopant composition dissolves in or reacts with the molten metallic composition, with a new phase precipitating from the melt acting as heterogenous nucleation sites. In some embodiments, the new phase may provide nucleation sites for re-crystallisation of the metallic composition when cooled, thus beneficially modulating the grain structure. Thus, in some embodiments the dopant composition is a grain-refining dopant composition operating via an inoculation mechanism. In some embodiments, the new phase may impart desirable mechanical properties to the resolidified metallic composition, for example via secondary strengthening effects.

[0147]

[0147] In other embodiments, the dopant composition is a low-reactivity dopant composition which remains at least partially, and typically predominantly, unreacted when the metallic composition of the large metal particles melts. As used herein, a low-reactivity dopant composition refers to a solid dopant composition which does not undergo bulk reaction with, or full dissolution into, a metallic composition when melted, although surface reactions at the interface between dopant particles and the melt may occur, for example during wetting. In some embodiments, the low-reactivity dopant composition disperses in the melt, preferably as nanoparticles, and provides heterogenous nucleation sites for solidification without excessive undercooling of the metallic composition when cooled, thus beneficially modulating the grain structure by encouraging preferred solidification reactions. The low-reactivity dopant composition may be selected to have a small lattice mismatch compared to the recrystallising metallic composition, thus enhancing nucleation of the cooling melt. Thus, in some embodiments the dopant composition is a grain-refining dopant composition which itself provides heterogeneous nucleation sites that modulates grain structure, i.e. without reliance on formation of a new nucleating phase (hereafter a low-reactivity grain-refining dopant).

[0148]

[0148] In various additive manufacturing techniques, such as DED, a metallic feed is melted and selectively directed onto the metallic substrate, creating a moving melt pool on the substrate which builds up the additively manufactured article in successive build layers. For example, wire arc additive manufacturing (WAAM, a type of DED) uses a wire as feed. A notable issue with such additive manufacturing techniques is that epitaxial grain growth in the heat flow direction (build direction) across build layers can generate undesirable non-isotropic properties in the resultant additively manufactured metallic article. Well-controlled introduction of grain-refining dopant compositions, selected to operate either via an inoculation mechanism or as a low-reactivity grain-refining dopant, to the metallic feed can thus improve isotropic properties by inhibiting the formation of larger epitaxial grains in favour of nucleated equiaxed grains.

[0149]

[0149] In some embodiments, the dopant composition is a low-reactivity dopant composition which is wetted and encapsulated by the molten metallic composition, and the small particles of the low-reactivity dopant composition are incorporated in a sufficient quantity to become the secondary, typically discontinuous, phase of a metal matrix composite. The matrix phase may be derived mainly from the metallic composition of the large metal particles, and to a lesser extent from the metallic composition of the small metal particles. In such embodiments, the dopant composition does not necessarily modulate the grain structure of the metal matrix, but instead is selected for its capacity to impart desirable mechanical or other properties to the resultant metal matrix composite.

[0150]

[0150] Given that the methods disclosed herein are applicable to various different metallic compositions, and doping compositions of different classes compatible with those metallic compositions (e.g. grain refiners of inoculating and low-reactivity grain-refining dopant types, wettable particles for metal matrix composites), it will be appreciated that a wide range of doping compositions can be used. Dopant compositions for grain refining, metal matrix composite formation, and other doping applications have previously been reported for many different metallic compositions, and a person skilled in the art is able to apply the principles disclosed herein to select suitable dopant compositions for use with a given metallic composition in the methods disclosed herein without undue burden.

[0151]

[0151] Computational methods can also be used to identify suitable dopant compositions. For example, computational alloy analysis using software such as Themo-Calc™ can be used to define desired phase transformations of various metallic compositions during processing, e.g. for e.g. grain-refining of that composition, and thus to select suitable dopant compositions. Such software can predict phase transformation and the stability of particular phases and the promotion of favoured solidification reactions in accordance with detailed materials databases for various alloy systems, such as for ferrous, aluminium, titanium nickel, etc based alloys.

[0152]

[0152] In some embodiments, the dopant composition when incorporated into the non-spherical particles of the cold compactible metallic powder is capable of remaining solid and substantially unreacted at least until the metallic composition of the large metal particles is melted, whereafter it may react with, dissolve into or disperse into the molten metallic composition of the large metal particles. The dopant composition may therefore have one or both of the following properties: (i) thermally stable at least up to the melting point of the metallic composition of the large metal particles (i.e. in isolation, under vacuum), and (ii) a melting point higher than the melting point of the metallic composition of the large metal particles.

[0153]

[0153] In some embodiments, the dopant composition is selected from the group consisting of a ceramic, an intermetallic compound, a metal or alloy with melting point higher than the first metallic powder, and a non-metal element.

[0154]

[0154] In some embodiments, the dopant composition comprises at least one non-metal element, for example selected from boron, carbon, oxygen, nitrogen, silicon, hydrogen, and combinations thereof, optionally present together with one or more metals as a boride, carbide, oxide, nitride, silicide, silicate, hydride or combination thereof. In some embodiments, the dopant composition comprises at least one non-metal element selected from boron, carbon, oxygen, nitrogen, silicon, and combinations thereof.

[0155]

[0155] In some embodiments, the dopant composition comprises at least one metal element. In some embodiments, the dopant composition and the metallic composition of the large metal particles have at least one metal element in common, i.e. they comprise the same metal element. For example, if the first metallic powder comprises titanium, e.g. titanium alloy, the dopant composition may also comprise titanium.

[0156]

[0156] In some embodiments, the dopant composition is a ceramic. As used herein, a ceramic is an inorganic, non-metallic solid and includes metal oxides, metal carbides, metal nitrides, metal silicides, metal silicates, metal borides, ternary and quaternary phases of these materials (with or without solute additions) and inorganic phases of thermodynamically stable compounds that do not contain metallic elements, including boron-carbides, boron-nitrides, and oxynitrides. Ceramics are typically highly thermodynamically stable materials.

[0157]

[0157] The ceramic may comprise at least one non-metal element, such as boron, carbon, oxygen, nitrogen, silicon, and combinations thereof, preferably selected from boron, carbon, oxygen, nitrogen, silicon and combinations thereof. The ceramic may therefore be selected from a boride, carbide, oxide, nitride, silicide, silicate, and combinations thereof. Suitable combinations may be, for example, oxyborides, borosilicides, cubic boron nitride and boron carbides. The ceramic preferably comprises at least one metal element. In some embodiments, the ceramic and the metallic composition of the large metal particles have at least one metal element in common, i.e. they comprise the same metal element. For example, if the first metallic powder comprises titanium, e.g. titanium alloy, the ceramic may also comprise titanium.

[0158]

[0158] In some embodiments, the ceramic comprises boron and / or carbon. The ceramic may therefore be a boride e.g. metal boride or a carbide, e.g. metal carbide.

[0159]

[0159] In some embodiments, the ceramic comprises boron. The ceramic may therefore be a metal boride. In some embodiments, the metal boride and the metallic composition of the large metal particles comprise the same metal element. For example, if the first metallic powder comprises titanium alloy, the ceramic may be a titanium boride, such as titanium diboride (TiB2). However, it is also envisaged that borides of different metals may be used, depending on the intended dopant action. For example, aluminium magnesium boride (BAM) and other refractory metal borides can be formed into a composite with titanium and other metals. Titanium boride can be an inoculant for aluminium-based alloys.

[0160] Boron, optionally introduced as a metal boride, is particularly useful as an inoculating grainer refiner for titanium alloys. Without wishing to be bound by any theory, boron reacts with titanium to form titanium monoboride (TiB). TiB2 inoculating dopant may partly or fully dissolve via a peritectic reaction with molten Ti to form TiB needles.

[0160]

[0161] In some embodiments, the dopant composition is an intermetallic compound. For example, the intermetallic compound may be an aluminide. Without wishing to be bound by any theory, aluminides may be suitable grain-refining dopants for some metallic compositions.

[0161]

[0162] In some embodiments, the dopant composition is a metal or alloy with melting point higher than the first metallic powder. The high melting metal or alloy may thus remain at least partially solid when the metallic composition of the large metal particles melts, thus acting as a grain-refining dopant by nucleation or other mechanisms (i.e. a low-reactivity grain-refining dopant). For example, it has been reported that Ti-10AI-25Nb can act as an effective grainrefining dopant for Ti-6AI-4V due to its high melting point and small lattice mismatch (Kennedy et al. Materials Letters: X, Volume 8, December 2020, 100057).

[0162]

[0163] In some embodiments, the dopant composition is a non-metal element, i.e. in its elemental form. For example, the dopant composition may comprise at least one selected from elemental boron, elemental carbon and elemental silicon. Such elemental compositions may react with the metallic composition of the large metal particles when melted, thus acting as inoculating grain-refining dopant compositions.

[0163]

[0164] The dopant powder comprises small particles of the dopant composition, and thus has a d50 particle size less than that of the first metallic powder. The dopant composition is intended to adhere to the core and / or the protrusions of the non-spherical particles following the impact blending process step(s), for example by embedment. The dopant powder may thus have a particle size suitable for this purpose. In some embodiments, the the d50 particle size of the dopant powder is less than 20%, or less than 10%, such as less than 5%, for example less than 3%, of the d50 particle size of the first metallic powder. In some embodiments, the d50 particle size of the dopant powder is less than the d50 particle size of the second metallic powder, for example less than 50%, or less than 30%, or less than 20%, of the d50 particle size of the second metallic powder.

[0164]

[0165] It will be appreciated that suitable absolute particle sizes of the dopant powder may be dependent on the particle sizes of the first metallic powder and the intended function of the dopant, among other variables. Low-reactivity grain-refining dopants, which may be required to disperse in the melt and nucleate re-solidification during subsequent melt-processing, may thus in some embodiments be in the nanoparticle size range. Inoculating dopant compositions may be present in larger particles, for example microparticle sizes. Dopant particles destined to become the discontinuous phase of an MMC will have particle size and morphology suited to that application. In some embodiments, however, the d50 particle size of the dopant powder is between 0.1 and 10 pm, or between 0.5 and 5 pm, such as between 2 and 5 pm. Optionally, the dopant powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution.

[0165]

[0166] There is no need for the dopant powder particles to be spherical, and in some embodiments they are irregular in shape.

[0166]

[0167] In some embodiments, the small particles of the dopant powder are destined to be embedded in the surface of the core, and optionally also in the protrusions of the non-spherical particles, following the impact blending process step(s). The small particles of the dopant composition may thus be harder and less ductile than the large metal particles, allowing the dopant particles to penetrate the surface of the large metal particles, typically without substantial co-yielding of the small particles of dopant composition. At least some of the envisaged dopant compositions, for example ceramics, are inherently hard and brittle materials, facilitating embedment in the metallic composition of the large and small metal particles during impact blending.

[0167]

[0168] However it is not excluded that the small particles of the dopant composition may be softer and more ductile than the large metal particles. In such embodiments, the dopant composition may adhere as a film or coating on at least part of the surface of the core and / or on the protrusions of the non-spherical particles following the impact blending process step(s). The particle striking action occurring during impact blending may initially adhere small ductile particles to large metal particles and subsequently smear the ductile composition across the large particle surface.

[0168] Impact blending process steps

[0169]

[0169] The methods disclosed herein include a step of subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder.

[0170]

[0170] As used herein, an impact blending process step refers to any dry powder blending process which causes high velocity impacts between the particles with sufficient intensity to (i) adhere, and preferably metallurgically bond, the small metal particles to the large metal particles, and (ii) adhere, and preferably physically embed, the dopant composition to the metal particles. Suitable impact blending processes include rotational impact blending, sometimes referred to as hybridization, and various other dry particle coating technologies.

[0171] In some embodiments, at least one and preferably each impact blending process step is a rotational impact blending process. In such a process, a rotor is rotated at high speed in an impact blending chamber such that impact blades or other mechanical elements mounted on the rotor strike the powder particles at high velocity. A rotational impact blending process may be performed with a rotor-stator impact blending apparatus, which in some embodiments comprises a cylindrical stator and a rotor operable to rotate within the stator. The rotor may comprise a plurality of impact blades having a particle-striking impact face. The impact blades may have an outer edge at the periphery of the rotor, which may be separated from the rotor by a suitable gap, for example between 1 mm and 5 mm. In use, particles subjected to rotational impact blending are stuck at high velocity by the impact face of the rotating blades and subjected to high shear forces in the gap between the outer edges of the rotating blades and the stator.

[0171]

[0172] As seen in Figures 1, 2 and 3, the rotational impact blending process may be performed in an apparatus 100 that includes an impact blending chamber 108, the cylindrical outer wall of which is defined by a stator 110, and a rotor 112 which rotates in the impact blending chamber. Impact blending chamber 108 is enclosed by removable front panel 113 and rear wall 117. The generally disc-shaped rotor includes a plurality of radially oriented impact blades 114 having an impact face 116 and an outer edge 118 at the 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 117 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 blending chamber and an exit port 126 located in front panel 113 which is directed to the centre of the impact blending chamber. Powder is added to the impact blending chamber via inlet 128 and discharged through powder outlet port 133, also located in front panel 113, via discharge valve 137 to powder outlet 135. Cooling water is circulated through the stator via coolant ports 130, 132.

[0172]

[0173] In use, the required combination of powders is fed from a sealed vessel into the impact blending 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 particles 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 powder continuously circulates through the recirculation conduit during the impact blending. The accelerated particles thus collide with each other, strike the stator and impact blades 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.

[0173]

[0174] Such apparatus, called the Nara Hybridization System (NHS-0, NHS-1, NHS-3 depending on size), is commercially available from Nara Machinery Co., Ltd. Other suitable impact blending apparatus may include the Mechanofusion system and Cyclomix from Hosokawa Micron Ltd..

[0174]

[0175] A few apparatus and process parameters of the rotational 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 modifying pre-alloyed Ti-6AI-4V particles with particle sizes of about 105 to 250 pm, and it is expected that a similar gap distance will be suited for impact blending particles with a size in the range of 1 pm to 500 pm. Thus, in some embodiments, 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.

[0175]

[0176] 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, particularly between the large and small metal particles. The inventors have found that peripheral rotor velocities in the range of about 37.1 m / s to 61.8 m / s (corresponding to 6,000 rpm to 10,000 rpm in the NHS-0) were suitable for modifying pre-alloyed Ti-6AI-4V large particles with particle sizes of about 105 to 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.

[0176]

[0177] A further significant parameter is the impact blending time, which should be sufficient to adhere the particles as required in the impact blending process step, but not so long that, for example, the desired non-spherical particles become spheroidized. Particularly when the second metallic powder comprises a relatively ductile metallic composition, there is a risk that excessive impact blending times will cause the small particles to film across the entire surface of the large core particles, so that an undesirable core-shell morphology is obtained.

[0177]

[0178] In some embodiments, the powders are impact blended in at least one impact blending process step, and optionally in each impact blending process step, for a time between 1 second and 10 minutes, such as between 30 seconds and 5 minutes. The time required to create a desirable particle morphology may be inversely correlated with the intensity of the impact blending conditions. The inventors have found that impact blending times of as little as a few seconds may be sufficient to create a desirable morphology when modifying pre-alloyed Ti-6AI-4V particles with particle sizes of about 105 to 250 pm with small Ti-6AI-4V particles at very high impact conditions (peripheral rotor velocity of about 62 m / s). Under more moderate blending conditions (peripheral rotor velocity of about 37 - 49 m / s), impact blending times of 30 seconds to 4 minutes were typically suitable.

[0178]

[0179] The powders may be impact blended in a dry inert gas atmosphere. This advantageously limits oxidation of the metallic powders during the impact blending. Without limitation by theory, it is proposed that freshly exposed metal surfaces of the metal 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.

[0179]

[0180] The first metallic powder, the second metallic powder and the dopant powder may be subjected to one, or more than one, impact blending process steps to produce the cold compactible, doped metallic powder comprising non-spherical particles.

[0180]

[0181] In some embodiments, the method comprises combining the first metallic powder, the second metallic powder and the dopant powder, and subjecting the combined powders to an impact blending process step to produce the cold compactible metallic powder. In such embodiments, only one impact blending process step, typically involving continuous rotational impact blending, may be required to adhere a plurality of the small metal particles to the large metal particles, and to adhere the dopant composition to the metal particles, despite the dissimilar nature of the particle compositions and adhesion mechanisms involved.

[0181]

[0182] In some embodiments of the single-step process, the plurality of small metal particles adhere to the large particles via metallurgical bonding. In some embodiments, a plurality of small particles of the dopant composition are embedded in the surface of the large metal particles, and optionally also in the surfaces of the plurality small metal particles adhered to and protruding from the large metal particles. The inventors have found that such adhesion modes can be performed simultaneously to produce non-spherical particles comprising one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition embedded in the surfaces of both the core and the protrusions.

[0182]

[0183] Figure 4 schematically depicts a single impact blending process step 400 which converts a combined powder 410, comprising large metal particles 412 (derived from the first metallic powder), small metal particles 414 (derived from the second metallic powder) and small particles of dopant composition 415 (derived from the dopant powder), into non-spherical particles 416. Particles 416 comprise one of the large metal particles 412 as core 418, a plurality of the small metal particles 414 as protrusions 420 from the core, and a plurality of small particles of dopant composition 415 embedded in core 418 and optionally also protrusions 420. Some of the protrusions (420a) may comprise a single small particle 414 while other protrusions (420b) may comprise a cluster of small particles 414.

[0183]

[0184] In other embodiments, the method comprises subjecting the first metallic powder and the dopant powder to a first impact blending process step to produce a modified powder in which the dopant composition is adhered to the large metal particles. The modified powder is then combined with the second metallic powder, and the combined powders are subjected to a second impact blending process step to produce the cold compactible metallic powder. In such embodiments, two sequential impact blending process steps, both typically involving continuous rotational impact blending, are used to sequentially adhere the dopant composition to the large metal particles, and then to adhere a plurality of the small metal particles to the doped large metal particles.

[0184]

[0185] In some embodiments of the two-step approach, a plurality of small particles of the dopant composition are embedded in the surface of the large metal particles, and thus ultimately in the core of the non-spherical particles. In some embodiments, the plurality of small metal particles adhere to the large particles via metallurgical bonding. The small metal particles adhered to and protruding from the large metal particle core in the non-spherical particles may thus be substantially free of dopant, particularly when the dopant composition is quantitatively and non-reversibly adhered to the large metal particles in the first impact blending process step. Despite the presence of the dopant composition on the large metal particles, typically embedded in the surface thereof, the inventors have found that the small metallic particles may be securely adhered to the modified large metal particles in the second impact blending process step.

[0185]

[0186] Figure 5 schematically depicts a two-stage process comprising first impact blending process step 500a and second impact blending process step 500b. First impact blending process step 500a converts a combined powder 510 comprising large metal particles 512 (derived from the first metallic powder) and small particles of dopant composition 515 (derived from the dopant powder) into a modified powder comprising doped particles 517. Doped particles 517 comprise one of the large metal particles 512 and a plurality of small particles of dopant composition 515 embedded in the surface of large metal particles 512.

[0186]

[0187] After the first impact blending process step, the modified powder produced in process step 400 and the dopant powder are combined. Second impact blending process step 500b converts the combined powder 410 comprising doped particles 517 (derived from the modified powder) and small metal particles 514 (derived from the second metallic powder) into non-spherical particles 516. Particles 516 comprise one of the large metal particles 512 as core 518, a plurality of the small metal particles 514 as protrusions 520 from the core, and small particles of dopant composition 515 embedded in core 518. Some of the protrusions (520a) may comprise a single small particle 514 while other protrusions (520b) may comprise a cluster of small particles 514. Protrusions 520 may be substantially free of adhered dopant particles 515.

[0187]

[0188] In still other embodiments, the method comprises subjecting the second metallic powder and the dopant powder to a first impact blending process step to produce a modified powder in which the dopant composition is adhered to the small metal particles. The dopant composition may be either embedded in or coated on the small particles. The modified powder is then combined with the first metallic powder, and the combined powders are subjected to a second impact blending process step to produce the cold compactible metallic powder. In such embodiments, two sequential impact blending process steps, both typically involving continuous rotational impact blending, are used to sequentially adhere the dopant composition to the small metal particles, and then to adhere a plurality of the doped small metal particles to the large metal particles.

[0188]

[0189] The first metallic powder, the second metallic powder and the dopant powder may be combined in relative amounts suitable to achieve the desired morphology and amount of dopant composition in the non-spherical particles. In some embodiments, the amount of dopant powder relative to the combined amounts of first and second metallic powders, is less than 20 vol%, or less than 10 vol.%, such as less than 7 vol.%, for example less than 5 vol.%, or between 0.1 vol.% and 10 vol.%, such as between 0.5 vol.% and 10 vol.%, for example between 2 vol.% and 7 vol.%. Higher amounts of dopant powder may be useful, for example, where the dopant composition is destined to become the discontinuous phase of a MMC, for example in an amount of between 2 vol.% and 10 vol.%, such as between 2 vol.% and 7 vol.%. By contrast, grain-refining dopant compositions such as inoculants may be present in smaller amounts, such as less than 2 vol.%, or less than 1.5 vol.%, for example less than 1 vol.%, or between 0.1 vol.% and 2 vol.%, for example between 0.2 vol.% and 1.5 vol.%, all relative to the combined amounts of first and second metallic powders.

[0189]

[0190] As described above, it is typically envisaged that small particles of the dopant composition will be embedded in the large and / or small particles, as combined in the non-spherical particles of the cold compactible doped metallic powder. This is expected when the dopant composition is a hard material, such as a ceramic. However it is not excluded that the certain dopant compositions may instead adhere as a film or coating on at least part of the surface of the core and / or on the protrusions of the non-spherical particles following the impact blending process step(s).

[0190]

[0191] The amount of dopant composition incorporated into non-spherical particles of the cold compactible metallic powder is typically determined by the relative amount of dopant powder combined with the first and second metallic powders in the impact blending, provided that care is taken to avoid adding more dopant powder than can be adhered (typically by embedment) in the metallic particles. In some embodiment, the cold compactible metallic powder comprises the dopant composition, present in non-spherical particles as disclosed herein, in an amount of less than 10 vol.%, such as less than 7 vol.%, for example less than 5 vol.%, or less than 3 vol.%, or less than 2 vol.%. In some embodiments, for example for MMC applications, the amount is between 0.1 vol.% and 10 vol.%, such as between 0.5 vol.% and 7 vol.% or between 2 vol.% and 7 vol.%. In some embodiments, for example for inoculant applications, the amount is less than 2 vol.%, or less than 1.5 vol.%, such as between 0.2 vol.% and 1.5 vol.%.

[0191]

[0192] Impact blending of the first metallic powder and second metallic powder 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 powder product and thus derives from the modified particle morphology in the powder.

[0192]

[0193] Without wishing to be limited by any theory, the inventors propose 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 bonding is distinguished from mere mechanical embedding of the type produced when surface-modifying soft non-metallic large particles with hard small particles (as for the dopant, in some cases). The large and small metal particles are thus effectively integrated into a single non-spherical metallic particle as core and protrusions, with dopant composition adhered thereto. 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.

[0193]

[0194] The non-spherical particles typically comprise many protrusions distributed around the periphery of the core particle. Each protrusion may comprise a single small metal particle or a cluster of small metal particles. The small metal 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 small particle 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 metal 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.

[0194]

[0195] 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 number of the non-spherical particles is below 0.85, or below 0.8, such as in the range of 0.4 to 0.85, or in the range of 0.5 to 0.8. In some embodiments, the convexity number of the non-spherical particles is below 0.75, or below 0.7. Convexity numbers of particles can be calculated from scanning electron microscopy (SEM) images with image processing software commonly used in materials science, such as Image J image-processing software which is a general-purpose open-source image analysis platform used for SEM and optical microscopy.

[0195]

[0196] It is not required that all particles in the cold compactible, doped 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 doped metallic powder thus comprises at least 20 wt.%, such as at least 50 wt.%, or at least 60%, of the non-spherical particles comprising one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions (which typically have a convexity number of less than 0.85, such as less than 0.8).

[0196]

[0197] In some embodiments, the cold compactible doped metallic powder comprises non-spherical particles comprising a large titanium or titanium alloy particle as a core and a plurality of small metal particles as protrusions from the core. The small metal particles may also comprise titanium or titanium alloy. The small metal particles may be metallurgically bonded to the large titanium or titanium alloy particle. In some embodiments, the dopant composition is a grain-refining dopant composition for titanium or titanium alloy, such as an inoculating grainrefining dopant composition for example a boride. In some embodiments, small particles of the dopant composition are embedded in surfaces of the core, and optionally also in the protrusions. Method of producing a porous metallic article

[0197]

[0198] Also disclosed herein is a method of producing a porous doped metallic article. The method comprises providing a cold compactible doped metallic powder comprising non-spherical particles. The non-spherical particles comprise a large metal particle as a core, a plurality of small metal particles as protrusions from the core, and a dopant composition adhered to at least one of the core and the protrusions.

[0198]

[0199] In some embodiments, the small metal particles are metallurgically bonded to the large metal particle along at least a portion of the interface. In some embodiments, a plurality of small particles of the dopant composition are embedded in the surface of the core, and optionally also in the surfaces of the protrusions.

[0199]

[0200] The non-spherical particles may be produced by the methods previously disclosed herein.

[0200]

[0201] The method includes a step of subjecting the cold compactible doped metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible doped metallic powder, thereby producing a porous doped metallic article, otherwise referred to herein as a compact.

[0201]

[0202] 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 with sufficient structural integrity to withstand further processing, e.g. via sintering, extrusion 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.

[0202]

[0203] In cold isostatic pressing, the metallic powder is sealed in a forming mould with low deformation resistance, such as a rubber bag, and liquid pressure is applied to the mould. The powder is thus compressed uniformly over the entire surface of the compact because the liquid pressure is transmitted through the flexible mould. In cold die pressing, also known as metal mould pressing, the metallic powder is filled into a mould (cold die) between upper and lower punches. The powder is then compressed by narrowing the distance between the upper and lower punches. In direct powder rolling, the metallic powder is compressed between a pair of rollers to form a continuous green strip. In metal injection moulding, the metallic powder is blended with a binder to create an injectable feedstock which is injection moulded to produce a preform. After moulding, the preform must undergo further processing to remove the binder.

[0203]

[0204] In some embodiments, the cold compactible doped metallic powder is subjected to the cold compaction process in the absence of an organic binder, or in the absence of a non- metallic binder, or in the absence of any binder. The cold compactible doped metallic powders provided by the present disclosure are particularly useful for binderless cold compaction processes because the adhesion of the particles is enhanced by the core-corona particle morphology.

[0204]

[0205] 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 6 schematically depicts some proposed interlocking modes that may occur between adjacent non-spherical particles 616 in the porous metal compact. Non-spherical particles 616 may be particles 416 or 417 as described herein with reference to Figures 4 and 5 respectively. At some interparticle interfaces, such as interface 610ab between particles 616a and 616b, the particles engage via protrusions 620a and 620b of both particles. At other interparticle interfaces, such as interface 610bc between particles 616b and 616c, the engagement of the particles is assisted by protrusion 620c of one particle only. Where the protrusions comprise a relatively soft metal (e.g. CP Ti), some joining of the protrusions on adjacent particles may also assist the consolidation of the particles in the compact. However, such joining is not seen for non-spherical particles with harder protrusions (e.g. Ti-6AI-4V) where excellent compaction properties were nevertheless obtained.

[0205]

[0206] Despite the doping of the non-spherical particles with a dopant composition on the core and / or protrusions, including via embedment of hard, low ductility small particles of dopant composition in the core and / or protrusion surfaces, the cold compactible doped metallic powder may still be consolidated under pressure to form a compact with desirable properties.

[0206]

[0207] The cold compaction process consolidates the cold compactible doped metallic powder to produce a porous doped 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. Compaction of non-spherical particles comprising cores and protrusions formed from high yield strength pre-alloyed Ti-6AI-4V particles can produces compacts with good mechanical properties when a compact density of about 71% of theoretical density (based on the density of Ti-6AI-4V) is exceeded. In some embodiments, therefore, the porous doped 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.

[0207]

[0208] Because the dopant composition is adhered to the surface of the non-spherical particles, typically via distributed embedment of multiple small particles of dopant composition over the entire surfaces of the core and / or protrusions, the dopant composition may be consistently distributed throughout the porous doped metallic article, at the interfaces between the consolidated particles. The amount of dopant composition in the porous doped metallic article can also be accurately controlled, based on the amount of dopant powder combined with the two metallic powders in the impact blending process steps.

[0208]

[0209] By contrast, cold compaction of a powder comprising a physical mixture of spherical or non-spherical metallic particles and particles of dopant composition is susceptible to producing a porous metallic compact with a poorly controlled distribution of dopant composition. This may be caused by non-homogeneous mixing, or segregation, of the metallic and dopant particles in the bulk powder subjected to compaction.

[0209]

[0210] The cold compactible doped metallic powder may be compacted at any pressure sufficient to consolidate the cold compactible doped metallic powder and thus form a porous doped metallic article. The cold compactible doped metallic powders of the present disclosure, comprising non-spherical particles with a core-corona morphology, 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 and protrusions formed from high yield strength pre-alloyed Ti-6AI-4V particles, doped with TiB2, may be consolidated to form robust compacts at pressures of only 380 MPa, with success expected at still lower pressures if attempted. By contrast, the unmodified pre-alloyed Ti-6AI-4V particles were not cold compactible 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 doped metallic powder is compacted at a pressure below 450 MPa, or below 400 MPa, such as below 350 MPa, for example below 300 MPa.

[0210]

[0211] The porous doped metallic article may have any configuration, based on the intended application. In some embodiments, the porous doped metallic article is a rod-shaped billet, for example a cylindrical rod billet. Such a configuration may be produced, for example, via cold isostatic pressing in a suitably configured, elongated mould. In other embodiments, the porous doped metallic article is a porous sheet. Such a configuration may be produced by direct powder rolling. By contrast, spherical powders may be poorly adapted for consolidation by direct powder rolling.

[0211] Further metallurgical processing

[0212]

[0212] The porous doped metallic articles produced by the compaction methods disclosed herein are typically not final metal products but will instead be further metallurgically processed.

[0213]

[0213] In some embodiments, the porous doped metallic article, for example configured as a rod-shaped billet, may be extruded to form a doped metallic extrudate.

[0214]

[0214] The present disclosure therefore provides a method of producing a doped metallic extrudate, comprising subjecting a cold compactible, doped metallic powder as disclosed herein, or as produced by the methods disclosed herein, to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous metallic billet, and extruding the porous metallic billet to form a doped metallic extrudate.

[0215]

[0215] In some embodiments, the porous metallic billet is subjected to an incubation heattreatment step between its formation in the cold compaction process and its subsequent extrusion. The incubation step may be conducted at a temperature well below the melting point of the metallic composition, and the dopant thus typically remains unreacted, or at least incompletely reacted, on the compacted powder particles within the porous metallic billet. Without wishing to be limited by any theory, the incubation step may improve the electrical conductivity of the porous metallic billet, thus facilitating the subsequent induction heating required in the extrusion process.

[0216]

[0216] The doped metallic extrudate is preferably fully dense, i.e. it is no longer significantly porous, due to the extrusion. In some embodiments, the doped metallic extrudate has a diameter of between about 1.5 and 20 mm diameter, such as between about 3.2 and 12 mm diameter. In some embodiments, the doped metallic extrudate is a doped metallic wire.

[0217]

[0217] In some embodiments, the porous metallic billet is extruded in a continuous rotary extruder. In continuous rotary extruders, a metal feedstock such as a rod-shaped billet may be fed into a groove on the periphery of a continuously rotating wheel. The billet is drawn into an arcuate passageway defined by the groove of the rotating wheel and a fixed shoe, with the resultant deformation causing frictional heating, and contacted with an abutment that protrudes into the wheel groove. The metal is thus plastically deformed at elevated temperature below its melting point and forced through the extrusion die to produce an elongated extrudate with cross-sectional profile corresponding to the shape of the extrusion die. Multiple rod-shaped billets may be fed sequentially so as to produce an extrudate of any desired length.

[0218] The porous metallic billet may be pre-heated before engagement with the continuously rotating wheel. The pre-heating may lower the flow stress during the subsequent extrusion process.

[0218]

[0219] In some embodiments of the presently disclosed methods, the porous doped metallic article, such as a rod-shaped billet, is extruded via a continuous rotary extrusion process as disclosed in US patent 9,468,960. US patent 9,468,960 discloses a continuous rotary extrusion apparatus and process suitable for extrusion of high temperature formable and optionally oxygen-sensitive metals such as titanium, tantalum and niobium metals and alloys. The metal feed, such as a rod-shaped billet, is pre-heated to a temperature in excess of 390°C in an inert atmosphere, preferably argon. Suitable pre-heat temperatures depend on the metal feed, but may be above 760°C and up to about 1140°C for titanium and up to about 1200°C for tantalum and niobium. Induction heating may be used in the pre-heating step.

[0219]

[0220] In some embodiments of the presently disclosed methods, the porous doped metallic article is pre-heated to a temperature of at least 390°C, such as between 760°C and 1200°C, prior to extrusion in the continuous rotary extruder. The pre-heating may be produced via induction heating, for example continuous induction heating wherein the rod-shaped billet or series of billets passes through an induction field when being continuously fed towards the rotary extrusion apparatus. In some embodiments, the porous doped metallic article is preheated, and typically also extruded, under an inert atmosphere such as argon.

[0220]

[0221] As previously discussed, the methods disclosed herein facilitate the manufacture of porous metallic billets in which the dopant composition is consistently distributed throughout the compacted metallic structure, at the interfaces between the consolidated particles. Advantageously, this consistent doping throughout the billet may translate to consistent doping within the metallic composition of the doped metallic extrudate.

[0221]

[0222] By contrast, direct extrusion of a powder comprising a physical mixture of metallic particles and particles of dopant composition is susceptible to producing an inconsistently doped metallic extrudate. This may be caused by non-homogeneous mixing, or segregation, of the metallic and dopant particles in the bulk powder subjected to extrusion.

[0222]

[0223] Although formation of a porous metallic billet by cold compaction, prior to extrusion, provides significant advantages in the extrusion of some metallic compositions, it is also envisaged that the cold compactible, doped metallic powder could be directly subjected to extrusion, i.e. without an intermediate cold-compaction step. The non-spherical and thus compactible morphology is expected to address some of the practical issues associated with direct extrusion of high yield strength, low ductility and / or refractory metallic powders. Notably, the cold compactible, doped metallic powder may be less susceptible to slippage than more spherical powders as it is drawn into the tooling, mitigating wear and reducing unwanted porosity in the extrudate. Moreover, the dopant composition may still be favourably distributed throughout the extrudate due to the incorporation of the dopant composition into the non-spherical particles. For example, such an option may be attractive for metallic compositions which are less oxygen-sensitive or where control of pre-heating is less critical.

[0223]

[0224] Accordingly, the present disclosure provides a method of producing a doped metallic extrudate, comprising subjecting a cold compactible, doped metallic powder as disclosed herein, or as produced by the methods disclosed herein, to an extrusion process to form a doped metallic extrudate. The extrusion process may be a continuous rotary extrusion process, for example as disclosed in US patent 9,468,960.

[0224]

[0225] In some embodiments, the dopant composition in the doped metallic extrudate remains incompletely reacted, or substantially unreacted, with the metallic composition of the extrudate. Because extrusion is a solid state forming process, even an inoculating dopant composition may not react to a significant extent with the metallic composition during extrusion. Advantageously, the inoculating dopant composition thus remains available for inoculation of the metallic composition when the doped metallic extrudate is subjected to further metallurgical processing involving a melt-processing step, for example to achieve a grain refining effect. Similarly, low-reactivity dopant compositions may remain available for desired doping interactions in further metallurgical processing of the extrudate, for example via low-reactivity grain-refining of the metallic composition in a melt-processing step.

[0225]

[0226] In some embodiments, the doped metallic extrudate, e.g. a doped metallic wire, is used as a feedstock for a further metallurgical processing step, particularly a melt-processing step (fusion metallurgy process). In such processes, melting and re-solidification of the metallic composition causes mixing, reaction and / or grain-refining according to the principles disclosed herein.

[0226]

[0227] Metallic wires or extruded rods are currently used as feedstocks to various fusion additive manufacturing methods, including WAAM, laser wire AM (laser wire-fed AM; LWAM), laser cladding (depositing a harder material on a softer substrate material), weld repair, and fusion welding methods that use a filler wire (gas tungsten arc, plasma arc, gas metal arc, metal inert gas). The doped metallic extrudate may be used in any such methods.

[0227]

[0228] Wires used in some additive manufacturing techniques have a diameter of about 1.2 mm. Wires of such small diameter may be produced by drawing down the doped metallic wires, as produced by the extrusion methods disclosed herein, to lower diameters. For example, 2mm extruded wires may be drawn through multiple diamond dies to produce 1.2 mm or 1 mm diameter wires in a separate process.

[0229] Metallic extrudates (wires or rods) are also currently used as feedstocks to powder manufacturing processes, for example gas atomisation processes, particularly for reactive and high-temperature metals where molten pour atomisation methods are disfavoured. In such processes, the metal wire feedstock is melted and impinged with a high velocity gas jet to disperse droplets which then cool to form a powder. The doped metallic extrudates, e.g. wires, provided by the methods disclosed herein may be used in such applications.

[0228]

[0230] Use of the doped metallic extrudates provided by the methods disclosed herein in fusion-based application may take advantage of the consistent distribution of dopant composition in the doped metallic extrudate, imparting desirable mechanical or other properties to the product. For example, the inclusion of homogeneously distributed grain-refining dopant compositions in controllable amounts in a wire feed may improve isotropic properties of an additively manufactured product by inhibiting epitaxial grain growth (columnar grains) in the heat flow direction (build direction) across build layers.

[0229]

[0231] In some embodiments, the porous doped metallic article produced by the cold compaction process is subsequently sintered to produce a sintered metal structure which may be further processed by conventional metalworking techniques. Sintering will densify the metal article, while typically leaving the dopant composition at the prior particle boundaries. In some embodiments, the porous doped metallic article is sintered to produce a sintered preform for use in a solid-state metallurgical forming process

[0230]

[0232] In some embodiments, the porous doped metallic article produced by the cold compaction process is used directly as a feedstock for a hot working or melt-processing step (fusion metallurgy process), i.e. without an intermediate extrusion step. For example, in some embodiments the porous doped metallic article is a porous sheet (e.g. produced by direct powder rolling), and the porous sheet is subjected to a continuous hot rolling process to produce a densified sheet or strip.

[0231]

[0233] In some embodiments, either the porous doped metallic article (produced by cold compaction as disclosed herein) or the doped metallic extrudate (produced by extrusion as disclosed herein) is used as a feedstock or preform for a solid-state metallurgical processing step involving material flow. Such processes may include forging, rolling or a friction stir process such as friction stir deposition (to build a new part), friction stir processing to densify or improve the properties of the preform, or friction stir joining to use the feedstock as a filler for solid state welding. Friction stir processing generally involves forcibly inserting a rotating tool into the workpiece to create intense localised plastic deformation and mixing as the tool moves laterally across the workpiece. Friction stir processing may result in a highly deformed and grain-refined structure by severe plastic deformation, and may either react or redistribute the dopant in the workpiece, for example to produce an MMC composition with favourable properties.

[0232]

[0234] In one envisaged application, the porous metallic article or doped metallic extrudate is a MMC (with the dopant composition being the discontinuous phase), and is used to join or repair a MMC workpiece, either by fusion or solid-state method. Such operations on MMC workpieces are typically technically challenging, but these difficulties may be mitigated by tailoring the MMC wire composition to match that of the MMC workpiece. In another envisaged application, the porous doped metallic article is a porous sheet (e.g. produced by direct powder rolling), and the porous sheet is subjected to solid state rolling and / or friction stir processing.

[0233] EXAMPLES

[0234]

[0235] 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.

[0235] Materials

[0236]

[0236] Highly spherical, gas-atomized titanium alloy Ti-6AI-4V (Ti64) powders were received from a commercial manufacturer in two size fractions: an oversized (large) fraction (>140# mesh, 105-250 pm, hereafter Ti64-L1) and an undersized (small) fraction (5-25 pm, hereafter Ti64-S1). Both materials are lesser used I low value size fractions of the gas atomisation process. Commercially pure titanium powder (CP Ti; Grade 2) with an irregular / angular morphology was received from TIPRO International Co. Ltd. Stainless steel 410L powder, stainless steel 316L powder, Inconel 718 powder and H13 steel powder were obtained from Hdganas AB.

[0237]

[0237] The as-received Ti64 particles were characterised by scanning electron microscopy to determine the sphericity. 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 average sphericity values were as follows: Ti64-L1 = 0.795±0.038; Ti64-S1 = 0.930±0.050.

[0238]

[0238] A Ti64 titanium alloy swarf, produced as a by-product of a manufacturing process in the aerospace industry (“as-received swarf”), was received and characterised by scanning electron microscopy (SEM, ZEISS Merlin™ FE-SEM) to investigate its morphology. The swarf particles were observed to be flake-shaped with some curvature to the structure typical of material generated through machining processes. 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.

[0239]

[0239] The Ti64 particles were characterised by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) for their compositions, Scanning electron Microscopy (SEM) (ZEISS Merlin™ FE-SEM) for their morphologies and Mastersizer S for particle size distributions (PSD). However, the as-received swarf was subjected to sieve analysis according to ASTM B214 to estimate the PSD. The results are shown in Figure 7 (SEM image of Ti64-L1), Figure 8 (SEM image of Ti64-S1), and in Tables 1 and 2 below.

[0240] Table 1.

[0241] < < <

[0242]

[0243] Table 2.

[0244]

[0245]

[0240] The ICP-OES results for the swarf confirmed the composition as Ti-6AI-4V alloy containing about 0.19% oxygen, which is within Grade 5 specification for this alloy.

[0246]

[0241] 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 / cm3and 0.751 g / cm3respectively. The very low apparent density values, compared to a theoretical density of 4.429 g / cm3for Ti-6AI-4V alloy, is consistent with the low packing efficiency of the curved, flake shaped particles.

[0242] Titanium diboride powder (TiB2) was obtained from Hermann C. Starck Berlin, containing 27.5 wt% B, 1.18 wt% O and 0.18 wt% C (measured by ICP-AES and LEO combustion). A SEM image is shown in Figure 9. X-Ray diffraction analysis (XRD) showed the particles contain about 99.6 wt% titanium diboride (TiB2) with small amount of boron carbide (0.4 wt% B13C2). SEM imaging (ZEISS Merlin™ FE-SEM) indicated that the particles were irregular in shape, with the particle sizes ranging from 1.0 pm to 9.0 pm, and dso at 3.6 pm measured by Mastersizer S.

[0247]

[0243] Carbon tetraboride powder (2.5 pm) was obtained from H. C. Starck GmbH, Germany. Silicon carbide powder (1.5 pm) was obtained from Shandong Jinmeng New Material Co., Ltd. Alumina powder (2.5 pm) and alumina nanoparticles (20-30 nm) were obtained from GetNano, France)

[0248] Impact blending apparatus and methods

[0249]

[0244] 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 blending chamber, defined by a cylindrical stator, with a rotor and a recirculation duct. During impact blending 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.

[0250]

[0245] Schematic drawings of the impact blending chamber of the NHS-0 are shown in Figures 1, 2 and 3. The outer wall of the impact blending 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.

[0251]

[0246] The NHS-0 is operable at rotor rotation speeds of up to 16,000rpm. Generally, speeds of 6,000rpm to 10,000rpm were found 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.

[0252] Table 3.

[0253]

[0254]

[0247] The maximum batch size of the NHS-0 Hybridizer is about 50 g. Unless stated otherwise, batch sizes of 30g powder were processed in each experiment with the NHS-0. 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.

[0255] Cold isostatic pressing apparatus and methods

[0256]

[0248] A Cold Isostatic Press (CIP) with a maximum pressure of 413 MPa (60 ksi) was used for cold compaction studies. Elongate, cylindrical polyurethane bags with an internal diameter (ID) of 14 mm were used as flexible moulds for the cold isostatic pressing experiments, configured to produce cold compacted rods with dimensions of about 13 mm diameter and about 390-400 mm length. Such rods weigh about 160 to 200 g.

[0257]

[0249] The following general procedure was used for the cold isostatic pressure experiments:

[0258] • Fill the flexible bag with the powders, frequently tapping the bag to make the powder packing as uniform and as dense as possible,

[0259] • 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 such as an L-shaped metal angle (with the elongated bag secured into the angle) or a cylindrical aluminium lattice (with the elongated bag secured inside the cylinder), 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.

[0260] • Run the CIP to press the compacts isostatically to the pre-set pressure.

[0261] • After pressing, remove the green rod(s) from the flexible bags.

[0250] Cold compactability of the powders was defined as forming an intact and non-friable solid rod after retrieval from the bag (friable means that the compact is easily reduced back to powder during handling).

[0262] Example 1.

[0263]

[0251] The oversized Ti64 powder (Ti64-L1), the undersized Ti64 powder (Ti64-S1) and titanium diboride powder (TiB2) were combined and mixed in weight-based ratios of 85 : 14.4 : 0.6 or 85 : 14 : 1.0 respectively. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 9,000 rpm for 30 seconds. The impact blended powder product was then recovered and analysed by SEM to investigate the resultant morphology.

[0264]

[0252] The powder product was composed mainly of non-spherical particles comprising a large metal particle (derived from Ti64-L1) as the core and a plurality of the small metal particles (derived from Ti64-S1) as protrusions from the core. TiB2 particles were mostly embedded into the exposed surface of large metal particles (Ti64-L1), with some also embedded in protruding (satellited) small metal particles (Ti64-S1). A representative SEM image is shown in Figure 10. Higher magnification SEM images showed: (i) metallurgical bonding between the core particles (derived from Ti64-L1) and the protrusions (derived from Ti64-S1), (ii) that TiB2 particles were both embedded in the core particles and enclosed within the corona of attached small metal particles, and (iii) that the TiB2 particles retained a morphology similar to the starting TiB2 powder.

[0265]

[0253] Similar results were obtained within a window of impact blending conditions, including 7,000 rpm, 8,000rpm and 9,000rpm with duration from 30s to 60s, but the most preferable particle morphology was obtained at 9,000 rpm for 30 seconds.

[0266]

[0254] The powder product (9,000 rpm, 30 seconds) was then subjected to cold isostatic pressing at a pressure of 380 MPa (55 ksi). A strong solid rod compact was formed, with a diameter of 13 mm, length of 380 mm and weight of 165 g. The results show that even highly spherical particles of titanium alloys and similar metals can be simultaneously functionalised with ceramics and rendered cold compactible by an impact blending methodology.

[0267]

[0255] By contrast, a combined mixture of the large Ti64-L1 and small Ti64-S1 particles, i.e. without impact blending, was not cold compactible under such conditions. This result is consistent with earlier studies which found that pressures of 1000 MPa are needed to cold compress similar spherical pre-alloyed (Ti-10V-2Fe-3AI) powders (Luo et al, Journal of Materials Processing Technology, 2014, 214, 660- 666). Example 2.

[0268]

[0256] The oversized Ti64 powder (Ti64-L1) and titanium diboride powder (TiB2) were combined and mixed in weight-based ratios of 85 : 0.6. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 9,000 rpm for 30-60 seconds. The impact blended powder (intermediate powder product) was recovered and analysed by SEM to investigate the resultant morphology.

[0269]

[0257] This intermediate powder product comprised mainly spheroidal particles with discrete particles of TiB2 embedded in the surface, consistent with the brittle and very hard characteristics of TiB2. A representative SEM image is shown in Figure 11.

[0270]

[0258] The intermediate powder product and the undersized Ti64 powder (Ti64-S1) was then combined and mixed in weight-based ratios of 85.6 : 14.4. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 9,000 rpm for 30 seconds. The impact blended powder product was then recovered and analysed by SEM to investigate the resultant morphology.

[0271]

[0259] The powder product was composed mainly of non-spherical particles comprising a large metal particle (derived from Ti64-L1) 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 12. The particles morphology is similar as for Example 1, except that the TiB2 particles were only embedded in the large metal particles (core) and not the satellited small metal particles.

[0272]

[0260] The powder product was then subjected to cold isostatic pressing at a pressure of 380 MPa (55 ksi). A strong solid mini rod compact was formed, with a diameter of 8.5mm ID and a length of 30 to 50mm.

[0273] Example 3.

[0274]

[0261] The as-received swarf was subjected to rotational impact blending in the NHS-0 system at 10,000 rpm for 30 seconds, causing comminution of the swarf into smaller but still non-spherical particles. After this milling step, the comminuted material was sieved to -250 pm, and +250 pm was again milled (30s) and sieved (-250 pm). The combined -250 pm fraction was then subjected to rotational impact blending in the NHS-0 system at 9,000 rpm for 8-10 minutes. The temperature of the material, as measured in the recirculation duct, rose to about 80°C. The impact blended swarf was then recovered, sieved to -250 pm with more than 95 wt.% passing through the sieve, and analysed by SEM to investigate the resultant morphology. The d50 particle size (Mastersizer) of the sieved product was 118 pm.

[0262] A representative SEM image is shown in Figure 13. The swarf particles were comminuted to much smaller particle sizes, which occurs in the early stages of impact blending. Due to the effect of further impact blending for extended periods of time, the comminuted particles were then spheroidized and smoothed. While not perfectly spherical, the resultant spheroidized metallic powder nevertheless contained predominantly low aspect ratio particles with rounded and smooth surfaces.

[0275] Example 4.

[0276]

[0263] The spheroidized metallic powder produced in Example 3, the undersized Ti64 powder (Ti64-S1) and titanium diboride powder (TiB2) were combined and mixed in weightbased ratios of 85 : 14.4 : 0.6 respectively. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 9,000 rpm for 30 seconds. The impact blended powder product was then recovered and analysed by SEM to investigate the resultant morphology.

[0277]

[0264] The powder product was composed mainly of non-spherical particles comprising a large metal particle (derived from the spheroidized metallic powder) as the core and a plurality of the small metal particles (derived from Ti64-S1) as protrusions from the core. TiB2 particles were again mostly embedded in large metal particles (Ti64-L1). A representative SEM image is shown in Figure 14.

[0278]

[0265] The powder product was then subjected to cold isostatic pressing at a pressure of 380 MPa (55 ksi). A strong solid rod compact was formed, with a diameter of 13 mm, length of 390 mm and weight of 168 g.

[0279]

[0266] By contrast, the spheroidized metallic powder as produced in Example 3 was not cold compactible under such conditions. The results show that spheroidized particles produced from swarf can be simultaneously functionalised with ceramics and rendered cold compactible by an impact blending methodology.

[0280] Example 5. (reference)

[0281]

[0267] A portion of the Ti64-L1 powder was sieved to produce a narrower particle size distribution powder (passing 100# mesh, <150 pm). A portion of the CP Ti powder was sieved (passing 400# mesh, <38 pm). The sieved (<150 pm) Ti64 powder and the sieved CP Ti powder (<38 pm) were combined and mixed at a weight ratio of 80:20.

[0282]

[0268] The combined powder was then subjected to rotational impact blending in the NHS-0 system at various rotor rotational speeds (6,000 to 10,000 rpm) for different impact blending times (1 second to 10 minutes). The impact blended powders were recovered and analysed by SEM to investigate the resultant morphology.

[0269] The impact blended powders were then subjected to cold isostatic pressing at pressures of 206 MPa and / or 413 MPa to determine if cold compacted rods (mini rods with dimensions of about 8.5mm ID and a length of 30 to 50 mm) could be formed. The results are shown in Table 4 below, where “Yes” indicates that the impact blended powder was cold compactible at 206 MPa, “No” indicates that cold compaction was not achieved at 206 MPa and “No*” indicates that cold compaction was also not achieved at 413 MPa.

[0283] Table 4.

[0284]

[0285]

[0270] The results can be explained based on the morphology of the impact blended powders, as seen in SEM images. Impact blending at very low impact conditions (6,000 rpm for 2 min; 8,000 rpm for 1 s) produced no or an insufficient number of core-corona particles. As seen in Table 4, these powders were thus not cold compactible even at 413 MPa pressure, similar to the mixture before impact blending.

[0286]

[0271] Impact blending at medium impact conditions (6,000 rpm, 4 min; 8,000 rpm, 1-3 min; 10,000 rpm, 1s) produced powders with core-corona morphology which were thus cold compactible at only 206 MPa pressure.

[0287]

[0272] Impact blending at excessively high impact conditions (6,000 rpm for 6 min; 8,000 rpm for 4-10 min; 10,000 rpm for 2 min) produced a powder morphology where the ductile CP Ti had been impacted sufficiently to form a uniform coating over the entire surface of the spherical Ti64 core particle. Thus, the impact blended particles were similarly spherical to the initial Ti64-L1 particles and could not be cold compacted despite the ductile CP Ti film coating.

[0288]

[0273] These experiments demonstrate that the core-corona particle morphology of the impact blended powders is important to achieve satisfactory cold compactability. The presence of ductile CP Ti in the precursor powder was not, in itself, sufficient to facilitate cold compaction of powders composed mainly of spherical pre-alloyed Ti64 particles. Mixtures of separate CP Ti particles and spherical pre-alloyed Ti64 were not cold compactible unless CP Ti was the primary component (at least 70 wt.% CP Ti was required to cold compact a mixture at 206 MPa and at least 60 wt.% CP Ti was required at 413 MPa). Furthermore, spherical particles comprising a core of Ti64 coated with a uniform shell of CP Ti were also not cold compactible. Without wishing to be limited by any theory, it is proposed that the enhanced cold compactability properties result from improved interlocking of the non-spherical core-corona particles in the impact blended powders.

[0289]

[0274] The experiments in Example 5 were conducted without added dopant, but it is expected that similar correlations between particle morphology and cold compactability will apply for doped, impact blended metallic powders as disclosed herein.

[0290] Example 6.

[0291]

[0275] It is evident from the results in Examples 1, 2, 4 and 5 that the core-corona morphology of impact blended non-spherical particles provides improved cold compactability properties compared to the substantially spherical or spheroidised precursor particles. For a quantitative comparison, the convexity of the precursor and impact blended powders was measured. 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 the polygon of smallest perimeter that encloses the particle cross section with no point of the polygon bending inwards.

[0292] „ particle convex perimeter

[0293] Convexity = - particle perimeter

[0294]

[0276] 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.

[0295]

[0277] The convexity measurements of the precursor Ti64 powders and impact blended powders produced in Examples 1, 2, 3, 4 and 5 are shown in Table 5. The measurements were obtained from SEM images processed using Image J image-processing software. At least 10 measurements are done for each particle composition.

[0296] Table 5.

[0297]

[0298] < <

[0299]

[0300]

[0278] The unmodified Ti64 powder and spheroidized swarf powder had a convexity very close to 1 , consist with the spheroidal and convex nature of the particles. After impact blending these powders with different materials (undersized Ti64 powder, Cp Ti powder, with or without dopant particles), the convexity value reduces substantially because of the protrusions from the cores. The impact blended powders with a convexity number of less than 0.8 were found to be cold compactible at 380 MPa.

[0301] Example 7.

[0302]

[0279] Cold compactible, doped metallic powder for wire-making was produced by the method of Example 1 (9,000 rpm, 30 seconds, nominal 0.6 wt.% TiB2). Bulk elemental analysis of the powder gave an average boron content of 0.18 wt.%, corresponding to a calculated TiB2 content of 0.58 wt.%.

[0303]

[0280] Rod compacts with nominal diameter of 12 mm, length of 400 mm, and weight of 180g were then produced by cold isostatic pressing of the powder at a pressure of 380 MPa (55 ksi). The rods were then subjected to an incubation heat-treatment at 1050°C for 70 minutes, well below the sintering temperature of Ti64 (which is above 1200°C).

[0304]

[0281] A cross-section of one rod compact was hot mounted, then ground using SiC papers and diamond suspension followed by a polish using OP-S suspension. To identify and examine the interface between the particles in the compact, the polished samples were examined by SEM (Hitachi TM4000 Plus II Tabletop SEM) at high magnification. The SEM images revealed a porous structure in which (i) the core-corona morphology of many particles remains intact after compaction, demonstrating the strength of the bonding between the large Ti64-L1 core particles and the small Ti64-S1 particles which form the protrusions, (ii) the large core particles are not significantly deformed by the compaction, (iii) protrusions from one or both particles are present at many of the interfaces between adjacent particles, and (iv) there is no evidence of metallurgical bonding at the compacted interfaces between adjacent non-spherical particles (in contrast to the interfaces between core and protrusions within the non-spherical particles). It is proposed that the protrusions facilitate interlocking between the particles during compaction, so that the cold compact develops greater mechanical strength. Simple models for this interlocking engagement are shown in Figure 6.

[0305]

[0282] The SEM images also showed TiB2 in the compact, located at the periphery of the large Ti64 particles and in the protrusions of small Ti64 particles. The embedded TiB2 particles were transformed by the heat treatment to needle-like structures, up to 12 pm long and about 2 pm wide. Elemental analysis by energy-dispersive X-ray spectroscopy (SEM-EDX) confirmed that the needles were transformed TiB2.

[0306]

[0283] The rod compacts were then extruded via a continuous rotary extrusion process, as generally disclosed in US patent 9,468,960, to produce Ti64 wire. The compacts were preheated by induction heating to about 775°C under inert atmosphere, fed in succession for engagement with the rotary extrusion wheel (350 mm grooved wheel rotating at 6.5 rpm), extruded through a nominally 6 mm diameter extrusion die and quenched first with argon then water. The resultant wire had a diameter of 5.5 to 5.9 mm.

[0307]

[0284] The wire was cross-sectioned in both transverse and longitudinal directions and then hot-mounted, ground with SiC papers and diamond suspensions, and subsequently polished using an OP-S suspension. To identify and examine the interface between the particles in the compact, the polished samples were examined by SEM (Hitachi TM4000 Plus II Tabletop SEM) at high magnification. The SEM images revealed a fully dense structure, but with the prior particle boundaries clearly visible and with TiB2 particles decorating those boundaries. Elemental analysis by SEM-EDX confirmed that the observed decorating particles were indeed TiB2. Bulk elemental analysis of the wire gave an average boron content of 0.20 wt.%, corresponding to a calculated TiB2 content of 0.64 wt.% (similar to the powder feedstock). The results show that incompletely reacted TiB2 is well-distributed throughout the Ti64 wire, and thus remains available for grain-refining of the Ti64 composition during fusion metallurgy processes conducted with the Ti64 wire as feedstock.

[0308] Example 8.

[0309]

[0285] Stainless steel 410L powder (particles sieved to 53 to 150 pm) and carbon tetraboride powder (B4C; 2.5 pm) were combined and mixed in volume-based ratios of 99 : 1; 97 : 3; 95 : 5 and 93 : 7. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 10,000 rpm for 60 seconds. The impact blended powders were recovered and analysed by SEM to investigate the resultant morphology. In all cases, the impact blended powder comprised mainly spheroidal particles with B4C embedded in a composite surface layer. A representative SEM image from the 95 : 5 ratio experiment (particle in cross-section) is shown in Figure 15.

[0310]

[0286] Table 6 shows the Hall powder flow rate (measured according to ASTM B213-20, Standard Test Methods for Flow Rate of Metal Powders Using the Hall Flowmeter Funnel) of the impact blended powders, with comparison to the initial stainless steel 410L powder. It is evident that the flow rates were substantially unaffected, confirming that B4C was near-quantitatively adhered to the stainless steel particles and that the impact blending did not significantly affect the powder morphology.

[0311] Table 6.

[0312]

[0313]

[0287] Stainless steel 316L powder (d50 of 42 pm) and carbon tetraboride powder (B4C; 2.5 pm) were combined, mixed in volume-based ratios of 99 : 1; 97 : 3; and 95 : 5 and then subjected to impact blending as described above (10,000 rpm for 120 seconds). SEM images again showed that the impact blended powder comprised mainly spheroidal particles with B4C embedded in a composite surface layer.

[0314]

[0288] Table 7 shows the Hall powder flow rate (ASTM B213-20) of the mechanically mixed powders (i.e. before impact blending) and the impact blended powders, with comparison to the initial stainless steel 316L powder. The flow of the impact blended powders was substantially unaffected due to near-quantitative B4C adhesion, in contrast to the mechanically mixed powder where free B4C particles disrupted the flow.

[0315] Table 7.

[0316]

[0317]

[0289] The impact blended stainless steel powders produced herein are suitable to be combined with small metal particles, optionally a fine powder of the same stainless steel grade, and subjected to a second impact blending step as described in Example 2 to produce a cold compactible powder product composed mainly of non-spherical particles comprising a large metal particle as the core, a plurality of the small metal particles as protrusions from the core and B4C adhered mainly to the core. It is expected that such a powder can be cold compacted by cold isostatic pressing as described in Example 2 and then extruded to form a stainless steel wire with well-dispersed B4C using the same method described in Example 7. The B4C will thus remain available for desirable reactions with the stainless steel during fusion manufacturing with the wire as feedstock, thereby hardening or strengthening the manufactured steel article.

[0318] Example 9.

[0319]

[0290] Inconel 718 powder (nickel-based superalloy; particles sieved to 15 to 45 pm) and alumina powder (AI2O3; 2.5 pm) were combined and mixed in volume-based ratios of 99 : 1; 97 : 3; 95 : 5 and 93 : 7. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 10,000 rpm for 120 seconds. The impact blended powders were recovered and analysed by SEM to investigate the resultant morphology. In all cases, the impact blended powder comprised mainly spheroidal particles with AI2O3 adhered in a composite surface layer. A representative SEM image from the 97 : 3 ratio experiment (particle in cross-section) is shown in Figure 16.

[0320]

[0291] Table 8 shows the Hall powder flow rate (ASTM B213-20) of the impact blended powders, with comparison to the initial Inconel 718 powder. It is evident that the impact blended composite powder exhibits improved flowability compared to the raw Inconel 718 powder. Table 8.

[0321]

[0322]

[0292] A mixture of Inconel 718 powder (particles sieved to 15 to 45 pm) and alumina nanoparticles (AI2O3; 20-30 nm), combined at a volume-based ratios of 99 : 1, was also subjected to impact blending as described above (10,000 rpm for 120 seconds). SEM images again showed that the impact blended powder comprised mainly spheroidal particles with alumina adhered in a surface layer.

[0323]

[0293] The impact blended Inconel powders produced herein are suitable to be combined with small metal particles, optionally a fine powder of the same Inconel grade, and subjected to a second impact blending step as described in Example 2 to produce a cold compactible powder product composed mainly of non-spherical particles comprising a large metal particle as the core, a plurality of the small metal particles as protrusions from the core and AI2O3 adhered mainly to the core. It is expected that such a powder can be cold compacted by cold isostatic pressing as described in Example 2 and then extruded to form a stainless steel wire with well-dispersed AI2O3 using the same method described in Example 7. The AI2O3 will thus remain available for desirable reactions with the Inconel during fusion manufacturing with the wire as feedstock, thereby strengthening the manufactured Inconel article.

[0324] Example 10.

[0325]

[0294] H13 steel powder (particles sieved to 53 to 150 pm) and alumina powder (AI2O3; 2.5 pm) were combined and mixed in a volume-based ratio of 97 : 3. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 9,000 rpm for 60 seconds. The impact blended powder was recovered and analysed by SEM to investigate the resultant morphology. The impact blended powder comprised mainly spheroidal particles with AI2O3 adhered in a composite surface layer. A representative SEM image (particle in cross-section) is shown in Figure 17.

[0326]

[0295] The impact blended H13 steel powder produced herein is suitable to be combined with small metal particles, optionally a fine powder of H13 steel, and subjected to a second impact blending step as described in Example 2 to produce a cold compactible powder product composed mainly of non-spherical particles comprising a large metal particle as the core, a plurality of the small metal particles as protrusions from the core and AI2O3 adhered mainly to the core. It is expected that such a powder can be cold compacted by cold isostatic pressing as described in Example 2 and then extruded to form an H13 steel wire with well-dispersed AI2O3 using the same method described in Example 7. The AI2O3 will thus remain available for desirable reactions with the H13 steel during fusion manufacturing with the wire as feedstock, thereby hardening the manufactured steel article.

[0327] Example 11.

[0328]

[0296] Stainless steel 410L powder (particles sieved to 53 to 150 pm) and silicon carbide powder (SiC; 1.5 pm) were combined and mixed in a volume-based ratio of 97 : 3. The combined powders (30 g) were then subjected to rotational impact blending in the NHS-0 system at 10,000 rpm for 120 seconds. The impact blended powder was recovered and analysed by SEM to investigate the resultant morphology. The impact blended powder comprised mainly spheroidal particles with SiC embedded in a composite surface layer. A representative SEM image is shown in Figure 18.

[0329]

[0297] Stainless steel 316L powder (d50 of 42 pm) and silicon carbide powder (B4C; 1.5 pm) were combined, mixed in a volume-based ratio of 97 : 3 and then subjected to impact blending as described above (10,000 rpm for 120 seconds). SEM images again showed that the impact blended powder comprised mainly spheroidal particles with SiC embedded in a composite surface layer.

[0330]

[0298] The stainless steel impact blended powders produced herein are suitable to be combined with small metal particles, optionally a fine powder of the same grade of stainless steel, and subjected to a second impact blending step as described in Example 2 to produce a cold compactible powder product composed mainly of non-spherical particles comprising a large metal particle as the core, a plurality of the small metal particles as protrusions from the core and SiC adhered mainly to the core. It is expected that such a powder can be cold compacted to form a porous billet or preform, optionally extruded into an extrudate, and then subjected to a solid-state metallurgical processing step involving material flow, for example friction stir processing. The SiC dispersed through the billet will thus remain available for desirable interactions with and further mixing into the stainless steel composition during the solid state processing.

[0331]

[0299] 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. Claims1. A method of producing a cold compactible, doped metallic powder, the method comprising:3.providing (i) a first metallic powder comprising large metal particles, (ii) a second metallic powder comprising small metal particles and (iii) a dopant powder comprising small particles of a dopant composition; and4.subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles having a convexity number of less than 0.8,5.wherein the non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions.

2. The method according to claim 1, wherein the dopant composition is selected from the group consisting of a ceramic comprising a non-metal element, an intermetallic compound, and a metal or alloy with melting point higher than the first metallic powder.

3. The method according to claim 1 or claim 2, wherein the dopant composition comprises at least one non-metal element.

4. The method according to any one of claims 1 to 3, wherein the dopant composition comprises a boride.

5. The method according to any one of claims 1 to 4, wherein the dopant composition is a grain-refining dopant composition for the metallic composition of the first metallic powder.

6. The method according to any one of claims 1 to 5, wherein the dopant composition is an inoculating dopant composition capable of reacting with the metallic composition of the large metal particles when the metallic composition melts and resolidifies.

7. The method according to any one of claims 1 to 4, wherein the dopant composition is a low-reactivity dopant composition which remains at least partially unreacted when the metallic composition of the large metal particles melts and resolidifies, wherein (i) the low- reactivity dopant composition modifies the grain structure of the metallic compositionwhen resolidified, or (ii) the metallic composition wets and encapsulates dispersed solid particles of the dopant composition when melted, thereby forming a metal matrix composite when the metallic composition resolidifies.

8. The method according to any one of claims 1 to 7, wherein the cold compactible metallic powder comprises the dopant composition in an amount of less than 5 vol.%.

9. The method according to any one of claims 1 to 8, comprising:14.combining the first metallic powder, the second metallic powder and the dopant powder; and15.subjecting the combined powders to an impact blending process step to produce the cold compactible metallic powder.

10. The method according to any one of claims 1 to 9, wherein the d50 particle size of the second metallic powder is less than 40% of the d50 particle size of the first metallic powder.

11. The method according to any one of claims 1 to 10, wherein the d50 particle size of the dopant powder is less than 20% of the d50 particle size of the first metallic powder.

12. The method according to any one of claims 1 to 11, wherein the large metal particles, and optionally also the small metal particles, comprise a metallic composition having a yield stress of at least 600 MPa.

13. The method according to any one of claims 1 to 12, wherein the large metal particles, and optionally also the small metal particles, comprise a metallic composition having an elongation at break of at least 1%.

14. The method according to any one of claims 1 to 13, wherein the large metal particles, and optionally also the small metal particles, comprise a metallic composition selected from the group consisting of titanium, tantalum, rhenium, 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.

15. The method according to any one of claims 1 to 14, wherein the large metal particles, and optionally also the small metal particles, comprise a titanium alloy.

16. The method according to any one of claims 1 to 15, wherein each impact blending process step comprises impact blending particles in an impact blending chamber of an apparatus comprising (i) a stator which defines a cylindrical outer wall of the impact blending chamber, and (ii) a rotor operable to rotate in the impact blending chamber, the rotor comprising a plurality of impact blades having an impact face and an outer edge at the periphery of the rotor.

17. The method according to any one of claims 1 to 16, further comprising producing the first metallic powder by:23.providing a particulate metallic precursor comprising flake-shaped particles with a maximum dimension of greater than 250 micron;24.comminuting and spheroidizing the particulate metallic precursor in one or more impact blending process steps.

18. A cold compactible, doped metallic powder comprising non-spherical particles having a convexity number of less than 0.8, wherein the non-spherical particles comprise a large metal particle as a core, a plurality of small metal particles as protrusions from the core and a dopant composition adhered to at least one of the core and the protrusions.

19. The cold compactible, doped metallic powder according to claim 18, wherein the dopant composition is selected from the group consisting of a ceramic comprising a non-metal element, an intermetallic compound, and a metal or alloy with melting point higher than the first metallic powder.

20. The cold compactible, doped metallic powder according to claim 18 or claim 19, wherein the dopant composition is a grain-refining dopant composition for the metallic composition of the first metallic powder.

21. The cold compactible, doped metallic powder according to any one of claims 18 to 20, wherein the dopant composition is an inoculating dopant composition capable of reacting with the metallic composition of the large metal particles when the metallic composition melts and resolidifies.

22. The cold compactible, doped metallic powder according to any one of claims 18 to 21, wherein the large metal particles, and optionally also the small metal particles, comprise a titanium alloy.

23. A method of producing a porous doped metallic article, the method comprising providing (i) a first metallic powder comprising large metal particles, (ii) a second metallic powder comprising small metal particles and (iii) a dopant powder comprising small particles of a dopant composition; and30.subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions; and31.subjecting the cold compactible, doped metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous doped metallic article.

24. A method of producing a porous doped metallic article, the method comprising subjecting a cold compactible, doped metallic powder according to any one of claims 18 to 22, or a cold compactible, doped metallic powder produced by a method according to any one of claims 1 to 17, to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous doped metallic article.

25. The method according to claim 23 or claim 24, wherein the pressure is below 450 MPa.

26. The method according to any one of claims 23 to 25, wherein the cold compaction process is selected from cold isostatic pressing, cold die pressing and direct powder rolling.

27. A method of producing a doped metallic extrudate, the method comprising36.providing (i) a first metallic powder comprising large metal particles, (ii) a second metallic powder comprising small metal particles and (iii) a dopant powder comprising small particles of a dopant composition; and37.subjecting the first metallic powder, the second metallic powder and the dopant powder to one or more impact blending process steps to produce a cold compactible, doped metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core, a plurality of the small metal particles as protrusions from the core, and the dopant composition adhered to at least one of the core and the protrusions;38.subjecting the cold compactible, doped metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous metallic billet; and39.extruding the porous metallic billet to form a doped metallic extrudate.

28. A method of producing a doped metallic extrudate, the method comprising:41.subjecting a cold compactible, doped metallic powder according to any one of claims 18 to 22, or a cold compactible, doped metallic powder produced by a method according to any one of claims 1 to 17, to a cold compaction process at a pressure sufficient to consolidate the cold compactible, doped metallic powder, thereby producing a porous metallic billet; and42.extruding the porous metallic billet to form a doped metallic extrudate.

29. The method according to claim 27 or claim 28, comprising heating the porous metallic billet, and extruding the heated billet through a continuous rotary extruder to form the doped metallic extrudate.

30. A method of additive manufacturing, comprising melting a doped metallic extrudate, produced by a method according to any one of claims 27 to 29, to produce a molten composition, and selectively depositing the molten composition on a workpiece to fabricate or repair a metallic article.

31. Use of a porous doped metallic article, produced by a method according to any one of claims 23 to 26, as a feedstock or preform for a fusion, hot working or solid-state forming process.

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