METHOD OF PRODUCING A COLD COMPACTIBLE METALLIC POWDER
The invention provides a method of producing a cold compactible metallic powder, the method comprising: providing (i) a first metallic powder comprising large metal particles and (ii) a second metallic powder comprising small metal particles, wherein the d50 particle size of the second metallic powder is less than the d50 particle size of the first metallic powder; combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles; and subjecting the precursor powder to an impact blending process to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.
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The invention relates to a method of producing a cold compactible metallic powder by combining a first metallic powder comprising large metal particles and a second metallic powder comprising small metal particles to provide a precursor powder, and impact blending the combined powder to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles. The non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core. The invention also relates to a method of producing a porous metallic article by subjecting the cold compactible metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible metallic powder.
BACKGROUND OF INVENTIONCold 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, with or without a binder, at temperatures below the sintering temperature, to produce a porous metal object (a compact) with sufficient structural integrity to withstand further processing via sintering or other techniques to produce a final metal product.
Cold compaction techniques generally require 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.
Cold compaction with high yield strength metallic compositions, such as titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, including alloys and metal matrix composites thereof, and certain low ductility alloys of iron, zinc, magnesium, aluminium and copper, is particularly challenging. Indeed, 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.
It is therefore desirable to develop methods of upgrading metallic powder feedstocks which allow their use in cold compaction processes, which reduce the pressures required for consolidation in a cold compaction process, or which improve the properties of the compacts produced in cold compaction processes (such as strength or density).
There is an ongoing need for methods of producing a cold compactible metallic powder and methods of producing a porous metallic article by cold compaction, which at least partially address one or more of the above-mentioned short-comings, or provide a useful alternative.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.
SUMMARY OF INVENTIONThe inventors have now discovered that the cold compaction properties of a metallic powder may be improved by combining that powder with a second metallic powder composed of smaller sized particles, and subjecting the combined powder to an impact blending process. By selecting suitable impact blending conditions, the small particles (of the second metallic powder) adhere to the surface of the larger particles (of the first metallic powder), producing non-spherical particles having one large metal particle as a core and a plurality of the small metal particles as protrusions from the core.
Non-spherical particles having this “core-corona” type morphology can be consolidated in a cold compaction process to form compacts with desirable mechanical properties (e.g. high strength) or can be successfully consolidated to form a compact at significantly reduced pressures compared to the unmodified starting material. Indeed, certain metal powder feedstocks not otherwise suitable for cold compaction can be upgraded to become desirable feedstocks for binderless cold compaction processes by the methods of the present disclosure. Without wishing to be bound by any theory, it is proposed that the protrusions of the non-spherical particles facilitate particle interlocking during the compaction and increase the contact area at the interface between adjacent particles in the compact. Thus, compacts with excellent mechanical properties are obtained.
The methods disclosed herein are particularly useful for upgrading powders of high yield strength metallic compositions and/or relatively spheroidal particle morphologies, which are otherwise difficult to cold compact.
Impact blending has previously been used to modify powders by embedding small guest particles in the surface of the core particles, thus producing a non-spherical morphology. However, in such processes, as described for example in U.S. Pat. No. 4,915,987, the core particles comprised a soft or non-metallic composition to facilitate mechanical embedding of the hard guest particles in the surface.
It is considered surprising that impact blending of two metallic powders can produce a core-corona type morphology, particularly when the large particles which become the cores are composed of a high yield strength metallic composition such as titanium alloy. Without wishing to be bound by any theory, it is believed that the collisions between the small metal particles and large metal particles when impact blending under conditions of sufficient intensity cause at least a degree of metallurgical bonding, as opposed to mere mechanical embedding. Thus, the large and small particles are effectively integrated into a single non-spherical metallic particle as core and protrusions respectively.
In accordance with a first aspect the invention provides a method of producing a cold compactible metallic powder, the method comprising: providing (i) a first metallic powder comprising large metal particles and (ii) a second metallic powder comprising small metal particles, wherein the d50 particle size of the second metallic powder is less than the d50 particle size of the first metallic powder; combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles; and subjecting the precursor powder to an impact blending process to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.
In at least some embodiments, at least a portion of the small metal particles adhered to the large metal particles are metallurgically bonded to the large metal particles.
In some embodiments, the precursor powder has 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 no more than 40%, or no more than 30%, or no more than 25%, such as in the range of 10% to 25%, of the d50 particle size of the first 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.
In some embodiments, the hardness of the large metal particles is greater than or about equal to the hardness of the small metal particles.
In some embodiments, the large metal particles and the small metal particles have substantially the same metallic composition. In other embodiments, the large metal particles and the small metal particles have a different metallic composition.
In some embodiments, the large metal particles in the first metallic powder are substantially spherical. The sphericity of the large metal particles may be greater than 0.7, or greater than 0.75.
In some embodiments, subjecting the precursor powder to the impact blending process comprises impact blending the precursor powder 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. The apparatus may further comprise 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 precursor powder continuously recirculates through the recirculation conduit during the impact blending. The outer edge of each impact blade may be 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. The rotor may be rotated such that the outer edge of each impact blade has a peripheral velocity of above 35 m/s, or above 40 m/s, during the impact blending.
In some embodiments, the precursor powder is impact blended for a time between 1 second and 10 minutes, or between 30 seconds and 5 minutes.
In some embodiments, the precursor powder is impact blended in a dry inert gas atmosphere.
In some embodiments, the large metal particles comprise a metallic composition having a yield stress of at least 400 MPa, or at least 600 MPa, such as at least 1000 MPa.
In some embodiments, the large 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%.
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, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, alloys thereof, metal matrix composites thereof, alloys of iron, alloys of zinc, alloys of magnesium, and alloys comprising both aluminium and copper.
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.
In some embodiments, the large metal particles, and optionally also the small metal particles, comprise titanium or titanium alloy. In some embodiments, the large metal particles comprise titanium alloy. In some embodiments, the large metal particles comprise titanium alloy and the small metal particles comprise a metallic composition selected from the group consisting of commercially pure titanium, titanium alloy, a master alloy for titanium, and mixtures thereof.
In some embodiments, providing the first metallic powder comprises comminuting and/or spheroidizing precursor metal particles in an impact blending process to produce the large metal particles.
In some embodiments, the first metallic powder has a d50 particle size of between 1 μm and 500 μm, or between 80 μm and 500 μm, such as between 100 μm and 250 μm, for example between 110 μm and 180 μm.
In some embodiments, the second metallic powder has a d50 particle size of between 0.1 μm and 100 μm, or between 5 μm and 100 μm, such as between 5 μm and 60 μm, for example between 10 μm and 40 μm.
In some embodiments, the first metallic powder comprises at least 70 wt. % of the precursor powder, or at least 80 wt. % of the precursor powder, such as between about 80 wt. % and about 90 wt. % of the precursor powder.
In some embodiments, the cold compactible metallic powder comprises at least 20 wt. %, or at least 50 wt. %, of the non-spherical particles comprising one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.
In accordance with a second aspect the invention provides a cold compactible metallic powder produced by a method according to any embodiment of the first aspect.
In accordance with a third aspect the invention provides a cold compactible metallic powder comprising non-spherical particles, the 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, wherein at least a portion of the small metal particles are metallurgically bonded to the large titanium or titanium alloy particle.
In some embodiments, the non-spherical particles comprise a large titanium alloy particle as a core and the small metal particles comprise a metallic composition selected from the group consisting of commercially pure titanium, titanium alloy, a master alloy for titanium, and mixtures thereof.
In accordance with a fourth aspect the invention provides a method of producing a porous metallic article, the method comprising: providing (i) a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise a large metal particle as a core and a plurality of small metal particles as protrusions from the core, preferably wherein at least a portion of the small metal particles are metallurgically bonded to the large metal particle, or (ii) a cold compactible metallic powder produced by a method according to any embodiment of the first aspect, or (iii) a cold compactible metallic powder according to any embodiment of the third aspect; and subjecting the cold compactible metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible metallic powder, thereby producing a porous metallic article.
In accordance with a fifth aspect the invention provides a method of producing a porous metallic article, the method comprising: providing (i) a first metallic powder comprising large metal particles and (ii) a second metallic powder comprising small metal particles, wherein the d50 particle size of the second metallic powder is less than the d50 particle size of the first metallic powder; combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles; subjecting the precursor powder to an impact blending process to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core; and subjecting the cold compactible metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible metallic powder, thereby producing a porous metallic article.
In some embodiments of the fourth and fifth aspects, the porous metallic article has a density of at least 70% of theoretical density.
In some embodiments of the fourth and fifth aspects, the pressure is below 450 MPa, such as below 400 MPa, for example below 350 MPa.
In some embodiments of the fourth and fifth aspects, the cold compactible metallic powder is subjected to the cold compaction process in the absence of a binder.
In some embodiments of the fourth and fifth aspects, the cold compaction process is selected from cold isostatic pressing, cold die pressing and direct powder rolling. In some embodiments, the cold compaction process is a cold isostatic pressing process.
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.
As used herein, the terms “first”, “second”, “third” etc in relation to various features of the disclosed devices are arbitrarily assigned and are merely intended to differentiate between two or more such features that the device 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.
Further aspects of the invention appear below in the detailed description of the invention.
Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:
The present invention relates to a method of producing a cold compactible metallic powder. The method comprises providing first and second metallic powders, comprising large metal particles and small metal particles respectively. The d50 particle size of the second metallic powder is thus less than, and preferably no more than about 40% of, the d50 particle size of the first metallic powder. The method comprises a step of combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles. The precursor powder is then subjected to an impact blending process to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles. The non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.
First Metallic PowderThe methods of the disclosure may be used to modify the morphology of a first metallic powder comprising large metal particles with a second metallic powder comprising small metal particles. The metal particles in the first and second metallic powders are typically present in a range of particle sizes. The particle size distribution (PSD) of such metallic 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 d10, 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).
Metallic powders with a wide range of d50 particle sizes may be modified, provided that the particles of the second metallic powder are correspondingly smaller. The first metallic powder may thus have a d50 particle size of between 1 μm and 500 μm. In some embodiments, the d50 particle size is between 80 μm and 500 μm, or between 100 μm and 250 μm, such as between 110 μm and 180 μm. Optionally, the first metallic powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution.
The particles of the first metallic powder may comprise any type of metallic composition, including commercially pure metals, metal alloys and metal matrix composites. The methods of the disclosure are particularly useful for upgrading powders of high yield strength metallic compositions. Thus, in some embodiments, the particles of the first metallic powder comprise a metallic composition having a yield stress of at least 400 MPa, or at least 600 MPa, such as at least 1000 MPa. Commonly, such metallic compositions will also have a relatively low ductility. However, without wishing to be limited by any theory, it is believed that a minimum level of ductility may be required to avoid shattering of the large particles under impact blending conditions. Thus, in some embodiments, the particles of the first metallic powder 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%. As used herein, yield stress and elongation at break are bulk material properties of a metallic composition, and can be measured by ASTM E8/E8M-13.
Non-limiting examples of high yield strength and low ductility 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. 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 particles via surface embedment (rather than metallic bonding). Metallic compositions comprising such metal elements will generally be suitable only if the yield stress is high, such as at least 400 MPa. For example, certain ferritic and martensitic steels have appropriate yield strength, whereas pure iron and austenitic steels are expected to be too soft. In some embodiments, the 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.
The particles of the first metallic powder may also comprise a metallic composition having fracture properties suitable to avoid or 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 (K1c) in the range of 10 to 150 MPa·m1/2, such as in the range of 40 to 150 MPa·m1/2. K1c is a bulk material property of a metallic composition, and can be measured by ASTM E1820.
The methods of the disclosure are also particularly useful for upgrading 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.
In some embodiments, the metal particles of the first metallic powder comprise a metallic composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof. In some embodiments, the metal particles comprise alloys of titanium, tantalum, or niobium. In some embodiments, the metal particles of the first metallic powder comprise a titanium alloy. Non-limiting examples of suitable titanium alloys include Ti-6Al-4V (i.e. titanium alloyed with 6 wt. % Al, 4 wt. % V) and Ti-10V-2Fe-3Al (i.e. titanium alloyed with 10 wt. % V, 2 wt. % Fe, 3 wt. % Al).
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.
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.
The methods of the disclosure are particularly useful for upgrading 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.
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 μm) of pre-alloyed Ti-6Al-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.
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. 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 main impact blending process step.
Second Metallic PowderThe 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.
The second metallic powder may have a d50 particle size of between 0.1 μm and 100 μm, or between 5 μm and 100 μm, such as between 5 μm and 60 μm, for example between 10 μm and 40 μm. Optionally, the second metallic powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution.
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.
The particles of the second metallic powder may comprise any type of metallic composition, including commercially pure metals, metal alloys and metal matrix composites. 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. As used herein, a metallic composition refers to the elemental composition of the powder as a whole and not the metallurgical structure. Thus, a blended elemental (BE) powder comprising alloy component particles of different compositions may have the same metallic composition as a powder consisting of pre-alloyed metallic particles. The inventors have demonstrated that large pre-alloyed Ti-6Al-4V spherical particles may be modified with small particles of the same metallic composition, either as (i) pre-alloyed Ti-6Al-4V particles or (ii) BE powder for Ti-6Al-4V composed of commercially pure Ti and 60Al-40V master alloy, to produce a cold compactible metallic powder. In either case, the large metal particles may advantageously be modified without changing its metallic composition.
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. In this scenario too, the spheroidized large metal particles may advantageously be modified without changing the metallic composition.
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.
Whereas the first metallic powder may 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. By suitable selection of the impact blending conditions, the inventors have demonstrated that large pre-alloyed Ti-6Al-4V spherical particles may be modified with either high yield strength pre-alloyed Ti-6Al-4V small particles or relatively soft commercially pure titanium small particles. Thus, in some embodiments, the small metal particles comprise a metallic composition selected from the group consisting of commercially pure titanium, titanium alloy, a master alloy for titanium, and mixtures thereof.
Without wishing to be limited by any theory, it is proposed that the mechanical properties of the large and small particles should 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.
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 μm) of pre-alloyed Ti-6Al-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. It has been found that small particles with a sphericity of only 0.3 may be used to form the protrusions.
Precursor PowderThe methods disclosed herein include a step of combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles. The two powders may be well-mixed before the subsequent impact blending step, although this is not essential as mixing will occur during the impact blending. In some embodiments, the two metallic powders are first combined in the impact blending apparatus.
Because of the differentiation in particle sizes of the two metallic powders, the precursor powder prior to impact blending typically has 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.
The first metallic powder is typically the primary component, by mass, in the precursor powder. In some embodiments, the first metallic powder comprises at least 70 wt. % of the precursor powder, or at least 80 wt. % of the precursor powder, such as between about 80 wt. % and about 90 wt. % of the precursor powder. The second metallic powder typically forms the remainder of the precursor powder, although it is not excluded that additional metallic powders could be combined with the first and second metallic powders.
Impact BlendingThe methods disclosed herein include a step of subjecting the precursor powder to an impact blending process to adhere the small metal particles to the large particles. As used herein, an impact blending process refers to any dry powder blending process which causes high velocity impacts between the particles with sufficient intensity to adhere, and preferably metallurgically bond, the small metal particles to the large particles. Suitable impact blending processes include rotational impact blending, sometimes referred to as hybridization, and various other dry particle coating technologies. In some embodiments, the impact blending process 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.
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In use, the metallic precursor powder 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 metal particles of the precursor powder 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.
Such apparatus, called the Nara Hybridization System (NHS-0), is commercially available from Nara Machinery Co., Ltd. Other suitable impact blending apparatus may include the Mechanofusion system and Cyclomix from Hosokawa Micron Ltd..
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-6Al-4V particles with a d50 particle size of about 120-160 μm, and it is expected that a similar gap distance will be suited for impact blending particles with a size in the range of 1 μm to 500 μm. 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.
Another significant parameter is the rotation speed of the rotor. Without wishing to be limited by any theory, it is proposed that the rotational velocity of the impact blades near the rotor periphery must be sufficient to generate the high intensity collisions between the powder particles needed for robust particle adhesion. The inventors have found that peripheral rotor velocities in the range of about 37.1 m/s to 61.8 m/s (corresponding to 6,000 rpm to 10,000 rpm) were suitable for modifying pre-alloyed Ti-6Al-4V large particles with a d50 particle size in the range of 120-160 μm, 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.
A further significant parameter is the impact blending time, which should be sufficient to adhere the small metal particles to the large core particles but not so long that the modified particles become spheroidized. 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.
In some embodiments, the precursor powder is impact blended 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 by experiment that impact blending times of as little as a few seconds may be sufficient to create a desirable morphology when modifying pre-alloyed Ti-6Al-4V particles with a d50 of about 120-160 μm 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.
The precursor powder may be impact blended in a dry inert gas atmosphere. This advantageously limits oxidation of the metal powders during the impact blending. Without limitation by theory, it is proposed that freshly exposed metal surfaces of the particles, as formed during impact blending under inert atmosphere, are not immediately sealed by an oxidic layer and thus remain capable of adhering and intermixing with the metallic surfaces of other particles.
Cold Compactible Metallic Powder Comprising Non-Spherical ParticlesImpact blending of the precursor 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 modified powder and thus derives from the modified particle morphology in the powder.
Without wishing to be limited by any theory, the inventors propose, on the basis of scanning electron microscopy (SEM) analysis of cross-sectioned core-corona particles, that the small metal particles are metallurgically bonded to the large particles along at least a portion of the inter-particle interfaces. This mode of chemical bonding is distinguished from mere mechanical embedding of the type produced when surface-modifying soft non-metallic core particles with hard guest particles. The large and small metal particles are thus effectively integrated into a single non-spherical metallic particle as core and protrusions. The resultant mechanical integrity of the non-spherical particles is considered to be important for desirable cold compaction properties, since adhesion of the protrusions to the core must withstand the severe compressive forces when the particles are consolidated into a compact under pressure.
The non-spherical particles typically comprise many protrusions distributed around the periphery of the core particle. Each protrusion may comprise a single small particle or a cluster of small particles. The small particles may be deformed by the bond-forming impact with the core particle, or by subsequent high velocity impacts of the non-spherical particle during the impact blending process. The extent of deformation may depend on the yield strength and ductility of the 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 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.
The morphology of particles may be quantified with a convexity number, defined as the ratio of the perimeter of a particle's convex hull to the perimeter of the object itself (both measured with respect to a cross-section of the particles). In some embodiments, the convexity of the non-spherical particles is below 0.8, such as in the range of 0.4 to 0.8.
It is not required that all particles in the impact blended metallic powder exhibit the core-corona morphology; satisfactory cold compaction properties may be obtained when only a fraction of the particles have this morphology. In some embodiments, the cold compactible metallic powder thus comprises at least 20 wt. %, such as at least 50 wt. %, or at least 60%, of the non-spherical particles comprising one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.
In some embodiments, the cold compactible 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 comprise a metallic composition selected from the group consisting of commercially pure titanium, titanium alloy, a master alloy for titanium, and mixtures thereof. The small metal particles may be metallurgically bonded to the large titanium or titanium alloy particle.
Method of Producing a Porous Metallic ArticleThe invention also relates to a method of producing a porous metallic article. The method comprises providing a cold compactible metallic powder comprising non-spherical particles. The non-spherical particles comprise a large metal particle as a core and a plurality of small metal particles as protrusions from the core. The small metal particles may be metallurgically bonded to the large metal particle along at least a portion of the interface. The non-spherical particles may be produced by the methods previously disclosed herein. The method includes a step of subjecting the cold compactible metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible metallic powder, thereby producing a porous metallic article.
The cold compaction process may involve any cold compaction method wherein a metallic powder is consolidated under pressure, with or without a binder, at temperatures below the sintering temperature, to produce a porous metal object (a compact) with sufficient structural integrity to withstand further processing, e.g. via sintering or other metalworking techniques, to produce a final metal product. Suitable cold compaction techniques may include cold isostatic pressing, cold die pressing, direct powder rolling and metal injection moulding.
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.
In some embodiments, the cold compactible 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 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.
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.
The cold compaction process consolidates the cold compactible metallic powder to produce a porous metallic article. There are thus voids between the compacted metal particles in the metallic structure, which will be empty following a binderless compaction process. The porosity of the metallic article will depend on the morphology of the particles, the deformability of the particles under the compaction pressure (which may be low if the particle cores are formed of a high yield strength composition) and the compaction pressure. The inventors have found that compaction of non-spherical particles comprising cores and protrusions formed from high yield strength pre-alloyed Ti-6Al-4V particles produces compacts with good mechanical properties when a compact density of about 71% of theoretical density (based on the density of Ti-6Al-4V) is exceeded. In some embodiments, therefore, the porous metallic article has a density of at least 70% of theoretical density. However, it will be appreciated that the porosity of suitably robust compacts may vary in other implementations, depending on the factors mentioned above. Furthermore, it is envisaged that the density of the porous compact may be increased by adding small particles to the cold compactible metallic powder, the small particles sized to occupy a portion of the voids between the interlocked non-spherical particles after compaction.
The cold compactible metallic powder may be compacted at any pressure sufficient to consolidate the cold compactible metallic powder and thus form a porous metallic article. The cold compactible 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-6Al-4V particles may be consolidated to form robust compacts at pressures of only 345 MPa. Consolidation at even lower pressures (206 MPa) was achievable when the protrusions were formed of a more ductile material (CP Ti). By contrast, the unmodified pre-alloyed Ti-6Al-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 metallic powder is compacted at a pressure below 450 MPa, or below 400 MPa, such as below 350 MPa, for example below 300 MPa.
The porous metallic articles produced by the methods disclosed herein are typically not final metal products but will instead be further processed. In some embodiments, the porous metal article is sintered to produce a sintered metal structure which may be further processed by conventional metalworking techniques. In other embodiments, the porous metallic articles may be a feedstock for a metal manufacturing technique such as extrusion. In one exemplary application, the porous metallic article is a cylindrical rod titanium alloy compact, for example with dimensions of 12.5 mm diameter and 400 mm length, suitable for extrusion to make titanium alloy wire as disclosed in U.S. Pat. No. 9,468,960.
ExamplesThe 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.
MaterialsHighly spherical, gas-atomized titanium alloy Ti-6Al-4V (Ti64) powders were received from a commercial manufacturer in two size fractions: an oversized (large) fraction (>140 #mesh, 105-250 μm, hereafter Ti64-L1) and an undersized (small) fraction (5-25 μm, hereafter Ti64-S1). Commercially pure titanium powder (CP Ti; Grade 2) with an irregular/angular morphology was received from TIPRO International Co. Ltd. Al60V40 master alloy was received from Reading Alloys, Inc.
A portion of the Ti64-L1 powder was sieved to produce a narrower particle size distribution powder (passing 100 #mesh, <150 μm, hereafter Ti64-L2). A portion of the CP Ti powder was sieved (passing 400 #mesh, <38 μm, hereafter CPTi-S2). A portion of the Al60V40 master alloy was also sieved (passing 635 #mesh, <20 μm, hereafter Al60V40-S2).
The as-received 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; CP Ti=0.370±0.019; Al60V40=0.438±0.108.
The 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). The results are shown in Tables 1 and 2 below.
A titanium alloy swarf, produced as a by-product of a manufacturing process in the aerospace industry (“as-received swarf”), was characterised by inductively coupled plasma-optical emission spectroscopy (ICP-OES), confirming the composition as Ti-6Al-4V alloy containing about 0.19% oxygen, which is within Grade 5 specification for this alloy.
The as-received swarf was 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 μm, and a particle size (maximum dimension) of up to 5 mm in length.
The as-received swarf was subjected to sieve analysis according to ASTM B214 to determine the particle size distribution. It was estimated that the d10, d50 and d90 particle sizes for the swarf were 0.64 mm, 1.25 mm and 2.2 mm respectively.
The apparent density and tap density of the as-received swarf were determined, according to ASTM B417 and ASTM B527, to be 0.616 g/cm3 and 0.751 g/cm3 respectively. The very low apparent density values, compared to a theoretical density of 4.429 g/cm3 for Ti-6Al-4V alloy, is consistent with the low packing efficiency of the curved, flake shaped particles.
Impact Blending Apparatus and MethodsA Nara Hybridization System (NHS-0), available from Nara Machinery Co., Ltd. and schematically depicted in
Schematic drawings of the impact blending chamber of the NHS-0 are shown in
The NHS-0 is operable at rotor rotation speeds of up to 16,000 rpm. Generally, speeds of 6,000 rpm to 10,000 rpm 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.
Unless stated otherwise, batch sizes of 30 g powder were processed in each experiment with the NHS-0.
Cold Isostatic Pressing Apparatus and MethodsA Cold Isostatic Presses (CIP) with a maximum pressure of 413 MP (60 ksi) was used for cold compaction studies. Flexible moulds for the cold isostatic pressing experiments were elongate, cylindrical bags with an internal diameter (ID) ranging from 9 mm ID (latex bag) to 24 mm ID (latex/rubber bag). For one application of interest in wire-making by extrusion, the required dimensions of the “green rod” are about 12.5 mm diameter and 400 mm length, so that a 14 mm ID flexible bag was required to produce these rods. Each rod thus weighs about 180 to 200 g. However, due to limited capacity of the NHS-0 hybridiser, in most cases smaller size bags with 9 mm ID were used to form mini rods, thus demonstrating the powder compactability. Each mini rod had dimensions of about 8.5 mm ID and a length of 30 to 50 mm, and weighed about 6.0 grams.
For some studies, a “double bagging” method was used, where a thin polyethylene tubing (50-150 μm thickness) was placed inside the rubber/latex outer bag to reduce the risk of breakage of the green compacts during demoulding. There are three factors which affect the risk of breakage of the green compacts: (1) tensile stress inside the flexible mould, which is induced from adhesion and friction at the interface during isostatic pressing; (2) interfacial strength, which is induced from adhesion and friction at the interface, and (3) strength of the resultant green compacts. The flexible mould tends to return to its original shape after the isostatic stress is released. When tensile stress inside the flexible mould exceeds the interfacial strength, detachment occurs between the mould and the compact. However, the detachment does not happen at once and hence this creates non-uniform stress on the compact. Fracture perpendicular to longitudinal direction of the compact will occur if this non-uniform stress exceeds the strength of the green compact.
After cold isostatic pressing of some metallic powders, the green compacts are quite strong and hence a relatively rigid single rubber bag can be used. However, for cold isostatic pressing of harder-to-compact powders, the green compacts may not be strong enough to overcome the tensile stress generated inside the rigid rubber bag, resulting in the breakage of the compacts. The double bagging method assists to mitigate this issue because there is little gripping force between the outermost flexible bag and the green compact, therefore protecting the compact from breaking. After the cold isostatic pressing, the outermost bag can be recovered for further use and the thin innermost polyethylene tubing is peeled off from the intact green compact.
The following general procedure was used for the cold isostatic pressure experiments:
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- Fill the flexible bag with the powders, frequently tapping the bag to make the powder packing as uniform and as dense as possible,
- Insert a stopper into the mouth of the bag and tighten it with a rubber band to ensure the bag is sealed tightly and the compressing fluid cannot contact the compacts,
- Secure the bag to an elongate metal support, 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.
- Run the CIP to press the compacts isostatically to the pre-set pressure.
- After pressing, remove the green rod(s) from the flexible bags.
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). In
The as-received oversized Ti64 powder (Ti64-L1) and the undersized Ti64 powder (Ti64-S1) were combined and mixed at three different weight ratios: 80:20, 85:15, and 90:10. The combined powders were then subjected to rotational impact blending in the NHS-0 system at various rotor rotational speeds (6,000 to 10,000 rpm) and for different impact blending times (1 second to 4 minutes). Afterwards, the impact blended powders were recovered and analysed by SEM to investigate the resultant morphology. Selected samples were also analysed by ICP-OES.
For the mixed powder with an 80:20 ratio of Ti64-L1 to Ti64-S1, impact blending at low impact conditions (6,000 rpm for 2 min; 8,000 rpm for 1 second) did not significantly change the particle morphology from the initial bimodal particle distribution. However, impact blending at more intensive conditions changed the particulate morphology and particle size distribution, with the resultant powder 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 (impact blending at 8000 rpm for two minutes) is shown in
The impact blended powders with this “core-corona” morphology were then subjected to cold isostatic pressing at pressures of 413 MPa to determine if cold compacted rods (mini rods with dimensions of about 8.5 mm 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 413 MPa and “No” indicates that cold compaction was not achieved at that pressure (and “No IB” refers to the mixture of powders before impact blending).
Impact blending at medium impact conditions (8,000 rpm, 1-4 min; 9,000 rpm, 0.5-1 min) or high impact conditions (10,000 rpm, 1 min) thus produced powders with core-corona morphology which were found to be cold compactible at 413 MPa pressure to form robust compacts. By contrast, the initial mixture of 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-3Al) powders (Luo et al, Journal of Materials Processing Technology, 2014, 214, 660-666).
At one mild impact blending condition (6,000 rpm, 3 minutes), a core-corona morphology was evident in the SEM images yet the powder was not cold compactible at 413 MPa. Without wishing to be bound by any theory, it is believed that the small particles were not adhered to the core particles strongly enough to withstand the forces applied during the cold isostatic pressing.
For the mixed powder with an 85:15 ratio of Ti64-L1 to Ti64-S1, impact blending at medium impact conditions (8,000 rpm for 1-4 min; 9,000 rpm for 0.5-3 min) produced powders with core-corona morphology. As seen in Table 5, these powders were found to be cold compactible at 413 MPa pressure to form robust compacts. By contrast, the initial mixture of particles, i.e. without impact blending, was not cold compactible at such conditions.
For the mixed powder with a 90:10 ratio of Ti64-L1 to Ti64-S1, impact blending at very low impact conditions (6,000 rpm for 3 min; 8,000 rpm for 1-3 min) produced no or an insufficient number of core-corona particles. As seen in Table 6, these powders were thus not cold compactible at 413 MPa pressure. Impact blending at medium impact conditions (9,000 rpm for 0.5-1 min; 10,000 rpm for 1 s-0.5 min) produced powders with core-corona morphologies which were thus cold compactible at 413 MPa pressure. It is noteworthy that impact blending times of as little as 1 second (once the set rotation speed as reached) were sufficient to produce a core-corona morphology, as seen in
Although compactible, the compacts formed with a 90:10 ratio of Ti64-L1 to Ti64-S1 were relatively weak and thus easy to break. This suggests that 10 wt. % of the small particles is close to the lower limit necessary for cold compactability for the specific case Ti64-L1 and Ti64-S1 powder blends.
These experiments demonstrate that spherical metal powders (such as out-of-specification atomised pre-alloyed Ti64 powders; oversized and undersized), can be upgraded via the methods disclosed herein to become suitable feedstocks for cold compaction powder metallurgy techniques (such as cold isostatic compaction to form rod compacts use in an extrusion process for wire making). The cost of the oversized atomised Ti64 powder (100-250 μm) is about US$2.50/kg, which contrasts with the price of additive manufacturing grade Ti64 powder (15-45 μm) of US$250 to 650/kg.
Example 2Impact blended powders, prepared by impact blending the precursor powders having ratios of 80:20, 85:15 and 90:10 Ti64-L1 to Ti64-S1 at 9,000 rpm for 60 seconds, were further characterised by ICP-OES to determine their compositions and by the Mastersizer S to determine their particle size distributions (PSD). The ICP-OES results indicated that the oxygen content of the three impact blended powders were all in the range of 0.11-0.12%, similar to the starting concentration (0.09%) and well within the ASTM Grade 5 & 23 specification (max. 0.20% and 0.13%, respectively). No iron contamination occurred due to processing in the NHS-0 system.
The PSD results are shown in Table 7. There was minimal difference between the particle size distributions of the impact blended powders with different ratios of Ti64-L1 to Ti64-S1. The impact blended powders were slightly larger than the Ti64-L1 precursor due to the adhesion of the small particles to the larger core particles. The specific surface areas of the impact blended powders increased by about 16% to 20% compared to the Ti64-L1 precursor, consistent with the change in morphology.
Based on the experimental results of Example 1, an optimised set of impact blending conditions was identified for scale-up work: 30 g batch of precursor powder composed of Ti64-L1 to Ti64-S1 in a ratio of 85:15 was impacted blended in the NHS-0 at a rotor rotation speed of 9,000 rpm for one minute. Seven batches of impact blended powder were produced under these conditions to prepare enough material for manufacturing green rods (compacts) with dimensions of 12.5 mm diameter and 400 mm length.
The double bagging method was used in the cold isostatic pressing step to minimise the effect of the gripping force between the flexible mould and the green compact. Different compaction set pressures were investigated as follows: 206 MPa (30 ksi), 275 MPa (40 ksi), 345 MPa (50 ksi) and 413 MPA (60 ksi).
The green rods formed at 206 MPa and 275 MPa broke apart at several locations when demoulding. However, at 345 MPa and higher, an undamaged and robust green rod was formed. For the specific case of impact blended mixtures of Ti64-L1 and Ti64-S1 powders, compacted to a 12.5 mm diameter and 400 mm length rod, a minimum pressure of about 345 MPa was thus found necessary to form a sufficiently dense and compacted structure to withstand the stresses during demoulding.
The densities of the green rods were measured according to ASTM B962-17, and the results are shown in Table 8 below. It can be seen that the density increased with increasing pressure, and it appears that a minimum green density of about 71% of theoretical density of Ti64 is needed to provide sufficient green strength for demoulding and subsequent processing in an extrusion process.
Impact blending of atomised Ti64 powder together with CP Ti was investigated, based on an initial hypothesis that the ductile CP Ti might act as a binder to improve the compactability of the spherical, high yield strength Ti64 particles.
The sieved (<150 μm) Ti64 powder (Ti64-L2) and the sieved CP Ti powder (CPTi-S2) were thus combined and mixed at a weight ratio of 80:20. 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.
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 8.5 mm ID and a length of 30 to 50 mm) could be formed. The results are shown in Table 9 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.
The results can be explained based on the morphology of the impact blended powders, as seen in the 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 9, these powders were thus not cold compactible even at 413 MPa pressure, similar to the mixture before impact blending.
Impact blending at medium impact conditions (6,000 rpm, 4 min; 8,000 rpm, 1-3 min; 10,000 rpm, 1 s) produced powders with core-corona morphology which were thus cold compactible at only 206 MPa pressure. Representative SEM images are shown in
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. Representative SEM images are shown in
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.
Example 5Impact blending of a Ti64 powder with blended elemental Ti64 was investigated to determine if spherical, high yield strength Ti64 particles could also be upgraded for cold compaction applications without affecting the metallic composition. Particulate blended elemental Ti64 (BE Ti64) was obtained by combining appropriate ratios (90:10) of the sieved CP Ti powder (CPTi-S2) and the sieved Al60V40 master alloy (Al60V40-S2).
The sieved (<150 μm) Ti64 pre-alloyed powder (Ti64-L2) and BE Ti64 were combined and mixed at a weight ratio of 80:20. The combined powder was then subjected to rotational impact blending in the NHS-0 system at 8,000 rpm for 1 or 2 minutes. The impact blended powders were recovered and analysed by SEM to investigate the resultant morphology. While lower impact conditions may have been preferable, the impact blended powder nevertheless still contained a fraction of particles with a core-corona morphology, as seen in
The impact blended powders were then subjected to cold isostatic pressing at pressures of 206 and 413 MPa to determine if cold compacted rods could be formed. The impact blended powders were found to be cold compactible at 206 MPa, although the resultant compacts were relatively weak and thus easy to break. Nevertheless, the cold compactability results contrast against the initial mixture of particles, i.e. before impact blending, which was not cold compactible even at 413 MPa.
The results of ICP-OES analysis of the impact blended powder (8,000 rpm for 1 minute) are shown in Table 10 below, with comparison against the Ti64-L2 pre-alloyed starting material. As intended, the impact blended powder with enhanced cold compactability properties has a virtually identical composition to the Ti64-L2 precursor. The oxygen content of the impact blended powder was 0.13%, well within the ASTM Grade 5 & 23 specification (max. 0.20% and 0.13%, respectively).
In this example, a metallic powder comprising spheroidal large metal particles, suitable for upgrading according to the methods disclosed herein, was prepared from the titanium alloy swarf using the same impact blending apparatus used for rotational impact blending in the earlier examples.
The as-received swarf was subjected to five sequential rounds of milling in the NHS-0 system at 10,000 rpm for 30 seconds. In the first round of milling, 100 g of swarf was processed in 10 batches, with the limitation of 10 g per batch due to the low apparent density of the swarf. After the first round of milling, the comminuted material from the 10 batches was sieved into three size ranges: −250 micron, 250-500 micron and +500 micron. In the second and subsequent rounds of milling (using up to 50 g material per batch), the two larger fractions were separately milled and classified into the same three size fractions. The newly formed −250 micron fraction was combined with the prior-formed −250 micron fraction, the two newly formed 250-500 micron fractions were combined and the two newly formed +500 micron fractions were combined for the next round of milling.
After five rounds of milling, 65% of the initial swarf was converted to particle sizes less than 250 μm, 30% of the particles were in the range of 250-500 μm and only 3% of particles were larger than 500 μm. Advantageously, the yield of material recovered after the five rounds of milling was nearly 97%. The comminuted swarf particles still exhibited a flake-like, non-spheroidal morphology, albeit with smoothed surfaces compared to the initial swarf.
The −250 micron fraction of particles was then further classified by sieving into-75 μm, 75-150 μm and 150-250 μm fractions. The particles in each size range were then separately subjected to milling in the NHS-0 at 10,000 rpm for 14 minutes to spheroidize the particles. The amount of material processed in each batch varied from 10 g to 25 g, limited only by the amount of material available, thus demonstrating that the spheroidization step can be conducted at higher loading concentrations in the impact chamber than the comminution step.
Spheroidizing and surface-smoothing of the milled swarf particles occurred in all three particle size ranges, although the extents of sphering were slightly different. Under the same milling conditions, the most effective spheroidizing occurred in the particle size range of 150-250 μm. The particle morphologies of this fraction prior to and post spheroidizing are shown in
To quantify the effect of the milling steps on the particle morphology, the sphericity of the powders was characterised using image analysis of SEM images. The sphericity values for the as-received swarf, the input powders to spheroidization (comminuted material classified into −75 μm, 75-150 μm and 150-250 μm fractions) and resultant spheroidized powders (still in the −75 μm, 75-150 μm and 150-250 μm fractions) are shown in Table 11, where the average sphericity is the average of 10 particles. The as-received swarf is highly irregular, and the sphericity remains low for all fractions after comminution. After the spheroidizing step, the sphericities are greatly improved, particularly for the two large fractions.
The three portions of spheroidized particles prepared in Example 6 were re-combined (−250 μm) and mixed with the undersized gas-atomized Ti-6Al-4V alloy powder (Ti64-S1; 5-25 μm) in a weight ratio of 80:20. The combined powders were then subjected to rotational impact blending (15 g per batch) in the NHS-0 system at various rotor rotational speeds (6,000 to 9,000 rpm) for 30 seconds. Afterwards, the impact blended powders were recovered and analysed by SEM to investigate the resultant morphology.
Under low impact conditions (6000 rpm for 30 s), only a few of the small Ti64-S1 particles had attached to the larger spheroidized particles. Most Ti64-S1 particles remained unaffected by the milling. At slightly more intense milling conditions (7000 rpm for 30 s), more small particles had attached. After impact blending at medium intensity conditions (8000 rpm for 30 s), a substantial fraction of the small Ti64-S1 particles had attached to the larger spheroidized particles. The resultant powder thus comprised non-spherical particles comprising a large metal particle (derived from the previously spheroidized particles) as the core and a plurality of the small metal particles (derived from Ti64-S1) as protrusions from the core. A representative SEM image is shown in
Only minor changes to the particle size distribution of the spheroidized metal powder (−250 μm) were evident as a result of the modification. The d10, d50 and d90 values changed from 55 μm, 122 μm and 236 μm to 31 μm, 118 μm and 253 μm after the impact blending at 8,000 rpm, 30 seconds. However, the specific surface area increased from 0.0141 m2/g to 0.0193 m2/g due to the formation of the core-corona morphology.
Example 8The powders produced in Example 7, comprising non-spherical particles comprising a large spheroidized metal particle as the core and a plurality of the small metal particles as protrusions from the core, were then subjected to cold isostatic pressing (CIPing) at pressures of 380 MPa (55 ksi) to determine if the powders were cold compactible. The powders were compacted to form “mini rods” with dimensions of about 8.5 mm diameter and a length of 30 to 50 mm.
Each of the powders was found to be cold compactible, although the mini rod formed from powder produced at low intensity milling (6000 rpm, 30-s) had a lower strength than the other rods. In contrast, the metallic powder of the −250 μm spheroidized particles (i.e. unmodified with the Ti64-S1 small particles) was found to be non-compactible at 380 MPa. The results show that spheroidized metallic powders produced by comminution and spheroidizing of swarf can be upgraded for powder metallurgy applications involving cold compaction by the impact blending methodology of the present disclosure.
Example 9The impact blended Ti64 powder with a core-corona morphology produced in Example 3 (85:15 ratio of Ti64-L1 to Ti64-S1, impacted blended at 9,000 rpm for one minute) was used to study the mode of bonding between the core (derived from the large Ti64-L1 particles) and the protrusions (derived from the small Ti64-S1 particles).
The impact blended powder particles were cold mounted and ground to 1200 grit using SiC papers and final polished using OP-S suspension. To identify and examine the interface between the large Ti64-L1 core particles and the small Ti64-S1 particles, the polished samples were etched by Kroll's reagent. The impact blended powder was then characterized by SEM (ZEISS Merlin™ FE-SEM) at high magnification.
A SEM image of a cross-section through one core-corona Ti64 particle is shown in
The following conclusions and proposals were drawn from the observations of impact blending large and small Ti64 particles. (i) Early in the impact blending, the large particle surfaces are roughened and potentially hardened due to collisions with small particles, large particles and impact blender components. (ii) The higher stresses experienced by the smaller particles, due to their smaller size and higher critically resolved shear stress, cause them to preferentially deform when they collide into the larger particles and adhere to form protuberances. (iii) Yielding and some intermixing of the surfaces of both small and large particles during collisions allows for partial metallurgical bonding between the core and protrusions. (iv) In addition to single-particle protrusions, protrusions comprising a cluster of small particles form due to the probability of overlapping collisions of small particles with already-formed protrusions on the core particle and/or due to agglomeration of small particles into a cluster which subsequently collides with and adheres to a large, core particle. (v) Freshly exposed metal surfaces of the particles are not sealed by oxide (or much oxide) due to the argon atmosphere and thus more readily stick to each other and intermix to allow bonding. (vi) The heat generated by the impact blending process, in particular localised heat at the collision sites, may contribute to adhesion of the particles via metallic bond formation.
Impact blended powders with a core-corona morphology produced from hard large particles and softer small particles in Example 4 (Ti64-L2 large particles+CPTi-S2 small particles) and Example 5 (Ti64-L2 large particles+CPTi-S2 small particles+Al60V40-S2 small particles), were also analysed by SEM. The resultant protrusions were generally smaller than those on the non-spherical particles from Example 3, due in part to the greater ductility of the small particles. However, high resolution SEM analysis of the interface between core and protrusions again indicated that the CP Ti particles had metallurgically bonded to the Ti64 alloy core in both experiments.
Example 10To investigate how the impact blended powders respond to compaction, the Ti64 green compact (pre-alloyed Ti64 large particles+pre-alloyed Ti64 small particles) produced by cold isostatic pressing at 206 MPa in Example 3 was characterized by SEM (ZEISS Merlin™ FE-SEM) at high magnification. A cross-section of the green compact was cold mounted and ground to 1200 grit using SiC papers and final polished using OP-S suspension. To identify and examine the interface between the particles in the compact, the polished samples were etched by Kroll's reagent before the SEM analysis.
From the SEM images shown in
A cold compact produced in Example 4 from an impact blended powder comprising pre-alloyed Ti64 large particles+CP Ti small particles (Example 4:8,000 rpm, 2 minutes) was also characterized by SEM analysis. Again, protrusions from one or both particles were seen to be present at many of the interfaces between adjacent particles, suggesting that the protrusions facilitate interlocking between the particles during compaction. In contrast to the impact blended particles of Example 3, however, a degree of deformation of the soft CP Ti protrusions was seen and it appears that some joining of the protrusions on adjacent particles may assist the consolidation of the particles in the compact. Moreover, loose CP Ti particles were also observed in the compact. These particles either had not adhered to the core Ti 64 particles during impact blending or detached from the non-spherical particles in response to the stresses applied during the compaction process.
Example 11It is evident from the results in Examples 1, 3, 4, 5 and 8 that the core-corona morphology of impact blended core-corona non-spherical particles provides improved cold compactability properties compared to the substantially spherical 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 a polygon that encloses the particle cross section with no point of the polygon bending inwards.
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.
The convexity measurements of the precursor Ti64 powder and impact blended powders produced in Examples 3, 4, 5 and 8 are shown in Table 12. The measurements were obtained from SEM images processed using Image J image-processing software. At least 10 measurements are done for each particle composition.
The unmodified Ti64 powder and spheroidized swarf powder had a convexity very close to 1, consist with the spherical nature of the particles. After impact blending with different materials, the convexity value reduces substantially because of the protrusions from the cores. The impact blended powder produced in Example 3 (pre-alloyed Ti64 large particles+pre-alloyed Ti64 small particles) had the lowest convexity, because the small particle protrusions are relatively undeformed due to the hard metallic composition. The impact blended powders produced in Examples 4 and 5, with CP Ti or blended elemental Ti64 small particles, respectively, had a higher convexity because the softer metal protrusions were somewhat flattened after the impact blending. Nevertheless, all of the impact blended particles were cold compactable at 206 MPa.
Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.
Claims
1. A method of producing a cold compactible metallic powder, the method comprising:
- providing (i) a first metallic powder comprising large metal particles and (ii) a second metallic powder comprising small metal particles, wherein the d50 particle size of the second metallic powder is less than the d50 particle size of the first metallic powder;
- combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles; and
- subjecting the precursor powder to an impact blending process to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core.
2. The method of claim 1, wherein at least a portion of the small metal particles adhered to the large metal particles are metallurgically bonded to the large metal particles.
3. (canceled)
4. The method of claim 1, wherein the d50 particle size of the second metallic powder is no more than 40% of the d50 particle size of the first metallic powder.
5. (canceled)
6. (canceled)
7. (canceled)
8. The method of claim 1, wherein the large metal particles and the small metal particles have substantially the same metallic composition.
9. The method of claim 1, wherein the large metal particles in the first metallic powder are substantially spherical.
10. The method of claim 1, wherein subjecting the precursor powder to the impact blending process comprises impact blending the precursor powder 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, wherein 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 precursor powder continuously recirculates through the recirculation conduit during the impact blending.
11. (canceled)
12. The method of claim 10, wherein 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, and wherein the rotor is rotated such that the outer edge of each impact blade has a peripheral velocity of above 35 m/s during the impact blending.
13. (canceled)
14. The method of claim 1, wherein the precursor powder is impact blended for a time between 1 second and 10 minutes in a dry inert gas atmosphere.
15. (canceled)
16. The method of claim 1, wherein the large metal particles comprise a metallic composition having a yield stress of at least 600 MPa.
17. The method of claim 1, wherein the large metal particles comprise a metallic composition having an elongation at break of at least 1%.
18. The method of claim 1, 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, nickel, cobalt, alloys thereof, metal matrix composites thereof, alloys of iron, alloys of zinc, alloys of magnesium, and alloys comprising both aluminium and copper.
19. The method of claim 1, wherein 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.
20. The method of claim 1, wherein the large metal particles, and optionally also the small metal particles, comprise titanium or titanium alloy.
21. The method of claim 1, wherein the large metal particles comprise titanium alloy and the small metal particles comprise a metallic composition selected from the group consisting of commercially pure titanium, titanium alloy, a master alloy for titanium, and mixtures thereof.
22. (canceled)
23. The method of claim 1, wherein the first metallic powder has a d50 particle size of between 80 μm and 500 μm.
24. (canceled)
25. (canceled)
26. (canceled)
27. (canceled)
28. A cold compactible metallic powder comprising non-spherical particles, the 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, wherein at least a portion of the small metal particles are metallurgically bonded to the large titanium or titanium alloy particle.
29. (canceled)
30. A method of producing a porous metallic article, the method comprising:
- providing (i) a first metallic powder comprising large metal particles and (ii) a second metallic powder comprising small metal particles, wherein the d50 particle size of the second metallic powder is less than the d50 particle size of the first metallic powder;
- combining at least the first metallic powder and the second metallic powder to provide a precursor powder comprising the large metal particles and the small metal particles;
- subjecting the precursor powder to an impact blending process to adhere the small metal particles to the large particles, thereby producing a cold compactible metallic powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core and a plurality of the small metal particles as protrusions from the core; and
- subjecting the cold compactible metallic powder to a cold compaction process at a pressure sufficient to consolidate the cold compactible metallic powder, thereby producing a porous metallic article.
31. The method of claim 30, wherein the porous metallic article has a density of at least 70% of theoretical density.
32. The method of claim 30, wherein the pressure is below 450 MPa, and wherein the cold compactible metallic powder is subjected to the cold compaction process in the absence of a binder.
33. (canceled)
34. The method of claim 30, wherein the cold compaction process is selected from cold isostatic pressing, cold die pressing and direct powder rolling.
Type: Application
Filed: Dec 20, 2022
Publication Date: May 8, 2025
Applicant: COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION (Acton)
Inventors: Robert Wilson (Clayton), Shiqin Yan (Clayton), Kun Yang (Clayton), Ling Chen (Clayton)
Application Number: 18/722,512
