Method for producing cold compactable metal powders

Abstract

The present invention provides a method of producing a cold-formable metal powder comprising the steps of: (i) providing a first metal powder comprising large metal particles; and (ii) a second metal powder comprising small metal particles, wherein the d50 grain size of the second metal powder is smaller than the d50 grain size of the first metal powder; combining at least the first metal powder and the second metal powder to provide a precursor powder comprising large and small metal particles; and subjecting the precursor powder to an impact mixing process to adhere the small metal particles to the larger particles, thereby producing a cold-formable metal powder comprising non-spherical particles, wherein the non-spherical particles comprise one of the large metal particles as a core and a plurality of small metal particles as protrusions from the core.

Description

[Technical field]

[0001] [1] The present invention relates to a method for producing a cold-formable metal powder comprising non-spherical particles by combining a first metal powder comprising larger metal particles with a second metal powder comprising smaller metal particles to provide a precursor powder and impact blending the combined powders to adhere the smaller metal particles to the larger particles. The non-spherical particles comprise one of the larger metal particles as a core and a plurality of smaller metal particles as protrusions from the core. The present invention also relates to a method for producing a porous metal article by subjecting the cold-formable metal powder to a cold compaction process at a pressure sufficient to densify the cold-formable metal powder. [Background technology]

[0002] [2] Cold compaction of metal powders, which includes techniques such as direct powder rolling, cold die pressing, cold isostatic pressing, and metal injection molding, offers significant advantages over traditional hot processing techniques in a variety of metallurgical production processes. What these cold compaction methods have in common is that metal powders, with or without a binder, are densified under pressure at temperatures below the sintering temperature to produce a porous metal body (compact) that has sufficient structural integrity to withstand further processing by sintering or other techniques and to produce the final metal product.

[0003] [3] Cold compaction techniques generally require the application of significant pressure to bond the metal particles together and to increase the density of the compact to an acceptable level (usually greater than 70% of theoretical density). Very high pressures may be required, especially for powders of metals with high yield strength and low ductility and / or relatively spherical particle morphology. The pressure required for densification can be reduced by the use of binders, but the use of binders may be undesirable due to the risk of introducing impurities into the metal composition.

[0004] [4] Cold forming of metal compositions with high yield strength, including titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, their alloys and metal matrix composites, as well as certain low-ductility alloys of iron, zinc, magnesium, aluminum and copper, is particularly challenging. Indeed, some commercially available metal powder feedstocks, such as atomized titanium pre-alloyed powders and similar spherical high-yield strength metal powders, may not be able to be successfully densified without a binder at practically achievable forming pressures. These problems undesirably limit the range of feedstocks that can be used in metallurgical production processes involving cold forming processing steps. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 4,915,987 [Patent Document 2] U.S. Patent No. 9,468,960 [Non-patent literature]

[0006] [Non-Patent Document 1] Luo et al., Journal of Materials Processing Technology, 2014, 214, 660~666 Summary of the Invention [Problem to be solved by the invention]

[0007] [5] It would therefore be desirable to develop a method of modifying powder metal feedstocks to enable their use in cold-forming processes, to reduce the pressure required for densification in cold-forming processes, or to improve the properties (e.g., strength, density, etc.) of the compacts produced by cold-forming processes.

[0008] [6] There is a continuing need for methods of producing cold-formable metal powders, and methods of producing porous metal articles by cold forming, that at least partially address one or more of the shortcomings discussed above or provide a useful alternative.

[0009] [7] Reference in this specification to a patent or other document described as prior art shall not be construed as an admission that such document was publicly known or that the information contained in it was part of the common general knowledge at the priority date of any claim. [Means for solving the problem]

[0010] [8] The inventors have now discovered that the cold forming properties of a metal powder can be improved by combining the metal powder with a second metal powder composed of smaller sized particles and subjecting the combined powders to an impact mixing process. By selecting suitable impact mixing conditions, the smaller particles (of the second metal powder) adhere to the surfaces of the larger particles (of the first metal powder) to produce non-spherical particles having one larger metal particle as a core and multiple smaller metal particles as protrusions from the core.

[0011] [9] This non-spherical particle with a "core-corona" type morphology can be densified in a cold compaction process to form a compact with desirable mechanical properties (e.g., high strength) or can be successfully densified to form a compact at a significantly reduced pressure compared to the unmodified starting material. Indeed, certain metal powder feedstocks that are otherwise not suitable for cold compaction can be modified by the methods of the present disclosure to become desirable feedstocks for binderless cold compaction processes. Without wishing to be bound by any theory, it is proposed that the protrusions of the non-spherical particles promote interlocking of the particles during compaction and increase the contact area at the interface between adjacent particles in the compact. Thus, compacts with superior mechanical properties are obtained.

[0012]

[10] The methods disclosed herein are particularly useful for improving powders having high yield strength metal compositions and / or relatively spherical particle morphology that are otherwise difficult to cold compact.

[0013]

[11] Impact mixing has previously been used to modify powders and produce nonspherical morphologies by embedding small guest particles into the surface of a core particle. However, in such processes, the core particle was composed of a soft or nonmetallic composition to facilitate the mechanical embedding of the hard guest particles into its surface, as described, for example, in U.S. Patent No. 4,915,987.

[0014]

[12] It is believed to be surprising that core-corona morphology can be produced by impact mixing of two metal powders, especially when the larger core particle is of a high yield strength metal composition such as a titanium alloy. Without wishing to be bound by any theory, it is believed that when impact mixing is performed under conditions of sufficient intensity, the collisions between the smaller and larger metal particles result in at least some degree of metallurgical bonding, rather than merely mechanical embedding. As a result, the larger and smaller particles are effectively united into a single non-spherical metal particle as the core and protrusions, respectively.

[0015]

[13] According to a first aspect of the present invention, there is provided a method of producing a cold-formable metal powder comprising the steps of: (i) providing a first metal powder comprising large metal particles; and (ii) a second metal powder comprising small metal particles, wherein the d50 grain size of the second metal powder is smaller than the d50 grain size of the first metal powder; combining at least the first metal powder and the second metal powder to provide a precursor powder comprising large and small metal particles; and subjecting the precursor powder to an impact mixing process to adhere the small metal particles to the larger particles, thereby producing a cold-formable metal powder comprising non-spherical particles having one of the large metal particles as a core and a plurality of small metal particles as protrusions from the core.

[0016]

[14] In at least some embodiments, at least a portion of the smaller metal particles adhered to the larger metal particles are metallurgically bonded to the larger metal particles.

[0017]

[15] In some embodiments, the precursor powder has a multimodal particle size distribution including at least a first peak corresponding to the first metal powder and a second peak corresponding to the second metal powder.

[0018]

[16] In some embodiments, the d50 particle size of the second metal powder is no greater than 40%, or no greater than 30%, or no greater than 25%, such as within the range of 10% to 25%, of the d50 particle size of the first metal powder.

[0019]

[17] In some embodiments, the d50 grain size of the second metal powder is smaller than the d10 grain size of the first metal powder. In some embodiments, the d90 grain size of the second metal powder is smaller than the d10 grain size of the first metal powder.

[0020]

[18] In some embodiments, the hardness of the larger metal particles is greater than or about equal to the hardness of the smaller metal particles.

[0021]

[19] In some embodiments, the large metal particles and the small metal particles have substantially the same metal composition. In other embodiments, the large metal particles and the small metal particles have different metal compositions.

[0022]

[20] In some embodiments, the large metal particles in the first metal powder are substantially spherical. The sphericity of the large metal particles may be greater than 0.7, or greater than 0.75.

[0023]

[21] In some embodiments, subjecting the precursor powder to an impact mixing process includes impact mixing the precursor powder in an impact mixing chamber of an apparatus including (i) a stator defining an outer cylindrical wall of the impact mixing chamber, and (ii) a rotor rotatably operable within the impact mixing chamber, the apparatus including a plurality of impact blades at a periphery of the rotor having an impact surface and an outer edge. The apparatus may further include a recirculation passage between an inlet located at the outer cylindrical wall of the impact mixing chamber and an outlet directed toward a center of the impact mixing chamber, the precursor powder being continuously recirculated through the recirculation passage during impact mixing. The outer edge of each impact blade may be spaced apart from the outer cylindrical wall by a gap distance in the range of 1 mm to 5 mm, e.g., 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 speed of greater than 35 m / s, or greater than 40 m / s during impact mixing.

[0024]

[22] In some embodiments, the precursor powders are impact mixed for between 1 second and 10 minutes, or between 30 seconds and 5 minutes.

[0025]

[23] In some embodiments, the precursor powders are impact mixed in a dry inert gas atmosphere.

[0026]

[24] In some embodiments, the large metal particles have a metal composition that has a yield stress of at least 400 MPa, or at least 600 MPa, such as at least 1000 MPa.

[0027]

[25] In some embodiments, the large metal particles have a metal composition that has an elongation at break of at least 1%, such as at least 3%, for example in the range of 5% to 30%.

[0028]

[26] In some embodiments, the larger metal particles, and optionally the smaller metal particles, have a metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, chromium, rhenium, nickel, cobalt, alloys thereof, metal matrix composites thereof, alloys of iron, alloys of zinc, alloys of magnesium, and alloys containing both aluminum and copper.

[0029]

[27] In some embodiments, the larger metal particles, and optionally the smaller metal particles, have a metal composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof.

[0030]

[28] In some embodiments, the larger metal particles, and optionally the smaller metal particles, comprise titanium or a titanium alloy. In some embodiments, the larger metal particles comprise a titanium alloy. In some embodiments, the larger metal particles comprise a titanium alloy and the smaller metal particles have a metal composition selected from the group consisting of commercially pure titanium, titanium alloys, titanium master alloys, and mixtures thereof.

[0031]

[29] In some embodiments, the step of providing the first metal powder includes crushing and / or spheronizing precursor metal particles in an impact mixing process to produce larger metal particles.

[0032]

[30] In some embodiments, the first metal powder has a d50 particle size of 1 μm to 500 μm, or 80 μm to 500 μm, such as 100 μm to 250 μm, for example 110 μm to 180 μm.

[0033]

[31] In some embodiments, the second metal powder has a d50 particle size of 0.1 μm to 100 μm, or 5 μm to 100 μm, such as 5 μm to 60 μm, for example 10 μm to 40 μm.

[0034]

[32] In some embodiments, the first metal powder comprises at least 70 wt% of the precursor powder, or at least 80 wt% of the precursor powder, for example, between about 80 wt% and about 90 wt% of the precursor powder.

[0035]

[33] In some embodiments, the cold-formable metal powder comprises at least 20 wt% or at least 50 wt% non-spherical particles comprising one larger metal particle as a core and multiple smaller metal particles as protrusions from the core.

[0036]

[34] According to a second aspect, the present invention provides a cold-formable metal powder produced by a method according to any embodiment of the first aspect.

[0037]

[35] According to a third aspect of the present invention there is provided a cold-formable metal powder comprising non-spherical particles, the non-spherical particles comprising a larger titanium or titanium alloy particle as a core and a plurality of smaller metal particles as protrusions from the core, at least a portion of the smaller metal particles being metallurgically bonded to the larger titanium or titanium alloy particle.

[0038]

[36] In some embodiments, the non-spherical particles comprise a larger titanium alloy particle as a core and the smaller metal particles comprise a metal composition selected from the group consisting of commercially pure titanium, titanium alloys, master alloys for titanium, and mixtures thereof.

[0039]

[37] According to a fourth aspect of the present invention, there is provided a method of producing a porous metal article, the method comprising the steps of: (i) providing a cold-formable metal powder comprising non-spherical particles, the non-spherical particles comprising a larger metal particle as a core and a plurality of smaller metal particles as protrusions from the core, preferably at least a portion of the smaller metal particles being metallurgically bonded to the larger metal particles, or (ii) a cold-formable metal powder produced by a method according to any embodiment of the first aspect, or (iii) a cold-formable metal powder according to any embodiment of the third aspect; and producing a porous metal article by subjecting the cold-formable metal powder to a cold forming treatment at a pressure sufficient to densify the cold-formable metal powder.

[0040]

[38] According to a fifth aspect of the present invention, there is provided a method of producing a porous metal article comprising the steps of: providing (i) a first metal powder comprising large metal particles and (ii) a second metal powder comprising small metal particles, the d50 grain size of the second metal powder being smaller than the d50 grain size of the first metal powder; combining at least the first metal powder and the second metal powder to provide a precursor powder comprising large and small metal particles; subjecting the precursor powder to an impact mixing process to cause the small metal particles to adhere to the larger metal particles, thereby producing a cold-formable metal powder comprising non-spherical particles, the non-spherical particles comprising one of the large metal particles as a core and a plurality of small metal particles as protrusions from the core; and subjecting the cold-formable metal powder to a cold forming process at a pressure sufficient to densify the porous metal article.

[0041]

[39] In some embodiments of the fourth and fifth aspects, the porous metal article has a density of at least 70% of theoretical density.

[0042]

[40] In some embodiments of the fourth and fifth aspects, the pressure is less than 450 MPa, e.g., less than 350 MPa, less than 400 MPa.

[0043]

[41] In some embodiments of the fourth and fifth aspects, the cold-formable metal powder is subjected to a cold-forming process in the absence of a binder.

[0044]

[42] In some embodiments of the fourth and fifth aspects, the cold forming process is selected from cold isostatic pressing, cold die pressing, and direct powder rolling. In some embodiments, the cold forming process is a cold isostatic pressing process.

[0045]

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

[0046]

[44] As used herein, the terms "first," "second," "third," etc. in connection with various features of the disclosed devices are arbitrarily assigned and are intended merely to distinguish between two or more such features that the devices may incorporate in various embodiments. These terms do not, in themselves, indicate any particular orientation or order. Moreover, it should be understood that the presence of a "first" feature does not imply the presence of a "second" feature, the presence of a "second" feature does not imply the presence of a "first" feature, etc.

[0047]

[45] Further aspects of the invention are described in the detailed description below.

[46] Embodiments of the present invention are herein described, by way of example only, with reference to the accompanying drawings. [Brief description of the drawings]

[0048] [Figure 1]

[47] FIG. 1 is a schematic diagram of a rotary impact mixer for use in methods according to some embodiments of the present invention. [Diagram 2]

[48] ​​FIG. 1 is an isometric view of the impact mixing chamber of the rotary impact mixer (Nara Hybridization Systems, NHS-0) used in the Examples. [Diagram 3]

[49] FIG. 1 shows an impact mixing chamber and rotor of a rotary impact mixer for use in methods according to some embodiments of the present invention. [Figure 4]

[50] show a schematic of an impact mixing process for converting a precursor powder containing large and small particles into non-spherical particles containing one of the large metal particles as a core and multiple small metal particles as protrusions from the core. [Diagram 5]

[51] FIG. 5 illustrates diagrammatically the interlocking that may occur between adjacent non-spherical particles produced in accordance with embodiments of the present invention when compressed to form a porous metal article. [Figure 6]

[52] Contrast an intact compact (d) produced by cold isostatic pressing of a metal powder considered to be cold formable with damaged compacts (a-c) resulting from cold isostatic pressing of a metal powder that is not sufficiently cold formable. [Figure 7]

[53] SEM image of non-spherical particles containing a large pre-alloyed Ti-6Al-4V particle as the core and multiple small pre-alloyed Ti-6Al-4V particles as protrusions from the core, produced by impact mixing (8000 rpm for 2 minutes) the mixed powder containing 80:20 ratio of large and small particles in Example 1. [Figure 8]

[54] SEM image of a non-spherical particle containing a large pre-alloyed Ti-6Al-4V particle as the core and multiple small pre-alloyed Ti-6Al-4V particles as protrusions from the core, produced by impact mixing (10,000 rpm for 1 second) the mixed powder containing a 90:10 ratio of large and small particles in Example 1. [Figure 9]

[55] SEM image of non-spherical particles containing a large pre-alloyed Ti-6Al-4V particle as the core and multiple small CP Ti particles as protrusions from the core, produced by impact mixing (8000 rpm for 1 minute) the mixed powder containing 80:20 ratio of large and small particles in Example 4. [Figure 10]

[56] SEM image of non-spherical particles containing a large pre-alloyed Ti-6Al-4V particle as the core and multiple small CP Ti particles as protrusions from the core, produced by impact mixing (6000 rpm for 4 minutes) the mixed powder containing 80:20 ratio of large and small particles in Example 4. [Figure 11]

[57] SEM image of spheroidized particles containing large pre-alloyed Ti-6Al-4V particles as cores and a shell of CP Ti, produced by impact mixing (8,000 rpm for 4 min) the mixed powders containing Ti-6Al-4V particles and small CP Ti particles in a ratio of 80:20 in Example 4. [Figure 12]

[58] SEM image of spheroidized particles containing large pre-alloyed Ti-6Al-4V particles as cores and a shell of CP Ti, produced by impact mixing (10,000 rpm for 2 min) the mixed powders containing Ti-6Al-4V particles and small CP Ti particles in a ratio of 80:20 in Example 4. [Figure 13]

[59] SEM image of a non-spherical particle containing a large pre-alloyed Ti-6Al-4V particle as the core and multiple small BE Ti64 particles as protrusions from the core, produced by impact mixing (8,000 rpm for 1 minute) the mixed powder containing an 80:20 ratio of large and small particles in Example 5. [Figure 14]

[60] FIG. 11 is an SEM image of the 150-250 μm fraction of the milled Ti-6Al-4V alloy chips produced in Example 6. [Figure 15]

[61] SEM image of spheroidized metal particles produced by spheroidizing the 150-250 μm fraction of the milled Ti-6Al-4V alloy chips in Example 6. [Figure 16]

[62] SEM image of non-spherical metal particles comprising a large metal particle as a core and multiple small metal particles as protrusions from the core, produced in Example 7 by impact mixing a combination of spheroidized Ti-6Al-4V alloy particles (produced by spheroidization in Example 6) and small Ti-6Al-4V alloy particles at 8,000 rpm for 30 seconds. [Figure 17]

[63] Figure 11 is an SEM image of a cross-section of one non-spherical particle analyzed in Example 9, containing a large pre-alloyed Ti-6Al-4V particle as the core and multiple smaller pre-alloyed Ti-6Al-4V particles as protrusions from the core. [Figure 18]

[64] FIG. 11 is an SEM image of a cross-section of a compact containing densified non-spherical particles with core-corona morphology analyzed in Example 10. [Figure 19]

[65] FIG. 13 is another SEM image of a cross-section of a compact containing densified non-spherical particles with core-corona morphology analyzed in Example 10. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049]

[66] The present invention relates to a method for producing a cold-formable metal powder. The method comprises providing a first and a second metal powder, each comprising large and small metal particles. Thus, the d50 grain size of the second metal powder is smaller than the d50 grain size of the first metal powder, preferably by about 40% or less. The method comprises combining at least the first and second metal powders to provide a precursor powder comprising large and small metal particles. The precursor powder is then subjected to an impact mixing process to bond the small metal particles to the large particles, thereby producing a cold-formable metal powder comprising non-spherical particles. The non-spherical particles comprise one of the large metal particles as a core and a number of small metal particles as protrusions from the core.

[0050] First metal powder

[67] The disclosed method can be used to modify the morphology of a first metal powder containing large metal particles with a second metal powder containing small metal particles. The metal particles in the first and second metal powders are typically present in a range of particle sizes. The particle size distribution (PSD) of such metal powders can be characterized by the d10, d50 and d90 particle sizes, defined as 10% by volume of the powder being present in particles with dimensions less than the d10 particle size, 50% by volume of the powder being present in particles with dimensions less than the d50 particle size, and 90% by volume of the powder being present in particles with sizes less than the d90 particle size. The d10, d50 and d90 particle sizes can be measured by routine methods in materials science, such as laser diffraction methods. Suitable devices for measuring the PSD include the Mastersizer series of laser diffraction particle size analyzers available from Malvern Panalytical. Particle size, including d10, d50 and d90 sizes, can be measured according to ASTM B822 (Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Diffraction).

[0051]

[68] Metal powders with a wide d50 particle size range may be modified if the particles of the second metal powder are correspondingly small. Thus, the first metal powder may have a d50 particle size 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, for example between 110 μm and 180 μm. Optionally, the first metal powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution.

[0052]

[69] The particles of the first metal powder may comprise any type of metal composition, including commercially pure metals, metal alloys, and metal matrix composites. The methods of the present disclosure are particularly useful for improving powders having high yield strength metal compositions. Thus, in some embodiments, the particles of the first metal powder comprise a metal composition having a yield stress of at least 400 MPa, or at least 600 MPa, such as at least 1000 MPa. In general, such metal compositions will also have relatively low ductility. However, without wishing to be limited by any theory, it is believed that a minimum ductility may be required to avoid shattering of large particles under impact mixing conditions. Thus, in some embodiments, the particles of the first metal powder comprise a metal composition having a fracture elongation of at least 1%, such as at least 3%, for example in the range of 5-30%. As used herein, yield stress and fracture elongation are properties of the material as a whole of the metal composition and may be measured by ASTM E8 / E8M-13.

[0053]

[70] Non-limiting examples of metal compositions with high yield strength and low ductility 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 containing both aluminum and copper. In contrast, very soft and ductile metal compositions such as industrial pure copper or pure aluminum may be less suitable because they are highly deformable and / or adhere to the impact blender surface. Many metal compositions containing significant amounts of metals such as copper, lead, zinc, tin and iron are also expected to be soft and therefore susceptible to undesirable deformation by small particles via surface embedding (rather than metallic bonding) and / or functionalization. Metal compositions containing such metal elements will generally only be suitable if the yield stress is high, such as at least 400 MPa. For example, certain ferritic and martensitic steels have adequate yield strength, while pure iron and austenitic steels are expected to be too soft. In some embodiments, the particles of the first metal powder contain only small amounts (<20 wt%, preferably <10 wt%) of copper, lead, zinc, tin and iron as alloying elements in alloys with other metals, or are substantially free of these metals.

[0054]

[71] The particles of the first metal powder can also have a metal composition that has suitable fracture characteristics to avoid or minimize particle fragmentation during impact mixing, rather than the desired co-yielding and mixing that would occur during the formation of the desired core-corona morphology. Thus, in some embodiments, the particles of the first metal powder can have a pressure of 10 to 150 MPa m 1 / 2 Within the range of, for example, 40 to 150 MPa m 1 / 2 Fracture toughness (K 1c ) metal compositions. 1c is a bulk material property of a metal composition and can be measured by ASTM E1820.

[0055]

[72] The disclosed method is also particularly useful for improving powders of oxygen and / or carbon sensitive metal compositions. The compaction of such powders is preferably performed without binders to avoid contamination of the metal composition, and therefore a method for improving binder-free cold compactibility is needed. Thus, in some embodiments, the metal particles of the first metal powder comprise an oxygen and / or carbon sensitive metal composition selected from the group consisting of titanium, tantalum, niobium, vanadium, molybdenum, hafnium, zirconium, tungsten, alloys of these metals, and metal matrix composites.

[0056]

[73] In some embodiments, the metal particles of the first metal powder have a metal composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof. In some embodiments, the metal particles include alloys of titanium, tantalum, or niobium. In some embodiments, the metal particles of the first metal powder include titanium alloys. 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).

[0057]

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

[0058]

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

[0059]

[76] The disclosed method is particularly useful for modifying powders having relatively spherical morphology for cold compaction applications, since such materials (without modification) can be difficult to densify due to small interparticle interfaces. Thus, in some embodiments, the large metal particles in the first metal powder are substantially spherical. As used herein, a "substantially spherical" particle is spherical or near-spherical in shape, as opposed to a non-spherical powder morphology consisting of irregular or flaky particles. The morphology of a particle can be quantified by its sphericity, defined as the ratio of the radius of the inscribed circle to the radius of the circumscribed circle with respect to the cross-section of the particle (where the inscribed circle is the largest circle inside the cross-section of the particle, and the circumscribed circle is the smallest circle outside the cross-section of the particle, both circles being centered on the center of mass of the particle). In some embodiments, the sphericity of the large metal particles is greater than 0.7 or greater than 0.75.

[0060]

[77] For example, certain metal powders produced by gas atomization have a substantially spherical particle morphology. Industrial gas atomization processes generally produce a range of particle sizes, only a portion of which may be industrially valuable (e.g., as powders for additive manufacturing). Thus, out-of-specification particle sizes, including both oversized and undersized portions, may be a low-cost by-product, especially considering that reprocessing of these portions from gas atomization may require remelting of the metal composition. Thus, in some embodiments, the metal particles of the first metal powder are produced by gas atomization. The inventors have demonstrated that the oversized fraction (105-250 μm) of pre-alloyed Ti-6Al-4V particles produced by gas atomization can be improved for cold forming applications by the methods of the present invention, despite the highly spherical morphology of the unmodified material.

[0061]

[78] In other embodiments, the step of providing a first metal powder includes grinding and / or spheroidizing the precursor metal particles to form larger particles of the first metal powder, typically before combining with the second metal powder. This particle modification step may include cutting the precursor metal particles to reduce particle size in a suitable cutting device. The spheroidization of the precursor particles may be performed in any suitable device, for example, an impact mixing device, which may be the same or different from the device used to subsequently impact mix the combined first and second metal powders. Using impact mixing, for example, in a hybridizer device, irregularly shaped particles may be spheroidized into a more spherical shape, and the particles so modified may be suitable as core particles for further modification according to the principles of the present disclosure. As an additional advantage, it is expected that spheroidization by impact mixing may harden the larger particles, which may help them to compatibilize with the smaller particles to produce the desired core-corona morphology in the main impact mixing processing step.

[0062] Second metal powder

[79] The methods of the present disclosure modify the morphology of a first metal powder using a second metal powder that contains small metal particles and therefore has a d50 particle size smaller than the d50 particle size of the first metal powder. In some embodiments, the d50 particle size of the second metal powder is 40% or less, or 30% or less, or 25% or less of the d50 particle size of the first metal powder. In some embodiments, the d50 particle size of the second metal powder is at least 4% of the d50 particle size of the first metal powder. In some embodiments, the d50 particle size of the second metal powder is in the range of 10%-25% of the d50 particle size of the first metal powder.

[0063]

[80] The second metal powder may have a d50 particle size 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 metal powder may be classified, for example by sieving, to provide a relatively narrow particle size distribution.

[0064]

[81] It is not necessary that all particles of the second metal powder be smaller than all particles of the first metal powder. Nevertheless, these particle size distributions are typically sufficiently distinct that a combination of the first and second metal powders may have a multimodal particle size distribution that includes at least a first peak corresponding to the first metal powder and a second peak corresponding to the second metal powder. In some embodiments, the d50 particle size of the second metal powder is smaller than the d10 particle size of the first metal powder. In some embodiments, the d90 particle size of the second metal powder is smaller than the d10 particle size of the first metal powder.

[0065]

[82] The particles of the second metal powder may have any type of metal composition, including commercially pure metals, metal alloys, and metal matrix composites. The small metal particles of the second metal powder and the large metal particles of the first metal powder may have different metal compositions or may have substantially the same metal composition. As used herein, metal composition refers to the elemental composition of the powder as a whole, not the metal structure. Thus, an elementary blend (BE) powder containing alloy component particles of different compositions may have the same metal composition as a powder consisting of pre-alloyed metal particles. The inventors have demonstrated that large pre-alloyed Ti-6Al-4V spherical particles may be modified with small particles of the same metal composition as either (i) pre-alloyed Ti-6Al-4V particles, or (ii) BE powder for Ti-6Al-4V consisting of a master alloy of commercially pure Ti and 60Al-40V to produce a cold-formable metal powder. In either case, the large metal particles can be advantageously modified without changing their metal composition.

[0066]

[83] In embodiments in which the larger particles of the first metal powder are produced by milling and spheroidizing an irregular metal precursor, the smaller particles of the second metal powder may include fines produced by that milling. Again, the spheroidized larger metal particles may be advantageously modified without changing their metal composition.

[0067]

[84] Alternatively, a second metal powder having a different metal composition may be preferred to provide a cold-formable powder having a different composition than that of the first metal powder.

[0068]

[85] The first metal powder may advantageously be composed of a high yield strength metal composition, while the small particles of the second metal powder may be formed of either a high yield strength material, for example as disclosed herein for the first metal powder, or a lower strength, more ductile metal material. Thus, in some embodiments, the metal composition of the large metal particles has a yield stress greater than or approximately equal to the yield stress of the metal composition of the small metal particles. The inventors have demonstrated that by suitable selection of the impact mixing conditions, the large prealloyed Ti-6Al-4V spherical particles may be modified with either small particles of high yield strength prealloyed Ti-6Al-4V or small particles of relatively soft commercially pure titanium. Thus, in some embodiments, the small metal particles comprise a metal composition selected from the group consisting of commercially pure titanium, titanium alloys, titanium master alloys, and mixtures thereof.

[0069]

[86] Without wishing to be limited by any theory, it has been proposed that the mechanical properties of the larger and smaller particles should be matched to allow some mutual yielding and seizing between their surfaces upon impact to produce a core-corona morphology with metallurgical bonds between the core and the protrusions. In principle, only one surface should yield to allow for particle bonding. However, if the smaller particles are substantially harder than the larger particles, they are less likely to mutually yield and more likely to penetrate and embed (i.e., via mechanical bonds rather than metallurgical bonds). Thus, in some embodiments, the larger particles of the first metal powder have a harder or equal hardness and a higher or equal yield strength than the smaller particles of the second metal powder. However, it is believed that the smaller particles, despite having a higher inherent yield strength, may still be sufficiently deformed to form the desired core-corona morphology if the resolved shear stress of the smaller particles upon impact is higher. This is possible because the impact surface area of ​​the smaller particles may be smaller than the surface that impacts the larger particles. Thus, in a collision between a larger particle and a smaller particle, the smaller particle will experience a greater stress than the larger particle during at least a portion of the collision.

[0070]

[87] The small particles of the second metal powder may have any suitable particle shape. In some embodiments, the small particles may be substantially spherical particles, for example, as produced by gas atomization. The inventors have demonstrated that a small fraction (5-25 μm) of pre-alloyed Ti-6Al-4V particles produced by gas atomization can be used to form protrusions of non-spherical particles. In other embodiments, the small particles may have a non-spherical morphology, such as irregular, blocky, or angular. It has been found that small particles having a sphericity of only 0.3 can be used to form protrusions.

[0071] Precursor Powder

[88] The methods disclosed herein include combining at least a first metal powder and a second metal powder to provide a precursor powder including large and small metal particles. The two powders may be thoroughly mixed prior to the subsequent impact mixing step, but this is not required as mixing occurs during impact mixing. In some embodiments, the two metal powders are first combined in an impact mixer.

[0072]

[89] Due to the difference in particle size of the two metal powders, the precursor powder prior to impact mixing typically has a multimodal particle size distribution including at least a first peak corresponding to the first metal powder and a second peak corresponding to the second metal powder.

[0073]

[90] The first metal powder is typically the major component in the precursor powder by mass. In some embodiments, the first metal powder constitutes at least 70 wt% of the precursor powder, or at least 80 wt% of the precursor powder, for example between about 80 wt% and about 90 wt% of the precursor powder. The second metal powder typically forms the remainder of the precursor powder, although it is not excluded that additional metal powders may be combined with the first and second metal powders.

[0074] Impact Mixing

[91] The methods disclosed herein include subjecting the precursor powder to an impact mixing process to bond the small metal particles to the larger particles. As used herein, impact mixing refers to any dry powder mixing process that produces high velocity impacts between particles with sufficient strength to bond, and preferably metallurgically bond, the small metal particles to the larger particles. Suitable impact mixing processes include rotational impact mixing, sometimes referred to as hybridization, and various other dry particle coating techniques. In some embodiments, the impact mixing process is a rotational impact mixing process. In such processes, a rotor is rotated at high speed within an impact mixing chamber, and impact blades or other mechanical elements attached to the rotor impact the powder particles at high speed.

[0075]

[92] As seen in Figures 1, 2 and 3, the rotary impact mixing process can be carried out in an apparatus 100 that includes an impact mixing chamber 108 whose cylindrical outer wall is defined by a stator 110, and a rotor 112 that rotates within the impact mixing chamber. The impact mixing chamber 108 is surrounded by a removable front panel 113 and a rear wall 117. The generally disk-shaped rotor has an impact face 116 and an outer edge 118, and includes a plurality of radially arranged impact blades 114 at the rotor's periphery that are spaced apart from the cylindrical stator by a small gap 120. The rotor also includes radial ribs 115 on its rear side that are spaced apart from the rear wall 117 of the impact chamber by a narrow gap 119. The apparatus includes a recirculation passage 122 that extends between an inlet 124 located at the cylindrical outer wall of the impact mixing chamber and an outlet 126 located at the front panel 113 that is directed towards the center of the impact mixing chamber. Powder is added to the impact mixing chamber through inlet 128 and discharged through powder outlet 133, also located in the front panel 113, through discharge valve 137 and into powder outlet 135. Cooling water circulates through the stator through coolant ports 130, 132.

[0076]

[93] In use, the metal precursor powder is fed from a sealed container into the impact mixing chamber via a high-pressure flow of inert gas (e.g. argon). The rotor is rotated such that the impact blades sweep through the impact mixing chamber. Thus, the metal particles of the precursor powder are repeatedly struck at high speed by the impact faces of the impact blades. The rotating rotor also generates vortices that accelerate the particles towards the peripheral gap by centripetal force, and the blowing effect creates a strong recirculating gas flow through the recirculating passage, so that the powder circulates continuously through the recirculating passage during impact mixing. Thus, the accelerated particles collide with each other, impinge on the stator and the impact blades, and are sheared in the gap between the outer edge of the impact blades and the stator. The particles that move to the back of the rotor are pushed back to the periphery by the rotating ribs 115.

[0077]

[94] Such an apparatus, called the Nara Hybridization System (NHS-0), is commercially available from Nara Machinery Co., Ltd. Other suitable impact mixing devices include the Mechanofusion System and the Cyclomix from Hosokawa Micron Corporation.

[0078]

[95] Several parameters of the equipment and process in the rotary impact mixing process can be particularly important to the resulting powder morphology. Such parameters include the blade design, particularly the gap between the rotating blades and the stator (gap 120). The inventors have found that a gap distance of 3.5 mm is suitable for modifying pre-alloyed Ti-6Al-4V particles having a d50 particle size of about 120-160 μm, and it is expected that a similar gap distance will be suitable for impact mixing particles having sizes in the range of 1 μm to 500 μm. Thus, in some embodiments, the outer edge of each impact blade is spaced from the cylindrical outer wall by a gap distance in the range of 1 mm to 5 mm, for example in the range of 2 mm to 4 mm.

[0079]

[96] Another important parameter is the rotational speed of the rotor. Without wishing to be limited by any theory, it is proposed that the rotational speed of the impacting blades near the rotor periphery must be sufficient to generate the high intensity collisions between powder particles required for strong particle adhesion. The inventors have found that rotor peripheral speeds in the range of about 37.1 m / s to 61.8 m / s (equivalent to 6,000 rpm to 10,000 rpm) are suitable for modifying large particles of prealloyed Ti-6Al-4V having a d50 particle size in the range of 120 to 160 μm, although it will be appreciated that lower or higher speeds may be preferred for different powder compositions and particle sizes. In some embodiments, during impact mixing, the rotor is rotated such that the outer edge of each impacting blade has a peripheral speed of greater than 35 m / s, e.g., greater than 40 m / s.

[0080]

[97] A further important parameter is the duration of the impact mixing, which should be long enough to bond the small metal particles to the large core particles, but not so long that the modified particles become spheroidized. Particularly when the second metal powder contains a relatively ductile metal composition, excessive impact mixing times run the risk of the small particles coating the entire surface of the large core particles with a thin film, resulting in an undesirable core-shell morphology.

[0081]

[98] In some embodiments, the precursor powder is impact mixed for a time between 1 second and 10 minutes, for example between 30 seconds and 5 minutes. The time required to form the desired particle morphology may be inversely related to the intensity of the impact mixing conditions. The inventors have found through experimentation that when modifying pre-alloyed Ti-6Al-4V particles with a d50 of about 120-160 μm under very high impact conditions (rotor peripheral speed of about 62 m / s), impact mixing times of only a few seconds may be sufficient to form the desired morphology. Under milder mixing conditions (rotor peripheral speed of about 37-49 m / s), impact mixing times of 30 seconds to 4 minutes were typically suitable.

[0082]

[99] The precursor powders may be impact mixed in a dry inert gas atmosphere, which advantageously limits oxidation of the metal powders during impact mixing. Without being limited by theory, it is proposed that the newly exposed metal surfaces of particles formed during impact mixing in an inert atmosphere are not immediately sealed by an oxide layer and therefore remain capable of adhering to and intermixing with the metal surfaces of other particles.

[0083] Cold compactible metal powders containing non-spherical particles

[0100] Impact mixing of precursor powders under appropriate conditions results in adhesion of the smaller metal particles to the larger particles, producing non-spherical particles that contain one of the larger metal particles as a core and multiple smaller metal particles as protrusions from the core. This type of morphology is also referred to herein as a "core-corona" morphology. Impact mixed powders containing non-spherical core-corona particles have been found to have desirable cold compaction properties that cannot be attributed solely to the metal composition of the modified powder, and thus result from the modified particle morphology in the powder.

[0084]

[0101] 4 shows a schematic of an impact mixing process 400 for converting a precursor powder 410, including larger particles 412 (from a first metal powder) and smaller particles 414 (from a second metal powder), into a non-spherical particle 416 having a core-corona morphology. The particle 416 includes one of the larger metal particles 412 as a core 418 and multiple smaller metal particles 414 as protrusions 420 from the core. Some of the protrusions (420a) include a single small particle 414, while other protrusions (420b) include a collection of the smaller particles 414.

[0085]

[0102] Without wishing to be limited by any theory, the inventors propose, based on scanning electron microscope (SEM) analysis of traversed core-corona particles, that the small metal particles are metallurgically bonded to the larger particles along at least a portion of the interparticle interface. Such a state of chemical bonding is distinct from mere mechanical embedding of the kind that occurs during surface modification of soft nonmetallic core particles with hard guest particles. Thus, the large and small metal particles are effectively integrated into a single nonspherical metal particle as the core and protrusions. The mechanical integrity of the resulting nonspherical particles is believed to be important for desirable cold forming properties, since the adhesion of the protrusions to the core must withstand severe compressive forces when the particles are densified under pressure into a compact.

[0086]

[0103] Non-spherical particles typically contain multiple protrusions distributed around the periphery of the core particle. Each protrusion may contain a single small particle or a collection of small particles. The small particles may be deformed by impact forming a bond with the core particle or by high-velocity impact of the non-spherical particles during the subsequent impact mixing process. The degree of deformation may depend on the yield strength and ductility of the metal composition of the small particles, as well as the impact mixing conditions and time. Some deformation and spheroidization may be tolerated as long as the particles retain their core-corona morphology. However, if spheroidization is continued to the extent that the small particles attached to the core are beaten into a substantially uniform shell surrounding the core, it may adversely affect the cold forming properties of the powder.

[0087]

[0104] Particle morphology can be quantified by the convexity number, which is defined as the ratio of the perimeter of the convex hull of the particle to the perimeter of the object itself (both measured with respect to the cross section of the particle). In some embodiments, the convexity of a non-spherical particle is less than 0.8, for example in the range of 0.4 to 0.8.

[0088]

[0105] It is not necessary for all particles in the impact mixed metal powder to exhibit the core-corona morphology, and good cold forming properties may be obtained even if only a portion of the particles have this morphology. Thus, in some embodiments, the cold formable metal powder comprises at least 20 wt%, e.g., at least 50 wt%, or at least 60 wt%, of non-spherical particles comprising one larger metal particle as a core and multiple smaller metal particles as protrusions from the core.

[0089]

[0106] In some embodiments, the cold-formable metal powder comprises non-spherical particles including a larger titanium or titanium alloy particle as a core and a plurality of smaller metal particles as protrusions from the core. The smaller metal particles may comprise a metal composition selected from the group consisting of commercially pure titanium, titanium alloys, master alloys for titanium, and mixtures thereof. The smaller metal particles may be metallurgically bonded to the larger titanium or titanium alloy particles.

[0090] Method for producing porous metal articles

[0107] The present invention also relates to a method of producing a porous metal article. The method includes providing a cold-formable metal powder comprising non-spherical particles. The non-spherical particles include a larger metal particle as a core and a plurality of smaller metal particles as protrusions from the core. The smaller metal particles may be metallurgically bonded to the larger metal particle along at least a portion of an interface. The non-spherical particles may be produced by the methods disclosed herein above. The method includes producing a porous metal article by subjecting the cold-formable metal powder to a cold forming process at a pressure sufficient to densify the cold-formable metal powder.

[0091]

[0108] Cold compaction processing may involve any cold forming method in which metal powder, with or without a binder, is densified under pressure at a temperature below the sintering temperature to produce a porous metal body (compact) with sufficient structural integrity to withstand further processing, such as by sintering or other metal processing techniques, to produce the final metal product. Suitable cold compaction techniques may include cold isostatic pressing, cold die compaction, direct powder rolling, and metal injection molding.

[0092]

[0109] In cold isostatic pressing, metal powder is enclosed in a low-resistance forming mold, such as a rubber bag, and hydraulic pressure is applied to the mold. As a result, the powder is compressed uniformly over the entire surface of the green compact as hydraulic pressure is transmitted through the flexible mold. In cold mold pressing, also known as metal mould pressing, metal powder is filled into a mold (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, metal powder is compressed between a pair of rollers to form a continuous green sheet. In metal injection molding, metal powder is mixed with a binder to produce an injection material, which is injection molded to produce a preform. After molding, the preform must undergo further processing to remove the binder.

[0093]

[0110] In some embodiments, the cold-formable metal powders are subjected to cold-forming processing in the absence of organic binders, or in the absence of non-metallic binders, or in the absence of other binders. The cold-formable metal powders provided by the present disclosure are particularly useful for binderless cold-forming processing because particle adhesion is enhanced by the core-corona particle morphology.

[0094]

[0111] Without wishing to be bound by any theory, it is proposed that the protrusions of the non-spherical particles promote interlocking of the particles during compaction and increase the contact area at the interface between adjacent particles in the compact. This increases adhesion between the particles and improves the mechanical properties of the resulting compact. Figure 5 shows a schematic of some interlocking conditions that are proposed to occur between adjacent non-spherical particles 516 in a porous metal compact. At some interparticle interfaces, such as interface 510ab between particles 516a and 516b, the particles engage through protrusions 520a and 520b of both particles. At other interparticle interfaces, such as interface 510bc between particles 516b and 516c, the engagement of the particles is promoted by protrusions 520c of only one particle. When the protrusions comprise a relatively soft metal (e.g., CP Ti), the joining of some of the protrusions on adjacent particles may also promote densification of the particles in the compact. However, such bonding was not observed for non-spherical particles with harder protrusions (eg, Ti-6Al-4V), which still yielded excellent forming properties.

[0095]

[0112] The cold compaction process densifies the cold compactable metal powder to produce a porous metal article. Thus, voids exist between the compacted metal particles in the metal structure, which voids are vacant after the binderless compaction process. The porosity of the metal article depends on the particle morphology, the particle deformability under compaction pressure (deformability may be low if the particle core is formed of a high yield strength composition), and the compaction pressure. The inventors have found that compaction of non-spherical particles, including cores and protrusions formed from high yield strength pre-alloyed Ti-6Al-4V particles, produces compacts with good mechanical properties when the density of the compact is greater than about 71% of theoretical density (based on the density of Ti-6Al-4V). Thus, in some embodiments, the porous metal article has a density of at least 70% of theoretical density. However, it will be understood that the porosity of a suitably robust compact may vary in other implementations depending on the factors mentioned above. It is further contemplated that the density of a porous compact may be increased by adding small particles to a cold-formable metal powder that are sized to occupy some of the voids between the interlocking non-spherical particles after compaction.

[0096]

[0113] The cold-formable metal powder may be compacted at any pressure sufficient to densify the cold-formable metal powder to form a porous metal article. The cold-formable metal powder of the present disclosure, including non-spherical particles with a core-corona morphology, may be advantageously compacted at pressures significantly lower than those required for rounded (e.g., spherical) particles lacking protrusions but having a similar metal composition. The inventors have found that non-spherical particles with cores and protrusions formed from high yield strength pre-alloyed Ti-6Al-4V particles can be compacted to form robust compacts at pressures of only 345 MPa. When the protrusions are formed from a more ductile material (CP Ti), densification at even lower pressures (206 MPa) was achievable. In contrast, unmodified pre-alloyed Ti-6Al-4V particles are not cold-formable at 413 MPa, and it is expected that pressures in excess of 1000 MPa may be required to densify these spherical particles. Thus, in some embodiments, the cold-formable metal powder is formed at a pressure of less than 450 MPa, or less than 400 MPa, such as less than 350 MPa, for example less than 300 MPa.

[0097]

[0114] The porous metal articles produced by the methods disclosed herein are typically not final metal products, but rather are further processed. In some embodiments, the porous metal articles are sintered to produce sintered metal structures that can be further processed by conventional metal processing techniques. In other embodiments, the porous metal articles can be feedstock for metal production techniques such as extrusion. In one exemplary application, the porous metal article is a titanium alloy compact, e.g., a cylindrical bar having dimensions of 12.5 mm in diameter and 400 mm in length, suitable for extrusion to make titanium alloy wire as disclosed in U.S. Pat. No. 9,468,960. EXAMPLES

[0098]

[0115] The invention will now be described with reference to the following examples which should be understood as illustrative and not limiting of the invention described herein.

[0099] material

[0116] Highly spherical, gas-atomized titanium alloy Ti-6Al-4V (Ti64) powder was obtained from a commercial manufacturer as 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 irregular angular morphology was obtained from TIPRO International Co. Ltd. A master alloy of Al60V40 was obtained from Reading Alloys, Inc.

[0100]

[0117] A portion of the Ti64-L1 powder was sieved to prepare a powder with a narrower particle size distribution (passing 100# mesh, <150μm, hereafter referred to as Ti64-L2). A portion of the CP Ti powder was sieved (passing 400# mesh, <38μm, hereafter referred to as CPTi-S2). A portion of the Al60V40 master alloy was also sieved (passing 635# mesh, <20μm, hereafter referred to as Al60V40-S2).

[0101]

[0118] The as-received particles were characterized by scanning electron microscopy to determine sphericity. Sphericity is a measure of the degree to which the particles in the powder approach a spherical shape. Sphericity is measured using image analysis software (in this case "Image J") based on the imaged cross-section of the particle. Sphericity is defined as the ratio of the radius of the inscribed circle to the radius of the circumscribed circle with respect to the imaged cross-section of the particle (where the inscribed circle is the largest circle inside the particle cross-section and the circumscribed circle is the smallest circle outside the particle cross-section, both circles are centered on the center of mass of the particle). The average values ​​of sphericity were: Ti64-L1 = 0.795 ± 0.038; Ti64-S1 = 0.930 ± 0.050; CP Ti = 0.370 ± 0.019; Al60V40 = 0.438 ± 0.108.

[0102]

[0119] The particles were characterized for their composition by inductively coupled plasma optical emission spectroscopy (ICP-OES), their morphology by scanning electron microscopy (SEM) (ZEISS Merlin™ FE-SEM), and particle size distribution (PSD) by Mastersizer S. The results are shown in Tables 1 and 2 below.

[0103] [Table 1]

[0104] [Table 2]

[0105]

[0120] Titanium alloy shavings generated as a by-product of the aerospace production process ("as-received shavings") were characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) and found to have a composition of Ti-6Al-4V alloy containing approximately 0.19% oxygen, within the specification for Grade 5 of that alloy.

[0106]

[0121] The as-received chips were characterized by scanning electron microscopy (SEM, ZEISS Merlin™ FE-SEM) to investigate their morphology. The chip particles were observed to be flaky with some curvature, a structure typical of materials produced by machining processes. There were also numerous and varied defects on the surface of the chips, such as cracks, jagged edges, and pitting. The thickness of the flaky chip particles ranged from 30 to 100 μm, and the grain size (maximum dimension) was less than 5 mm in length.

[0107]

[0122] The as-received chips were subjected to sieve analysis to determine the particle size distribution according to ASTM B214. The d10, d50 and d90 particle sizes of the chips were estimated to be 0.64 mm, 1.25 mm and 2.2 mm, respectively.

[0108]

[0123] The apparent and tapped densities of the as-received chips were 0.616 g / cm3, according to ASTM B417 and ASTM B527, respectively. 3 and 0.751 g / cm 3 The theoretical density of Ti-6Al-4V alloy was measured to be 4.429 g / cm. 3 Its very low apparent density value compared to is consistent with the poor packing efficiency of the curved flaky particles.

[0109] Impact mixing apparatus and method

[0124] The Nara Hybridization System (NHS-0), available from Nara Machinery Co., Ltd. and shown diagrammatically in Figure 1, was used as the particle modification device. The system includes an impact mixing chamber defined by a cylindrical stator, a rotor, and a recirculation tube. During impact mixing, particles can exit the impact mixing chamber via the outlet of the stator and are re-fed to the center of the impact mixing chamber via the recirculation tube. The chamber is surrounded by a jacket in which a coolant is circulated to keep the internal processing temperature below 100°C, typically below 50°C. The NHS-0 was operated under a high purity argon (3 ppm O2) atmosphere to keep oxygen levels as low as possible and reduce the chance of oxygen contamination of the titanium powder material.

[0110]

[0125] Schematic diagrams of the NHS-0 impingement mixing chamber are shown in Figures 1, 2 and 3. The outer wall of the impingement mixing chamber is defined by a stator 110. A rotor 112 of diameter 118 mm includes six radially arranged impact blades 114 at the rotor periphery, having an impact face 116 and an outer edge 118. The impact blades have a radial length of 20 mm, a thickness of 5 mm and flat edges. The gap 120 between the outer edge 118 of the blades and the stator 110 was 3.5 mm. The rotor also includes radial ribs 115 on the rear side, which are spaced from the rear wall 117 of the impingement chamber by a gap 119 of 0.9 mm.

[0111]

[0126] The NHS-0 can be operated at rotor rotation speeds up to 16,000 rpm. In general, speeds between 6,000 rpm and 10,000 rpm have been found to be most suitable. The corresponding conversion from rotation speed to peripheral speed (i.e., the speed of the outer edge 118 of the blade) is shown in Table 3. The device 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 mixing times referred to in the following examples refer to the time after the set rotation speed is reached.

[0112] [Table 3]

[0113]

[0127] Unless otherwise stated, a single dose of 30 g of powder was treated with NHS-0 in each experiment.

[0114] Cold isostatic pressing apparatus and method

[0128] Cold isostatic pressing (CIP) at a maximum pressure of 413 MP (60 ksi) was used for the cold compaction studies. The flexible molds for the cold isostatic pressing experiments were elongated cylindrical bags with inner diameters (ID) ranging from 9 mm (latex bags) to 24 mm (latex / rubber bags). For an interesting application in wire fabrication by extrusion, the required dimensions of the "green rods" were about 12.5 mm in diameter and about 400 mm in length, and flexible bags with an ID of 14 mm were required to produce these rods. Each rod thus weighed about 180-200 g. However, due to the limited processing capacity of the NHS-0 hybridizer, in most cases smaller bags with an ID of 9 mm were used to form mini-rods to demonstrate the compactibility of the powder. Each mini-rod had dimensions of about 8.5 mm ID and 30-50 mm in length, and weighed about 6.0 grams.

[0115]

[0129] Some studies used a "double bagging" method, in which a thin polyethylene tube (50-150 μm thick) was placed inside the rubber / latex outer bag to reduce the risk of failure of the green compacts during demolding. There are three factors that affect the risk of failure of the green compacts: (1) the tensile stress inside the flexible mold induced from adhesion and friction at the interface during isostatic molding, (2) the interfacial strength induced from adhesion and friction at the interface, and (3) the strength of the resulting green compacts. The flexible mold tends to return to its original shape after the hydrostatic pressure is released. If the tensile stress inside the flexible mold exceeds the interfacial strength, delamination occurs between the mold and the compact. However, such delamination does not occur all at once, resulting in non-uniform stress in the compact. If this non-uniform stress exceeds the strength of the green compact, fracture perpendicular to the longitudinal direction of the compact can occur.

[0116]

[0130] After cold isostatic pressing of some metal powders, the green compacts are quite strong so that a relatively stiff single rubber bag can be used. However, when cold isostatic pressing of more difficult to mold powders, the green compacts may not be strong enough to overcome the tensile stresses that arise inside the stiff rubber bag, resulting in breakage of the compact. The double bagging method helps to alleviate this problem, as there is little gripping force between the outermost flexible bag and the green compact, protecting the compact from breakage. After cold isostatic pressing, the outermost bag can be retrieved for further use and the innermost thin polyethylene tube is peeled off from the intact green compact.

[0117]

[0131] The following general procedure was used for the cold isostatic pressing experiments. Fill the flexible bag with powder, tapping it frequently to ensure the powder is as even and dense as possible. Insert a stopper into the opening of the bag and fasten it with a rubber band to tightly seal the bag and ensure that the pressing liquid cannot come into contact with the powder compact. Fixing the bag to an elongated metal support, such as an L-shaped metal angle (fixing the elongated bag inside the angle) or a cylindrical aluminum grille (fixing the elongated bag inside a cylinder), to keep the bag upright during CIP and also to apply uniform pressure to the outside of the bag; The secured bag is placed into a cage configured to be placed in a CIP pressure chamber. · Carry out CIP and hydrostatically press the green compact to a pre-set pressure. After pressing, remove the green rod from the flexible bag.

[0118]

[0132] Cold formability of the powder was defined as the formation of an intact, non-friable solid bar after removal from the bag (friable means that the compact breaks down easily back into powder during handling). In Figure 6, titanium alloy powders in (a), (b) and (c) are considered "non-formable". However, powder in (d) is considered "formable".

[0119] Example 1

[0133] As-received oversized Ti64 powder (Ti64-L1) and undersized Ti64 powder (Ti64-S1) were combined and mixed in three different weight ratios: 80:20, 85:15, and 90:10. The combined powders were then subjected to rotational impact mixing in an NHS-0 system at various rotor rotation speeds (6,000-10,000 rpm) and different impact mixing times (1 s-4 min). The impact mixed powders were then collected and analyzed by SEM to investigate the resulting morphology. Selected samples were also analyzed by ICP-OES.

[0120]

[0134] For the mixed powders with an 80:20 ratio of Ti64-L1 and Ti64-S1, impact mixing at low impact conditions (6,000 rpm for 2 min; 8,000 rpm for 1 sec) did not significantly change the particle morphology from the initial bimodal particle distribution. However, impact mixing at stronger impact conditions changed the particle morphology and size distribution, and the resulting powder was composed primarily of non-spherical particles with a large metal particle (from Ti64-L1) as the core and multiple small metal particles (from Ti64-S1) as protrusions from the core. A representative SEM image (impact mixing at 8000 rpm for 2 min) is shown in Figure 7. Similar morphologies were evident for other impact mixed powders as shown in Table 4 below.

[0121]

[0135] The impact mixed powders having such a "core-corona" morphology were then subjected to cold isostatic pressing at a pressure of 413 MPa to determine whether they could be used to form cold-formed bars (mini-bars having dimensions of about 8.5 mm inside diameter and 30-50 mm in length). The results are shown in Table 4 below, where "OK" indicates that the impact mixed powders were cold formable at 413 MPa, and "NO" indicates that cold forming was not achieved at that pressure (and "No IB" refers to the powder mixture before impact mixing).

[0122] [Table 4]

[0123]

[0136] Impact mixing at moderate (8,000 rpm, 1-4 min; 9,000 rpm, 0.5-1 min) or high (10,000 rpm, 1 min) impact conditions thus produced powders with core-corona morphology that were cold compactable at pressures of 413 MPa to form robust compacts. In contrast, initial mixtures of particles, i.e., not impact mixed, were not cold compactable under these conditions. This result is consistent with previous work that found that pressures of 1000 MPa were required to cold compact a similar spherical prealloy (Ti-10V-2Fe-3Al) powder (Luo et al., Journal of Materials Processing Technology, 2014, 214, 660-666).

[0124]

[0137] At one of the milder impact mixing conditions (6,000 rpm, 3 min), the core-corona morphology was evident in the SEM images, but the powder was not cold compactable 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 cold isostatic pressing.

[0125]

[0138] For the powder mix of Ti64-L1 and Ti64-S1 with a ratio of 85:15, impact mixing at moderate impact conditions (8,000 rpm for 1-4 min; 9,000 rpm for 0.5-3 min) produced powders with a core-corona morphology. As seen in Table 5, these powders were found to be cold compactable at a pressure of 413 MPa and to form robust compacts. In contrast, the initial mixture of particles, i.e., not impact mixed, was not cold compactable at these conditions.

[0126] [Table 5]

[0127]

[0139] For the mixed powders with a 90:10 ratio of Ti64-L1 and Ti64-S1, impact mixing at very low impact conditions (6,000 rpm for 3 min; 8,000 rpm for 1-3 min) produced no core-corona particles or only an insufficient number of core-corona particles. Thus, these powders were not cold compactable at a pressure of 413 MPa, as seen in Table 6. Impact mixing at moderate impact conditions (9,000 rpm for 0.5-1 min; 10,000 rpm for 1 s-0.5 min) produced powders with core-corona morphology and thus cold compactable at a pressure of 413 MPa. It is noteworthy that when a sufficiently high impact mixing speed (10,000 rpm) was used, impact mixing times as little as 1 s (once the set rotation speed was reached) were sufficient to produce the core-corona morphology, as seen in Figure 8.

[0128]

[0140] Although formable, the compacts made from a 90:10 ratio of Ti64-L1 and Ti64-S1 were relatively weak and brittle, suggesting that 10 wt% small particles is close to the lower limit required for cold formability in the particular case of Ti64-L1 and Ti64-S1 powder mixtures.

[0129] [Table 6]

[0130]

[0141] These experiments demonstrate that spherical metal powders (off-spec atomized pre-alloyed Ti64 powder; over- and under-sized; etc.) can be improved by the methods disclosed herein to be suitable feedstocks for cold compaction powder metallurgy techniques (such as cold isostatic pressing to form rod-shaped compacts for use in extrusion processing to fabricate wire). The cost of over-atomized Ti64 powder (100-250 μm) is approximately US$2.50 / kg, which contrasts with the price of additive manufacturing grade Ti64 powder (15-45 μm) which is US$250-650 / kg.

[0131] Example 2

[0142] Impact mixed powders prepared by impact mixing precursor powders with Ti64-L1 and Ti64-S1 ratios of 80:20, 85:15, and 90:10 at 9,000 rpm for 60 s were further characterized by measuring their compositions by ICP-OES and their particle size distributions (PSDs) by Mastersizer S. ICP-OES results showed that the oxygen contents of the three impact mixed powders were all in the range of 0.11-0.12%, which was similar to the starting concentration (0.09%) and well within the specifications of ASTM grades 5 and 23 (maximum 0.20% and 0.13%, respectively). No iron contamination occurred due to processing in the NHS-0 system.

[0132]

[0143] The PSD results are shown in Table 7. There was minimal difference between the particle size distributions of the shock-mixed powders with different ratios of Ti64-L1 and Ti64-S1. The shock-mixed powders were slightly larger than the Ti64-L1 precursor due to the smaller particles adhering to the larger core particles. The specific surface area of ​​the shock-mixed powders increased by approximately 16%-20% compared to the Ti64-L1 precursor, consistent with a change in its morphology.

[0133] [Table 7]

[0134] Example 3

[0144] Based on the experimental results of Example 1, a set of impact mixing conditions optimized for scale-up work was identified, which consisted of impact mixing a 30 g batch of precursor powder composed of Ti64-L1 and Ti64-S1 in a ratio of 85:15 for 1 minute in an NHS-0 with a rotor speed of 9,000 rpm. Seven batches of impact mixed powder were generated under these conditions to prepare enough material to produce a green bar (green compact) with dimensions of 12.5 mm in diameter and 400 mm in length.

[0135]

[0145] In the cold isostatic pressing step, a double bagging method was used to minimize the effect of gripping forces between the flexible mould and the green compacts. Different set compaction pressures were investigated: 206 MPa (30 ksi), 275 MPa (40 ksi), 345 MPa (50 ksi) and 413 MPa (60 ksi).

[0136]

[0146] The green bars formed at 206 MPa and 275 MPa broke apart in several places during demolding. However, above 345 MPa, intact and robust green bars were formed. Thus, in the specific case of a mixture of impact mixed Ti64-L1 and Ti64-S1 powders, formed into a bar of 12.5 mm diameter and 400 mm length, a minimum pressure of approximately 345 MPa was found to be required to form a compacted structure dense enough to withstand the stresses of demolding.

[0137]

[0147] The density of the green bars was measured according to ASTM B962-17 and the results are shown below in Table 8. It was seen that density increased with increasing pressure and a minimum green density of about 71% of the theoretical density of Ti64 appears to be necessary to provide sufficient green strength for demolding and subsequent processing in the extrusion process.

[0138] [Table 8]

[0139] Example 4

[0148] To improve the formability of spherical, high-yield strength Ti64 particles, impact mixing with CP Ti was investigated along with atomized Ti64 powder, based on the initial hypothesis that the ductile CP Ti acts as a binder.

[0140]

[0149] Therefore, sieved (<150 μm) Ti64 powder (Ti64-L2) and sieved CP Ti powder (CPTi-S2) were combined and mixed in a weight ratio of 80:20. The combined powders were then subjected to rotational impact mixing in an NHS-0 system at various rotor rotation speeds (6,000-10,000 rpm) for different impact mixing times (1 s-10 min). The impact mixed powders were collected and analyzed by SEM to investigate the resulting morphologies.

[0141]

[0150] The impact mixed powder was then subjected to cold isostatic pressing at pressures of 206 MPa and / or 413 MPa to determine whether cold formed bars (mini bars having dimensions of about 8.5 mm inner diameter and 30-50 mm length) could be formed. The results are shown in Table 9 below, where "OK" indicates that the impact mixed powder was cold formable at 206 MPa, "NO" indicates that cold forming was not achieved at 206 MPa, and "NO*" indicates that cold forming was not achieved even at 413 MPa.

[0142] [Table 9]

[0143]

[0151] These results can be explained based on the morphology of the impact mixed powders as seen in the SEM images. Impact mixing at very low impact conditions (6,000 rpm for 2 min; 8,000 rpm for 1 sec) produced no or insufficient number of core-corona particles. Thus, as seen in Table 9, these powders were not cold compactable even at a pressure of 413 MPa, similar to the pre-impact mixed mixtures.

[0144]

[0152] Impact mixing at moderate impact conditions (6,000 rpm, 4 min; 8,000 rpm, 1–3 min; 10,000 rpm, 1 s) produced powders that had a core-corona morphology and were therefore cold compactable at pressures of only 206 MPa. Representative SEM images are shown in Figure 9 (8000 rpm for 1 min) and Figure 10 (6000 rpm for 4 min).

[0145]

[0153] Impact mixing 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 powder morphologies in which the ductile CP Ti was impacted and formed a uniform coating over the entire surface of the spherical Ti64 core particles. Representative SEM images are shown in Figure 11 (8,000 rpm for 4 min) and Figure 12 (10,000 rpm for 2 min). Thus, the impact mixed particles were spherical, similar to the pristine Ti64-L2 particles, and could not be cold compacted despite the ductile CP Ti membrane coating.

[0146]

[0154] These experiments indicate that the core-corona particle morphology of the impact mixed powder is important to achieve adequate cold formability. The presence of ductile CP Ti in the precursor powder was not sufficient in itself to facilitate cold forming of powders composed primarily of spherical prealloyed Ti64 particles. Mixtures of separate CP Ti particles and spherical prealloyed Ti64 were not cold formable unless CP Ti was the major component (at least 70 wt% CP Ti was required to cold form the mixture at 206 MPa, and at least 60 wt% CP Ti was required at 413 MPa). Furthermore, spherical particles containing a Ti64 core coated with a uniform shell of CP Ti were also not cold formable. Without wishing to be limited by any theory, it is proposed that the improved cold formability properties result from improved interlocking of the non-spherical core-corona particles in the impact mixed powder.

[0147] Example 5

[0155] Impact mixing of Ti64 powder with premixed Ti64 powder was investigated to determine whether spherical, high yield strength Ti64 particles could be improved for cold forming applications without affecting its metal composition. Particulate premixed Ti64 powder (BE Ti64) was obtained by combining sieved CP Ti powder (CPTi-S2) with sieved Al60V40 master alloy (Al60V40-S2) in an appropriate ratio (90:10).

[0148]

[0156] Sieved (<150 μm) Ti64 prealloy powder (Ti64-L2) and BE Ti64 were combined and mixed in a weight ratio of 80:20. The combined powders were then subjected to rotary impact mixing in an NHS-0 system at 8,000 rpm for 1 or 2 minutes. The impact mixed powders were collected and analyzed by SEM to investigate the resulting morphology. While lower impact conditions may have been desirable, the impact mixed powders still contained some particles with core-corona morphology, as seen in FIG. 13 (8,000 rpm for 1 minute).

[0149]

[0157] The impact mixed powder was then subjected to cold isostatic pressing at pressures of 206 MPa and 413 MPa to determine whether cold-formed bars could be formed. The impact mixed powder was found to be cold formable at 206 MPa, but the resulting green compacts were relatively weak and brittle. Nevertheless, the cold formability results contrasted with the initial particle mixture, i.e., before impact mixing, which was not cold formable even at 413 MPa.

[0150]

[0158] ICP-OES analysis of the impact mixed powder (8,000 rpm for 1 minute) is shown below in Table 10 along with a comparison to the Ti64-L2 prealloy starting material. As intended, the impact mixed powder with improved cold forming properties has substantially the same composition as the Ti64-L2 precursor. The oxygen content of the impact mixed powder was 0.13%, well within the ASTM Grade 5 and 23 specifications (maximum 0.20% and 0.13%, respectively).

[0151] [Table 10]

[0152] Example 6

[0159] In this example, a metal powder containing large, spherical metal particles suitable for modification according to the methods disclosed herein was prepared from titanium alloy chips using the same impact mixing equipment used for rotary impact mixing in the previous examples.

[0153]

[0160] The as-received chips were subjected to five consecutive grinding runs at 10,000 rpm for 30 seconds in an NHS-0 system. In the first grinding run, 10 batches of 100 g chips were processed, limited to 10 g per run due to the low apparent chip density. After the first grinding run, the ground material from the 10 batches was sieved into three size ranges: ~250 microns, 250-500 microns, and +500 microns. In the second and subsequent grinding runs (using up to 50 g of material per batch), the two larger fractions were ground separately and sorted into the same three size fractions. The newly formed ~250 micron fraction was combined with the previously formed ~250 micron fraction, the two newly formed 250-500 micron fractions, and the two newly formed +500 micron fractions were combined for the next grinding run.

[0154]

[0161] After five grinding passes, 65% of the initial chips had a particle size less than 250 μm, 30% of the particles were in the range of 250-500 μm, and only 3% of the particles were larger than 500 μm. Advantageously, the yield of recovered material after five grinding passes was nearly 97%. The crushed chip particles still exhibited a flaky, non-spherical morphology, although the surface was smoother compared to the initial chips.

[0155]

[0162] The ~250 μm fraction of particles was then further divided by sieving into ~75 μm, 75-150 μm and 150-250 μm fractions. Each size range of particles was then separately subjected to grinding in NHS-0 at 10,000 rpm for 14 min to spheronize 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 indicating that the spheronization step can be performed at a higher packing concentration in the impact chamber than the grinding step.

[0156]

[0163] Spheroidization and surface smoothing of the ground chip particles occurred in all three particle size ranges, although the degree of spheroidization differed slightly. Under the same grinding conditions, the most effective spheroidization occurred in the particle size range of 150–250 μm. The particle morphology of this fraction before and after spheroidization is shown in Figures 14 and 15, respectively.

[0157]

[0164] To quantify the effect of the grinding step on particle morphology, the powder sphericity was characterized using image analysis of SEM images. The sphericity values ​​of the as-received chips, the powder fed for spheroidization (ground material divided into fractions of ~75 μm, 75–150 μm, and 150–250 μm), and the resulting spheroidized powder (still in fractions of ~75 μm, 75–150 μm, and 150–250 μm) are shown in Table 11, where the average sphericity is the average of 10 particles. The as-received chips are very irregular and their sphericity remains low for all fractions even after grinding. After the spheroidization step, the sphericity is significantly improved, especially for the two larger fractions.

[0158] [Table 11]

[0159] Example 7

[0165] The three portions of spheroidized particles prepared in Example 6 were again combined (~250 μm) and mixed with 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 rotary impact mixing (15 g per batch) in an NHS-0 system at various rotor rotation speeds (6,000-9,000 rpm) for 30 seconds. The impact mixed powders were then collected and analyzed by SEM to investigate the resulting morphology.

[0160]

[0166] Under low impact conditions (6000 rpm for 30 s), only a few of the small Ti64-S1 particles were attached to the larger spheroidized particles. The majority of the Ti64-S1 particles remained unaffected by grinding. Under slightly more intense grinding conditions (7000 rpm for 30 s), more small particles were attached. After impact mixing under medium intensity conditions (8000 rpm for 30 s), a significant portion of the small Ti64-S1 particles were attached to the larger spheroidized particles. The resulting powder thus contained non-spheroidal particles with a large metal particle (originating from the previously spheroidized particle) as a core and multiple small metal particles (originating from Ti64-S1) as protrusions from the core. Representative SEM images are shown in Figure 16. It was also evident that under even higher intensity conditions (9000 rpm for 30 s), most of the small Ti64-S1 particles were attached to the larger spheroidized particles, and the resulting protrusions were somewhat flattened.

[0161]

[0167] As a result of the modification, only slight changes were evident in the particle size distribution of the spheroidized metal powders (~250 μm). 10 , d 50 and d 90 The values ​​of α,β,β changed from 55 μm, 122 μm, and 236 μm to 31 μm, 118 μm, and 253 μm after impact mixing at 8,000 rpm for 30 s. However, the specific surface area was 0.0141 m due to the formation of a core-corona morphology. 2 / g to 0.0193m 2 / g.

[0162] Example 8

[0168] The powder produced in Example 7, which contained non-spherical particles with a large spheroidized metal particle as a core and multiple smaller metal particles as protrusions from the core, was then subjected to cold isostatic pressing (CIPing) at a pressure of 380 MPa (55 ksi) to determine whether the powder was cold compactable. The powder was compacted to form "mini-bars" having dimensions of approximately 8.5 mm in diameter and 30-50 mm in length.

[0163]

[0169] Although each powder was found to be cold compactable, the mini-bars formed from the powders produced by low intensity grinding (6000 rpm, 30 seconds) had lower strength than the other bars. In contrast, the metal powders with spheroidized particles of ∼250 μm (i.e., not modified with small particles of Ti64-S1) were found not to be compactable at 380 MPa. These results indicate that spheroidized metal powders produced by milling and spheroidization of chips can be improved for powder metallurgy applications involving cold compaction by the impact mixing method of the present disclosure.

[0164] Example 9

[0170] The impact mixed Ti64 powder with core-corona morphology produced in Example 3 (Ti64-L1 and Ti64-S1 in a ratio of 85:15, impact mixed at 9,000 rpm for 1 min) was used to investigate the bonding state between the core (derived from the large Ti64-L1 particles) and the protrusions (derived from the small Ti64-S1 particles).

[0165]

[0171] The impact mixed powder particles were cold mounted and polished to 1200 grit using SiC paper and final polished using OP-S suspension. The polished samples were etched with Kroll's reagent to identify and inspect the interface between the large Ti64-L1 core particles and the small Ti64-S1 particles. The impact mixed powder was then characterized by SEM (ZEISS Merlin™ FE-SEM) at high magnification.

[0166]

[0172] SEM images of the cross section of one core-corona Ti64 particle are shown in Figure 17. There were approximately seven sections (B-H) where small particles were attached to and protruding from the core particle, and these sections were examined at high magnification to determine the nature of the interfaces. In one section (section B), many small particles were present in clusters that were bonded to the core particle. The interfaces between the core and small particles, and between adjacent small particles within a cluster, were also identifiable as dark grey lines in the magnified SEM image. Despite some very small voids along the interfaces, the integrity of the interfaces is generally consistent with metallurgical bonding between the particles along at least part of the interface. Similar interfaces between the core particle and protruding particles were also seen in other parts of the cross section.

[0167]

[0173] From the observation of impact mixing of small and large Ti64 particles, the following conclusions and suggestions were derived: (i) In the initial stage of impact mixing, the surfaces of the large particles are roughened and potentially hardened due to collisions with the small particles, the large particles, and the impact mixer parts. (ii) The small particles, which are more stressed due to their smaller size and higher critical resolved shear stress, deform preferentially when impacting the large particles and bond to each other to form protrusions. (iii) The surfaces of both the small and large particles yield and intermix to some extent during impact, allowing partial metallurgical bonding between the core and the protrusions. (iv) In addition to single particle protrusions, protrusions containing clusters of small particles are formed due to the probability of small particles overlapping and colliding with protrusions already formed on the core particle and / or due to the aggregation of small particles into clusters, which subsequently collide with and bond to the larger core particles. (v) The metal surfaces of the newly exposed particles are not sealed by oxides (or near oxides) due to the argon atmosphere, and therefore can more easily adhere to each other and intermix to allow bonding. (vi) The heat generated in the impact mixing process, especially the localized heat at the site of impact, can contribute to particle adhesion via the formation of metallurgical bonds.

[0168]

[0174] The impact mixed powders with core-corona morphology generated from hard large and soft 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 analyzed by SEM. The resulting protrusions were generally smaller than those on the non-spherical particles in Example 3, due in part to the greater ductility of the small particles. However, high resolution SEM analysis of the core-protrusion interface again showed that the CP Ti particles were metallurgically bonded to the Ti64 alloy core in both experiments.

[0169] Example 10

[0175] To investigate how the impact mixed powders respond to compaction, the Ti64 green compacts (large grains of pre-alloyed Ti64 + small grains of pre-alloyed Ti64) produced by cold isostatic pressing at 206 MPa in Example 3 were characterized by SEM (ZEISS Merlin™ FE-SEM) at high magnification. Cross sections of the green compacts were cold fixed and polished to 1200 grit using SiC paper and final polished using OP-S suspension. To identify and examine the interfaces between the grains in the green compacts, the polished samples were etched with Kroll's reagent prior to SEM analysis.

[0170]

[0176] From the SEM images shown in Figures 18 and 19, it is clear that: (i) the core-corona morphology of many particles remains intact after compaction, indicating the strength of the bond between the large Ti64-L1 core particles and the small Ti64-S1 particles that form the protrusions, (ii) the large core particles are not significantly deformed by compaction, (iii) at many interfaces between adjacent particles (highlighted by circles), protrusions from one or both particles are present, and (iv) the compacted interfaces between adjacent non-spherical particles are devoid of any evidence of metallurgical bonding (in contrast to the core-protrusion interfaces within non-spherical particles). It is proposed that these protrusions promote interlocking between particles during compaction, resulting in greater mechanical strength in the cold compact. A simple model of such interlocking engagement is shown in Figure 5.

[0171]

[0177] Cold compacts made from impact mixed powders containing large particles of prealloyed Ti64 + small particles of CP Ti in Example 4 (Example 4: 8,000 rpm, 2 min) were also characterized by SEM analysis. Again, at many interfaces between adjacent particles, protrusions from one or both particles were found to be present, suggesting that these protrusions promote interlocking between particles during compaction. However, in contrast to the impact mixed particles in Example 3, some deformation was observed in the soft CP Ti protrusions, and it is believed that the joining of some protrusions on adjacent particles may promote densification of the particles in the compact. In addition, free CP Ti particles were also observed in the compact. These particles either did not adhere to the core Ti64 particles during impact mixing or peeled off from the non-spherical particles in response to the applied stress during the compaction process.

[0172] Example 11

[0178] From the results of Examples 1, 3, 4, 5 and 8, it is clear that the core-corona morphology of the impact mixed core-corona non-spherical particles results in improved cold compaction properties compared to the substantially spherical precursor particles. For quantitative comparison, the convexity of the precursor and impact mixed 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 measured in cross section. The convexity measurement is obtained by forming the ratio of the perimeter of the convex hull of the particle to the perimeter of the object itself according to the following formula: A convex hull is a polygon that encloses the cross section of the particle without any point of the polygon bending inwards.

[0173]

number

[0174]

[0179] For objects with convex particle cross-sections (such as round particles or ellipses with smooth surfaces), the convex hull and the perimeter of the object are the same, so the convexity is 1. If the object has irregular boundaries, the value will be less than 1. For impact-mixed particles, if their convexity value is close to 1, their shape is close to the original core particle before impact mixing.

[0175]

[0180] The perimeter convexity measurements of the precursor Ti64 powder and the impact mixed powders produced in Examples 3, 4, 5 and 8 are shown in Table 12. The measurements were taken from SEM images processed using the image processing software Image J. At least 10 measurements were taken for each particle composition.

[0176] [Table 12]

[0177]

[0181] The convexity of the unmodified Ti64 powder and the spheroidized chip powder was very close to 1, consistent with the spherical nature of the particles. After impact mixing with different materials, the convexity value is significantly reduced due to the protrusions from the core. The impact mixed powder (large particles of prealloyed Ti64 + small particles of prealloyed Ti64) produced in Example 3 had the smallest convexity, because the protrusions of the small particles were relatively undeformed due to the hard metal composition. The powders impact mixed with small particles of CP Ti or mixed raw powder Ti64, respectively, produced in Examples 4 and 5, had higher convexity because the softer metal protrusions were somewhat flattened after impact mixing. Nevertheless, all impact mixed particles were cold compactable at 206 MPa.

[0178]

[0182] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, and it is understood that the invention includes all such variations and modifications that fall within the spirit and scope of the claims.

Claims

1. 1. A method for producing a cold-compactable metal powder, comprising: (i) providing a first metal powder comprising large metal particles and (ii) a second metal powder comprising small metal particles, wherein a d50 particle size of the second metal powder is smaller than a d50 particle size of the first metal powder; combining at least the first metal powder and the second metal powder to provide a precursor powder comprising the large metal particles and the small metal particles; and subjecting the precursor powder to an impact mixing process to cause the small metal particles to adhere to the large metal particles, thereby producing a cold-compactable metal powder comprising non-spherical particles, 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.

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. 10. The method of claim 1, wherein the d50 particle size of the second metal powder is no greater than 40% of the d50 particle size of the first metal powder.

4. The method of claim 1 , wherein the large metal particles and the small metal particles have substantially the same metal composition.

5. The method of claim 1 , wherein the large metal particles in the first metal powder are substantially spherical.

6. the step of subjecting the precursor powder to the impact mixing process comprises impact mixing the precursor powder in an impact mixing chamber of an apparatus comprising: (i) a stator defining an outer cylindrical wall of the impact mixing chamber; and (ii) a rotor rotatably operable within the impact mixing chamber, the rotor including a plurality of impact blades on a periphery of the rotor, the impact blades having an impact surface and an outer edge; 10. The method of claim 1, wherein the apparatus further comprises a recirculation passage between an inlet located on the outer cylindrical wall of the impact mixing chamber and an outlet directed toward the center of the impact mixing chamber, the precursor powder being continuously recirculated through the recirculation passage during the impact mixing.

7. an outer edge of each impact blade is spaced from the outer cylindrical wall by a gap distance in the range of 1 mm to 5 mm; 7. The method of claim 6, wherein the rotor is rotated during the impact mixing such that the outer edge of each impact blade has a peripheral speed greater than 35 m / s.

8. 10. The method of claim 1, wherein the precursor powder is impingement mixed in a dry inert gas atmosphere for a time between 1 second and 10 minutes.

9. 9. The method of claim 1, wherein the large metal particles have a metal composition with a yield stress of at least 600 MPa.

10. 9. The method of claim 1, wherein the large metal particles have a metal composition with an elongation at break of at least 1%.

11. 9. The method of any one of claims 1 to 8, wherein the large metal particles, and optionally the small metal particles, have a metal 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 containing both aluminum and copper.

12. 9. The method of any one of claims 1 to 8, wherein the large metal particles, and optionally the small metal particles, have a metal composition selected from the group consisting of titanium, tantalum, niobium, and alloys thereof.

13. 9. The method of any one of claims 1 to 8, wherein the large metal particles, and optionally also the small metal particles, comprise titanium or a titanium alloy.

14. 9. The method of any one of claims 1 to 8, wherein the larger metal particles comprise a titanium alloy and the smaller metal particles have a metal composition selected from the group consisting of commercially pure titanium, titanium alloys, master alloys for titanium, and mixtures thereof.

15. 9. The method of any one of claims 1 to 8, wherein the first metal powder has a d50 particle size between 80 μm and 500 μm.

16. 1. A cold-formable metal powder comprising non-spherical particles, the non-spherical particles comprising a large titanium or titanium alloy particle as a core and a plurality of smaller metal particles as protrusions from the core, at least a portion of the smaller metal particles being metallurgically bonded to the large titanium or titanium alloy particle.

17. 1. A method of making a porous metal article, comprising: (i) providing a first metal powder comprising large metal particles and (ii) a second metal powder comprising small metal particles, wherein a d50 particle size of the second metal powder is smaller than a d50 particle size of the first metal powder; combining at least the first metal powder and the second metal powder to provide a precursor powder comprising the large metal particles and the small metal particles; producing a cold-formable metal powder comprising non-spherical particles by subjecting the precursor powder to an impact mixing process to cause the small metal particles to adhere to the large metal particles, 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; producing a porous metal article by subjecting the cold-formable metal powder to a cold-forming process at a pressure sufficient to densify the cold-formable metal powder.

18. The method of claim 17, wherein the porous metal article has a density of at least 70% of theoretical density.

19. the pressure is less than 450 MPa; 19. The method of claim 17 or 18, wherein the cold-formable metal powder is subjected to the cold-forming treatment in the absence of a binder.

20. 19. The method of claim 17 or 18, wherein the cold compaction process is selected from cold isostatic pressing, cold die compaction and direct powder rolling.

Images