Composite powder metal compositions

Abstract

 Metal powder compositions for producing a high-strength and high-density metal component by powder metallurgical techniques. A metal powder composition of the present invention comprises acicular metal particles (10) associated with a polymeric binder (25) or a combination of inorganic binders (35) and polymeric binders (25), such as by coating or mixing. When the powder composition is compacted by a powder metallurgical compaction technique, the resulting compact (40) has an improved density and an improved strength. The compact (40) can be used in applications such as a structural component or as a magnetic core component for AC or DC applications.

Description

Field of the Invention

[0001] This invention generally relates to powder metallurgy materials and processes and, more particularly, relates to metal powder compositions which, when compacted, yield compacts having a high strength and a high density.

Background of the Invention

[0002] Powder metallurgy techniques offer significant advantages over conventional metalworking processes that utilize multiple processing steps to form finished components by casting or making it as a wrought product. Among other advantages, powder metallurgy techniques reduce costs and labor by providing a finished component having the net-shape or the near-net-shape of the finished component. Powder metallurgy techniques provide rapid, high-volume and economic production methods for mass producing precision metal parts. These metal parts include common structural components with engineered shapes, such as gears, cams, brackets and the like, and magnetic core components for alternating current (AC) or direct current (DC) applications, such as transformers, inductors, AC and DC motors, generators, relays and the like.

[0003] Powder metallurgy techniques consolidate a quantity of metal powder to form a shaped structure known as a compact. The elemental profile and composition of the metal powder is selected as appropriate to provide physical, metallurgical, mechanical and electromagnetic properties suitable for the intended application. For example, a metal powder of an iron or an iron alloy is usually used to form magnetic core components. To improve the properties of the compact, the composition of the metal powder may be modified by adding a binder. For example, binders comprising insulating additives can be added to the metal powder before compaction to provide interparticle electrical insulation essential for optimizing the performance of magnetic core components. Some binders act as a cementing agent for enhancing the strength of the green compact and, after compaction, can be removed by a heat treatment. Other binders can improve the compactibility or moldability of the metal powder for enhancing the density of the compact.

[0004] The tensile properties, such as tensile strength and yield strength, of the compact are determined to a great extent by density and pore size. Strength can be improved by reducing the porosity of the compact, such as by sintering after pressing or adding an appropriate binder to the powder before pressing. The improvement in density due to sintering intimately relates to the green density which, if low, limits the consolidation of the pore structure. In addition, the ability of a binder to improve the strength is offset by the continued presence of the binder in the green compact. Before sintering, the binder is usually removed by a preliminary heat treatment or delube. However, for AC magnetic core components, the green compact is not heat treated because the high temperatures may compromise or otherwise disrupt the insulating layer separating the metal particles. Thus, sintering is not a viable option for densifying compacts destined for such AC applications.

[0005] Secondary processes can also be used to increase the density of the compact, such as infiltration or double-pressing/double-sintering. Infiltration reduces the porosity and, thereby, improves the strength by filling the pores of the compact with a low-melting point metal. Because the infiltrant metal usually has a high conductivity, infiltration is particularly unsuitable for reducing the porosity of AC and DC magnetic core components. Double pressing/double sintering significantly increases the manufacturing expense and production time so that powder metallurgy loses its advantages over conventional metalworking processes. Therefore, traditional methods for improving the density and strength of a compact have deficiencies and shortcomings that limit the application of powder metallurgy techniques for making precision metal parts.

[0006] The size and geometry of the metal particles in the metal powder are factors that influence the green density of the compact. Common metal powders have a spherical or near-spherical geometry. Compacts formed by cold uniaxial single-pressing of spherical metal powders have green densities of about 90% of the theoretical density. Warm pressing can improve the density of a green compact formed from a spherical powder to about 95% of the theoretical density.

[0007] U.S. Pat. No. 5,594,186 (Krause et al.) discloses substantially linear, acicular metal particles having a substantially triangular cross section that, when compacted by a single, cold uniaxial pressing operation, provide compacts having a green density of at least 95% of full theoretical density. The acicular shape enhances density by improving deformation of the particles during compaction and by improving the interlocking between adjacent metal particles. However, compacts formed from the acicular metal particles, in general, have a low strength that is inadequate for many magnetic core and structural components.

[0008] Compacts must have adequate physical and mechanical properties to fulfill the functional performance requirements of the metal component. Compacts formed from traditional metal powders have heretofore had densities and strengths that are unacceptable for use as structural and magnetic core components in many applications. Although methods are available to improve one or more of these properties, the degree of improvement is limited and components fabricated from traditional metal powders have been limited in their application. Moreover, magnetic core components compacted from traditional metal powders incorporating an insulating binder have heretofore had outputs unacceptably low for critical AC applications.

[0009] There is thus a need for a metal powder composition which, when compacted, yields a green compact having a high strength and a high density.

Summary of the Invention

[0010] The present invention provides methods of manufacturing a high-density and high-strength metal component. The methods comprise providing a plurality of acicular metal particles, wherein the acicular metal particles are substantially linear, nonspiraled particles having a substantially triangular cross section. A polymeric binder is associated with the exterior surfaces of a large fraction of the plurality of acicular metal particles to form a composite metal powder. Finally, the composite metal powder is compacted to form a green compact, wherein the presence of the binder improves the strength and density of the compact for use as a metal component.

[0011] The present invention also provides a composite metal powder composition suitable for forming a high-density and high-strength compact. The composition comprises a plurality of acicular metal particles, wherein the acicular metal particles are substantially linear, nonspiraled, and have a substantially triangular cross-sectional profile, and a polymeric binder is associated with the exterior surfaces of a large fraction of the plurality of acicular particles. The binder promotes the strength and density of a compact formed from the composite metal powder.

[0012] The presence of the binder in the composite powder improves the green strength of the compact while retaining the improvement in green density afforded by the acicular particles.

Brief Description of the Drawings

[0013] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. In the drawings wherein like reference numerals represent like parts:

FIG. 1 is a perspective view of an acicular metal particle of the present invention;

FIG. 2 is an enlarged cross-sectional view of a coated surface of the acicular particle of FIG. 1 having a layer of polymeric material;

FIG. 3 is an enlarged cross-sectional view of an alternative embodiment of the coated surface of the acicular particle of FIG. 1, similar to FIG. 2, wherein the coated surface has an inner layer of an inorganic binder and an outer layer of a polymeric binder; and

FIG. 4 is a perspective view of a high-density, high-strength compact formed by a powder-metallurgy compaction technique from a collection of metal particles similar to the metal particles of FIG. 1.

Detailed Description

[0014] According to the present invention, a powder metal composition is provided in which a polymeric binder or a combination of a polymeric binder and an inorganic binder is associated with the exterior of acicular metal particles. When appropriately compacted, the association of the binder with the acicular metal particles promotes the strength of the green compact while retaining the increase in density afforded by the acicular shape of the metal particles. The powder metal composition may also include other additives, such as lubricants, that promote the flow of the particles during compaction or that promote various physical, metallurgical, mechanical and electromagnetic properties desired for the resulting metal component.

[0015] Referring to FIG. 1, an acicular metal particle, in accordance with the present invention and indicated generally by reference numeral 10, has an elongated rod-like geometry or morphology which is substantially linear and nonspiraled. The acicular metal particle 10 has three longitudinal faces 11, 12 and 13 and a substantially triangular cross-sectional profile when viewed parallel to a first end 14 or a second end 15. The longitudinal faces 11, 12 and 13 may be substantially flat, concave, convex or combinations thereof. The substantially triangular cross-sectional profile at first end 14 may differ from the substantially triangular cross-sectional profile present at second end 15. In addition, the acicular metal particle 10 has substantially triangular cross-sectional profile for any normal cross-sectional cut through the acicular metal particle 10 at a point along the length of the longitudinal faces 11, 12 and 13.

[0016] The triangular cross-sectional profile at end 14 comprises a base 16, which constitutes the longest side, and two shorter sides 17 and 18. The triangular cross-sectional profile is further defined by an altitude or height 19 perpendicular to the base 16 and extending to the vertex 20 opposite base 16. The base 16 and sides 17 and 18 may be linear or curvilinear. Those of ordinary skill understand that the acicular metal particle 10 has a triangular cross-sectional profile at any point along the length of each longitudinal face 11, 12, and 13.

[0017] Nonspiraled, substantially linear acicular metal particles, similar to particle 10, having a substantially triangular cross section of a type particularly useful in the present invention are disclosed in U.S. Pat. No. 5,594,186 (Krause et al.). The disclosure of the Krauss patent is hereby incorporated by reference herein in its entirety. Generally, the triangular cross section of each acicular metal particle has a height to base ratio of about 0.08:1 to about 1:1. The acicular metal particles have a length-to-base ratio of at least about 3 to 1. Generally, the acicular metal particles have a length of about 0.006 to about 0.20 inches, a base of about 0.002 to about 0.05 inches, and a height of about 0.002 to about 0.05 inches. When filling a die, the acicular metal particles have a die fill ratio of less than 3 to 1. The acicular metal particles may be compacted to produce a compact having a green density of at least 95% of full theoretical density.

[0018] Acicular metal particles, similar to acicular particle 10 and suitable for use in the present invention, are a comminuted metal powder formed by machining or milling a bulk source of a metal or a metal alloy to generate shavings or chips having suitable size, dimensions, and geometry. Suitable bulk sources for the acicular metal particles include rolled sheets or blocks but the present invention is not so limited. The operational parameters of the milling or machining operation determine the size, dimension and geometry of the acicular metal particles. The processes and apparatus used to form the acicular metal particles are familiar to those of ordinary skill in the art.

[0019] Suitable elemental compositions for the bulk material used to form the acicular particles will depend on the particular component and application and may encompass ferrous and non-ferrous metals and metal alloys. Elemental compositions for the bulk material to form acicular particles suitable for AC and DC electromagnetic components include iron, iron alloys (iron-silicon alloys, iron-phosphorus alloys, Fe-Si-Al alloys, ferrites, magnetic stainless steels, etc.), nickel, nickel alloys, cobalt, and cobalt alloys. Suitable elemental compositions for the bulk material to form acicular particles for use as structural components include ferrous materials such as iron and iron alloys (stainless steels, high-performance steels, low-alloy steels, etc.) and nonferrous materials such as noble metals (rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, etc.), aluminum, copper, magnesium, titanium, tungsten, zinc, and their alloys.

[0020] The green strength of a compact depends upon the compressibility of the material forming the acicular particles. Exemplary low-compressibility metal powders from the above listed elemental compositions that would benefit from the present invention include acicular particles composed of stainless steels, low-alloy steels, tungsten, titanium, and other low-compressibility materials recognized by those of ordinary skill in the art of powder metallurgy.

[0021] As a preliminary step after the milling or machining process, the acicular particles may be mechanically blended to thoroughly intermingle and homogeneously distribute acicular particles of varying size and geometry throughout the metal powder. Conventional mechanical blenders and mixers are suitable for mixing the acicular particles and include drum blenders, cubical-shaped blenders, double-cone mixers, twin-shell AV≅ blenders, conical screw blenders, and other mixing and blending devices known to those of ordinary skill in the art of powder metallurgy.

[0022] Metallurgical additives and inorganic materials are added to the acicular particles to promote the physical, metallurgical, mechanical and electromagnetic properties desired for the product. In situations where a permanent binder is desired, such as for interparticle electrical insulation in AC electromagnetic components, a polymeric binder or a combination of a polymeric binder and an inorganic binder may be associated with the acicular particles prior to compaction. Because the binder promotes an increase in green strength and green density, binders have also been employed as temporary additives to powder mixtures. In structural components and DC electromagnetic components, a polymeric binder may be employed to promote the strength and density of the green compact. For these applications, the green compact is usually delubed to remove the binder and sintered to obtain the physical, metallurgical, mechanical and electromagnetic properties desired for the component.

[0023] In accordance with one embodiment of the present invention, a composite metal powder is produced by associating a small amount of a polymeric binder with the acicular particles. Referring to Figs. 1 and 2, the acicular particle 10 is covered by a thin layer 25 of a polymeric binder. The amount of polymeric binder associated with each acicular particle ranges from about 0.1 wt.% to about 10 wt.% and typically ranges from about 0.2 wt.% to about 0.5 wt.%. Suitable materials for the polymeric binder of the present invention include thermoplastic resins, thermoset resins, and combinations thereof. Suitable thermoplastic resins include, but are not limited to, polyether-imides, polyether sulfones, polyamide-imides, polyamides, polyimides, polycarbonates, and combinations thereof. Suitable thermosetting resins include, but are not limited to, phenolics, polyesters, epoxies, alkyds, silicones, and combinations thereof.

[0024] Because the melting point of most polymeric materials is less than the melting point of the acicular metal particles, the polymeric binder may be expelled, if desired, by a heat treatment following compaction. For example, the polymeric binder may be pyrolyzed by a relatively low-temperature heat treatment that is often an initial cycle of the sintering process. However, compacts destined for AC applications are not sintered because the polymeric binder provides valuable interparticle electrical insulation required for such applications.

[0025] Polymeric materials suitable for the polymeric binder of the present invention include thermoplastic resins, thermoset resins, and combinations thereof. Suitable thermoplastic resins include, but are not limited to, polyether-imides, polyether sulfones, polyamide-imides, polyamides, polyimides, polycarbonates, and combinations thereof. Suitable thermosetting resins include, but are not limited to, phenolics, polyesters, epoxies, alkyds, silicones, and combinations thereof. An exemplary family of polyether-imide resins is available commercially from General Electric Company under the trade name ULTEM⁷. An exemplary polyether sulfone resin is commercially available from BP Amoco under the trade name UDEL⁷. A suitable polyamide-imide resin is available commercially from BP Amoco under the trade name TORLON⁷. Exemplary commercial thermosetting resins include the family of DUREZ⁷ phenolic compounds available from Occidental Chemical Corporation.

[0026] Polymeric binders may be associated with the acicular metal particles by various methods, as understood by those of ordinary skill in the art of powder metallurgy. To achieve the optimum results, the binder should be associated with a significant fraction, if not all, of the total number of acicular metal particles in a collection to be compacted. Conventional mechanical mixing methods may be used to homogeneously distribute a powdered binder with the acicular chips to particles of varying geometry. Common mechanical mixers used to perform the dry mixing include drum blenders, cubical-shaped blenders, double-cone mixers, twin-shell AV≅ blenders, and conical screw blenders. Alternatively, the polymeric binder, may be applied as a coating in a fluidized bed process, such as by using a Wurster coater. In a Wurster coating process, the acicular particles are fluidized in air. The polymeric binder is dissolved in an appropriate organic solvent and the solution is sprayed through an atomizing nozzle to contact and wet the fluidized bed of acicular particles. When the solvent evaporates, a layer of the polymeric binder remains which coats the exteriors of the acicular particles. The layer of polymeric binder may be uniform in thickness and coverage so as to encapsulate each metal particle. Processes that combine mechanical mixing and solution coating, such as solution-blending, solution AV≅-blending and slurry-mixing, for associating an organic binder with the acicular metal particles are also within the spirit and scope of the present invention.

[0027] In an alternative embodiment, the acicular particles may be coated with an inorganic binder before the association of the polymeric binder. The inorganic binder may be added to promote one or more of the physical, metallurgical, mechanical and electromagnetic properties of the green compact. Typically, the inorganic binding agent is added to improve the electromagnetic properties of the compact, such as providing interparticle electrical insulation or supplementing a magnetic property of the compact. The inorganic binder is applied to, associated with, deposited on, or grown on the exteriors of the acicular particles by various methods and processes understood by those of ordinary skill in the art. The amount of inorganic binder added to the metal powder may range from about 0.01 wt.% to about 1 wt.% and usually ranges from about 0.025 wt.% to about 0.2 wt.%. However, the desired amount of inorganic binder in the admixture will depend upon the particular AC application.

[0028] With regard to acicular powders having both polymeric and inorganic binders for use in fabricating AC magnetic core components, the inorganic binder is usually applied prior to applying the polymeric binder. FIG. 3 shows the acicular particle 10 with an outer thin layer 30 of a polymeric binder overlying an inner thin layer 35 of an inorganic binder.

[0029] Exemplary inorganic binders include silicates (sodium silicate, potassium silicate, silica, etc.), metal oxides (alumina, zirconia, calcia, beryllia, steatite, etc.), phosphates (ferrous phosphate, ferric phosphate, etc.), borides (aluminum boride, etc.), nitrides (boron nitride, silicon nitride, titanium nitride, etc.), carbides (silicon carbide, boron carbide, zirconium carbide, titanium carbide, etc.), ferrites (magnesium ferrite, aluminum ferrite, manganese ferrite, copper ferrite, zinc ferrite, nickel ferrite, cobalt ferrite, strontium ferrite, potassium ferrite, iron ferrite, etc.), or magnesium-based formulations lacking oxygen, such as magnesium methylate.

[0030] A lubricant may be added to the metal powder for reducing friction between adjacent particles and improving the flow rate of particles during compaction. To that end, the composite metal powders of the present invention may be mechanically mixed with a powdered lubricant to produce a homogeneous mixture of metal powder and powdered lubricant. Generally, powdered lubricant is added to the composite metal powder in an amount of about 0.01 to about 1.0 wt.%, by total weight of the composite powder, and typically about 0.025 wt.%. Suitable lubricants for use with the present invention include stearates, fluorocarbons, waxes, low-melting polymers and synthetic waxes. One particularly suitable lubricant is ethylene bis-stearamide, which is commercially available, for example, under the trade name ACRAWAX⁷ from Lonza, Inc.

[0031] Any conventional powder metallurgy method may be used to compact the composite metal powder and create a compact having a significant increase in green strength without a reduction in the green density. Accordingly, the metal powder may be compacted by conventional powder metallurgy compaction techniques such as uniaxial or die pressing, cold and hot isostatic pressing, and dynamic magnetic compaction. In uniaxial pressing, for example, a predetermined amount of the composite acicular powder is fed into a precision die body operably positioned in a mechanical or hydraulic compacting press. The die may include one or more core rods for forming holes in the compact. The composite acicular powder in the die body may be pressed between opposing punches both moving relative to the die or pressed in a stationary die body between one moving punch and one fixed punch. The moving punch or punches transmit a compaction pressure in a uniaxial direction to the composite acicular powder confined in the die body. For uniaxial pressing, a typical compaction pressure ranges from about 5 tons/in² to about 100 tons/in². It is understood that the required compaction pressure is dependent upon the composition and compactability of the metal powder and, therefore, depends upon the compressibility of the selected composite metal powder. The compaction can be performed at a chilled temperature, at ambient or room temperature, or at an elevated temperature. Warm pressing involves heating the die to a given temperature and heating the powders within the die to a lesser temperature. Typically, warm pressing with a compaction temperature between about 150EC to about 180EC is optimal for producing a high-density compact. It is generally known that a moderate increase in the compaction temperature increases the green density for a given applied pressure.

[0032] Compaction consolidates the composite acicular powder into a green compact having a near-net shape or net shape of the desired finished magnetic core or structural component. The strength and the density of a compact are directly related parameters. A compact formed using the composite acicular metal powder of the present invention has a significantly higher green strength than a compact formed in an identical fashion from a metal powder of acicular particles absent the associated binder. A typical density for a green compact compacted by uniaxial pressing from one of the composite metal powders of the present invention has a density at least 95% of the theoretical density. When compared with metal powders of spherical or near-spherical particle geometry, the composite metal powder of the present invention affords a significant increase in density, as well as strength.

[0033] Green compacts formed from the composite metal powder of the present invention may have improved magnetic properties that are directly related to the improvement in density. For example, green compacts compacted from the composite metal powder of the present invention are more like to be suitable for use as AC electromagnetic components, such as core elements for generators or motors, because the compacts have significantly better physical and mechanical properties and a significantly higher output. With regard to the improved output, the high permeability, high flux carrying capacity, and low hysterisis loss of a green compact are believed to arise from the morphology of the particles of the composite metal powder. In particular, the composite metal powder of acicular particle and binder is a hybrid with attributes of laminations and of a compacted spherical metal powder in its magnetic properties.

[0034] An example of a high-density, high-strength compact 40 suitable for use as an AC magnetic core component is shown in FIG. 4. Compact 40 has been compacted from a collection of acicular metal particles, similar to acicular metal particle 10 as shown in FIG. 2, by a conventional powder-metallurgy compaction technique, such as uniaxial pressing. Compact 40 is a magnetic core component, commonly known as a stator core, suitable for use in an AC motor or an AC generator, and demonstrates the intricately shaped parts that can be mass-produced by powder metallurgy techniques using the composite metal powders of the present invention.

[0035] In DC applications or in structural applications that produce engineered shapes, green compacts formed from the composite metal powders of acicular particles and polymeric binder may be heat treated. After compaction, the green compact may be heated in a burn off cycle at a first temperature and for a sufficient time to decompose, or pyrolyze, the polymeric binder and any lubricant. The delubed compact may then be sintered at a second temperature sufficiently high and for a sufficient time to develop metallurgical bonds by mass transfer between the acicular metal particles for increasing the density and strength of the component.

[0036] Any conventional powder metallurgy sintering technique may be used to heat treat and sinter the green compact. Suitable conventional sintering techniques include low temperature sintering, high temperature belt/atmosphere sintering, hot isostatic pressing, and vacuum furnace sintering. For example, in a high temperature belt/atmosphere sintering, the green compact is conveyed through a multi-stage furnace filled having a controlled atmosphere, such as argon, nitrogen, hydrogen or combinations thereof. Other types of furnaces and furnace atmospheres are within the scope of the present invention as determined by one skilled in the art. In an exemplary high temperature belt/atmosphere sintering, the lubricant is pyrolyzed in a burn-off cycle at about 760°C, and held at that temperature for about one hour. After the burn-off cycle, the temperature is raised to a sintering temperature in excess of 1000EC, typically about 1100°C, and the compact is held at the sintering temperature for about 20 to about 90 minutes. The sintering temperature is generally higher than one-half of the melting point of the metal composing the composite powder.

[0037] Sintering metallurgically fuses adjacent acicular particles and reduces the porosity of the compact. Therefore, sintering improves the mechanical and physical properties of the compact by densifying and strengthening the structure. A compact formed by compacting the acicular particles of the present invention has a significantly higher strength and a significantly higher density, after sintering, than a compact processed in an identical fashion from spherical or near-spherical particles lacking an associated binder.

[0038] The following example illustrates the improvement in green strength for a compact formed from the composite metal powder compositions of the invention.

EXAMPLE

[0039] An iron powder comprising acicular particles was provided by milling a rolled sheet of iron. A first portion of the iron powder was solution V-blended with a magnesium methylate solution. When the solution dried, the acicular particles had a residual coating of magnesium methylate. Thereafter, the acicular particles were V-blended with 0.025 wt.% of ACRAWAX⁷ lubricant. Compacts were produced by uniaxially compacting quantities of the first portion of the iron powder in a press at a compaction pressure of 55 tons/in² and a temperature of 175EC. The compacts had green densities of about 7.60 g/cm³ and had green strengths of about 1000 to about 2000 psi.

[0040] A second portion of the iron powder was also solution V-blended with magnesium methylate. A third portion of the iron powder was left uncoated. The second and third portions were coated with a binder of phenolic resin. Compacts were formed by uniaxially compacting quantities of the second and third portions of the iron powder in a press at a compaction pressure of 55 tons/in² and a temperature of 125EC. Generally, compacts formed from the second and third portions of the iron powder had green densities of about 7.60 g/cm³ and green strengths ranging from about 12000 to about 15000 psi. The significant improvement in the green strength is directly related to the presence of the binder and is achieved while retaining the improved green density of the compact arising from the acicular shape of the iron particles.

[0041] While the present invention has been illustrated by the description of embodiments and an example thereof, and while the embodiments and example have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of applicant=s general inventive concept.

Claims

1. A method of manufacturing a high density and high strength metal component comprising:

providing a plurality of acicular metal particles (10), wherein the acicular metal particles (10) are substantially linear, nonspiraled particles (10) having a substantially triangular cross section and an exterior surface;

associating a polymeric binder (25) with the exterior surfaces of a large fraction of the plurality of acicular metal particles (10) to form a composite metal powder; and

compacting the composite metal powder to form a green compact (40), wherein the presence of the polymeric binder (25) improves the strength and density of the compact (40) for use as a metal component.

2. The method of claim 1, further comprising heating the green compact (40) for a sufficient time and at a sufficient temperature to pyrolyze the binder (25).

3. The method of claim 1, further comprising heating the green compact (40) for a sufficient time and at a sufficient temperature to form bonds between adjacent metal particles (10) and, thereby, form a sintered compact (40).

4. The method of claim 1, wherein the step of providing a plurality of acicular metal particles (10) comprises milling a bulk source of a metal or a metal alloy to generate the acicular particles (10).

5. The method of claim 1, further comprising, before associating the polymeric binder (25), associating an inorganic binder (35) with the exterior surfaces of a large fraction of the acicular metal particles (10), wherein the presence of the inorganic binder (35) promotes a physical, metallurgical, mechanical or electromagnetic property of a compact (40) formed from the composite metal powder.

6. The method of claim 5, wherein the inorganic binder (35) is selected from the group consisting of silicates, metal oxides, phosphates, borides, nitrides, carbides, ferrites and combinations thereof.

7. The method of claim 5, wherein the inorganic binder (35) is present in an amount ranging from about 0.01 wt.% to about 1 wt.%.

8. The method of claim 7, wherein the inorganic binder (35) is present in an amount ranging from about 0.025 wt.% to about 0.2 wt.%.

9. The method of claim 5, wherein the associating of the inorganic binder (35) comprises associating as a substantially uniform coating of the inorganic binder (35) that substantially encapsulates each acicular metal particle (10).

10. The method of claim 1, wherein the polymeric binder (25) is a thermoplastic resin.

11. The method of claim 10, wherein the thermoplastic resin is selected from the group consisting of polyether-imides, polyether sulfones, polyamide-imides, polyamides, polyimides, polycarbonates, and combinations thereof.

12. The method of claim 1, wherein the polymeric binder (25) is a thermoset resin.

13. The method of claim 12, wherein the thermoset resin is selected from the group consisting of phenolics, polyesters, epoxies, alkyds, silicones, and combinations thereof.

14. The method of claim 1, wherein the polymeric binder (25) is present in an amount ranging from about 0.1 wt.% to about 10 wt.%.

15. The method of claim 14, wherein the polymeric binder (25) is present with a range from about 0.2 wt.% to about 0.5 wt.%.

16. The method of claim 1, wherein the associating of the polymeric binder (25) comprises associating as a substantially uniform coating of the polymeric binder (25) that substantially encapsulates each acicular metal particle (10).

17. The method of claim 1, wherein each acicular metal particle (10) comprises a metal selected from the group of iron and iron alloys, nickel and nickel alloys, cobalt and cobalt alloys, and combinations thereof.

18. The method of claim 1, wherein each acicular metal particle (10) comprises a metal selected from the group of iron and iron alloys, rhenium and rhenium alloys, ruthenium and ruthenium alloys, rhodium and rhodium alloys, palladium and palladium alloys, silver and silver alloys, osmium and osmium alloys, iridium and iridium alloys, platinum and platinum alloys, gold and gold alloys, aluminum and aluminum alloys, copper and copper alloys, magnesium and magnesium alloys, titanium and titanium alloys, tungsten and tungsten alloys, zinc and zinc alloys, and combinations thereof.

19. A composite metal powder composition for forming a high-density and high-strength compact (40), comprising:

a plurality of acicular metal particles (10), wherein the acicular metal particles (10) are substantially linear, nonspiraled particles (10) having a substantially triangular cross-sectional profile and an exterior surface; and

a polymeric binder (25) associated with the exterior surfaces of a large fraction of the plurality of acicular particles (10), wherein said polymeric binder (25) promotes the strength and density of a compact (40) formed from the composite metal powder.

20. The composition of claim 19, wherein said polymeric binder (25) comprises a substantially uniform coating that encapsulates said acicular particles (10).

21. The composition of claim 19, wherein the polymeric binder (25) is a thermoplastic resin.

22. The composition of claim 21, wherein the thermoplastic resin is selected from the group consisting of polyether-imides, polyether sulfones, polyamide-imides, polyamides, polyimides, polycarbonates, and combinations thereof.

23. The composition of claim 19, wherein the polymeric binder (25) is a thermoset resin.

24. The composition of claim 23, wherein the thermoset resin is selected from the group consisting of phenolics, polyesters, epoxies, alkyds, silicones, and combinations thereof.

25. The composition of claim 19, wherein the polymeric binder (25) is present with a range from about 0.1 wt.% to about 10 wt.%.

26. The composition of claim 25, wherein the polymeric binder (25) is present with a range from about 0.2 wt.% to about 0.5 wt.%.

27. The composition of claim 19, wherein each acicular metal particle (10) comprises a metal selected from the group of iron and iron alloys, nickel and nickel alloys, cobalt and cobalt alloys, and combinations thereof.

28. The composition of claim 19, wherein each acicular metal particle (10) comprises a metal selected from the group of iron and iron alloys, rhenium and rhenium alloys, ruthenium and ruthenium alloys, rhodium and rhodium alloys, palladium and palladium alloys, silver and silver alloys, osmium and osmium alloys, iridium and iridium alloys, platinum and platinum alloys, gold and gold alloys, aluminum and aluminum alloys, copper and copper alloys, magnesium and magnesium alloys, titanium and titanium alloys, tungsten and tungsten alloys, zinc and zinc alloys, and combinations thereof.

29. A compact (40) comprising the composite metal powder composition of claim 19 having a density of at least about 95% of theoretical density.

30. A composite metal powder composition for forming a high-density and high-strength compact (40), comprising:

a plurality of acicular metal particles (10), wherein the acicular metal particles (10) are substantially linear, nonspiraled particles (10) having a substantially triangular cross-sectional profile and an exterior surface;

an inorganic binder coating (35) on the exterior surfaces of a large fraction of the plurality of acicular particles (10); and

a polymeric binder (25) associated with the coated exterior surfaces of the large fraction of the plurality of acicular particles (10), wherein the polymeric binder (25) promotes the strength and density of a compact (40) formed from the composite metal powder.

31. The composition of claim 30, wherein said polymeric binder (25) comprises a substantially uniform coating that encapsulates said acicular particles (10).

32. The composition of claim 30, wherein the polymeric binder (25) is a thermoplastic resin.

33. The composition of claim 30, wherein the thermoplastic resin is selected from the group consisting of polyether-imides, polyether sulfones, polyamide-imides, polyamides, polyimides, polycarbonates, and combinations thereof.

34. The composition of claim 30, wherein the polymeric binder (25) is a thermoset resin.

35. The composition of claim 34, wherein the thermoset resin is selected from the group consisting of phenolics, polyesters, epoxies, alkyds, silicones, and combinations thereof.

36. The composition of claim 30, wherein the polymeric binder (25) is present with a range from about 0.1 wt.% to about 10 wt.%.

37. The composition of claim 36, wherein the polymeric binder (25) is present with a range from about 0.2 wt.% to about 0.5 wt.%.

38. The composition of claim 30, wherein each acicular metal particle (10) comprises a metal selected from the group of iron and iron alloys, nickel and nickel alloys, cobalt and cobalt alloys, and combinations thereof.

39. The composition of claim 30, wherein each acicular metal particle (10) comprises a metal selected from the group of iron and iron alloys, rhenium and rhenium alloys, ruthenium and ruthenium alloys, rhodium and rhodium alloys, palladium and palladium alloys, silver and silver alloys, osmium and osmium alloys, iridium and iridium alloys, platinum and platinum alloys, gold and gold alloys, aluminum and aluminum alloys, copper and copper alloys, magnesium and magnesium alloys, titanium and titanium alloys, tungsten and tungsten alloys, zinc and zinc alloys, and combinations thereof.

40. The composition of claim 30, wherein the inorganic binder (35) is selected from the group consisting of silicates, metal oxides, phosphates, borides, nitrides, carbides, ferrites and combinations thereof.

41. The composition of claim 30, wherein the inorganic binder (35) is present in an amount ranging from about 0.01 wt.% to about 1 wt.%.

42. The composition of claim 41, wherein the inorganic binder (35) is present in an amount ranging from about 0.025 wt.% to about 0.2 wt.%.

43. The composition of claim 30, wherein the inorganic binder (35) comprises a substantially uniform coating that encapsulates said acicular particles (10).

44. A compact (40) comprising the composite metal powder composition of claim 30 having a density of at least about 95% of theoretical density.

Drawing