SPHEROIDAL DEHYDROGENATED METALS AND METAL ALLOY PARTICLES

Abstract

Methodologies, systems, and devices are provided for producing metal spheroidal powder products. Dehydrogenated and spheroidized particles are prepared using a process including introducing a metal hydride feed material into a plasma torch. The metal hydride feed material is melted within a plasma in order to dehydrogenate and spheroidize the materials, forming dehydrogenated and spheroidized particles. The dehydrogenated and spheroidized particles are then exposed to an inert gas and cooled in order to solidify the particles into dehydrogenated and spheroidized particles. The particles are cooled within a chamber having an inert gas.

Claims

1.-29. (canceled)

30. A method of producing spheroidized particles, the method comprising: introducing a titanium or titanium alloy feed material into a plasma torch; directing the feed material toward a plasma within the plasma torch; and melting and spheroidizing the feed material within the plasma to form spheroidized titanium or titanium alloy particles, wherein the spheroidized particles comprise spheroidized titanium or titanium alloy powder particles having a martensitic microstructure.

31. The method of claim 30, further comprising deoxidizing the feed material within the plasma.

32. The method of claim 30, wherein the plasma exposes the feed material to a temperature profile between about 4,000 K and 8,000 K.

33. The method of claim 30, further comprising exposing the feed material to a partial vacuum while the feed material is exposed to the plasma.

34. The method of claim 30, further comprising exposing the feed material to a pressure higher than atmospheric pressure while the feed material is exposed to the plasma.

35. The method of claim 30, further comprising: screening the feed material prior to introducing the feed material into the plasma torch; and maintaining an average particle size distribution from the feed material to the spheroidized particles.

36. The method of claim 30, wherein the particle size of feed material is no less than 1.0 micrometers and no more than 300 micrometers.

37. The method of claim 30, further comprising purging the feed material with an inert gas to remove oxygen prior to introducing the feed material into the plasma torch.

38. The method of claim 30, wherein the plasma torch is a microwave generated plasma torch.

39. The method of claim 30, further comprising: exposing the spheroidized particles to an inert gas; and cooling and solidifying the spheroidized particles in a chamber having the inert gas, the spheroidized particles having more than 90% spheroidization consistency.

40. Spheroidized particles prepared by a process comprising: introducing a titanium feed material into a plasma torch; and melting and spheroidizing the feed material within the plasma to form spheroidized titanium particles, wherein the spheroidized particles comprise spheroidized titanium powder particles having a martensitic microstructure.

41. The particles of claim 40, wherein the spheroidized particles are further deoxidized within the plasma.

42. The particles of claim 40, wherein the feed material is exposed to a temperature profile between about 4,000 K and 8,000 K within the plasma.

43. The particles of claim 40, wherein the feed material is exposed to a partial vacuum within the plasma.

44. The particles of claim 40, wherein the feed material is exposed to a pressure greater than atmospheric pressure within the plasma.

45. The particles of claim 40, wherein a particle size of the feed material is no less than 1.0 micrometers and no more than 300 micrometers.

46. The particles of claim 40, wherein the feed material is continuously introduced into the plasma.

47. The particles of claim 40, wherein the feed material is purged with an inert gas prior to introducing the feed material into the plasma torch.

48. The particles of claim 40, wherein the plasma torch comprises a microwave generated plasma torch.

49. The particles of claim 40, wherein the spheroidized particles have more than 90% spheroidization consistency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The features and advantages of the present disclosure will be more fully understood from the following description of exemplary embodiments when read together with the accompanying drawings, in which:

[0017] FIG. 1 illustrates an example method of producing spheroidal metallic and metallic alloy particles according to the present disclosure, compared against a conventional method for producing similar particles.

[0018] FIG. 2 illustrates another example method of producing dehydrogenated spheroidal particles according to the present disclosure.

[0019] FIG. 3 illustrates another example method of producing dehydrogenated spheroidal particles from metal hydride material according to the present disclosure.

[0020] FIG. 4 illustrates an exemplary microwave plasma torch that can be used in the production of spheroidal and dehydrogenated metal or metal alloy powders, according to embodiments of the present disclosure.

DETAILED DESCRIPTION

[0021] One aspect of the present disclosure involves a process of spheroidization of metals and metal alloy hydrides using a microwave generated plasma. The process uses readily available existing pre-screened or non-prescreened raw materials made of metal hydrides as feedstock. The powder feedstock is entrained in inert and/or reducing and/or oxidizing gas environment and injected into the microwave plasma environment. Upon injection into a hot plasma, the feedstock is simultaneously dehydrogenated and spheroidized and released into a chamber filled with an inert gas and directed into hermetically sealed drums where is it stored. This process can be carried out at atmospheric pressure, in a partial vacuum, or at a slightly higher pressure than atmospheric pressure. In alternative embodiments, the process can be carried out in a low, medium, or high vacuum environment. The process can run continuously and the drums are replaced as they fill up with spheroidized dehydrogenated and deoxydated metal or metal alloy particles. The process not only spheroidizes the powders, but also eliminates the dehydrogenation and deoxydation steps from the traditional process of manufacturing metal and metal alloy powders using Hydride-De-hydride (HDH) process, which leads to cost reduction. By reducing the number of processing steps and providing a continuous process, the possibilities for contamination of the material by oxygen and other contaminants is reduced. Furthermore, provided the homogeneity of the microwave plasma process, particle agglomeration is also reduced, if not totally eliminated, thus leading to at least maintaining the particle size distribution of the original hydride feed materials.

[0022] In the powdered metallurgy industry, the Hydride-Dehydride (HDH) process is used to resize large metallic or metallic alloy pieces down to a finer particle size distribution through crushing, milling, and screening. Metal and alloy powders are manufactured using the HDH process, where bulk feedstock, such as coarse metal powders or metal/metal alloy scraps, etc., are heated in a hydrogen-containing atmosphere at high temperature (˜700° C.) for a few days. This leads to the formation of a brittle metal hydride, which can readily be crushed into a fine power and sifted to yield a desired size distribution determined by the end user. To be useful in powdered metallurgy, hydrogen must be dissociated and removed from the metal by heating the metal hydride powder within vacuum for a period of time. The dehydrogenated powder must then be sifted to remove large particle agglomerations generated during process due to sintering. The typical resulting powder particles have an irregular or angular shape. The powder is submitted to a deoxydation process to remove any oxygen picked up by the powder during sifting and handling. Conventional HDH processes produce only coarse and irregular shaped particles. Such HDH processes must be followed by a spheroidization process to make these particles spheroidal.

[0023] Conventional HDH processes are primarily carried out as solid-state batch processes. Typically, a volume of metal hydride powder is loaded into a crucible(s) within a vacuum furnace. The furnace is pumped down to a partial vacuum and is repeatedly purged with inert gas to eliminate the presence of undesired oxygen. Diffusion of the inert gas through the open space between the powder particles is slow making it difficult to fully eliminate oxygen, which otherwise contaminates the final product. Mechanical agitation may be used to chum powder allowing for more complete removal of oxygen. However, this increases the complexity of the system and the mechanical components require regular maintenance, ultimately increasing cost.

[0024] Following oxygen purging the, hydrogenation may begin. The furnace is filled with hydrogen gas and heated up to a few days at high temperature to fully form the metal hydride. The brittle nature of the metal hydride allows the bulk material to be crushed into fine powders which are then screened into desired size distributions.

[0025] The next step is dehydrogenation. The screen hydride powder is loaded into the vacuum furnace then heated under partial vacuum, promoting dissociation of hydrogen from the metal hydride to form H.sub.2 gas and dehydrided metal. Dehydrogenation is rapid on the particle surface where H.sub.2 can readily leave the particles. However, within the bulk of the powder, H.sub.2 must diffuse through the bulk of the solid before it reaches surface and leave the particle. Diffusion through the bulk is a rate-limiting process “bottle-neck” requiring relatively long reaction time for complete dehydrogenation. The time and processing temperatures required for dehydrogenation are sufficient to cause sintering between particles, which results in the formation of large particle agglomerations in the final product. Post-process sifting eliminates the agglomerations, which adds process time and cost to the final product. Before the powder can be removed from the furnace, it must be sufficiently cooled to maintain safety and limit contamination. The thermal mass of the large furnaces may take many hours to sufficiently cool. The cooled powders must then be spheroidized in a separate machine. Generally this is carried out within an RF plasma, which are known to exhibit large temperature gradients resulting in partially spheroidized products.

[0026] Techniques are disclosed herein for manufacturing spheroidal metal and metal alloy powder products in a continuous process that simultaneously dehydrogenates, spheroidizes, and deoxidizes feed materials. According to exemplary embodiments, the dehydrogenation, deoxydation, and spheroidization steps of an HDH process can be eliminated in favor of a single processing step using a microwave generated plasma. Such embodiments can reduce the cost of spheroidizing metal powders by reducing the number of processing steps, reducing the energy per unit volume of processed material, and increasing the consistency of the final product. Reduction in the number of processing steps also reduces the possibility for powder contamination by oxygen and other contaminants. Additionally, the continuous dehydrogenation processes disclosed herein improves the consistency of the end products by reducing or eliminating variations associated with typical batch-based dehydrogenation processes.

[0027] The rate of cooling of the dehydrogenated, deoxidized, and spheroidized metal and metal alloys can be controlled to strategically influence the microstructure of the powder. For example, rapid cooling of α-phase titanium alloys facilitates an acicular (martensite) structure. Moderate cooling rates produce a Widmanstätten microstructure, and slow cooling rates form an equiaxed microstructure. By controlling the process parameters such as cooling gas flow rate, residence time, etc., microstructure of the metal and metal alloys can be controlled. The precise cooling rates required to form these structures is largely a function of the type and quantity of the alloying elements within the material.

[0028] In one exemplary embodiment, inert gas is continually purged surrounding a powdered metal hydride feed to remove oxygen within a powder-feed hopper. A continuous volume of powder feed is then entrained within an inert gas and fed into the microwave generated plasma for dehydrogenation. In one example, the microwave generated plasma may be generated using a microwave plasma torch, as described in U.S. Patent Publication No. US 2013/0270261, and/or U.S. Patent Publication No. US 2008/0173641 (issued as U.S. Pat. No. 8,748,785), each of which is hereby incorporated by reference in its entirety. In some embodiments, the particles are exposed to a uniform temperature profile at between 4,000 and 8,000 K within the microwave generated plasma. Within the plasma torch, the powder particles are rapidly heated and melted. Liquid convection accelerates H.sub.2 diffusion throughout the melted particle, continuously bringing hydrogen (H.sub.2) to the surface of the liquid metal hydride where it leaves the particle, reducing the time each particle is required to be within the process environment relative to solid-state processes. As the particles within the process are entrained within an inert gas, such as argon, generally contact between particles is minimal, greatly reducing the occurrence of particle agglomeration. The need for post-process sifting is thus greatly reduced or eliminated, and the resulting particle size distribution could be practically the same as the particle size distribution of the input feed materials. In exemplary embodiments, the particle size distribution of the feed materials is maintained in the end products.

[0029] Within the plasma, the melted metals are inherently spheroidized due to liquid surface tension. As the microwave generated plasma exhibits a substantially uniform temperature profile, more than 90% spheroidization of particles could be achieved (e.g., 91%, 93%, 95%, 97%, 99%, 100%), eliminating the need for separate dehydrogenation and deoxydation steps. After exiting the plasma, the particles are cooled before entering collection bins. When the collection bins fill, they can be removed and replaced with an empty bin as needed without stopping the process.

[0030] Referring to FIG. 1, shown is a comparison of a conventional process for producing spheroidized titanium powder (100) versus a method (200) in accordance with the present technology. The process flow (101) on the left of FIG. 1 presents an example process that combines a HDH process (100) with spheroidization of titanium powders. The process starts with Ti raw material (step a, 105) that is hydrogenated (step b, 110), and then crushed and sifted to size (step c, 115). Pure titanium is recovered through dehydrogenation (step d, 120). It is then screened for agglomerations and impurities, then sifted to the size specified by the customer (step e, 125). The powder then goes through a deoxidation step to reduce or eliminate oxygen that it picked up during the sifting and screening processes. Deoxidation is required especially for small particle sizes, such as particles below 50 microns, where the surface to volume ratio is substantial (step f, 130). The titanium particles are then spheroidized (step g, 135) and collected (step h, 140). A similar process can be used to create a Ti alloy, such as Ti 6-4, instead of pure titanium powder.

[0031] As discussed above, embodiments of the present disclosure combine the dehydrogenation, deoxydation, and spheroidization steps shown on the left side of FIG. 1 (101, 130, 135) in favor of a single step to produce spheroidized metals and/or metal alloys from corresponding hydride feedstock. An example of this technique is illustrated in the process flow (201) shown on the right side of FIG. 1. The method starts with a crushed and sifted metal hydride feed material (i.e., step c. 115, without performing the dehydride step). In this particular embodiment, the feed material is a titanium hydride powder, and the powder resulting from process 200 is a spherical titanium powder. (It is noted that process 200 can also be used with crushed and sifted metal alloy hydride feed material, such as titanium alloy hydride feed material, and the powder resulting from completion of process 200 is a spherical metal alloy powder, such as a spherical titanium alloy powder.) The powder is entrained within an inert gas and injected into a microwave generated plasma environment exhibiting a substantially uniform temperature profile between approximately 4,000 K and 8.000 K and under a partial vacuum. The hermetically scaled chamber process can also run at atmospheric pressure or slightly above atmospheric pressure to eliminate any possibility for atmospheric oxygen to leak into the process. The particles are simultaneously melted and dehydrogenated in the plasma, spheroidized due to liquid surface tension, re-solidifying after exiting the plasma (200). The particles are then collected in sealed drums in an inert atmosphere (140). Within the plasma, the powder particles are heated sufficiently to melt and cause convection of the liquid metal, causing dissociation of the hydrogen according to the reversible reaction where M=an arbitrary metal:

[00001]$M_x{H}_y.Math.\left(x\right)M+\left(\frac{y}{2}\right)H_2$

[0032] Within the partial vacuum, dissociation of hydrogen from the metal to form hydrogen gas is favored, driving the above reaction to the right. The rate of dissociation of hydrogen from the liquid metal is rapid, due to convection, which continually introduces H.sub.2 to the liquid surface where it can rapidly leave the particle.

[0033] FIG. 2 is a flow chart illustrating an exemplary method (250) for producing spherical powders, according to an embodiment of the present disclosure. In this embodiment, the process (250) begins by introducing a feed material into a plasma torch (255). In some embodiments, the plasma torch is a microwave generated plasma torch or an RF plasma torch. Within the plasma torch, the feed materials are exposed to a plasma causing the materials to melt, as described above (260). During the same time (i.e., time that feed material is exposed to plasma), hydrogen within the feed material dissociates from the metal, resulting in dehydrogenation (260a). Simultaneously the melted materials are spheroidized by surface tension, as discussed above (260b). Note that the step 260 includes 260a and 260b. That is, by exposing the feed material to the plasma both dehydrogenation and spheroidization are achieved; no separate or distinct processing steps are needed to achieve dehydrogenation and spheroidization. After exiting the plasma, the products cool and solidify, locking in the spherical shape and are then collected (265).

[0034] FIG. 3 is a flow chart illustrating another exemplary method (300) for producing spherical powders, according to another embodiment of the present disclosure. In this example, the method (300) begins by introducing a substantially continuous volume of filtered metal hydride feed materials into a plasma torch. As discussed above, the plasma torch can be a microwave generated plasma or an RF plasma torch (310). In one example embodiment, an AT-1200 rotating powder feeder (available from Thermach Inc.) allows a good control of the feed rate of the powder. In an alternative embodiment, the powder can be fed into the plasma using other suitable means, such as a fluidized bed feeder. The feed materials may be introduced at a constant rate, and the rate may be adjusted such that particles do not agglomerate during subsequent processing steps. In another exemplary embodiment, the feed materials to be processed are first sifted and classified according to their diameters, with a minimum diameter of 1 micrometers (μm) and a maximum diameter of 22 μm, or a minimum of 22 μm and a maximum of 44 μm, or a minimum of 44 μm and a maximum of 70 μm, or a minimum of 70 μm and a maximum of 106 μm, or a minimum of 106 μm and a maximum of 300 μm. As will be appreciated, these upper and lower values are provided for illustrative purposes only, and alternative size distribution values may be used in other embodiments. This eliminates recirculation of light particles above the hot zone of the plasma and also ensures that the process energy present in the plasma is sufficient to melt the particles without vaporization. Pre-screening allows efficient allocation of microwave power necessary to melt the particles without vaporizing material.

[0035] Once introduced into the microwave plasma torch, the feed materials can be entrained within an axis-symmetric laminar and/or turbulent flow toward a microwave or RF generated plasma (320). In exemplary embodiments, each particle within the process is entrained within an inert gas, such as argon. In some embodiments, the metal hydride materials are exposed to a partial vacuum within the plasma (330).

[0036] Within the plasma, the feed materials are exposed to a substantially uniform temperature profile and are melted (340). In one example, the feed materials are exposed to a uniform temperature profile of approximately between 4,000 and 8,000 K within the plasma. Melting the feed materials within the plasma brings hydrogen to the surface of the liquid metal hydride where it can leave the particle, thus rapidly dehydrogenating the particles (350). The H.sub.2 acts as a reducing agent simultaneously deoxidizing the metal. Surface tension of the liquid metal shapes each particle into a spherical geometry (360). Thus, dehydrogenated, deoxidized, and spherical liquid metal particles are produced, which cool and solidify into dehydrogenated, deoxidized, and spherical metal powder products upon exiting the plasma (370). These particles can then be collected into bins (380). In some embodiments, the environment and/or sealing requirements of the bins are carefully controlled. That is, to prevent contamination or potential oxidation of the powders, the environment and or seals of the bins are tailored to the application. In one embodiment, the bins are under a vacuum. In one embodiment, the bins are hermetically sealed after being filled with powder generated in accordance with the present technology. In one embodiment, the bins are back filled with an inert gas, such as, for example argon. Because of the continuous nature of the process, once a bin is filled, it can be removed and replaced with an empty bin as needed without stopping the plasma process.

[0037] The methods and processes in accordance with the invention (e.g., 200, 250, 300) can be used to make spherical metal powders or spherical metal alloy powders. For example, if the starting feed material is a titanium hydride material, the resulting powder will be a spherical titanium powder. If the starting feed material is a titanium alloy hydride material, the resulting powder will be a spherical titanium alloy powder. In one embodiment that features the use of a starting titanium alloy hydride material, the resulting spherical titanium alloy powder comprises spherioidized particles of Ti Al6-V4, with between 4% to 7% weight aluminum and 3% to 5% weight vanadium.

[0038] FIG. 4 illustrates an exemplary microwave plasma torch that can be used in the production of spheroidal and dehydrogenated metal or metal alloy powders, according to embodiments of the present disclosure. As discussed above, metal hydride feed materials 9, 10 can be introduced into a microwave plasma torch 3, which sustains a microwave generated plasma 11. In one example embodiment, an entrainment gas flow and a sheath flow (downward arrows) may be injected through inlets 5 to create flow conditions within the plasma torch prior to ignition of the plasma 11 via microwave radiation source 1. In some embodiments, the entrainment flow and sheath flow are both axis-symmetric and laminar, while in other embodiments the gas flows are swirling. The feed materials 9 are introduced axially into the microwave plasma torch, where they are entrained by a gas flow that directs the materials toward the plasma. As discussed above, the gas flows can consist of a noble gas column of the periodic table, such as helium, neon, argon, etc. Within the microwave generated plasma, the feed materials are melted, as discussed above, in order to dehydrogenate, deoxidize and spheroidize the materials. Inlets 5 can be used to introduce process gases to entrain and accelerate particles 9, 10 along axis 12 towards plasma 11. First, particles 9 are accelerated by entrainment using a core laminar gas flow (upper set of arrows) created through an annular gap within the plasma torch. A second laminar flow (lower set of arrows) can be created through a second annular gap to provide laminar sheathing for the inside wall of dielectric torch 3 to protect it from melting due to heat radiation from plasma 11. In exemplary embodiments, the laminar flows direct particles 9, 10 toward the plasma 11 along a path as close as possible to axis 12, exposing them to a substantially uniform temperature within the plasma. In some embodiments, suitable flow conditions are present to keep particles 10 from reaching the inner wall of the plasma torch 3 where plasma attachment could take place. Particles 9, 10 are guided by the gas flows towards microwave plasma 11 were each undergoes homogeneous thermal treatment. Various parameters of the microwave generated plasma, as well as particle parameters, may be adjusted in order to achieve desired results. These parameters may include microwave power, feed material size, feed material insertion rate, gas flow rates, plasma temperature, and cooling rates. In some embodiments, the cooling or quenching rate is not less than 10.sup.+3 degrees C./sec upon exiting plasma 11. As discussed above, in this particular embodiment, the gas flows are laminar, however, in alternative embodiments, swirl flows or turbulent flows may be used to direct the feed materials toward the plasma.

[0039] In describing exemplary embodiments, specific terminology is used for the sake of clarity and in some cases reference to a figure. For purposes of description, each specific term is intended to at least include all technical and functional equivalents that operate in a similar manner to accomplish a similar purpose. Additionally, in some instances where a particular exemplary embodiment includes a plurality of system elements, device components or method steps, those elements, components or steps may be replaced with a single element, component or step. Likewise, a single element, component or step may be replaced with a plurality of elements, components or steps that serve the same purpose. Moreover, while exemplary embodiments have been shown and described with references to particular embodiments thereof, those of ordinary skill in the art will understand that various substitutions and alterations in form and detail may be made therein without departing from the scope of the invention. Further still, other functions and advantages are also within the scope of the invention.