Patent Application
20190218650
Embodiments of the present disclosure generally relate to methods of forming spherical titanium-based metallic particles. More particularly, embodiments of the present disclosure relate to methods of forming spherical titanium alloy particles using microwave plasma.
An important aspect of preparing some forms of industrial powders is the spheroidization process, which transforms irregularly shaped or angular powders produced by conventional crushing methods, into spherical low-porosity particles. Spherical powders are homogenous in shape, dense, less porous, and exhibit better flowability. Such powders may exhibit superior properties in applications such as injection molding, thermal spray coatings, or additive manufacturing.
Titanium and titanium-alloy particles are particularly useful in additive manufacturing of industrial grade components. Additive manufacturing of titanium components may require high-quality, low-cost spherical titanium or titanium alloy powder as a feedstock for good flowability. Conventional methods for processing of titanium alloys to produce spherical powders typically involve multiple steps, such as, producing titanium ingots from sponges and utilizing melting and atomization processes on the titanium ingots to produce spherical powder. The formation of titanium powder can be facilitated by one of several approaches, such as, the Kroll process, the Hunter process, or the Armstrong process. However, most of these commercial processes are typically carried out as large-scale processes and are batch segregated, which increases the complexity and associated cost. Furthermore, the intermediate metallurgical processes for conversion to alloys may add to the cost of the resulting spherical titanium alloy powder.
Other methods of forming spherical titanium particles employ thermal arc plasma or radio-frequency (RF) generated plasma for spheroidization of titanium-based feedstock material. However, these two methods may present limitations inherent to the thermal non-uniformity of radio-frequency and thermal arc plasmas. Some other spheroidization methods employ inductively coupled plasma (ICP), where angular powder obtained from a hydride-dehydride (HDH) process is entrained within a gas and injected though a hot plasma environment to melt the powder particles. However, this method also suffers from non-uniformity of the microwave plasma, which leads to incomplete spheroidization of feedstock.
In one aspect, the present disclosure relates to method of forming spherical metallic particles including titanium. The method includes performing a hydride-dehydride process on a meltless metallic sponge to form a feedstock material including a metallic powder. The method further includes introducing the feedstock material into a microwave plasma discharge to form the spherical metallic particles.
In another aspect, the present disclosure relates to a plurality of spherical metallic particles including titanium. The plurality of spherical metallic particles is formed by performing a hydride-dehydride process on a meltless metallic sponge to form a feedstock material including a metallic powder; and introducing the feedstock material into a microwave plasma discharge.
In yet another aspect, the present disclosure relates to a method of forming spherical titanium alloy particles. The method includes performing a hydride-dehydride process on a meltless titanium alloy sponge to form a feedstock material including acicular titanium alloy powder. The method further includes introducing the feedstock material into a microwave plasma discharge to form the spherical titanium alloy particles.
These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:
In the following specification and the claims, which follow, reference will be made to a number of terms, which shall be defined to have the following meanings. The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “or” is not meant to be exclusive and refers to at least one of the referenced components being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value solidified by a term or terms, such as “about”, and “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Similarly, “free” may be used in combination with a term, and may include an insubstantial number, or trace amounts, while still being considered free of the solidified term. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.
As mentioned earlier, conventional methods for producing spherical titanium-based particles may involve multiple intermediate metallurgical steps and batch processing of the feedstock material, which in turn may affect the cost and consistency of the final product. Embodiments of the present disclosure described herein address the noted shortcomings in the art.
A method of forming spherical metallic particles including titanium is presented. The method includes performing a hydride-dehydride process on a meltless metallic sponge to form a feedstock material including a metallic powder. The feedstock material is introduced into a microwave plasma discharge to form the spherical metallic particles.
The term “metallic particles” as used herein refers to a plurality of particles including an elemental metal, a metal alloy, or a combination thereof. Therefore, the term metallic particles as used herein includes elemental titanium, a titanium-based metal alloy, or a combination thereof. The term “elemental metal” as used herein means that an amount of a base metal in the metallic particles is greater than 97 weight percent. In certain embodiments, an amount of the base metal in the metallic particles is greater than 99 weight percent. Therefore, the term “elemental titanium” as used herein means than an amount of titanium in the metallic particles is greater than 97 weight percent. In certain embodiments, the spherical metallic particles include a metal alloy including titanium. The metal alloy may further include aluminum, vanadium, or a combination thereof. In certain embodiments, the spherical metallic particles include titanium alloy particles, such as, Ti6Al4V. In some such instances, the amount of aluminum in the titanium alloy may be in a range of from about 4 weight percent to about 7 weight percent, and the amount of vanadium in the titanium alloy may be in a range from about 3 weight percent to about 5 weight percent.
The term “spherical metallic particles” as used herein refers to a plurality of particles having an average aspect ratio that is less than 1.1. In some embodiments, the spherical metallic particles may have an average aspect ratio that is less than 1.05. The spherical metallic particles may have an average diameter in a range of from about 1 micron to about 500 microns. In some embodiments, the spherical metallic particles may have an average diameter in a range of from about 10 microns to about 150 microns.
As noted herein, the method includes performing a hydride-dehydride process on the meltless metallic sponge to form the feedstock material. In some embodiments, where the spherical metallic particles include elemental titanium, the meltless metallic sponge includes elemental titanium. Also, in some embodiments, where the spherical metallic particles include a metal alloy, the meltless metallic sponge includes a metal alloy including titanium. In such instances, the metal alloy may further include aluminum, vanadium, or a combination thereof. In certain embodiments, the meltless metallic sponge includes a titanium-based metal alloy.
The term “meltless metallic sponge’ refers to a metallic material present in the form of a sponge that has been produced without melting of the metallic material. In some embodiments, the meltless metallic sponge may be produced by chemically reducing suitable precursors for the metallic material, without melting the metallic material. As used herein, “without melting,” “no melting,” and related concepts mean that the material is not macroscopically or grossly melted, so that it liquefies and loses its shape. There may be, for example, some minor amount of localized melting as low-melting-point elements melt and are diffusionally alloyed with the higher-melting-point elements that do not melt. Even in such cases, the gross shape of the material remains unchanged.
In some embodiments, the Hunter Process or the Armstrong Process may be used to form the meltless metallic sponge by reduction of metal halide precursors with sodium. In some embodiments, the Kroll Process may be used to produce the meltless metallic sponge by reducing titanium tetrachloride with magnesium.
In certain embodiments, the method of forming a meltless metallic sponge of a metal alloy includes contacting a chemically reducible nonmetallic base-metal precursor compound with a chemically reducible nonmetallic alloying-element precursor compound. “Nonmetallic precursor compounds” are nonmetallic compounds of the metals that eventually constitute the metal alloy. Any operable nonmetallic precursor compounds may be used. For example, oxides of the metals may be employed as nonmetallic precursor compounds in solid-phase reduction, but other types of nonmetallic compounds such as sulfides, carbides, halides, and nitrides may also be employed. The “base-metal” is a metal that is present in a greater percentage by weight than any other element in the metal alloy. In certain embodiments, the base-metal is titanium, and the chemically reducible nonmetallic base-metal precursor compound includes titanium oxide, TiO2. The alloying element may be any element that is available in the chemically reducible form of the precursor compound. A few illustrative examples are aluminum and vanadium.
The chemically reducible nonmetallic precursor compounds are selected to provide the desired metals in the final meltless metallic sponge, and are mixed together in the proper proportions to yield the desired proportions of these metals in the meltless metallic sponge. For example, if the final meltless metallic sponge was to have particular proportions of titanium, aluminum, and vanadium, the chemically reducible nonmetallic precursor compounds may include titanium oxide, aluminum oxide, and vanadium oxide, for solid-phase reduction in the particular proportions. Chemically reducible nonmetallic precursor compounds that serve as a source of more than one of the metals in the final meltless metallic sponge may also be used. These precursor compounds are furnished and mixed together in the correct proportions such that the ratio of titanium, aluminum, and vanadium in the mixture of chemically reducible nonmetallic precursor compounds is equivalent to the one desired in the meltless metallic sponge. In certain embodiments, the final meltless metallic sponge includes a titanium-base alloy, which has more titanium by weight than any other element.
The chemically reducible nonmetallic base-metal precursor compound and the chemically reducible nonmetallic alloying-element precursor compound may be in the form of finely divided solids to ensure that they are chemically reacted in the subsequent step. The finely divided chemically reducible nonmetallic base-metal precursor compound and the chemically reducible nonmetallic alloying-element precursor compound may be in the form of, for example, powders, granules, flakes, liquids, or the like.
The chemically reducible nonmetallic base-metal precursor compound and the chemically reducible nonmetallic alloying-element precursor compound may be mixed to form a compound mixture. The mixing may be performed by conventional procedures used to mix powders in other applications, for solid-phase reduction. After, or, during mixing, the compound mixture may be compacted to form a preform. The compacting may be conducted by cold or hot pressing of the compound mixture, but not at such a high temperature that there is any melting of the compound mixture. The compacted shape may be sintered in the solid-state to temporarily bind the particles together. The compacting desirably forms a shape similar to, but larger in dimensions than, the shape of the final meltless metallic sponge.
The compacted compound mixture may be then reduced using solid-phase reduction. A non-limiting example of a suitable method to perform the solid-phase reduction includes fused salt electrolysis. Briefly, in fused salt electrolysis, the compound mixture is immersed in an electrolysis cell in a fused salt electrolyte, such as a chloride salt, at a temperature below the melting temperatures of the metals that form the compound mixture. The compound mixture is made the cathode of the electrolysis cell, with an inert anode. The oxygen, in the case of oxide nonmetallic precursor compounds, is removed from the mixture by chemical reduction (i.e., the reverse of chemical oxidation). The reaction is performed at an elevated temperature to accelerate the diffusion of the oxygen or other gases away from the cathode. The cathodic potential is controlled to ensure that the reduction of the nonmetallic precursor compounds occurs, rather than other possible chemical reactions such as the decomposition of the molten salt. The electrolyte is typically a salt that is more stable than the equivalent salt of the metals being refined and suitably stable to remove the oxygen. In some embodiments, the chemical reduction may be carried to completion, so that the nonmetallic precursor compounds are completely reduced. In some other embodiments, the chemical reduction may instead be partial, such that some nonmetallic precursor compounds remain.
The physical form of the metallic material at the completion of the solid-phase reduction process depends upon the physical form of the mixture of chemically reducible nonmetallic precursor compounds at the beginning of the solid-phase reduction process. As noted herein, as the mixture of chemically reducible nonmetallic precursor compounds is a compressed mass. Therefore, the final physical form of the metallic material is typically in the form of a porous metallic sponge.
The process for forming a meltless metallic sponge is described above in the context of forming a meltless metal alloy sponge. A similar method may be employed for forming a meltless metallic sponge composed primarily of an elemental metal. In such instances, a non-metallic precursor compound (e.g., TiO2) may be compacted and chemically reduced using a suitable solid-phase reduction process, e.g., fused salt electrolysis, to form the meltless metallic sponge.
The meltless metallic sponge may be further characterized by a packing density. The term “packing density” as used in this context refers to the percentage volume of the total volume of the meltless metallic sponge, occupied by the metallic material. In some embodiments, the meltless metallic sponge has a packing density less than 20%. In certain embodiments, the meltless metallic sponge has a packing density in a range from about 10% to about 20%.
The term “hydride-dehydride” process as used herein refers to a process in which a metallic material (e.g., the meltless metallic sponge) is first subjected to a hydrogenation step, followed by milling and a dehydrogenation step, resulting in the feedstock material. In some embodiments, the feedstock material includes acicular or angular metallic powder. In certain embodiments, the feedstock material includes acicular or angular titanium-based metal alloy powder.
The acicular metallic powder may be further characterized by a packing density. The term “packing density” as used in this context refers to the percentage volume of the total volume of the acicular metallic powder, occupied by the metallic material. In some embodiments, the acicular metallic powder has a packing density greater than 50%. In certain embodiments, the acicular metallic powder has a packing density in a range from about 50% to about 90%.
The microwave plasma discharge may be generated using a suitable microwave plasma torch. The method may include introducing the feedstock material into the microwave plasma torch using any suitable means, for example, a suitable powder feeder. Within the microwave plasma torch, the feedstock material is exposed to a plasma discharge causing the materials to melt. Because of the uniformity of the microwave plasma discharge, the feedstock material may be exposed to a substantially uniform temperature profile, and rapidly heated and melted. In one example, the feedstock material may be exposed to a uniform temperature profile in a range from about 4,000 K to about 8,000 K, within the microwave plasma.
In certain embodiments, the feedstock material is introduced into the microwave plasma discharge in the presence of a non-reactive gas. The term “non-reactive gas” as used herein refers to a gas or a gas mixture that does not react with the feedstock material or the spherical metallic particles, in the presence of the microwave plasma discharge. A non-limiting example of a suitable non-reactive gas may include argon. Thus, in contrast to conventional methods of forming spherical particles using thermal arc/RF plasma, the methods, as described herein, may not require use of additional reductants along with the microwave plasma, such as, magnesium or hydrogen.
After the melting of the feedstock material within the microwave plasma discharge, the melted metals may be inherently spheroidized, at least in part, due to liquid surface tension. As the microwave generated plasma exhibits a substantially uniform temperature profile, greater than 90% spheroidization of particles may be achieved.
Various parameters of the microwave plasma discharge, as well as feedstock material parameters, may be adjusted in order to achieve the desired results. These parameters may include one or more of microwave power, feedstock material size, feedstock material insertion rate, gas flow rates, plasma temperature, and cooling rates.
After the spheroidization step in the microwave plasma discharge, the plurality of spherical metallic particles may exit the microwave plasma discharge, resulting in cooling and further solidification of the particles. In some embodiments, the spherical metallic particles exiting from the microwave plasma discharge may be further subjected to one or more additional cooling steps to facilitate solidification and collection. The cooled and solidified spherical metallic particles may be subsequently collected using appropriate collection mechanisms, e.g., collection bins.
A flow chart for a method of forming spherical metallic particles is further illustrated in
A method of forming spherical titanium alloy particles is also presented in flow chart 300 illustrated in
A plurality of spherical metallic particles including titanium, formed by the methods described herein, is also presented. The plurality of spherical metallic particles includes an elemental metal, a metal alloy, or a combination thereof. In some embodiments, the spherical metallic particles include elemental titanium, a titanium alloy, or a combination thereof. In certain embodiments, the plurality of spherical metallic particles includes a titanium alloy. The titanium alloy may further include aluminum, vanadium, or a combination thereof.
The spherical titanium-based metallic particles and methods of producing such particles, in accordance with embodiments of the present disclosure, may provide a number of advantages. For example, the methods as described herein may allow for fewer number of processing steps for spheroidization of the meltless metallic sponge, using a microwave plasma discharge. Reduction in the number of intermediate steps may reduce the cost of the resulting spherical metallic particles.
The methods as described herein can further achieve additional improvements in consistency due to the homogeneity and control of the energy source (i.e., microwave plasma). Specifically, if the microwave plasma conditions are well controlled, particle agglomeration can be reduced, if not eliminated, thus leading to a better particle size distribution, which could result in high-quality, low-cost, high flowability titanium-based powder. As mentioned earlier, high-quality, low-cost, high flowability titanium-based powder may be particularly desirable for additive manufacturing of titanium-based components.
The appended claims are intended to claim the invention as broadly as it has been conceived and the examples herein presented are illustrative of selected embodiments from a manifold of all possible embodiments. Accordingly, it is the Applicants' intention that the appended claims are not to be limited by the choice of examples utilized to illustrate features of the present disclosure. As used in the claims, the word “comprises” and its grammatical variants logically also subtend and include phrases of varying and differing extent such as for example, but not limited thereto, “consisting essentially of” and “consisting of” Where necessary, ranges have been supplied; those ranges are inclusive of all sub-ranges there between. It is to be expected that variations in these ranges will suggest themselves to a practitioner having ordinary skill in the art and where not already dedicated to the public, those variations should where possible be construed to be covered by the appended claims. It is also anticipated that advances in science and technology will make equivalents and substitutions possible that are not now contemplated by reason of the imprecision of language and these variations should also be construed where possible to be covered by the appended claims.
