Patent Description:
With the growing interest in rapid prototyping and manufacturing, commonly known as additive manufacturing or <NUM>-D printing, a number of techniques have been developed for the production of dense spherical powders, which are useful for such technologies. The success of additive manufacturing and <NUM>-D printing depends in a large extent on the availability of materials usable for parts manufacturing. Such materials need to be provided in the form of highly pure, fine (e.g. diameter less than <NUM>), dense, spherical, and free-flowing powders that have well-defined particle size distributions. Conventional melt atomization techniques such as gas, liquid and rotating disc atomization are unable to produce such high quality powders.

More recent techniques avoid the use of crucible melting, which is often responsible for material contamination. These recent techniques provide spherical, free-flowing powders.

For example, some plasma atomization processes are based on the use of a plurality of plasma torches producing plasma jets that converge toward an apex. By feeding a material to be atomized in the form of a wire or rod into the apex, the material is melted and atomized by thermal and kinetic energy provided by the plasma jets. It has also been proposed to feed a material to be atomized in the form of a continuous molten stream directed towards an apex where several plasma jets converge. These types of plasma atomization processes are rather delicate and require laborious alignment of at least three plasma torches in order to have at least three plasma jets converging toward the apex. Due to the physical size of such plasma torches, the apex location is bound to be a few centimeters away from an exit point of the plasma jets. This causes a loss of valuable thermal and kinetic energy of the plasma jets before they reach the apex position and impinge on the material. Overall, these processes involve several difficulties in terms of requirements for precise alignment and power adjustment of the torches and for precise setting of the material feed rate.

Document <CIT> teaches inserting a rod or wire into a plasma atomization system within which three plasma torches produce jets that converge into an apex where plasma jets impinge on the rod or wire. The plasma jets are generated by three non-transferred D.

Other technologies are based on the use of direct induction heating and melting of a wire or rod of a material to be atomized while avoiding contact between the melted material and a crucible. Melt droplets from the rod fall into a gas atomization nozzle system and are atomized using a high flow rate of an appropriate inert gas. A main advantage of these technologies lies in avoiding possible contamination of the material to be atomized by eliminating any possible contact thereof with a ceramic crucible. These technologies are however limited to the atomization of pure metals or alloys. Also, these technologies are complex and require fine adjustment of operating conditions for optimal performance. Furthermore, large amounts of inert atomizing gases are consumed.

Therefore, there is a need for techniques for efficient and economical production of powder particles from a broad range of feed materials.

The invention is an apparatus according to claim <NUM>.

The foregoing and other features will become more apparent upon reading of the following non-restrictive description of illustrative embodiments thereof, given by way of example only with reference to the accompanying drawings. Like numerals represent like features on the various figures of drawings.

Embodiments of the disclosure will be described by way of example only with reference to the accompanying drawings, in which:.

Generally speaking, the present disclosure addresses one or more of the problems of efficiently and economically producing powder particles from a broad range of feed materials.

More particularly, the present disclosure describes a plasma atomization process and an apparatus therefor, usable to produce powder particles from a broad range of feed materials, including for example pure metals, alloys, ceramics and composites. The disclosed technology may be used in the manufacture of a wide range of dense spherical metal, ceramic or composite powders from a feed material of the same nature in the form of an elongated member such as, as non-limitative examples, a rod, a wire or a filled tube. A powder may be defined as comprising particles with a diameter of less than one (<NUM>) millimeter, a fine powder may be defined as comprising of particles of diameter less than <NUM> micrometers, while an ultrafine powder may be defined as comprising particles of less than one (<NUM>) micrometer in diameter.

In a non-limitative embodiment, the plasma torch, which may optionally be an inductively coupled plasma torch, is supplied with the feed material along a central, longitudinal axis thereof. A speed of movement and/or a distance of travel of the feed material in an optional preheating zone of the plasma torch may be controlled to allow the material to heat to a temperature as close as possible to its melting point while avoiding premature melting thereof within the plasma torch. In one embodiment, a forward end of the optionally preheated feed material enters the atomization nozzle to emerge from its downstream side and enter a cooling chamber. Due to its passage in the atomization nozzle, the forward end or tip of the feed material is exposed to a plurality of plasma jets, for example high velocity plasma jets, including, though not limited to, supersonic fine plasma jets. Upon impinging on the feed material, the plasma jets melt its surface and strip out molten material resulting in finely divided, spherical molten droplets of the material entrained with the plasma gas from the atomization nozzle. In another embodiment, the forward end of the optionally preheated feed material is exposed to an annular plasma jet within the atomization nozzle, the annular plasma jet also causing surface melting of the feed material. Resulting droplets are entrained by the plasma gas into the cooling chamber. In both embodiments, the droplets cool down and freeze in-flight within the cooling chamber, forming for example small, solid and dense spherical powder particles. The powder particles can be recovered at the bottom of the cooling chamber, for example in a downstream cyclone or in a filter, depending on their particle size distribution.

In the context of the present disclosure, powder particles obtained using the disclosed process and apparatus may include, without limitation, micron sized particles that may be defined as particles in a range from <NUM> to <NUM> micrometer in diameter.

The following terminology is used throughout the present disclosure:
Powder particle: a grain of particulate matter, including but not limited to micron sized and nanoparticles.

Atomization: reduction of a material into particles.

Feed material: a material to be transformed by a process.

Filled tube: feed material provided in the form of a tube, made as non-limitative examples of metal, plastic or any other suitable material, filled with a powder composed of a pure metal, alloys, ceramic material, any other suitable material, or composed of a mixture of materials, so that melting the powder can give rise to the formation of an alloy or composite.

Plasma: a gas in a hot, partially ionized state.

Plasma torch: a device capable of turning a gas into plasma.

Inductively coupled plasma torch: a type of plasma torch using electric current as an energy source to produce electromagnetic induction of the energy into the plasma.

Injection probe: an elongated conduit that may be cooled using a cooling fluid, for insertion or supply of a feed material.

Preheating zone: area in a plasma torch in which feed material is elevated to a temperature below its melting point.

Atomization nozzle: element to produce plasma jets and to allow feed material to transfer from a plasma torch to a cooling chamber.

In-flight freezing: cooling of liquid droplets becoming solid particles while suspended within a gas.

Cooling chamber: a container in which in-flight freezing takes place.

Referring now to the drawings, <FIG> is a front elevation view of a plasma torch usable for atomization of feed material in the form of an elongated member such as, as non-limitative examples, a wire, rod or filled tube. Obviously, other types of elongated member could potentially be used in the disclosed process and apparatus for atomization of feed material.

<FIG> is a detailed, front elevation view of the plasma torch of <FIG>, having an atomization nozzle according to an embodiment and a configuration for direct preheating of the elongated member by the plasma, while <FIG> is a detailed, front elevation view of the plasma torch of <FIG>, having the atomization nozzle of <FIG> and a configuration in which the elongated member is indirectly heated by the plasma through a radiation tube. <FIG> is a front elevation view of an apparatus for atomization of feed material in the form of an elongated member, the apparatus including the plasma torch of <FIG>.

Referring at once to <FIG>, <FIG> and <FIG>, an apparatus <NUM> for producing powder particles by atomization of a feed material <NUM> in the form of an elongated member such as, as non-limitative examples, a wire, a rod or a filled tube, comprises a plasma torch <NUM> producing plasma <NUM>, and a cooling chamber <NUM>. Without limiting the present disclosure, the plasma torch <NUM> as shown is an inductively coupled plasma torch. Use of other types of plasma torches is also contemplated. The apparatus <NUM> may further comprise a powder collector <NUM>.

The plasma torch <NUM> comprises an injection probe <NUM> in the form of an elongated conduit mounted onto the head <NUM> coaxial with the inductively coupled plasma torch <NUM>. As illustrated in <FIG>, the injection probe <NUM> extends through the head <NUM> and through the plasma confinement tube <NUM>. The feed material <NUM> can be inserted in the plasma torch <NUM> via the injection probe <NUM> so that it is coaxial with the torch body <NUM>. The feed material <NUM> may be supplied to the injection probe <NUM>, in continuous manner, by a typical wire, rod or tube feeding mechanism (not shown) for example similar to commercially available units currently used in wire arc welding such as the units commercialized by Miller for MIG/Wire welding, and comprising a first set of wheels operated to control the feed rate of the elongated member to the injection probe <NUM>. The feeding mechanism may be either preceded or followed by two successive sets of straightening wheels to straighten the elongated member within two perpendicular planes. Of course, in some situations, only one set or more of straightening wheels may be required to straighten the elongated member within one plane only or multiple planes. The set(s) of straightening wheels are useful when the feed material is supplied under the form of rolls. In a variant, the feeding mechanism may be adapted to rotate the feed material <NUM> about a longitudinal axis thereof, specifically about a longitudinal axis of the plasma torch <NUM>.

A preheating zone <NUM> for preheating a forward portion <NUM> of the feed material <NUM>, either by direct contact with the plasma <NUM> as illustrated in <FIG> or by radiation heating from a radiation tube <NUM> surrounding the feed material <NUM>, the radiation tube <NUM> itself being heated by direct contact with the plasma <NUM>, as illustrated in <FIG>. The radiation tube <NUM> may be made, for example, of refractory material such as graphite, tungsten or hafnium carbide. The plasma torch <NUM> also comprises an atomization nozzle <NUM> with a channel through which the forward portion <NUM> of the feed material <NUM> from the preheating zone <NUM> travels to expose a forward end <NUM> of the feed material <NUM> to a plurality of plasma jets <NUM> and atomize the feed material. The channel may comprise a central aperture <NUM> allowing the forward portion <NUM> of the feed material <NUM> to exit the plasma torch <NUM> and enter the cooling chamber <NUM>, and with radial apertures <NUM> for producing the plurality of plasma jets <NUM>. The cooling chamber <NUM> is mounted to the lower end of the plasma torch <NUM>, downstream of the nozzle <NUM>. In the cooling chamber <NUM>, the forward end <NUM> of the feed material <NUM> is exposed to the plurality of plasma jets <NUM>.

Still referring to <FIG>, <FIG> and <FIG> and although other types of plasma torches could eventually be used, the plasma torch <NUM> is an inductively coupled plasma torch and comprises an outer cylindrical torch body <NUM>, an inner cylindrical plasma confinement tube <NUM>, and at least one induction coil <NUM> in a coaxial arrangement. The outer cylindrical torch body <NUM> may be made of moldable composite material, for example a moldable composite ceramic material. The inner cylindrical plasma confinement tube <NUM> may be made of ceramic material and, as indicated hereinabove, is coaxial with the torch body <NUM>. The at least one induction coil <NUM> is coaxial with and embedded in the torch body <NUM> to produce a RF (radio frequency) electromagnetic field whose energy ignites and sustains the plasma <NUM> confined in the plasma confinement tube <NUM> including preheating zone <NUM>. The plasma is produced from at least one gas such as argon, helium, hydrogen, oxygen, nitrogen or a combination thereof, supplied within the plasma confinement tube <NUM> through a head <NUM> of the inductively coupled plasma torch <NUM> at the upper end of the torch body <NUM>. RF current is supplied to the induction coil(s) <NUM> via power leads <NUM>. Water or another cooling fluid is fed via inlets such as <NUM>, flows in cooling channels such as <NUM>, in particular through an annular spacing between the torch body <NUM> and the plasma confinement tube <NUM>, for cooling the inductively coupled plasma torch. The water or other cooling fluid exits the apparatus <NUM> via outlets such as <NUM>. Water or other cooling fluid may also flow (a) within a shield <NUM> of the injection probe <NUM> and into the induction coil(s) <NUM> which is (are) then tubular.

Exposure of the forward end <NUM> of the feed material <NUM> to the plurality of plasma jets <NUM> causes local melting of the feed material followed by instantaneous stripping and breakdown of the formed molten layer of feed material into small droplets <NUM>. The droplets <NUM> fall into the cooling chamber <NUM>, which is sized and configured to allow in-flight freezing of the droplets <NUM>. The droplets <NUM>, when freezing, turn into powder particles <NUM> collected in the collector <NUM>.

The apparatus <NUM> of <FIG> is configured to let the droplets <NUM> fall towards the collector <NUM> by gravity. However, other configurations in which the droplets <NUM> do not fall vertically, being propelled by a gas or by a vacuum, are also contemplated. In the embodiment of <FIG> and in such other configurations, an exit pipe <NUM> may connect a lower part of the cooling chamber <NUM> toward a vacuum pumping system (not shown) to withdraw gas from the cooling chamber <NUM>.

The apparatus <NUM> includes other components such as casings, flanges, bolts, and the like, which are illustrated on <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG> and <FIG>. These elements are believed to be self-explanatory and are not described further herein. The precise configuration of the various components illustrated on these and other Figures do not limit the present disclosure.

<FIG> is a perspective view of the atomization nozzle <NUM> with a support flange <NUM> according to an embodiment. <FIG> is a cross-sectional view of the atomization nozzle <NUM> and support flange <NUM> of <FIG> are top, bottom and perspective views showing details of the atomization nozzle <NUM> of <FIG>, including the central aperture <NUM> surrounded by radial apertures <NUM> for forming plasma jet channels, for example micro-plasma jet channels. Without limitation, the atomization nozzle <NUM> may be formed of a water-cooled metal or of a radiation cooled refractory material or a combination of both.

The nozzle <NUM> is supported by the flange <NUM>. As shown in <FIG> and <FIG>, the flange <NUM> can be secured between the lower end of the plasma torch <NUM> and a mounting annular member <NUM> in a sealing arrangement between the plasma torch <NUM> and the cooling chamber <NUM>. Still referring to <FIG> and <FIG>, the nozzle <NUM> comprises an annular, inner surface <NUM> which may define a portion of the cooling channels <NUM> to provide at the same time for cooling of the nozzle <NUM>. The nozzle <NUM> also defines an annular groove <NUM> to receive the lower end <NUM> of the plasma confinement tube <NUM> in a proper sealing arrangement.

The nozzle <NUM> of <FIG> comprises, on the inner side, a central tower <NUM> defining the central aperture <NUM>, which is co-axial with the injection probe <NUM>. The central aperture <NUM> has an input funnel-shaped enlargement <NUM>. This configuration of the tower <NUM> facilitates alignment and insertion of the forward portion <NUM> of the feed material <NUM>. The central aperture <NUM> of the nozzle <NUM> allows the forward portion <NUM> of the feed material <NUM> to exit the plasma torch <NUM> toward the inside of the cooling chamber <NUM>.

The atomization nozzle <NUM> also comprises, around the central tower <NUM>, a bottom wall formed with the plurality of radial apertures <NUM> equally, angularly spaced apart from each other. The radial apertures <NUM> are designed for allowing respective fractions of the plasma <NUM> to flow toward the cooling chamber <NUM> and generate the plasma jets <NUM>. The number of radial apertures <NUM> and their angle of attack with respect to the central, geometrical longitudinal axis of the plasma torch <NUM> may be selected as a function of a desired distribution of the plasma jets <NUM> around the longitudinal axis of the plasma torch <NUM>.

The central aperture <NUM> may be sized and configured to closely match a cross-section of the feed material <NUM> so that the central aperture <NUM> becomes substantially closed by insertion of the forward portion <NUM> of the feed material <NUM> therein. By closing the central aperture <NUM>, a pressure of the plasma <NUM> in the plasma torch <NUM> builds up. This in turn causes respective fractions of the plasma <NUM> to be expelled from the zone <NUM> in the plasma confinement tube <NUM> via the radial apertures <NUM>. These expelled fractions of the plasma <NUM> form the plasma jets <NUM>. The radial apertures <NUM> are sized and configured to expel the plasma jets <NUM> at high velocity, which could possibly attain sonic or supersonic velocities.

In cases where the cross-section of the feed material <NUM> is smaller than the opening of the central aperture <NUM>, the aperture <NUM> is not entirely blocked and pressure build-up within the plasma torch <NUM> may be of a lesser magnitude. Regardless, the sheer action of the plasma torch <NUM> and the partial blockage of the central aperture <NUM> by the feed material <NUM> still cause the plasma <NUM> to be at a significant pressure level. The plasma jets <NUM> may still be present, though potentially reduced in terms of flow and pressure. A portion of the plasma <NUM> is expelled through the central aperture <NUM>, in a gap left between the feed material <NUM> and the opening of the central aperture <NUM>. This portion of the plasma <NUM> forms an annular plasma jet, or flow, that surrounds the forward end <NUM> of the feed material <NUM>. As it passes through the central aperture <NUM>, the forward end <NUM> can be, in such cases, atomized in part by the annular plasma jet. The forward end <NUM> may further be atomized in a further part by plasma jets <NUM> that, though weaker, may still be expelled from the radial apertures <NUM> of the atomization nozzle <NUM> at a significant speed.

The radial apertures <NUM> may each be oriented so that the plasma jets <NUM> converge toward the forward end <NUM> of the feed material <NUM> in the form of an elongated member such as, as non-limitative examples, a wire, a rod or a filled tube, within the cooling chamber <NUM> to enhance the atomization process. More particularly, <FIG> show, respectively, top and bottom views of the atomization nozzle <NUM>. It may be observed that the radial apertures <NUM> are angled inwardly about the central, geometrical longitudinal axis of the plasma torch <NUM> from top to bottom of the atomization nozzle <NUM>. In this manner, the plasma jets <NUM> formed therein will converge within the cooling chamber <NUM> toward a convergence point in axial alignment with the central aperture <NUM>. Without limitation, the radial apertures <NUM> may be cylindrical and have a diameter in the range of <NUM> up to <NUM> to produce sonic or supersonic plasma micro-jets and may be oriented at <NUM>° to <NUM>° angles with respect to the central, geometrical longitudinal axis of the plasma torch <NUM>. Other shapes and diameters of the radial apertures <NUM> may of course be contemplated.

As expressed hereinabove, the atomization nozzle <NUM> generates a plurality of converging plasma jets and may further generate an annular plasma jet. Another embodiment of the atomization nozzle that only generates an annular plasma jet will now be described.

<FIG> is a detailed, front elevation view of the plasma torch of <FIG>, showing an atomization nozzle according to another embodiment. In this embodiment, the plasma torch <NUM> is modified to comprise an atomization nozzle <NUM> arranged centrally on a bottom closure piece of the torch <NUM> secured to the lower end of the torch body <NUM>. The atomization nozzle <NUM> has a central aperture <NUM> at its exit end and an internal face <NUM> that tapers off toward the central aperture <NUM>. In a non-limitative embodiment, the central aperture <NUM> of the atomization nozzle <NUM> is sized and configured to substantially match a cross-section of the elongated member forming the feed material <NUM> so moving the forward end <NUM> of the feed material <NUM> into the atomization nozzle <NUM> causes building up of a pressure of the plasma <NUM> in the plasma torch <NUM>. The pressure of the plasma <NUM> in the plasma torch <NUM> causes some of the plasma to be expelled through the atomization nozzle <NUM>, forming an annular plasma jet <NUM> between the forward end <NUM> of the feed material <NUM> and the internal face <NUM> of the atomization nozzle <NUM>. Exposure of the forward end <NUM> of the feed material <NUM> to the annular plasma jet <NUM> causes surface melting and atomization of the feed material <NUM>. The atomized feed material exits the plasma torch <NUM> through the central aperture <NUM> and enters the cooling chamber <NUM> in the form of fine or ultrafine droplets <NUM>. The droplets <NUM> fall into the cooling chamber <NUM>, which is sized and configured to allow in-flight freezing of the droplets <NUM>. The droplets <NUM>, when freezing, turn into powder particles <NUM> collected in the collector <NUM>. Some of the plasma, forming the annular plasma jet <NUM>, also enters the cooling chamber <NUM>.

<FIG> is a detailed, front elevation view of a variant of the plasma torch of <FIG>, showing the atomization nozzle of <FIG> and further including a sheath gas port surrounding the exit end of the atomization nozzle. In this variant, the plasma torch <NUM> of earlier Figures is supplemented by the addition of an input port <NUM> for receiving a sheath gas <NUM>. The sheath gas <NUM> is constrained underneath the plasma torch <NUM> by a cover <NUM> that forms with the bottom closure piece of the torch an annular cavity surrounding the central aperture <NUM> of the atomization nozzle <NUM>. The sheath gas <NUM> is expelled from the annular sheath gas output port <NUM> to form a sheath gas curtain <NUM> surrounding the plasma and the droplets <NUM> expelled from the atomization nozzle <NUM>. Presence of the axial sheath gas curtain <NUM> prevents the droplets <NUM> from reaching and depositing on any downstream surface of the plasma torch <NUM>, including the atomization nozzle <NUM>. Specifically, the sheath gas curtain <NUM> prevents rapid expansion of the plasma flow emerging from the atomization nozzle <NUM> and, therefore, the droplets <NUM> from impinging on any downstream surfaces of the cooling chamber. As shown on <FIG>, the central aperture <NUM> of the atomization nozzle <NUM> may be extended slightly in a short annular flange <NUM> to better deflect the sheath gas <NUM> around the flow formed by the plasma gas and the droplets <NUM>. The sheath gas may be of a same nature as the source of the plasma gas, including for example inert gases such as argon and helium to their mixtures with hydrogen, oxygen and/or nitrogen. The sheath gas may alternatively consist of a different gas.

The apparatus <NUM> may integrate either of the atomization nozzles <NUM> and <NUM>. Though not illustrated, a further variant of the apparatus <NUM> including a combination of the atomization nozzle <NUM> with components providing the sheath gas <NUM> via the sheath gas port <NUM> is also contemplated.

<FIG> is a flow chart showing operations of a process of producing powder particles by atomization of a feed material in the form of an elongated member such as, as non-limitative examples, a wire, rod or filled tube. On <FIG>, a sequence <NUM> comprises a plurality of operations that may be executed in variable order, some of the operations possibly being executed concurrently, some of the operations being optional.

The sequence <NUM> for producing powder particles by atomization of a feed material in the form of an elongated member such as, as non-limitative examples, a wire, a rod or a filled tube is initiated at operation <NUM> by introducing the feed material in a plasma torch, for example in an inductively coupled plasma torch. Introduction of the feed material in the plasma torch may be made via an injection probe in continuous manner, using a typical wire, rod or tube feeding mechanism to control the feed rate of the elongated member and, if required, to straighten the elongated member sometimes provided in the form of rolls.

Within the plasma torch, a forward portion of the feed material may be preheated by either direct or indirect contact with plasma at operation <NUM>. When an injection probe is used, a section of the plasma torch beyond an end of the injection probe, specifically between the end of the injection probe and may form a preheating zone for preheating the forward portion of the feed material. Operation <NUM> comprises moving a forward portion of the feed material from into an atomization nozzle of the plasma torch, a forward end of the feed material reaching a central aperture of the atomization nozzle.

One or more plasma jets are produced by the atomization nozzle. The one or more plasma jets may include an annular plasma jet surrounding the forward end of the feed material, a plurality of converging plasma jets expelled by the atomization nozzle, or a combination of the annular and converging plasma jets. Generating additional plasma jets using a secondary plasma torch operably connected to the cooling chamber is also contemplated. Operation <NUM> comprises surface melting the forward end of the feed material by exposure to the one or more plasma jets formed in the atomization nozzle.

Droplets formed by atomization of the feed material are frozen in-flight within the cooling chamber, at operation <NUM>. Then operation <NUM> comprises collecting powder particles resulting from in-flight freezing of the droplets.

Production of the powder particles using the sequence <NUM> of <FIG> may be made continuous by continuously advancing the feed material into the plasma torch while maintaining the plasma and plasma jets at proper temperature levels. Generally, a duration of the travel of the forward portion of the feed material in the preheating zone, whether by direct contact between the feed material and the plasma or indirect radiation heating by the plasma through a radiation tube is controlled so that the forward portion of the feed material reaches a predetermined temperature before moving into the atomization nozzle. The predetermined temperature obtained in the preheating operation <NUM> is below a melting point of the feed material. Control of the duration of the preheating time of the feed material may be made by controlling a rate of feeding of the feed material and/or a length of the preheating zone in the plasma torch.

Through temperature control of the plasma and of the plasma jets, production of the powder particles using the sequence <NUM> may apply to a broad range of materials such as pure metals, for example titanium, aluminum, vanadium, molybdenum, copper, alloys of those or other metals including for example titanium alloys, steel and stainless steel, any other metallic materials having a liquid phase, ceramics including for example those of oxide, nitride, or carbide families, or any combination thereof, or any other ceramic material that has a liquid phase, composites or compounds thereof. The foregoing list of materials is not intended to limit the application of the process and apparatus for producing powder particles by atomization of a feed material in the form of an elongated member.

According to a first example, the process for producing powder particles by atomization of a feed material in the form of an elongated member may comprise the following operations. This first example may make use of the apparatus <NUM> illustrated in whole or in parts in <FIG> that includes the plasma torch <NUM> for heating, melting and atomizing the feed material <NUM>. The process involves an axial introduction of the feed material <NUM> in the form of an elongated member such as, as non-limitative examples, a wire, a rod or a filled tube, through the injection probe <NUM>, into the center of a discharge cavity where the plasma <NUM> is generated. The feed material <NUM> may be supplied to the injection probe <NUM> in continuous manner by a typical wire, rod or tube feeding mechanism (not shown) for example similar to commercially available units currently used in wire arc welding such as the units commercialized by Miller for MIG/Wire welding, and comprising, as indicated in the foregoing description, wheels operated to control the feed rate of the elongated member and, if required, to straighten the elongated member sometimes provided in the form of rolls. As the feed material <NUM> emerges from the injection probe <NUM> and traverses the plasma <NUM>, it is heated in the preheating zone <NUM> before entering into the downstream atomization nozzle <NUM> at the lower end of the plasma torch <NUM>. A distance between the end of the injection probe <NUM> and the entrance point of the atomization nozzle <NUM> defines a length of the preheating zone <NUM>. A time of heating of the feed material <NUM> by the plasma in the preheating zone <NUM> depends on the length of the preheating zone <NUM> and on a linear speed at which the elongated member travels in the plasma torch <NUM>. An amount of energy received by the feed material <NUM> in the preheating zone <NUM> depends in turn not only on the time of preheating of the feed material <NUM> in the preheating zone <NUM> but also on thermo-physical properties of the plasma <NUM> as well as on a diameter of the elongated member forming the feed material <NUM>. Through control of the length of the preheating zone <NUM>, the linear speed of the elongated member forming the feed material <NUM>, and the plasma temperature, it is possible to control the temperature of the forward end <NUM> of the feed material <NUM> as it enters into the atomization nozzle <NUM>. For optimal results, the temperature of the feed material <NUM>, as it penetrates into the atomization nozzle <NUM>, may be as high as possible, though preferably not too close to the melting point of the feed material <NUM> in order to avoid premature melting of the feed material <NUM> in the discharge cavity of the plasma torch <NUM>.

As the preheated forward end <NUM> of the feed material <NUM> emerges from the atomization nozzle <NUM> in the cooling chamber <NUM>, it is exposed to a plurality of plasma jets, for example a high velocity, sonic or supersonic, micro-plasma jets <NUM> that impinge on the surface of the forward end <NUM> of the elongated member forming the feed material <NUM>, melts the material and, in statu nascendi, strips out molten material in the form of finely divided, spherical molten droplets <NUM> that are entrained by the plasma gas. As the atomized droplets <NUM> are transported further downstream into the cooling chamber <NUM>, they cool down and freeze in-flight forming dense spherical powder particles <NUM> of the feed material. The powder particles <NUM> are recovered in the container <NUM> located at the bottom of the cooling chamber <NUM>, or may be collected in a downstream cyclone (not shown) or collection filter (also not shown), depending on their particle size distribution.

Again, this second example may make use of the apparatus <NUM> that includes the plasma torch <NUM> for heating, melting and atomizing the feed material <NUM>. According to the second example usable to manufacture powders of dense spherical particles of metals, metal alloys and ceramics, the process for producing powder particles by atomization of a feed material in the form of an elongated member comprises the following operations:.

According to a third example, which may make use of the apparatus <NUM>, the process for producing powder particles by atomization of a feed material in the form of an elongated member comprises the following operations.

Feed material <NUM> in the form of an elongated member such as, as non-limitative examples, a wire, a rod or a filled tube is introduced through the injection probe <NUM> axially oriented along a centerline of the plasma torch <NUM>.

As the feed material <NUM> emerges from the injection probe <NUM>, at a downstream end of the plasma torch <NUM>, its forward portion <NUM> is heated either by direct contact with the plasma <NUM> or indirectly using the radiation tube <NUM> in the preheating zone <NUM>. A distance of travel in the preheating zone <NUM> and a speed of movement of the feed material <NUM> may be adjusted to allow sufficient time for the forward portion <NUM> of the elongated member to heat to a temperature as close as possible to the melting point of the feed material, without actually reaching that melting point.

At this point, the forward end <NUM>, or tip, of the feed material <NUM> reaches the atomization nozzle <NUM> and penetrates through its central aperture <NUM>, which in this third example has substantially the same diameter as that of the elongated member. As the forward end <NUM> of the feed material <NUM> emerges in the cooling chamber <NUM> from a downstream side of the atomization nozzle <NUM>, it is exposed to the plurality of plasma jets <NUM>, for example the high-velocity plasma micro-jets <NUM> impinging thereon. Since the forward end of the feed material <NUM>, being already preheated in the preheating zone <NUM>, i.e. in the discharge cavity, to near its melting point, it rapidly melts at its surface and is stripped away by the plasma jets <NUM>, turning into fine or ultrafine droplets <NUM> that are entrained by a plasma flow resulting from the plasma jets <NUM>. As the droplets <NUM> travel down the cooling chamber <NUM>, they cool down and solidify in the form of dense spherical particles <NUM> that deposits by gravity in the container <NUM> at the bottom of the cooling chamber <NUM> or are transported by the plasma gas to a downstream powder collection cyclone or to a fine metallic filter.

According to a fourth example, which may make use of the apparatus <NUM>, the process for producing powder particles by atomization of a feed material in the form of an elongated member comprises the following operations.

Feed material <NUM> in the form of an elongated member such as, as non-limitative examples, a wire, a rod or a filled tube has smaller diameter than that of the central aperture <NUM>. The feed material <NUM> is introduced through the injection probe <NUM> axially oriented along a centerline of the plasma torch <NUM>.

As in the third example, the feed material <NUM> emerges from the injection probe <NUM>, at a downstream end of the plasma torch <NUM>, its forward portion <NUM> is heated either by direct contact with the plasma <NUM> or indirectly using the radiation tube <NUM> in the preheating zone <NUM>. A distance of travel in the preheating zone <NUM> and a speed of movement of the feed material <NUM> may be adjusted to allow sufficient time for the forward portion <NUM> of the elongated member to heat to a temperature as close as possible to the melting point of the feed material, without actually reaching that melting point.

At this point, the forward end <NUM>, or tip, of the feed material <NUM> reaches the atomization nozzle <NUM> and penetrates through its central aperture <NUM>, which in this fourth example has a larger diameter than that of the elongated member. As the forward end <NUM> of the feed material <NUM> travels through the central aperture <NUM> of the atomization nozzle <NUM>, it is exposed to an annular plasma jet present in a gap formed of a difference between the diameter of the central aperture <NUM> and the diameter of the elongated member. Since the forward end <NUM> of the feed material <NUM>, is already preheated in the preheating zone <NUM>, i.e. in the discharge cavity, to near its melting point, exposition of the forward end <NUM> of the feed material <NUM> to this annular plasma jet causes a rapid melting at its surface, being stripped away by the annular plasma jet, turning into fine or ultrafine droplets <NUM> that are entrained by a plasma flow resulting from the annular plasma jet. If the forward end <NUM> is not entirely atomized by the annular plasma jet, remaining feed material emerges in the cooling chamber <NUM> from a downstream side of the atomization nozzle <NUM>. The remaining feed material is exposed to the plurality of plasma jets <NUM> impinging thereon. The remaining feed material continues melting at its surface and, being stripped away by the plasma jets <NUM>, turning into more fine or ultrafine droplets <NUM> that are entrained by a plasma flow resulting from the annular plasma jet and from the plasma jets <NUM>. As the droplets <NUM> travel down the cooling chamber <NUM>, they cool down and solidify in the form of dense spherical particles <NUM> that deposits by gravity in the container <NUM> at the bottom of the cooling chamber <NUM> or are transported by the plasma gas to a downstream powder collection cyclone or to a fine metallic filter.

An overall view of a typical plasma atomization apparatus <NUM> is shown in <FIG>. The basic dimensions and shapes of the shown components of the apparatus <NUM> may widely vary depending on the material to be atomized and depending on desired production rates. A power level of the plasma torch <NUM> may, without loss of generality, vary between <NUM> or <NUM> kW up to hundreds of kW for a commercial production scale unit.

Referring again to <FIG>, an example of design of the atomization nozzle <NUM> is shown. The nozzle <NUM> comprises the flange <NUM>. The atomization nozzle <NUM> may be made of fluid-cooled copper or stainless steel. Alternatively, the atomization nozzle <NUM> may be made of a refractory material such as graphite, in combination with a water-cooled flange <NUM>.

The atomization nozzle <NUM> has a central aperture <NUM> optionally adapted to closely match a diameter of the elongated member forming the feed material <NUM>. The atomization nozzle <NUM> has a plurality of radial apertures <NUM> equally distributed around the central aperture <NUM> and which, according to an embodiment, are directed at an angle of <NUM>° about the central, geometrical longitudinal axis of the plasma torch <NUM>. Successful operation was obtained using sixteen (<NUM>) radial apertures <NUM> having a diameter of <NUM>, the radial apertures <NUM> being equally distributed around the central aperture <NUM>. The diameter, the number and the angle of the radial apertures <NUM> can be adjusted depending on thermo physical properties of the materials to be atomized and on a desired particle size distribution.

It should be pointed out that the atomized material may change its chemical composition during atomization through the reaction between different components premixed into the feed material. A non-limitative example is the production of an alloy by mixing different metals forming the particles filling a tube forming the feed material. Another non-limitative example is a chemical reaction between the chemical components forming the particles in the filled tube. It should also be pointed out that the atomized material may change its chemical composition during atomization as a result of a chemical reaction between the plasma gas(es) and/or sheath gas(es) and the atomized material, for example by oxidation, nitration, carburization, etc..

Based on fluid dynamic modeling of the flow and temperature field in the discharge cavity of the plasma torch it is possible to calculate the temperature profile in the elongated member forming the feed material as it traverses the preheating zone in the torch. <FIG> is a schematic view, including a graph showing modelling results for heating a <NUM> stainless steel wire introduced in an argon/hydrogen induction plasma at <NUM> kW. <FIG> provides typical results that can be obtained using an inductively coupled plasma torch as shown on <FIG>. <FIG> shows, on its left hand side a two-dimensional temperature field in the discharge cavity for the argon/hydrogen plasma operated with a radio frequency power supply with an oscillator frequency of <NUM>, and a plate power of <NUM> kW. At the bottom of <FIG>, a corresponding temperature field in a <NUM> diameter stainless steel rod is given for rod translation velocities of <NUM> and <NUM>/s. As expected the overall temperature of the rod drops with the increase of its translation speed across the preheating zone in the discharge cavity of the plasma torch. The center of <FIG> is a graph showing a variation of the maximum temperature achieved at the tip of the elongated member, for different speeds, and different length of the preheating zone <NUM>, identified on the left hand side of <FIG> as 'z'. It may be noted that depending on the length of the preheating zone <NUM>, maintaining the rod translation velocity within a relatively narrow window allows to avoid the premature melting of the material in the discharge cavity or its arrival at the atomization nozzle at too low a temperature, which would have a negative impact on the quality of the atomized product.

<FIG> is an electron micrograph of powder particles obtained by atomization of a <NUM> diameter stainless steel wire and a graph of corresponding particle size distribution. Such particles can be obtained using the plasma torch of <FIG>. Stainless steel powder particles were obtained using the induction plasma atomization process. The powder particles had a mean particle diameter, d<NUM> of about <NUM> and the powder production rate was about <NUM>/hour. The powder was mostly composed of dense spherical particles. A certain number of splats and satellites were observed depending on the operating conditions and process optimization.

<FIG> illustrates electron micrographs of different stainless steel spherical powder fractions produced using the process and apparatus for producing powder particles by atomization of a feed material in the form of an elongated member. Such particles can be obtained using the inductively coupled plasma torch of <FIG>, <FIG> and <FIG>. Again, the powder was mostly composed of dense spherical particles; only few splats and satellites were observed depending on the operating conditions and process optimization.

Those of ordinary skill in the art will realize that the description of the process and apparatus for producing powder particles and the description of powder particles so produced are illustrative only and are not intended to be in any way limiting. Other embodiments will readily suggest themselves to such persons with ordinary skill in the art having the benefit of the present disclosure. Furthermore, the disclosed process, apparatus and powder particles may be customized to offer valuable solutions to existing needs and problems related to efficiently and economically producing powder particles from a broad range of feed materials.

Various embodiments of the process for producing powder particles by atomization of a feed material in the form of an elongated member, of the apparatus therefor, and of the powder particles so produced, as disclosed herein, may be envisioned. Such embodiments may comprise a process for the production of a broad range of powders including, tough not limited to, fine and ultrafine powders of high purity metals, alloys and ceramics in an efficient cost effective way that is scalable to an industrial production level. The process is applicable for the production of powders of pure metals, alloys and ceramics, causes minimal or no contamination of the atomized material, causes minimal or no oxygen pickup especially for reactive metals and alloys, produces fine or ultrafine particle size, for example with particle diameter less than <NUM>, the particles being dense and spherical, with minimal or no contamination with satellites.

In the interest of clarity, not all of the routine features of the implementations of process, apparatus, and use thereof to produce powder particles are shown and described. It will, of course, be appreciated that in the development of any such actual implementation of the process, apparatus, and use thereof to produce powder particles, numerous implementation-specific decisions may need to be made in order to achieve the developer's specific goals, such as compliance with application-, system-, and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the field of materials processing having the benefit of the present disclosure.

Claim 1:
An apparatus for producing powder particles (<NUM>) by plasma atomization of a feed material (<NUM>) in the form of an elongated member, comprising:
a) a plasma torch (<NUM>) including: an injection probe (<NUM>) for receiving the feed material (<NUM>); and an atomization nozzle (<NUM>) configured to: receive a forward portion of the feed material (<NUM>) from the injection probe (<NUM>), be supplied with plasma, produce one or more plasma jets, and melt a surface of a forward end of the feed material (<NUM>) by exposure to the one or more plasma jets; wherein the one or more plasma jets are selected from an annular plasma jet and a combination of an annular plasma jet and a plurality of converging plasma jets, the atomization nozzle (<NUM>) comprising a central aperture (<NUM>) for receiving the forward end of the feed material (<NUM>) and an internal face (<NUM>) tapering off toward the central aperture (<NUM>);
b) a cooling chamber (<NUM>) mounted to the plasma torch (<NUM>) downstream of the atomization nozzle (<NUM>).