PROCESS FOR PREPARING POWDER

Abstract

A preparation process for preparing powder from a first material and a second material, having: a step of dispensing wires made of the materials by means of a feeder and an additional feeder, a melting step of melting the wires, a spraying step of spraying in such a way as to form droplets, a cooling step of cooling the droplets in such a way as to form solid particles, a separation and collection step of separating the solid particles from the carrier gas and collecting the solid particles so as to form the powder, and, during the dispensing step, the wires are firstly pushed by the feeder, then pulled by the additional feeder, and then pushed by the additional feeder towards said electric arc.

Claims

1. A process for preparing powder from a first material and a second material, comprising: dispensing a first wire and a second wire made of the first material and the second material, respectively, said dispensing step being carried out for each wire by at least a feeder and an additional feeder; melting the first and second materials, with an electric arc formed by the application of an electric current between said first and second materials; spraying the first and second molten materials thereby forming droplets; cooling the droplets with a carrier gas thereby forming solid particles; separating the solid particles from the carrier gas and collecting the solid particles thereby forming the powder, wherein, during said dispensing step, said first wire and said second wire are pushed by said feeder towards said electric arc, then said first wire and said second wire are pulled towards said electric arc by said additional feeder, and then said first wire and said second wire are pushed by said additional feeder towards said electric arc.

2. The process according claim 1, wherein said first wire and said second wire each have an end which is melted during said melting step, said first wire and said second wire are each pushed by said additional feeder over a distance (L), each distance (L) being equal to the length measured between said additional feeder and said respective end of the wires.

3. The process according to claim 1, wherein said first and second wires are dispensed at a rate of between 5 and 10 m/min.

4. The process according to claim 1, further comprising an additional step of analyzing the powder to determine the density of said powder and/or the oxygen content of said powder and/or the particle size of said powder.

5. The n process according to claim 1, further comprising an enriching step of enriching the droplets and/or the particles with an active substance which is carried out during the cooling step, the enriching step being preceded by a step of ionizing the active substance.

6. The process according to claim 1, wherein the enriching step is carried out during the spraying and cooling steps.

7. The process according to claim 1, wherein the cooling step is carried out using a cooling gas in addition to the carrier gas.

8. The preparation process according claim 5, wherein the active substance comprises: at least one inert gas; and at least one active compound comprising at least one of the following atoms: oxygen, nitrogen, carbon or hydrogen; each active compound being in the gaseous, liquid or solid phase, the content of each active compound being between 5 ppm and 20000 ppm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0023] The invention will be understood more clearly from reading the following description and from studying the accompanying figures. These figures are provided purely by way of illustration and do not in any way limit the invention.

[0024] FIG. 1 is a schematic representation of a device implementing the invention.

[0025] FIG. 2 is a schematic representation of a process for preparing the powders according to the invention.

[0026] FIG. 3 is a schematic representation of the values for the voltage and current strength during the preparation process for one embodiment of the invention.

[0027] FIG. 4 is a schematic representation of a dispensing device for the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] FIG. 1 shows a preparation device 200 configured to implement the preparation process 100 according to the invention. This preparation device 200 comprises at least: a spraying means 300; an atomization chamber 400; a first collection means 500; and an exhaust means 600. This preparation device 200 may also comprise complementary elements (not shown), such as: a gas/particle separation system; and a second collection means.

[0029] With reference to FIG. 2, the preparation process 100 comprises at least the following steps: a melting step 110 of melting two materials 1a, 1b using an electric arc 314; a spraying step 120 of spraying each material 1a, 1b in such a way as to form droplets 2; a cooling step 130 of cooling the droplets 2, using a carrier gas 11, in such a way as to form solid particles 3; and a separation and collection step 140 of separating the particles 3 from the carrier gas 11 and collecting the solid particles 3 so as to form a first powder and a second powder 5.

[0030] Each material 1a, 1b is an electrical conductor. It may, for example, be a pure metal such as titanium or aluminium or an alloy such as a titanium-based alloy, an aluminium-based alloy, a nickel-based alloy, a copper-based alloy or an iron-based alloy. The materials 1a, 1b may be of the same type or even identical. The choice of the composition of each material 1a, 1b partially determines the composition of the powders 5 obtained. In one embodiment, said first or second material is a Ti6Al4V alloy. These materials 1a, 1b are supplied in the form of conductive wires 312a, 312b, respectively, and dispensed towards the spraying means 300 shown in FIG. 1, and more specifically into the enclosure 311, by a feeding system, not shown, for example at a predefined speed. In one embodiment, the diameter of the wires 312a, 312b ranges between 0.8 and 2 mm, preferably between 1.2 and 2 mm. The wires can thus be easily manipulated.

[0031] During the melting step 110, this spraying means 300 is configured to carry out the melting step 110 of melting each material 1a, 1b using an electric arc 314. The spraying means 300 comprises an electric-arc source 310, also referred to as wire-arc torch. The wire-arc torch 310 is configured to generate an electric arc 314. The electric arc 314 may be created from the carrier gas 11, such as argon, nitrogen or helium, or a mixture of these gases. The wire-arc torch 310 comprises an enclosure 311 which is filled with the carrier gas 11 and in which the electric arc 314 is generated. The pressure of the carrier gas 11 in the enclosure 311 may be greater than or equal to atmospheric pressure. The wire-arc torch 310 is configured to generate the electric arc 314 between the first material 1a and the second material 1b. The wire-arc torch comprises the two conductive wires 312a, 312b, which are disposed one on each side of the enclosure 311, are mutually separate and are configured to trigger and sustain the electric arc 314 using the electric current. During operation, the distance between the two conductive wires 312a, 312b is preferably kept less than 5 mm and depends on the energy supplied. When the wire-arc torch 310 is in operation, the electric arc 314 is adjacent to the two facing ends 313a, 313b of the two wires 312a, 312b. The carrier gas 11 is introduced in jet form into the enclosure 311 through an inlet 313. The jet of carrier gas 11 is configured to strike the ends 313a, 313b of the two wires 312a, 312b. In one embodiment, the spraying means 300 comprises multiple wire-arc torches 310 for increasing the amount of powder produced by the preparation device 200. During this melting step 110, the mode of operation of the wire-arc torch 310 is chosen such that the temperature of the plasma at the electric arc 314 is greater than the melting temperature of each material 1a, 1b. Thus, during operation, said plasma causes the ends 313a, 313b of the two wires 312a, 312b to melt.

[0032] The spraying step 120 is also carried out using the spraying means 300. The spraying step 120 of spraying each material 1a, 1b from the liquefied ends 313a, 313b allows droplets 2 to form. During this spraying step 120, the jet of carrier gas 11 is focused directly onto the liquefied ends 313a, 313b of the wires 312a, 312b such that the melted ends 313a, 313b are sprayed and droplets 2 are created. In order to maintain a fixed spacing between the ends 313a, 313b in spite of the spraying of material, the wires 312a, 312b are always introduced into the enclosure 311 by a feeding system, not shown, at a predefined speed.

[0033] FIG. 1 schematically shows the atomization chamber 400 and the exhaust means 600, which are configured in such a way as to carry out the cooling step 130 of cooling the droplets 2, using the carrier gas 11, such that solid particles 3 are formed. In one embodiment, in order to accelerate the cooling 130, a cooling gas 12 can be injected into the atomization chamber 400. The cooling gas 12 in contact with the carrier gas forms a gas mixture 13. Thus, during the cooling step 130, the droplets 2, which are in contact with the gas mixture 13, establish a transfer of heat with the gas mixture 13. Preferably, the temperature of the cooling gas 12 injected is chosen such that it is lower than the lowest of the solidification temperatures of the materials 1a, 1b or of the alloys formed by the materials 1a, 1b within the droplets 2. The cooling mixture 12 is, for example, injected at ambient temperature. As a result, the expanded carrier gas 11 and the cold cooling gas 12 cause heat to be transferred from the droplets 2 to the gas mixture 13, and this cools the droplets 2. When the temperature of the droplets 2 is lower than the solidification temperature of the droplets 2, the droplets 2 are solidified in such a way as to form the solid particles 3. The cooling step 130 allows the droplets 2 to form spheres, i.e. they adopt a spherical shape by virtue of the surface tension on the surface of the melted droplets 2 and the interaction with the gas mixture 13. Thus, as they solidify, the droplets 2 form particles 3 which have a sphericity greater than 0.9 and as close as possible to 1.

[0034] The preparation process 100 may also comprise an enriching step 160 of enriching the droplets 2 or/and the particles 3. The enriching 160 is carried out by means of an active substance 16. The enriching 160 is at least implemented during the cooling step 130. However, the enriching 160 may also start during the spraying 120 and continue during the cooling 130. Enriching is understood to mean a metallurgical treatment of the materials 1a, 1b and of the alloys formed within the droplets 2 by means of an active substance 16 in such a way as to give the resulting particles 3 specific physico-chemical properties.

[0035] The active substance 16 implemented in the enriching step 160 comprises: at least one inert gas, advantageously of the same composition as the carrier gas 11; and at least one active compound comprising at least one of the following atoms: oxygen, nitrogen, carbon and hydrogen. Each active compound may be in the gaseous, liquid or solid phase, for example in the form of droplets or suspended particles. The content of each active compound within the active substance 16 ranges between 5 ppm and 20000 ppm and preferably between 5 ppm and 1000 ppm. It may, for example, be carbon monoxide or methan. The active compound in the active substance 16 may be a hydrocarbon, such as methane, which is rich in carbon and hydrogen. If the active substance 16 comprises carbon monoxide or methane, the enriching 160 corresponds to a carburizing of the materials 1a, 1b. If the active substance 16 comprises nitrogen, the enriching 160 corresponds to a nitriding. If the active substance 16 comprises oxygen or hydrogen, the enriching 160 corresponds to an oxidation or, conversely, a reduction of the materials 1a, 1b. The active substance 16 may react with the materials 1a, 1b whether they are in the form of droplets 2 or solid particles 3. The active substance 16 is preferably injected into the atomization chamber 400 of the device 200. As a result, the active substance 16 reacts with the particles 3. Advantageously, the active substance 16 is involved in the spraying step 120. In this way, the active substance 16 reacts with the droplets 2. Alternatively, the active substance 16 is also injected at the spraying means 300. The partial pressures of the inert gas and of each active compound of the active substance 16 are monitored within the device 200 throughout the process 100 so that the content of each active compound remains between 5 ppm and 20000 ppm and preferably between 5 ppm and 1000 ppm. Since the chemical reactions take place between the active substance 16 and the surfaces of the droplets 2 and of the particles 3, it is possible to optimize the exchange surface area. In this way, the enriching step 160 is carried out efficiently. As a result, the enriching step 160 makes it possible to control the final chemical composition of the resulting particles 3.

[0036] Then, there is a separation and collection step 140 of separating the solid particles and collecting the solid particles 3 so as to form the powder 5. This step is firstly carried out by the first collection means 500 which is connected to the atomization chamber 400. With reference to FIG. 1, the first collection means 500 comprises a main vessel 520 configured to receive a first portion of the solid particles 3 thus forming the powder 5. Then, a gas/particle separation system, not shown, is configured to separate the second portion of the particles 3 from the gas mixture 13. This second portion of particles is mainly formed of the lightest particles. The gas/particle separation system may, for example, be a filtration means, a settler or a cyclone.

[0037] In FIG. 2, the schematically shown preparation process 100 comprises several combinable steps, illustrated in dashed lines, which will now be described. An ionization step 150 can be combined with the enriching step 160 in order to improve the kinetics of the chemical reactions taking place between the droplets 2, the particles 3 and the active substance 16. The ionization step 150 comes before the enriching step 160, in which case the enriching step can start during the spraying 120. In this step, the active substance 16 may be introduced into the enclosure 311 of the spraying means 300 in such a way that it is ionized by the electric arc 314. The electric arc 314 ionizes each component of the active substance 16 so that reactive free ions are produced. The high-energy reactive free ions improve the reaction kinetics during the enriching step 160. The enriching reactions therefore balance before the droplets 2 are solidified. As a result, the chemical composition of the resulting particles 3 is controlled and reproducible. The concentration of reactive free ions is highest within the enclosure 311. Outside the enclosure, the concentration of reactive free ions decreases owing to the recombination reactions. Advantageously, the reactive free ions follow the path taken by the droplets 2 in the atomization chamber 400 in order to increase the duration of the enriching step 160.

[0038] At the end of the collection step 140, the passivation step 170 of passivating the surface of the particles 3 can be carried out, for example, if the first and second powders 5 are prepared from materials that are flammable, i.e. have a strong affinity with oxygen. This is the case for example with powders 5 formed from titanium, alloys of titanium or aluminium. The passivation step 170 is carried out using a passivation gas 14. The passivation gas 14 may for example comprise a noble gas and an active gas such as oxygen, the active gas preferably having a concentration of between 20 ppm and 2%. The passivation step 170 is carried out systematically on the two powders 5. The following example will show the passivation step 170 carried out on the first powder 5 in the first collection means 500. The passivation step 170 can be transferred to the gas/particle separation system.

[0039] In order to obtain a first and a second powder 5 that satisfy particle size distribution characteristics, an additional sieving step 180 may be carried out on the first and the second powder 5. The sieving 180 makes it possible, for example, to remove aggregates of particles 3 or particles 3 that exceed a limit size from the powders 5. The particle size distribution can be characterized by three specific diameters denoted D10, D50 and D90. 10% of the particles 3 have a diameter less than D10, 50% of the particles 3 have a diameter less than D50 and 90% of the particles 3 have a diameter less than D90. The sieving 180 may for example be carried out in order to adjust the distribution of the powders 5, in particular the diameter D50, corresponding to the median of the distribution.

[0040] In order for the chemical composition of the powders 5 to be reproducible, the preparation device 200 can undergo an additional inerting step 101. The inerting step 101 is carried out using an inerting gas and cycles of compression and expansion, in order to purge the air present in the device 200 until the oxygen content is less than 100 ppm, preferably less than 10 ppm, before starting the melting step 110. The inerting gas may for example comprise an inert gas or a mixture of inert gases.

[0041] Conventionally, in this process the transfer regimen used is a spray transfer regimen. This affords a continuous electric arc 314 by means of a continuous electric current during the melting and spraying steps. Moreover, a constant length of the electric arc 314 as the wire of material 1a, 1b is dispensed is obtained. However, the yields obtained by this powder preparation process are not satisfactory enough; specifically, the particle size distribution is not controlled enough. A possible result of this process is thus an excessive amount of fine particles, i.e. powders having a size of less than 1 micron. Furthermore, the inventors have noted that this process causes a sharp rise in temperature, and therefore the powder has a tendency to oxidize. In addition, sometimes the powder is sintered.

[0042] The inventors have thus sought to avoid these drawbacks. They then reduced the voltage of the electric current applied, while still keeping a constant supply of material, in order to reduce the length of the electric arc 314 and obtain powders 5 of larger diameter. The transfer regime then changes to a short-circuit transfer regime. In this short-circuit regime, the inventors noted a considerable improvement in the control of the particle size distribution of the powders 5. In this transfer regime, the electric current applied is no longer a continuous electric currentit is a short-circuit current. As a result, during the melting step, the heat given off by the electric arc 314 makes the wire ends 313a, 313b melt until a droplet 2 is formed, and the two wires 312a, 312b will then be connected. A liquid bridge is also formed between the two wires 312a, 312b and the droplets 2. At this moment, a short circuit is created and the voltage of the electric current applied is then equal to 0 V. With reference to FIG. 3, the electric current applied is regulated by increasing the strength of the applied current. At this instant, the increase in the current strength, caused by the short circuit, generates electromagnetic fields which, together with the surface tension effect, will cut off the liquid bridge between the wires 312a, 312b and the droplet 2. The breaking of the bridge causes droplets 2 to spray out during the spraying step, these droplets 2 subsequently forming the powder 5. Depending on the rate of the increase in current intensity, the formation of large droplets 2 is avoided. This means that the particle size distribution of the powders 5 is better controlled. The powders 5 produced comprise, in particular, less fine powder than the powders 5 produced with a spray transfer regime. Furthermore, the powder is less polluted and the composition obtained is also better controlled. Indeed, the phenomenon of oxidation of the powders is less pronounced, and the composition is thus better controlled. Then, after the spraying of the droplets 2, the voltage is re-established, resulting in the re-creation of an electric arc 314 and the cycle of melting and spraying starts again.

[0043] In one embodiment, the voltage applied between the two conductive wires 312a, 312b can range between 10 V and 30 V, preferably between 11 V and 20 V, more preferably between 14 V and 19 V. The length of the electric arc 314 induced by the application of the electric current to the two materials 1a and 1b is smaller than it is for a spray transfer regime. These ranges of values make it possible to ensure a controlled particle size, i.e. a particle size distribution desired by the user of the process. Specifically, if the voltage is too low, the droplet 2 of metal can no longer drop off and the conductive wires 312a, 312b run the risk of becoming connected. There is then no short circuit. If the voltage is much too low, the electric arc 314 is not created and the wires 312a, 312b are fed without creating droplets. By contrast, if the voltage is too high, the particle size distribution of the powders and the composition of the powder will not be satisfactory. Specifically, the particle size of the powders can be excessively heterogeneous and/or include particles of powder 5 that are excessively fine and have too small a diameter, for example have a size less than 1 micron. Furthermore, the quality of the powder may be adversely affected at high voltages and a condensation phenomenon can occur, generating nanoparticles that are sprayed out. The inventors have also observed, using a scanning electron microscope, that agglomerates of nanoparticles that have been sprayed out become attached to the surface of larger particles. These nanoparticles or sprayings increase the exchange surface area of the powders 5 produced and these sprayings thus contribute to increasing the oxygen content in the powders 5 produced. The powder to be prepared is for example a Ti6Al4V powder, and the materials 1a and 1b are Ti6Al4V titanium alloys. In one embodiment of the Ti6Al4V powder, the voltage ranges between 15 and 19 V, preferably 16 V. It has been noted that this process makes it possible to obtain powders that meet the specifications defined in the ASTM F3001-14 and ASTM F2924-14 standards.

[0044] In one embodiment, the frequency of the short circuit ranges between 40 and 200 Hz. Moreover, if the voltage applied is increased, the frequency of the short circuit can be reduced. These frequency values make it possible to improve the quality of the powder 5 developed. The powder to be prepared is for example a Ti6Al4V powder, and the materials 1a and 1b are Ti6Al4V titanium alloys. It has been noted that this process makes it possible to obtain powders that meet the specifications defined in the ASTM F3001-14 and ASTM F2924-14 standards. Furthermore, the particle size distribution is better controlled.

[0045] In one embodiment, said melting and spraying steps are carried out under inert gas, for example under argon, at a pressure P of between 4 and 12 bar. Reducing the pressure P increases the diameter of the droplets 2. Preferably in one embodiment, the pressure P is between 6 and 10 bar. This range of pressure P makes it possible in particular to obtain a particle size distribution suitable for the preparation processes that implement adapted metal powders, such as powder-bed laser melting or electron beam melting.

[0046] In one embodiment, the electric current applied between the two conductive wires 312a, 312b has a current strength of between 50 and 400 A, preferably between 150 and 250 A, more preferably between 180 and 220 A.

[0047] In one embodiment, the first and second materials 1a, 1b are dispensed at a dispensing rate and said current strength is determined as a function of said dispensing rate of said first and second materials 1a, 1b. This makes it possible to regulate the productivity of the preparation process.

[0048] In one embodiment, said first and second materials are dispensed at a rate of between 5 and 10 m/min. This reduces the mechanical stresses induced in the wire. Furthermore, if the rate is excessively high, the wire 312a, 312b will not have enough time to melt and if the rate is too low the electric arc 314 is interrupted.

[0049] In one embodiment, when said first voltage is equal to 0, the current strength increases between 100% and 150% during said spraying step.

[0050] In one embodiment, the melting and spraying steps are repeated at least once and said applied electric current has a second voltage different from said first voltage. This results in a perfectly controlled particle size of the powder prepared.

[0051] In one embodiment, the preparation process 100 comprises an additional step of analyzing the powder to determine the density of said powder and/or the oxygen content of said powder and/or the particle size of said powder. As a result, the parameters of the process can be adjusted depending on the analysis of the powder.

[0052] In one example, the material 1a, 1b used is a Ti6Al4V titanium alloy. The regime used is a short-circuit regime. The voltage of the electric current applied is fixed at 16 V. The target current strength is fixed at 200 A and the pressure P at 8 bar. The diameter of the wire used is 1.6 mm. The process has made it possible to obtain a powder 5 which meets the specifications for compositions in the ASTM F3001-14 and ASTM F2924-14 standards, and the specifications for apparent density defined by the ASTM-B-212 standard or for flowability defined by the ASTM-B-213 standard. The process of this embodiment makes it possible to obtain a better controlled particle size distribution of the powders. As a result, at least 75% of the particles of the powders have a size less than 150 microns. Moreover, at least 50% of the particles of the powders have a size between 20 and 100 microns, which is the size of the powders usually used in additive manufacturing processes that implement metal powders. More particularly, more than 20% of the particles of powders 5 have a size between 20 and 63 microns. This is a size suitable for powder-bed laser melting processes. More than 25% of the particles of powders 5 have a size between 63 and 100 microns. This is a size suitable for electron beam melting processes.

[0053] According to the invention, the process of the invention comprises an additional step of dispensing. With reference to FIG. 1 and FIG. 4, before the melting step, the wires 312a, 312b stored in the form of wire coils are each dispensed upstream of the preparation device 200 by a feeder 1000, for example rollers. The wires 312a, 312b are each dispensed in a dispensing direction, i.e. from the coil towards the electric arc 314. These feeders 1000 come after the coils in the dispensing direction. The feeders 1000 make it possible to push the wires 312a, 312b towards the preparation device 200.

[0054] Then, as close as possible to the electric arc 314, at least one additional feeder 2000 is added to the device for each wire 312a, 312b dispensed. The additional feeder 2000 makes it possible to stabilize the wire 312a, 312b by pulling it and then pushing it towards the electric arc 314, for example by means of rollers. The additional feeder 2000 is distinct from the feeder 1000.

[0055] The feeder 1000 and the additional feeder 2000 together form a push-pull system. The wires 312a, 312b are thus pushed over a distance D in the dispensing direction. This distance D is the length measured between the feeder 1000 and the additional feeder 2000. Then, the wires 312a, 312b are drawn from the additional feeder 2000 towards the electric arc 314 in the dispensing direction. Then, the wires 312a, 312b are pushed from the additional feeder 2000 towards the electric arc 314, up to the ends of the wires 313a, 313b. The feeder 1000 and the additional feeder 2000 are synchronous, i.e. they dispense the wire at the same rate. This push-pull feeder makes it possible to reduce the take-up of oxygen by the powder prepared by the process 100 during the change in state, i.e. when the wire is being transformed into powder.

[0056] In one embodiment, said first wire 312a and said second wire 312b each have an end 313a, 313b which is melted during the melting step. Said first wire 312a and said second wire 312b are each pushed over a distance L, each distance L being equal to the length measured between said additional feeder 2000 and said respective ends 313a, 313b of the wires 312a, 312b. There are therefore fewer mechanical jolts on the wire 312a, 312b. As a result, the oxygen content in the powders 5 prepared is lower than the oxygen content obtained without this embodiment.

[0057] In one embodiment, said first wire 312a and said second wire 312b are pulled over a distance L equal to at least 1% of the distance D, preferably equal to at least 10% of the distance D. As a result, the tension of the wires 312a, 312b is well controlled. Moreover, the oxygen content in the powders 5 prepared is lower than the oxygen content obtained without the process of the invention.

[0058] For example, the length of the wire between the feeding system and the electric arc 314 is 3 metres. The additional feeder is at a distance of between 20 and 50 cm from the ends 313a, 313b. Those skilled in the art will be able to easily adjust these distances on the basis of the teaching provided by the invention to obtain the desired effect, which is an oxygen content in the prepared powders 5 which is lower than the oxygen content obtained without this embodiment.

[0059] In one embodiment, the wires 312a, 312b are more particularly pushed through sheaths. The sheaths come after the feeder 1000 in the dispensing direction. The sheaths make it possible to guide the wires 312a and 312b, and to minimize the curvatures of the wires 312a, 312b. This guidance through the sheaths makes it possible to avoid deformations of the wires 312a, 312b and thus limit the stresses in the material 1a, 1b. In one embodiment, the sheaths are between the feeder 1000 and the additional feeder 2000, and this also reduces the friction in the sheaths. The dispensing of the wire is also better regulated.

[0060] In one embodiment, the wire feed rate ranges between 5 and 10 m/min.

[0061] When the current applied is a short-circuit current, the use of the additional feeder 2000 makes it possible to especially stabilize the forward movement of the wire 312a, 312b.

[0062] In one embodiment, an additional step of straightening makes it possible to straighten the wires 312a, 312b by means of a wire straightener. As a result, the wires 312a, 312b are perfectly rectilinear, and are not deformed. The mechanical forces in the sheaths are lower. Moreover, the dispensing becomes easier. As a result, the electric arc 314 is well controlled, without interruption. Moreover, the quality of the powder in terms of composition is also well controlled. The particle size and the sphericity of the powders 5 are better controlled.

[0063] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0064] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0065] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of: comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0066] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0067] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0068] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0069] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.