Process for Preparing Powder

This application claims the benefit of priority under 35 U.S.C. § 119(a) and (b) to French patent application No. FR 2406345, filed Jun. 14, 2024, the entire contents of which are incorporated herein by reference.

The present invention relates to a process for preparing powder, and more particularly to a process for preparing powder for an additive manufacturing process.

Powder preparation processes, for example electrode induction melting gas atomization (EIGA), are known from the prior art. This process involves melting the material from a slowly rotating metal rod using a high-frequency induction coil. A stream of liquid is then formed which flows through an atomizing nozzle. The flow is then fragmented and solidified by a high velocity pulsed gas stream from the atomizing nozzle to form fine powder particles. These processes from the prior art are used to obtain spherical metal powders with limited pollution of the constituent material of the powders obtained. However, these processes, like the EIGA process, consume a lot of energy and are not simple to implement.

A process for preparing metal powders from a first material and a second material, the preparation process notably comprising a step of melting the materials using an electric arc, a step of spraying the molten materials in such a way as to form droplets, a step of cooling the droplets with a carrier gas in such a way as to form solid particles, and a step of separating the solid particles from the carrier gas and collecting the solid particles to form the powder, is also known from the application EP4221916A1.

It is not possible to obtain a powder having a completely controlled composition using this disclosed process. For example, it is not possible to obtain a well-controlled oxygen content in the powders prepared using the process. In addition, the process does not make it possible to obtain an oxygen content that can be adjusted depending on the needs of a user of the prepared powder, for example a lower oxygen content.

The aim of the present invention is to effectively overcome the drawbacks of the prior art by providing a powder preparation process which makes it possible to obtain a powder with a lower oxygen content than the processes of the prior art.

The invention thus relates to a process for preparing powder from a first material and a second material, the preparation process comprising:

According to one embodiment, 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, each distance being equal to the length measured between said additional feeder and said respective end of the wires.

According to one embodiment, said first and second wires are dispensed at a rate of between 5 and 10 m/min.

According to one embodiment, the preparation process comprises a step of enriching the droplets and/or the particles by means of an active substance which is carried out during the cooling step, the enriching step being preceded by a step of ionizing the active substance.

According to one embodiment, the preparation process 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.

According to one embodiment, the enriching step is carried out during the spraying and cooling steps.

According to one embodiment, the cooling step is carried out using a cooling gas.

According to one embodiment, the active substance comprises:

shows a preparation deviceconfigured to implement the preparation processaccording to the invention. This preparation devicecomprises at least: a spraying means; an atomization chamber; a first collection means; and an exhaust means. This preparation devicemay also comprise complementary elements (not shown), such as: a gas/particle separation system; and a second collection means.

With reference to, the preparation processcomprises at least the following steps: a melting stepof melting two materials,using an electric arc; a spraying stepof spraying each material,in such a way as to form droplets; a cooling stepof cooling the droplets, using a carrier gas, in such a way as to form solid particles; and a separation and collection stepof separating the particlesfrom the carrier gasand collecting the solid particlesso as to form a first powder and a second powder.

Each material,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,may be of the same type or even identical. The choice of the composition of each material,partially determines the composition of the powdersobtained. In one embodiment, said first or second material is a Ti6Al4V alloy. These materials,are supplied in the form of conductive wires,, respectively, and dispensed towards the spraying meansshown in, and more specifically into the enclosure, by a feeding system, not shown, for example at a predefined speed. In one embodiment, the diameter of the wires,ranges between 0.8 and 2 mm, preferably between 1.2 and 2 mm. The wires can thus be easily manipulated.

During the melting step, this spraying meansis configured to carry out the melting stepof melting each material,using an electric arc. The spraying meanscomprises an electric-arc source, also referred to as wire-arc torch. The wire-arc torchis configured to generate an electric arc. The electric arcmay be created from the carrier gas, such as argon, nitrogen or helium, or a mixture of these gases. The wire-arc torchcomprises an enclosurewhich is filled with the carrier gasand in which the electric arcis generated. The pressure of the carrier gasin the enclosuremay be greater than or equal to atmospheric pressure. The wire-arc torchis configured to generate the electric arcbetween the first materialand the second material. The wire-arc torch comprises the two conductive wires,, which are disposed one on each side of the enclosure, are mutually separate and are configured to trigger and sustain the electric arcusing the electric current. During operation, the distance between the two conductive wires,is preferably kept less than 5 mm and depends on the energy supplied. When the wire-arc torchis in operation, the electric arcis adjacent to the two facing ends,of the two wires,. The carrier gasis introduced in jet form into the enclosurethrough an inlet. The jet of carrier gasis configured to strike the ends,of the two wires,. In one embodiment, the spraying meanscomprises multiple wire-arc torchesfor increasing the amount of powder produced by the preparation device. During this melting step, the mode of operation of the wire-arc torchis chosen such that the temperature of the plasma at the electric arcis greater than the melting temperature of each material,. Thus, during operation, said plasma causes the ends,of the two wires,to melt.

The spraying stepis also carried out using the spraying means. The spraying stepof spraying each material,from the liquefied ends,allows dropletsto form. During this spraying step, the jet of carrier gasis focused directly onto the liquefied ends,of the wires,such that the melted ends,are sprayed and dropletsare created. In order to maintain a fixed spacing between the ends,in spite of the spraying of material, the wires,are always introduced into the enclosureby a feeding system, not shown, at a predefined speed.

schematically shows the atomization chamberand the exhaust means, which are configured in such a way as to carry out the cooling stepof cooling the droplets, using the carrier gas, such that solid particlesare formed. In one embodiment, in order to accelerate the cooling, a cooling gascan be injected into the atomization chamber. The cooling gasin contact with the carrier gas forms a gas mixture. Thus, during the cooling step, the droplets, which are in contact with the gas mixture, establish a transfer of heat with the gas mixture. Preferably, the temperature of the cooling gasinjected is chosen such that it is lower than the lowest of the solidification temperatures of the materials,or of the alloys formed by the materials,within the droplets. The cooling mixtureis, for example, injected at ambient temperature. As a result, the expanded carrier gasand the cold cooling gascause heat to be transferred from the dropletsto the gas mixture, and this cools the droplets. When the temperature of the dropletsis lower than the solidification temperature of the droplets, the dropletsare solidified in such a way as to form the solid particles. The cooling stepallows the dropletsto form spheres, i.e. they adopt a spherical shape by virtue of the surface tension on the surface of the melted dropletsand the interaction with the gas mixture. Thus, as they solidify, the dropletsform particleswhich have a sphericity greater than 0.9 and as close as possible to 1.

The preparation processmay also comprise an enriching stepof enriching the dropletsor/and the particles. The enrichingis carried out by means of an active substance. The enrichingis at least implemented during the cooling step. However, the enrichingmay also start during the sprayingand continue during the cooling. “Enriching” is understood to mean a metallurgical treatment of the materials,and of the alloys formed within the dropletsby means of an active substancein such a way as to give the resulting particlesspecific physico-chemical properties.

The active substanceimplemented in the enriching stepcomprises: at least one inert gas, advantageously of the same composition as the carrier gas; 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 substanceranges 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 substancemay be a hydrocarbon, such as methane, which is rich in carbon and hydrogen. If the active substancecomprises carbon monoxide or methane, the enrichingcorresponds to a carburizing of the materials,. If the active substancecomprises nitrogen, the enrichingcorresponds to a nitriding. If the active substancecomprises oxygen or hydrogen, the enrichingcorresponds to an oxidation or, conversely, a reduction of the materials,. The active substancemay react with the materials,whether they are in the form of dropletsor solid particles. The active substanceis preferably injected into the atomization chamberof the device. As a result, the active substancereacts with the particles. Advantageously, the active substanceis involved in the spraying step. In this way, the active substancereacts with the droplets. Alternatively, the active substanceis also injected at the spraying means. The partial pressures of the inert gas and of each active compound of the active substanceare monitored within the devicethroughout the processso 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 substanceand the surfaces of the dropletsand of the particles, it is possible to optimize the exchange surface area. In this way, the enriching stepis carried out efficiently. As a result, the enriching stepmakes it possible to control the final chemical composition of the resulting particles.

Then, there is a separation and collection stepof separating the solid particles and collecting the solid particlesso as to form the powder. This step is firstly carried out by the first collection meanswhich is connected to the atomization chamber. With reference to, the first collection meanscomprises a main vesselconfigured to receive a first portion of the solid particlesthus forming the powder. Then, a gas/particle separation system, not shown, is configured to separate the second portion of the particlesfrom the gas mixture. 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.

In, the schematically shown preparation processcomprises several combinable steps, illustrated in dashed lines, which will now be described. An ionization stepcan be combined with the enriching stepin order to improve the kinetics of the chemical reactions taking place between the droplets, the particlesand the active substance. The ionization stepcomes before the enriching step, in which case the enriching step can start during the spraying. In this step, the active substancemay be introduced into the enclosureof the spraying meansin such a way that it is ionized by the electric arc. The electric arcionizes each component of the active substanceso that reactive free ions are produced. The high-energy reactive free ions improve the reaction kinetics during the enriching step. The enriching reactions therefore balance before the dropletsare solidified. As a result, the chemical composition of the resulting particlesis controlled and reproducible. The concentration of reactive free ions is highest within the enclosure. 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 dropletsin the atomization chamberin order to increase the duration of the enriching step.

At the end of the collection step, the passivation stepof passivating the surface of the particlescan be carried out, for example, if the first and second powdersare prepared from materials that are flammable, i.e. have a strong affinity with oxygen. This is the case for example with powdersformed from titanium, alloys of titanium or aluminium. The passivation stepis carried out using a passivation gas. The passivation gasmay 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 stepis carried out systematically on the two powders. The following example will show the passivation stepcarried out on the first powderin the first collection means. The passivation stepcan be transferred to the gas/particle separation system.

In order to obtain a first and a second powderthat satisfy particle size distribution characteristics, an additional sieving stepmay be carried out on the first and the second powder. The sievingmakes it possible, for example, to remove aggregates of particlesor particlesthat exceed a limit size from the powders. The particle size distribution can be characterized by three specific diameters denoted D10, D50 and D90. 10% of the particleshave a diameter less than D10, 50% of the particleshave a diameter less than D50 and 90% of the particleshave a diameter less than D90. The sievingmay for example be carried out in order to adjust the distribution of the powders, in particular the diameter D50, corresponding to the median of the distribution.

In order for the chemical composition of the powdersto be reproducible, the preparation devicecan undergo an additional inerting step. The inerting stepis carried out using an inerting gas and cycles of compression and expansion, in order to purge the air present in the deviceuntil the oxygen content is less than 100 ppm, preferably less than 10 ppm, before starting the melting step. The inerting gas may for example comprise an inert gas or a mixture of inert gases.

Conventionally, in this process the transfer regimen used is a spray transfer regimen. This affords a continuous electric arcby means of a continuous electric current during the melting and spraying steps. Moreover, a constant length of the electric arcas the wire of material,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.

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 arcand obtain powdersof 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. In this transfer regime, the electric current applied is no longer a continuous electric current—it is a short-circuit current. As a result, during the melting step, the heat given off by the electric arcmakes the wire ends,melt until a dropletis formed, and the two wires,will then be connected. A liquid bridge is also formed between the two wires,and the droplets. 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, 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,and the droplet. The breaking of the bridge causes dropletsto spray out during the spraying step, these dropletssubsequently forming the powder. Depending on the rate of the increase in current intensity, the formation of large dropletsis avoided. This means that the particle size distribution of the powdersis better controlled. The powdersproduced comprise, in particular, less fine powder than the powdersproduced 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, the voltage is re-established, resulting in the re-creation of an electric arcand the cycle of melting and spraying starts again.

In one embodiment, the voltage applied between the two conductive wires,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 arcinduced by the application of the electric current to the two materialsandis 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 dropletof metal can no longer drop off and the conductive wires,run the risk of becoming connected. There is then no short circuit. If the voltage is much too low, the electric arcis not created and the wires,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 powderthat 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 powdersproduced and these sprayings thus contribute to increasing the oxygen content in the powdersproduced. The powder to be prepared is for example a Ti6Al4V powder, and the materialsandare 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.

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 powderdeveloped. The powder to be prepared is for example a Ti6Al4V powder, and the materialsandare 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.

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. 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.

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

In one embodiment, the first and second materials,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,. This makes it possible to regulate the productivity of the preparation process.

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,will not have enough time to melt and if the rate is too low the electric arcis interrupted.

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

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.

In one embodiment, the preparation processcomprises 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.

In one example, the material,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 powderwhich 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 powdershave 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 powdershave a size between 63 and 100 microns. This is a size suitable for electron beam melting processes.

According to the invention, the process of the invention comprises an additional step of dispensing. With reference toand, before the melting step, the wires,stored in the form of wire coils are each dispensed upstream of the preparation deviceby a feeder, for example rollers. The wires,are each dispensed in a dispensing direction, i.e. from the coil towards the electric arc. These feederscome after the coils in the dispensing direction. The feedersmake it possible to push the wires,towards the preparation device.

Then, as close as possible to the electric arc, at least one additional feederis added to the device for each wire,dispensed. The additional feedermakes it possible to stabilize the wire,by pulling it and then pushing it towards the electric arc, for example by means of rollers. The additional feederis distinct from the feeder.

The feederand the additional feedertogether form a push-pull system. The wires,are thus pushed over a distance D in the dispensing direction. This distance D is the length measured between the feederand the additional feeder. Then, the wires,are drawn from the additional feedertowards the electric arcin the dispensing direction. Then, the wires,are pushed from the additional feedertowards the electric arc, up to the ends of the wires,. The feederand the additional feederare 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 processduring the change in state, i.e. when the wire is being transformed into powder.

In one embodiment, said first wireand said second wireeach have an end,which is melted during the melting step. Said first wireand said second wireare each pushed over a distance L, each distance L being equal to the length measured between said additional feederand said respective ends,of the wires,. There are therefore fewer mechanical jolts on the wire,. As a result, the oxygen content in the powdersprepared is lower than the oxygen content obtained without this embodiment.

In one embodiment, said first wireand said second wireare 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,is well controlled. Moreover, the oxygen content in the powdersprepared is lower than the oxygen content obtained without the process of the invention.

For example, the length of the wire between the feeding system and the electric arcis 3 metres. The additional feeder is at a distance of between 20 and 50 cm from the ends,. 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 powderswhich is lower than the oxygen content obtained without this embodiment.

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

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

When the current applied is a short-circuit current, the use of the additional feedermakes it possible to especially stabilize the forward movement of the wire,

In one embodiment, an additional step of straightening makes it possible to straighten the wires,by means of a wire straightener. As a result, the wires,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 arcis 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 powdersare better controlled.

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.