FR3105036A1 - IN SITU treatment of powders for additive manufacturing - Google Patents

Abstract

The invention relates to an additive manufacturing process by selective melting of metal or ceramic powder comprising the steps of: - Deposition of a layer (C) of powder; - Selective melting of at least one zone of the powder layer, in which the process comprises, after the deposition and prior to the selective melting, a coating step during which grains of metallic or ceramic powder ( 1) are at least partially covered with a layer of an addition material (2), the grains of metal or ceramic powder (1) having a first reflectivity, the addition material (2) having a second reflectivity more lower than the first reflectivity. Figure for the abstract: Fig. 2

Description

IN SITU treatment of powders for additive manufacturing

FIELD OF THE INVENTION

The invention relates to the general technical field of additive manufacturing.

More particularly, the invention relates to the treatment of metal or ceramic powders for additive manufacturing by melting metal or ceramic powders.

Metal powders are powders made up of elements from the family of metals such as iron, titanium, aluminum, vanadium, chromium, manganese, cobalt, nickel, copper, platinum gold, etc. the powder can be mono-component or in the form of an alloy or even a composite.

Ceramic powders are powders made up of non-metallic and non-organic materials obtained by the action of high temperatures. It is these high temperatures which induce, at the heart of the raw material, an irreversible transformation which gives the ceramic produced new properties: solidity and resistance to wear, resistance to heat, insulating properties, etc.

The invention is addressed in particular, but not only, to ceramics that can be obtained from metallic elements such as:

- oxides: aluminum oxide, zirconium oxide; etc;

- non-oxides: carbides, borides, nitrides, ceramics composed of silicon and atoms such as tungsten, magnesium, platinum or titanium, etc.;

- composite ceramics: combination of oxides and non-oxides.

STATE OF THE ART

Conventionally, a process for manufacturing a part by adding metal or ceramic powders is carried out by depositing successive layers of powders on a flat support surface, thus forming a powder bed.

After the deposition of a powder layer C, a power source selectively heats certain areas of the powder bed, so as to cause the powders to melt in the heated areas and thus form a coherent part stratum after cooling.

The layers of powders are thus successively deposited and selectively solidified so as to form a part in successive layers.

Conventionally, the power sources used to carry out the selective melting of the powders can comprise one or more laser sources, or one or more electron or particle beam sources, or even a combination of sources using a laser, a beam of electrons or particles.

When producing metal or ceramic parts, the thermal and electrical conductivity and the reflection coefficient of the powders used limit the energy absorption rate of the powders.

The energy efficiency of the selective melting operation is therefore greatly reduced.

In some special cases, for example when using powders of metals with high reflectivity during additive manufacturing, such as copper or aluminum, most of the laser light, particularly in infrared (IR), is reflected (up to 99%), greatly reducing the efficiency of this type of process, thereby greatly increasing the duration of carrying out such processes.

It is classically understood by reflectivity the proportion of electromagnetic energy reflected on the surface of a material compared to the total energy of the electromagnetic signal reaching the material. In other words, the electromagnetic energy returns to the source medium of the incident electromagnetic beam, after the interaction with the surface.

Estimation methods known in the literature make it possible to determine the reflectivity of certain alloys. The reflectivity of a surface can vary depending on the wavelength of the radiation. This quantity is generally expressed in decibels (for high frequencies) or percentage (for infrared, visible, or even ultraviolet light).

During the reflection of a wave on a more or less reflecting material, part of the wave is returned to the transmitter and a continuous part in the material. The part of the wave which continues its propagation in the material can be absorbed by the material, or come out on the other side, after having crossed it. The reflection coefficient R _c is the ratio of the amplitudes between the reflected wave A _r and the incident wave A _i : R _c = A _r / A _i

Depending on the Fresnel coefficients, this value can be a complex number.

The reflectivity Ref is the reflected energy relative to the incident energy. As the energy is proportional to the square of the amplitude of the waves, it happens to be the following ratio: Ref = A _r ²/A _i ² =R _c ²

Reflectivity is therefore the squared reflection coefficient, always positive and a real number.

Additive manufacturing processes therefore suffer from limited energy efficiency, particularly in the case of additive manufacturing from metal or ceramic powders fused using laser beam sources.

An object of the invention is to improve the energy efficiency of such a process.

Another goal is to reduce the duration of a selective melting cycle.

Another object is to achieve nuances in the material of the powder layer.

Another object is to improve the thermal conductivity between the powder grains, in particular for additive manufacturing processes operating by thermal conductivity such as manufacturing processes using a laser beam alone or in combination with another source of energy or heat. .

Another object is also to improve the electrical conductivity between the powder grains, in particular for additive manufacturing processes operating by electrical conductivity such as manufacturing processes using an electron beam alone or in combination with another energy source. or heat.

In order to respond to these problems, the invention proposes an additive manufacturing process by selective melting of metal or ceramic powder comprising the steps of:

- Deposition of a layer of powder;

- Selective melting of at least one zone of the powder layer, in which the method comprises, after the deposition and prior to the selective melting, a coating step during which grains of metallic or ceramic powder are at least partially covered with a film of an addition material, the grains of metal or ceramic powder having a first reflectivity, the addition material having a second reflectivity lower than the first reflectivity.

The invention may advantageously comprise the following characteristics, taken alone or in combination:

- the powder is metallic and comprises a metal or an alloy having a reflectivity greater than 90%;

- the addition material comprises at least one material from among carbon, tungsten or molybdenum, or a mixture thereof;

- the powder is metallic and comprises at least one metal or an alloy of metals chosen from copper, aluminium, gold, silver and platinum;

- the coating step is carried out by means of a surface treatment device configured to generate a plasma and to project the plasma according to a substantially linear pattern, the surface treatment device being adapted to be moved above the metal powder or ceramic so as to project the plasma onto the metal or ceramic powder.

- the surface treatment device comprises an electric arc device comprising an electrode comprising added material, the electrode being positioned facing the layer of powder in such a way that added material is projected towards the layer of powder when an electric arc is established between the electrode and the layer;

- the surface treatment device comprises a vapor phase coating device comprising an electrode comprising a wall delimiting an internal cavity, and a gaseous precursor injection system configured to inject a precursor gas into the cavity, the wall further having a orifice located opposite the layer, the precursor gas being configured to generate addition element particles when excited by the electrode, the addition element particles being expelled through the orifice onto the powder coat;

- the surface treatment device comprises a magnetron device;

- the surface treatment device comprises a blown wire device, comprising a wire connected to a power source, in which the wire extends above the powder bed and is made of the addition material, the power source being configured to inject a current into the wire such that the add-on material vaporizes and deposits on the powder layer;

- the layer of addition material has a thickness of less than 100 nm;

- the selective melting step is carried out by means of a power source comprising at least one laser source;

- a step of surface treatment of metal or ceramic powders is carried out successively at each step of placing a layer of metal or ceramic powder;

- the powder layer is covered with a film of addition material containing one or more components among carbon black, carbon nanotubes and graphene;

- the film is deposited by spraying a liquid solution of addition material in a volatile solvent at high temperature;

- the film is deposited mechanically by covering the layer (C) of powders with a composite film comprising a matrix volatile at high temperature and loaded with added material.

According to another aspect, the invention proposes an additive manufacturing device configured to carry out an additive manufacturing process according to the invention, the additive manufacturing device comprising:

- a layering element configured to lay out a layer of metal or ceramic powder that is substantially planar;

- a surface treatment device configured to carry out a method for surface treatment of the layer of metallic or ceramic powder according to the invention;

- a power source configured to carry out a selective melting of at least one zone of the layer of powders.

Advantageously, in such an additive manufacturing device, the layering element can be configured to deposit a layer of powder when it is moved on the surface of a support, and the surface treatment device can be integral in movement. of the layering element, such that upon movement of the layering element, the layering element deposits a layer of powder and the surface treatment device processes simultaneously or with a time lag the layer of powder deposited by the layering element.

DESCRIPTION OF FIGURES

Other characteristics and advantages of the invention will emerge from the following description, which is purely illustrative and not limiting, and must be read in conjunction with the appended figures in which:

- Figure 1 is a diagram representing an additive manufacturing device according to the invention;

- Figure 2 is a generic diagram representing the implementation of a surface treatment method according to the invention;

- Figure 3 is a diagram representing a variant of an embodiment surface treatment device operating preferably around atmospheric pressure according to the invention;

- Figure 4 is a diagram representing a variant of an embodiment of a chemical surface treatment device preferably operating under a "primary" vacuum (1 to 10 ^-3 mbar) according to the invention;

- Figure 5 shows a variant of an embodiment of a magnetron surface treatment device operating preferably at low pressure according to the invention;

- Figure 6 shows a diagram representing a variant of an embodiment of a wire-blown surface treatment device according to the invention;

- Figure 7 is a schematic representation of an embodiment of a spray surface treatment device according to the invention;

- Figure 8 is a schematic representation of an embodiment of a composite film roller surface treatment device according to the invention.

DESCRIPTION OF ONE OR MORE MODES OF IMPLEMENTATION AND REALIZATION

General

The figures illustrate an example of an additive manufacturing process by selective melting of metal or ceramic powder comprising the steps of:

- Deposit of a layer C of metal or ceramic powder;

- Selective melting of at least one zone of the powder layer, in which the process comprises, after the deposition and prior to the selective melting, a coating step during which grains of metal or ceramic powder 1 are at least partially covered with a film of an addition material 2, the grains of metal or ceramic powder 1 having a first reflectivity, the addition material 2 having a second reflectivity lower than the first reflectivity.

Such a surface treatment makes it possible to increase the rate of energy absorption of the powder grains, which makes it possible to heat metal or ceramic powders 1 more quickly and more efficiently. This makes it possible to improve the energy efficiency of the process while making it possible to reduce the duration of a melting cycle of metal or ceramic powder 1 during an additive manufacturing process by selective melting.

Advantageously, the powder treatment is carried out "in situ", that is to say within the additive manufacturing machine, unlike the "mass" powder treatment processes carried out by different installations outside the additive manufacturing machine. additive manufacturing.

Advantageously, the in situ treatment of the powders is carried out during an additive manufacturing process comprising at least one step of depositing a layer C of metal or ceramic powder and at least one step of selective melting of at least one zone layer C of metal or ceramic powder 1.

The manufacturing method thus advantageously comprises a step of in situ treatment of the metal or ceramic powders 1, which can be carried out in one embodiment after the step of depositing the layer C of metal or ceramic powder 1 and prior to the selective melting step.

This avoids any breakdown in the manufacturing process, which in turn reduces the cycle time.

In a preferred embodiment, a metal or ceramic powder treatment step 1 is carried out before each selective melting step.

Advantageously, in one embodiment, a surface treatment step is carried out after each step of depositing a layer C of metal or ceramic powder 1.

In one embodiment, the powder 1 is metallic and comprises a metal having a reflectivity greater than 90%, or optionally greater than 95%. The metal powder can therefore comprise copper, gold, silver, platinum or a mixture of one or more of these metals, and the addition material 2 can comprise carbon or aluminum or tungsten, or an alloy comprising one or more of these elements, or others with equivalent properties.

The additive material may include one or a combination of Zinc, Silver, Copper, Chromium, Nickel, Aluminum, Niobium, Zirconium, Tungsten, Titanium, Carbon and tantalum.

Preferably, the addition material 2 comprises at least one material from among carbon, tungsten or molybdenum, or a mixture thereof. These refractory materials have a very high melting point and will therefore store heat and return it to the copper by conduction, thus promoting the melting of the copper. Their interest also lies in their low thermal conductivity and high melting point. In fact, they will absorb a lot of energy before passing into the liquid phase or being evaporated and thus the energy transfer time between these materials and the powder grains will heat up by conduction, which is not the case if the laser is sent directly on the powder.

For example, carbon is not miscible in copper. When the temperature rises, the carbon diffuses to the surface of the copper and becomes structured, generally in graphitic form (stable phase of carbon).

The graphite (or graphene, i.e. a single sheet of graphite) on the surface has a lower reflectivity than copper and makes it possible to increase the energy absorption at the surface of the copper.

Furthermore, if the carbon is present on the surface of the copper in the form of one or a few monolayers (graphene), then the fraction of the electromagnetic radiation of the incident wave which is not reflected propagates and can cross the graphene to join the copper underneath.

If the carbon is deposited deep in the material (for example by ion implantation), then it rises to the surface with the increase in temperature, including after the establishment of a melt pool. It is thus possible to bring up towards the surface, after melting layer by layer, a fraction of the carbon present on the surface of the previous layer, and in fact contain (or even control) the level of carbon present in the copper for the final part.

In addition, graphite passes directly from the solid phase to the vapor phase (it has no liquid phase at ambient pressure, we speak of sublimation) with a much higher boiling temperature (liquid-vapor passage) (3825 °C) than that of copper (2560°C), which moreover first passes into the liquid phase at 1083°C. In fact, part of the energy necessary for the fusion of the copper can be evacuated by the graphite, thus reducing the cooling time of the liquid bath and reducing the quantity of evaporated copper (evaporation also called emission of fumes).

Advantageously, the carbon can be combined with other elements (such as iron, molybdenum, etc.).

In another example, tungsten or any other refractory metal exhibits behaviors close to carbon, although they pass through a liquid phase before evaporation. For example, the melting point of tungsten is 3422°C and the boiling point is 5555°C.

In another example, the presence of aluminum on the copper grains and between these grains makes it possible to significantly reduce the reflectivity of the grains. Indeed, aluminum has a melting temperature of only 660°C. As the reflection of the liquid aluminum bath is much lower (77%) than that of the same solid metal (95%, for a wavelength of 1 µm), the electromagnetic energy is favorably absorbed by the liquid aluminum and transmitted to the copper powder below. Moreover, suitably, the two metals, aluminum and copper, although they have different melting temperatures, they have very close boiling temperatures (2470°C for Al versus 2560°C for Cu), limiting the vaporization of aluminum before melting copper.

In another example, in the event of pollution, aluminum can also capture traces of oxygen to form alumina. Unlike metal, the melting point of alumina (2072°C) is higher than that of copper. Alumina has a lower reflectivity than copper and absorbs photons better, and will also use some of the heat to evaporate.

Such a process can also make it possible to produce nuances in the material, by forming an alloy between the material of the metal or ceramic powders 1 and the doping material 2, which makes it possible to modify certain physical properties of the final material constituting the manufactured part, for example the mechanical, chemical or thermal characteristics of the alloy.

This also makes it possible to deposit the addition material only on the visible part of the powder layer. Visible is here understood as exposed to radiation from the power source. This greatly limits the pollution that could be generated in the powder layer by the addition of additive material in the material of the powder grains. This makes it possible to limit the pollution to 100 ppm, preferably to 10 ppm.

Additive manufacturing device

Such a manufacturing process is implemented using an additive manufacturing device 3 comprising a layering element 4 configured to deposit a layer C of metal or ceramic powder, a surface treatment device 5 configured to process the metal or ceramic powder and a power source 10 configured to perform selective melting of at least one zone of the layer C of powder.

An embodiment of an additive manufacturing device 3 is shown in Figure 1.

Additive manufacturing device 3 includes:

- a support such as a horizontal plate 7 on which are successively deposited the different layers of additive manufacturing powder (metal powder, ceramic powder, etc.) making it possible to manufacture a three-dimensional object 8 (object in the shape of a fir tree in the figure ),

- a powder tank 9 located above the plate 7,

- the layering element 4 to spread the different successive layers of powder (movement along the double arrow A),

- the surface treatment device 5 configured to process additive manufacturing powders,

- a source of power 10 to provide the energy necessary for the melting (total or partial) of the thin spread layers,

- a control unit 11 which controls the various components of the device 3 according to pre-stored information (memory M),

- A mechanism 12 to allow the tray support 7 to be lowered as the layers are deposited (movement along the double arrow B).

In the example described with reference to Figure 1, set 10 includes two sources of consolidation:

- an electron beam gun 13 and

- A source 14 of the laser type.

As a variant, the assembly 10 may comprise only one source, of the laser type for example, or only one localized energy source under vacuum or at very low pressure (<0.1 mbar), for example electron gun.

Still as a variant, the assembly 10 can also comprise several sources of the same type, such as for example several electron guns and/or laser sources, or means making it possible to obtain several beams from the same source.

In the example described in FIG. 1, at least one galvanometric mirror 15 makes it possible to orient and move the laser beam coming from the source 14 with respect to the object 8 according to the information sent by the control unit 11 , and deflection and focusing coils 16 and 17 make it possible to locally deflect and focus the electron beam on the zones of layers to be sintered or fused.

Any other deflection system can of course be envisaged.

In another example not illustrated, the assembly 10 comprises several laser-type sources 14 and the movement of the different laser beams is obtained by moving the different laser-type sources 14 above the layer C of powder to be fused.

A heat shield 18 can be interposed between the beam or beams of assembly 10 and the walls of enclosure 19 in the case where at least one source is of the electron gun type.

The components of the device 3 are arranged inside a sealed enclosure 19 which can be connected to at least one vacuum pump 20 which maintains a secondary vacuum inside said enclosure 19 (typically approximately 10 ^-2 / 10 ^-3 mbar, or even 10 ^-4 / 10 ^-6 mbar) in the case where at least one source is of the electron gun type.

Layering element 4 can advantageously be configured to place a layer C of metal or ceramic powder that is substantially planar on support 7 so as to form a bed of powder.

The layering element 4 can be advantageously configured to inject metal or ceramic powder according to a substantially linear pattern along a longitudinal axis, and can then be translated along a transverse direction so as to spread the powder deposited linearly. in the form of a layer C of powder on the surface of the powder bed.

Surface treatment device

In one embodiment represented in FIG. 2, the surface treatment device 5 is configured to project particles of addition material 2 onto the surface of the layer C of powder.

In one embodiment, the surface treatment device 5 projects the addition material 2 according to a substantially linear pattern extending along a first direction, and is moved parallel to the layer C of powder along a second direction so as to treat the surface of layer C of powder.

In one embodiment, shown in particular in FIG. 1, the surface treatment device 5 is integral in movement with the layering element 4, both being mounted on the same motor module 21 configured to move them. Thus, the surface treatment device 5 treats the layer C of powder deposited by the layering element 4 during the same step, which makes it possible to shorten the cycle time.

In a first embodiment, the surface treatment device 5 and the layering element 4 can be parallel, so as to promote the compactness of the assembly comprising the surface treatment device 5 and the layering element 4 .

In another embodiment not shown, the surface treatment device 5 can be driven individually, and can for example be moved so as to treat only the zones of the layer C of powder likely to be heated to merge and then solidify. while cooling. This saves addition material.

In one embodiment, as represented in FIG. 2, the surface treatment device 5 comprises a plasma spraying device, configured to spray the addition material 2. The arrow indicates the direction of movement above the bed of the layer of powder C leading to the creation of a coating at the rear of the device 5. This plasma device can be produced according to different variants.

In one embodiment of the surface treatment device 5 as represented in FIG. 3, it is possible to use an electric arc, preferably operating at atmospheric pressure, and more generally at high pressure (> 1 mbar).

In another embodiment of the plasma device as shown in Figure 4, it is possible to use a plasma-assisted vapor phase coating system, which can operate over a wide range of pressures, from atmospheric pressure to vacuum " primary” (1 to 10 ^-3 mbar).

In another embodiment of the plasma device as represented in FIG. 5, it is possible to use a magnetron-type coating system, preferably operating at low pressure or under secondary vacuum (<0.1 mbar).

In another embodiment as represented in FIG. 6, the surface treatment device 5 comprises a blown wire device.

Electric arc device

An electric arc device as represented in FIG. 3 comprises an electrode 501, preferably negatively polarized (and playing, in this case, the role of cathode) vis-à-vis the layer of powder C, generally connected to ground . The electrode 501 consists of the addition material 2 which is projected towards the layer of powder C when an electric arc 201 or more is established between the electrode 501 and the layer C of powder.

A device not shown in FIG. 3 makes it possible to modify the distance d between the electrode and the layer C of powder to facilitate the establishment of electric arcs. In addition, this system allows the alignment of the electrode 501 substantially parallel to the powder layer.

An electrical power supply 111 connected to electrode 502 makes it possible to create an electric arc between electrode 501 and the powder layer. The power supply 111 is controlled by a control unit 11. The power supply 111 can be continuous, radiofrequency or pulsed.

During the establishment of the electric arc, part of the electrode 501 is vaporized and this vapor condenses favorably just below, that is to say on the surface of the layer C of powder, thus modifying its surface properties and ensuring the desired surface treatment.

Vapor phase coating device

A plasma-assisted vapor phase coating device as represented in FIG. 4 comprises an electrode 502 having a wall delimiting an internal cavity forming the reaction chamber, and a gaseous precursor injection system 52 configured to inject precursor gas into the cavity of the electrode 502. The gaseous precursor must contain the addition element 2 in its composition. For example, to achieve a metallic coating of the powder layer, it is better to use an organometallic. In another example, where addition 2 would be carbon, the precursor can be a hydrocarbon. More generally, any gas capable of producing a coating increasing the absorption of photons and improving the electrical conductivity can be used as a precursor.

A power supply 112 connected to the electrode 502 powers the electrode so as to create a plasma in the cavity of the electrode 502. The power supply 112 is controlled by a control unit 11. The power supply 112 can be continuous, radiofrequency or pulsed.

The plasma thus created decomposes the precursor by the action of free and energetic electrons on the gas and activates it chemically. These species 202 can leave the cavity 502 through one or more orifices 51 made through the wall of the electrode 502. The orifice 51 is advantageously oriented towards the layer of powder C to be treated.

A device not shown in FIG. 4 makes it possible to modify the distance d between the electrode and the layer of powder C. Thus the surface treatment adjusts below the slot in an optimal manner. The entire powder layer C can be coated by moving the device over the powder layer C, as indicated by the double arrow.

In a variant, not represented in FIG. 4, the gas containing the precursor can be thermally activated, by a heating system for example, which is controlled by a control unit 11.

Magnetron device

A magnetron device as represented in FIG. 5 comprises an electrode 503, preferably negatively polarized (and playing, in this case, the role of cathode).

An arrangement of magnets 54, arranged at the rear of electrode 503, generates a magnetic trap 55 which allows the plasma electrons to be confined opposite the other face of electrode 503. In order to further increase the effectiveness of the trap, an alternate arrangement is generally made (outside north and south in the center, or the opposite) to make a closed magnetic track, not shown.

Magnets can be permanent or electromagnets, or a combination of both. This confinement is very effective at low pressure (< 0.1 mbar) making the surface treatment particularly advantageous.

Depending on the needs, the electrode 503 can be supplied (source 113) with direct current (DC - direct current, in English), with Radio Frequency (RF) or in high power pulsed mode (HiPIMS - High Power Impulse Magnetron Sputtering, in English), but generally receiving a negative voltage. The source 113 is controlled by a control unit 11.

The electrode 503 is advantageously made from the addition material 2 used to treat the metallic or ceramic powder 1.

Indeed, under the action of the plasma (in particular of the ions), particles 203 originating from the material of the electrode 503 are sputtered and are projected at high speed towards the surface of the layer C of powders. A protection 53 surrounds the rear of the electrode 503 in order to avoid loss of material and to make the treatment effective towards layer C.

This deposit of material makes it possible to carry out the surface treatment of metal or ceramic powders.

A circulation, not shown, of a cooling fluid (for example water, glycol, etc.) is provided near the magnets, supplied by an external system.

In certain embodiments, the electrode 503 is a cylindrical roller as in FIG. 5, but the electrode 503 can also be planar, in the form of a disc or even rectangular (not shown).

During the operation of the magnetron device, the electrode 503 is rotated. In this way, the part of the electrode 503 which is exposed to the plasma changes regularly, limiting the heating of a particular zone, the plasma being always confined at the level of the magnetic trap 55 generated by the arrangement of magnets 54 which has a fixed orientation with respect to the magnetron device, in particular towards the layer C of powder, as illustrated in FIG. 5. In the case where the electrode 503 is driven in rotation, the protection 53 also retains a fixed orientation with respect to the magnetron device.

In a variant not shown, the doping material 2 can be sputtered by ion bombardment only. Thus a flux of high energy ions (>100 eV) coming from an ion source (ion gun, for example) is sent towards the surface of the doping material 2 and induces its sputtering. By suitably choosing the position and the power of the ion source, it is possible to keep the ion source fixed and to bombard the doping material 2 while moving it parallel to the layer C of powder, thus making it possible to deposit a surface film to layer C of powder. In this variant, the doping material 2 can be a simple plate of material, which makes it possible to dispense with an electrode supply.

Device to blown yarn

A blown wire device as represented in FIG. 6 comprises a wire 504 of reduced section (diameter<0.5 mm). The 504 wire consists of the addition material 2 which is thrown towards the powder layer C when a very strong electric current (> 10 A) passes through it.

A device, not shown, makes it possible to modify the distance between the electrode and the layer C. In addition, this system makes it possible to align the electrode substantially parallel to the layer of powder.

A power supply 114 injects a strong current into the wire 504 which heats up by Joule effect until it is completely vaporized. The power supply 114 is controlled by a control unit 11.

During the evaporation of the yarn 504 this vapor condenses favorably just below, i.e. on the surface of the layer C, thus modifying the surface properties of the layer C and ensuring the desired surface treatment. A screen 182 is placed just above wire 504 to avoid any metallization towards enclosure 19 or towards heat shield 18.

The typical coating deposition rates for the various devices which have just been described are from 0.2 to 2 nm/s, i.e. between 1 and 8 ML/s (mono-layers of English 'MonoLayer' per second, i.e. ML/s). Even taking the lowest deposition rate, it is therefore possible to deposit 2 monolayers (round trip), therefore to significantly modify the surface state of the powder grains without significantly modifying the stoichiometry of the powder grains.

The advantages of the treatment obtained are multiple, particularly in the case of copper if the addition material is different from the powder, thus favoring:

- covering the surface of the powder with a tiny quantity of material (~2 ML);

- the decrease in surface reflectivity, allowing better absorption of photons;

- the increase in roughness and/or surface texturing, allowing better trapping of photons during multiple reflections;

- improving the thermal conduction between the powder grains, i.e. improving the thermal conductivity of the powder bed;

- improving the electrical conductivity of the powder bed;

- the limitation of the repulsion (uplift) of the powders by the formation of atomic bridges between the grains of powder, facilitating the flow of charges during selective melting. During the melting of the powders these bridges should be destroyed by simple local evaporation;

- the increase in the contact surface between the grains, making it possible to reduce the hot spots at the contacts between the powders and thus the "suction" of the grains joined to the zone under the beam;

- improved control of the amount of material of add-on material 2 to create shades for the final part. Thus, depending on the fraction of addition material 2 added, the final properties of the part made from the powders can be improved, without going through a modification of the powder manufacturing process.

Liquid spray coating

In one embodiment of the surface treatment device 5 represented in FIG. 7, a liquid solution of absorbing agent containing a nanometric precipitate (of sub-micron size) with strong laser absorption can be sprayed in the form of microdroplets 204 above the powder bed, by a spray device 301 comprising one or more nozzles 311 configured to project droplets of liquid absorbent solution onto the grains of powder and supplied from reservoir 400. The liquid absorbent solution can be conveyed from the reservoir 400 to the spray device 301 by means of a flexible conduit.

Preferably, the liquid solution is oxygen-free, and the precipitate contains one or more of carbon black, carbon nanotubes and graphene, thereby forming the addition material.

Optionally, the spray device 301 comprises a plurality of micro-nozzles or electro-nozzles configured to control the size of the drops of liquid solution of absorbent agent, so as to control the quantity deposited on the powder grains.

In a preferred embodiment, the liquid chosen for the liquid solution of absorbent agent is a volatile solvent at high temperature, for example an organometallic solvent.

In this way, during a selective melting step, only the absorbent agent remains on the surface of the powder bed, which significantly modifies the absorption of the laser radiation. The liquid vaporized during the melting of the powder grains can also be evacuated in the form of fumes, with the other elements released during the formation of the liquid metal bath.

The assembly can be controlled by the control unit 11 which controls the various components of the surface treatment device 5.

Composite film coating

In one embodiment of the surface treatment device 5 represented in FIG. 8, the surface treatment device 5 comprises a drive device 601 configured to unroll, on the surface of the layer C of powders, a roll 210 of composite film 205 formed of a very fine matrix (preferably having a thickness of the order of a micrometer) and volatile at high temperature and preferably loaded with absorbent material with respect to the wavelength of the laser, for example a polymer film, which in turn can be loaded with conductive additions. The polymer used can be a polyester, a thermoplastic polymer or a resin conventionally used for producing matrices of composite materials.

The composite film 205 can for example comprise one or more components among carbon black, carbon nanotubes, and graphene, thus forming the addition material. This purely mechanical operation leads to the covering of the upper surface of the powder bed by the composite film 205.

Claims (17)

Additive manufacturing process by selective melting of metal or ceramic powder comprising the steps of:
- Deposition of a layer (C) of powder
- Selective melting of at least one zone of the powder layer, in which the method comprises, after the deposition and prior to the selective melting, a coating step during which grains of metallic or ceramic powder ( 1) are at least partially covered with a film of an addition material (2), the grains of metal or ceramic powder (1) having a first reflectivity, the addition material (2) having a second reflectivity more lower than the first reflectivity. Additive manufacturing process according to claim 1, in which the powder is metallic and comprises a metal or an alloy having a reflectivity greater than 90%. Additive manufacturing process according to claim 1 or 2, in which the additive material (2) comprises at least one material among carbon, tungsten or molybdenum, or a mixture thereof. Additive manufacturing process according to one of Claims 1 to 3, in which the powder is metallic and comprises at least one metal or an alloy of metals chosen from copper, aluminium, gold, silver and platinum. . Additive manufacturing method according to one of Claims 1 to 4, in which the coating step is carried out by means of a surface treatment device (5) configured to generate a plasma and project the plasma according to a substantially linear pattern, the surface treatment device (5) being adapted to be moved above the metallic or ceramic powder so as to project the plasma onto the metallic or ceramic powder. Additive manufacturing method according to claim 5, in which the surface treatment device (5) comprises an electric arc device comprising an electrode (501) comprising addition material (2), the electrode (501) being positioned in facing the powder layer (C) in such a way that addition material (2) is projected towards the powder layer (C) when an electric arc (201) is established between the electrode (501) and layer (C). Additive manufacturing process according to claim 5, in which the surface treatment device (5) comprises a vapor phase coating device comprising an electrode (502) comprising a wall delimiting an internal cavity, and an injection system (52) precursor gas configured to inject a precursor gas into the cavity, the wall further having an orifice (51) located facing the layer (C), the precursor gas being configured to generate particles of addition element when it is excited by the electrode (502), the addition element particles being expelled through the orifice (51) onto the powder layer (C). Additive manufacturing method according to claim 5, in which the surface treatment device (5) comprises a magnetron device (503). An additive manufacturing method according to claim 5, wherein the surface treatment device (5) comprises a blown wire device, comprising a wire (504) connected to a power source, in which the wire (504) extends to the above the powder bed and is comprised of the add material, the power source being configured to inject a current into the wire (504) such that the add material vaporizes and deposits on the layer (C ) powder. Additive manufacturing process according to one of Claims 1 to 9, in which the layer of added material has a thickness of less than 100 nm. Additive manufacturing process according to one of Claims 1 to 10, in which the selective melting step is carried out by means of a power source (10) comprising at least one laser source (14). Additive manufacturing process according to one of Claims 1 to 11, in which a step of surface treatment of the metal or ceramic powders is carried out successively at each step of placing a layer (C) of metal or ceramic powder. Additive manufacturing process according to one of Claims 1 to 4, in which the layer of powder (C) is covered with a film of additive material containing one or more components from among carbon black, carbon nanotubes and graphene. Additive manufacturing process according to claim 13, in which the film is deposited by spraying a liquid solution of additive material in a volatile solvent at high temperature. Additive manufacturing process according to claim 13, in which the film is deposited mechanically by covering the layer (C) of powders with a composite film (205) comprising a matrix volatile at high temperature and loaded with addition material. Additive manufacturing device configured to carry out an additive manufacturing process according to one of Claims 1 to 15, the additive manufacturing device comprising:
- a layering element (4) configured to lay out a layer (C) of metallic or ceramic powder (1) which is substantially planar;
- a surface treatment device (5) configured to perform a surface treatment process for the layer (C) of metallic or ceramic powder;
- a power source (10) configured to carry out a selective melting of at least one zone of the layer (C) of powders. Additive manufacturing device according to claim 16, in which the layering element (4) is configured to deposit a layer (C) of powder when it is moved on the surface of a support (7), and in which the surface treatment device (5) is integral in movement with the layering element (4), so that when the layering element (4) is moved, the layering element layer (4) deposits a layer (C) of powder and the surface treatment device (5) treats the layer (C) of powder deposited by the layering element (4).