FR3079773A1 - HEATING DEVICE FOR ADDITIVE MANUFACTURING APPARATUS - Google Patents

Abstract

The invention relates to a device for heating a bed of powder in an additive manufacturing apparatus, characterized in that it comprises: - a plasma generating device (28), said device being adapted to be arranged and moved to the above the powder bed, at a distance from the powder bed for generating plasma thereon, - a unit for the power supply (22) of said heating device, - a control unit (9) for controlling feeding and moving the heater.

Description

HEATING DEVICE

FOR ADDITIVE MANUFACTURING APPARATUS

GENERAL TECHNICAL AREA AND PRIOR ART

The present invention relates to the general field of selective additive manufacturing.

More particularly, it relates to the heating treatments that are implemented on the powder beds before the selective melting.

Selective additive manufacturing consists of making three-dimensional objects by consolidating selected areas on successive layers of powdery material (metallic powder, ceramic powder, etc.). The consolidated areas correspond to successive sections of the three-dimensional object. Consolidation is done, for example, layer by layer, by a total or partial selective fusion carried out with a power source (high power laser beam, electron beam, etc.).

Conventionally, in order to avoid projections due to the electrostatic repulsion of adjacent powder particles which are charged under the effect of the beam of the power source, the powder bed is previously consolidated by preheating. This preheating ensures a rise in temperature of the powder bed at temperatures which can be quite substantial (approximately 750 ° C. for the titanium alloys).

However, it has a high energy cost.

It also represents a significant loss of cycle time.

In addition, when the geometry of the part does not require it, it is not necessary to heat the entire powder bed at each selective melting step.

In addition, the total preheating of a layer of powder results in cohesion of the powders which hinders the cleaning of the part when the manufacturing process is finished, in particular when the part has internal cavities with complex geometry.

OVERVIEW OF THE INVENTION

A general object of the invention is to overcome the drawbacks of the configurations proposed so far.

In particular, an object of the invention is to provide a solution which allows preheating and / or post-treatment by heating without loading and raising the powder.

Yet another object is to propose a solution which makes it possible to reduce the costs and the preheating and / or post-treatment times by heating in the manufacturing cycles.

Another object of the invention is to propose a simple construction solution.

Another aim is also to provide an efficient heating solution over a wide range of pressures.

Another object is to limit the heating of the powder bed to only the zones which will be melted during the following melting phase, for the stratum during manufacture.

Thus, according to a first aspect, the invention provides a device for heating a bed of powder in an additive manufacturing device, characterized in that it comprises:

a plasma generation device, said device being adapted to be placed and moved above the powder bed, at a distance from the powder bed allowing the generation of the plasma thereon,

a unit for the electrical supply of said plasma generation device,

- a control unit to control the supply and movement of the plasma generation device.

Such a device is advantageously supplemented by the following different characteristics, taken alone or in combination:

- The plasma generation device comprises an elongated electrode adapted to be moved with a main component of displacement perpendicular to the direction in which said electrode extends;

- The heating device comprises at least one elongated electrode surrounded by a metal sheath, said sheath being, in its zone above the powder bed, provided with a slot;

the unit for the electrical supply of said plasma generation device comprises an alternating source operating in RF and / or an alternating pulse source;

- The heating device includes a mechanism for adjusting the distance of the plasma generation device from the powder bed;

- The electrode has a cross section in an arc;

- the developed internal surface of the electrode is greater than the area of the plasma projection zone on the powder bed;

- The sheath comprises shutter means, the slot being delimited by said shutter means;

the plasma generation device comprises several localized independent plasma generators, each of these generators being capable of generating a localized plasma above the powder bed, the control unit controlling said generators, during the movement of the plasma generation device, according to sequences depending on the zone or zones of the powder bed to be preheated and the heating desired for these;

- the localized plasma generators comprise a plurality of electrodes juxtaposed linearly;

the electrodes are distributed in one or more parallel planes, the electrodes of the same plane being distant from each other, the electrodes of a first plane being staggered relative to the electrodes of a second plane, so that an electrode of a plane is placed opposite the space separating two electrodes of another plane;

the electrical supply device is configured to independently supply each localized plasma generation device in a bipolar manner;

- The control unit is adapted to move the plasma generation device according to several movement speeds;

- the plasma is produced inside a cavity electrode and projected through a diaphragm.

According to a second aspect, the invention relates to an apparatus for manufacturing a three-dimensional object by selective additive manufacturing comprising in an enclosure:

- a support for depositing successive layers of additive manufacturing powder,

a distribution arrangement suitable for applying a layer of powder on said tray or on a previously consolidated layer,

- at least one power source suitable for the selective consolidation of a layer of powder applied by the distribution arrangement, characterized in that it comprises a heating device, including the plasma generation device, said device being adapted to be placed and moved above the powder bed, at a distance from the powder bed allowing the generation of the plasma thereon.

In particular, in one embodiment, the dispensing arrangement comprises a squeegee or a layering roller, the heating device extending close to said squeegee or roller and being movable therewith or independently of it. this.

According to a third aspect, the invention relates to a method of manufacturing a three-dimensional object by selective additive manufacturing comprising the steps of:

- Depositing a layer of powder on a support or on a previously solidified layer,

- Heating of the powder layer by means of a preheating device as defined above,

- Consolidation of the preheated zone, consolidation being carried out by means of a power source.

In particular, in one embodiment of the manufacturing method, during the heating step, the heating device comprises several electrode segments and has two operating modes:

- a mode of movement, during which the speed of movement of the heating device is maximized;

- A heating mode, during which at least one electrode segment is supplied for a determined period, the heating device moving at a determined speed, thereby heating at least one zone of the powder bed.

PRESENTATION OF THE FIGURES

Other characteristics and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and should be read with reference to the appended figures in which:

- Figure 1 is a schematic representation of an additive manufacturing apparatus comprising a heating device according to a possible embodiment of the invention;

- Figure 2 illustrates a possible embodiment for such a heating device;

- Figure 3a is a schematic sectional view of a heating device comprising a cylinder-cylinder type electrode;

FIG. 3b is a schematic sectional view of a heating device comprising an electrode of the wire-plane type,

FIG. 3c is a schematic sectional view of a heating device comprising a knife-type electrode;

- Figure 3d is a schematic sectional view of a heating device comprising an electrode of the semi-cylindrical type;

- Figure 4 schematically illustrates a powder bed of which only certain areas have been heated locally, in accordance with the invention;

- Figure 5 schematically illustrates a localized heating device according to the invention; more precisely, FIG. 5a represents a variant in which the generators are arranged in two parallel planes, FIG. 5b illustrating an embodiment in which the generators are arranged on the same plane;

- Figure 6 schematically illustrates a powder bed of which only certain areas have been locally heated, according to the invention;

- Figure 7 is a perspective diagram of an embodiment of a localized heating device according to the invention.

DESCRIPTION OF ONE OR MORE MODES OF IMPLEMENTATION AND IMPLEMENTATION

Overview

The apparatus 1 for selective additive manufacturing of FIG. 1 comprises:

- A support such as a horizontal plate 3 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 (object 2 in the shape of a fir tree in the figure )

- a powder reservoir 7 located above the plate 3,

an arrangement 4 for the distribution of said metallic powder on the tray, this arrangement 4 comprising for example a squeegee 5 or a layering roller for spreading the different successive layers of powder (displacement according to the double arrow A),

- a set 8 of energy sources for the fusion (total or partial) of the spread thin layers,

a control unit 9 which controls the various components of the device 1 as a function of pre-stored information (memory M),

- A mechanism 10 to allow the support of the plate 3 to descend as the layers are deposited (displacement according to the double arrow B).

In the example described with reference to FIG. 1, the assembly 8 comprises a source 12 of the laser type.

As a variant, the assembly 8 may comprise any other type of localized energy source making it possible to merge a powder under primary vacuum (10 ^-3 to 1 bar) such as for example a source of particles.

Still alternatively, the assembly 8 can also include several sources of the same type, such as for example several laser sources, or means making it possible to obtain several beams from the same source.

In the example described with reference to FIG. 1, at least one galvanometric mirror 14 makes it possible to orient and move the laser beam coming from the source 12 relative to the object 2 as a function of the information sent by the control 9.

Any other deviation system can of course be envisaged.

In another example not illustrated, the assembly 8 comprises several sources 12 of the laser type and the displacement of the different laser beams is obtained by moving the different sources 12 of the laser type above the layer of powder to be fused.

A heat shield T can be interposed between the source or sources of the assembly 8.

The components of the device 1 are arranged inside a sealed enclosure 17, if necessary connected to a vacuum pump 18.

The apparatus further comprises a heating device 19 disposed above the powder bed and able to move linearly with respect thereto.

This heating device 19 can be placed behind and / or in front of the squeegee 5 or the layering roller on the same sliding carriage. It can also be mounted on an independent trolley or on a robot arm.

In the case of a heater 19 located in front of the layering roller 5 (in the direction of movement of the layering roller 5), when moving the layering roller 5 to deposit a layer of powder , the heating device 19 performs the heating of the powder layer which has been deposited during the deposition of the previous layer.

The heated layer is then covered by the next layer, thus limiting the losses of energy by convection between the heated layer and the medium of the enclosure 17, and heating by conduction the next layer.

As a variant, one can imagine two or more heating devices in succession, placed before and / or after the layering device 5.

To obtain a certain heating power, it is preferable to use several devices of lesser power, making it possible to total the desired heating power together.

For example, the heat losses by the Joule effect being proportional to the square of the intensity of the supply current of the device 19, the use of two devices of identical power P / 2 makes it possible to individually reduce their energy losses by four compared to a single device with a power of P. The overall losses of the device are therefore reduced by half.

The movement of said device 19, its supply and its residence time in front of the different zones which it is desired to heat on the surface of the powder bed are also controlled by unit 9.

Various examples of such heating solutions are described below.

Linear discharge heating

In the example illustrated in FIG. 2, the heating device 19 includes a plasma generation device 28 which is moved above the metal powder bed (solid or granular surface 21, made up of micro- or nano-powder) .

This plasma generation device 28 comprises an elongated electrode 20 supplied by an electrical excitation source 22 and itself controlled by the control unit 9.

This electrode 20 generates, under the effect of said source 22, electric discharges between said electrode 20 and the surface 21 and creates a plasma by the supply of energetic particles of positive and negative charge, the plasma ensuring the heating of the surface. 21.

The electrode 20 extends substantially parallel to the surface 21. It is moved parallel to said surface 21, perpendicular to the direction in which the electrode extends.

Such a configuration allows homogeneous heating over a surface of the powder bed corresponding to the length of the electrode 21 and to its displacement distance.

This solution for generating an electric discharge by an elongated electrode is effective over a wide range of pressures ranging from 1 to a few hundred millibar.

The distance between the electrode and the surface of the powder bed must be as small as possible, so that the electrical discharge (plasma) develops between the electrode and the surface of the powder bed 21 only.

A mechanism (not shown) allows the adjustment of this distance in addition to the mechanism 10.

In the above-mentioned pressure ranges, the electrode 20 is spaced from the surface 21 to be heated by a space (gas) of a few millimeters.

The elongated electrode 20 is preferably made of a metal which has a high melting temperature (By high temperature, it is understood that the melting temperature of the material of the electrode is at least equal to or higher than the temperature powder fusion).

The electrode 20 can be made of dielectric material, or of semiconductor material or ceramic.

Preferably, the electrode 20 is made of a material which is a good electrical conductor (conductivity greater than 5 × 10 ⁶ S / m).

The resistance to heat depends on the possible cooling of the electrode.

The surface 21 of the powder bed is for example connected to ground.

The source 22 allows the application of a high voltage (typically> 0.5 kV, for a heating power of some 100 W) between the electrode 20 and the surface 21 of the powder bed.

The power supply thus produced by the source 22 can be alternated, either sinusoidal low frequency, or any other signal profile, (square, triangular, etc.) or radio frequency (RF), or pulse.

The operation of the radio frequency excitation (RF) discharge ensures perfect neutrality of the powder surface and thus avoids mutual repulsion of the powders.

The electrode 20 can also - as illustrated in FIG. 3a in the particular case of an electrode with a substantially annular section - be protected by a metallic sheath 26 (“guard electrode”), floating or connected to ground , provided with a slot 27 on the surface side 21. The electrode 20 can also be provided with a slot.

The electrode 20 may also, as a variant, have a section of different geometry, such as for example circular, elliptical, parabolic or hyperbolic.

Several other configurations are possible for the elongated electrode (knife geometry electrode, flat wire electrode, flat cylinder, etc.). A preferred configuration, but not limiting, is that of a cylinder-cylinder electrode as described below.

Cylinder-cylinder electrode (Fig. 3a)

In the embodiment illustrated in FIG. 3a, the electrode 20 comprises a metal cylinder 25 which extends in a cylindrical metal sheath 26. The cylinder 25 and the sheath 26 can however be made from non-metallic materials.

Said cylinder 25 is centered on the axis of the sheath 26.

Said sheath 26 further has a linear slot 27 which extends parallel to the cylinder 25, along the generating line of said sheath 26 which is closest to the surface 21 to be preheated, and which thus directly faces said surface 21.

The cylinder 25 has a linear groove 251 located opposite the slot 27 formed on the sheath 26, so that the groove 251 faces the surface 21 to be heated.

The ignition of the discharge towards the powder bed is favored by the sheath 26 - floating or connected to ground - and placed at a very short distance from the cylinder 25 so as to prevent the discharge from developing over the entire surface of cylinder 25 and thus favor discharges inside cylinder 25.

In order to keep the sheath 26 at a short distance from the electrode 25, the cylinder 25 is centered in the sheath 26 using a centering element 252, which can be made of dielectric material.

Such centering means 252 can be used regardless of the type of electrode used (wire, cylinder, knife, etc.).

In the illustrated embodiment, the centering element 252 can comprise, for example, two capillaries or two fine ceramic cylinders situated between the cylinder 25 and the sheath 26.

Optionally, the cylinder 25 and the sheath 26 can be physically but not electrically connected to each other at the level of the groove 251 and the slot 27 by means of a sealing element 253 of dielectric material, so as to create a closed space hermetic 254 between the cylinder 25 and the sheath 26. In one embodiment, the sealing element 253 comprises two linear plugs extending respectively on either side of the slot 27 and of the groove 251, each of the plugs joining on the one hand the cylinder 25 and on the other hand the sheath 26.

Optionally, a fluid can be circulated through this closed space 254 so as to cool the cylinder 25 and the sheath 26.

Such a configuration allows the plasma to develop inside the cylinder 25 and to project through the groove 251 and the slot 27 onto a controlled area on the surface of the powder bed 21.

Preferably, the area of the projection area on the surface 21 is less than the developed internal surface of the cylinder 25.

In this way, in fact, it is avoided that the electrode 20 and the sheath 26 absorb most of the power, while the plasma has a volume sufficient to develop in order to achieve effective heating of the powder bed 21.

Optionally, the cylinder 25 is equipped at its ends with plugs (not shown) preventing the plasma from being evacuated through these openings. In one embodiment, these plugs may include a metal mesh causing the formation of a Faraday cage and limiting the leakage of plasma from the ends of the cylinder

25.

The excitation of the cylinder 25 can be carried out by radio frequency (RF), while the mass is connected to the powder bed (and to the frame of the enclosure 17). The positive-negative alternation of the applied RF voltage ensures the neutrality of the charge on the surface of the powder bed, thus avoiding charging and ultimately the electrostatic rise of the powders.

Other types of excitations are possible: high voltage pulses (IHT), mono-polar or bipolar excitations.

Like RF power, positive-negative pulse alternation ensures the neutrality of the charge on the surface of the powder bed.

Bi-polar power has the advantage, unlike RF power, of not requiring an impedance matching system (essential for RF polarization), while ensuring the neutrality of the treated powder.

Also, it allows supplies at much higher voltages (> 10 kV) than in RF (~ 1 kV).

Furthermore, the distance between on the one hand the assembly constituted by the cylinder 25 and the sheath 26 and on the other hand the surface 21 of the powder bed is adjustable. The distance to the surface 21 is notably adjusted as a function, for example, of the pressure conditions in the enclosure 17.

The generation of electric discharges depends on the pressure-distance product (Paschen's law) and on the nature of the gas present in the enclosure. Thus, depending on the working pressure in the enclosure and the nature of the gas, it is possible to adjust this distance in order to obtain optimum heating.

variants

In a variant, the electrode 20 comprises a wire 29 which extends inside the sheath 26.

This embodiment, called the plan line, is shown in FIG. 3b.

When the wire 29 is excited, the plasma forms between the wire 29 and the powder bed 21, the slit 27 of the sheath 26 delimiting the zone of projection of the plasma onto the powder bed 21.

Preferably, the wire 29 is coaxial with the sheath 26. When the wire is excited and the plasma heats the powder bed, the temperature of the wire increases and the wire expands, thereby reducing its rigidity.

To prevent the thread from slackening and deviating from the axis of the sheath 26, it is possible to link to the ends of the thread 29 a mechanical tensioning device, configured to maintain a constant tension in the thread 29 and therefore maintain the wire 29 along the axis of the sheath 26 whatever the temperature and the elongation of the wire 29.

In a second variant, the electrode 20 has a knife type geometry, as shown in FIG. 3c.

The electrode 20 is a massive electrode, which ends, at its zone intended for the generation of the plasma, by a linear sharp edge 20a.

The edge 20a extends opposite the bed of powder to be heated.

The ignition of the discharge towards the powder bed is favored by the sheath 26 - floating or connected to ground - and placed at a very short distance from the electrode 20 so as to prevent the plasma from settling on the sides side of the electrode 20.

In another embodiment, the sheath 26 and the cylinder 25 may not have an annular section but have a section in an arc of a circle (or ellipse), as shown in FIG. 3d. The sheath 26 then comprises sealing means 30 delimiting the slot 27 so as to delimit the plasma formation zone. The sealing means may comprise plates, preferably made of dielectric material, fixed on the sheath 26 and extending longitudinally relative to the axis of the sheath 26.

It is understood that the length of the arc of the section of the electrode 20 is not limited to the illustrated representation, and may extend over an angular range between 10 ° and 359 °.

This geometry makes it possible in particular to reduce the wear and the heating of the cylinder 25 and of the sheath 26, in particular at the level of the slot 27 and the groove 251, caused by the ejection of the plasma through the slot 27 and the groove 251.

Implementation of full field heating

By placing an elongated electrode 20 (cylinder 25 / sheath 26 assembly for example) in front of the powder surface 21, it is possible to maintain a homogeneous and confined plasma between said electrode 20 and the powder bed.

By moving this electrode 20 it is possible to scan the surface 21. By keeping the plasma on and by performing a complete scan of the surface 21, this surface is heated superficially.

Optionally, depending on the duration of ignition of the plasma (time ti, t2 or ts) and the position of the system 20 above the surface 21, only certain zones can be heated, over the entire width of the surface 21 , as illustrated in figure 4.

By limiting the duration of plasma ignition, energy consumption can be optimized while achieving the desired heating.

Energy is thus efficiently transferred to the powder, which enables it to be heated.

Energy is transmitted to the powder by several means coexisting simultaneously in a plasma.

These are charged species, electrons and ions, but also energetic neutral species, in particular excited non-radiative (metastable) states, and photons. As the surface (powder) receives the two charged species, the charge effects (Coulomb repulsion) are reduced or even eliminated.

In addition, all visible and infrared (IR) photons heat the material when absorbed, while ultra-violet (UV) photons can also modify the surface superficially (pickling the surface for example) if their energy is greater than the binding energy of the species adsorbed on the surface.

The denser the plasma, the greater the energy transmitted to the surface. It is possible to create a plasma from a few 10 W and up to 5 kW or more. The amount of energy brought by the plasma to the surface can be easily adjusted by the RF or pulse power injected.

Localized heating device

In an optimized embodiment, the plasma generation device 28 comprises a plurality of independent plasma generators (in this case twelve referenced from SI to S12) distributed over the length of said device 28 illustrated in FIG. 5a.

These different generators SI to S12 are supplied independently of each other by an excitation source 22, which is associated with a distributor 23.

The plasma generator 28 comprises for example an electrode 20 divided into a plurality of independent electrodes.

In the illustrated embodiment, the electrodes E1 to E12 are separated from each other and supplied independently of each other.

Each of the generators SI to S12 comprises at least one electrode.

The length of these electrodes E1 to E12 is adapted to the desired precision on the location of the heating. This length is at least one millimeter.

Optionally, the separation distance between electrodes can be of the order of a millimeter.

In the illustrated embodiment, the plasma generation device 28 comprises two rows of six electrodes supplied by an electrical excitation source 22 and itself controlled by the control unit 9.

The two rows are located in two parallel planes A-A ’(odd electrodes) and B-B’ (even electrodes).

In the illustrated embodiment, the electrodes E1 to E12 are staggered, so that an even electrode is located opposite the space separating two odd electrodes, and vice versa.

The distance between the two planes depends on the size of the electrodes, depending on the type of electrode chosen. It is not dictated by plasma.

It is understood that this arrangement can vary, the electrodes possibly being more or less numerous and arranged on the same plane or on a greater number of planes, as in an embodiment illustrated in FIG. 5b, in which the electrodes E1 to E12 are formed from the same elongated electrode.

Optionally, the electrodes E1 to E12 can be separated by a dielectric or have shapes different from each other.

Optionally, these electrodes E1 to E12 can be placed on one or more articulated arms.

The electrodes E1 to E12 are excited to generate, under the effect of said source 22, localized electrical discharges between said electrodes and the surface 21 and thus create a plasma of limited size, which provides localized heating of the surface 21.

The distance between the electrodes E1 to E12 and the surface 21 must be as small as possible, so that the localized electrical discharges develop between the electrodes and the surface only, so as to generate a localized plasma.

The electrodes E1 to E12 are for example elongated electrodes (knife, wire, cylinder, etc.) which extend substantially parallel to the surface 21. A metal sheath can also be provided. The assembly is moved parallel to said surface 21, perpendicular to the direction in which it extends.

Such a configuration allows homogeneous heating over a limited length (segment) of the surface of the powder bed corresponding to the length of each electrode and the heated surface depends on the distance of movement with the plasma of each segment (discharge) activated.

This solution for generating electrical discharges by one, two or more electrodes is effective over a wide range of pressures, ranging from 1 to a few hundred millibar.

A mechanism (not shown) allows adjustment of the distance between the electrode (s) in addition to the mechanism 10.

Implementation of localized heating

The sequence of the excitation durations of each electrode is controlled by the distributor 23, as a function of the speed of advance of the assembly of the plasma generation device 28 so as to locally heat only certain predefined areas of the surface 21, such as than in the example illustrated in FIG. 5a or 5b, and depending on the heating desired for them.

Each localized heated zone is of a width substantially equal to the length of an electrode and of a length corresponding to the distance vxt, where v represents the scanning speed of the plasma generation device 28 (or that of the delivery system in layer, if the two are integral) and t denotes the time of ignition of the discharge (which corresponds substantially to the excitation time).

The combination of the different zones thus heated locally is adapted (by the distributor 23 controlled by the unit 9) as a function of the zone which it is desired to subject thereafter to selective melting on the surface 21.

This is illustrated in FIG. 6, in which several zones heated over the entire surface of the powder bed 21 have been represented by a plasma generation device 28 moving in the direction indicated by the arrow on the Fig.

These zones correspond to an excitation of the only electrodes El, E2, E3, E7 and E10 triggered at a first instant for the electrode E10, at the same second instant for the electrodes El and E7.

The electrodes E1, E2 and E3 are excited at the same third instant, then the electrode E2 is extinguished at a fourth instant, excited at a fifth instant, extinguished at a sixth instant and excited at a seventh instant.

The electrode E10 is lit for a duration t10, the electrode E7 for a duration t7 and the electrode El for a duration il and therefore heat three local surface areas vxilO, vxt7 and vxtl.

The electrodes E1, E2 and E3 are then lit for a period t, then the electrode E2 is switched off for a period t 'during which the electrodes E1 and E3 are kept lit.

The electrode E2 is then relit for a period t, the electrodes El and E3 being always supplied.

The electrode E2 is then extinguished for a period t ′ during which the electrodes El and E3 are kept in excitation.

The electrode E2 is finally supplied for a period t, simultaneously with the electrodes El and E3.

The fact of dividing the plasma generation device 20 into different independent plasma generators makes it possible to excite only the generators SI to S12 which heat the surfaces concerned by the fusion, thus making it possible to minimize the amount of energy necessary for the phase heating, and to avoid possible cohesion of the powders in the non-molten areas facilitating their subsequent reuse.

Furthermore, by changing the speed of movement when the plasma generation device 28 passes over an area of the powder bed which will not be heated, the time of the heating phase can also be drastically reduced.

This time can be, in certain cases also reduced by installing independent plasma sources on articulated arms (not shown) allowing different trajectories from the example of a rectilinear trajectory of the sources described above.

The productivity and the efficiency of the heating phase are therefore improved.

An exemplary embodiment of a segmented plasma generation device 28 is shown in FIG. 7.

In the embodiment shown, the plasma generation device comprises four plasma generators SI, S2, S3 and S4 arranged in staggered rows, the odd generators SI and S3 being located in a first plane AA 'and the even generators S2 and S4 lying in a second plane BB '.

The spacing of the planes AA 'and BB' depends on the type of electrode chosen, here of the cylindrical type.

By choosing another type of electrode, for example of the knife type, the difference between the planes AA 'and BB' could be reduced.

It would also be possible as a variant to place all the electrodes E1 to E4 on the same plane, as presented in FIG. 5b.

Each of the plasma generators SI to S4 includes an electrode El to E4.

Each of the electrodes E1 to E4 is independently supplied by an excitation source 22, which is associated with a distributor 23 itself controlled by the control unit 9.

Alternatively, for very short electrode segments (of the order of a centimeter), it is possible to have for each independent source an ejection diaphragm (instead of the slit) of the plasma created inside. of a cavity electrode and to obtain a reduced heated region (spot type). By cavity, it is understood that the electrode 20 forms a cavity in which the plasma is formed, in a similar manner to the embodiments illustrated in FIG. 3a or 3d. This source variant (not shown) is particularly well suited to be installed on an articulated arm. By diaphragm is meant an electrode provided with an orifice, the shape and position of which are adjusted so as to allow the plasma formed in the cavity to exit through said orifice.

Claims (18)

1. Device for heating a powder bed in an additive manufacturing device, characterized in that it comprises: - a plasma generation device (28), said device being adapted to be placed and moved above the powder bed, at a distance from the powder bed allowing the generation of the plasma thereon, - a unit for the electrical supply (22) of said plasma generation device, - a control unit (9) for controlling the supply and movement of the plasma generation device. 2. Heating device according to claim 1, in which the plasma generation device (28) comprises an elongated electrode (20, 25) adapted to be moved with a main component of displacement perpendicular to the direction in which said electrode (20 , 25) extends. 3. Heating device according to claim 2, comprising at least one elongated electrode (20, 25) surrounded by a metal sheath (26), said sheath (26) being, in its region above the powder bed, provided with 'a slot (27). 4. Heating device according to one of the preceding claims, in which the unit (22) for the electrical supply of said plasma generation device (28) comprises an alternating source operating in RF and / or an alternating pulse source. 5. Heating device according to one of the preceding claims, comprising a mechanism for adjusting the distance of the plasma generation device (28) relative to the powder bed. 6. Heating device according to one of claims 2 to 5, wherein the electrode (20, 25) has a cross section in an arc. 7. Heating device according to claim 6, in which the developed internal surface of the electrode (20, 25) is greater than the area of the zone of projection of the plasma onto the powder bed. 8. Heating device according to one of claims 3 to 7, wherein the sheath (26) comprises closure means (30), the slot (27) being delimited by said closure means (30). 9. Heating device according to one of claims 3 to 7, in which the plasma is produced inside a cavity electrode and projected through a diaphragm. 10. Heating device according to one of claims 1 to 9, in which the plasma generation device (28) comprises several independent localized plasma generators (SI, S2, etc.), each of these generators being capable of the generation of a plasma located above the powder bed, the control unit controlling said generators, during movement of the plasma generation device (28), according to sequences depending on the zone or zones of the powder bed to be preheated and the desired heating for them. 11. Heating device according to claim 10, wherein the localized plasma generators (SI, S2, ...) comprise a plurality of electrodes (El, E2, ...) juxtaposed linearly. 12. Heating device according to claim 11, in which the electrodes (E1, E2, ...) are distributed along one or more parallel planes (A-A ', B-B', ...), the electrodes of the same plane being spaced from each other, the electrodes of a first plane (A-A ') being staggered relative to the electrodes of a second plane (B-B'), so that a electrode of a plane is arranged opposite the space separating two electrodes from another plane. 13. Heating device according to one of claims 11 or 12, wherein the electrical supply device (22) is configured to independently supply each localized plasma generation device (SI to S12) in a bipolar manner. 14. Heating device according to one of the preceding claims, in which the control unit (9) is adapted to move the plasma generation device (28) according to several movement speeds. 15. Apparatus for manufacturing a three-dimensional object by selective additive manufacturing comprising in an enclosure: - a support (3) for depositing successive layers of additive manufacturing powder, a distribution arrangement (4) suitable for applying a layer of powder on said support (3) or on a previously consolidated layer, - at least one power source (8) suitable for the selective consolidation of a layer of powder applied by the distribution arrangement (4), characterized in that it comprises a heating device (19) according to one of the preceding claims, said heating device (19) being adapted to be placed and moved above the powder bed, at a distance from the powder bed allowing the generation of the plasma thereon. 16. Apparatus according to claim 15, wherein the dispensing arrangement (4) comprises a squeegee (5) or a layering roller, the heating device (19) extending near said squeegee (5) or roller and being movable therewith or independently thereof. 17. Method for manufacturing a three-dimensional object by selective additive manufacturing, comprising the steps of: - Depositing a layer of powder on a support (3) or on a previously solidified layer, - Heating of at least one zone of the powder layer by means of a heating device (19) according to one of claims 1 to 13, - Consolidation of the preheated zone, consolidation being carried out by means of a power source (8). 18. Method according to claim 17, in which during the heating step, the heating device (19) comprises several electrode segments (E1 to E12) and has two operating modes: - a mode of movement, during which the speed of movement of the heating device (19) is maximized; - a heating mode, during which at least one electrode segment (E1 to E12) is supplied for a determined duration, the heating device (19) moving at a determined speed, thus heating the minus one area of the powder bed.