BACKGROUND

Technical Field

[0001] The present invention relates to a method and an apparatus for additive manufacturing.

Description of Related Art

[0002] Freeform fabrication or additive manufacturing is a method for forming three- dimensional articles through successive fusion of chosen parts of powder layers applied to a worktable.

[0003] An additive manufacturing apparatus may comprise a work table on which the three- dimensional article is to be formed, a powder dispenser, arranged to lay down a thin layer of powder on the work table for the formation of a powder bed, an energy beam for delivering energy to the powder whereby fusion of the powder takes place, elements for control of the energy given off by the energy beam over the powder bed for the formation of a cross section of the three-dimensional article through fusion of parts of the powder bed, and a controlling computer, in which information is stored concerning consecutive cross sections of the three- dimensional article. A three-dimensional article is formed through consecutive fusions of consecutively formed cross sections of powder layers, successively laid down by the powder dispenser.

[0004] In additive manufacturing it is important to control the powder distribution. It is desirable to distribute a predefined amount of powder over a predetermined area. When fusing the powder at selected locations according to a 3 -dimensional model with an electron beam the powder may start to "smoke", i.e., the electron beam may charge the powder particles, which in turn may start to repel each other and lift from the powder bed. Such "smoke" of powder is highly undesirable because, if it happens, the additive manufacturing process is most likely to be stopped. In order to prohibit powder "smoke" there are different methods used such as preheating of the powder layer to be fused and/or reduced power of the fusing beam. These methods may increase the build time which may be a problem. BRIEF SUMMARY

[0005] An object of the various embodiments of the present invention is to provide a method and apparatus which may eliminate or at least reduce the above mentioned problem in the additive manufacturing process. The abovementioned object is achieved by the features in the method and apparatus claimed herein.

[0006] Various embodiments provide a method for forming a three-dimensional article through successive fusion of parts of at least one layer of a powder bed provided on a work table in a vacuum chamber, which parts corresponds to successive cross sections of the three- dimensional article. The method comprising the steps of: providing a layer of predetermined thickness of powder particles on the work table in the vacuum chamber, providing a coating on at least a portion of the powder particles while the powder is inside the vacuum chamber, which coating is at least partially covering the powder particles, and fusing the powder particles on the work table with an energy beam.

[0007] A non-limiting and exemplary advantage of various embodiments of the present invention is that the powder is coated in the vacuum chamber, i.e., the coating is performed under vacuum conditions, which means that the coated powder may be free of surface oxides. In an example embodiment the vacuum condition means a pressure which is less than lxlO ² mbar. In

-3 another example embodiment the vacuum condition means a pressure which is less than 1x10 ^" mbar. Creating a coated powder without surface oxides means that the electrical conductivity of the powder is much higher compared to a pre coated powder which always has a thin layer of surface oxides and/or surface nitrides. Powder without surface oxides may be sintered at a lower temperature and faster compared to a powder which has surface oxides and/or surface nitrides. The increased conductivity of the powder also means that the probability of powder smoke may be greatly reduced.

[0008] In still other exemplary embodiments the coating is provided prior to providing the powder particles on the work table. The coating may for instance be provided on the top surface of the powder in the powder container which may be provided inside the vacuum chamber. The advantage of providing the coating prior to providing the powder on the work table where the three- dimensional article is to be manufactured in that coated and uncoated powder may be mixed during powder distribution on the powder table, which further may improve the electrical conductivity of the powder layer. [0009] In yet still another example embodiment the coating is provided on the powder particles while the powder particles are provided on the work table. The powder in the powder container may be unsuitable for coating the powder to be distributed over the work table. In such cases the coating of the powder may be performed while the powder is already distributed over the worktable just before the fusion of the powder is to be performed. In another example embodiment the powder may first be coated while being in the powder container and thereafter recoated while being provided on the work table. In an example embodiment the coating while being in the powder container may be of a first material and the coating while being on the work table may be of a second material. The first and second material may be the same or different materials with higher electrical conductivity than the clean powder itself.

[0010] In still a further example embodiment of the present invention the work table may be vibrating while coating the powder on the work table. The advantage of vibrating the work table may be two fold, firstly the particles may rotate which means that a surface which is not visible for the coating device may be coated, secondly the particles may group together more homogenously, i.e., with less voids, thereby creating a better overall electrical conductivity and packing degree of the powder layer.

[0011] In still another example embodiment of the present invention the coating is provided by at least one of the group of: sputtering, chemical vapor deposition, physical vapor deposition, laser ablation, resistive melting of a target, laser beam melting of a target and/or electron beam melting of a target. In an example embodiment the coating is made of the same material as the powder particles. The advantage of providing a coating of the same material as the powder particles is that the material characteristics are not changed.

[0012] In still another example embodiment of the present invention the coating is made of another material compared to the powder particles. This may be advantageous if one wants to tweak the material properties with a doping material. In an example embodiment the material properties may be changed for specific layers of the three dimensional article, for instance the outer layer. One may choose a doping material which may amend the ductility of the surface of the three dimensional article.

[0013] In still another example embodiment the coating has at least one material component in common with the powder particles. If the powder material is TiAl, the coating may be made of Al, Ti, or TiAl. [0014] In still another example embodiment of the present invention the coating has none material component in common with the powder particles. Small amount of some material may change the microstructure of the fused powder layer.

[0015] In still another example embodiment the coating material may have an electrical conductivity which may be higher than the powder particles for increasing the electrical conductivity of the powder. This may be advantageous if the clean powder without coating is having a low conductivity which may not allow fusing with an electron beam which requires an electrically conductive powder. Such powder may for instance be a ceramic or polymer powder. The coating is in such cases made of a material with a low electric resistance.

[0016] In another aspect of the present invention it is provided an apparatus for forming a three-dimensional article through successive fusion of parts of at least one layer of a powder bed provided on a work table, which parts corresponds to successive cross sections of the three- dimensional article, the apparatus comprising: an energy beam source for fusing the powder, a powder distributor for distributing the powder on top of the work table, and a coating device for coating at least a portion of the powder with a coating material. In an example embodiment the work table and the coating device are provided in a vacuum chamber. In an example embodiment the work table may be provided with a vibrator which may be activated when coating the powder on the work table.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0017] Various embodiments of the invention will be further described in the following, in a non-limiting way with reference to the accompanying drawings. Same characters of reference are employed to indicate corresponding similar parts throughout the several figures of the drawings:

[0018] Figure 1 A shows, in a schematic view, an apparatus for producing a three

dimensional product according to prior art;

[0019] Figure IB depicts, in a schematic view, a first embodiment of a device according to the present invention for coating powder for use in an additive manufacturing process;

[0020] Figure 2 depicts, in a schematic view, a second embodiment of a device according to the present invention for coating powder for use in an additive manufacturing process; and

[0021] Figure 3 depicts, in a schematic view, a third embodiment of a device according to the present invention for coating powder for use in an additive manufacturing process. DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0022] To facilitate the understanding of various embodiments of the present invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.

[0023] The term "three-dimensional structures" and the like as used herein refer generally to intended or actually fabricated three-dimensional configurations (e.g. of structural material or materials) that are intended to be used for a particular purpose. Such structures, etc. may, for example, be designed with the aid of a three-dimensional CAD system.

[0024] The term "electron beam" as used herein in various embodiments refers to any charged particle beam. The sources of charged particle beam can include an electron gun, a linear accelerator and so on. Instead of using an electron beam a laser beam may be used for fusing the powder layers.

[0025] Figure 1A depicts an embodiment of a freeform fabrication or additive manufacturing apparatus 21 according to prior art. The apparatus 21 comprising an electron beam gun 6; deflection coils 7; two powder hoppers 4, 14; a build platform 2; a build tank 10; a powder distributor 28; a powder bed 5; and a vacuum chamber 20.

[0026] The vacuum chamber 20 is configured to maintain a vacuum environment by means of a vacuum system, which system may comprise a turbomolecular pump, a scroll pump, an ion pump and one or more valves which are well known to a skilled person in the art and therefore need no further explanation in this context. The vacuum system is controlled by a control unit 8.

[0027] The electron beam gun 6 is generating an electron beam which is used for melting or fusing together powder material provided on the build platform 2. At least a portion of the electron beam gun 6 may be provided in the vacuum chamber 20. The control unit 8 may be used for controlling and managing the electron beam emitted from the electron beam gun 6. At least one focusing coil (not shown), at least one deflection coil 7, an optional coil for astigmatic correction (not shown) and an electron beam power supply (not shown) may be electrically connected to the control unit 8. In an example embodiment of the invention the electron beam gun 6 generates a focusable electron beam with an accelerating voltage which may be about 15-60kV and with a beam power which may be in the range of 3-1 OKw. The pressure in the vacuum chamber may be lxlO ³ mbar or lower when building the three-dimensional article by fusing the powder layer by layer with the energy beam.

[0028] In certain embodiments a laser beam may be used for melting or fusing the powder material. In such case tiltable mirrors may be used in the beam path in order to deflect the laser beam to a predetermined position.

[0029] The powder hoppers 4, 14 comprise the powder material to be provided on the build platform 2 in the build tank 10. The powder material may for instance be pure metals or metal alloys such as titanium, titanium alloys, aluminum, aluminum alloys, stainless steel, Co-Cr alloys, nickel based superalloys, and the like.

[0030] The powder distributor 28 is arranged to lay down a thin layer of the powder material on the build platform 2. During a work cycle the build platform 2 will be lowered successively in relation to a fixed point in the vacuum chamber. In order to make this movement possible, the build platform 2 is in one embodiment of the invention arranged movably in vertical direction, i.e., in the direction indicated by arrow P. This means that the build platform 2 starts in an initial position, in which a first powder material layer of necessary thickness has been laid down. Means for lowering the build platform 2 may for instance be through a servo engine equipped with a gear, adjusting screws, and the like.

[0031] An electron beam may be directed over the build platform 2 causing the first powder layer to fuse in selected locations to form a first cross section of the three-dimensional article. The beam is directed over the build platform 2 from instructions given by the control unit 8. In the control unit 8 instructions for how to control the electron beam for each layer of the three- dimensional article is stored.

[0032] After a first layer is finished, i.e., the fusion of powder material for making a first layer of the three-dimensional article, a second powder layer is provided on the build platform 2. The second powder layer is typically distributed according to the same manner as the previous layer. However, there might be alternative and/or additional methods in the same additive manufacturing machine for distributing powder onto the work table. [0033] After having distributed the second powder layer on the build platform, the energy beam is directed over the work table causing the second powder layer to fuse in selected locations to form a second cross section of the three-dimensional article. Fused portions in the second layer may be bonded to fused portions of the first layer. The fused portions in the first and second layer may be melted together by melting not only the powder in the uppermost layer but also remelting at least a fraction of a thickness of a layer directly below the uppermost layer.

[0034] A first example embodiment of a method according to the present invention comprises at least the step of forming a three-dimensional article 1 10 through successive fusion of parts of at least one layer of a powder bed provided on a work table 102 in an additive manufacturing machine 100, which parts corresponds to successive cross sections of the three-dimensional article. The exemplary embodiment may further comprise the steps of providing a layer of predetermined thickness of powder particles on the work table 102, providing a coating 122 on at least a portion of the powder particles which coating is at least partially covering the powder particles, and fusing the powder particles on the work table with an electron beam 155, as may be understood from Figure IB.

[0035] The coating may be provided prior to providing the powder particles on the work table 102. Figure IB depicts an example embodiment of a method for coating powder particles prior to providing the powder particles on the work table 102.

[0036] In Figure IB the coating of powder may be performed on a top surface of the powder in a powder container 128. The coating may be applied on the powder particles via a coating device 1 16. The coating on the powder particles may be provided by at least one of the group of: sputtering, chemical vapor deposition, physical vapor deposition, laser ablation, resistive melting of a target, laser beam melting of a target and/or electron beam melting of a target. The electron beam or laser beam used for melting the target may be the same electron beam or laser beam which is used for fusing the powder layers.

[0037] In an example embodiment of the present invention a vibrating device 192 may be provided on the powder container 128. The vibrating device may introduce vibrating power on the powder in the powder container. This may cause the powder particles to move around on the top surface of the powder in the powder container 128, allowing for a coating on all surface positions of the powder particles in the top surface layer of the powder in the powder container 128. [0038] When the top surface of the powder 120 in the powder container 128 has been coated a movable floor 124 may be moved upwards by suitable means attached to a piston 126 which in turn is attached to the movable floor 124. The means for moving the movable floor may be an electric motor, a pneumatic motor, a hydraulic motor etc. The movable floor may be increased a certain distance allowing the powder distributor 118 to rake off a predetermined amount of material from the powder container 128 and distribute the powder evenly on top of the work table 102 in the build tank 106. When moving the powder particles from the powder container, the coated powder particles may be mixed with the non-coated powder particles. When distributing a powder layer on top of the work table 102, the powder which is distributed may be an evenly distributed mix of coated and non-coated powder particles allowing for a good control of the material properties when the powder particles is fused.

[0039] The powder rake 1 18 may remove a predetermined thickness of the powder material from a powder tank 128 to the build tank 106. In an example embodiment the rake removes the same thickness as the distance in which the movable floor 124 in the powder tank 128 is raised. In another embodiment a fraction of the powder thickness, corresponding to a fraction of a height in which the movable floor 124 is raised, is removed and transferred from the powder tank 128 to the build tank 106. The powder tank 128 in Figure IB is very similar to the build tank 106. The powder tank 128 comprises a movable bottom plate 124, which may change its position in the same way as the work table 102 in the build tank 106. In Figure IB it is only illustrated one powder tank 128 to the right of the build tank 106. In an another embodiment there may be provided another powder tank 128 to the left of the build tank 106.

[0040] A first type of powder material may be provided in a first powder tank and a second powder material may be provided in a second powder tank. A first coating device may be coating the top surface of the powder in the first powder tank. A second coating device may be coating the top surface of the powder in the second powder tank. The coating material in the first coating device may be different to the coating material in the second coating device. The coating material in the first coating device may be equal to the coating material in the second coating device.

[0041] In an example embodiment the coating on the powder particles may be made of the same material as the powder particles itself. For instance, if the powder particles are made of titanium, the coating may be made of titanium. [0042] In another example embodiment the coating may be made of another material compared to the powder particles. For instance, if the powder particles are made of a ceramic material the coating may be made of metal, for instance copper.

[0043] In still another example embodiment the coating may have at least one material component in common with the powder particles. For instance, if the powder particles are made of TiAl, the coating may be made of titanium or aluminum.

[0044] In yet another example embodiment the coating may have no material component in common with the powder particles. If the powder particles are made of Ti, the coating may be made ofAl.

[0045] The material characteristics may be altered for at least one layer of the three-dimensional article by providing a predetermined thickness of the coating on a predetermined portion of the powder particles. For example, if the powder particles are made of Ti, the coating may be made of Al. The coating may be applied for every second layer, thereby forming a sandwiched structure of the material having different properties for different layers. Instead of applying the coating on the powder particles layer wise, the coating may be applied for different parts of the three dimensional structure to be manufactured, i.e., a first portion may have the material of the powder particles only and a second portion may have the alloy of the powder particle material and the coating material. This means that three-dimensional articles may be manufactured, which have different material characteristics for different portions of the finalized article, although the same powder particles are used throughout the manufacturing process. The application of the coating of the powder particles allows for customized and/or fine-tuned material properties of the three dimensional articles produced with additive manufacturing.

[0046] In still another example embodiment of the present invention, the powder particles may be made of polymer material or ceramic material and the coating may be made of electrically conductive material, for instance metal. The coating material may also be made of carbon. This allows for manufacturing three dimensional articles made essentially of electrically insulating material such as polymer or ceramic material by using an electron beam. Without the electrically conductive coating, which makes the electrically isolating powder particles electrically conductive, the electron beam is not suitable for manufacturing three dimensional articles using powder material which have little or no electric conductivity. By applying a thin coating of a material which is electrically conductive, an electron beam may be used for additively manufacturing a three dimensional article by using powder material which is more or less regarded as electrically isolating.

[0047] In an example embodiment the coating material may have an electrical conductivity which is higher than the powder particles.

[0048] In another embodiment the coating may be provided prior to providing the powder particles in the additive manufacturing machine. The powder coating device may in this embodiment be separated from the additive manufacturing machine.

[0049] In another example embodiment the coating may be provided on the powder particles while the powder particles are provided on the work table 202, see Figure 2. In Figure 2 an energy beam source 208, which may be a laser source or an electron beam source, may be used for fusing powder layers provided on a worktable at selected locations according to a model. A coating device 216 may be used for coating one or several layers of the powder material 212 which is to be fused. The coating process may be performed in between the powder distribution process and the fusing process, i.e., as soon as a powder layer has been provide on the work table 202 the coating device 216 may start to coat the top surface layer of the powder in the build tank 206. A shutter (not illustrated) may be provided in between the coating device and the top surface of the powder on the work table. The shutter may determine which region of the top surface of the powder on the work table to coat.

[0050] In an example embodiment the coating device may be provided movable in a vertical direction and/or a horizontal direction. By moving the coating device 216, the coating device may cover different areas of the powder surface. The coating device may scan the complete top surface of the powder on the work table for coating powder particles on each and every position of the top surface. Varying the height, i.e., moving the coating device in a vertical direction may also influence the covering area of the coating device, a larger distance from the coating device to the top surface of the powder on the work table may cover a larger area compared to if the coating device is provided closer to the top surface of the powder on the work table.

[0051] The work table and/or the build tank may be provided with a vibrator which is activated while coating the powder on the work table.

[0052] The present invention also relates to an apparatus for forming a three-dimensional article through successive fusion of parts of at least one layer of a powder bed provided on a work table, which parts corresponds to successive cross sections of the three-dimensional article. The apparatus comprises an energy beam source for fusing the powder, a powder distributor for distributing the powder on top of the work table, and a coating device for coating at least a portion of the powder with a coating material.

[0053] The energy beam source may be a laser beam source or an electron beam source. The powder distributor may be a rake arranged at a predetermined distance above the top surface of the work table or the top surface of the previous powder layer provided on the work table. Powder material may be provided in front of the rake. The rake may have a length which is longer than the width of the work table. The rake is movable horizontally at a predetermined distance above the top surface of the work table or the top surface of the previous powder layer provided on the work table. Powder material provided in front of the rake is evenly distributed on the work table or the previous powder layer on the work table.

[0054] The coating device may be a sputtering device, chemical vapor deposition device, physical vapor deposition device, laser ablation device, a device for resistive melting of a target, a device for laser beam melting of a target and/or a device for electron beam melting of a target.

[0055] The coating device may be arranged in the apparatus for coating the powder particles before being provided on the work table. An example embodiment of this situation is illustrated in Figure IB.

[0056] The coating device may be arranged in the apparatus for coating the powder while being arranged on the work table. An example embodiment of this situation is shown in Figure 2.

[0057] The coating device may be arranged in the apparatus for coating free-floating powder. An example embodiment of this situation is shown in Figure 3. Powder particles 360 may be provided in an external container 362. The powder particles 360 in the external container 362 may be transferred from the external container 362 to a powder hopper 352. Freely falling powder 370 from the external container 362 to the powder tank 352 may be coated with a coating device 316. Coated powder particles 350 are filling the powder tank 352.

[0058] The coated powder particles may be used in an additive manufacturing process. An schematic illustration of an example embodiment of an additive manufacturing device is illustrated to the right of the powder tank 352. A powder distributor 318 may catch coated powder material 350 from the scree of powder falling out of the powder tank 352 by moving the powder distributor 318 a predetermined distance into the scree of powder. The powder caught by the powder distributor is distributed over the work table 302. A thickness of a powder layer to be fused may be determined by the distance which the work table 302 has been lowered in relation to the previous layer. An energy beam source 308 may melt the powder layer in selected locations according to a model.

[0059] The powder which may be raked from the powder tank 352 to the build tank is distributed evenly on top of the work table inside the build tank. The evenly distribution may be performed with the powder rake 318, but may also be performed with another distribution device such as another rake or a vibration or oscillation mechanism.

[0060] A first layer of the three-dimensional article may be formed by fusing the layer of powder provided on the work table in predetermined locations.

[0061] The work table 302 may be lowered a predetermined distance in order to allow a further layer of powder material to be provided on the already applied powder layers on the work table. The steps of raking new powder material from the powder hopper to the build tank, distribution of the powder on the work table, fusing of the powder layers on predetermined location and lowering of the work table is repeated until the three dimensional article is finalized.

[0062] It should be understood that the present invention is not limited to the above- described embodiments and many modifications are possible within the scope of the following claims. Such modifications may, for example, involve using a different source of energy beam than the exemplified electron beam such as a laser beam. Additionally or otherwise, materials other than metallic powder may be used, such as the non-limiting examples of powder of polymers or powder of ceramics.