Patent Grant
9079248
1. Related Field
The present invention relates to a method for increasing the resolution in additively manufactured three-dimensional articles according to the preamble of claim 1.
2. Description of Related Art
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.
Such an apparatus may comprise a work table on which said 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 said powder bed for the formation of a cross section of said three-dimensional article through fusion of parts of said 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.
When an energy beam in the form of an electron beam hits the powder, a charge distribution develops around the electron target area. Desirably, this charge will be led through a produced part of the article to be made and/or the powder bed towards ground. If the charge distribution density exceeds a critical limit, an electrical field having field strength above a predetermined level will develop around the position where the beam is radiating. The electrical field having electrical field strength above the predetermined level will be referred to as Emax. An electrical field will cause the powder particles to repel each other such that particles leave the uppermost surface layer of the particle and create a distribution of particles floating above the surface. The floating particles resemble a cloud positioned above the surface. When the electrical field has field strength above Emax, the electrical field, i.e. the particle cloud or smoke of powder, will influence the resolution of the device in a negative way. This is partly due to the fact that the particles in the particle cloud will diverge the electron beam. When the electrical field has field strength below Emax, the electrical field, i.e. the particle cloud, will not influence the resolution of the device in a significant way. A field strength below Emax is thus desirable.
Since the particles are charged they will seek a ground contact and thereby some may leave the cloud and will then contaminate different parts of the device being positioned inside the vacuum chamber. A result of such a critical electrical field is that the structure of the powder surface will be destroyed.
One solution to the problem of avoiding charging of powder is disclosed in WO 2008/147306. In said document the amount of ions present in close vicinity to the position where the electron beam radiates the powder material is controlled. This is according to one example embodiment performed by introducing a supplementary gas into the vacuum chamber, which is capable of producing ions when irradiated by the electron beam.
The problem with said solution is that the supplementary gas increases the electron beam spot dimension and thereby affects the resolution of the additive manufacturing process in a negative way.
An object of the invention is to provide a method and apparatus for increasing the resolution in the additive manufacturing process while maintaining the electrical field distribution in the powder material so that free floating particles is prohibited.
The abovementioned object is achieved by the features in the method according to claim 1.
In a first aspect of the invention it is provided a method for increasing the resolution when forming a three-dimensional article through successive fusion of parts of a powder bed, which parts corresponds to successive cross sections of the three-dimensional article. Said method comprising the steps of: providing a vacuum chamber, providing an electron gun, providing a first powder layer on a work table inside said vacuum chamber, directing an electron beam from said electron gun over said work table causing said first powder layer to fuse in selected locations to form a first cross section of said three-dimensional article, providing a second powder layer on said work table, directing the electron beam over said work table causing said second powder layer to fuse in selected locations to form a second cross section of said three-dimensional article, wherein said second layer is bonded to said first layer, reducing the pressure in the vacuum chamber from a first pressure level to a second pressure level between the steps of providing said first powder layer and said second powder layer.
An advantage of the present invention is that the pressure level and/or the amount of ions in the vacuum chamber can be adapted to the process step, i.e., by providing a relatively high first pressure level during a preheating step when smoke of powder is most likely and thereafter decreasing the pressure to a relatively low second pressure level. By so doing, the preheating step is performed at the first pressure level where the number of ions present is high enough in order to prohibit smoke of powder. Since the preheating can be performed with any kind of energy beam quality, the spread of the energy beam due to the interaction of the electron beam with the gas atoms will not affect the quality of the final three dimensional article. The actual fusing of said powder is performed at the second pressure level, which is considerably smaller than said first pressure level, so that the electron beam resolution can be as high as possible. The number of gas atoms in the vacuum chamber during the fusion step is much smaller than during the preheating step. Since the powder is already sintered, particle smoke is not an issue. A decreased pressure level during the fusion step will affect the quality of the electron beam, i.e., the lower the pressure level the better the beam quality. This in turn will increase the resolution of the final three dimensional article to be produced.
Another advantage of the present invention is that a lifetime of a filament of an electron beam source can be prolonged since a mean pressure during the additive manufacturing process may be decreased compared to the state of the art technique. In an example embodiment the relatively high first pressure level may only be present during less than 20% of the total process time of each layer. The reminder of the process time for each layer the second pressure level is present, which is considerably lower than the first pressure level. In an example embodiment said second pressure level may be present during more than 80% of the process time of each layer. The relatively high first pressure level may only be present in a first part of a preheating step. The second pressure level, being considerably lower than said first pressure level may be present during the complete fusion step and a second part of the preheating step and any other process step for each layer.
In one example embodiment of the present invention said method further comprising the step of providing a first supplementary gas into the vacuum chamber, which first supplementary gas is capable of providing ions when irradiated by the electron beam during said preheating.
An advantage of said embodiment is that one can provide an amount of gas not more than necessary for suppressing the smoke of powder.
In still another example embodiment of the present invention said first supplementary gas is at least one inert gas, wherein said inert gas is one or more in the group of: Helium, Argon, Carbon dioxide, Nitrogen, Neon, Krypton, Xenon, Radon.
An advantage of the present invention is that other gases may be used in the vacuum chamber. Usually gases with light atoms, for instance helium, were used since gases with heavier atoms scattered the electron beam too much resulting in bad resolution. A decrease of pressure from the preheating step to the fusion step makes it possible to use gases with heavier atoms while maintaining the resolution of the electron beam. Argon, which has heavier atoms compared to Helium, is a better gas for prohibiting smoke of powder. Since the quality of the beam during preheating is not important Argon or any one of or combinations of Carbon dioxide, Nitrogen, Neon, Krypton, Xenon, Radon may be used to prohibit powder smoke during preheating. If the pressure during fusion is decreased sufficiently the electron beam quality will not be degraded as would have been the case if the pressure would have been the same during preheating and fusion.
In yet another example embodiment of the present invention said method further comprising the step of providing a second supplementary gas into the vacuum chamber, which second supplementary gas is capable of, at least when having been exposed to the electron beam, reacting chemically and/or physically with the powder layer on the work table.
Since the pressure in the vacuum chamber has been decreased during the fusion step to at around 10−5 mbar or lower, a second supplementary gas may be provided into the chamber during the fusion step. Said second supplementary gas may influence the characteristic of the final article by for example increasing its strength, ductility or any other mechanical property.
In still another example embodiment of the present invention said second supplementary gas is one or more selected from the group of: Hydrogen, deuterium, hydrocarbons, gaseous organic compounds, ammonia, nitrogen, oxygen, carbon monoxide, carbon dioxide, nitrogen oxides and/or nitrous oxide.
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:
To facilitate the understanding of this 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.
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.
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.
Said 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.
The vacuum chamber 20 is capable of maintaining 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.
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 said control unit 8. In an example embodiment of the invention said electron beam gun 6 generates a focusable electron beam with an accelerating voltage of about 15-60 kV and with a beam power in the range of 3-10 Kw. The pressure in the vacuum chamber may be 10−3 mbar or lower when building the three-dimensional article by fusing the powder layer by layer with the energy beam.
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 etc.
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 etc.
An electron beam may be directed over said build platform 2 causing said first powder layer to fuse in selected locations to form a first cross section of said three-dimensional article. The beam is directed over said 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.
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 said build platform 2. The second powder layer is preferably distributed according to the same manner as the previous layer. However, there might be alternative methods in the same additive manufacturing machine for distributing powder onto the work table. For instance, a first layer may be provided by means of a first powder distributor 28, a second layer may be provided by another powder distributor. The design of the powder distributor is automatically changed according to instructions from the control unit 8. A powder distributor 28 in the form of a single rake system, i.e., where one rake is catching powder fallen down from both a left powder hopper 4 and a right powder hopper 14, the rake as such can change design.
After having distributed the second powder layer on the build platform, the energy beam is directed over said work table causing said second powder layer to fuse in selected locations to form a second cross section of said three-dimensional article. Fused portions in the second layer may be bonded to fused portions of said 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 said uppermost layer.
In the case where an electron beam is used, it is necessary to consider the charge distribution that is created in the powder as the electrons hit the powder bed 5. The invention is, at least partly, based on the realization that the charge distribution density depends on the following parameters: beam current, electron velocity (which is given by the accelerating voltage), beam scanning velocity, powder material and electrical conductivity of the powder, i.e. mainly the electrical conductivity between the powder grains. The latter is in turn a function of several parameters, such as temperature, degree of sintering and powder grain size/size distribution.
Thus, for a given powder, i.e. a powder of a certain material with a certain grain size distribution, and a given accelerating voltage, it is possible, by varying the beam current (and thus the beam power) and the beam scanning velocity, to affect the charge distribution.
By varying these parameters in a controlled way, the electrical conductivity of the powder can gradually be increased by increasing the temperature of the powder. A powder that has a high temperature obtains a considerably higher conductivity which results in a lower density of the charge distribution since the charges quickly can diffuse over a large region. This effect is enhanced if the powder is allowed to be slightly sintered during the pre-heating process. When the conductivity has become sufficiently high, the powder can be fused together, i.e. melted or fully sintered, with predetermined values of the beam current and beam scanning velocity.
A general function for describing the charge density that develops in the powder in an arbitrary scanning procedure will be a rather complex function of time and beam position since the charge density generated along one scanned path will be affected by the charge density generated along another scanned path if these paths are not very well separated in space and time. Thus, charge summation effects between different paths must be taken into account.
A next process step is the preheating step 32. During preheating the powder provided on the build platform 2 is about to be brought to an appropriate temperature before a fusing step 33. This preheating step is performed by scanning the electron beam over the powder bed in an appropriate manner for heating the powder bed without creating powder smoke. This is performed by leaving enough spacing between two consecutive scanning lines so the summation of charges in a first scanning line is not affecting the charges provided in the second scanning line. During preheating the pressure in the additive manufacturing apparatus 21 is at a first pressure level P2, which is high enough for suppressing or eliminating smoke of powder. The first pressure level P2 may be around 10−3 mbar. The pressure level may be at a constant high level throughout the preheating. Alternatively the pressure level is varying during said preheating but always high enough in order to suppress smoke of powder. In
During a fusion step 33 the pressure in the additive manufacturing apparatus 21 may be kept as low as possible and in
In the inventive method it is provided a reduction of the pressure in the vacuum chamber of said additive manufacturing apparatus from a first pressure level, here indicated by said first pressure level P2, to a second pressure level, here indicated by P3, between the steps of providing said first powder layer and said second powder layer.
The preheating of the powder bed may be performed at least partially with said first pressure level P2 which is higher than said second pressure level P3. Said powder bed may be fused in said selected locations at least partially with said second pressure level P3.
When the fusion step is finalized the powder and fused powder may need some heating in order to be within a predetermined temperature interval. This heating step is denoted in
When the second valve 54 is open said valve 23 is closed. With the second valve 54 open and the valve 23 closed gas from the gas source 52 will be provided into the reservoir 56. The pressure and time said second valve is open will determine the amount of gas provided to the reservoir given a constant volume of the reservoir 56. In an example embodiment said time said valve 54 is opened may be 1 second.
When the second valve 54 is closed said valve 23 may be opened in order to let the gas from the reservoir into the additive manufacturing device 21. In an example embodiment said time said valve 23 is opened may be 1 second.
The embodiment in
A next process step is the preheating step 62. During preheating the powder provided on the build platform 2 is about to be brought to an appropriate temperature before a fusing step 63. This preheating step is performed by scanning the electron beam over the powder bed in an appropriate manner for heating the powder bed without creating powder smoke as described in connection with
Since smoke of powder is most likely to be present in the beginning of the preheating when no powder is sintered at all said decrease of gas pressure will not affect the suppression of smoke of powder.
During the fusion step 63 the pressure in the additive manufacturing apparatus 21 may be kept as low as possible and in
In an alternative embodiment the first pressure level P2 may be around 10−2 mbar and the second pressure level P1 may be 10−3 mbar or lower. In still another example embodiment the first pressure level P2 may be 2×10−3 mbar and the second pressure level P1 may be 10−4 mbar. In yet another example embodiment said first pressure level P2 may be 2×10−2 mbar and said second pressure level P1 may be 2×10−5 mbar. The first pressure level P2 may be kept at a relatively high pressure in order to ensure a safe suppression of smoke of powder which is very material dependent given the same power of the electron beam. The second pressure level P1 may be kept at a relatively low pressure in order to keep the electron beam quality as good as possible, i.e., as little as possible blurred by interaction of the atoms during the path from a electron beam filament to the powder bed.
In the inventive method it is provided a reduction of the pressure in the vacuum chamber of said additive manufacturing apparatus from a first pressure level, here in
The preheating of the powder bed may, as in
When the fusion step 63 is finalized the powder and fused powder may need some heating in order to be within a predetermined temperature interval. This heating step is denoted in
A first supplementary gas provided into the vacuum chamber may be capable of providing ions when irradiated by the electron beam during said preheating. Said first supplementary gas may be at least one inert gas. Said inert gas is one or more in the group of: Helium, Argon, Carbon dioxide, Nitrogen, Neon, Krypton, Xenon, Radon.
Said first pressure level P2 may be about 10−3 mbar. The first pressure level may be changed depending on the type of first supplementary gas chosen, a lighter atom may need a somewhat higher pressure than a heavier atom chosen among the supplementary gases given above.
A second supplementary gas may be provided into the vacuum chamber of said additive manufacturing device 21. Said second supplementary gas may be capable of, at least when having been exposed to the electron beam, reacting chemically and/or physically with the powder layer on the work table.
The first and second supplementary gases may be present at the same time in the vacuum chamber.
The second supplementary gas may be provided to the vacuum chamber of said additive manufacturing apparatus 21 during the fusion step. The second supplementary gas may be let into the vacuum chamber at the end of the preheating step in order to ensure a constant and sufficient amount of second supplementary gas throughout the fusion step. This is to ensure that all powder layers will be melted by the same amount of second supplementary gas which in turn will ensure the same material property throughout the three-dimensional article to be produced.
The second supplementary gas may be one or more gases selected from the group of: Hydrogen, deuterium, hydrocarbons, gaseous organic compounds, ammonia, nitrogen, oxygen, carbon monoxide, carbon dioxide, nitrogen oxides and/or nitrous oxide.
The second pressure level P1, P3 may be about 10−5 mbar. A heavier gas atom may need a somewhat lower pressure than a lighter atom. This has to do with the fact that a heavier atom may scatter the electron beam more than a light gas atom and therefore less heavier atoms is needed in order to reach the same resolution of the electron beam as compared to the lighter gas atoms.
The powder provided on the work table may be made of metal.
A mean pressure level during the preheating may be higher than a mean pressure level during the fusion of the selected locations. The reason of having a higher pressure level of said at least first supplementary gas is to reduce or eliminate the likelihood of powder smoke. A certain number of ions are needed in the vacuum chamber in order to neutralize or decreasing the amount of the charges in the powder created by the ion beam when hitting said powder. For this reason there have to be a certain gas pressure in the vacuum chamber either by letting in said first supplementary gas as disclosed above or by providing an ion generator to said vacuum chamber which ions are let in to the vacuum chamber during said preheating step.
For one or a plurality of powder layers the preheating of powder layer N may be performed at a first pressure level and the fusion of powder layer N may be performed at a second pressure level, where said first pressure level may be higher than said second pressure level, where N may be any integer≧1.
During the fusion one wants to keep the pressure level of the gases in the vacuum chamber at a minimum since the gas atoms may more or less influence the resolution of the electron beam. Depending on the type of ions present in the vacuum chamber there may be some differences in the pressure allowed for maintaining the same electron beam resolution for reasons as explained above.
In another aspect of the present invention it is provided an apparatus for forming a three-dimensional article through successive fusion of parts of a powder bed, which parts corresponds to successive cross sections of the three-dimensional article. Said apparatus comprising: a vacuum chamber, an electron gun, a powder distributor for providing a first powder layer on a work table inside said vacuum chamber, a control unit for controlling an electron beam path over said work table causing said first powder layer to fuse in selected locations to form a first cross section of said three-dimensional article, wherein said control unit is configured to reduce the pressure in the vacuum chamber from a first pressure level, which is used during at least a part of a preheating of the powder layer, to a second pressure level, which is used during at least a part of the fusion of the powder layer.
The electron beam path may be controlled by means of one or a plurality of deflection coils arranged in the electron beam gun which is well known to a skilled person in the art. The control unit may control the deflection coil(s) according to the pattern (selected locations) which is to be fused on said powder layer.
Preheating may be used for heating the powder layer to a predetermined temperature and the pressure in the vacuum chamber may be higher during preheating compared to when the fusion of the powder is taking place. The control unit may control the vacuum pump(s) and one or a plurality of gas supply valves. A supplementary gas may be let into the vacuum chamber during preheating. Said supplementary gas may for instance be a gas which is capable of providing ions when irradiated by the electron beam during said preheating. Said supplementary gas may be Helium, Argon, Carbon dioxide, Nitrogen, Neon, Krypton, Xenon, Radon.
The control unit may change the pressure in the vacuum chamber from said first pressure level during preheating, to a second pressure level during fusion. The second pressure level has a lower pressure than the first pressure level. Fusion may take place with as little as possible disturbance of the electron beam by the molecules present in the vacuum chamber. For one or a plurality of powder layers the preheating of powder layer N may be performed at a first pressure level and the fusion of powder layer N may be performed at a second pressure level, where said first pressure level may be higher than said second pressure level, where N may be any integer≧1.
The 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 laser beam. Other materials than metallic powder may be used such as powder of polymers or powder of ceramics.
This application is a national stage application, filed under 35 U.S.C. §371, of International Application No. PCT/EP2012/074383, filed Dec. 4, 2012, which claims priority to U.S. Provisional Application No. 61/580,768, filed Dec. 28, 2011, the contents of both which are hereby incorporated by reference in their entirety.
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| Number | Date | Country | |
|---|---|---|---|
| 20140370323 A1 | Dec 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61580768 | Dec 2011 | US |
