The present invention relates to a build plate for use in an additive manufacturing (AM) process and to a method of manufacturing a build plate for use in an additive manufacturing (AM) process.

The main differences between processes for additive manufacturing (AM) are in the way layers are deposited to create parts and in the materials that are used. Some methods melt or soften the material to produce the layers, for example selective laser melting (SLM) or direct metal laser sintering (DMLS), selective laser sintering (SLS), fused deposition modeling (FDM), or fused filament fabrication (FFF), while others cure liquid materials using different sophisticated technologies.

Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power source to sinter powdered material, aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to direct metal laser sintering (DMLS). Both are instantiations of the same concept but differ in technical details. Selective laser melting (SLM) uses a comparable concept, but in SLM the material is fully melted rather than sintered, allowing different properties (crystal structure, porosity, and so on). SLS involves the use of a high power laser (for example, a fibre laser or diode laser) to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that has a desired three-dimensional shape. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed. Because finished part density depends on peak laser power, rather than laser duration, a SLS machine typically uses a pulsed laser.

Selective laser melting (SLM) is a particular rapid prototyping, 3D printing, or Additive Manufacturing (AM) technique designed to use a high power-density laser to melt and fuse metallic powders together. SLM is considered to be a subcategory of Selective Laser Sintering (SLS). The SLM process has the ability to fully melt the metal material into a solid 3D-dimensional part unlike SLS. With selective laser melting, thin layers of atomized fine metal powder are evenly distributed using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen or various gas mixtures at oxygen levels below 5000 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.

Electron-beam additive manufacturing or electron-beam melting (EBM) is a type of additive manufacturing, or 3D printing, for metal parts. The raw material (metal powder or wire) is placed under a vacuum and fused together from heating by an electron beam. This technique is distinct from selective laser sintering as the raw material fuses having completely melted.

Metal powders can be consolidated into a solid mass using an electron beam as the heat source. Parts are manufactured by melting metal powder, layer by layer, with an electron beam in a high vacuum. This powder bed method produces fully dense metal parts directly from metal powder with characteristics of the target material. The EBM machine reads data from a 3D CAD model and lays down successive layers of powdered material. These layers are melted together utilizing a computer-controlled electron beam. In this way it builds up the parts. The process takes place under vacuum, which makes it suited to manufacture parts in reactive materials with a high affinity for oxygen, e.g. titanium. The process is known to operate at higher temperatures (up to 1000 °C), which can lead to differences in phase formation through solidification and solid-state phase transformation.

The powder feedstock is typically pre-alloyed, as opposed to a mixture. That aspect allows classification of EBM with selective laser melting (SLM), where competing technologies like SLS and DMLS require thermal treatment after fabrication. Compared to SLM and DMLS, EBM has a generally superior build rate because of its higher energy density and scanning method. Direct metal laser sintering (DMLS) is an additive manufacturing metal fabrication technology, occasionally referred to as selective laser sintering (SLS) or selective laser melting (SLM), that generates metal prototypes and tools directly from computer aided design (CAD) data. It is unique from SLS or SLM because the process uses a laser to selectively fuse a fine metal powder. DMLS uses a variety of alloys, allowing prototypes to be functional hardware made out of the same material as production components. Since the components are built layer by layer, it is possible to design organic geometries, internal features and challenging passages that could not be cast or otherwise machined. DMLS produces strong, durable metal parts that work well as both functional prototypes or end-use production parts. The DMLS process begins with a 3D CAD model whereby a .stl file is created and sent to the machine’s computer program. The DMLS machine uses a high-powered fiber optic laser or diode laser with a power of up to 1000 watts. Inside the build chamber area, there is a material dispensing platform and a build plate along with a recoater blade used to move new powder over the build plate. The technology fuses metal powder into a solid part by melting it locally using the focused laser beam. Parts are built up additively layer by layer, typically using layers 20 micrometers thick.

In general, additive manufacturing is a manufacturing technology that includes a methodology whereby a heat source melts a feedstock of material which is deposited onto a substrate. Computer control of the movement of the heat source, and the source of the feedstock, makes it possible to build complex components. In additive manufacturing the processing is not limited to the above-mentioned methods in which metal powders are processed but composites or polymers are processed. The heat source can include (but is not limited to) the already mentioned laser beam, an arc, or other plasma-based heat sources.

Typically, the AM part or AM component is produced layer by layer on a build plate. The initial layer is formed directly on a surface of the build plate. This requires that the AM material is identical to or at least compatible with the material of the build plate. Otherwise the initial layer will not properly join to the build plate and/or the AM material will be contaminated by the material of the build plate. However, manufacturing the build plate from a material identical to the AM material, can be very cost-intensive especially when high-grade and expensive AM material is used to produce the AM component.

It is an object of the present invention to provide a build plate and a method of manufacturing a build plate used in additive manufacturing (AM) of an AM part that at least partially mitigate the problems described above.

One or more of these problems are solved by a method and a specially fabricated build plate according to the independent claims. Advantageous embodiments are defined in the sub-claims.

According to the present invention, the body of the build plate and the surface of the build plate on which the AM part is to be built consist of different materials. This is achieved by applying a coating on the surface of the build plate. The body of the build plate is made of a first material and the coating consists of a second material. The AM part is built layer by layer on the coated surface only. The initial layer of AM material is placed onto the coated surface, heated up and preferably melted, and joined to the surface.

The term "material" shall mean metallic and non-metallic materials as well as composite material. The term "material" shall in particular relate to metallic material, for example in the form of pure chemical substances or chemical compounds or in the form of mixtures or complexes of different substances.

The term "AM material" shall mean the material from which the AM part is manufactured. For example, in powder bed additive manufacturing "AM material" shall mean the powder, especially a metallic powder, which is placed on the build plate and melted by the laser or by another heat source.

The second material, i.e. the coating material, and the AM material have to be compatible so that the AM material can be properly joined to the surface of the build plate and that unwanted intermetallic phases are avoided. The body of the build plate is not in direct contact with the AM material and the AM material is not joined to the body of the build plate. Therefore, the AM material and the first material do not need to be compatible to each other with respect to welding or bonding. The invention uses different material for the body of the build plate and for the surface of the build plate. The body of the build plate is the main portion of the build plate and comprises more than 50% by volume, preferably more than 80%, preferably more than 90%, preferably more than 95%, preferably more than 99% by volume of the entire build plate.

The body of the build plate can be made of a first cheap material and only the second material for the surface coating has to be compatible with the AM material. For example, in additive manufacturing of a titanium article a titanium feedstock, for example titanium powder, is placed onto the build plate and melted. The initial layers are thereby joined, welded or otherwise connected to the upper surface of the build plate. In the prior art, the whole build plate has to be made of titanium in order to ensure the required compatibility of the build plate material and the titanium feedstock. When using such high-grade and high-quality material as titanium, the costs for the build plate are considerably high.

The invention suggests to apply to the surface of the build plate a metal coating. Only this coating layer has to be compatible with the AM material. In the above example, a thin titanium coating could be applied to the surface of the build plate whereas the body of the build plate is made of a relative cheap material such as steel. Since the AM part is then formed directly on the titanium surface layer, intermetallic phases and a contamination of the AM part are prevented.

Compatibility of the AM material and the second material, that is of the material used for manufacturing the AM part and the material of the applied surface coating, shall mean that both materials can be joined together by the laser or heat source used in the additive manufacturing process. Further, intermetallic phases which negatively affect the properties and/or quality of the manufactured AM part shall be prevented.

In an embodiment, the additive manufacturing process comprises selective laser melting.

In another preferred embodiment, the metal coating is applied by thermal spraying. Advantageously, this allows to distribute the coating evenly and homogeneously. Further, thermal spraying allows to join many material combinations. In general, all or most metals can be thermally sprayed onto a metal substrate. Therefore, thermal spraying allows to select the optimal material for the body of the build plate and the optimal material for the metal coating.

In yet another embodiment, the metal coating is applied by a coating deposition method. Advantageously, these deposition methods allow to produce a coating or a film of a controlled thickness and, in turn, to decrease recycling or manufacturing costs.

In another preferred embodiment, the coating deposition method is cold spraying. This method employs low temperatures compared to other spraying techniques and allows to keep the depositing powder in a solid state. This is advantageous as the powder is not molten and therefore can retain its primary properties during and after deposition. Furthermore, it allows for depositing dissimilar materials, where there is a need for creating a composite coating made up from the materials with notably differing properties.

In yet another embodiment, the metal coating is applied by deposition welding. Advantageously, this method incorporates both laser deposition welding and TIG deposition welding and therefore allows to use a wide variety of metals to be deposited onto the build plate surface.

In another embodiment, the build plate and the metal coating comprise a composite material.

In yet another embodiment, the metal coating comprises any of titanium, copper, nickel, aluminium, steel or stainless steel or any alloys thereof. Silver, gold or other precious metals can also be used for the metal coating. This arrangement is in particular advantageous since expensive material such as titanium or nickel are used for the surface coating only. The body of the build plate can be fabricated of another material.

The body of the build plate is preferably made of steel, stainless steel, copper, aluminium or alloys thereof. Copper may be used as it shows a very good thermal conductivity. On the other hand, stainless steel has a low thermal conductivity. Depending on the thermal requirements, it can be advantageous to make the body of the build plate from copper or from stainless steel. Aluminium shows a good thermal conductivity, too, and in addition it has the advantages of being light and stable. Or if costs are an issue, one might select steel as it is stable and relative cheap.

The second material, that is the material of the surface coating, is preferably selected such that a high affinity between the AM material and the surface coating is achieved. The first material, that is the material of the body of the build plate, is preferably selected to optimize other parameters, such as heat conductivity, rigidity, stiffness, or material costs. The invention decouples the requirements to the body of the build plate from the requirements to the surface of the build plate. Both can be individually optimized.

For example, when heat control is to be optimized and when a better control of the heat balance is required, it is advantageous to use as the first material for the build plate body a material with good thermal conductivity and proper heat conducting properties, for example copper. Thus, according to this embodiment it is possible to combine two or more materials with different thermal conductivity, such as copper and titanium, wherein the bulk copper of the build plate distributes the heat more evenly, whereas the titanium coating allows for affinity of the surface with the AM material and with the AM part to be manufactured.

In another embodiment, the build plate comprises one or more cooling channels. This is advantageous when extra ventilation and/or heat dissipation and, in turn, process efficiency optimization is needed. It is also possible to selectively heat the build plate to reach different properties in the generated material on different areas on the build plate. As such, the build plate may comprise separate heating channels through which hot fluid may flow. Alternatively, hot fluid may flow through the cooling channels when required.

The invention is explained below with the aid of an embodiment shown in the drawings.

The drawings show in: Figure 1 a rough schematic view of a system for powder bed additive manufacturing by Selective Laser Sintering (SLS) or Selective Laser Melting (SLM).

Figure 2 a rough schematic view of the surface before applying the inventive coating.

Figure 3 a rough schematic view of the surface after applying the inventive coating.

Figure 4 a rough schematic view of the thermal spraying process.

Figure 5 a cross-section through a build plate showing cooling channels.

Figure 1 shows an apparatus for additive manufacturing 1 according to an embodiment of the present invention. The apparatus shown in figure 1 is an apparatus for use in selective laser melting (SLM) or selective laser sintering (SLS). However, it is clear that the present invention is applicable for all methods and devices for additive manufacturing disclosed in this description.

The apparatus 1 comprises a production cylinder 2, a delivery cylinder 3 and a heat source 4.

The heat source 4 according to a preferred embodiment comprises a laser and a corresponding scanner system for melting metal powder (not shown). However, other heat sources, such as an electron beam in combination with a scanning system, are also possible.

The delivery cylinder 3 comprises a housing 5 with a wall 6 wherein a powder delivery piston 7 is disposed inside the housing 5.

A powder applying device 8, for example a roller, is provided for pushing a metal powder from the delivery cylinder 3 to the production cylinder 2.

The production cylinder 3 comprises a housing 9 with a wall 10.

A lift table 11 with a build platform 12 is disposed inside the wall 10 of the housing 9. The lift table 11 and the corresponding build platform 12 embody a fabrication piston. The wall 10 of the housing 9 and the build platform 12 of the lift table 11 of the production cylinder 2 define a build space 13.

The build space 13 houses the fabrication powder bed and therefore the AM part being fabricated.

The build plate 19 is a separate part connected to the build platform 12. The build platform 12 remains permanently in the machine irrespective of what component is being made, and a suitable build plate 19 is placed over the build platform 12 for each manufacturing process. In the illustrated embodiment the bulk of the build plate 19 is made from a different material to the component (AM part) to be manufactured, with a surface coating of a material that is the same material as the component to be manufactured. This reduces the costs of the build plate 19 in the event that the component is to be made from an expensive alloy. The surface coating has been applied by thermal spraying, preferably by cold spraying.

The build plate provides a flat, planar surface on which the additive manufacture process can take place. In the prior art, use of a build plate that is made from the same material as the component (AM part) to be manufactured ensured that no contamination with undesired material occurred during the manufacturing process. But as explained above, this is costly when the component is made of expensive highgrade material. According to a preferred embodiment of the invention a coating is applied to the surface of the build plate wherein the coating material matches the material of the to-be-built components while it is different from the material of the body of the build plate.

Furthermore, a processing chamber 17 is provided surrounding the production cylinder 2, the delivery cylinder 3 and the heat source 4.

The manufacturing space 20 according to the present invention is therefore the build space 13 of the production cylinder 2 defined by the 10 wall of the housing 9 of the production cylinder 2 and the lift table 11 with the build platform 12 disposed inside the wall 10 of the housing 9, and/or the manufacturing space is the room within the processing chamber 17. The apparatus 1 may be used to produce parts by Selective Laser Melting (SLM) in the conventional way. In this case the heat source 4 is a high-power density laser. Briefly, at the start of the process, a first layer of powder is applied on top of the build plate 19 by the powder applying device 8. Optionally, the powder and build plate 19 may be predetermined to a temperature below the melting point of the powder, at which the process is to take place. The scanner system then controls the laser 4 to heat selected portions of the first layer of powder in order to melt these portions. A further layer of powder is then applied on top of the powder bed, and when the powder is to be preheated then the laser 4 is controlled to heat the further layer of powder to the predetermined temperature. The laser 4 is then controlled to further heat selected regions so as to melt the selected regions. The process is repeated until all portions of the component (AM part) that are required to be fused have been melted by the laser 4. At this point, formation of the part is complete, and the completed AM part and build plate 19 are removed from the manufacturing space 20. The AM part 13 is then removed from the build plate 19, for example using a band saw.

After the removal of the AM part from the build plate 19, the planar surface of the build plate may have been degraded, forming a non-planar surface 21 as shown in Figure 2. This is due to partial sintering of the AM part onto the surface of the build plate 19. The AM part component is generally removed by sawing, which leaves a non-planar surface.

Depending upon the severity of the degrading of the planar surface, it may be necessary to regenerate the surface to make it substantially planar (flat) prior to the next use. Hitherto, regeneration of build plates has typically been performed by mechanical processing, such as machining. This leads to a thinning of the build plate 19, and after a number of repeated regeneration cycles the build plate 19 has to be discarded because the thickness of the plate becomes unsuitable to support the printed component.

As described above, the invention suggests to provide a metal coating onto the build plate wherein the material of the metal coating matches the AM material. This inventive provision of a surface coating 22 on the build plate has the additional benefit that, as shown in figure 3, such a coating covers the defects that make the surface non-planar, thereby providing a new, planar surface on the build plate 19. The metallic coating may be the same material as the AM material from which the AM part is manufactured or a material compatible with the AM material. In any case the coating material and the AM material are compatible in a sense that the AM material properly joins to the surface of the build plate and that the AM material is not contaminated or negatively affected by the surface material. However, the materials of the metallic surface coating and the rest of the build plate are different.

Figure 4 shows an example system 30 for applying a coating to a build plate by thermal spraying of the metal onto the build plate 19 to get a surface 22 which is compatible with the AM material. Further, the sprayed surface layer 22 helps to restore the planarity of the surface, as shown in Figure 3. It comprises of a spray gun 24 and a feeder 26 that supplies metal powder to the torch through conduits. The spray gun 24 is connected to a power supply 28 and a media supply 29 that supplies gases or liquids for the generation of the flame or plasma jet within the spray gun 24. The feeder supplies the metal powder into the flame or plasma jet within the spray gun 24, and a continuous stream of molten metal powder 25 suspended in a jet of flame or plasma is ejected from the feeder 24 through a feeder nozzle 27 and covers the non-planar surface of the build plate 19 to form a layer of metallic coating 22. A control unit for controlling all parts of the spray system can be a standalone unit, or optionally there can be a separate control unit for each part of the system. The spray gun 24 may be operated by a robot (not shown).

Although the system shown in figure 4 is for applying a coating by thermal spraying, it will be understood that other coating methods may also be suitable. For example, suitable coating deposition methods may further include gas dynamic cold spraying or deposition welding. These are known methods and include either acceleration of solid particles in a supersonic gas to cause plastic deformation or using electrodes to melt the solid particles prior to deposition. In all cases the molten (or plastically deformed) particles are deposited onto the surface 21 of the building plate 19. A layer of metallic coating 22 is formed on the surface of the building plate, as shown in Figure 3.

In some embodiments, it may be necessary to lightly machine the coated surface to produce a sufficiently flat surface. However, the amount of machining required is reduced by the presence of the coating, and a reduction in the overall thickness is avoided by only machining after application of the coating. In operation, a process of using the build plate is as follows. First, the AM part is produced by SLM using the apparatus shown in figure 1 . However, it will be understood that any suitable additive manufacture technique or method could also be used. The completed AM part 13 and build plate 19 are then removed from the manufacturing space 20. The AM part 13 is then detached from the build plate 19 using any suitable technique, for instance a band saw.

As discussed above, the surface coating 22 of the build plate 19 is normally selected to match the composition of the printed AM part. Thus, for example, if the printed AM part is made, without limitation, of steel or titanium alloy, it is desirable that the coating on the build plate is also made of the same steel or titanium alloy.

In some embodiments it may also be desirable to improve the process parameters by changing physical properties of the body of the build plate such as its thermal conductivity. In this case the build plate 19 can be made of a material with enhanced thermal conductivity, for instance copper. In another embodiment the body of the build plate is made of a material with low thermal conductivity.

However, when the steel or titanium alloy is to be printed, it is required that the composition of the surface of the build plate matches the composition of the printed AM part. Therefore the build plate can be made of copper and can be covered with a thin layer of the matching steel or titanium alloy, thus resulting in the formation of a so called composite build plate. It is understood that any suitable metal and/or alloy can be used to form the coating on the build plate. The second material for the coating depends on the AM material and composition of the printed AM part. For example, the second material includes without limitation include titanium, nickel, aluminium, copper and steel.

In another embodiment, in order to improve the thermal conductivity of the build plate, the build plate 19 may have a plurality of cooling channels 18, as shown in figure 5. These channels may be positioned in a longitudinal or transverse direction. The channels 18 may accelerate the removal of heat from the build plate by allowing air circulation. Alternatively, a liquid coolant may be pumped through the cooling channels 18, which may allow further control over the rate of heat removal by controlling the flow rate of the coolant in the channels. If it is desired to heat a part of the build plate, then hot fluid may be passed through one or more of the cooling channels. In other embodiments, separate heating channels may be provided. Unless otherwise stated, all technical features described in connection with the build plate are applicable in connection with method steps for the method according to the present invention.

Reference signs

1 device

2 production cylinder (build cylinder)

3 delivery cylinder (supply cylinder)

4 heat source

5 housing

6 wall

7 powder delivery piston

8 powder applying device

9 housing

10 wall

11 lift table

12 build platform

13 build space

17 processing chamber

18 cooling channels

19 build plate

20 manufacturing space

21 non-planar surface

22 coating

23 planar surface

24 spray gun

25 molten metal powder

26 feeder

27 nozzle

28 power supply

29 media supply