Calibration technology for the irradiation system of additive manufacturing equipment

Abstract

A method for calibrating an illumination system of an additive manufacturing apparatus is provided, the method including applying a powder layer to a work area of ​​the apparatus, measuring a topography of at least a portion of the powder layer, determining at least one lateral correction value for an illumination beam of the illumination system based on the measured topography, and applying the lateral correction value to scan data used by the illumination system to scan the illumination beam over the work area. Further provided is an additive manufacturing apparatus.

Description

[Technical Field]

[0001] The present invention relates generally to the calibration of an illumination system of an additive manufacturing apparatus, which may be an apparatus for powder bed fusion bonding, such as, but not limited to, selective laser sintering, selective laser melting or electron beam melting. [Background technology]

[0002] Powder bed fusion is an additive layering process that can process powdered raw materials, especially metal and / or ceramic, into three-dimensional workpieces with complex shapes. For this purpose, a layer of raw material powder is applied to a carrier, and the powder layer is irradiated with radiation (e.g., laser or particle light) in a selective manner depending on the desired shape of the workpiece to be manufactured. The radiation that penetrates the powder layer heats the raw material powder particles, resulting in their melting or sintering. Additional layers of raw material powder are then applied successively to the previously irradiated carrier layer until the workpiece has the desired shape and size. Powder bed fusion can be used to manufacture or repair prototypes, tools, replacement parts, high-value components, or medical prostheses, such as dental or orthopedic prostheses, based on CAD data. Examples of powder bed fusion techniques include selective laser melting, selective laser sintering, and electron beam melting.

[0003] Apparatuses for manufacturing one or more workpieces according to the above techniques are known. For example, EP2961549A1 and EP2878402A1 describe apparatuses for manufacturing three-dimensional workpieces according to the selective laser melting technique. The general principles described in these documents are also applicable to the technique of the present disclosure.

[0004] Several calibration techniques are known for accurately determining the position of the irradiation beam relative to the workpiece area onto which it is irradiated. In particular, when the irradiation system of an additive manufacturing machine emits multiple irradiation beams that can be scanned independently, it is important to calibrate each irradiation beam relative to the other. In other words, when the irradiation beams are directed to the same location on the workpiece area by the machine's control unit, it is important to achieve the desired workpiece quality (without unwanted edges or other irregularities in the finished workpiece structure) that all irradiation beams hit the same location on the workpiece area.

[0005] For example, WO2019 / 161886A1 describes a method for aligning a multi-beam irradiation system used in an apparatus for manufacturing three-dimensional workpieces by irradiating a layer of raw material powder with electromagnetic radiation or particle radiation. The method in WO2019 / 161886A1 includes the steps of: (i) applying a first raw material powder layer on a carrier to define an irradiation surface onto which a radiation beam emitted from the irradiation system is irradiated; (ii) generating a first test structure in the first raw material powder layer in an overlapping region of the irradiation surface using a first radiation beam emitted from a calibrated first irradiation unit of the irradiation system; (iii) generating a second test structure in the first raw material powder layer in an overlapping region of the irradiation surface using a second radiation beam emitted from a calibrated second irradiation unit of the irradiation system; (iv) determining an offset between the first test structure and the second test structure in the irradiation surface; and (v) aligning at least one of the first and second calibrated irradiation units based on the determined offset between the first and second test structures so that the offset does not exceed a threshold value.

[0006] Furthermore, EP3907021A1 describes a method for automatic alignment of scanning optics for additive manufacturing, which includes the steps of irradiating at least one radiation beam onto a target area of ​​a layer of powder material provided on a build platform, irradiating at least one radiation beam onto a calibration area of ​​the layer of powder material, guiding the first radiation beam onto an intermediate top surface using a first scanning optics to thereby fuse a first calibration pattern onto the intermediate top surface, guiding the second radiation beam onto the intermediate top surface using a second scanning optics to thereby fuse a second calibration pattern onto the intermediate top surface, acquiring at least one image of the intermediate top surface, using the at least one image to identify image points associated with geometric features of the calibration pattern from the image points, deriving a spatial offset between the second geometric features, and aligning the at least one scanning optics taking the spatial offset into account.

[0007] Furthermore, calibration methods are known that use a calibration foil that can be brought into the working area of ​​the additive manufacturing machine and that prints predetermined patterns onto the calibration foil using one or more lasers, the relative positions of which can be observed, for example, by a camera, and one or more correction values ​​can be determined for each laser.

[0008] Instead of a calibration foil, it is known to install sensors (particularly two-dimensional sensors) in the bottom of the build chamber within or adjacent to the work area. These sensors may be CCD or CMOS sensors configured to determine the location of the laser beam relative to the sensor surface. Based on the determined location, correction values ​​can be determined so that one or more lasers of the device hit the desired location. Summary of the Invention [Problem to be solved by the invention]

[0009] However, the above techniques assume ideal conditions during a build job, especially with regard to the properties of the applied raw material layers, but in real-world scenarios, the above calibration techniques may still not be accurate enough.

[0010] It is therefore an object of the present invention to provide an improved technique for calibrating the illumination system of an additive manufacturing apparatus, particularly, but not exclusively, to provide a technique for improving the accuracy of calibration of one or more illumination beams of an additive manufacturing apparatus.

[0011] This object is addressed by the subject matter of the independent claims. Advantageous embodiments are set forth in the dependent claims.

[0012] The techniques described above do not take into account the actual topography (e.g., curvature or slope) of the applied powder layer. However, the topography (especially if the topography is not exactly flat or sloped) can affect the location where one or more irradiation beams impinge on the raw powder. The techniques disclosed in this disclosure take into account the shape of the powder layer to calibrate one or more irradiation beams. [Means for solving the problem]

[0013] According to a first aspect, there is provided a method of calibrating an illumination system of an apparatus for additive manufacturing, the method comprising: applying a powder layer to a work area of ​​the apparatus, measuring a topography of at least a portion of the powder layer, determining at least one lateral correction value for an illumination beam of the illumination system based on the measured topography, and applying the lateral correction value to scan data used by the illumination system to scan the illumination beam over the work area.

[0014] One or more of the following features of the method aspect may also be applicable to the apparatus of the apparatus aspect described below: In this disclosure, whenever the term "workpiece" is used, it refers to a "three-dimensional workpiece."

[0015] The additive manufacturing process by which the workpiece is produced may be additive manufacturing from a powder bed, such as selective laser sintering or selective laser melting, or other additive manufacturing process in which the workpiece is constructed from raw material powder by irradiating the raw material powder with a radiation beam to solidify the raw material powder in desired locations.

[0016] The powder layer can be applied by a powder applicator (also referred to as a powder coating device). The powder applicator includes a powder hopper, which can store, for example, an amount of powder necessary to apply at least one entire powder layer. The powder applicator can further include one or more rollers and one or more blades. The one or more rollers can be configured to compress the raw material powder and / or create a uniform surface. The one or more blades can be configured to create a smooth and uniform surface having a uniform thickness. The one or more blades can also be configured to remove excess raw material powder from the work area. For example, the one or more blades can be configured to push the excess raw material powder into an overflow container for excess powder. The powder applicator can have an elongated shape extending along a first direction (e.g., y direction) and can be configured to move along a second direction (e.g., x direction) perpendicular to the first direction, for example, guided by one or more rails.

[0017] The powder applicator, or at least a part of the powder applicator (e.g., a blade), may be movable vertically (i.e., in the z-direction) to control the thickness of the applied powder layer. Furthermore, the thickness of the powder layer can be controlled by setting the vertical position of the carrier of the apparatus onto which the raw material powder is applied in layers.

[0018] The work area can be defined as the area where the raw material powder is applied and irradiated. The work area may be flush with the bottom of the processing chamber of the apparatus. In other words, the work area defines a (virtual) area or plane parallel to the surface of the carrier and / or substrate plate of the apparatus. In an ideal case, all new raw material powder layers are applied so that they extend exactly within or above the work area (without curvatures and / or irregularities).

[0019] However, in real situations, the applied powder layer may have a topography that is different from a plane. This topography (shape) is measured in at least a portion of the powder layer. The term "topography" may be synonymous and therefore interchangeable with "three-dimensional structure" or "height profile." In particular, measuring the topography provides a height profile of the measured part, where curvatures, protrusions, and / or depressions can be identified both in terms of position in the xy plane (i.e., work area) and depth / height along the z direction.

[0020] In the measuring step, the entire powder layer may be measured, or only a predetermined portion of the powder layer may be measured, for example, the topography may be measured only in a portion of the powder layer where two or more irradiation areas corresponding to two or more irradiation beams overlap (i.e., an overlapping region of two or more irradiation beams).

[0021] The lateral correction value can be determined so that the position of the irradiation beam is corrected to hit the lateral position (in the xy plane) where the irradiation beam would hit the raw material layer if the powder layer were perfectly flat and parallel to the xy plane.

[0022] The lateral correction values ​​may be applied at different (logical) positions with respect to the scan data. For example, the "original" scan data may be kept unchanged and the correction values ​​may be added to the scan data before being fed to the projection system. Additionally, the scan data may be modified by adding the correction values ​​and the modified scan data may be fed to the projection system.

[0023] The at least one lateral correction value may be a single correction value for the entire powder layer. However, it may be desirable to provide multiple correction values, particularly one correction value for each lateral position (x-y position) in the powder layer. In this way, local unevenness in the powder layer can be corrected. For example, if there is a bump in a specific area of ​​the powder layer, a correction value (different from zero) can be provided only for that specific area.

[0024] The lateral correction value may indicate an offset in the xy plane (eg, in mm or μm) or an angular offset (eg, in degrees) relating to the illumination angle of the illumination beam.

[0025] The method may further include irradiating the powder layer with the irradiation beam according to the scan data to which the lateral correction value has been applied.

[0026] In the step of irradiating the powder layer, the irradiation beam creates a melt pool at a desired location in the powder layer, causing the powder to melt and solidify at the desired location, forming a predetermined shape of the work layer of the workpiece to be manufactured.

[0027] The method may further include calibrating a lateral position of the illumination beam relative to a horizontal plane within the work area.

[0028] In this calibration step, the irradiation beam is calibrated with respect to an ideal plane that spans the work area. For example, the calibration methods described in WO 2019 / 161886 A1 or EP 2961549 A1 can be used. This calibration can already calibrate the position of one or more irradiation beams emitted from the irradiation system very well. The calibration step can be performed, for example, before a powder layer is applied to the work area, especially before the first powder layer is applied to the work area. However, the calibration step can also be performed after the powder layer is applied, for example, by irradiating a part of the work area that is not used for the workpiece. Furthermore, during the calibration step, one or more detectable areas, sensors, or calibration foils that are not located within the work area but adjacent to it, for example, in the bottom area of ​​the processing chamber, can be irradiated.

[0029] Therefore, the method described herein can assume that a (primary) calibration has already been performed with respect to horizontal surfaces within the work area. Therefore, the illumination beam is already calibrated to some extent, and the method of the present disclosure improves on this (primary) calibration. Furthermore, the calibration step can be performed every N layers (N being, for example, 1, 2, 3, 4, or 5), and the method of the first aspect can be performed for each layer of raw material powder applied to the work area. Therefore, drifts caused by, for example, thermal effects can be corrected by the calibration with respect to horizontal surfaces, and unevenness in the powder layers can be corrected by the method of the first aspect.

[0030] The topography can be measured by a stripe light projection method.

[0031] A striped light pattern can be projected onto the powder layer and observed by one or more cameras, particularly from different angles. This technique is also called 3D scanning. In particular, a projector can project a striped light pattern onto the powder layer, and two cameras can observe the resulting pattern from different angles. Based on the generated camera images, the topography of the workpiece layer can be calculated. This technique is known to those skilled in the art. The device used to measure the topography is also called a structured light 3D scanner.

[0032] Further methods for measuring the topography of a powder layer applicable to the methods of the present disclosure may include at least one of line scanning, optical coherence tomography, and laser triangulation, which methods are known to those skilled in the art.

[0033] The method may further include determining a predetermined lateral position at which the illumination beam will strike the powder layer. The lateral correction value may be determined such that the illumination beam strikes the powder layer at the predetermined lateral position.

[0034] The predetermined lateral position may be represented by an x-coordinate and a y-coordinate within the work area. Depending on the topography of the powder layer, even if calibration is performed with respect to an ideal horizontal plane, the irradiation beam may not strike the predetermined lateral position despite being directed at that position. For example, if the thickness of the raw powder layer is thicker than expected (due to calibration with respect to the horizontal plane), the deflection angle of the laser beam (i.e., deflection from normal irradiation) may need to be increased to strike the desired x-y position. Also, if the thickness of the raw powder layer is thinner than expected (due to calibration with respect to the horizontal plane), the deflection angle of the laser beam (i.e., deflection from normal irradiation) may need to be decreased to strike the desired x-y position. These calculations can be performed by the control unit of the additive manufacturing device. These involve standard geometric calculations known to those skilled in the art and will not be described in detail herein.

[0035] The lateral correction value can be determined so that the irradiation beam strikes the powder layer at a lateral position that corresponds to the lateral position of the intersection of the irradiation beam, not including the lateral correction value, with a horizontal plane within the work area.

[0036] Therefore, a virtual intersection (in the x and y directions) of the horizontal plane and the irradiation beam may be considered. This intersection may correspond to the x and y irradiation data shown in the irradiation data of each workpiece. A lateral correction value can be calculated so that the actual irradiation beam (i.e., the irradiation beam with the correction value applied) hits the (real) powder layer exactly at this (desired) lateral position.

[0037] The method may further include determining at least one further lateral correction value for a further illumination beam of the illumination system based on the measured topography, and applying the further lateral correction value to further scanning data used by the illumination system to scan the further illumination beam over the work area.

[0038] All details and aspects described above with respect to the determination of the lateral correction value of the illumination beam are equally applicable to the determination of the further lateral correction value of the further illumination beam.

[0039] The lateral correction value and the further lateral correction value can be determined such that when the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value irradiate the same spot on a horizontal plane within the work area, the irradiation beam and the further irradiation beam irradiate the same point on the powder layer.

[0040] The method may further include irradiating a first structure in the powder layer using an irradiation beam; irradiating a second structure in the powder layer using an additional irradiation beam; determining a lateral position of the first structure and a lateral position of the second structure; and determining at least one of at least one lateral correction value and at least one additional correction value based on the measured topography, the lateral position of the first structure, and the lateral position of the second structure.

[0041] In this way, both calibration with respect to the work area and calibration with respect to the topography of the powder layer can be performed, for example, substantially simultaneously. The first and second structures are not necessarily formed or generated from the melt pool of the irradiation section. It is sufficient that these structures are visible, for example, by a camera, during or immediately after irradiation. In this regard, the term "structure" refers to irradiation that can be detected within a certain period of time during or after irradiation by the irradiation beam, but may also refer to irradiation that does not cause permanent changes in the irradiated material. In other words, the structure may be a purely "optical" structure that is visible by scattered light during irradiation but is invisible after irradiation. If it is sufficient to calibrate the two irradiation beams relative to each other (i.e., only relative, not absolute), it may be sufficient to determine only one of the lateral correction value and the further lateral correction value. However, if absolute calibration (i.e., with respect to the coordinate system of the apparatus, in other words, the work area) is desired, both the lateral correction value and the further lateral correction value can be determined.

[0042] The method can be performed on multiple successive layers of powder.

[0043] In particular, the method can be performed for each layer of powder applied and irradiated to a workpiece to be produced (i.e., for each powder layer of a build job), but the method may also be performed every M layers, where M may be, for example, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

[0044] The at least one lateral correction value can be determined based on the measured topography of the applied powder layer and the measured topography of a previously applied powder layer.

[0045] Therefore, not only the topography of the current layer can be considered, but also the topography of the underlying layer, since the topography of the underlying layer can affect the height of the solidified structure of the current layer.

[0046] The method may further include determining at least one vertical correction value for an illumination beam of the illumination system based on the measured topography, and applying the vertical correction value to a focusing optics of the illumination system.

[0047] In other words, not only the lateral position of the irradiation beam can be corrected, but also the focal position. For example, if the topography profile is high and / or the actual powder layer is thicker than expected, the focal length can be shortened, e.g., only at lateral positions where this is the case (ridges). Similarly, the focal length can be increased at lateral positions where the topography includes depressions.

[0048] According to a second aspect, there is provided an additive manufacturing apparatus comprising: an illumination system configured to illuminate a work area of ​​the apparatus with at least one illumination beam; a powder application device configured to apply a powder layer to the work area of ​​the apparatus; a topography measurement device configured to measure a topography of at least a portion of the powder layer; and a control unit configured to determine at least one lateral correction value for the illumination beam of the illumination system based on the measured topography, and to apply the lateral correction value to scan data used by the illumination system to scan the illumination beam across the work area.

[0049] All of the above aspects and details described with respect to the method aspect (first aspect) are also applicable to the apparatus aspect. In particular, the apparatus of the second aspect is configured to perform the method of the first aspect, and implements one or more of the details described with respect to the method aspect.

[0050] The control unit can be configured to calibrate the lateral position of the illumination beam relative to a horizontal plane within the work area.

[0051] The topography measurement device can be configured to measure the topography by a stripe light projection method.

[0052] The control unit can be configured to determine a predetermined lateral position at which the illumination beam strikes the powder layer. The control unit can be configured to determine a lateral correction value such that the illumination beam strikes the powder layer at the predetermined lateral position.

[0053] The control unit may be configured to determine the lateral correction value such that the irradiation beam strikes the powder layer at a lateral position corresponding to the lateral position of an intersection of the irradiation beam without the lateral correction value and a horizontal plane within the work area.

[0054] The control unit can further be configured to determine at least one further lateral correction value for the further illumination beam of the illumination system based on the measured topography, and to apply the further lateral correction value to further scan data used by the illumination system to scan the further illumination beam over the work area.

[0055] The control unit can be configured to determine the lateral correction value and the further lateral correction value such that when the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value irradiate the same point on a horizontal plane within the work area, the irradiation beam and the further irradiation beam irradiate the same point on the powder layer.

[0056] The control unit can be further configured to control the illumination system to irradiate a first structure into the powder layer using the illumination beam, control the illumination system to irradiate a second structure into the powder layer using the further illumination beam, determine a lateral position of the first structure and a lateral position of the second structure, and determine at least one of the at least one lateral correction value and the at least one further correction value based on the measured topography, the lateral position of the first structure, and the lateral position of the second structure.

[0057] The control unit can be configured to determine at least one lateral correction value based on the measured topography of the applied powder layer and the measured topography of a previously applied powder layer.

[0058] The control unit may be configured to determine at least one vertical correction value for the illumination beam of the illumination system based on the measured topography, and to apply the vertical correction value to the focusing optics of the illumination system.

[0059] Preferred embodiments of the present invention will now be described in more detail with reference to the accompanying schematic drawings. [Brief explanation of the drawings]

[0060] [Figure 1] FIG. 1 is a schematic side view of an additive manufacturing apparatus having an illumination system configured to illuminate one laser beam, according to an embodiment of the present disclosure. [Figure 2] FIG. 1 is a schematic side view of an additive manufacturing apparatus having an illumination system configured to illuminate two laser beams, according to an embodiment of the present disclosure. [Figure 3] 1A-1C are schematic diagrams of powder bed topographies illustrating embodiments of the calibration method of the present disclosure. [Figure 4] FIG. 1 illustrates a flowchart of a method according to an embodiment of the present disclosure. [Figure 5] FIG. 1 illustrates a control unit of a device with modules according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0061] 1 is a schematic diagram of an additive manufacturing apparatus 10. The apparatus 10 may also be referred to as an apparatus 10 for manufacturing a three-dimensional workpiece 12. The apparatus 10 may be based on, for example, a typical additive manufacturing apparatus, and the method for calibrating an illumination system according to the present disclosure may be programmed into a control unit 40 of the apparatus 10. Furthermore, additional components may be provided, such as three-dimensional scanning devices 28a, 28b, 29, and optionally a calibration sensor 42.

[0062] The principles of the apparatus 10 are well known to those skilled in the art of additive manufacturing and will only be briefly described. For example, such an apparatus 10 may be an apparatus for selective laser melting or an apparatus for selective laser sintering, in which one or more laser beams 14 can be used to selectively irradiate and solidify subsequent layers of raw material powder.

[0063] An apparatus 10 for performing a selective laser melting process as described below is provided as an example. A typical feature of powder bed fusion is that raw material powder is applied in layers, and each layer is selectively irradiated and solidified to produce one layer of the workpiece 12 to be manufactured. After removing excess powder and after any post-processing steps (e.g., removal of one or more support structures), the final workpiece 12 is obtained.

[0064] 1 shows an apparatus 10 for manufacturing a three-dimensional workpiece 12 by selective laser melting. The apparatus 10 includes a process chamber 16. The process chamber 16 is sealable from the ambient atmosphere, i.e., the environment surrounding the process chamber 16.

[0065] A powder applicator 18 arranged in the processing chamber 16 applies raw material powder onto a support (carrier) 20. For this purpose, the powder applicator 18 may comprise at least one of a roller, a blade, and a raw material hopper. The powder applicator 18 is configured to apply a uniform layer of raw material onto a previously applied and irradiated layer of raw material. Uniformity here means, in particular, that the layer thickness is uniform. However, this cannot be guaranteed, especially with regard to small local variations in the layer thickness, as will be explained below. The layer thickness can be selected, for example, by vertical positioning of the powder applicator 18 and / or the carrier 20.

[0066] The work area 21 is defined by the footprint of the carrier 20. The work area 21 is flush with the bottom area of ​​the process chamber 16 and defines the area where powder deposition occurs by the powder deposition device 18 and where one or more laser beams 14 irradiate the top layer of material.

[0067] The powder application device 18 extends in the y direction over at least the entire work area 21, making it possible to apply a layer of raw material powder "in one run". For this purpose, a horizontal movement device is provided which is configured to move the powder application device 18 horizontally, i.e., in the x direction according to Figure 1.

[0068] A vertical movement unit 22 is provided to displace the carrier 20 in the vertical direction, so that as the build height of the workpieces 12 stacked in layers from the raw material powder on the carrier 20 increases, the carrier 20 can be moved vertically downward.

[0069] The ability of the carrier 20 to be moved by the vertical movement unit 22 is well known in the field of selective laser melting and will not be described in detail here. As an alternative to a movable carrier 20, the carrier 20 may be provided as a static (or fixed) carrier (particularly with respect to the vertical z-direction), in which case the irradiation device 24 (see below) and the processing chamber 16 are configured to be moved upward during the build process (i.e., as the build height of the workpiece 12 increases). Furthermore, both the carrier 20 and the irradiation device 24 may be independently movable along the z-direction.

[0070] The carrier surface of the carrier 20 defines a horizontal plane (xy plane), and the direction perpendicular to this plane is defined as the vertical or build direction (z direction). Thus, each top layer of raw material powder and each layer of the workpiece 12 extends in a plane parallel to the horizontal plane (xy plane) defined above. As defined above, the workpiece area 21 also extends in a plane parallel to the horizontal plane (xy plane).

[0071] The apparatus 10 further includes a gas inlet 26 for supplying an inert gas (e.g., argon) into the processing chamber 16. A gas outlet (not shown) may be provided to form a gas circuit to generate a continuous gas flow through the processing chamber 16. In a preferred embodiment, a unidirectional laminar flow is generated along the x-direction above the top layer of raw material powder.

[0072] The apparatus 10 further includes an irradiation device 24 (also referred to as an irradiation unit or an optical unit) for selectively irradiating the laser beam 14 onto the top layer of the raw material powder applied on the carrier 20. The irradiation device 24 allows the raw material powder applied on the carrier 20 to be selectively irradiated with the laser beam in accordance with the desired shape of the workpiece 12 to be manufactured.

[0073] In this embodiment, the illumination system can be defined as consisting of one illumination device 24 configured to emit one laser beam 14. However, as shown in Figure 2 for example, the illumination system of the apparatus 10 may comprise two or more illumination devices 24 (more precisely, 24a, 24b), each configured to emit one laser beam 14. Thus, the present technology is not limited to apparatuses having only one illumination device 24, but rather multiple (e.g., 2, 4, 5, 6, 8, 10, 12, 14, etc.) illumination devices may be provided.

[0074] The irradiation device 24 of the apparatus 10 of Fig. 1 includes a scanning unit 30 configured to selectively irradiate the raw material powder provided on the carrier 20 with the laser beam 14. The scanning unit 30 is controlled by a control unit 40 of the apparatus 10. The scanning unit 30 may include a single mirror tiltable about two perpendicular axes. Alternatively, the scanning unit 30 may include two tiltable mirrors, each configured to tilt about a corresponding axis. The tiltable mirrors may be, for example, galvanometer mirrors.

[0075] The irradiation device 24 is supplied with laser light from a laser light source 32. The laser light source 32 may be provided inside the irradiation device 24 or may be provided outside the irradiation device 24 as shown in FIG. 1. In the former case, the laser light source 32 may be considered to be part of the irradiation device 24. In the latter case, the laser beam is generated by the laser light source 32 and directed into the irradiation device 24 via an optical fiber 34. Alternatively, the laser beam may be directed into the irradiation device 24 through air or a vacuum, for example, by using one or more mirrors.

[0076] A laser beam is directed from the laser source 32 towards the scanning unit 30. The laser source 32 may include, for example, a diode-pumped ytterbium fiber laser that emits laser light at a wavelength of approximately 1070-1080 nm (i.e., in the infrared wavelength range).

[0077] Illumination device 24 further includes two lenses 36 and 38 configured to focus laser beam 14 to a desired focal position along the z-axis. In the embodiment shown in FIG. 1, both lenses 36 and 38 have positive optical power. Lens 38, which is upstream in the beam path, is configured to collimate the laser light emitted by fiber 34, thereby producing a collimated or substantially collimated laser beam. Lens 36, which is downstream in the beam path, is configured to focus the collimated (or substantially collimated) laser beam to a desired z-position.

[0078] The control unit 40 includes a processor and memory, and the memory stores instructions for controlling the individual components of the apparatus 10. For example, the control unit 40 can be configured to control one or more of the vertical motion unit 22, the powder applicator 18, the gas flow rate provided by the gas inlet 26, and the irradiation device 24 of the irradiation system. A user input / output interface can be provided and connected or connectable to the control unit 40. The control unit 40 also has an interface for receiving workpiece data representing the three-dimensional shape of the workpiece 12 to be manufactured.

[0079] The location of the control unit 40 in the drawings (FIGS. 1 and 2) is merely schematic and is not limiting. The control unit 40 may be provided at any suitable location within the device 10, or may be provided remotely from the device 10 (e.g., connected to the device 10 via a network such as a LAN). The control unit 40, or at least a part thereof, may be provided in the form of a cloud computing device.

[0080] The device 10 of this embodiment includes the following elements in addition to the above-mentioned relatively common elements of an additive manufacturing device.

[0081] A three-dimensional scanning device 28a, 28b, 29 is provided, which includes a projector 29 and two cameras 28a and 28b. In alternative embodiments, the number of projectors and / or cameras may vary, for example, two or more projectors and three or more cameras may be provided. Furthermore, at least one of camera 28a, camera 28b, and the projector may be movable. Still further, other suitable devices for determining the topography of the powder layer may be used.

[0082] The three-dimensional scanning device operates as a structured light 3D scanner, the operation of which is generally known to those skilled in the art and will not be described in detail herein. Projector 29 is configured to project a stripe pattern onto the powder layer in work area 21, and cameras 28a and 28b capture images of the stripe pattern from two different angles. Note that the two different angles are not only different from each other but also different from the optical axis of projector 29. Based on the two images, a topography (also referred to as a height profile) can be calculated, for example, by a control unit (not shown) of the three-dimensional scanning device or a control unit 40 of device 10. The measured topography can be stored in the memory of control unit 40. For example, the topography can assign one height value (height along the z direction) to each xy value of work area 21. In this case, the number of xy values ​​can be determined by the resolution of the three-dimensional scanning device.

[0083] Furthermore, the apparatus 10 includes a calibration sensor 42 located in the bottom region of the processing chamber 16, adjacent to the top layer of raw material powder, i.e., the work area 21. Alternatively, only one calibration sensor 42, or three or more calibration sensors 42, may be provided. The calibration sensors 42 are used to perform (absolute) calibration of the laser beam 14 with respect to the coordinate system of the apparatus 10. For this purpose, the laser beam 14 is directed to at least one calibration sensor 42, and a predetermined pattern is projected onto the laser beam 14. The position of the predetermined pattern is detected by the respective calibration sensor 42, and the control unit 40 calculates appropriate correction values ​​for the laser beam 14. This ensures that the laser beam is projected exactly at the desired position with respect to the coordinate system of the apparatus 10.

[0084] The calibration sensor 42 is optional and can be replaced, for example, by at least one detectable area. The detectable area is a predetermined area that can be reached and illuminated by the laser beam 14. In some embodiments, the laser beam does not leave a permanent mark on the detectable area, but an illuminated structure on the detectable area is observed by a camera (e.g., camera 28a or 28b) during illumination. In other words, scattered light and / or thermal radiation is detected by the camera. Calibration can be performed based on the position of the detected structure.

[0085] In other embodiments, each calibration (i.e., calibration with respect to the horizontal plane of the work area 21) can be performed by directly irradiating one or more patterns onto the raw material of the applied raw material layer. During this irradiation, the patterns may or may not melt the powder (in particular, may not cause any permanent changes to the irradiated material). A camera observing the work area (e.g., one of cameras 28a and 28b) captures an image of the pattern or observes the pattern during irradiation (see above), and control unit 40 determines appropriate correction values ​​for laser beam 14, thereby directing laser beam 14 to the desired position with respect to the coordinate system of apparatus 10.

[0086] FIG. 2 shows a different embodiment of an apparatus 10 similar to the embodiment of the apparatus 10 of FIG. 1. The only difference between the two apparatuses 10 is that the illumination system of the apparatus 10 of FIG. 2 includes two illumination devices 24a and 24b instead of the single illumination device 24 of the apparatus 10 of FIG. 1. However, the remainder of the apparatus 10 of FIG. 2 has the same components and functions as those described above with respect to FIG. 1, and therefore a repetition of this description will be omitted. Also, the components of the illumination devices 24a and 24b are given the same reference numerals as in FIG. 2. However, the suffixes "a" and "b" are used to distinguish between the components of the illumination device 24a (suffix a) and the components of the further illumination device 24b (suffix b). The functions of the individual components within the illumination devices 24a and 24b are the same as those described above with respect to the illumination device 24 of FIG. 1.

[0087] Hereinafter, the use of reference numerals without the suffix (a or b) also refers to the respective elements with the suffixes a and b, unless otherwise specified. For example, when referring to "scanning unit 30," it also refers to scanning units 30a and 30b.

[0088] In the apparatus 10 of FIG. 2, the irradiation device 24a is configured to scan a first predetermined area (i.e., a first scanning field) of the top powder layer. Similarly, the additional irradiation device 24b is configured to scan a second predetermined area (i.e., a second scanning field) of the top powder layer. The first scanning field and the second scanning field overlap each other in an overlapping region. In other words, there is a region of the top powder layer (i.e., an overlapping region) that can be reached and selectively irradiated by both the laser beams 14a and 14b. The first scanning field and the second scanning field may each be rectangular or circular, and the size and / or shape of each scanning field may be predetermined by the movement range of the scanning unit 30a of the irradiation device 24a and the scanning unit 30b of the irradiation device 24b. The overlapping region can cover the entire work area 21, thereby allowing both the laser beams 14a and 14b to reach any position in the work area 21.

[0089] To produce the three-dimensional workpiece 12, both laser beams 14a and 14b can simultaneously irradiate different portions of the same powder layer, with each laser beam 14a and 14b irradiating a portion of the workpiece 12 in a corresponding scanning field. In this manner, the workpiece 12 can be constructed more quickly than if only one laser beam 14 were used (see, for example, FIG. 1 ). Furthermore, the two laser beams 14a and 14b may have different parameters, such as laser power, beam profile, spot diameter, and / or wavelength. For example, the control unit 40 can instruct the first laser beam 14a to irradiate the core portion of the workpiece layer and the second laser beam 14b to irradiate the shell portion of the workpiece layer (e.g., with a smaller spot diameter and / or lower laser power).

[0090] To ensure high quality of the resulting workpiece 12, it is important that the two lasers 14a and 14b be calibrated to one another. In other words, it is important that the relative position of the first laser beam 14a with respect to the second laser beam 14b be known. For example, if both laser beams 14a and 14b are irradiating the same spot (or if one laser beam continues irradiating a line started by the other laser beam), proper relative calibration is necessary.

[0091] Furthermore, whether there is only one irradiator 24 (see FIG. 1) or multiple irradiators 24a, 24b (see FIG. 2), it may be important to perform absolute calibration, i.e., calibration relative to the position of the top powder layer. For example, in hybrid manufacturing where the object to be repaired is embedded in a powder bed below the top powder layer, it may be important to hit a specific, predetermined location in the top powder layer to continue building the object to be repaired.

[0092] 1, calibration sensors 42 can be used to provide both absolute and relative calibration, at least to a first order. For example, predetermined patterns from both laser beams can be projected onto the same or different calibration sensors 42, and correction values ​​for both lasers can be determined (for absolute calibration) based on the lateral offset of the two patterns (and optionally, based on the known positions of the calibration sensors 42 relative to each other). If only relative calibration is to be provided, it may be sufficient to determine correction values ​​for only one of lasers 14a, 14b.

[0093] Furthermore, as mentioned above, a pattern can be projected onto the raw material powder layer and observed with a camera, see also the calibration process described in WO2019 / 161886A1.

[0094] However, in this calibration method, the single laser beam 14 in Figure 1 and the two laser beams in Figure 2 are only calibrated relative to an (ideal) horizontal plane within the work area 21. Topographic irregularities are taken into account by the techniques described below.

[0095] Figure 3 is a schematic side view showing the situation of Figure 2, where two laser beams 14a and 14b are directed onto a powder layer 50 having a non-planar topography. The depiction in Figure 2 is for illustrative purposes only and is not to scale (i.e., relative sizes and distances may not be correct).

[0096] Powder layer 50 is applied by powder applicator 18 (see FIG. 1 or FIG. 2) onto an underlying powder layer 52 that has already been partially solidified in a previous irradiation process. The solidified portion is not shown in FIG. 3. In addition to underlying powder layer 52, there may be further powder layers below powder layer 52. However, powder layer 50 may also be applied directly onto carrier 20.

[0097] As shown in FIG. 3, the topography of powder layer 50 is not ideally flat, but has ridges (or hills) 54. These irregularities in the work layer topography can have a variety of causes, only a few of which will be mentioned here: The solidified portions of sublayer 52 (or layers below layer 52) may "bump up," creating the ridges; The guide rails along which powder applicator 18 travels may be curved, tilted, or have other structural defects; There may be lumps in the powder; The powder applicator 18 may be clogged; The powder applicator 18 (particularly the blades of powder applicator 18) may be bent or tilted.

[0098] 3 is that both guide rails of powder applicator 18 bend upward in the middle, causing powder applicator 18 to apply more powder (i.e., thicker powder) to the center of work area 21 than to the edges of work area 21. The situation shown in FIG. 3 is merely an example, and the techniques disclosed herein can also be applied to other irregularities (e.g., bumps, depressions, etc. along the y-direction).

[0099] 3 also shows an irradiation angle 56 of the laser beam 14a and an irradiation angle 58 of the laser beam 14b. The irradiation angles are determined based on the surface normals of the xy plane (indicated by dashed lines 60 and 62, respectively, in FIG. 3), i.e., the z axis.

[0100] The irradiation devices 24a and 24b (see FIG. 1 or FIG. 2) are pre-calibrated so that when the laser beams are directed at the same position in the x-y plane of an ideally flat powder layer, the laser beams are irradiated at that position. This situation is shown in FIG. 3, where the (hypothetical) ideally flat powder layer is indicated by dashed line 64, and the (hypothetical) laser beams irradiated at the same x-y position 66 are indicated by dashed and dotted lines 68a and 68b, respectively. The dashed and dotted lines 64 and 68a and 68b represent a hypothetical situation, i.e., a situation in which the ridge 54 does not exist. However, in the example of FIG. 3, the ridge 54 exists, so the irradiation angle of each laser beam must be corrected to irradiate the same desired position 66. More precisely, the irradiation angle of each laser beam must be corrected so that the laser beams are irradiated as shown by reference numerals 14a and 14b, rather than as shown by reference numerals 68a and 68b.

[0101] When bumps 54 are present in powder layer 50, lateral corrections must be applied to both laser beams 14a and 14b to hit the same desired location 66. As shown in FIG. 3, laser beam 14a must be corrected from its location 68a to the left by lateral correction 70. Similarly, laser beam 14b must be corrected from its location 68b to the right by lateral correction 72. Once the two laser beams are corrected with lateral corrections 70 and 72, respectively, they will hit the same desired x-y location 66, which is where uncorrected beams 68a and 68b would hit in an ideally flat powder layer 64.

[0102] It should be noted that the lateral correction value can be calculated and provided in the form of a distance (as shown in FIG. 3) or in the form of an angular correction (in degrees) for the irradiation angles 56 and 58. More precisely, the lateral distances 70, 72 shown in FIG. 3 refer to the lateral distances above the imaginary flat powder layer 50, i.e., the work area. From these distances, the angular correction value (in degrees) can be easily calculated (and vice versa) since the distance between the scanning units 30a, 30b and the plane on which the work area extends (corresponding to the lengths of the dashed lines 60 and 62 in FIG. 3) is known.

[0103] Furthermore, the lateral correction value can be calculated and set not only in the x direction but also in the y direction.

[0104] 3, it is clear that the correction values ​​70, 72 can be easily calculated using standard geometries once the topography of the powder layer 50 is known. The determination of the correction values ​​70, 72 is performed by the control unit 40.

[0105] An appropriate correction value can be determined for each xy position of the work area 21. Furthermore, it is possible to set one correction value for the entire work area 21, but this may not be sufficient to correct local irregularities in the topography. Furthermore, it is also possible to set one or more correction values ​​only for areas where the topography of the powder layer 50 is different from a flat surface (i.e., areas where irregularities exist).

[0106] The topography of powder layer 50 is measured by 3D scanning devices 28a, 28b, 29 described above and shown, for example, in Figures 1 and 2. In some embodiments, it may not be necessary to measure the topography of the entire powder layer (i.e., the powder layer within the entire work area 21). For example, only the topography within the overlap area may be measured, and corrections may be provided only for xy positions within the overlap area.

[0107] It should be noted that although two laser beams 14a and 14b are shown in Figure 3, the techniques of the present disclosure are also applicable to an apparatus 10 having only one laser beam 14, as shown in Figure 1. In this case, only laser beam 14 is calibrated and a corresponding correction value is determined. Furthermore, an apparatus may have more than two (e.g., 3, 4, 5, 6, 8, 10, 12, or more) illumination devices 24, in which case a corresponding correction value can be determined for each of the illumination devices 24 in a manner similar to that described above with respect to Figure 3.

[0108] Furthermore, the correction values ​​can be determined not only in the x and y directions but also for the focal positions of the laser beams 14 and 14b. As shown in Figure 3, at the location of the protrusion 54, the focal lengths set for the corresponding irradiation devices 24a and 24b can be shortened to accurately focus the laser spots on the top surface of the protrusion 54. Similarly, if there is a depression, the focal lengths can be lengthened.

[0109] FIG. 4 shows a flowchart of a method for calibrating the illumination system 24 of the additive manufacturing apparatus 10 according to an embodiment of the present disclosure.

[0110] The method is performed by an apparatus for additive manufacturing (for example, the apparatus 10 described with reference to FIGS. 1 and 2) during the manufacturing process of a three-dimensional workpiece 12.

[0111] The method begins with step 70 of applying a layer of powder onto a work area of ​​the apparatus. This step is performed by powder application device 18 of apparatus 10 under the control of control unit 40.

[0112] The method further includes a step 72 of measuring the shape of at least a portion of the powder layer. This step is performed by the 3D scanning devices 28a, 28b, 29 of the apparatus 10 under the control of the control unit 40.

[0113] The method further comprises a step 74 of determining at least one lateral correction value for the illumination beam of the illumination system based on the measured topography. This step is performed by the control unit 40 of the apparatus 10.

[0114] The method further includes a step 76 of applying the lateral correction value to scan data used by the illumination system to scan the illumination beam over the work area. This step is performed by the control unit 40 of the apparatus 10.

[0115] Figure 5 is a schematic diagram of a control unit 40 in one of the devices 10 of Figure 1 or Figure 2. The control unit 40 has at least modules 80 and 82, each of which may be represented as hardware and / or software. In one example, the control unit 40 comprises a processor and memory. The memory has instructions stored therein that cause the processor to perform the method shown in Figure 4. For this purpose, the software stored in the memory may be considered to include modules 80 and 82 shown in Figure 5.

[0116] In detail, these modules are: A determination module 80 that determines at least one lateral correction value for the illumination beam of the illumination system based on the measured topography. An application module 82 that applies the lateral correction value to scan data used by the illumination system to scan the illumination beam over the work area.

[0117] The other elements of the device 10 (ie, elements other than the "logical" elements of the control unit 40) are described and illustrated above with reference to FIGS.

[0118] One or more embodiments of the present technology may have at least one of the following advantages: By performing the calibration described herein, the topography of the applied material layer can be taken into account and corrected. Without the technology described herein, the laser spot may not be irradiated at the desired location on the work layer. The present technology ensures that the desired location is irradiated even when the powder layer being irradiated has unevenness.

Claims

1. 1. A method for calibrating an illumination system of an apparatus for additive manufacturing, comprising: applying a layer of powder onto a work area of ​​the apparatus; measuring the topography of at least a portion of the powder layer; determining at least one lateral correction value for an illumination beam of the illumination system based on the measured topography; applying the lateral correction value to scan data used by the illumination system to scan the illumination beam over the work area; A method comprising:

2. The method of claim 1 , further comprising irradiating the powder layer with the irradiation beam according to the scan data to which the lateral correction value has been applied.

3. The method of claim 1 or 2, further comprising calibrating the lateral position of the illumination beam with respect to a horizontal plane of the work area.

4. The method according to any one of claims 1 to 3, wherein the topography is measured by a stripe light projection method.

5. determining a predetermined lateral position at which the radiation beam impinges on the powder layer; The method according to any one of claims 1 to 4, wherein the lateral correction value is determined such that the radiation beam strikes the powder layer at the predetermined lateral position.

6. 6. The method according to claim 1, wherein the lateral correction value is determined so that the irradiation beam strikes the powder layer at a lateral position corresponding to a lateral position of an intersection of the irradiation beam and a horizontal plane within the work area, not including the lateral correction value.

7. - determining at least one further lateral correction value for a further illumination beam of a further illumination system based on the measured topography; applying the further lateral correction value to further scan data used by the further illumination system to scan the further illumination beam over the work area; The method of any one of claims 1 to 6, further comprising:

8. 8. The method of claim 7, wherein the lateral correction value and the further lateral correction value are determined such that when the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value irradiate the same spot on a horizontal plane within the work area, the irradiation beam and the further irradiation beam irradiate the same point on the powder layer.

9. irradiating a first structure in the powder layer using the irradiation beam; irradiating a second structure in the powder layer using the further irradiation beam; determining a lateral position of the first structure and a lateral position of the second structure; determining at least one of the at least one lateral correction value and the at least one further lateral correction value based on the measured topography, the lateral position of the first structure, and the lateral position of the second structure; 9. The method of claim 7 or 8, further comprising:

10. The method according to any one of claims 1 to 9, wherein the method is carried out on multiple successive layers of powder.

11. 11. The method of any one of claims 1 to 10, wherein the at least one lateral correction value is determined based on the measured topography of the applied powder layer and a measured topography of a previously applied powder layer.

12. determining at least one vertical correction value for the illumination beam of the illumination system based on the measured topography; applying the vertical correction value to focusing optics of the illumination system; The method of any one of claims 1 to 10, further comprising:

13. 1. An apparatus for additive manufacturing, comprising: an illumination system configured to project at least one illumination beam onto a work area of ​​the apparatus; a powder application device configured to apply a layer of powder onto the work area of ​​the device; a topography measurement device configured to measure a topography of at least a portion of the powder layer; a control unit; The control unit determining at least one lateral correction value for an illumination beam of the illumination system based on the measured topography; configured to apply the lateral correction value to scan data used by the illumination system to scan the illumination beam over the work area. Device.

14. The apparatus of claim 13 , wherein the control unit is configured to calibrate a lateral position of the illumination beam with respect to a horizontal plane of the work area.

15. 15. The apparatus according to claim 13 or 14, wherein the topography measurement device is configured to measure the topography by a stripe light projection method.

16. the control unit is configured to determine a predetermined lateral position at which the irradiation beam impinges on the powder layer; 16. The apparatus of any one of claims 13 to 15, wherein the control unit is configured to determine the lateral correction value such that the irradiation beam strikes the powder layer at the predetermined lateral position.

17. 17. The apparatus of claim 13, wherein the control unit is configured to determine the lateral position such that the irradiation beam strikes the powder layer at a lateral position that corresponds to a lateral position of an intersection of the irradiation beam without the lateral correction value and a horizontal plane within the work area.

18. The Koreki control unit further determining at least one further lateral correction value for a further illumination beam of the illumination system based on the measured topography; 18. Apparatus according to any one of claims 13 to 17, configured to apply the further lateral correction value to further scan data used by the illumination system to scan the further illumination beam over the work area.

19. 20. The apparatus of claim 18, wherein the control unit is configured to determine the lateral correction value and the further lateral correction value such that when the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value irradiate the same spot on a horizontal plane within the work area, the irradiation beam and the further irradiation beam irradiate the same point on the powder layer.

20. The control unit further comprises: controlling the illumination system to irradiate a first structure in the powder layer using the illumination beam; controlling the illumination system to irradiate a second structure in the powder layer using the further illumination beam; determining a lateral position of the first structure and a lateral position of the second structure; 20. The apparatus of claim 18 or 19, configured to determine at least one of the at least one lateral correction value and the at least one further lateral correction value based on the measured topography, a lateral position of the first structure, and a lateral position of the second structure.

21. 21. The apparatus of any one of claims 13 to 20, wherein the control unit is configured to determine the at least one lateral correction value based on the measured topography of the applied powder layer and a measured topography of a previously applied powder layer.

22. The control unit determining at least one vertical correction value for the illumination beam of the illumination system based on the measured topography; Apparatus according to any one of claims 13 to 21, configured to apply the vertical correction value to focusing optics of the illumination system.

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