Techniques for calibrating an illumination system of an apparatus for additive manufacturing

Abstract

A method of calibrating an illumination system of an apparatus for additive manufacturing is provided. The method includes applying a layer of powder onto a working area of a device; measuring the morphology of at least one section 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 for scanning the illumination beam over the working area. In addition, an apparatus for additive manufacturing is provided.

Description

Technical Field

[0001] The present invention generally relates to calibration of an irradiation system of an apparatus for additive manufacturing. More precisely and without limitation, the apparatus for additive manufacturing may be an apparatus for powder bed fusion, such as selective laser sintering, selective laser melting or electron beam melting. Background Art

[0002] Powder bed fusion is an additive layering process by which powdered (particularly metal and / or ceramic) raw materials can be processed into three-dimensional workpieces with complex shapes. To this end, layers of raw powder are applied to a carrier and subjected to radiation (e.g., laser or particle radiation) in a position-selective manner according to the desired geometry of the workpiece to be produced. The radiation penetrating into the powder layer causes heating and therefore melting or sintering of the raw powder particles. Further layers of raw powder are then applied in sequence to the layer on the carrier that has already been subjected to the radiation treatment until the workpiece has the desired shape and size. Powder bed fusion can be used to produce prototypes, tools, replacement parts, high-value parts or medical prostheses (e.g., dental or orthopedic prostheses) based on CAD data. Examples of powder bed fusion technologies include selective laser melting, selective laser sintering and electron beam melting.

[0003] Apparatus for producing one or more workpieces according to the above-mentioned techniques are known. For example, EP 2 961 549 A1 and EP 2 878 402 A1 each describe an apparatus for producing three-dimensional workpieces according to the selective laser melting technique. The general principles described in these documents can also be applied to the technique of the present disclosure.

[0004] In order to accurately determine the position of an illumination beam relative to the work area it illuminates, various calibration techniques are known. In particular, where more than one illumination beam can be emitted and independently scanned by an illumination system of an additive manufacturing apparatus, it is important that the illumination beams are calibrated relative to one another. In other words, to achieve the desired quality of the workpiece (the absence of undesirable edges and / or other irregularities in the structure of the finished workpiece), it is important that all illumination beams, when directed to the same point in the work area by the apparatus's control unit, strike precisely that same point.

[0005] For example, WO 2019 / 161886 A1 describes a method for aligning a multi-beam irradiation system for use in an apparatus for producing a three-dimensional workpiece by irradiating a raw material powder layer with electromagnetic radiation or particle radiation. The method of WO 2019 / 161886 A1 comprises the following steps: i) applying a first raw material powder layer to a carrier to define an irradiation plane to be irradiated with a radiation beam emitted by the irradiation system; ii) generating a first test structure in the first raw material powder layer in an overlapping region of the irradiation plane using a first radiation beam emitted by 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 plane using a second radiation beam emitted by 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 plane; and v) aligning at least one of the calibrated first irradiation unit and the calibrated second irradiation unit based on the determined offset between the first test structure and the second test structure so that the offset does not exceed a threshold value.

[0006] Furthermore, EP 3 907 021 A1 describes a method for automatically aligning scanning optics for additive manufacturing. The method of EP 3 907 021 A1 comprises the following steps: irradiating a target area of a powder material layer arranged on a building platform with at least one irradiation beam, irradiating a calibration area of the powder material layer with at least one irradiation beam, guiding the first irradiation beam over an intermediate top surface with a first scanning optic, thereby fusing a first calibration pattern onto the intermediate top surface, guiding a second irradiation beam over the intermediate top surface with a second scanning optic, thereby fusing 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, identifying 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 at least one of the scanning optics taking into account the spatial offset.

[0007] Furthermore, calibration methods are known that use a calibration foil that can be placed in the working area of an additive manufacturing system and into which a predetermined pattern is ablated by one or more lasers. The relative position of the pattern or patterns can be observed, for example, by a camera, and one or more correction values can be determined for the corresponding laser or lasers.

[0008] Instead of a calibration foil, it is known to provide a sensor (in particular a two-dimensional sensor) in the bottom area of the build chamber in or near the working area. These sensors may be CCD sensors or CMOS sensors configured to determine the impingement position of the laser beam relative to the surface of the sensor. Based on the determined impingement position, correction values can be determined so that the laser or lasers of the device impinge at the desired position.

[0009] However, the above techniques assume ideal conditions during the build operation, particularly with regard to the properties of the applied material layers. In real-life scenarios, however, the above calibration techniques may still not be accurate enough.

[0010] Therefore, it is an object of the present invention to provide an improved technique for calibrating an illumination system of an apparatus for additive manufacturing. In particular, but not limited to, it is desirable to provide a technique that improves the calibration accuracy of one or more illumination beams of an apparatus for additive manufacturing.

[0011] This object is solved by the subject matter of the independent claims. Advantageous embodiments are defined in the dependent claims.

[0012] The techniques discussed above do not take into account the actual topography (e.g., curvature or tilt) of the applied powder layer. However, topography (especially if the topography is not completely flat or tilted) may have an impact on the location where one or more irradiation beams impinge on the raw powder. The techniques disclosed in this disclosure take into account the topography of the powder layer for calibrating one or more irradiation beams. Summary of the Invention

[0013] According to a first aspect, a method of calibrating an illumination system of an apparatus for additive manufacturing is provided. The method comprises applying a powder layer to a working area of the apparatus; measuring a topography of at least a section 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 for scanning the illumination beam over the working area.

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

[0015] The additive manufacturing process (via which the workpiece is produced) may be powder bed additive manufacturing (such as selective laser sintering or selective laser melting), or any other additive manufacturing process in which a workpiece is built up from raw material powder by directing an irradiation beam onto the raw material powder and thereby solidifying the raw material powder at desired locations.

[0016] The powder layer can be applied by a powder applying device (also referred to as a powder coating device). The powder applying device may include a powder hopper in which a certain amount of powder (e.g., at least as much powder as is required to apply a complete powder layer) may be stored. In addition, the powder applying device may include one or more rollers, and one or more scrapers. The one or more rollers may be configured to compress the raw material powder and / or produce a uniform surface. The one or more scrapers may be configured to produce a smooth and uniform surface with a uniform thickness. The one or more scrapers may also be configured to remove excess raw material powder from the working area. For example, the one or more scrapers may be configured to push excess raw material powder into an excess powder overflow container. The powder applying device may have an elongated shape extending along a first direction (e.g., y direction) and may be configured to move along a second direction (e.g., x direction) perpendicular to the first direction, for example, guided by one or more guide rails.

[0017] In order to control the thickness of the applied powder layer, the powder application device or at least a portion of the powder application device (e.g., a scraper) may be vertically movable (i.e., in the z-direction). Furthermore, the thickness of the powder layer may 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 working area can also be defined as the area where the raw material powder is applied and irradiated. The working area can be flush with the bottom of the processing chamber of the device. In other words, the working area defines a (virtual) area or plane parallel to the surface of the device's carrier and / or substrate. Ideally, each new raw material powder layer is applied so that it extends precisely within or above the working area (without any curvature and / or protrusions or indentations).

[0019] However, in real life, the applied powder layer can have a topography that deviates from a plane. This topography is measured in at least a section of the powder layer. The term "topography" can be used synonymously and can therefore be replaced by "three-dimensional structure" or "height profile." In particular, the topography measurement can produce a height profile of the measured section, wherein curvatures, protrusions, and / or indentations can be identified relative to the position in the xy plane (i.e., within the working area) and relative to the depth / height along the z-direction.

[0020] In the measuring step, the entire powder layer or only a predetermined section of the powder layer may be measured. For example, the topography may be measured only in a section of the powder layer where two or more illumination regions of two or more corresponding illumination beams overlap (i.e., are within the overlapping region of the two or more illumination beams).

[0021] A lateral correction value can be determined such that the position of the illumination beam is corrected and such that the illumination beam impinges on the raw material layer at a lateral position (in the xy plane) at which the illumination beam would impinge on the raw material layer if the powder layer were completely flat and parallel to the xy plane.

[0022] The lateral correction values can be applied at different (logical) locations relative to the scan data. For example, the "raw" scan data can remain unchanged, and the correction values can be added to the scan data before it is fed to the illumination system. Furthermore, the scan data can be modified by adding the correction values, and the modified scan data can be fed to the illumination system.

[0023] The at least one lateral correction value may be one correction value for the entire powder layer. However, it may be desirable to provide multiple correction values, in particular one correction value for each lateral position (xy position) of the powder layer. In this way, local irregularities in the powder layer can be compensated. For example, if a protrusion is present in a specific area of the powder layer, a correction value (different from zero) may be provided only for that specific area.

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

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

[0026] In the step of irradiating the powder layer, the irradiation beam may generate a molten pool at a desired position of the powder layer, so that the powder melts at the desired position and solidifies to form a predetermined geometry of the workpiece layer of the workpiece to be produced.

[0027] The method may further comprise calibrating a lateral position of the illumination beam relative to a horizontal plane in the working area.

[0028] In this calibration step, the irradiation beam is calibrated relative to an ideal plane extending in the working area. For example, a calibration method as described in WO 2019 / 161886 A1 or EP 2 961 549 A1 can be used. This calibration can already provide a fairly good calibration of the position of one or more irradiation beams emitted by the irradiation system. The calibration step can be carried out, for example, before applying the powder layer to the working area, in particular before applying the first powder layer to the working area. However, the calibration step can also be carried out after applying the powder layer, for example by irradiating a section of the working area that is not used for the workpiece. In addition, during the calibration step, one or more detectable areas, sensors or calibration foils can be irradiated, and one or more detectable areas, sensors or calibration foils can be irradiated, and one or more detectable areas, sensors or calibration foils are not arranged in the working area but are arranged next to the working area, for example, in the bottom area of the process chamber.

[0029] Thus, for the methods described herein, it can be assumed that a (first-order) calibration has already been performed relative to a horizontal plane in the working area. Thus, the irradiation beam is already calibrated to a certain extent, and the method of the present disclosure provides a refinement of this (first-order) calibration. Furthermore, a calibration step can be performed for every N (e.g., N can be 1, 2, 3, 4, or 5) layers, and the method of the first aspect can be performed for each layer of raw powder applied to the working area. Thus, for example, drifts caused by thermal effects, for example, can be compensated by calibration relative to a horizontal plane, and irregularities in a powder layer can be compensated by the method of the first aspect.

[0030] The topography can be measured by fringe light projection.

[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 referred to as 3D scanning. Specifically, 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 resulting camera images, the topography of the workpiece layer can be calculated. This technique is well known to those skilled in the art. Devices for measuring topography are also referred to as structured light 3D scanners.

[0032] Additional methods for measuring the topography of the powder layer that can be applied in the method of the present disclosure can include at least one of line scanning, optical coherence tomography, and laser triangulation. These methods are known to those skilled in the art.

[0033] The method may further comprise determining a predetermined lateral position on the powder layer at which the illumination beam is to impinge.A lateral correction value may be determined such that the illumination beam impinges on the powder layer at the predetermined lateral position.

[0034] The predetermined lateral position can be represented by an x-coordinate and a y-coordinate in the working area. Due to the morphology of the powder layer, even if calibration relative to an ideal horizontal plane has been performed, when the irradiation beam is directed to the predetermined lateral position, the irradiation beam may not be irradiated at the predetermined lateral position. For example, in the case where the thickness of the raw material powder layer is greater than the expected thickness (due to calibration relative to the horizontal plane), it may be necessary to increase the deflection angle of the laser beam (i.e., the angle away from vertical radiation) so that the laser beam is irradiated at the desired xy position. In the case where the thickness of the raw material powder layer is less than the expected thickness (due to calibration relative to the horizontal plane), it may be necessary to reduce the deflection angle of the laser beam (i.e., the angle away from vertical radiation) so that the laser beam is irradiated at the desired xy position. These calculations can be performed by a control unit of the device for additive manufacturing. These calculations involve standard geometric calculations, which are known to those skilled in the art and are therefore not described in detail herein.

[0035] The lateral correction value may be determined such that the illumination beam impinges on the powder layer at a lateral position that corresponds to the lateral position of an intersection of the illumination beam and a horizontal plane in the working area without the lateral correction value.

[0036] Therefore, an imaginary intersection point (in the xy direction) of the horizontal plane and the illumination beam can be considered. This intersection point can correspond to the xy illumination data represented in the illumination data for the corresponding workpiece. A lateral correction value can be calculated so that the actual illumination beam (i.e., the illumination beam to which the correction value is applied) illuminates the (real) powder layer at exactly this (desired) lateral position.

[0037] The method may further comprise 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 for scanning the further illumination beam over the working area.

[0038] All details and aspects described above with respect to determining the transverse correction value for an illumination beam may correspondingly apply to determining the further transverse correction value for the further illumination beam.

[0039] The lateral correction value and the further lateral correction value can be determined so that the illumination beam and the further illumination beam illuminate the same point on the powder layer when the illumination beam without the lateral correction value and the further illumination beam without the further lateral correction value will illuminate the same point on a horizontal plane in the working area.

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

[0041] In this way, for example, calibration with respect to the working area and calibration with respect to the topography of the powder layer can be performed essentially simultaneously. The first and second structures do not necessarily consist of or result from a melt pool in the irradiation zone. It is sufficient that the structure is visible (e.g., using a camera) during or for a short period after irradiation. In this regard, the term structure can also refer to an illumination beam that can be detected for a period of time during or after exposure without causing a permanent change in the exposed material. In other words, the structure can be a purely "optical" structure that is visible during irradiation due to scattered light but is invisible after irradiation. In cases where it is sufficient to calibrate the two illumination beams relative to each other (i.e., only relatively, not absolutely), only one of the lateral correction value and the additional lateral correction value can be determined. However, in cases where absolute calibration is required (i.e., with respect to the coordinate system of the device, in other words, with respect to the working area), both the lateral correction value and the additional lateral correction value can be determined.

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

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

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

[0045] Therefore, not only the topography of the current layer may be taken into account, but also the topography of the underlying layer, since the topography of the underlying layer may have an influence on the height of the solidified structure of the current layer.

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

[0047] In other words, not only the lateral position can be corrected, but also the focal position of the irradiation beam. For example, in cases where the topological profile is higher and / or the actual powder layer is thicker as expected, the focal length can be reduced, for example, only in the lateral position (convexity) in this case. Similarly, the focal length can be increased in the lateral position where the topography includes indentations.

[0048] According to a second aspect, an apparatus for additive manufacturing is provided. The apparatus comprises an illumination system configured to illuminate at least one illumination beam onto a working area of the apparatus; a powder application device configured to apply a powder layer onto the working area of the apparatus; a topography measurement device configured to measure a topography of at least a section of the powder layer; and a control unit. The control unit is 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 scanning data used by the illumination system to scan the illumination beam over the working area.

[0049] All the above aspects and details discussed with respect to the method aspect (first aspect) may apply to the device aspect. Specifically, the device of the second aspect may be configured to perform the method of the first aspect, wherein one or more of the details discussed with respect to the method aspect are implemented.

[0050] The control unit may be configured to calibrate a lateral position of the illumination beam relative to a horizontal plane in the working area.

[0051] The topography measurement device may be configured to measure the topography via a fringe light projection method.

[0052] The control unit may be configured to determine a predetermined lateral position on the powder layer at which the illumination beam is to impinge.The control unit may be configured to determine a lateral correction value such that the illumination beam impinges on the powder layer at the predetermined lateral position.

[0053] The control unit may be configured to determine the lateral correction value such that the illumination beam impinges on the powder layer at a lateral position corresponding to a lateral position of an intersection of the illumination beam and a horizontal plane in the working area without using the lateral correction value.

[0054] The control unit may also be configured to: determine at least one further lateral correction value for a further illumination beam of the illumination system based on the measured topography; and apply the further lateral correction value to further scanning data used by the illumination system for scanning the further illumination beam over the working area.

[0055] The control unit can be configured to determine the lateral correction value and the further lateral correction value so that when the illumination beam without using the lateral correction value and the further illumination beam without using the further lateral correction value will irradiate the same point on the horizontal plane in the working area, the illumination beam and the further illumination beam will irradiate the same point on the powder layer.

[0056] The control unit can also be configured to: control the irradiation system to irradiate a first structure into the powder layer using an irradiation beam; control the irradiation system to irradiate a second structure into the powder layer using another irradiation beam; determine the lateral position of the first structure and the lateral position of the second structure; and determine at least one lateral correction value and at least one additional correction value based on the measured morphology, the lateral position of the first structure and the lateral position of the second structure.

[0057] The control unit may be configured to determine at least one lateral correction value based on the measured topography of the applied powder layer and based on 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 an illumination beam of the illumination system based on the measured topography and to apply the vertical correction value to focusing optics of the illumination system. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] A preferred embodiment of the present invention will now be described in more detail with reference to the accompanying schematic drawings, in which:

[0060] Figure 1 shows a schematic side view of an apparatus for additive manufacturing according to an embodiment of the present disclosure, the apparatus having an illumination system configured to emit a laser beam;

[0061] Figure 2 shows a schematic side view of an apparatus for additive manufacturing according to an embodiment of the present disclosure, the apparatus having an illumination system configured to emit two laser beams;

[0062] Figure 3 A schematic diagram showing the morphology of a powder layer is provided, based on which an embodiment of the method for calibration according to the present disclosure is explained;

[0063] Figure 4 A flowchart illustrating a method according to an embodiment of the present disclosure; and

[0064] Figure 5 A control unit having modules of a device according to an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0065] Figure 1A schematic diagram of an apparatus 10 for additive manufacturing is shown. The apparatus 10 may also be referred to as an apparatus 10 for producing a three-dimensional workpiece 12. The apparatus 10 may, for example, be based on a typical additive manufacturing apparatus, wherein the method for calibrating an irradiation system according to the present disclosure is 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.

[0066] 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, wherein one or more laser beams 14 may be used to selectively irradiate and solidify subsequent layers of raw material powder.

[0067] The apparatus 10 for performing the selective laser melting process described below can be used as an example. A typical feature of powder bed fusion is that the raw powder is applied in layers, and each layer is selectively irradiated and cured to produce a layer of the workpiece 12 to be produced. After removing excess powder and after optional post-processing steps (e.g., removing one or more support structures), the final workpiece 12 is obtained.

[0068] Figure 1 An apparatus 10 for producing a three-dimensional workpiece 12 by selective laser melting is shown. The apparatus 10 comprises a process chamber 16. The process chamber 16 may be sealed with respect to the surrounding atmosphere, ie with respect to the environment surrounding the process chamber 16.

[0069] A powder application device 18, disposed in processing chamber 16, is used to apply raw material powder to carrier 20. To this end, powder application device 18 may include at least one of a roller, a scraper, and a raw material hopper. Powder application device 18 is configured to apply a substantially uniform layer of raw material powder on top of a previously applied and irradiated layer of raw material powder. In this context, uniform specifically means that the layer should have a uniform thickness. However, as will be described below, this cannot be guaranteed in real-life situations, particularly with regard to small local variations in layer thickness. For example, the layer thickness can be selected by vertically positioning powder application device 18 and / or carrier 20.

[0070] The working area 21 is defined by the size of the footprint of the carrier 20. The working area 21 is flush with the bottom area of the process chamber 16 and is defined as the area in which powder application is performed using the powder application device 18 and the uppermost raw material layer is irradiated with one or more laser beams 14.

[0071] The powder application device 18 extends at least over the entire working area 21 in the y direction so that the powder application device can apply a layer of raw material powder "in one operation". For this purpose, a horizontal movement device is provided, which is configured to move the powder application device 18 in the horizontal direction, i.e. in accordance with Figure 1 Move in the x direction.

[0072] The vertical movement unit 22 is provided so that the carrier 20 can be displaced in the vertical direction, so that when the workpiece 12 is layered from the raw material powder on the carrier 20 , the carrier 20 can be moved downward in the vertical direction as the build height of the workpiece 12 increases.

[0073] Since the movability of the carrier 20 by means of the vertical movement unit 22 is well known in the field of selective laser melting, it will not be explained in detail herein. As an alternative to a movable carrier 20, the carrier 20 can be arranged as a stationary (or fixed) carrier (in particular with respect to the vertical z-direction), wherein the irradiation device 24 (see below) and the process chamber 16 are configured to move upwards during the build process (i.e., as the build height of the workpiece 12 increases). In addition, both the carrier 20 and the irradiation device 24 can be moved independently along the z-direction.

[0074] The carrier surface of carrier 20 defines a horizontal plane (xy plane), wherein the direction perpendicular to this plane is defined as the vertical direction or build direction (z direction). Therefore, each uppermost layer of raw material powder and each layer of workpiece 12 extends in a plane parallel to this horizontal plane (xy plane). As described above, working area 21 also extends in a plane parallel to the horizontal plane (xy plane).

[0075] The apparatus 10 further includes a gas inlet 26 for supplying an inert gas (e.g., argon) into the process chamber 16. A gas outlet (not shown) may be provided so that a continuous gas flow through the process chamber 16 can be generated by implementing a gas circuit. In a preferred embodiment, a unidirectional laminar flow is generated in the x-direction over the uppermost layer of feedstock powder.

[0076] The apparatus 10 further comprises an irradiation device 24 (also referred to as an irradiation unit or optical unit) for selectively irradiating the laser beam 14 onto the uppermost layer of raw material powder applied to the carrier 20. By means of the irradiation device 24, the raw material powder applied to the carrier 20 can be subjected to laser irradiation in a position-selective manner depending on the desired geometry of the workpiece 12 to be produced.

[0077] In this embodiment, the irradiation system can also be defined as being composed of one irradiation device 24 configured to emit one laser beam 14. However, as in Figure 2As shown in FIG, the irradiation system of the apparatus 10 may include two or more irradiation devices 24 (more precisely, irradiation devices 24a, 24b), wherein each irradiation device is configured to emit one laser beam 14. Therefore, the present technology is not limited to an apparatus having only one irradiation device 24, but a plurality of irradiation devices (e.g., 2, 4, 5, 6, 8, 10, 12, 14, etc.) may be provided.

[0078] Figure 1 The irradiation device 24 of the apparatus 10 includes a scanning unit 30, which is configured to selectively irradiate the laser beam 14 onto the raw material powder applied to the carrier 20. The scanning unit 30 is controlled by a control unit 40 of the apparatus 10. The scanning unit 30 may include a single mirror that is tiltable relative to two perpendicular axes. Alternatively, the scanning unit 30 may include two tiltable mirrors, each tiltable mirror being configured to tilt relative to a corresponding axis. The tiltable mirror may be, for example, a galvanometer mirror.

[0079] Laser radiation is supplied from a laser beam source 32 to the irradiation device 24. Figure 1 As shown, the laser beam source 32 can be arranged inside the irradiation device 24 or outside the irradiation device 24. In the first case, the laser beam source 32 can be considered as part of the irradiation device 24. In the latter case, the laser beam is generated by the laser beam source 32 and guided into the irradiation device 24 via the optical fiber 34. Alternatively, the laser beam can be guided into the irradiation device 24 through air or through a vacuum, for example by using one or more mirrors.

[0080] The laser beam is directed to the scanning unit 30 from a laser beam source 32. For example, the laser beam source 32 may include a diode-pumped ytterbium fiber laser that emits laser light having a wavelength of approximately 1070 nm to 1080 nm (ie, in the infrared wavelength range).

[0081] The irradiation device 24 further includes two lenses 36 and 38, which are configured to focus the laser beam 14 along the z-axis to a desired focus position. Figure 1 In the embodiment shown, both lenses 36 and 38 have positive optical power. Lens 38, further upstream in the beam path, is configured to collimate the laser light emitted by optical fiber 34, thereby producing a collimated or substantially collimated laser beam. Lens 36, further downstream in the beam path, is configured to focus the collimated (or substantially collimated) laser beam to a desired z position.

[0082] The control unit 40 includes a processor and a memory on which instructions for controlling the various components of the apparatus 10 are stored. For example, the control unit 40 may be configured to control one or more of the vertical movement unit 22, the powder application device 18, the gas flow supplied by the gas inlet 26, and the irradiation device 24 of the irradiation system. A user input and output interface may be provided, which may be connected or connectable to the control unit 40. In addition, the control unit 40 has an interface for receiving workpiece data representing the three-dimensional shape of the workpiece 12 to be produced.

[0083] Multiple graphs ( Figure 1 and Figure 2 ) is merely illustrative and non-limiting. The control unit 40 may be provided at any suitable location on 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 portion of the control unit 40 may be provided in the form of a cloud computing device.

[0084] In addition to the above-mentioned rather common elements of an apparatus for additive manufacturing, the apparatus 10 of the present embodiment includes the following elements.

[0085] A three-dimensional scanning device 28a, 28b, 29 is provided, comprising a projector 29 and two cameras 28a and 28b. In alternative embodiments, the number of projectors and / or cameras may vary; for example, more than one projector and more than two cameras may be provided. Furthermore, at least one of the cameras 28a, 28b, and projectors may be movable. Furthermore, any other suitable device for determining the topography of the powder layer may be used.

[0086] The three-dimensional scanning device operates as a structured light 3D scanner, the operation of which is well known to those skilled in the art and will not be described in detail herein. The projector 29 is configured to project a stripe pattern onto the powder layer in the working area 21, and the cameras 28a and 28b obtain images of the stripe pattern from two different angles. It should be noted that the two different angles are not only different from each other, but also different from the optical axis of the projector 29. Based on these two images, the topography (also known as the height profile) can be calculated, for example, by a control unit (not shown) of the three-dimensional scanning device or by a control unit 40 of the device 10. The measured topography can be stored in a memory of the control unit 40. For example, the topography can assign a height value (height along the z direction) to each xy value of the working area 21, where the number of xy values can be determined by the resolution of the three-dimensional scanning device.

[0087] Furthermore, the apparatus 10 comprises a calibration sensor 42, which is arranged in the bottom region of the process chamber 16, close to the uppermost layer of the raw material powder, i.e., close to the working area 21. Alternatively, only one calibration sensor 42 may be provided, or two or more calibration sensors 42 may be provided. The calibration sensors 42 can be used to perform (absolute) calibration of the laser beam 14 relative to the coordinate system of the apparatus 10. To this end, the laser beam 14 is directed to at least one of the calibration sensors 42, where the laser beam 14 irradiates a predetermined pattern. The position of the predetermined pattern is detected by the respective calibration sensor 42, and the control unit 40 calculates a suitable correction value for the laser beam 14. Thus, it can be ensured that the laser beam is directed to the exact desired position relative to the coordinate system of the apparatus 10.

[0088] Calibration sensor 42 is optional. Calibration sensor 42 can be replaced, for example, by at least one detectable area. These detectable areas are predetermined areas that can be reached and irradiated by laser beam 14. According to some embodiments, the laser beam does not leave any permanent marks in the detectable areas, but during irradiation, structures irradiated on the detectable areas are observed via a camera (e.g., camera 28a or 28b). 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 structures.

[0089] In other embodiments, the corresponding calibration (i.e., calibration relative to the horizontal plane in the working area 21) can be performed by irradiating one or more patterns directly into the raw material of the applied raw material layer. During this irradiation, the pattern may or may not cause the powder to melt (in particular, without causing any permanent changes to the irradiated material). A camera observing the working area (e.g., one of cameras 28a and 28b) captures an image of the pattern or observes the pattern during the irradiation (see above), and the control unit 40 determines appropriate correction values for the laser beam 14 so that the laser beam 14 can be directed to the desired position relative to the coordinate system of the device 10.

[0090] Figure 2 Shown with Figure 1 The embodiment of the device 10 is similar to the different embodiment of the device 10. As the only difference between the two devices 10, Figure 2 The irradiation system of the apparatus 10 comprises two irradiation devices 24a and 24b, and Figure 1 The device 10 comprises only one irradiation device 24. However, Figure 2 The remainder of the device 10 has the same Figure 1 The components and functions discussed in the present invention are the same as those discussed in the present invention, so repetition of the description is omitted. In addition, the components of the irradiation devices 24a, 24b have the same Figure 2The same reference numerals are used for the illumination devices of FIG. 24a and FIG. 24b. However, the suffixes "a" and "b" are used to distinguish between the components of illumination device 24a (suffix a) and the components of the separate illumination device 24b (suffix b). The functions of the various components within illumination devices 24a and 24b are the same as those discussed above with respect to illumination device 24.

[0091] In the following, unless otherwise explicitly stated, reference numerals without a suffix (a or b) are used to refer also to corresponding elements with the suffixes a and b. For example, when reference is made to "scanning unit 30", reference is thereby also made to scanning units 30a and 30b.

[0092] Figure 2 The device 10 is configured so that the irradiation device 24a is configured to scan a first predetermined area of the uppermost powder layer (i.e., a first scanning field). Similarly, the additional irradiation device 24b is configured to scan a second predetermined area of the uppermost powder layer (i.e., a second scanning field). The first scanning field and the second scanning field overlap each other in the overlapping area. In other words, there is an area of the uppermost powder layer that both laser beams 14a and 14b can reach and selectively irradiate (i.e., an overlapping area). The first scanning field and the second scanning field can each be rectangular or circular, and the size and / or shape of the corresponding scanning field can be predefined by the movement range of the corresponding scanning units 30a and 30b of the irradiation devices 24a and 24b, respectively. The overlapping area can also cover the entire working area 21, so that both laser beams 14a, 14b can reach every position in the working area 21.

[0093] In order to produce a three-dimensional workpiece 12, two laser beams 14a and 14b can simultaneously irradiate different sections of the same powder layer, wherein each laser beam 14a and 14b irradiates a section of the workpiece 12 in its corresponding scanning field. In this way, compared to the case where only one laser beam 14 is used (see, for example, Figure 1 ), the workpiece 12 can be built faster than with a conventional laser beam 14a. Furthermore, the two laser beams 14a and 14b can have different parameters, such as laser power, beam profile, spot diameter, and / or wavelength. For example, the control unit 40 can direct the first laser beam 14a to irradiate the core portion of the workpiece layer, and can direct 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).

[0094] To achieve high quality in the resulting workpiece 12, it is important that the two laser beams 14a and 14b are aligned relative to each other. In other words, it is important that the relative position of the first laser beam 14a relative to the second laser beam 14b is known. For example, when the two laser beams 14a and 14b are to illuminate the same spot (or one laser beam is to continue illuminating a line started by the other laser beam), proper relative alignment is required.

[0095] Furthermore, for only one irradiation device 24 (see Figure 1 ) and for more than one irradiation device 24a, 24b (see Figure 2 ), it is also important to provide absolute calibration (i.e. calibration with respect to the position of the uppermost powder layer). For example, for hybrid manufacturing scenarios (where the object to be repaired is embedded in a powder bed below the uppermost powder layer), it may be important to reach a specific predetermined position within the uppermost powder layer in order to continue building the object to be repaired.

[0096] By using the Figure 1 The calibration sensor 42 discussed can, at least in a first order, implement both absolute and relative calibration. For example, two laser beams can emit a predetermined pattern into the same or different calibration sensors 42, and based on the lateral offset of the two patterns (optionally, based on the known positions of the calibration sensors 42 relative to each other), correction values can be determined for both lasers (in the case of absolute calibration). In the case of only relative calibration, it is sufficient to determine a correction value for only one of the lasers 14a, 14b.

[0097] Alternatively, as mentioned above, a pattern can be irradiated into the raw powder layer and observed by a camera. In this regard, reference is also made to the calibration process described in WO 2019 / 161886 A1.

[0098] However, Figure 1 A laser beam 14 and Figure 2 The two laser beams are only aligned by this alignment method relative to the (ideal) horizontal plane in the working area 21. Irregularities in the topography are taken into account by the technique discussed below.

[0099] Figure 3 Shown Figure 2 Schematic side view of a situation in which two laser beams 14a, 14b are irradiated onto a powder layer 50 having a topography different from a plane. It should be noted that Figure 2 The illustrations are for illustration purposes only and are not drawn to scale (meaning that relative sizes and distances may be incorrect).

[0100] The powder layer 50 has been applied by the powder applying device 18 (see Figure 1 or Figure 2 ) is applied on top of the underlying powder layer 52 which has been partially cured in the previous irradiation process. Figure 3 In addition to the underlying powder layer 52 , further powder layers may be present beneath the powder layer 52 . However, the powder layer 50 may also be applied directly to the carrier 20 .

[0101] like Figure 3 As shown, the topography of the powder layer 50 is not ideally flat, but includes protrusions (or bumps) 54. Such irregularities in the topography of the workpiece layer can have different causes, of which only a few are mentioned: Solidified portions in the underlying layer 52 (or the layer below layer 52) can "rise" and thereby form protrusions. The guide rails on which the powder application device 18 moves can be curved, tilted, or can have other structural defects. There may be lumps in the powder. There may be congestion in the powder application device 18. The powder application device 18 (in particular the scraper of the powder application device 18) can be curved or tilted.

[0102] Therefore, Figure 3 One possible explanation for the shown bulge is that the two guide rails of the powder application device 18 bend upward in the middle, so that the powder application device 18 applies more powder (ie, a higher thickness) in the middle of the working area than at the edges of the working area 21 . Figure 3 The situation shown is only exemplary, and the technology disclosed herein is also applicable to other irregularities (e.g., irregularities along the y direction, indentations, etc.)

[0103] In addition, Figure 3 The irradiation angle 56 of the laser beam 14a and the irradiation angle 58 of the laser beam 14b are shown in FIG. Figure 3 , denoted by dashed lines 60 and 62, respectively), that is, the illumination angle is determined relative to the z-axis.

[0104] Illumination devices 24a and 24b (see Figure 1 or Figure 2 ) are precalibrated so that when each irradiation device 24a and 24b directs a laser beam to the same position in the xy plane of an ideally flat powder layer, the irradiation devices 24a and 24b will irradiate that position. Figure 3 , where a (hypothetical) ideally flat powder layer is represented by dashed line 64, and (hypothetical) laser beams directed to the same xy position 66 are represented by dashed lines 68a and 68b, respectively. The dashed line 64 and the dashed lines 68a and 68b illustrate a hypothetical situation, i.e., a situation where no protrusion 54 exists. However, in Figure 3 In the example of FIG, there is a protrusion 54, and the irradiation angle of each laser beam must be corrected in order to irradiate the same desired position 66. More precisely, the irradiation angle of each laser beam must be corrected so that the laser beam is not irradiated as shown by reference numerals 68a and 68b, but is irradiated as shown by reference numerals 14a and 14b.

[0105] In order for both laser beams 14a and 14b to impinge on the same desired position 66 in the presence of protrusions 54 in the powder layer 50, a lateral correction value must be applied to both laser beams 14a and 14b. Figure 3 As shown, laser beam 14a must be corrected to the left from its position 68a by a lateral correction value 70. Similarly, laser beam 14b must be corrected to the right from its position 68b by a lateral correction value 72. When the two laser beams are corrected using their respective lateral correction values 70, 72, they impinge on the same desired xy position 66, which is where the uncorrected beams 68a, 68b would impinge in the case of an ideally flat powder layer 64.

[0106] It should be noted that the lateral correction value can be expressed in the form of distance (e.g. Figure 3 ) or in the form of angle correction values (in degrees) for the illumination angles 56 and 58. More precisely, Figure 3 The lateral distances 70, 72 shown refer to the lateral distances on a hypothetical flat powder layer 50 (i.e. on the working area). From these distances, the angle correction value (in degrees) can be easily calculated (and vice versa), since the distance between the scanning units 30a, 30b and the plane in which the working area extends (corresponding to Figure 3 The lengths of the dashed lines 60 and 62 in FIG. 1 are known.

[0107] Furthermore, lateral correction values can be calculated and provided not only for the x-direction but also for the y-direction.

[0108] from Figure 3 As is clear from the example shown, the correction values 70 , 72 can be easily calculated by 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 .

[0109] An appropriate correction value may be determined for each xy position of the working area 21. Furthermore, one correction value may be provided for the entire working area 21, although this may not be sufficient to compensate for local irregularities in the topography. Furthermore, one or more correction values may be provided only for areas of the powder layer 50 where the topography differs from a flat surface (i.e., where protrusions or indentations are present).

[0110] The morphology of the powder layer 50 is determined by the methods discussed above and for example in Figure 1 and Figure 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 working area 21). For example, only the topography in the overlapping area may be measured, and correction values may be provided only for the xy positions in the overlapping area.

[0111] It should also be noted that although Figure 3 Two laser beams 14a and 14b are shown in FIG, but the technology of the present disclosure can also be applied to an apparatus 10 having only one laser beam 14, such as Figure 1 In this case, only the laser beam 14 is calibrated and the corresponding correction value is determined. In addition, the device may include more than two (for example 3, 4, 5, 6, 8, 10, 12 or more) irradiation devices 24, wherein for each irradiation device 24, a similar method as described above for Figure 3 The discussed approach determines the corresponding correction value.

[0112] Furthermore, correction values may be determined not only for the xy directions but also for the focal positions of the laser beams 14a and 14b. Figure 3 As shown, at the location of the protrusion 54, the focal length set in the corresponding irradiation device 24a, 24b can be reduced to focus the laser spot accurately on the top surface of the protrusion 54. Similarly, in the case of the indentation, the focal length can be increased.

[0113] Figure 4 A flow chart illustrating a method of calibrating the illumination system 24 of the apparatus 10 for additive manufacturing according to an embodiment of the present disclosure is shown.

[0114] The method is performed during the construction process of the three-dimensional workpiece 12 by an apparatus for additive manufacturing (such as the one described with reference to FIG. Figure 2 The device 10) shown and discussed is performed.

[0115] The method begins with a step 70 of applying a layer of powder to the working area of the apparatus. This step is performed under the control of the control unit 40 using the powder application device 18 of the apparatus 10.

[0116] The method further comprises a step 72 of measuring the topography of at least a section of the powder layer. This step is performed by the three-dimensional scanning devices 28a, 28b, 29 of the apparatus 10 under the control of the control unit 40.

[0117] 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 device 10.

[0118] The method further comprises a step 76 of applying the lateral correction value to the scanning data used by the illumination system for scanning the illumination beam over the working area. This step is performed by the control unit 40 of the device 10.

[0119] Figure 5 Shown Figure 1 or Figure 2Schematic diagram of a control unit 40 of one of the devices 10. The control unit 40 includes at least modules 80 and 82, each of which can be represented in the form of hardware and / or software. In one example, the control unit 40 includes a processor and a memory. The memory stores instructions that cause the processor to execute Figure 4 To this end, the software stored on the memory may be considered to include Figure 5 Modules 80 and 82 are shown.

[0120] Specifically, these modules are:

[0121] A determination module 80 is configured to determine at least one lateral correction value for an illumination beam of the illumination system based on the measured topography.

[0122] An application module 82 is used to apply the lateral correction value to the scanning data used by the illumination system for scanning the illumination beam over the work area.

[0123] References Figure 1 and Figure 2 Other elements of device 10 (ie, elements other than the "logic" elements of control unit 40) are shown and discussed.

[0124] One or more embodiments of the present technology may have at least one of the following advantages. By performing calibration as described herein, the topography of the applied raw material layer can be accounted for and compensated for. Without the techniques described herein, the laser spot may not be directed to the desired location in the workpiece layer. The present technique ensures that the desired location can be irradiated even when irregularities are present in the irradiated powder layer.

Claims

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

2. The method according to claim 1, further comprising: The powder layer is irradiated with the irradiation beam based on the scan data to which the lateral correction value has been applied.

3. The method according to claim 1 or 2, further comprising: A lateral position of the illumination beam relative to a horizontal plane in the working area is calibrated.

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

5. The method according to any one of claims 1 to 4, further comprising: determining a predetermined lateral position on the powder layer at which the illumination beam is to impinge, The lateral correction value is determined so that the irradiation beam irradiates the powder layer at the predetermined lateral position.

6. The method according to any one of claims 1 to 5, wherein The lateral correction value is determined such that the illumination beam impinges on the powder layer at a lateral position corresponding to a lateral position of an intersection of the illumination beam and a horizontal plane in the working area without using the lateral correction value.

7. The method according to any one of claims 1 to 6, further comprising: determining at least one further lateral correction value for a further illumination beam of the illumination system based on the measured topography; and The further lateral correction value is applied to further scanning data used by the illumination system for scanning the further illumination beam over the working area.

8. The method according to claim 7, wherein: The lateral correction value and the further lateral correction value are determined so that the irradiation beam and the further irradiation beam irradiate the same point on the powder layer when the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value are to irradiate the same point on the horizontal plane in the working area.

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

10. The method according to any one of claims 1 to 9, wherein The method is performed on a plurality of successive powder layers.

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

12. The method according to any one of claims 1 to 11, further comprising: determining at least one vertical correction value for the illumination beam of the illumination system based on the measured topography; and The vertical correction value is applied to focusing optics of the illumination system.

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

14. The apparatus according to claim 13, wherein The control unit is configured to: A lateral position of the illumination beam relative to a horizontal plane in the working area is calibrated.

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

16. The apparatus according to any one of claims 13 to 15, wherein The control unit is configured to: determining a predetermined lateral position on the powder layer at which the illumination beam is to impinge, The control unit is configured to determine the lateral correction value so that the illumination beam impinges on the powder layer at the predetermined lateral position.

17. The apparatus according to any one of claims 13 to 16, wherein The control unit is configured to determine the lateral correction value such that the illumination beam impinges on the powder layer at a lateral position corresponding to a lateral position of an intersection of the illumination beam and a horizontal plane in the working area without using the lateral correction value.

18. Apparatus according to any one of claims 13 to 17, wherein The control unit is further configured to: determining at least one further lateral correction value for a further illumination beam of the illumination system based on the measured topography; and The further lateral correction value is applied to further scanning data used by the illumination system for scanning the further illumination beam over the working area.

19. The apparatus according to claim 18, wherein The control unit is configured to determine the lateral correction value and the additional lateral correction value so that when the illumination beam without using the lateral correction value and the additional illumination beam without using the additional lateral correction value will illuminate the same point on the horizontal plane in the working area, the illumination beam and the additional illumination beam illuminate the same point on the powder layer.

20. The apparatus according to claim 18 or 19, wherein The control unit is further configured to: controlling the illumination system to illuminate a first structure into the powder layer using the illumination beam; controlling the illumination system to illuminate a second structure into the powder layer using the further illumination beam; determining a lateral position of the first structure and a lateral position of the second structure; as well as At least one of the at least one lateral correction value and at least one further correction value is determined based on the measured topography, the lateral position of the first structure and the lateral position of the second structure.

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

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

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