DE102022135050A1 - TECHNIQUE FOR CALIBRATING AN IRRADIATION SYSTEM OF AN ADDITIVE MANUFACTURING DEVICE - Google Patents

Abstract

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

Description

The present invention relates generally to the calibration of an irradiation system of an additive manufacturing apparatus. More specifically, and without limitation, the additive manufacturing apparatus may be a powder bed fusion apparatus, such as selective laser sintering, selective laser melting, or electron beam melting.

Powder bed fusion is an additive layering process by which powdered, in particular metallic and/or ceramic raw materials can be processed into three-dimensional workpieces of complex shapes. For this purpose, a raw material powder layer is applied to a carrier and subjected to radiation (e.g. laser or particle radiation) in a location-selective manner depending on the desired geometry of the workpiece to be produced. The radiation penetrating the powder layer causes heating and consequently melting or sintering of the raw material powder particles. Further raw material powder layers are then applied one after the other to the layer on the carrier, which has already been subjected to radiation treatment, until the workpiece has the desired shape and size. Powder bed fusion can be used to produce prototypes, tools, spare parts, high-quality 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.

Devices for producing one or more workpieces according to the above technique are known. For example, EP 2 961 549 A1 and EP 2 878 402 A1 each discloses an apparatus for producing a three-dimensional workpiece according to the technique of selective laser melting. The general principles described in these documents may also apply to the technique of the present disclosure.

In order to precisely determine a position of the irradiation beam with respect to a work area where the irradiation beam is irradiated, several calibration techniques are known. In particular, when more than one irradiation beam can be emitted from an irradiation system of an additive manufacturing device and scanned independently, it is important that the irradiation beams are calibrated relative to each other. In other words, in order to achieve a desired quality of the workpiece (without unwanted edges and/or other irregularities in a structure of the finished workpiece), it is important that all irradiation beams hit exactly the same point of the work area when directed to the same point by a control unit of the device.

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

Furthermore, EP 3-907-021 A1 a process for the automated alignment of scanning optics for additive manufacturing. The process of EP 3-907-021 A1 includes the following steps: irradiating an object region of a layer of a powdery material provided on a building platform with at least one irradiation beam, irradiating a calibration region of the layer of the powdery material with at least one irradiation beam, guiding the first irradiation beam with the first scanning optics over the upper interface and thereby melting a first calibration pattern into the upper interface, guiding the second irradiation beam with the second scanning optics over the upper interface and thereby melting a second calibration pattern into the upper interface, using the at least one image, identifying image points with respect to the geometric features of the calibration patterns from the image points, deriving a spatial offset between the second geometric features and aligning at least one of scanning optics taking into account the spatial offset.

Furthermore, methods for calibration are known that use calibration foils that can be brought into the work area of an additive manufacturing device and into which predefined patterns are burned by one or more lasers. A relative position of this one or more patterns can be viewed, e.g. by a camera, and one or more correction values can be determined for the respective laser(s).

Instead of the calibration foils, it is known to provide sensors (in particular two-dimensional sensors) in the work area or next to the work area at a lower area of the build chamber. These sensors can be CCD sensors or CMOS sensors that are configured to determine an irradiation point of a laser beam with respect to a surface of the sensor. Based on the determined irradiation point, correction values can be determined so that one or more lasers of the device impinge on a desired point.

However, the above techniques assume an ideal situation during the build application, especially regarding the properties of the deposited raw material layer. However, in real-world scenarios, the above calibration techniques may still not be precise enough.

The invention is therefore directed to the object of providing an improved technique for calibrating an irradiation system of an additive manufacturing device. In particular and without limitation, it is desired to provide a technique that leads to improved precision in calibrating one or more irradiation beams of an additive manufacturing device.

This object is solved by the subject matter of the independent claims. Advantageous embodiments are specified in the dependent claims.

The techniques discussed above do not take into account an actual topography of a deposited powder layer (e.g., a curvature or a slope). However, the topography (particularly if it is not exactly flat or sloped) can have an impact on the locations where one or more irradiation beams impinge on the raw material powder. The technique disclosed in the present disclosure takes into account the topography of the powder layer to calibrate one or more irradiation beams.

According to a first aspect, a method for calibrating an irradiation system of an additive manufacturing device is provided. The method comprises applying a powder layer to a work area of the device, measuring a topography of at least a portion of the powder layer based on the measured topography, determining at least one lateral correction value for an irradiation beam of the irradiation system, and applying the lateral correction value to the scan data used by the irradiation system to scan the irradiation beam over the work area.

One or more of the following features of the method aspect may also apply to the apparatus of the apparatus aspect described below. Whenever the term "workpiece" is used in the present disclosure, it always refers to the "three-dimensional workpiece".

The additive manufacturing process by which the workpiece is created may be additive manufacturing from a powder bed, 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 irradiating the raw material powder with an irradiation beam and thereby solidifying the raw material powder at desired locations.

The powder layer may be applied by a powder application device (also referred to as a powder coating device). The powder application device may include a powder hopper in which a certain amount of powder can be stored, e.g. at least as much powder as is required to apply a full layer of powder. Further, the powder application device may include one or more rollers and one or more squeegees. The one or more rollers may be configured to compress the raw material powder and/or create a uniform surface. The one or more squeegees may be configured to create a smooth and uniform surface with a uniform thickness. The one or more squeegees may further be configured to remove excess raw material powder from the work area. For example, the one or more squeegees may be configured to push excess raw material powder into an overflow container for excess powder. The powder application 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 perpendicular to the first direction (e.g. x-direction), e.g. guided over one or more rails. To control a thickness of the applied powder layer, the powder application device or at least parts of the powder application device (e.g. a doctor blade) may be vertically movable (i.e. along a z-direction). Furthermore, the thickness of the powder layer may be controlled by adjusting a vertical position of a carrier of the device on which the raw material powder is applied in layers.

The working area may be defined as an area in which the deposition of the raw material powder and the irradiation of the raw material powder are carried out. The working area may be flush with a bottom of a process chamber of the device. In other words, the working area defines a (virtual) area or plane parallel to a surface of a carrier and/or a substrate plate of the device. Ideally, each new raw material powder layer is applied such that it extends exactly into or over the working area (without any curvature and/or bumps or dents).

However, in real situations, the applied powder layer may have a topography that is different from a flat plane. This topography is measured in at least one section of the powder layer. The term "topography" may be synonymous and can therefore be replaced by "three-dimensional structure" or "height profile". In particular, the measurement of the topography may result in a height profile of the measured section, where a curvature, a hump and/or a dent can be identified both with respect to a location in the x-y plane (i.e. in the work area) and with respect to a depth/height along the z-direction.

In the measuring step, either the entire powder layer can be measured or only a predefined portion thereof. For example, the topography can be measured only in a portion of the powder layer where two or more irradiation areas of corresponding two or more irradiation beams overlap (i.e. in an overlap area of two or more irradiation beams).

The lateral correction value can be determined such that a position of the irradiation beam is corrected and the irradiation beam hits the raw material layer at a lateral position (in the x-y plane) where it would hit if the powder layer were completely flat and parallel to the x-y plane.

The lateral correction value can be applied to different (logical) positions with respect to the scan data. For example, the "original" scan data can remain unchanged and the correction value can be added to the scan data before it is fed to the irradiation system. Furthermore, the scan data can be modified by adding the correction value and the modified scan data can be fed to the irradiation system.

The at least one lateral correction value may be a correction value for the entire powder layer. However, it may be desirable to provide a plurality of correction values, in particular one correction value for each lateral position (x-y position) of the powder layer. In this way, local irregularities in the powder layer can be compensated. For example, in a case where a hump is present in a certain region of the powder layer, correction values (non-zero) may only be provided for that certain region.

The lateral correction value may specify an offset in the x-y plane (e.g. specified in mm or pm) or it may specify an angular offset with respect to an irradiation angle of the irradiation beam (e.g. specified in degrees).

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

In the step of irradiating the powder layer, the irradiation beam can create a melt pool at desired locations on the powder layer so that the powder melts and solidifies at the desired locations to predefinedly form a geometry of a workpiece layer of the workpiece to be produced.

The method may further comprise calibrating a lateral position of the irradiation beam with respect to a horizontal plane in the work area.

In this calibration step, the irradiation beam is calibrated with respect to an ideal flat plane extending in the working area. For example, a calibration method as in WO 2019/161886 A1 or EP 2-961-549 A1 This calibration can already provide a fairly good calibration for the position of one or more irradiation beams emitted by the irradiation system. The calibration step can, for example, be carried out before the powder layer is applied to the work area and in particular before a first powder layer is applied to the work area. However, the calibration step can also be carried out after the powder layer has been applied, for example by irradiating a portion 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 can be irradiated that are not provided within the work area but next to it, for example in a lower area of the process chamber.

Thus, for the method described herein, it can be assumed that a calibration (first order) has already been carried out with respect to a horizontal plane in the work area. The irradiation beams are therefore already calibrated to a certain degree and the method of the present disclosure provides a refinement of this calibration (first order). Furthermore, the step of calibrating can be carried out for all N layers (N can be e.g. 1, 2, 3, 4 or 5) and the method of the first aspect can be carried out for each layer of raw material powder applied to the work area. Thus, for example, a drift caused e.g. by thermal effects can be compensated by the calibration with respect to the horizontal plane and irregularities in the powder layer can be compensated by the method of the first aspect.

The topography can be measured using a stripe projection method.

A stripe light pattern can be projected onto the powder layer and can be viewed by one or more cameras, in particular from different angles. This technique can also be referred to as 3D scanning. In particular, a projector can project a stripe light pattern onto the powder layer and two cameras can view the resulting pattern from different angles. Based on the camera images generated, 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 can also be referred to as a structured light 3D scanner.

Other methods for measuring the topography of the powder layer that may be used in the method of the present disclosure may include at least one of line scanning, optical coherence tomography, and laser triangulation. These methods are known to those skilled in the art.

The method may further comprise determining a predefined lateral position at which the irradiation beam is to impinge on the powder layer. The lateral correction value may be determined such that the irradiation beam impinges on the powder layer at the predefined lateral position.

The predefined lateral position may be specified via x and y coordinates in the workspace. Due to a topography of the powder layer, the irradiation beam does not hit the predefined lateral position when directed there, even if a calibration has been performed with respect to an ideal horizontal plane. For example, in case a thickness of the raw material powder layer is larger than expected (due to the calibration with respect to the horizontal plane), a deflection angle (i.e., a deflection from the perpendicular radiation) of the laser beam needs to be increased to hit the desired x-y location. In case a thickness of the raw material powder layer is smaller than expected (due to the calibration with respect to the horizontal plane), a deflection angle (i.e., a deflection from the perpendicular radiation) of the laser beam needs to be decreased to hit the desired x-y location. These calculations may be performed by a control unit of the additive manufacturing apparatus. These are standard geometric calculations known to those skilled in the art and are therefore not described in detail here.

The lateral correction value can be determined such that the irradiation beam strikes the powder layer at a lateral position corresponding to a lateral position of an intersection point of the irradiation beam without the lateral correction value and a horizontal plane in the working area.

Thus, an imaginary intersection point (in x-y direction) of the horizontal plane with the irradiation beam can be taken into account. This intersection point can correspond to x-y irradiation data specified in irradiation data for the respective workpiece. The lateral correction value can be calculated so that the actual irradiation beam (i.e. the irradiation beam to which the correction value is applied) hits the (real) powder layer exactly in this (desired) lateral position.

The method may further determine, based on the measured topography, at least one further lateral correction value for a further Determine the irradiation beam of the irradiation system and apply the additional lateral correction value to additional scan data used by the irradiation system to scan the additional irradiation beam across the work area.

All details and aspects described above with regard to determining the lateral correction value for the irradiation beam can apply accordingly to determining the further lateral correction value for the further irradiation beam.

The lateral correction value and the further lateral correction value can be determined such that the irradiation beam and the further irradiation beam irradiate the same point on the powder layer if the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value would irradiate the same point on a horizontal plane in the working area.

The method may further comprise irradiating a first structure into the powder layer using the irradiation beam, wherein a second structure is irradiated into the powder layer using the further irradiation beam, wherein a lateral position of the first structure and a lateral position of the second structure are determined, and based on the measured topography of the lateral position of the first structure and the lateral position of the second structure, at least one of the at least one lateral correction value and the at least one further correction value is determined.

In this way, both a calibration with respect to the working area and a calibration with respect to the topography of the powder layer can be carried out, e.g. essentially at the same time. The first and the second structure do not necessarily consist of a melt pool at the irradiated sections. It may be sufficient that the structures are visible either during irradiation or shortly thereafter, e.g. with a camera.

In this respect, the term structure may also refer to irradiation that can be detected during or within a period of time after exposure to the irradiation beam, but which does not result in permanent changes to the exposed material. In other words, the structure may be a purely "optical" structure that is visible due to scattered light during irradiation, but is invisible after irradiation. In case it is sufficient that the two irradiation beams are calibrated to each other (i.e. only relatively and not absolutely), it may be sufficient to determine only one of the lateral correction value and the further lateral correction value. In case an absolute calibration (i.e. with respect to the coordinate system of the device, in other words with respect to the working area) is desired, both the lateral correction value and the further lateral correction value may be determined.

The process can be carried out for a plurality of successive powder layers.

In particular, the method can be carried out for each powder layer that is applied and irradiated for the workpiece to be produced (i.e. each powder layer of the build-up application). However, the method can also be carried out for each M layer, where M can be, for example, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

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

Thus, not only the topography of the current layer, but also a topography of an underlying layer can be taken into account, since the topography of the underlying layer can influence a height of the solidified structures of the current layer.

The method may further determine at least one vertical correction value for the irradiation beam of the irradiation system based on the measured topography and apply the vertical correction value to a focusing optics of the irradiation system.

In other words, not only a lateral position can be corrected, but also a focus position of the irradiation beam. For example, a focus length can be reduced if the topological profile is higher and/or an actual powder layer is thicker as expected, e.g. only at lateral positions where this is the case (humps). Similarly, a focus length can be increased at lateral positions if the topography includes dents.

According to a second aspect, a device for additive manufacturing is provided. The device comprises an irradiation system configured to irradiate at least one irradiation beam onto a working area of the device, a powder application device configured to apply a powder layer onto the working area of the device, and a direction, a topography measuring device configured to measure a topography of at least a portion of the powder layer, and a control unit. The control unit is configured to determine, based on the measured topography, at least one further lateral correction value for a further irradiation beam of the irradiation system and to apply the further lateral correction value to scan data used by the irradiation system to scan the irradiation beam across the work area.

All of the above aspects and details discussed with respect to the method aspect (first aspect) may be applied to the apparatus aspect. In particular, the apparatus of the second aspect may be configured to perform the method of the first aspect, implementing one or more of the details discussed above with respect to the method aspect.

The control unit may be configured to calibrate a lateral position of the irradiation beam with respect to a horizontal plane in the work area.

The topography measuring device may be configured to measure the topography via a strip light projection method.

The control unit can be configured to determine a predefined lateral position at which the irradiation beam is to impinge on the powder layer. The control unit can be configured to determine the lateral correction value such that the irradiation beam impinges on the powder layer at the predefined lateral position.

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

The control unit may be further configured to determine, based on the measured topography, at least one further lateral correction value for a further irradiation beam of the irradiation system and to apply the further lateral correction value to further scan data used by the irradiation system to scan the further irradiation beam across the work area.

The control unit can be configured to determine the lateral correction value and the further lateral correction value such that the irradiation beam and the further irradiation beam irradiate the same point on the powder layer if the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value would irradiate the same point on a horizontal plane in the work area.

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

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

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

• 1 eine schematische Seitenansicht einer Vorrichtung zur additiven Fertigung mit einem Bestrahlungssystem zeigt, das konfiguriert ist, um einen Laserstrahl gemäß einer Ausführungsform der vorliegenden Offenbarung zu emittieren;
• 2 eine schematische Seitenansicht einer Vorrichtung zur additiven Fertigung mit einem Bestrahlungssystem zeigt, das konfiguriert ist, um zwei Laserstrahlen gemäß einer Ausführungsform der vorliegenden Offenbarung zu emittieren;
• 3 eine schematische Darstellung einer Pulverschichttopographie, basierend darauf, welche Ausführungsform des Verfahrens zur Kalibrierung gemäß der vorliegenden Offenbarung erläutert wird, zeigt;
• 4 ein Flussdiagramm eines Verfahrens gemäß einer Ausführungsform der vorliegenden Offenbarung zeigt; und
• 5 eine Steuereinheit der Vorrichtung mit Modulen gemäß einer Ausführungsform der vorliegenden Offenbarung zeigt.

Preferred embodiments of the invention will be described in more detail with reference to the accompanying schematic drawings, in which

• 1 shows a schematic side view of an additive manufacturing apparatus with an irradiation system configured to emit a laser beam according to an embodiment of the present disclosure;
• 2 shows a schematic side view of an additive manufacturing apparatus with an irradiation system configured to emit two laser beams according to an embodiment of the present disclosure;
• 3 a schematic representation of a powder layer topography, based on which embodiment of the method for calibration ization according to the present disclosure;
• 4 shows a flowchart of a method according to an embodiment of the present disclosure; and
• 5 shows a control unit of the device with modules according to an embodiment of the present disclosure.

1 shows a schematic representation of a device 10 for additive manufacturing. The device 10 can also be referred to as a device 10 for producing a three-dimensional workpiece 12. The device 10 can, for example, be based on a typical additive manufacturing device, wherein the method for calibrating the irradiation system according to the present disclosure is programmed into a control unit 40 of the device 10. Furthermore, additional components such as a three-dimensional scanning device 28a, 28b, 29 and optionally calibration sensors 42 can be provided.

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

The apparatus 10 for performing a selective laser melting process as described below may serve as an example. Typical features of powder bed fusion are that a raw material powder is applied layer by layer and each layer is selectively irradiated and solidified to produce a layer of a workpiece 12 to be manufactured. After removal of excess powder and optional post-processing steps (e.g. removal of one or more support structures), the final workpiece 12 is obtained.

1 shows a device 10 for producing a three-dimensional workpiece 12 by selective laser melting. The device 10 comprises a process chamber 16. The process chamber 16 can be closed off from the ambient atmosphere, ie from the environment surrounding the process chamber 16.

A powder application device 18 arranged in the process chamber 16 serves to apply a raw material powder to a carrier 20. For this purpose, the powder application device 18 may comprise a roller and/or a doctor blade and/or a raw material hopper. The powder application device 18 is configured to apply a substantially uniform layer of raw material to a previously applied and irradiated layer of raw material. In this context, uniform means that the layer should have a uniform thickness. However, as discussed below, this cannot be guaranteed in real situations, in particular with regard to small local variations in the thickness of the layer. The layer thickness can be selected, for example, by vertically positioning the powder application device 18 and/or the carrier 20.

A working area 21 is defined by a size of the base area of the carrier 20. The working area 21 is flush with the lower area of the process chamber 16 and is defined as the area in which the powder application is carried out with the powder application device 18 and in which an uppermost raw material layer is irradiated with one or more laser beams 14.

The powder application device 18 extends in the y-direction at least over the entire working area 21, so that it can coat layers of raw material powder "in one pass". For this purpose, a horizontal movement device is provided which is configured to move the powder application device 18 in a horizontal direction, ie in the x-direction according to 1 , to move.

A vertical movement unit 22 is provided so that the carrier 20 can be displaced in a vertical direction so that as the build-up height of the workpiece 12 increases when it is built up in layers from the raw material powder on the carrier 20, the carrier 20 can be moved downward in the vertical direction.

Since the mobility 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 more detail here. Alternatively to the movable carrier 20, the carrier 20 can be provided 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 be moved upwards during the build process (i.e., with increasing build height of the workpiece 12). Furthermore, both the carrier 20 and the irradiation device 24 can be individually movable along the z-direction.

A support surface of the support 20 defines a horizontal plane (an xy plane), wherein a direction perpendicular to the plane is defined as a vertical direction or build direction (z direction). Thus, each uppermost layer of raw material powder and each layer of the workpiece 12 in a plane parallel to the horizontal plane (xy plane) defined above. As defined above, the work area 21 also extends in a plane parallel to the horizontal plane (xy plane).

The device 10 further comprises a gas inlet 26 for supplying an inert gas (e.g. argon) into the process chamber 16. Furthermore, a gas outlet (not shown) can be provided such that a continuous flow of gas can be generated through the process chamber 16 by implementing a gas circuit. In a preferred embodiment, a unidirectional laminar flow is generated across the top raw material powder layer along the x-direction.

The device 10 further comprises an irradiation device 24 (also referred to as irradiation unit or optical unit) for selectively emitting the laser beam 14 onto the uppermost layer of raw material powder that is applied to the carrier 20. By means of the irradiation device 24, the raw material powder applied to the carrier 20 can be exposed to laser radiation in a location-selective manner depending on the desired geometry of the workpiece 12 to be produced.

In the present embodiment, an irradiation system may be defined as consisting of the one irradiation device 24 configured to emit a laser beam 14. As in 2 However, as shown, an irradiation system of the apparatus 10 may include two or more irradiation devices 24 (more specifically, 24a, 24b), each irradiation device configured to emit a laser beam 14. The present technique is therefore 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.

The irradiation device 24 of the device 10 of 1 comprises a scanning unit 30 configured to irradiate the laser beam 14 selectively onto the raw material powder applied to the carrier 20. The scanning unit 30 is controlled by a control unit 40 of the device 10. The scanning unit 30 may comprise a mirror tiltable with respect to two perpendicular axes. Alternatively, the scanning unit 30 may comprise two tiltable mirrors, each configured to be tilted with respect to a corresponding axis. The tiltable mirrors may be, for example, galvanometer mirrors.

The irradiation device 24 is supplied with laser radiation from a laser beam source 32. The laser beam source 32 can be provided inside the irradiation device 24 or outside the irradiation device 24, as in 1 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 a glass fiber 34. Alternatively, the laser beam can be guided into the irradiation device 24 through the air or through a vacuum, e.g. using one or more mirrors.

The laser beam is directed from the laser beam source 32 to the scanning unit 30. The laser beam source 32 may comprise, for example, a diode-pumped ytterbium fiber laser that emits laser light at a wavelength of about 1070 to 1080 nm (i.e., in the infrared wavelength range).

The irradiation device 24 further comprises two lenses 36 and 38 configured to focus the laser beam 14 along the z-axis to a desired focus position. In the 1 In the embodiment shown, both lenses 36 and 38 have a positive refractive power. The lens 38 further upstream of the beam path is configured to collimate the laser light emitted by the fiber 34 such that a collimated or substantially collimated laser beam is produced. The lens 36 further downstream of the beam path is configured to focus the collimated (or substantially collimated) laser beam to a desired z-position.

The control unit 40 includes a processor and a memory, with instructions for controlling the individual components of the device 10 being stored on the memory. 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, a gas stream supplied from the gas inlet 26, and the irradiation device 24 of the irradiation system. A user input and output interface may be provided and connected or connectable to the control device 40. Furthermore, the control unit 40 has an interface for receiving workpiece data representative of a three-dimensional shape of the workpiece 12 to be manufactured.

The figures ( 1 and 2 ) is purely schematic and not limiting. The control unit 40 may be provided at any suitable position of the device 10 and may also be remote from the Device 10 may be provided (e.g. connected to device 10 via a network such as 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.

In addition to the more common elements of an additive manufacturing apparatus described above, the apparatus 10 of the present embodiment includes the following elements.

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

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 is therefore not described in detail herein. The projector 29 is configured to project a stripe pattern onto the powder layer in the work area 21, and the cameras 28a and 28b obtain an image 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 an optical axis of the projector 29. Based on the two images, a topography (also referred to as a height profile) may be calculated, for example by a control unit of the three-dimensional scanning device (not shown) or by the control unit 40 of the device 10. The measured topography may be stored in a memory of the control unit 40. For example, the topography may assign a height value (height along the z-direction) to each x-y value of the work area 21, where the number of x-y values may be determined by a resolution of the three-dimensional scanning device.

Furthermore, the device 10 comprises calibration sensors 42 arranged in a lower region of the process chamber 16, next to the top layer of raw material powder, i.e. next to the working area 21. Alternatively, only one calibration sensor 42 or more than two calibration sensors 42 can be provided. The calibration sensors 42 can be used to carry out an (absolute) calibration of the laser beam 14 with respect to a coordinate system of the device 10. For this purpose, the laser beam 14 is directed at at least one of the calibration sensors 42, where it emits a predefined pattern. A location of the predefined pattern is detected by the respective calibration sensor 42 and the control unit 40 calculates a suitable correction value for the laser beam 14. In this way, it can be ensured that the laser beam is directed to an exactly desired location with respect to the coordinate system of the device 10.

The calibration sensors 42 are optional. They can be replaced, for example, by at least one detectable area. These are predefined areas that can be reached and irradiated by the laser beam 14. According to some embodiments, the laser beam does not leave permanent markings in the detectable area, but a structure irradiated onto the detectable area is observed during irradiation via a camera (e.g. camera 28a or 28b). In other words, scattered light and/or thermal radiation is detected by the camera. Based on a position of the detected structure, a calibration can be carried out.

In other embodiments, a respective calibration (i.e. calibration with respect to a horizontal plane in the work area 21) may be performed by irradiating one or more patterns directly into the raw material of a deposited raw material layer. During this irradiation, the pattern may melt the powder or may not melt the powder (in particular, it may not cause permanent changes in the irradiated material). A camera observing the work area (e.g. one of the cameras 28a and 28b) takes an image of the pattern or observes the pattern during irradiation (see above), and the control unit 40 determines an appropriate correction value for the laser beam 14 so that it can be directed to a desired location with respect to the coordinate system of the device 10.

2 shows another embodiment of a device 10, similar to the device 10 of the embodiment of 1 . The only difference between the two devices 10 is that the irradiation system of the device 10 of 2 instead of the one irradiation device 24 of the device 10 of 1 two irradiation devices 24a and 24b. The rest of the device 10 of 2 However, it has the same components and functions as described above with regard to 1 discussed above, so that a repetition of this description is omitted. Furthermore, the components of the irradiation devices 24a and 24b have the same reference numerals as those of 2 However, suffixes “a” and "b" are used to distinguish between components of the irradiation device 24a (suffix a) and the components of the further irradiation device 24b (suffix b). The function of the individual components within the irradiation devices 24a and 24b is the same as discussed above with regard to the irradiation device 24.

In the following, the use of a reference symbol without a suffix (a or b) also refers to the respective elements with the suffixes a and b, unless explicitly stated otherwise. For example, when reference is made to the "scanning unit 30", this also refers to the scanning units 30a and 30b.

The device 10 of 2 is configured such that the irradiation device 24a is configured to scan a first predefined area (ie, a first scan field) of the topmost powder layer. Similarly, the further irradiation device 24b is configured to scan a second predefined area (ie, a second scan field) of the topmost powder layer. The first scan field and the second scan field overlap each other in an overlap region. In other words, there is an area of the topmost powder layer that can be reached and selectively irradiated by both laser beams 14a and 14b (ie, the overlap region). The first scan field and the second scan field can each be rectangular or circular, and a size and/or shape of the respective scan field can be predetermined by a range of movement of the respective scanning unit 30a and 30b of the irradiation devices 24a and 24b, respectively. The overlap area can also cover the entire working area 21 so that every position of the working area 21 can be reached by both laser beams 14a, 14b.

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 its respective scan field. In this way, the workpiece 12 can be built up more quickly than in a case where only one laser beam 14 is used (see e.g. 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 may instruct the first laser beam 14a to irradiate a core portion of a workpiece layer and may instruct the second laser 14b to irradiate a sleeve portion of the workpiece layer (e.g., with a smaller spot diameter and/or with less laser power).

In order to achieve a high quality of the produced workpiece 12, it is important that the two lasers 14a and 14b are calibrated with respect to each other. In other words, it is important that a relative position of the first laser beam 14a with respect to the second laser beam 14b is known. For example, if both laser beams 14a and 14b are to irradiate the same point (or one laser beam is to continue irradiating a line started by the other laser beam), a proper relative calibration is needed.

Furthermore, it can also be the case in the case of only one irradiation device 24 (see 1 ) and in the case of more than one irradiation device 24a, 24b (see 2 ) it may be important to provide an absolute calibration, i.e. a calibration with respect to positions of the top powder layer. For example, for a case of hybrid manufacturing where an object to be repaired is embedded in the powder bed below the top powder layer, it may be important to hit a specific predefined position within the top powder layer to continue a build of the object to be repaired.

Both absolute and relative calibration can be achieved at least to the first order by using the calibration sensors 42, as described with respect to 1 discussed above. For example, both laser beams may emit a predefined pattern either into the same or different calibration sensors 42 and based on a lateral offset of the two patterns (and optionally based on a known position of the calibration sensors 42 relative to each other), a correction value may be determined for both lasers (in the case of absolute calibration). If only relative calibration is to be achieved, it may be sufficient to determine a correction value for only one of the lasers 14a, 14b.

Furthermore, as discussed above, a pattern can be blasted into the raw material powder layer and observed by a camera. See also the WO 2019/161886 A1 described calibration process.

The one laser beam 14 from 1 and the two laser beams from 2 However, with this calibration method they are only calibrated with respect to an (ideal) horizontal plane in the working area 21. Irregularities in the topography are taken into account by the technique discussed below.

3 shows a schematic side view of the situation of 2 , whereby two laser beams 14a and 14b are irradiated onto a powder layer 50 that has a topography that differs from a flat plane. It should be noted that the representation of 2 is illustrative only and is not to scale (meaning relative sizes and spacing may not be correct).

The powder layer 50 was applied by the powder application device 18 (see 1 or 2 ) is applied to an underlying powder layer 52, which has already been partially solidified in a previous irradiation process. The solidified sections are in 3 not shown. In addition to the underlying powder layer 52, further powder layers may be present under the powder layer 52. However, the powder layer 50 may also have been applied directly to the carrier 20.

As in 3 , the topography of the powder layer 50 is ideally not ideally flat, but includes a hump (or hill) 54. Such irregularities in the topography of a workpiece layer can have various causes, only a few of which are mentioned: A solidified part in an underlying layer 52 (or layer under layer 52) can "pop up" and thereby cause a hump. A guide rail over which the powder applicator 18 is moved can be curved, tilted, or have other structural defects. There can be lumps in the powder. There can be a jam in the powder applicator 18. The powder applicator 18 (in particular a doctor blade of the powder applicator 18) can be curved or tilted.

This provides a possible explanation for the 3 shown humps that both guide rails of the powder application device 18 are bent upwards in the middle, causing the powder application device 18 to apply more powder (ie a higher thickness) in the middle of the working area than at the edges of the working area 21. The in 3 The situation shown is only exemplary and the technique disclosed herein also applies to other irregularities (e.g. irregularities along the y-direction, dents, etc.)

Furthermore, 3 an irradiation angle 56 of the laser beam 14a and an irradiation angle 58 of the laser beam 14b are specified. The irradiation angle is determined with respect to a surface normal of the xy plane (in 3 indicated as dashed lines 60 and 62 respectively), ie with respect to the z-axis.

The irradiation devices 24a and 24b (see 1 or 2 ) are pre-calibrated so that if each of the irradiation devices 24a and 24b directs a laser beam to a same position within the xy plane of an ideally flat powder layer, they would hit that position. This situation is in 3 where the (hypothetical) ideally flat powder layer is indicated by a dashed line 64 and the (hypothetical) laser beams directed to the same xy position 66 are indicated with dashed lines as 68a and 68b, respectively. The dashed line 64 and the dashed lines 68a and 68b show a hypothetical situation, i.e. a situation in which the bump 54 is not present. In the example of 3 However, the bump 54 is present and an irradiation angle of each of the laser beams must be corrected to irradiate the same desired position 66. More specifically, an irradiation angle of each of the laser beams must be corrected so that they are not irradiated as shown by reference numerals 68a, 68b but as shown by reference numerals 14a, 14b.

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

It should be noted that the lateral correction values are given in the form of a distance (as in 3 shown) or in the form of an angle correction (in degrees) of the irradiation angles 56 and 58 respectively. More specifically, the values in 3 indicated lateral distances 70, 72 to lateral distances on the hypothetical flat powder layer 50, i.e. on the working area. From these distances an angle correction value (in degrees) can easily be calculated (and vice versa) since the distances between the scanning units 30a, 30b and a plane in which the working area extends are known (corresponding to the length of the dashed lines 60 and 62 of 3 ).

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

From the 3 example shown it is clear that a correction value 70, 72 can be easily calculated using standard geometry as soon as the topography of the powder layer 50 is known. The determination of the correction values 70, 72 is carried out by the control unit 40.

For each x-y position of the working area 21, an appropriate correction value may be determined. 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 where the topography of the powder layer 50 differs from a plane (i.e., where a bump or dent is present).

The topography of the powder layer 50 is measured by the three-dimensional scanning device 28a, 28b, 29 discussed above and is, for example, in 1 and 2 In some embodiments, it may not be necessary to measure a topography of the entire powder layer (i.e., the powder layer within the entire working area 21). For example, only the topography in an overlap region may be measured and correction values may only be provided for xy positions within the overlap region.

It should also be noted that although two laser beams 14a and 14b in 3 shown, the technique of the present disclosure can also be applied to a device 10 with only one laser beam 14, as shown in 1 In this case, only the laser beam 14 is calibrated and corresponding correction values are determined. Furthermore, a device can comprise more than two irradiation devices 24 (for example 3, 4, 5, 6, 8, 10, 12 or more), wherein for each of the irradiation devices 24 corresponding correction values can be determined, similar to the above with regard to 3 discussed.

Furthermore, correction values can be determined not only for the xy directions, but also for a focus position of the laser beams 14a and 14b. As in 3 As shown, at the position of the bump 54, a focal length set in the corresponding irradiation devices 24a, 24b can be reduced in order to focus the laser spot precisely on an upper surface of the bump 54. Likewise, in the case of a dent, the focal length can be increased.

4 shows a flowchart of a method for calibrating an irradiation system 24 of an apparatus 10 for additive manufacturing according to an embodiment of the present disclosure.

The method is performed by an additive manufacturing apparatus (such as the apparatus 10 shown and discussed with respect to Figures 1 and 2) during a build process of a three-dimensional workpiece 12.

The method starts with a step of applying 70 a powder layer to a working area of the device. This step is carried out with the powder application device 18 of the device 10 under the control of the control unit 40.

The method further comprises a step of measuring 72 a topography of at least a portion of the powder layer. This step is carried out by the three-dimensional scanning device 28a, 28b, 29 of the device 10 under the control of the control unit 40.

The method further comprises a step of determining 74, based on the measured topography, at least one lateral correction value for an irradiation beam of the irradiation system. This step is carried out by the control unit 40 of the device 10.

The method further comprises a step of applying 76 the lateral correction value to scan data used by the irradiation system to scan the irradiation beam over the work area. This step is carried out by the control unit 40 of the device 10.

5 shows a schematic representation of the control unit 40 of one of the devices 10 of 1 or 2 . The control unit 40 includes at least modules 80 and 82, each of which may be embodied in 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 the 4 For this purpose, the software stored on the memory may be used as the software described in 5 Modules 80 to 82 shown can be considered in detail.

In detail, these modules are:

Determination module 80 for determining, based on the measured topography, at least one lateral correction value for the irradiation beam of the irradiation system.

Application module 82 for applying the lateral correction value to scan data received from the irradiation system for scanning the irradiation beam over the work area.

The other elements of the device 10 (ie the elements apart from the “logical” elements of the control unit 40) are described above with reference to 1 and 2 shown and discussed.

One or more embodiments of the present technique may have at least one of the following advantages. By performing the calibration described herein, a topography of a deposited raw material layer can be taken into account and compensated. Without the technique described herein, the laser spot cannot be directed to the desired position of the workpiece layer. The present technique provides that a desired position can be irradiated even if irregularities are present in the irradiated powder layer.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• EP 2961549 A1 [0003, 0027]
• EP 2878402 A1 [0003]
• WO 2019161886 A1 [0005, 0027, 0092]
• EP 3907021 A1 [0006]

Claims (22)

A method for calibrating an irradiation system of an additive manufacturing apparatus, 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; based on the measured topography, determining at least one lateral correction value for an irradiation beam of the irradiation system; and applying the lateral correction value to scan data used by the irradiation system to scan the irradiation beam across the work area. Procedure according to 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. Procedure according to Claim 1 or 2 , further comprising: calibrating a lateral position of the irradiation beam with respect to a horizontal plane in the work area. Method according to one of the Claims 1 until 3 , where the topography is measured using a stripe projection method. Procedure according to one of the Claims 1 until 4 , further comprising: determining a predefined lateral position at which the irradiation beam is to impinge on the powder layer, wherein the lateral correction value is determined such that the irradiation beam impinges on the powder layer at the predefined lateral position. Method according to one of the Claims 1 until 5 , wherein the lateral correction value is determined such that the irradiation beam strikes the powder layer at a lateral position which corresponds to a lateral position of an intersection point of the irradiation beam without the lateral correction value and a horizontal plane in the working area. Procedure according to one of the Claims 1 until 6 , further comprising: based on the measured topography, determining at least one further lateral correction value for a further irradiation beam of the irradiation system; and applying the further lateral correction value to further scan data used by the irradiation system to scan the further irradiation beam across the work area. Procedure according to Claim 7 , wherein the lateral correction value and the further lateral correction value are determined such that the irradiation beam and the further irradiation beam irradiate the same point on the powder layer if the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value would irradiate the same point on a horizontal plane in the working area. Procedure according to Claim 7 or 8th , 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 based on the measured topography, the lateral position of the first structure and the lateral position of the second structure, determining at least one of the at least one lateral correction value and the at least one further correction value. Procedure according to one of the Claims 1 until 9 , wherein the method is carried out for a plurality of successive powder layers. Procedure according to one of the Claims 1 until 10 , wherein the at least one lateral correction value is determined based on the measured topography of the applied powder layer and based on a measured topography of a previously applied powder layer. Procedure according to one of the Claims 1 until 11 , further comprising: based on the measured topography, determining at least one vertical correction value for the irradiation beam of the irradiation system; and applying the vertical correction value to a focusing optics of the irradiation system. Apparatus for additive manufacturing, the apparatus comprising: an irradiation system configured to irradiate at least one irradiation beam onto a working area of the apparatus; a powder application device configured to apply a layer of powder onto the working area of the apparatus; a topography measuring device configured to measure a topography of at least a portion of the powder layer; a control unit configured to: based on the measured topography, determine at least one lateral correction value for the irradiation beam of the irradiation system; and apply the lateral correction value to scan data used by the irradiation system to scan the irradiation beam across the work area. Device according to Claim 13 , wherein the control unit is configured to: calibrate a lateral position of the irradiation beam with respect to a horizontal plane in the work area. Device according to Claim 13 or 14 , wherein the topography measuring device is configured to measure the topography via a strip light projection method. Device according to one of the Claims 13 until 15 , wherein the control unit is configured to: determine a predefined lateral position at which the irradiation beam is to impinge on the powder layer, wherein the control unit is configured to determine the lateral correction value such that the irradiation beam impinges on the powder layer at the predefined lateral position. Device according to one of the Claims 13 until 16 , wherein the control unit is configured to determine the lateral correction value such that the irradiation beam impinges on the powder layer at a lateral position corresponding to a lateral position of an intersection point of the irradiation beam without the lateral correction value and a horizontal plane in the working area. Device according to one of the Claims 13 until 17 , wherein the control unit is further configured to: based on the measured topography, determine at least one further lateral correction value for a further irradiation beam of the irradiation system; and apply the further lateral correction value to further scan data used by the irradiation system to scan the further irradiation beam across the work area. Device according to Claim 18 , wherein the control unit is configured to determine the lateral correction value and the further lateral correction value such that the irradiation beam and the further irradiation beam irradiate the same point on the powder layer if the irradiation beam without the lateral correction value and the further irradiation beam without the further lateral correction value would irradiate the same point on a horizontal plane in the work area. Device according to Claim 18 or 19 , wherein the control unit is further configured to: control the irradiation system to irradiate a first structure into the powder layer using the irradiation beam; control the irradiation system to irradiate a second structure into the powder layer using the further irradiation beam; determine a lateral position of the first structure and a lateral position of the second structure; and based on the measured topography, the lateral position of the first structure and the lateral position of the second structure, determine at least one of the at least one lateral correction value and the at least one further correction value. Device according to one of the Claims 13 until 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 based on a measured topography of a previously applied powder layer. Device according to one of the Claims 13 until 21 , wherein the control unit is configured to: based on the measured topography, determine at least one vertical correction value for the irradiation beam of the irradiation system; and apply the vertical correction value to a focusing optics of the irradiation system.

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