Calibration plate and calibration technique

Abstract

We describe a calibration plate for calibrating an apparatus for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation, the calibration plate comprising a surface feature representative of a defect of a layer to be irradiated. We further describe a method of calibrating an apparatus, the method comprising obtaining at least one image of the calibration plate arranged within a build chamber of the apparatus, detecting one or more of the at least one surface feature of the calibration plate based on the obtained at least one image, and calibrating the apparatus based on the detected one or more surface features. We further describe a system comprising the calibration plate and an apparatus configured to perform the method.

Description

[0001] Calibration plate and calibration technique

[0002] The present invention generally relates to a calibration plate and a calibration technique using such a calibration plate. In particular, a calibration plate for an apparatus for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation is provided, and a system comprising the calibration plate. The invention further relates to the apparatus, and to a method of calibrating such an apparatus.

[0003] In additive layering methods, workpieces are produced layer-by-layer by generating a sequence of solidified and interconnected workpiece layers. These processes may be distinguished by the type of raw material and / or the way of solidifying said raw material in order to produce the workpiece.

[0004] For example, powder bed fusion is a kind of additive layering process by which pulverulent, in particular metallic and / or ceramic raw material powders, can be processed to three-dimensional workpieces of complex shapes. To that end, a raw material powder layer is applied (e.g., deposited) onto a carrier (e.g., a build platform) and subjected to, for example, electromagnetic (e.g., laser) or particle (e.g., electron beam) radiation in a site selective manner in dependence on the desired geometry of the workpiece that is to be produced. The radiation penetrating into the powder layer causes heating and consequently selective solidification of the powder layer (e.g., by melting or sintering of raw material powder particles). Further raw material powder layers are then applied and processed successively to the layers on the carrier that have already been subjected to radiation treatment, until the workpiece has the desired shape and size. Selective melting or sintering can be used in particular for the production of prototypes, tools, replacement parts or medical prostheses, such as, for example, dental or orthopedic prostheses, on the basis of computer aided design (CAD) data.

[0005] In some cases, a deposited powder layer to be irradiated may comprise one or more defects, which may negatively affect subsequent processing steps, in particular the selective solidification of the defective powder layer and / or the deposition of a subsequent powder layer onto the defective powder layer. Defects of powder layers to be irradiated may also have a negative influence on the physical properties (e.g., rigidity and / or surface smoothness) of the produced workpiece. At least one defect in a deposited powder layer to be irradiated may be detected and / or classified, for example using a detection system. The detection system may be part of the apparatus for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation. The detection system may be implemented by a control unit of the apparatus.

[0006] The at least one defect in the powder layer to be irradiated may be detected and / or classified based on (e.g., in) an (e.g., two-dimensional or three-dimensional) image of at least a portion of the powder layer to be irradiated. The image may include spatial and / or depth information. The image may be acquired by an image acquisition unit (e.g., of the apparatus and / or the detection system), for example by a (e.g., stereo-) camera comprised in the apparatus.

[0007] The image acquisition unit may be configured to acquire a single image comprising the whole powder layer or the whole build area of the powder layer, wherein the build area is the portion of the powder layer that can be irradiated by the electromagnetic or particle radiation. Alternatively, or in addition, the image acquisition unit may be configured to acquire a plurality of images, each of a different portion of the powder layer and / or the build area. A defect of the powder layer may be detected and / or classified (e.g., by the detection system and / or the control unit) based on the single image or based on one or more (e.g., all) of the plurality of images. The single image and / or the plurality of images may be acquired via an optical scanning system of the apparatus, the optical scanning system being configured to direct the electromagnetic radiation toward the powder layer to be irradiated.

[0008] Detecting the at least one defect may comprise identifying one or more portions in the at least one image that match at least one predefined criterion. Classifying the at least one defect may comprise classifying a detected defect based on the at least one predefined criterion, or based on another predefined criterion. The respective at least one predefined criterion may comprise one or more predefined geometrical properties and / or one or more predefined optical properties. The geometrical properties may comprise one or more of a shape, an outline, a roughness, a height, a depth, a width, a length and a radius of curvature. The optical properties may comprise one of more of a texture, a color, a brightness and a reflectivity. The detection and / or classification of the at least one defect may be performed by or based on at least one of: (i) a pattern recognition algorithm, (ii) a blob detection algorithm, (iii) an edge detection algorithm, (iv) a shape detection algorithm, and (v) a trained machine learning model. Detecting and / or classifying the at least one defect may comprise adjusting (e.g., filtering, color-correcting, color spaceconverting, cropping, rotating, shifting, distorting and / or scaling) the obtained at least one image of the (e.g., portion of the) powder layer to be irradiated, and detecting the one or more surface features based on (e.g., in) the adjusted image. The at least one image may be adjusted (e.g., by the image acquisition unit) before it is obtained for the detection and / or classification of the at least one defect.

[0009] Based on the detected and / or classified at least one defect, a warning may be output (e.g., to a user or to a software component). Alternatively, or in addition, one or more process parameters used by the apparatus for producing a three-dimensional workpiece (e.g., for irradiating the layer to be irradiated, for depositing one or more subsequent powder layers and / or for irradiating one or more subsequent powder layers) may be set or adjusted. The detected and / or classified at least one defect may be correlated with a predefined (e.g., set and / or value of) process parameter(s) to be set. Such process parameters may include one or more of a gas composition within a build chamber of the apparatus, a gas flow rate within the build chamber of the apparatus, a temperature of powder material to be deposited, a temperature of deposited powder material (e.g., to be irradiated), a control parameter (e.g., a pose, speed, path, powder deposition rate and / or powder deposition area) of a powder deposition unit configured to deposit a powder layer and comprised in the apparatus, a pose or movement of a build platform of the apparatus, and an irradiation control parameter (e.g., an irradiation position, an irradiation beam scan speed, an irradiation power, an irradiation beam shape and an irradiation beam focus point). Other process parameters that may be set or adapted based on the detected and / or classified at least one defect of the powder layer to be irradiated may be apparent to those skilled in the art.

[0010] The present disclosure provides for a method of calibrating an (e.g., the) apparatus for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation.

[0011] The method may or may not comprise the steps mentioned above. In any case, the method comprises obtaining at least one image of at least a portion of a calibration plate arranged within a (e.g., the) build chamber of the apparatus, for example in a (e.g., the) build area. The at least one image may be acquired by and / or obtained from the image acquisition unit (e.g., when, or in response to the calibration plate being arranged in a predefined pose within the build chamber). As explained above with reference to the image of the (e.g., portion of the) powder layer, the at least one image of the (e.g., portion of the) calibration plate may either be a single image of the (e.g., portion of the) calibration plate, or the at least one image may comprise a plurality of images, each of a different sub-portion of the (e.g., portion of the) calibration plate.

[0012] In accordance with the present disclosure, and as described below in detail, the calibration plate comprises at least one surface feature representative of (e.g., mimicking, simulating, replicating and / or reproducing) a defect of a layer to be irradiated.

[0013] The method further comprises detecting (e.g., by the detection system and / or the control unit) one or more of the at least one surface feature of the calibration plate based on (e.g., in) the obtained at least one image (e.g., of at least the portion of the calibration plate arranged within the build chamber of the apparatus).

[0014] Detecting the one or more surface features may comprise identifying one or more portions in the at least one image that match (e.g., the) at least one predefined criterion. This at least one predefined criterion may comprise (e.g., the) one or more predefined geometrical properties and / or (e.g., the) one or more predefined optical properties. As in the detection and / or classification of the defect, these geometrical properties may comprise one or more of a shape, an outline, a roughness, a height, a depth, a width, a length and a radius of curvature, whereas the optical properties may comprise one of more of a texture, a color, a brightness and a reflectivity. The detection of the one or more surface features may be performed by or based on at least one of: (i) a (e.g., the) pattern recognition algorithm, (ii) a (e.g., the) blob detection algorithm, (iii) an (e.g., the) edge detection algorithm, (iv) a (e.g., the) shape detection algorithm, and (v) a (e.g., the) trained machine learning model. Detecting the one or more surface features may comprise adjusting (e.g., filtering, color-correcting, color space-converting, cropping, rotating, shifting, distorting and / or scaling) the obtained at least one image of (e.g., the portion of) the calibration plate, and detecting the one or more surface features based on (e.g., in) the adjusted image. The at least one image may be adjusted before it is obtained for the detection of the one or more surface features. A same algorithm, or multiple same algorithms, used for the detection and / or classification of the at least one defect may be used for the detection of the one or more surface features and vice versa. The method further comprises calibrating the apparatus based on the detected one or more surface feature(s) (e.g., of the calibration plate).

[0015] The apparatus may be calibrated by setting or adjusting one or more parameters based on the detected one or more surface features. At least one of the one or more parameters may be used (e.g., by the apparatus and / or by the detection system and / or by the control unit) to (e.g., later on) detect and / or classify a defect of a layer to be irradiated (e.g., based on the acquired image(s) of the (e.g., portion of the) powder layer to be irradiated). Such a parameter may comprise an upper threshold value, a lower threshold value and / or a range associated with one or more geometrical properties and / or one or more optical properties of features to be detected and / or classified as (e.g., acceptable, inacceptable or critical) defects. Calibrating the apparatus (e.g., setting or adjusting one or more parameters) may comprise correlating (e.g., one or more geometrical properties and / or one or more optical properties of) the one or more detected surface features with (e.g., one or more geometrical properties and / or one or more optical properties of) an associated (e.g., type, size, degree of acceptability or criticality of a) defect as represented by the respective one or more detected surface features.

[0016] For example, the at least one parameter that is set for calibrating the apparatus may define a minimum height as geometrical property of features to be detected and / or classified as (e.g., acceptable, inacceptable or critical) defects. As another example, the at least one parameter that is set for calibrating the apparatus may define a maximum radius of curvature as geometrical property of features to be detected and / or classified as (e.g., acceptable, inacceptable or critical) defects. As a still further example, the at least one parameter that is set for calibrating the apparatus may define a maximum reflectivity or a maximum brightness as optical property of features to be detected and / or classified as (e.g., acceptable, inacceptable or critical) defects.

[0017] After calibrating the apparatus, the method may comprise producing, by the apparatus, a three-dimensional work piece by irradiating layers of raw material powder with electromagnetic or particle radiation. Before producing the three- dimensional workpiece, the calibration plate (e.g., and a holder thereof) may be removed from the build chamber. This removal may be performed automatically or manually. The method may comprise triggering or instructing deposition of a powder layer to be irradiated (e.g., on the build platform). The method may comprise depositing the powder layer to be irradiated (e.g., on the build platform and / or at least within the build area).

[0018] The method may comprise obtaining at least one image of at least a portion of the (e.g., deposited) powder layer to be irradiated, as described above. This at least one image may be acquired during the production of the three-dimensional workpiece, for example after (e.g., at least partial) deposition of the powder layer, or after (e.g., at least partial) irradiation of the powder layer.

[0019] The method may further comprise detecting and / or classifying at least one defect in the powder layer to be irradiated based on the obtained at least one image of the (e.g., portion of the) powder layer to be irradiated, as described above. This step may be performed based on (e.g., the) at least one parameter that was set or adjusted when calibrating the apparatus.

[0020] The method may comprise outputting the warning based on the detected and / or classified at least one defect. Alternatively, or in addition, the method may comprise setting or adjusting the one or more process parameters based on the detected and / or classified at least one defect, the process parameters for example being used in subsequent processing steps for manufacturing the three-dimensional workpiece.

[0021] As indicated above, the present disclosure provides for a calibration plate for calibrating an apparatus, namely an (e.g., the) apparatus for producing three- dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation, which calibration plate comprises at least one surface feature representative of (e.g., mimicking, simulating, replicating and / or reproducing) a (e.g., typical, regularly observed, anticipated and / or likely) defect of a layer to be irradiated.

[0022] The defect of the layer to be irradiated may be a local deviation of the (e.g., upper and / or to-be-irradiated) surface of this layer from a (e.g., optimal and / or predetermined) planar powder layer surface. A roughness of the powder layer surface resulting only from the powder's particle sizes may be neglected in this regard. A defect of the layer to be irradiated may result from an erroneous deposition of the raw material powder of the layer to be irradiated, a deposition of spatter or debris produced by irradiating a previous layer of the raw material powder or by irradiating a portion of the layer to be irradiated and / or a damage of a previously irradiated portion of the raw material powder (e.g., of a previous layer).

[0023] The at least one surface feature may be a permanent surface feature of the calibration plate. For example, the at least one surface feature may be a rigid (e.g., manually non-deformable), immobile, solid (e.g., single-piece) and / or non-temporary surface feature of the calibration plate. The at least one surface feature may not consist of unsolidified powder material.

[0024] The calibration plate may be configured such that a pose (e.g., at least one of a position and an orientation) of one or more of the at least one surface feature on the calibration plate can be (e.g., manually and / or automatically) adjusted. Also in this case, the respective surface feature may be a rigid, solid and / or non-temporary surface feature of the calibration plate, and the surface feature may optionally not consist of unsolidified powder material.

[0025] The calibration plate may be configured such that one or more of the at least one surface feature can be (e.g., manually and / or automatically) replaced with another surface feature representative of a defect of a (e.g., the) layer to be irradiated. Also in this case, the respective surface feature may be a rigid and / or solid surface feature of the calibration plate, and the surface feature may optionally not consist of unsolidified powder material.

[0026] In one exemplary embodiment, the at least one surface feature has fixed (e.g., predetermined, permanent and / or non-temporary) geometrical properties. These geometrical properties may comprise one or more of a shape, an outline, a roughness, a height, a depth, a width, a length and a radius of curvature. Alternatively, or in addition, the at least one surface feature may have fixed (e.g., predetermined, permanent and / or non-temporary) optical properties. These optical properties may comprise one of more of a texture, a color, a brightness and a reflectivity.

[0027] The at least one surface feature may be representative of a defect of a layer to be irradiated by mimicking or reproducing the defect's geometrical and / or optical properties. The at least one surface feature may have similar geometrical properties as the defect of the layer to be irradiated (e.g., the defect represented by the surface feature). Alternatively, or in addition, the at least one surface feature may have similar optical properties as the defect of the layer to be irradiated (e.g., the defect represented by the surface feature).

[0028] In one variant, the calibration plate comprises a coating defining the optical properties of the at least one surface feature. The coating may be configured to mimic the optical properties of the (e.g., metallic) powder material of the layer to be irradiated. The coating may comprise silver paint.

[0029] In one variant, the at least one surface feature comprises selectively solidified (e.g., sintered or at least partially melted) powder material (e.g., metallic powder material). The at least one surface feature may be made from the same material as the raw material powder of the layer to be irradiated.

[0030] The at least one surface feature and / or the calibration plate comprising the surface feature may be made from selectively solidified powder material, for example selectively solidified powder material of the same type, composition, material and / or granularity as the powder material of the layer to be irradiated. The at least one surface feature and / or the calibration plate comprising the surface feature may be made from selectively solidified powder material, for example selectively solidified powder material of the same type, composition, material and / or granularity as the powder material used by the apparatus to produce a workpiece.

[0031] The at least one surface feature may be representative of a defect resulting from at least one of the following events:

[0032] (i) an erroneous deposition of the raw material powder of the layer to be irradiated;

[0033] (ii) a deposition of spatter or debris produced by irradiating a previous layer of the raw material powder or by irradiating a portion of the layer to be irradiated;

[0034] (iii) a damage of a previously irradiated portion of the raw material powder.

[0035] The calibration plate may be configured to be removably arranged (e.g., in one or more predefined poses, for example relative to the build area and / or the image acquisition unit) within a build chamber of the apparatus. The calibration plate may be configured to be arranged within the build chamber such that a surface comprising the surface feature is arranged in a pose (e.g., height) similar to a surface of a layer of raw material powder to be irradiated. The calibration plate may be configured to be arranged within the build chamber such that the at least one surface feature, all surface features of the calibration plate, or the whole calibration plate lie(s) in the build area. The calibration plate may be configured to be arranged within the build chamber such that the image(s) acquired by the image acquisition unit depict at least the at least one surface feature, for example all such surface features of the calibration plate or a complete surface of the calibration plate comprising the surface feature.

[0036] In one exemplary embodiment, the calibration plate is configured to be removably arranged in a holder. The holder is removably arrangeable (e.g., in one or more predefined poses, for example relative to the build area and / or the image acquisition unit) within the build chamber and configured to arrange (e.g., position) the calibration plate (e.g., in one or more predefined poses) within the build chamber (e.g., when holding the calibration plate). The holder may be configured to be mounted on a (e.g., the) build plate within the build chamber and / or to be inserted into a recess of the build chamber configured to hold (e.g., deposited and / or selectively irradiated) layers of powder material.

[0037] The present disclosure also provides for an apparatus for producing three- dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation, the apparatus comprising a control unit (e.g., a processor) configured to carry out the method(s) described herein.

[0038] The present disclosure also provides for a system. The system comprises at least one calibration plate as disclosed herein. The system further comprises at least one of the following entities: (i) the apparatus, and (ii) the holder.

[0039] The system may comprise two or more calibration plates, the two or more calibration plates being configured to be replaceably arranged in the same holder and differing from one another at least in the (e.g., type, size, pose, number of and / or alignment of the) at least one surface feature(s).

[0040] These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, in which:

[0041] Figure 1 shows a schematic illustration of an apparatus in accordance with the present disclosure; Figure 2 shows a schematic illustration of a calibration plate in accordance with the present disclosure;

[0042] Figures 3a-3i show schematic illustrations of different types of defects in accordance with the present disclosure;

[0043] Figures 4a-4f show schematic illustrations of different types of defects in accordance with the present disclosure;

[0044] Figure 5 shows a schematic illustration of a system in accordance with the present disclosure; and

[0045] Fig. 6 shows a flow diagram of a method in accordance with the present disclosure.

[0046] Figure 1 shows a schematic illustration of an apparatus 100 in accordance with the present disclosure. The apparatus 100 comprises a build chamber 2 in which three- dimensional workpieces can be produced by means of selective irradiation of layers of powder material. A recess 4 in the build chamber 2 is configured to receive the respective layers of powder material to be irradiated. At a bottom of the recess 4, a build plate 6 is arranged in a movable manner. The build plate 6 can be controllably moved downwards to give room for an additional powder layer within the recess 4. Such powder layers can be deposited with a powder deposition unit 8 slidably arranged on two tracks 10. In the illustrated variant, the powder deposition unit 8 comprises a main body 12 configured to hold powder material to be deposited, and a scraper 14 configured to smooth powder material deposited in front of the scraper 14 when the powder deposition unit 8 moves along the tracks 10. It should be noted that other variants of depositing a powder layer are possible and shall not be excluded.

[0047] The apparatus 100 comprises an energy source 16 configured to emit electromagnetic or particle radiation 18. A beam steering system 20 is configured to controllably direct the radiation 18 towards the build plate 6. In case a powder layer is deposited on the build plate 6, the powder layer (e.g., within a build area) may be irradiated with the radiation 18 for selective solidification thereof.

[0048] An image acquisition unit 20 (e.g., a camera such as a stereo-camera) is configured to acquire an image of a predefined (e.g., adjustable) area 22 in the build chamber 2. The predefined area 22 may comprise the build area in which powder material can be solidified by the radiation 18. The build area may be defined by an outline of the recess 4 when viewed along the z-direction and / or may lie within the recess 4.

[0049] The image acquisition unit 20 in the illustrated example acquires the image via a same beam steering system 23 that is used for directing the radiation 18 towards the build plate 6. In this variant, the radiation 18 is electromagnetic radiation and the beam steering system 23 may comprise an optical scanning mirror 24 and / or a beam-splitter 26.

[0050] It should be noted that other arrangements of the image acquisition unit 20 differing from Fig. 1 are possible and shall not be excluded. For example, an image acquisition unit 20 may be provided which is configured to acquire an image of the predefined (e.g., adjustable) area 22 without using the beam steering system 23. That is, an optical path of the image acquisition unit 20 and a path of the radiation 18 may (e.g., at least partially) coincide or not. For example, in case the radiation 18 is particle radiation, the beam steering system 23 may comprise electromagnetic beam adjustment components (e.g., electromagnetic lenses and / or electromagnetic deflectors), which may not be used to define the optical path of the image acquisition unit 20. Furthermore, instead of acquiring a single image of the predefined area 22, it is also possible to acquire, with the image acquisition unit 20, a plurality of images, each covering a different portion of the predefined area 22.

[0051] The apparatus 100 further comprises a control unit 28 (e.g., a processor). The control unit 28 is configured to control operation of the apparatus 100, for example operation of the build plate 6, the powder deposition unit 8, the energy source 16, the beam steering system 23, the image acquisition unit 20 and other components (e.g., a gas supply system, a heating system and / or a powder supply system) of the apparatus 100. The control unit 28 is configured to perform the method(s) as described herein. To this end, the control unit 28 may be communicatively coupled to a memory 30 storing instructions which, when executed by the control unit 28, cause the control unit 28 to perform the method(s) described herein.

[0052] Figure 2 shows a schematic illustration of an exemplary calibration plate 200 in accordance with the present disclosure. The calibration plate 200 comprises a body 31 having an upper surface 32. The calibration plate 200 comprises at least one surface feature 34 on the upper surface 32. Each of the at least one surface feature 34 is representative of a defect of a powder layer to be irradiated by the radiation 18. Put differently, each surface feature 34 mimics a surface defect that may be present when a powder layer has been deposited (e.g., and been at least partially irradiated) on the build plate 6.

[0053] Any deviation (e.g., elevation, indentation, incompleteness and / or erroneous solidification) of the powder layer surface from a planar (e.g., optimal, preferred and / or intended) powder layer surface may be considered as a surface defect. As no completely smooth surface can be formed using a powder material, a roughness of the powder layer surface resulting only from the powder's particle sizes may be neglected in this regard.

[0054] In the illustrated example, a plurality of surface features 34a represent powder ridges as a defect of the layer to be irradiated. The ridges have different predetermined sizes and extend across the whole width of the calibration plate 200 in the x-direction. A plurality of surface features 34b represent powder grooves as a defect of the layer to be irradiated. The grooves have different predetermined sizes and extend across the whole width of the calibration plate 200 in the x-direction. A plurality of surface features 34c represent raised overhangs as a defect of the layer to be irradiated. The raised overhangs have different sizes and are located at different portions of the calibration plate 200. A plurality of surface features 34d represent spatter as a defect of the layer to be irradiated. The spatter-simulating features have different sizes and are located at different portions of the calibration plate 200. It is to be understood that the calibration plate 200 shown in Fig. 2 is an example and that a calibration plate 200 in accordance with the present disclosure may comprise different numbers, sizes, arrangements and / or types of surface features 34.

[0055] In one example, each surface feature 34 is a solid and permanent surface feature of the calibration plate 200, and has fixed, predefined geometrical and optical properties. Each surface feature 34 may have similar optical and geometrical properties as the defect represented by the respective surface feature 34.

[0056] The calibration plate 200 and / or the surface feature(s) 34 may be made from a plastic material such as PA12 or PA-GF. To mimic the optical properties of a defect of a metallic powder layer, the calibration plate 200 may comprise a coating of silver paint that defines the optical properties of the surface feature. In the example of Fig. 2, a portion of the calibration plate 200 comprises such a coating 33, which covers the surface features 34c and 34d and thus defines their optical properties. In another variant, the calibration plate 200 and / or the surface feature(s) 34 may be made from a metallic material, for example selectively solidified metallic powder material (e.g., the same material as the raw material powder to be irradiated by the radiation 18). Examples of such metallic materials include AISi Mg, Ti6AI4V, IN718 and 316L. The surface feature(s) 34 may be made by irradiating such metallic powder material with a relatively low irradiation intensity to ensure that the powder material is not completely melted. Such a partial melting or sintering may result in a similar surface texture and roughness as non-irradiated powder material.

[0057] Figures 3a-4g show schematic illustrations of different types of defects 35 in accordance with the present disclosure. One or more of these defects 35 may be represented by a respective surface feature 34 of the calibration plate 200. When reference is made to an x-, y- or z-direction, the directions indicated in Fig. 2 are meant. When the calibration plate 200 is arranged in its predefined pose within the build chamber, these directions coincide or are parallel to the x-, y- and z-directions indicated in Fig. 1. In Figs. 3a-4g, the build plate 6 of the apparatus 100 is shaded with parallel lines, non-solidified powder material of the layer to be irradiated is indicated with dots, whereas solidified powder material is highlighted with white filling.

[0058] Fig. 3a-3i illustrate different defects 35, each resulting from an erroneous deposition of the raw material powder of the layer to be irradiated.

[0059] Fig. 3a illustrates powder clumps as a defect 35a of the layer to be irradiated. Typical dimensions of the individual clumps lie within 200-1000pm in the x- and y-directions and within 30-5000pm in the z-direction.

[0060] Fig. 3b illustrates a patchy powder delivery as a defect 35b of the layer to be irradiated. Typical dimensions of the defect lie within 200-2000pm in the x- and in the y-direction, and between 30-120pm in the z-direction depending on the selected layer thickness.

[0061] Fig. 3c illustrates an incomplete powder layer spreading as a defect 35c of the layer to be irradiated. This defect is also known under the term "short feed". The lack of powder of the layer typically extends 1000pm up to the complete width of the powder bed in the x-direction, 1000pm up to the complete length of the powder bed in the y-direction and 30-120|jm in the z-direction depending on the selected layer thickness.

[0062] Fig. 3d illustrates a powder groove as a defect 35d of the layer to be irradiated. Typical dimensions of the defect lie within 500-5000pm in the x-direction and 10.000pm up to the complete powder layer length in the y-direction. The powder groove may extend in the z-direction above 1000pm.

[0063] Fig. 3e illustrates ejected powder as a defect 35e of the layer to be irradiated, which may occur due to the scraper 14 bending when traveling over a previously solidified portion and returning to its unbent shape immediately afterwards. Typical dimensions of such defects lie between 500-5000pm in the x- and y-directions, and between 100-500pm in the z-direction.

[0064] Fig. 3f illustrates a powder ridge as a defect 35f of the layer to be irradiated. Typical dimensions of the defect lie within 500-5000pm in the x-direction and 10.000pm up to the complete powder layer length in the y-direction. The powder groove may extend in the z-direction above 1000pm.

[0065] Fig. 3g illustrates a powder heap as a defect 35g of the layer to be irradiated, which may occur due to the scraper 14 bending when traveling over a previously solidified portion. Typical dimensions of the defect lie within 500-50.000pm in the x- and y- directions, and 30-2000pm in the z-direction.

[0066] Fig. 3h illustrates parallel waves as a defect 35h of the layer to be irradiated. Such waves typically extend in the x-direction between 10.000pm up to the complete width of the powder layer, in the y-direction between 2000-5000pm and in the z- direction between 100-2000pm.

[0067] Fig. 3i illustrates parallel stripes as a defect 35i of the layer to be irradiated. Such stripes typically extend in the x-direction between 10.000pm up to the complete width of the powder layer, in the y-direction between 200-1000pm and in the z- direction between 100-2000pm.

[0068] Fig. 4a illustrates deposited spatter as a defect 35j of the layer to be irradiated. Such spatter may be produced and deposited by irradiating a previous layer of the raw material powder or by irradiating a portion of the layer to be irradiated. Typical dimensions of spatter lie between 200-500pm in the x-, y- and z-directions. Fig. 4b illustrates a defect 35k resulting from a deposition of debris. Such debris may be produced and deposited by irradiating a previous layer of the raw material powder or by irradiating a portion of the layer to be irradiated. Typical dimensions of debris lie between 200-2000pm in the x-, y- and z-directions.

[0069] Figs. 4c-4f illustrates different defects 35 resulting from a damage of a previously irradiated portion of the raw material powder.

[0070] Fig. 4c illustrates a raised overhang as a defect 351 of the layer to be irradiated. Typical dimensions of such defects lie between 500-5000pm in the x- and y- directions and between 100-5000pm in the z-direction.

[0071] Fig. 4d illustrates an elevated build sample as a defect 35m of the layer to be irradiated. Typical dimensions of such defects lie between 500-5000pm in the x- and y-directions and between 100-5000pm in the z-direction.

[0072] Fig. 4e illustrates snapped off support structures as a defect 35n of the layer to be irradiated. Typical dimensions of such defects lie between 200-5000pm in the x- and y-directions and between 100-5000pm in the z-direction.

[0073] Fig. 4f illustrates broken-away solidified material as a defect 35o of the layer to be irradiated. Typical dimensions of such defects lie between 200-5000pm in the x- and y-directions and between 100-5000pm in the z-direction.

[0074] Fig. 4g illustrates elevated edges as a defect 35p of the layer to be irradiated. Due to the elevated edges of a previously solidified portion, scraper 14 may deform, resulting in an uneven distribution of the deposited powder layer to be irradiated. Typical dimensions of such defects lie in the range of 200-5000pm in the x- and y- directions and between 30-200pm in the z-direction.

[0075] The calibration plate 200 may be configured to be removably arranged within the build chamber 2 of the apparatus 100. The calibration plate 200 maybe arranged in the build chamber, for example in the recess 4 and / or on the build plate 6, in order to calibrate the apparatus 100 using the method described herein. The calibration plate 200 may be removed from the build chamber before the apparatus 100 produces a three-dimensional workpiece by selective layerwise radiation of powder material. The calibration plate 200 may be (e.g., configured to be) arranged in the build chamber 200 in a predefined position and orientation such that the x-, y- and z- directions in Figs. 1 and 2 align (e.g., extend in parallel or coincide).

[0076] Figure 5 shows a schematic illustration of a system 1000 in accordance with the present disclosure. The system 1000 comprises multiple calibration plates 200a, 200b and a holder 300 removably arrangeable within the build chamber 2 of the apparatus 2. The calibration plates 200a, 200b each correspond to the calibration plate 200 as described herein, but differ from one another in the one or more surface feature(s) 34. For example, the surface feature(s) 34 of the calibration plates 200a, 200b may differ from one another in the defect they represent, in a size and / or in a position on the respective calibration plate.

[0077] The holder 300 is configured to arrange one of the calibration plates 200a, 200b at a time in a predefined position and orientation within the build chamber 2. The holder 300 may be adapted to snugly fit into the recess 4 and / or to mount onto the build plate 6. In one exemplary variant, the calibration plate 200, 200a, 200b and / or the holder 300 may be temporarily fixed to the apparatus using fixtures (e.g., screws or bolts), for example in a predefined position and orientation relative to the build area.

[0078] Fig. 6 shows a flow diagram of a method in accordance with the present disclosure. Optional steps of the method are indicated with dashed lines. The method may be performed by the control unit 28.

[0079] In a step 602, at least one (e.g., first) image II of at least a portion of the calibration plate 200 arranged within the build chamber 2 is obtained. The at least one image II may be acquired by the image acquisition unit 20 (e.g., once the calibration plate 200 is arranged in its predefined position and orientation within the build chamber 2).

[0080] In a step 604, one or more surface features 34 of the calibration plate 200 are detected based on the obtained at least one image II. The detection may be based on an (e.g., pattern recognition) algorithm configured to identify portions in the obtained image II that correspond to the surface feature(s) 34.

[0081] In a step 606, the apparatus 100 is calibrated based on the detected surface feature(s) 34. Calibrating the apparatus 100 may comprise setting or adjusting, based on the detected surface feature(s) 34, one or more parameters (e.g., subsequently) used to detect and / or classify a defect of a layer to be irradiated, for example the predefined geometrical and / or optical criteria used by the same (e.g., pattern recognition) algorithm to identify the portions in an image that represent an (e.g., inacceptable) defect. The algorithm may be tuned by setting or adjusting the one or more parameters, such that the algorithm only detects and / or classifies defects in powder layers that are inacceptable by exhibiting inacceptable predefined geometrical and / or optical properties (e.g., a height above 250pm). These inacceptable predefined geometrical and / or optical properties may be derived from the detected surface feature(s) 34 that are known to represent inacceptable defects.

[0082] For example, in step 606 each identified portion of the image II may be correlated to predefined geometrical and / or optical properties associated with the respective surface feature 34. This allows mapping the real, predefined geometrical and / or optical properties of the surface features 34 to their depictions in the at least one image II. One may say that calibrating the apparatus comprises teaching the apparatus about which depictions of surface features 34 and / or defects 35 in an obtained image represent acceptable surface features 34 and / or defects 35, and which do not.

[0083] In an optional step 608, the calibration plate 200 may be removed from the build chamber 2 (e.g., automatically). In case the calibration plate 200 is removed manually, this step may not be part of the method as performed by the control unit 28.

[0084] In an optional step 610, a powder layer to be irradiated may be deposited, for example by instructing, via the control unit 28, the deposition unit 8 to deposit a powder layer.

[0085] In an optional step 612, at least one (e.g., second) image 12 of at least a portion of the powder layer to be irradiated is obtained. The at least one image 12 may be acquired by the image acquisition unit 20.

[0086] In an optional step 614, at least one defect 35 in the deposited powder layer is detected and / or classified based on the at least one image 12. Due to the calibration in step 606, the detection and / or classification of the at least one defect 35 may be conducted under consideration of the geometrical and / or optical properties of the surface feature(s) 34 of the calibration plate. For example, only portions of the image 12 that depict inacceptable defects are identified in step 614, based on the calibration in step 606. This may ensure that inacceptable defects in the powder layer are reliably detected and / or classified.

[0087] In an optional step 616, based on the detected and / or classified at least one defect 35, a warning is output. Alternatively, or in addition, one or more process parameters may be adjusted and / or set based on the detected and / or classified at least one defect 35.

[0088] It is to be understood that the steps 602-608 may be repeated during the manufacturing process of a three-dimensional workpiece. That is, the manufacturing process, including the layerwise deposition and selective solidification, may be interrupted to calibrate the apparatus using the calibration plate 200, and then be continued after the calibration of the apparatus, for example including performance of the steps 612-616.

[0089] In one exemplary configuration, one or more of the steps 602-606 and / or one or more of the steps 612-616 may be performed by a detection system or a processor that is separate (e.g., remote) from the apparatus 100. The detection system or processor may comprise or be communicatively coupled to the image acquisition unit 20 and / or the control unit 28. Other distributed processing arrangements are also possible. In these variants, the entity performing at least the steps 612-616 may be calibrated in step 606 (e.g., instead of calibrating the apparatus 100, the detection system and / or the processor may be calibrated).

[0090] No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and example implementations and encompasses modifications apparent to those skilled in the art and lying within the scope of the claims appended hereto.

Claims

Claims1. A calibration plate (200; 200a; 200b) for calibrating an apparatus (100) for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation (18), the calibration plate (200) comprising at least one surface feature (34) representative of a defect (35a-35p) of a layer to be irradiated.

2. The calibration plate (200; 200a; 200b) of claim 1, wherein the at least one surface feature (34) is a permanent surface feature of the calibration plate.

3. The calibration plate (200; 200a; 200b) of claim 1 or 2, wherein the at least one surface feature (34) has fixed geometrical and / or optical properties.

4. The calibration plate (200; 200a; 200b) of any one of claims 1 to 3, wherein the at least one surface feature (34) has similar optical properties as the defect (35a- 35p) of the layer to be irradiated.

5. The calibration plate (200; 200a; 200b) of claim 3 or 4, comprising a coating (33) defining the optical properties of the at least one surface feature (34).

6. The calibration plate (200; 200a; 200b) of any one of claims 1 to 5, wherein the at least one surface feature (34) has similar geometrical properties as the defect (35a-35p) of the layer to be irradiated.

7. The calibration plate (200; 200a; 200b) of any one of claims 1 to 6, wherein the at least one surface feature comprises selectively solidified powder material.

8. The calibration plate (200; 200a; 200b) of any one of claims 1 to 7, wherein the at least one surface feature (34) is made from the same material as the raw material powder of the layer to be irradiated.

9. The calibration plate (200; 200a; 200b) of any one of claims 1 to 8, wherein the at least one surface feature (34) is representative of a defect (35a-35p) resulting from at least one of the following events:(i) an erroneous deposition of the raw material powder of the layer to be irradiated;(ii) a deposition of spatter or debris produced by irradiating a previous layer of the raw material powder or by irradiating a portion of the layer to be irradiated;(iii) a damage of a previously irradiated portion of the raw material powder.

10. The calibration plate (200; 200a; 200b) of any one of claim 1 to 9, configured to be removably arranged within a build chamber (2) of the apparatus (100).

11. The calibration plate (200; 200a; 200b) of claim 10, configured to be removably arranged in a holder (300) that is removably arrangeable within the build chamber (2) and configured to arrange the calibration plate (200) within the build chamber (2).

12. A method of calibrating an apparatus (100) for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation (18), the method comprising: obtaining at least one image (II) of the calibration plate (200; 200a; 200b) of any one of claims 1 to 11 arranged within a build chamber (2) of the apparatus (100); detecting one or more of the at least one surface feature (34) of the calibration plate (200) based on the obtained at least one image (II); and calibrating the apparatus (100) based on the detected one or more surface features (34).

13. The method of claim 12, wherein the apparatus (100) is calibrated by setting one or more parameters, used by the apparatus (100) to detect and / or classify a defect (35a-35p) of a layer to be irradiated, based on the detected one or more surface features.

14. An apparatus (100) for producing three-dimensional work pieces by irradiating layers of raw material powder with electromagnetic or particle radiation (18), the apparatus comprising a control unit (28) configured to carry out the method according to claim 12 or 13.

15. A system (1000) comprising: at least one calibration plate (200; 200a; 200b) according to any one of claims 1 to 11; andat least one of the following entities:(i) the apparatus (100), for example as defined in claim 14; and(ii) the holder (300).

16. The system (1000) of claim 15, comprising two or more calibration plates(200a, 200b) according to claim 11, the two or more calibration plates (200a, 200b) being configured to be replaceably arranged in the same holder (300) and differing from one another at least in the at least one surface feature (34).

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