WO2019161886A1

WO2019161886A1 - Method for aligning a multi beam irradiation system

Info

Publication number: WO2019161886A1
Application number: PCT/EP2018/054179
Authority: WO
Inventors: Daniel Brueck
Original assignee: SLM Solutions Group AG
Priority date: 2018-02-20
Filing date: 2018-02-20
Publication date: 2019-08-29
Also published as: CN111867754A; CN111867754B; EP3755485A1; JP2021514309A

Abstract

A method for aligning a multi beam irradiation system (20) for use in an apparatus (10) for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation comprises the steps of: i) applying a first raw material powder layer onto a carrier (16) so as to define an irradiation plane (S) to be irradiated with radiation beams (24a, 24b) emitted by the irradiation system (20); ii) producing a first test structure (34) in the first raw material powder layer in an overlap zone (18c) of the irradiation plane (S) using a first radiation beam (24a) emitted by a calibrated first irradiation unit (22a) of the irradiation system (20); iii) producing a second test structure (36) in the first raw material powder layer in the overlap zone (18c) of the irradiation plane (S) using a second radiation beam (24b) emitted by a calibrated second irradiation unit (22b) of the irradiation system (20); iv) determining an offset (dx_t, dy_t) between the first and the second test structure (34, 36) in the irradiation plane (S); and v) aligning at least one of the first and the second calibrated irradiation unit (22a, 22b) based on the determined offset (dx_t, dy_t) between the first and the second test structure (34, 36) in such a manner that the offset does not exceed a threshold value.

Description

Method for aligning a multi beam irradiation system The present invention relates to a method for aligning a multi beam irradiation sys- tem for use in an apparatus for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation.

Further, the invention relates to a method for operating an apparatus for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation which is equipped with a multi beam irradiation system.

Powder bed fusion is an additive layering process by which pulverulent, in particular metallic and/or ceramic raw materials can be processed to three-dimensional work pieces of complex shapes. To that end, a raw material powder layer is applied onto a carrier and subjected to laser radiation in a site selective manner in dependence on the desired geometry of the work piece that is to be produced. The laser radiation penetrating into the powder layer causes heating and consequently melting or sintering of the raw material powder particles. Further raw material powder layers are then applied successively to the layer on the carrier that has already been subjected to laser treatment, until the work piece has the desired shape and size. Selective laser melting or laser sintering can be used in particular for the production of prototypes, tools, replacement parts or medical prostheses, such as, for example, dental or orthopaedic prostheses, on the basis of CAD data.

An apparatus for producing moulded bodies from pulverulent raw materials by a powder bed fusion process is described, for example, in EP 1 793 979 Bl. The prior art apparatus comprises a process chamber which accommodates a plurality of carriers for the shaped bodies to be manufactured. A powder layer preparation system comprises a powder reservoir holder that can be moved to and fro across the carriers in order to apply a raw material powder to be irradiated with a laser beam onto the carriers. The process chamber is connected to a protective gas circuit comprising a supply line via which a protective gas may be supplied to the process chamber in order to establish a protective gas atmosphere within the process chamber.

An exemplary irradiation system which may be employed in an apparatus for producing three-dimensional work pieces by irradiating pulverulent raw materials is de- scribed in EP 2 335 848 Bl. The irradiation system comprises a radiation source, in particular a laser source, and an optical unit The optical unit which is supplied with a radiation beam emitted by the radiation source comprises a beam expander, a scanner unit and an objective lens which is designed in the form of an f-theta lens.

For calibrating the irradiation system and in particular the optical unit employed in an apparatus for producing three-dimensional work pieces by irradiating pulverulent raw materials, a so-called burn-off foil may be applied to the carrier which, during normal operation of the apparatus, carries the raw material powder layers to be irradiated. The burn-off foil then is irradiated according to a predetermined pattern resulting in the development of a burn-off image of the irradiation pattern on the foil. The burn- off image is digitalized and compared to a digital reference image of the irradiation pattern. Based on the result of the comparison between the digitalized burn-off image and the reference image, the irradiation unit is calibrated in order to compensate for deviations between the actual burn-off image and the reference image. Alternatively, a digital image of an irradiation pattern produced by a radiation beam incident on a sensor arrangement arranged in an irradiation plane maybe used for calibrating an irradiation system as described in EP 3 241 668 Al. In particular for the production of larger objects or for increasing the production speed, multi beam irradiation systems as described, for example, in EP 2 875 897 Bl or EP 2 862 651 Al may be employed. These multi beam irradiation systems are equipped with a plurality of irradiation units for emitting a plurality of radiation beams. Each of the radiation beams typically operates in a designated exclusive irradiation zone and in one or more overlapping zone(s).

Even if the irradiation units of a multi beam irradiation system are individually calibrated, for example as described above, the radiation beams emitted by the irradiation system in the overlapping zone(s) typically still show a certain misalignment. In particular, radiation beams that in an overlapping zone are directed to the same x-y coordinate may still be offset relative to each other. This offset may affect the quality of the work piece to be produced.

EP 3 202 524 Al discloses a method for aligning a pair of calibrated lasers of a laser additive manufacturing system in an overlap region in which the pair of calibrated lasers selectively operate. In this method, in a first step, a first plurality of layers of a test structure is formed in the overlap region solely using a first calibrated laser of the pair of calibrated lasers. In a second step, a second plurality of layers of a test structure is formed in the overlap region solely using a second calibrated laser of the pair of calibrated lasers. Thereafter, a dimension of an offset step created between the first plurality of layers and the second plurality of layers in an outer surface of the test structure is measured and the pair of calibrated lasers is aligned by applying the dimension of the offset step as an alignment correction to at least one of the pair of calibrated lasers.

The invention is directed at the object of providing a reliable and efficient method for aligning a multi beam irradiation system for use in an apparatus for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation. Further, the invention is directed at the object of providing a method for operating an apparatus for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation which is equipped with a multi beam irradiation system aligned in accordance with such a reliable and efficient aligning method.

These objects are addressed by a method for aligning a multi beam irradiation system as defined in claim 1 and a method for operating an apparatus for producing a three-dimensional work piece as defined in claim 19.

In a method for aligning a multi beam irradiation system for use in an apparatus for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation, a first raw material powder layer is applied onto a carrier so as to define an irradiation plane to be irradiated with radiation beams emitted by the multi beam irradiation system. The carrier may be arranged within a process chamber which may be sealable against the ambient atmosphere, in order to be able to maintain a controlled atmosphere, in particular an inert atmosphere, within the process chamber. The raw material powder may be a metallic powder, but may also be a ceramic powder or a plastic material powder or a powder containing different materials. The powder may have any suitable particle size or particle size distribution. It is, however, preferable to process powders of particle sizes < 100 pm. The irradiation plane is a plane of the raw material powder layer which is subjected to the radiation beams emitted by the multi beam irradiation system. Typically, the irradiation plane coincides with an upper surface plane of the raw material powder layer. A position of the upper surface plane of the raw material powder layer relative to a surface plane of the carrier in turn may depend on the positioning of a powder application device for applying raw material powder layers onto the carrier, in particular on the positioning of a leveling slider of the powder application device which serves to level and smoothen the upper surface of the raw material powder layer.

The multi beam irradiation system may comprise a radiation source, in particular a laser source, for example a diode pumped Ytterbium fibre laser. The multi beam irradiation system may be provided with only one radiation source. It is, however, also conceivable that the multi beam irradiation system is equipped with a plurality of radiation sources. Further, the multi beam irradiation system may comprise two or more than two irradiation units. Each of the irradiation units may comprise a plurality of optical elements including, for example, a beam expander for expanding a radia- tion beam emitted by the radiation source, a scanner and an object lens. Alternative- ly, the plurality of optical elements provided in each of the irradiation units may comprise a beam expander including a focusing optic and a scanner unit. By means of the scanner unit, the position of a focus of the radiation beam both in the direction of the beam path and in a plane perpendicular to the beam path can be changed and adapted. The scanner unit may be designed in the form of a galvanometer scanner and the object lens may be an f-theta object lens.

In a further step of the method for aligning a multi beam irradiation system, a first test structure is produced in the first raw material powder layer in an overlap zone of the irradiation plane using a first radiation beam emitted by a calibrated first irradia- tion unit. In addition, a second test structure is produced in the first raw material powder layer in the overlap zone of the irradiation plane using a second radiation beam emitted by a calibrated second irradiation unit. The term "overlap zone" here designates a region of the irradiation plane which can be irradiated with the radiation beams of both the first and the second irradiation unit. The first and the second test structure thus are produced in a region of the irradiation plane which is accessible for both the first radiation beam emitted by the first irradiation unit and the second radiation beam emitted by the second irradiation unit.

The first and the second irradiation unit of the multi beam irradiation system may be calibrated prior to performing the method for aligning the multi beam irradiation system. Further, the calibration of the individual irradiation units may be performed either simultaneously or one after the other. For calibrating the irradiation units, any calibration method, for example a calibration method involving the use of a burn-off foil or the calibration method described in EP 3 241 668 Al, may be used as long as it is suitable to substantially eliminate an offset of a radiation beam emitted by an irradiation unit relative to a set coordinate or a reference pattern in the irradiation plane.

In a next step, an offset between the first and the second test structure in the irradiation plane is determined. Basically, it is conceivable to determine an offset between the first and the second test structure in only one direction within the irradiation plane. Preferably, however, an offset between the first and the second test structure in two directions within the irradiation plane is determined. If need be, for example in case an offset between the first and the second test structure in two directions within the irradiation plane should be determined or in case the multi beam irradiation system comprises more than two irradiation units, a plurality of pairs of first and second test structures may be produced.

Thereafter, at least one of the first and the second irradiation unit is aligned based on the determined offset between the first and the second test structure in the irradiation plane. In particular, at least one of the first and the second irradiation unit is aligned in such a manner that the offset between the first and the second test struc- ture in the irradiation plane does not exceed, in particular is reduced below a threshold value. Preferably, the first and the second test structure are shaped and arranged in such a manner that the determined offset between the first and the second test structure in the irradiation plane can directly be used for aligning at least one of the first and the second irradiation unit.

In the method for aligning a multi beam irradiation system, both the first test structure and the second test structure are produced in the first raw material powder layer, i.e. the first and the second test structure are produced in the same raw mate- rial powder layer. As a result, the first and the second test structure may be produced simultaneously which reduces the time required for building the test structures. Further, the test structures can be built with a low built height while still allowing a reliable determination of an offset between the first and the second test structure which occurs in the irradiation plane when the first and the second irradiation unit are not aligned. In the method for aligning a multi beam irradiation system the step of applying a raw material powder layer onto the carrier and at least one of the steps of producing a first test structure in the applied powder layer and of producing a second test struc- ture in the applied powder layer may be repeatedly performed before determining the offset between the first and the second test structure and before aligning at least one of the first and the second calibrated irradiation unit. In this way, a multilayer first test structure and/or a multilayer second test structure may be produced.

In the method for aligning a multi beam irradiation system, only one first test struc- ture and only one second test structure may be produced. It is, however, also conceivable that the method involves the production of a plurality of first test structures and a plurality of second test structures. The test structures may be produced in pairs with each pair of test structures comprising a first and a second test structure, for example in order to determine an offset between the first and the second test structure in two directions within the irradiation plane and/or in order to determine multiple offset values which may be averaged.

The first test structure and the second test structure may be produced during an alignment operation of the apparatus for producing a three-dimensional work piece. The alignment operation of the apparatus may be performed separate from normal operation of the apparatus for producing the three-dimensional work piece and may, for example, be performed after calibrating the individual irradiation units of the multi beam irradiation system, but before starting normal operation of the apparatus for producing a three-dimensional work piece. In other words, calibration/alignment of the apparatus and normal operation of the apparatus may be separated and normal operation of the apparatus for producing a three-dimensional work piece may be started only after completion of the calibration/alignment process. Alternatively or additionally thereto it is, however, also conceivable that the first test structure and the second test structure are produced during normal operation of the apparatus for producing a three-dimensional work piece, i.e. the first and the second test structure and the three-dimensional work piece may be produced simultaneously in a single building process.

The first test structure and the second test structure may be produced in the form of a massive component. A test structure produced in the form of a massive component may be connected to a substrate plate which is arranged on the carrier and covered with subsequent raw material powder layers in order to build up the test structure on the substrate plate. Further, a test structure produced in the form of a massive component may comprise a number of layers that corresponds to a number of layers of a simultaneously produced three-dimensional work piece. Thus, a test structure in the form of a massive component may be produced with a built height that corresponds to a built height of the three-dimensional work piece. It is, however, also conceivable that a test structure produced in the form of a massive component comprises a number of layers that is lower than a number of layers of a simultaneously produced three-dimensional work piece. Also single layer test structures are conceivable. In case the first and the second test structure are produced in the form of a massive component, the dimensions and geometry of the test structures may be measured for determining the offset between the first and the second test structure, while, for determining the offset between the first and the second test structure, the test structures may remain connected to the carrier or the substrate plate or may be released from the carrier or the substrate plate.

Alternatively, the first test structure and the second test structure may be produced in the form of a lost component which is formed separate from a substrate plate and the carrier. Preferably, a test structure produced in the form of a lost component comprises a number of layers that is less than a number of layers of a simultaneously produced three-dimensional work piece. Also single layer test structures are conceivable.

In case the first and the second test structure are produced in the form of a lost component, at least one layer of the first test structure and the second test structure may be arranged coplanar with an intermediate layer of a simultaneously produced three-dimensional work piece that is built at a distance from the carrier. Further, it is conceivable that test structure layers are produced on a regular basis or random or algorithm based. For example, five test structure layers may be produced coplanar with five workpiece layers after each 20 workpiece layers.

The offset between the first and the second test structure may be determined after completion of the production of the first and the second test structure. For example, at least one dimensions and/or a geometry of the test structures may be manually measured after completion of the production of the test structures with the aid of a suitable manual measuring tool. Alternatively or additionally, the offset between the first and the second test structure may be determined during the production of the first and the second test structure, for example after completion of a layer of the first and the second test structure and before starting the production of a further layer of the first and the second test structure.

For determining the offset between the first and the second test structure, an optical measuring device, for example a camera or an optical sensor may be used. In a particular preferred embodiment of the method for the alignment of a multi beam irradiation system, the offset between the first and the second test structure is de termined by means of an optical measuring device which constitutes a component of a melt pool monitoring system, i.e. a system for observing a melt pool created in the course of the introduction of radiation energy into the raw material powder. Alterna- tively, the optical measuring device which is used for determining the offset between the first and the second test structure may be constituted by a component of a layer control system, i.e. a system for monitoring the raw material powder layers applied onto the carrier in order to detect defects or irregularities in the raw material powder layers. The use of an optical measuring device allows the determination of the offset between the first and the second test structure during the production of the first and the second test structure. It is, however, also conceivable to use an optical measur- ing device for manually or automatically determining the offset between the first and the second test structure after completion of the production of the first and the sec- ond test structure.

The optical measuring device used for determining the offset between the first and the second test structure may be designed in the form of a pyrometric detection device that is configured to detect thermal radiation and to output an intensity value indicative of an intensity of the detected thermal radiation. Typically, the intensity of thermal radiation emitted from a previously solidified test structure surface differs from thermal radiation emitted from non-solidified raw material powder surrounding the solidified test structure. Thus, a location of a boundary between a test structure and raw material powder surrounding the test structure may be determined based on the thermal radiation detected by the pyrometric detection device.

For determining the offset between the first and the second test structure, after completion of a layer of the first and the second test structure, a region of the irradi ation plane containing the first and the second test structure may be scanned with a test radiation beam emitted by at least one of the first and the second irradiation unit. In case an optical measuring device which forms a component of a melt pool monitoring system is associated to one of the first and the second irradiation unit, only the irradiation unit associated to the optical measuring device may be used for scanning the region of the irradiation plane containing the first and the second test structure. The region of the irradiation plane to be scanned is set so as to include not only a region in which first and second test structures produced by perfectly aligned first and second irradiation units are expected to be located, but also a surrounding region in order to make sure that the region to be irradiated also contains first and second test structures that are offset relative to each other due to a misalignment of the first and second irradiation units. During scanning of the region of the irradiation plane containing the first and the second test structure, an interaction of the at least one of the first and the second test radiation beam with either the first and the second test structure or raw material powder surrounding the first and the second test structure within the irradiation plane is monitored by means of the optical measuring device. When the at least one of the first and the second test radiation beam is incident on non-solidified raw material powder, an intensity value detected by the optical measuring device typically is higher than an intensity value detected by the optical measuring device when the at least one of the first and the second test radiation beam is incident on a previously solidified test structure surface.

As a result, a location of a boundary between the first and the second test structure and the raw material powder surrounding the first and the second test structure may be determined based on the interaction of the at least one of the first and the second test radiation beam with the first and the second test structure and the raw material powder surrounding the first and the second test structure in the region of the irradiation plane containing the first and the second test structure. In this way, also an offset between the first and the second test structure may be detected.

Preferably, the region of the irradiation plane containing the first and the second test structure is scanned according to a pattern comprising a first plurality of scan vectors extending within the irradiation plane and a second plurality of scan vectors extending within the irradiation plane at an angle, in particular perpendicular to the first plurality of scan vectors. The distance between parallel scan vectors of the scan pattern should be kept small enough to allow an accurate determination of the loca- tion of the boundary between the first and the second test structure and the raw material powder surrounding the first and the second test structure. Preferably, parallel scan vectors of the first and/or the second plurality of scan vectors are unidirectional. A signal detected by an optical measuring device, in particular a pyrometric measuring device of a melt pool monitoring system in a transition region from raw material powder to a previously solidified test structure surface differs from a signal detected by the optical measuring device in a transition region from a previously solidified test structure surface to raw material powder. This signal difference can be disregarded if an unidirectional scanning strategy is employed upon scanning the region of the irradiation plane containing the first and the second test structure.

The region of the irradiation plane containing the first and the second test structure preferably is scanned with the test radiation beam at a beam power and/or a scan speed that is/are lower than a beam power and/or a scan speed of the first or the second radiation beam during production of the first and the second test structure. A lower beam power and a lower scan speed help to distinguish between a signal indicating an interaction of the test radiation beam with a previously solidified test structure surface and an interaction of the test radiation beam with non-solidified raw material powder. In addition, a lower scan speed reduces signal noise and increases the amount of detected data. Consequently, detection accuracy can be increased. The beam power of the test radiation beam may also be zero, which means that the region of the irradiation plane containing the first and the second test structure may be scanned without being irradiated with a test radiation beam having a positive beam power. However, an irradiation of the region of the irradiation plane containing the first and the second test structure with the test radiation beam at a beam power that is above zero has the advantage that the thermal radiation signal to be detected is amplified and hence can easily be distinguished from background noise.

In addition to or as an alternative to the use of an optical measuring device that is configured to detect thermal radiation emitted by the first and the second test struc- ture and the surrounding raw material powder, it is conceivable to determine the offset between the first and the second test structure with the aid of an optical measuring device that is adapted to capture an image of the irradiation plane. For example, an image pickup device of a layer control system of the apparatus for producing a three-dimensional work piece may be used for this purpose.

In particular, the step of determining the offset between the first and the second test structure may include a step of capturing an image, in particular a two-dimensional image of the irradiation plane after completion of a layer of the first and the second test structure. The captured image may contain the entire irradiation plane or only a region of the irradiation plane containing the first and the second test structure. Further, a location of a boundary between the first and the second test structure and the raw material powder surrounding the first and the second test structure may be determined based on the captured image of the irradiation plane, either by means of automatic image processing or by manually, i.e. visually evaluating the captured image. Thus, the image pickup device of the layer control system may comprise a camera and optionally an image processing system.

At least one of the first and the second calibrated irradiation unit may be aligned based on the determined offset between the first and the second test structure in such a manner that the offset does not exceed a threshold value after completion of the production of the first and the second test structure. In other words, the first and the second test structure may be produced. In a next step, the offset between the first and the second test structure in the irradiation plane may be measured and only thereafter, the first and the second irradiation unit may be aligned in dependence on the measured offset. Alternatively or additionally thereto, it is, however, also conceivable that at least one of the first and the second calibrated irradiation unit is aligned based on the deter mined offset between the first and the second test structure in such a manner that the offset does not exceed a threshold value during the production of the first and the second test structure. For example, an alignment procedure for aligning at least one of the first and the second irradiation unit may be performed after completion of a layer of the first and the second test structure, but before generating a further layer of the first and the second test structure. Aligning at least one of the first and the second irradiation unit in order to compensate for an offset between the first and the second test structure in the course of the production of the first and the second test structure is particularly advantageous in case also the offset determination is performed in the course of the production of the first and the second test structure.

Preferably, the first test structure and the second test structure are produced so as to be shaped and arranged that at least one edge of the first test structure is arranged flush with at least one edge of the second test structure when the first irradiation unit and the second irradiation unit are aligned. An offset between the edges of the first and the second test structure then corresponds to an offset between the first radiation beam which is emitted by the first irradiation unit and used for producing the first test structure and the second radiation beam which is emitted by the second irradiation unit and used for producing the second test structure. The edges of the first and the second test structure which are arranged flush with each other when the first irradiation unit and the second irradiation unit are aligned may, for example, extend parallel to an x-direction within the irradiation plane. An offset between the edges then indicates an offset between the first and the second radiation beam in a y-direction within the irradiation plane, i.e. in a direction within the irradiation plane that is perpendicular to the x-direction. To the contrary, if the edges of the first and the second test structure which are arranged flush with each other when the first irradiation unit and the second irradiation unit are aligned extend parallel to the y-direction within the irradiation plane, an offset between the edges then indicates an offset between the first and the second radiation beam in a x- direction within the irradiation plane. In order to determine the offset between the first and the second radiation beam in both the x-direction and the y-direction, test structures with edges parallel to the x-direction and the y-direction or pairs of test structures that are rotated relative to each other by 90° may be produced. In one embodiment, the first test structure may have a rectangular shape and may be produced adjacent to a second test structure which also has a rectangular shape. In particular, the first and the second test structure may have congruent rectangular shapes and may be arranged side-by-side in such a manner that an edge of the first test structure is arranged parallel to the x-direction or the y-direction within the irra- diation plane and flush with an edge of the second test structure when the first irradiation unit and the second irradiation unit are aligned. These test structures are easy to produce. Further, an offset between the test structures can easily be determined. However, in case an offset in two directions within the irradiation plane should be determined, two pairs of test structures have to be built up.

Alternatively, the first and the second test structure may be substantially line-shaped and may form a cross when the first irradiation unit and the second irradiation unit are aligned. Such a design of the test structures allows the determination of an offset of the radiation beams of the irradiation units within the irradiation plane in two directions with the aid of a single pair of test structures. However, line-shaped test structures are difficult to be handled for manually measuring an offset between the test structures after completion of the built-up of structures. Line-shaped test struc- tures therefore are preferably employed for offset measurements that are performed during the production of the first and the second test structure as explained above.

In a further embodiment, the first test structure has a rectangular shape that fits into a cut-out provided in a substantially L-shaped second test structure when the first irradiation unit and the second irradiation unit are aligned. The first and the second test structure may be arranged so as to contact each other. It is, however, also con- ceivable to arrange the first and the second test structure at a distance from each other in order to avoid that a raw material powder region is irradiated by both of the first and the second radiation beams due to a misalignment of the irradiation units. A test structure design with a first test structure that fits into a cut-out provided in a substantially L-shaped second test structure is particular advantageous for aligning more than two, for example four, irradiation units of a multi beam irradiation system. For example, the L-shaped second test structure may be produced with the aid of an irradiation system which later is used as a reference system for aligning the other irradiation systems relative thereto.

In further preferred embodiment of the method for aligning a multi beam irradiation system, the first and the second test structure are produced on a common base. The test structures then may be disconnected from a substrate platform and/or the carri er together with the common base and can easily be handled during measuring the offset, for example with the aid of a manual measurement tool.

A method for operating an apparatus for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radia tion which is equipped with a multi beam irradiation system comprises the step of aligning the multi beam irradiation system of the apparatus in accordance with a method as described above. Further raw material powder layers are then applied onto the carrier and irradiated with the first and the second irradiation beam emitted by the calibrated and aligned first and second irradiation units in order to produce a three-dimensional work piece while maintaining a constant distance between a beam emission plane and the irradiation plane. As a result, the alignment of the first and the second irradiation unit can be reliably maintained, not only during the production of a single work piece, but also during subsequent production processes. This en sures a high quality of the work pieces to be produced also in the overlap zone. In a preferred embodiment of the method of operating an apparatus for producing a three-dimensional work piece, in order to maintain a constant distance between the beam emission plane and the irradiation plane, a powder application device config- ured to apply raw material powder layers onto the carrier is suitably positioned rela- tive to the beam emission plane. In particular a levelling slider of the powder application device which serves to level the raw material powder layer applied onto the carrier may be suitably positioned relative to the beam emission plane in order to ensure that a constant distance between the beam emission plane and the irradiation plane is maintained, also after maintenance of the powder application device which involves an exchange of the leveling slider of the powder application device.

The distance between the beam emission plane and the irradiation plane should be set as accurate as possible. The powder application device therefore may be positioned relative to the beam emission plane with the aid of a positioning tool that allows a positioning of the powder application device relative to the carrier and hence the setting of the distance between the beam emission plane and the irradiation plane, in particular with an accuracy of ± 2 pm, preferably ± 1 pm.

Preferred embodiments of the invention now are described in greater detail with reference to the appended schematic drawings wherein

Fig. 1 shows an apparatus for producing a three-dimensional work pieces by selectively irradiating electromagnetic or particle radiation onto a raw material powder;

Figs. 2a to e illustrate the dependency of an offset between a first radiation beam emitted by a first irradiation unit and a second radiation beam emitted by a second irradiation unit of the apparatus according to figure 1 on a distance between a beam emission plane and an irradiation plane;

Figs. 3a and b show a first embodiment of a test structure design;

Figs. 4a and b show a second embodiment of a test structure design;

Figs. 5a and b show a third embodiment of a test structure design; Figs. 6a and b show a modification of the third embodiment of a test structure design;

Figs. 7a and b illustrate a manual determination of an offset between a first and a second test structure of the test structure design according to Figs. 6a and b;

Fig. 8 shows a positioning tool for positioning a powder application device of the apparatus according to figure 1 in a state posi- tioned in a build chamber of the apparatus; and

Fig. 9 shows the positioning of the powder application device 14 with the aid of the positioning tool according to figure 8.

Figure 1 shows an apparatus 10 for producing a three-dimensional work piece. The apparatus 10 comprises a process chamber 12. A powder application device 14, which is disposed in the process chamber 12, serves to apply a raw material powder onto a carrier 16. In the arrangement of figure 1, a raw material powder layer 15 is applied onto the carrier 16. As indicated by the arrow A, the carrier 16 is designed to be displaceable in a vertical direction into a build chamber 19 so that, with increasing construction height of a work piece, as it is built up in layers from the raw material powder on the carrier 16, the carrier 16 can be moved downwards in the vertical direction. The powder application device 14 comprises a leveling slider 17 which serves to level and smoothen the raw material powder layer 15 applied onto the carrier 16.

A first and a second irradiation zone 18a, 18b are defined on a surface of the raw material powder layer 15 and the carrier 16, respectively, in a side-by-side arrangement, i.e. in the apparatus 10 depicted in Figure 1, a left half of the raw material powder layer 15 and the carrier 16, respectively, defines the first irradiation zone 18a, while a right half of the raw material powder layer 15 and the carrier 16, respectively, defines the second irradiation are 18b. In the region of a boundary between the first and the second irradiation zone 18a, 18b, an overlap zone 18c is defined.

The apparatus 10 comprises an irradiation system 20 for selectively irradiating the raw material powder applied onto the carrier 16. The operation of the irradiation system 20 is controlled by means of a control device 21. By means of the irradiation system 20, the raw material powder applied onto the carrier 16 may be subjected to radiation in a site-selective manner in dependence on the desired geometry of the work piece that is to be produced. The irradiation system 20 comprises a first and a second irradiation unit 22a, 22b. The first irradiation unit 22a is adapted to irradiate the first irradiation zone 18a and the overlap zone 18c defined on the surface of the raw material powder layer 15 with a first radiation beam 24a. The second irradiation unit 22b is adapted to irradiate the second irradiation zone 18b and the overlap zone 18c defined on the surface of the raw material powder layer 15 with a second radia- tion beam 24b.

The radiation beams 24a, 24b exit the first and the second irradiation unit 22a, 22b in a beam emission plane B. The upper surface of the raw material powder layer 15 defines an irradiation plane S where the radiation beams 24a, 24b emitted by the irradiation units 22a, 22b are incident on the raw material powder. A distance between the irradiation plane S and the beam emission plane B is indicated with D. As will be described in more detail below, the position of the irradiation plane S relative to the beam emission plane B and hence the distance D between the irradiation plane S and the beam emission plane B depends on the position of the powder appli- cation device 14 and in particular a lower surface of the leveling slider 17 relative to the beam emission plane B. In order to accurately position the powder application device 14 as desired, the apparatus 10 therefore comprises a positioning tool 27 which is configured to position the powder application device 14 and hence the lower leveling surface of the leveling slider 17 relative to the beam emission plane B with an accuracy of ± 2 pm, in particular ± 1 pm. The positioning tool 27 may be provided separate from the apparatus 10 and be inserted into the process chamber 12 of the apparatus 10 only if necessary.

As shown in figure 8, the positioning tool 27 comprises two leveling plates 40, 42 which may be arranged in the irradiation plane S. In order to position the leveling plates 40, 42 in the irradiation plane S, the positioning tool 27 comprises a support structure 44 which is adapted to span the build chamber 19 of the apparatus 10.

Each of the leveling plates 40, 42 is connected to a distance measuring device 46, 48 which is adapted to measure a distance of an object to the respective leveling plate 40, 42. A lever 50 serves to lower or raise the leveling plates 40, 42 by a fixed distance between an upper position and a lower position. In use of the positioning tool 27, the positioning tool 27 is arranged in the process chamber 12 and the position of the leveling plates 40, 42 is set so that the leveling plates 40, 42 are arranged in the irradiation plane S. Thereafter, the leveling plates 40, 42 are lowered by activating the lever 50 in order to allow the powder application device 14 to be positioned above the leveling plates 40, 42 without colliding therewith. After the powder application device 14 has reached the desired location, the leveling plates 40, 42 are raised again into the irradiation plane S such that the leveling plates 40, 42 abut against the lower leveling surface of the leveling slider 17 as shown in figure 9. Thereafter, the leveling slider 17 is raised with the aid of a mi- crometer screw 52 provided on the powder application device 14 until it's lower level- ing surface is arranged at a desired distance of for example 200 pm ± 10 pm from the leveling plates 40, 42. When the position of the leveling slider 17 is set, the level- ing plates 40, 42 can again be lowered to by activating the lever 50, the powder application device 14 can be moved and the positioning tool 27 can be removed from the process chamber 12.

Both irradiation units 22a, 22b are associated with a laser beam source 26, for example a diode pumped Ytterbium fibre laser emitting laser light at a wavelength of approximately 1070 to 1080 nm. Each of the first and the second irradiation unit 22a, 22b comprises an optical unit for guiding and/or processing the radiation beam emitted by the laser beam source 26 which may, for example include a beam expander for expanding the radiation beam, a scanner and an object lens. Alternatively, the optical unit of the irradiation units 22a, 22b may comprise a beam expander including a focusing optic and a scanner unit. By means of the scanner unit, the position of the focus of the radiation beam both in the direction of the beam path and in a plane perpendicular to the beam path can be changed and adapted. The scanner unit may be designed in the form of a galvanometer scanner and the object lens may be an f- theta object lens. In the embodiment of an apparatus 10 depicted in figure 1, the radiation beams 24a, 24b emitted by the irradiation units 22a, 22b are laser beams.

The apparatus 10 further comprises a melt pool monitoring system 28 for monitoring a melt pool that is generated due to the interaction of a radiation beam 24a, 24b with the raw material powder. The melt pool monitoring system 28 comprises an optical measuring device 30, which in the embodiment of an apparatus 10 shown in figure 1 is designed in the form of a pyrometric detection device. The optical measur- ing device 30 is configured to receive and detect thermal radiation that may be elec tromagnetic radiation in the visual and/or the infrared wavelength range having an intensity maximum at a specific wavelength depending on the temperature of the melt pool where the raw material powder is heated and melted. Based on the detected thermal radiation, the optical measuring device 30 outputs an intensity value that is indicative of the detected radiation intensity.

Further, the apparatus 10 comprises a layer control system 31 which serves to monitor the raw material powder layers 15 applied onto the carrier 16 in order to detect defects or irregularities in the raw material powder layers 15. The layer control system 31 comprises an optical measuring device 32, which in the embodiment of an apparatus 10 shown in figure 1 is designed in the form of an image pickup device comprising a camera and an image processing system. The optical measuring device 32, in particular the camera of the optical measuring device 32, is configured to capture a two-dimensional image of the irradiation plane S. Further, the image processing system of the optical measuring device 32 is configured to automatically process the two-dimensional image of the upper surface of the raw material powder layer 15 in order to detect defects or irregularities. As an alternative, the image processing system may be omitted and the images captured by the optical measuring device 32 may be visually evaluated by an operator of the apparatus 10. In figure 2, figures 2a, 2b and 2c depict a schematic plan view, a schematic front view and a schematic side view, respectively, of the irradiation units 22a, 22b and a scan field plate 33 used for calibrating the first and the second irradiation unit 22a, 22b. An upper surface of the scan field plate 33 defines a calibration irradiation plane C which is arranged at a defined distance from the beam emission plane B. Like the irradiation plane S, also the calibration irradiation plane C comprises the first irradiation zone 18a which can be irradiated by the first irradiation unit 22a, the second irradiation zone 18b which can be irradiated by the second irradiation unit 22b and the overlap zone 18c which can be irradiated by both the first and the second irradiation unit 22a, 22b.

The first irradiation unit 22a of the irradiation system 20 is calibrated, for example as described in EP 3 241 668 Al, in such a manner that, in the calibration irradiation plane C, an offset of the first radiation beam 24a emitted by the first irradiation unit 22a relative to a reference pattern does not exceed a threshold value. Further, also the second irradiation unit 22b of the irradiation system 20 is calibrated, for example as described in EP 3 241 668 Al, in such a manner that, in the calibration irradiation plane C, an offset of the second radiation beam 24b emitted by the second irradiation unit 22a relative to a reference pattern does not exceed a threshold value.

After calibrating the first and the second irradiation unit 22a, 22b, an offset between the first and the second radiation beam 24a, 24b in the overlap zone 18c of the d calibration irradiation plane C is negligible, i.e. the first and the second irradiation unit 22a, 22b can be controlled by the control device 21 in such a manner that an incident spot of the first radiation beam 24a in the overlap zone 18c of the designat ed calibration irradiation plane C coincides with an incident spot of the second radia- tion beam 24b in the overlap zone 18c of the calibration irradiation plane C, see figures 2a to 2c.

As shown in figures 2d and 2e, for producing a three-dimensional work piece during normal operation of the apparatus 10, the powder layer 15 to be irradiated with the first and the second radiation beam 24a, 24b emitted by the first and the second irradiation unit 22a, 22b is applied onto the carrier 16 by means of the powder appli cation device 14. As a result, the irradiation plane S of the raw material powder layer 15 which is defined by the surface of the raw material powder layer 15 and which, during operation of the apparatus 10, is subjected to the radiation beams 24a, 24b emitted by the irradiation units 22a, 22b, is arranged substantially parallel to and at a distance d from the calibration irradiation plane C. The distance d depends on the position of the powder application device 14 and in particular a leveling slider 17 of the powder application device 14 relative to the beam emission plane B. As depicted in figures 2d and 2e, the offset existing between the calibration irradiation plane C and the irradiation plane S in a z-direction inevitably leads to an offset dx_r, dy_r of the radiation beams 24a, 24b emitted by the irradiation units 22a, 22b in an x-direction and a y-direction within the irradiation plane S. As becomes apparent from figure 2d, an offset dx_r of the radiation beams 24a, 24b in the x-direction within the irradiation plane S typically is less critical, since the incidence spots of both radiation beams 24a, 24b on the irradiation plane S are shifted by the same amount dx_r. To the contrary, an offset dy_r of the radiation beams 24a, 24b in the y-direction within the irradiation plane S results in incidence spots of the radiation beams 24a, 24b on the irradiation plane S that are spaced from each other in the y-direction by 2dy_r.

For correcting the offset between the incidence spots of the radiation beams 24a,

24b emitted by the irradiation units 22a, 22b, a first test structure 34 is produced in the overlap zone 18c of the irradiation plane S using the calibrated first irradiation unit 22a. In addition, a second test structure 36 is produced in the overlap zone 18c of the irradiation plane S using the calibrated second irradiation unit 22b. Different embodiments of first and second test structures 34, 36 are shown in figures 3 to 6. However, all pairs of test structures 34, 36 have in common that they are produced adjacent to each other in the same raw material powder layers.

In a next step, an offset dx_t, dy_t between the first and the second test structure 34, 36 in the irradiation plane S is determined. Thereafter, at least one of the first and the second irradiation unit 22a, 22b is aligned based on the determined offset dx_t, dy_t between the first and the second test structure 34, 36 in the irradiation plane S in such a manner that the offset dx_t, dy_t between the first and the second test structure 34, 36 in the irradiation plane S is reduced and finally no longer exceeds a threshold value.

The exemplary test structures 34, 36 shown in figures 3 to 6, which will be discussed in more detailed further below, are shaped and arranged in such a manner that the determined offset dx_t, dy_t between the first and the second test structure 34, 36 in the irradiation plane S can directly be used for aligning at least one of the first and the second irradiation unit 22a, 22b. In order to compensate for a determined offset dxt, dy_t of the test structures 34, 36 either only the first irradiation unit 22a may be corrected in order to shift the incidence spot of the first radiation beam 24a on the irradiation plane S, or only the second irradiation unit 22b may be corrected in order to shift the incidence spot of the second radiation beam 24b on the irradiation plane S by -dx_r and -dy_r, or both irradiation units 22a, 22b may be corrected in order to bring the incidence spots of the first and the second radiation beam 24a, 24b into alignment. This may, for example, be achieved by shifting one of the irradiation units 22a, 22b by -dx_t, -dy_t or by shifting both irradiation units 22a, 22b by an appropriate amount in order to compensate for the offset dx_t, dy_t.

The first and the second test structure 34, 36 may be produced during an alignment operation of the apparatus 10 which is performed separate from normal operation of the apparatus 10 for producing a three-dimensional work piece. For example, the first and the second test structure 34, 36 may be produced in the course of an alignment operation of the apparatus 10 which is performed after completion of a building process and prior to starting a new building process. It is, however, also possible that the first and the second test structure 34, 36 are produced during nor- mal operation of the apparatus 10, i.e. the first and the second test structure 34, 36 and a three-dimensional work piece may be produced simultaneously in the course of a single building process. The first and the second test structure 34, 36 may be produced in the form of a massive component which is connected to a substrate plate arranged on the carrier 16 for building the test structures 34, 36 and optionally also a three-dimensional work piece thereon. In case the test structures 34, 36 are produced during normal operation of the apparatus 10, the test structures 34, 36 may be formed with a num- ber of layers that corresponds to a number of layers of the simultaneously produced three-dimensional work piece. Test structures 34, 36 in the form of a massive component, after completion of the building process, may be measured, for example with the aid of a micrometer screw as shown in figure 7, in order to determine the offset dx_t, dy_t between the first and the second test structure 34, 36. For measuring the offset dx_t, dy_t between the first and the second test structure 34, 36, the test struc- tures 34, 36 may remain connected to the carrier 16 or the substrate plate or may be released from the carrier 16 or the substrate plate. In order to improve the handling of the test structures 34, 36 during releasing the test structures 34, 36 from the carrier 16 and during measuring the offset dx_t, dy_t between the test structures 34, 36, the test structures 34, 36 may be produced on a common base.

Alternatively, the first test structure 34 and the second test structure 36 may be produced in the form of a lost component which is formed separate from a substrate plate. In particular, test structures 34, 36 produced in the form of lost components may be produced with a number of layers that is less than a number of layers of a simultaneously produced three-dimensional work piece. At least one layer of the first test structure 34 and the second test structure 36 may be arranged coplanar with an intermediate layer of a simultaneously produced three-dimensional work piece that is built at a distance from the carrier 16. Further, it is conceivable that test structure layers are produced on a regular basis or random or algorithm based. For example, five test structure layers may be produced coplanar with five workpiece layers after each 20 workpiece layers.

The offset dx_t, dy_t between the first and the second test structure 34, 36 may be determined only after completion of the production of the first and the second test structure 34, 36. Alternatively or additionally, the offset dx_t, dy_t between the first and the second test structure 34, 36 may be determined already during the production of the first and the second test structure 34, 36, for example after completion of a layer of the first and the second test structure 34, 36 and before starting the production of a further layer of the first and the second test structure 34, 36. In the apparatus 10 depicted in figure 1, the offset dx_t, dy_t between the first and the second test structure 34, 36 may be determined with the aid of the optical measuring device 30 of the melt pool monitoring system 28 and/or the with the aid of the optical measuring device 32 of the layer control system 31. In order to measure the offset dx_t, dy_t between the first and the second test structure 34, 36 with the aid of the optical measuring device 30 of the melt pool monitoring system 28, after completion of a layer of the first and the second test structure 34, 36, a region of the irradiation plane S containing the first and the second test structure 34, 36 is scanned with a test radiation beam emitted by at least one of the first and the second irradiation unit 22a, 22b. The region of the irradiation plane S to be scanned with the test radiation beam is set so as to include not only a region in which first and second test structures 34, 36 produced by perfectly aligned first and second irradiation units 22a, 22b are expected to be located, but also a surrounding region in order to make sure that the region to be scanned also contains first and second test structures 34, 36 that are offset relative to each other due to a misalignment of the first and second irradiation units 22a, 22b.

In particular, the region of the irradiation plane S containing the first and the second test structure 34, 36 is scanned with the test radiation beam according to a pattern comprising a first plurality of scan vectors extending within the irradiation plane S and a second plurality of scan vectors extending within the irradiation plane S at an angle, in particular perpendicular to the first plurality of scan vectors. The parallel scan vectors of the first and the second plurality of scan vectors are unidirectional. Further, the region of the irradiation plane S containing the first and the second test structure 34, 36 is scanned with the test radiation beam at a beam power and a scan speed that are lower than a beam power and a scan speed of the first and the second radiation beam 24a, 24b during production of the first and the second test structure 34, 36. Finally, the region of the irradiation plane S containing the first and the second test structure 34, 36 is unidirectionally irradiated. During scanning of the region of the irradiation plane S containing the first and the second test structure 34, 36 an interaction of the test radiation beam with either the first and the second test structure 34, 36 or raw material powder surrounding the first and the second test structure 34, 36 within the irradiation plane S is monitored by means of the optical measuring device 30. When the test radiation beam is incident on non-solidified raw material powder, an intensity value detected by the optical measuring device 30 is higher than an intensity value detected by the optical meas- uring device 30 when the test radiation beam is incident on a previously solidified test structure surface. As a result, a location of a boundary between the first and the second test structure 34, 36 and the raw material powder surrounding the first and the second test structure 34, 36 and thus an offset dx_t, dy_t between the first and the second test structure 34, 36 can be determined.

In case the optical measuring device 32 of the layer control system 31 should be used for determining the offset dx_t, dy_t between the first and the second test struc^¬ ture 34, 36, an image, in particular a two-dimensional image of the irradiation plane S after completion of a layer of the first and the second test structure 34, 36 may be captured. The captured image may contain the entire irradiation plane S or only a region of the irradiation plane S containing the first and the second test structure 34, 36. A location of a boundary between the first and the second test structure 34, 36 and the raw material powder surrounding the first and the second test structure 34, 36 may be determined based on the captured image of the irradiation plane S, either by means of automatic image processing or by manually, i.e. visually evaluating the captured image.

The irradiation units 22a, 22b may be aligned based on the determined offset dx_t, dy_t between the first and the second test structure 34, 36 in such a manner that the offset dx_t, dy_t does not exceed a threshold value only after completion of the production of the first and the second test structure 34, 36. Alternatively or additionally thereto, it is, however, also conceivable that the irradiation units 22a, 22b are aligned based on the determined offset dx_t, dy_t between the first and the second test structure 34, 36 in such a manner that the offset dx_t, dy_t does not exceed a thresh- old value already during the production of the first and the second test structure 34, 36. For example, an alignment procedure for aligning the irradiation units 22a, 22b may be performed after completion of a layer of the first and the second test structure 34, 36, but before generating a further layer of the first and the second test structure 34, 36. Like the determination of the offset dx_t, dy_t between the first and the second test structure 34, 36, also the alignment of the irradiation units 22a, 22b may be performed either manually or automatically for example, under the control of the control device 21. Various exemplary test structure designs are shown in figures 3 to 6. As becomes apparent from these figures, the design of the test structures 34, 36 allows that the first and the second test structure 34, 36 are produced adjacent to each other in the same raw material powder layers, i.e. in the same plane substantially parallel to a surface of the carrier 16. As a result, the first and the second test structure 34, 36 may be produced simultaneously and with a low built height. Further, the various test structure designs according to figures 3 to 6 have in common that the first test structure 34 and the second test structure 36 are shaped and arranged that at least one edge of the first test structure 34 is arranged flush with at least one edge of the second test structure 36 when the first irradiation unit 22a and the second irradiation unit 22b are aligned. In other words, the test structures 34, 36 are shaped and ar ranged in such a manner that an offset dx_r, dy_r between the radiation beams 24a, 24b emitted by the first and the second irradiation unit 22a, 22b results in a corre- sponding offset dx_t, dy_t between the edges of the first and the second test structure 34, 36 that extend flush relative to each other when the irradiation units 22a, 22b are aligned.

Figure 3 shows a first embodiment of a test structure design, wherein figure 3a illus- trates the appearance of a test structure arrangement produced by aligned irradia^¬ tion units 22a, 22b, whereas figure 3b illustrates the appearance of a test structure arrangement produced by irradiation units 22a, 22b emitting radiation beams 24a, 24b that are offset relative to each other both in the x-direction and the y-direction within the irradiation plane S. The test structure arrangement comprises two pairs of first and second test structures 34, 36. Both first test structures 34 have a rectangular shape and are produced and arranged adjacent to a second test structure 36 in the same raw material powder layers. The second test structures 36 also have a rectangular shape that is congruent to the rectangular shape of the first test structures 34. In each of the pairs of test structures 34, 36, the first and the second test structure 34, 36 are arranged side-by-side.

In the first pair of test structures 34, 36, which in figure 3 is shown on the left-hand side, a lower edge 38 of the first test structure 34 is arranged parallel to the x- direction within the irradiation plane S and flush with a lower edge 40 of the second test structure 36 when the first irradiation unit 22a and the second irradiation unit 22b are aligned, see figure 3a. To the contrary, when the first irradiation unit 22a and the second irradiation unit 22b are not aligned, an offset dy_t in the y-direction within the irradiation plane S occurs between the lower edge 38 of the first test structure 34 and the lower edge 40 of the second test structure 36 which corre- sponds to an offset dy_r between the radiation beams 24a, 24b emitted by the irradia^¬ tion units 22a, 22b.

In the second pair of test structures 34, 36, which in figure 3 is shown on the right side and which is rotated relative to the first pair of test structures 34, 36 by 90°, a left edge 42 of the first test structure 34 is arranged parallel to the y-direction within the irradiation plane S and flush with a left edge 44 of the second test structure 36 when the first irradiation unit 22a and the second irradiation unit 22b are aligned, see figure 3a. To the contrary, when the first irradiation unit 22a and the second irradiation unit 22b not are aligned, an offset dx_t in the x-direction within the irradiation plane S occurs between the left edge 42 of the first test structure 34 and the left edge 44 of the second test structure 36 which corresponds to an offset dx_r between the radiation beams 24a, 24b emitted by the irradiation units 22a, 22b.

The test structure arrangement according to figure 3 can easily be produced. Fur- ther, the offset(s) between the test structures 34, 36 can easily be measured, for example with the aid of a manual measuring tool or by means of an optical measur- ing device 30, 32 as described above. However, in case an offset in both the x- direction and the y-direction within the irradiation plane S should be determined, two pairs of test structures 34, 36 need to be produced and measured.

An alternative test structure arrangement is depicted in figure 4 wherein figure 4a illustrates the appearance of a test structure arrangement produced by aligned irradiation units 22a, 22b, whereas figure 4b illustrates the appearance of a test structure arrangement produced by irradiation units 22a, 22b emitting radiation beams 24a, 24b that are offset relative to each other both in the x-direction and the y- direction within the irradiation plane S. Each of the first and the second test structure 34, 36 is substantially line-shaped. The first test structure 34 defines a left upper part of a cross, whereas the second test structure 36 defines a right lower part of the cross.

As becomes apparent from figure 4b, an offset dx_r, dy_r between the radiation beams 24a, 24b emitted by the irradiation units 22a, 22b leads to an offset dx_t, dy_t between the test structures 34, 36 so that an offset dx_r, dy_r of the radiation beams 24a, 24b in both directions within the irradiation plane S can be determined with the aid of a single pair of test structures 34, 36. However, a manual determination of the offset dx_t, dy_t between the line-shaped test structures 34, 36 is not possible or at least very difficult, so that the test structure arrangement according to figure 4 is particularly advantageous for an offset determination process which is performed already during the production of the first and the second test structure 34, 36.

A further alternative test structure arrangement which is in particular suitable for use in a process for aligning more than two irradiation units, in particular four irradiation units is depicted in figure 5. Figure 5a illustrates the appearance of a test structure arrangement produced by aligned irradiation units, whereas figure 5b illustrates the appearance of a test structure arrangement produced by irradiation units emitting radiation beams that are offset relative to each other both in the x-direction and the y-direction within the irradiation plane S. An upper part of figure 5 shows test structures 34, 36 that are built up in an overlap zone 18c₃₄ that can be irradiated by a third and a fourth irradiation unit. A central part of figure 5 shows test structures 34, 36 that are built up in an overlap zone 18c₂₃ that can be irradiated by a second and third irradiation unit. A lower part of figure 5 shows test structures 34, 36 that are built up in an overlap zone 18CI₂ that can be irradiated by a first and the second irradiation unit.

Each of the first test structures 34 has a rectangular shape that fits into a cut-out provided in a corresponding substantially L-shaped second test structure 36 when the irradiation units used for producing the test structures 34, 36 are aligned, see figure 5a. To the contrary, an offset between the radiation beams emitted by the irradiation units that a used for producing the respective test structures 34, 36 leads to an offset between the test structures 34, 36, see figure 5b. Thus, measuring the offset dx_t43, dy_t43 of the test structures 34, 36 depicted in the upper part of figure 5b allows the determination of an offset between the radiation beams emitted by the third and the fourth irradiation unit in both the x-direction and the y-direction within the irradiation plane S. Measuring the offset dx_t32, dy_t32 of the test structures 34, 36 depicted in the central part of figure 5b allows the determination of an offset between the radiation beams emitted by the second and the third irradiation unit in both the x-direction and the y-direction within the irradiation plane S. Finally, meas- uring the offset dx_ti2, dy_ti2 of the structures 34, 36 depicted in the lower part of figure 5b allows the determination of an offset between the radiation beams emitted by the first and the second irradiation unit in both the x-direction and the y-direction within the irradiation plane S. The first and the second test structure 34, 36 may be arranged so as to contact each other as shown in figure 5. It is, however, also conceivable to arrange the first and the second test structure 34, 36 at a distance from each other as shown in figure 6, wherein figure 6a illustrates the appearance of a test structure arrangement pro- duced by aligned irradiation units, whereas figure 6b illustrates the appearance of a test structure arrangement produced by irradiation units emitting radiation beams that are offset relative to each other both in the x-direction and the y-direction within the irradiation plane S. The arrangement according to figure 6 minimizes the risk of double irradiation of a raw material powder region due to a misalignment of the irradiation units that a used for producing the test structures 34, 36.

As shown in figure 7, the test structures 34, 36 may be measured, for example with the aid of a micrometer screw. As shown in figure 7a, in a first step, a set extension in the x-direction may be determined by measuring the extension of the solid (in figure 7a upper) part of the second test structure 36 in the x-direction. In a second step, the extension of the combination of the first and the second test structure 34, 36 in the x-direction may be measured as shown in figure 7b in order to determine the actual extension in the x-direction. An offset dx_t between the first and the second test structure 34, 36 then may be determined by forming the difference between the set extension in the x-direction as measured according to figure 7a and the actual extension in the x-direction as measured according to figure 7b. The determination of an offset dy_t between the first and the second test structure 34, 36 in the y- direction may be performed accordingly.

For aligning four irradiation units based on an offset measurement using the test structure arrangement according to figure 5, in a first step, the third irradiation unit may be aligned in order to compensate for the measured offset dx_t3₂, dyo2, for example by shifting the third irradiation unit by -dx_t32 and -dy_t32 in order to shift the radiation beam emitted by the first irradiation unit by -dx_r32 and -dy_r3₂. The first irradiation unit is shifted by -dx_ti2 and -dy_ti2 in order to shift the radiation beam emitted by the first irradiation unit by -dx_ri2 and -dy_ri2. Thereafter, the fourth irradiation unit is shifted by (-dx_t43 - dx_t32) and (dy_t43 - dym) in order to shift the radiation beam emitted by the fourth irradiation unit by (-dx_t43 - dx_t32) and (dy_t43 - dy_t32) while con- sidering the previous shifting of the third irradiation unit. The position of the second irradiation unit is maintained constant - the second irradiation unit thus is used as a kind of reference irradiation unit. It is, however, also conceivable to use another irradiation unit, for example the third irradiation unit, as a fixed position reference irradiation unit.

During normal operation of the apparatus 10 after aligning the irradiation units 22a, 22b, a plurality of raw material powder layers 15 are applied onto the carrier 16 and irradiated with the first and the second irradiation beam 24a, 24b emitted by the calibrated and aligned first and second irradiation units 22a, 22b in order to produce a three-dimensional work piece, while a constant distance D between the beam emission plane B and the irradiation plane S is maintained. As a result, the calibration and alignment status of the irradiation units 22a, 22b is maintained and a continuing high quality of the three-dimensional work piece can be obtained. The distance D between the beam emission plane B and the irradiation plane S should be maintained constant not only during the production of a single work piece, but also during sub- sequent work piece production processes. As a result, additional calibration and/or alignment procedures between the subsequent work piece production processes can be dispensed with.

In order to maintain a constant distance D between the beam emission plane B and the irradiation plane S, the powder application device 14 and in particular the leveling slider 17 of the powder application device 14 is suitably positioned relative to the beam emission plane B by means of the positioning tool 27. In particular, the positioning tool 27 is used for positioning the powder application device 14 in such a manner that a constant distance D between the beam emission plane B and the irradiation plane S is maintained also after an exchange of the leveling slider 17.

Claims

1. Method for aligning a multi beam irradiation system (20) for use in an appa- ratus (10) for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation, the method comprising the steps of:

i) applying a first raw material powder layer onto a carrier (16) so as to define an irradiation plane (S) to be irradiated with radiation beams (24a, 24b) emitted by the irradiation system (20);

ii) producing a first test structure (34) in the first raw material powder layer in an overlap zone (18c) of the irradiation plane (S) using a first radiation beam (24a) emitted by a calibrated first irradiation unit (22a) of the irradiation system (20);

iii) producing a second test structure (36) in the first raw material powder layer in the overlap zone (18c) of the irradiation plane (S) using a second radiation beam

(24b) emitted by a calibrated second irradiation unit (22b) of the irradiation system

(20);

iv) determining an offset (dx_t, dy_t) between the first and the second test structure (34, 36) in the irradiation plane (S); and

v) aligning at least one of the first and the second calibrated irradiation unit (22a,

22b) based on the determined offset (dx_t, dy_t) between the first and the second test structure (34, 36) in such a manner that the offset does not exceed a threshold value. 2. The method according to claim 1,

wherein step i) and at least one of steps ii) and iii) are repeatedly performed before carrying out steps iv) and v) in order to produce at least one of a multilayer first test structure (34) and a multilayer second test structure (36). 3. The method according to claim 1 or 2,

wherein a plurality of first test structures (34) and a plurality of second test structures (36) are produced.

4. The method according to any one of claims 1 to 3,

wherein the first test structure (34) and the second test structure (36) are produced: - during an alignment operation of the apparatus (10) which is performed separate from normal operation of the apparatus (10) for producing a three-dimensional work piece and/or

- during normal operation of the apparatus (10) for producing a three-dimensional work piece.

5. The method according to anyone of claims 1 to 4,

wherein the first test structure (34) and the second test structure (36) are produced:

- in the form of a massive component in particular being connected to a substrate plate and/or in particular comprising a number of layers that corresponds to a number of layers of a simultaneously produced three-dimensional work piece, or

- in the form of a lost component formed separate from a substrate plate in particular comprising a number of layers that is less than a number of layers of a simultaneously produced three-dimensional work piece.

6. The method according to claim 5,

wherein, when the first test structure (34) and the second test structure (36) are produced in the form of a lost component, at least one layer of the first test structure (34) and the second test structure (36) is arranged coplanar with an intermediate layer of the three-dimensional work piece that is built at a distance from the carrier

(16).

7. The method according to anyone of claims 1 to 6,

wherein the offset (dx_t, dy_t) between the first and the second test structure (34, 36) is determined:

- after completion of the production of the first and the second test structure (34, 36) and/or

- during the production of the first and the second test structure (34, 36). 8. The method according to anyone of claims 1 to 7,

wherein the offset (dx_t, dy_t) between the first and the second test structure (34, 36) is determined by means of an optical measuring device (30, 32), the optical measuring device (30) in particular forming a component of a melt pool monitoring system (28) or a component of a layer control system (3).

9. The method according to claim 8, wherein the optical measuring device (30) is designed in the form of a pyrometric detection device that is configured to detect thermal radiation and to output an intensity value indicative of an intensity of the detected thermal radiation. 10. The method according to claim 8 or 9,

wherein determining the offset (dx_t, dyt) between the first and the second test structure (34, 36) includes:

- after completion of a layer of the first and the second test structure (34, 36), scanning a region of the irradiation plane (S) containing the first and the second test structure (34, 36) with a test radiation beam emitted by at least one of the first and the second irradiation unit (22a, 22b);

- monitoring an interaction of the test radiation beam with either the first and the second test structure (34, 36) or raw material powder surrounding the first and the second test structure (34, 36) by means of the optical measuring device (30); and - determining a location of a boundary between the first and the second test structure (34, 36) and the raw material powder surrounding the first and the second test structure (34, 36) based on the interaction of the test radiation beam beam with the first and the second test structure (34, 36) and the raw material powder surrounding the first and the second test structure (34, 36).

11. The method according to claim 10,

wherein the region of the irradiation plane (S) containing the first and the second test structure (34, 36) is scanned with the test radiation beam according to a pattern comprising a first plurality of scan vectors extending within the irradiation plane (S) and a second plurality of scan vectors extending within the irradiation plane (S) at an angle, in particular perpendicular to the first plurality of scan vectors.

12. The method according to claim 11,

wherein parallel scan vectors of the first and/or the second plurality of scan vectors are unidirectional.

13. The method according to anyone of claims 10 to 12,

wherein the region of the irradiation plane (S) containing the first and the second test structure (34, 36) is scanned with the test radiation beam at a beam power and/or a scan speed that is/are lower than a beam power and/or a scan speed of the first and/or the second radiation beam (24a, 24b) during production of the first and the second test structure (34, 36).

14. The method according to any one of claims 9 to 13,

wherein determining the offset (dx_t, dy_t) between the first and the second test structure (34, 36) includes:

- after completion of a layer of the first and the second test structure (34, 36) capturing an image, in particular a two-dimensional image of the irradiation plane (S); and

- determining a location of a boundary between the first and the second test structure (34, 36) and the raw material powder surrounding the first and the second test structure (34, 36) based on the captured image of the irradiation plane (S).

15. The method according to anyone of claims 1 to 14,

wherein at least one of the first and the second calibrated irradiation unit (22a, 22b) is aligned based on the determined offset (dx_t, dy_t) between the first and the second test structure (34, 36) in such a manner that the offset does not exceed a threshold value:

- during the production of the first and the second test structure (34, 36).

16. The method according to anyone of claims 1 to 15,

wherein the first test structure (34) and the second test structure (36) are produced so as to be shaped and arranged that at least one edge (38, 42) of the first test structure (34) is arranged flush with at least one edge (40, 44) of the second test structure (36) when the first irradiation unit (22a) and the second irradiation unit (22b) are aligned.

17. The method according to anyone of claims 1 to 16,

wherein the first test structure (34) has a rectangular shape and is produced adja- cent to a second test structure (36) which also has a rectangular shape.

18. The method according to anyone of claims 1 to 17,

wherein the first and the second test structure (34, 36) are substantially line-shaped and form a cross when the first irradiation unit (22a) and the second irradiation unit (22b) are aligned.

19. The method according to anyone of claims 1 to 17, wherein the first test structure (34) has a rectangular shape that fits into a cut-out provided in the second test structure (36) which is substantially L-shaped when the first irradiation unit (22a) and the second irradiation unit (22b) are aligned. 20. The method according to anyone of claims 1 to 19,

wherein the first and the second test structure (34, 36) are produced on a common base.

21. Method for operating an apparatus (10) for producing a three-dimensional work piece by irradiating layers of a raw material powder with electromagnetic or particle radiation which is equipped with a multi beam irradiation system (20) the method comprising the steps of:

- aligning the multi beam irradiation system (20) in accordance with a method as defined in anyone of claims 1 to 20; and

- applying further raw material powder layers onto the carrier (16) and irradiating said raw material powder layers with the first and the second irradiation beam (24a, 24b) emitted by the calibrated and aligned first and second irradiation units (22a, 22b) in order to produce a three-dimensional work piece while maintaining a constant distance (D) between a beam emission plane (B) and the irradiation plane (S).

22. The method according to claim 21,

wherein, in order to maintain a constant distance (D) between the beam emission plane (B) and the irradiation plane (S), a powder application device (14) configured to apply raw material powder layers onto the carrier (16) is suitably positioned rela- tive to the beam emission plane (B), wherein the positioning of the powder application device (14) in particular is effected by means of a positioning tool (27) that is configured to allow a positioning of the powder application device (14) relative to the beam emission plane (B) in particular with an accuracy of ± 2 pm, preferably ± 1 pm.