Improvements in or relating to coaxial melt pool sensors in additive manufacturing equipment

Abstract

The present invention relates to a method for aligning a coaxial melt pool sensor (123) in an additive manufacturing device. The method comprises: using a first optical chain (106a, 106b, 106c, 106d) to scan a first laser beam (118a, 118b, 118c, 118d) on a work surface (110) along a first scanning path (L1) to generate a melt pool (130) along the first scanning path (L1); and using a second optical chain (106a, 106b, 106c, 106d) to scan a field of view (132) of a coaxial sensor (123) on the work surface along a second scanning path (L2) for steering a second laser beam (118a, 118b, 118c, 118d). The first scanning path and the second scanning path (L1, L2) intersect. When the field of view (132) is scanned along the second scanning path (L2), the adjustment to be made to the alignment of the field of view (132) of the coaxial sensor (123) and the optical axis of the second optical chain (106a, 106b, 106c, 106d) is determined based on the change in the signal generated by the coaxial sensor (123).

Description

Technical Field

[0001] The present invention relates to improvements in or relating to coaxial melt pool sensors in additive manufacturing apparatus, in particular additive manufacturing apparatus using lasers to melt material. The invention applies particularly, but not exclusively, to laser-based powder bed fusion apparatus, and more particularly to powder bed fusion apparatus comprising a plurality of optical chains, each for independently manipulating a laser beam, and at least one of the optical chains comprising a coaxial melt pool sensor. Background Art

[0002] Powder bed fusion equipment produces objects by solidifying materials such as metal powder materials layer by layer using a high-energy beam such as a laser beam. A powder layer is formed on a powder bed contained in a build sleeve by lowering a build platform in the build sleeve to lower the powder bed, dropping a pile of powder near the lowered powder bed, and spreading the pile of powder (from one side of the powder bed to the other side) on the powder bed with a recoater to form the layer. Portions of the powder layer corresponding to the cross-section of the workpiece to be formed are then solidified by irradiating these areas with a beam. The beam melts or sinters the powder to form a solidified layer. After selective solidification of the layer, the powder bed is reduced by the thickness of the newly solidified layer, and another layer of powder is spread on the surface as needed and solidified.

[0003] During the selective laser melting of materials, particularly metals, a hot, high-speed vapor plume is emitted from the molten pool, which cools to form a fine mist of metal "condensate" nanoparticles. In addition, larger, irregular spatter particles are ejected from the boiling molten pool. Furthermore, the pressure drop caused by the movement of the vapor plume draws powder near the molten pool, thereby throwing it upward and away from the powder bed.

[0004] WO 2017 / 085470 discloses an optical module comprising multiple optical chains for independently steering multiple laser beams onto a powder bed.

[0005] International patent application WO 2019 / 092414 discloses a detector module that can be mounted to an optical module (such as the optical module disclosed in WO 2017 / 085470) to detect radiation emitted from the melt pool region during powder melting and collected by an optical chain. For example, the radiation can be radiation emitted by the melt pool itself (usually infrared wavelengths) or radiation emitted by a plasma plume generated above the melt pool during the melting process (usually visible wavelengths). The detector module includes a flexure for adjusting the relative position of the optical axis of the sensor to the mounting position on the optical module.

[0006] It has been found that changing the scan direction of the laser beam over the powder bed affects the intensity of the signal generated by the coaxial melt pool sensor, even when the change in conditions at the melt pool is expected to be small for the change in scan direction. This intensity change may cause the signal intensity to be too low for some scan directions, so that the signal indicating anomalies (such as spatter particles or changes in porosity) is masked by the underlying noise in the signal. Summary of the invention

[0007] According to a first aspect of the present invention, there is provided a method for aligning a coaxial melt pool sensor in an additive manufacturing device, the method comprising: using a first optical chain to scan a first laser beam on a work surface along a first scanning path to generate a melt pool along the first scanning path; and using a second optical chain to scan a field of view of the coaxial sensor on the work surface along a second scanning path for manipulating a second laser beam, wherein the first scanning path and the second scanning path intersect; and when the field of view is scanned along the second scanning path, determining adjustments to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the second optical chain based on changes in a signal generated by the coaxial sensor.

[0008] In this way, when the second laser beam is used to form a molten pool, the field of view of the coaxial molten pool sensor can be aligned with the optical axis to compensate for the misalignment of the field of view with the emission generated by the second laser beam. The alignment may include moving an image plane relative to the coaxial molten pool sensor maintained in a fixed position, or moving the coaxial molten pool sensor relative to a fixed image plane. This can prevent the signal intensity from dropping for certain scanning directions of the second laser beam. For a calibrated system, one or more expected intersections of the first scanning path and the second scanning path are known. However, due to the misalignment of the field of view of the coaxial sensor with the optical axis of the second optical chain, the molten pool generated by the first laser beam may not be centered in the field of view at one or more expected intersections, but may be centered before or after one or more expected intersections within the field of view. The actual position of the molten pool generated by the first laser beam centered in the field of view of the coaxial sensor can be determined based on properties (such as, the intensity of the signal generated by the coaxial sensor). This mismatch between the actual position(s) and the expected intersection(s) can be used to determine adjustments to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the second optical chain. This alignment can be performed manually, for example by an operator or service personnel, by adjusting an alignment mechanism (such as a flexure) associated with the coaxial sensor to adjust the position of the field of view of the coaxial sensor relative to the optical axis of the second optical chain. The method may include generating an output (such as a display on a screen) indicating the adjustments to be made. Alternatively, the additive manufacturing apparatus may include a motorized system for adjusting the alignment of the field of view of the coaxial sensor relative to the optical axis, and the method may include controlling the motorized system to adjust the position of the field of view of the coaxial sensor relative to the optical axis of the second optical chain.

[0009] It will be understood that the term "intersecting" as used herein refers to both space and time. Thus, the movement of the first laser beam and the field of view can be moved and timed spatially relative to each other so that the molten pool falls within the field of view at a point along which they travel along the first scan path and the second scan path.

[0010] The coaxial sensor may include an integrating sensor (such as a photodiode) that generates a signal by integrating the signal over the field of view, and the attribute is the intensity of the signal generated by the integrating sensor. In an alternative embodiment, the coaxial sensor is a position sensitive device (PSD), such as a CCD or CMOS device, which can generate a spatial representation of the intensity in the field of view. In this alternative embodiment, the attribute can be the intensity integrated over the field of view or at a specified position (such as the center of the PSD), or a property of the melt pool and / or plume obtainable from the spatial representation of the intensity in the field of view, such as a measurement of whether the melt pool and / or plume is centered on the PSD (e.g., the area of ​​the melt pool and / or plume that falls within the field of view). Alignment of the field of view of a coaxial sensor comprising an array of photodetector elements may include selecting a subset of the photodetector elements of the array from which values ​​are recorded, for example, such that the field of view of the subset of photodetector elements is aligned with the optical axis of the second optical chain. For example, the entire array may be large enough so that the area of ​​interest (molten pool and plume) falls on the photodetector array even if misaligned. However, reading out the values ​​from all photodetector elements of such a large array may be too slow. Thus, alignment may include selecting a subset of photodetector values ​​from which to read out values ​​based on an adjustment determined according to the first aspect of the invention. The selected subset of photodetector elements may be those that image the molten pool and / or plume. Reading out values ​​from only a subset of photodetector elements may allow for faster readout speeds than reading out values ​​from all photodetector elements of the array. Fast readout rates are advantageous in additive manufacturing when an operator wants to obtain feedback on the evolution of the molten pool / plume in a time comparable to the solidification time of the molten pool.

[0011] The second scanning path may include a change in the scanning direction on the working surface, for example, the scanning path may include a scan in a first direction and a scan in a second direction orthogonal to the first direction. In this way, information about the misalignment of the field of view with the optical axis of the second optical chain can be obtained for orthogonal directions (X and Y). The second scanning path may include an oscillating scanning path, such as a sinusoidal scanning path. The second scanning path may start and end at the same position, for example, the coincidence of an oscillating scanning path on a closed contour such as a circular path. A second scanning path with a smooth transition between directions is preferred so as to move within the dynamic capabilities of the manipulation optics of the second optical chain.

[0012] The first scanning path may be symmetrical (with respect to reflection or rotation) to the second scanning path. In this way, synchronization of the first laser beam and the field of view is maintained. In one embodiment, the first scanning path and the second scanning path are oscillating scanning paths that are π out of phase.

[0013] The method may include: for each of a plurality of optical chains of the additive manufacturing apparatus, aligning the field of view of the corresponding coaxial melt pool sensor with the optical axis of the optical chain using the method according to the first aspect of the invention. For two or more of the optical chains, the first laser beam used to determine the alignment of the field of view of the coaxial melt pool sensor may be the same laser beam.

[0014] In one embodiment, the additive manufacturing device includes three or more optical chains (for example, four optical chains), each optical chain is used to manipulate the corresponding laser beam to the work surface, and the method includes: using the first optical chain among the optical chains to scan the first laser beam on the work surface along the first scanning path to generate a molten pool along the first scanning path; and using the second optical chain among the optical chains to scan the field of view of the coaxial sensor on the work surface along the second scanning path for manipulating the second laser beam, wherein the first scanning path and the second scanning path intersect; and using the third optical chain among the optical chains to scan the third laser beam on the work surface along the third scanning path to generate a molten pool along the third scanning path; and using the second optical chain among the optical chains to scan the field of view of the coaxial molten pool sensor on the work surface along a fourth scanning path, wherein the third scanning path and the fourth scanning path intersect; and when the field of view is scanned along the second scanning path and the fourth scanning path, determining the adjustment to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the second optical chain based on the change of the signal generated by the coaxial sensor.

[0015] According to a second aspect of the present invention, there is provided an additive manufacturing apparatus for manufacturing an object layer by layer by laser melting, the additive manufacturing apparatus comprising: a first optical chain comprising a first movable optical component for manipulating a first laser beam on a working surface; a second optical chain comprising a second movable optical component for manipulating a second laser beam on the working surface; a coaxial melt pool sensor for detecting radiation emitted by a melt pool and collected by the second movable optical component of the second optical chain; and a controller, the controller being arranged to control the first movable component and the second movable component of the first optical chain and the second optical chain, so that the first laser beam is scanned on the working surface along a first scanning path using the first optical chain to generate a melt pool along the first scanning path, the field of view of the coaxial sensor is scanned on the working surface along a second scanning path using the second optical chain, and adjustments to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the second optical chain are determined based on changes in a signal generated by the coaxial sensor when the field of view is scanned along the second scanning path, wherein the first scanning path and the second scanning path intersect.

[0016] The additive manufacturing device may include a third optical chain, which includes a third movable optical component for manipulating a third laser beam on the working surface, and the controller is arranged to: use the third optical chain to scan the third laser beam on the working surface along a third scanning path to generate a molten pool along the third scanning path; and use the second optical chain to scan the field of view of the coaxial sensor on the working surface along a fourth scanning path, wherein the third scanning path and the fourth scanning path intersect; and when the field of view is scanned along the second scanning path and the fourth scanning path, determine the adjustment to be made to the alignment of the field of view of the coaxial sensor and the optical axis of the second optical chain according to the change of the signal generated by the coaxial sensor.

[0017] The additive manufacturing apparatus may include an adjustment mechanism for adjusting the position of the field of view of the coaxial melt pool sensor relative to the optical axis of the second optical chain. The adjustment mechanism may be manually adjustable, and the additive manufacturing apparatus includes a display, the controller being arranged to control the display to output an indication of adjustments to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the second optical chain. Alternatively or in addition, the adjustment mechanism may include a motor and a drive mechanism for moving the optics of a detector module including the coaxial melt pool sensor and / or the coaxial melt pool sensor, the controller being arranged to control the motor to align the field of view of the coaxial sensor with the optical axis of the second optical chain based on the determined alignment.

[0018] The additive manufacturing apparatus may include a position sensor (such as an encoder) for measuring the position of the optical device of the detector module and / or the coaxial melt pool sensor relative to the optical axis of the second optical chain. The controller may control the display to provide feedback about the position of the coaxial sensor and / or control the motor based on a signal from the position sensor.

[0019] The additive manufacturing apparatus may include a powder bed fusion additive manufacturing apparatus, the powder bed fusion additive manufacturing apparatus comprising a building platform that can be lowered in a building sleeve and a layer forming device for forming a powder layer on the building platform to form a powder bed, wherein the working surface is the upper surface of the powder bed. However, in an alternative embodiment, the working surface is the upper surface of a building base plate or a sacrificial plate mounted on the building platform.

[0020] These movable optical components may include optical components, such as mirrors, rotatable around an axis under the control of a motor. Alternatively or additionally, these movable optical components may include optical components, such as mirrors, movable along a linear axis, such as on a stage system.

[0021] According to a third aspect of the present invention, there is provided a controller for an additive manufacturing device for manufacturing an object layer by layer by laser melting, the additive manufacturing device comprising: a first optical chain, the first optical chain comprising a first movable optical component for manipulating a first laser beam on a working surface; a second optical chain, the second optical chain comprising a second movable optical component for manipulating a second laser beam on the working surface; a coaxial melt pool sensor, the coaxial melt pool sensor being used to detect radiation emitted by a melt pool and collected by the second movable optical component of the second optical chain, the controller being configured to control the additive manufacturing device to perform the method according to the first aspect of the present invention.

[0022] According to a fourth aspect of the present invention, there is provided a data carrier having instructions stored thereon, which, when executed by a controller of an additive manufacturing device for manufacturing an object layer by layer by laser melting, causes the controller to control the additive manufacturing device to perform a method according to the first aspect of the present invention, wherein the additive manufacturing device comprises: a first optical chain comprising a first movable optical component for manipulating a first laser beam on a working surface; a second optical chain comprising a second movable optical component for manipulating a second laser beam on the working surface; and a coaxial melt pool sensor for detecting radiation emitted by a melt pool and collected by the second movable optical component of the second optical chain.

[0023] According to a fifth aspect, there is provided a method of normalizing signals from a plurality of coaxial melt pool sensors in an additive manufacturing apparatus, the additive manufacturing apparatus comprising a plurality of optical chains, wherein each optical chain is arranged to independently steer a corresponding laser beam to a work surface, and each melt pool sensor is arranged to observe the work surface through a corresponding one of the optical chains, the method comprising:-

[0024] forming a molten pool on the work surface using one of the corresponding laser beams;

[0025] controlling a first one of the optical chains so that the molten pool falls within a field of view of a first one of the molten pool sensors, the first molten pool sensor observes the work surface through the first one of the optical chains, and recording a first signal using the first one of the molten pool sensors;

[0026] controlling a second one of the optical chains so that the molten pool, or another molten pool generated on the work surface using one of the corresponding laser beams having scanning parameters set to achieve the same predetermined flux at the work surface as that used to form the molten pool, falls within the field of view of a second one of the molten pool sensors, which observes the work surface through the second one of the optical chains, and records a second signal using the second one of the molten pool sensors; and

[0027] The first signal and the second signal are compared, and a scaling of the signal from one or both melt pool sensors is determined based on the comparison.

[0028] In this way, the response to the coaxial melt pool sensors can be adjusted so that both the first melt pool sensor and the second melt pool sensor give the same response to a melt pool formed under the same flux conditions. In this way, the signals from the first melt pool sensor and the second melt pool sensor can be displayed / processed together without generating artifacts due to the differential response between the first melt pool sensor and the second melt pool sensor for the same melting conditions.

[0029] In a preferred embodiment, the second optical chain is controlled so that the second melt pool sensor observes the same melt pool as the first melt pool sensor. In this way, any differences in the melt pools observed by the first melt pool sensor and the second melt pool sensor are reduced or eliminated. However, it will be understood that in another embodiment, the second optical chain is controlled so that the second melt pool sensor observes a different melt pool but formed under the same flux conditions. It will be expected that this other melt pool will generate substantially the same melt pool and plume, and therefore generate substantially the same response from the melt pool sensor, thereby allowing scaling to be determined based on a comparison of the first signal and the second signal. This may be necessary, for example, if the scanning fields of the first optical chain and the second optical chain do not overlap.

[0030] Scaling may be selected such that the first signal and the second signal become equal after scaling.

[0031] The method may include performing the above method multiple times to obtain multiple pairs of first signals and second signals. Each pair of first signals and second signals may be obtained for different positions of the molten pool and / or another molten pool on the work surface. For example, due to changes in the shape of the laser spot and / or changes in the transmission of the emitted energy through the optical components of these optical chains (such as, f-θ lenses), the molten pool sensor response may be different for different positions of the molten pool on the work surface. Thus, it may be desirable to determine the scaling of these molten pool sensor signals based on the position of the molten pool on the work surface. Each pair of first signals and second signals may be obtained for a molten pool / another molten pool formed by different laser beams. Different laser beams may result in slightly different molten pool sensor responses, and therefore, it may be desirable to determine different scalings of molten pools generated using different laser beams.

[0032] This method can be performed, for example, after the first melt pool sensor and the second melt pool sensor are aligned with the optical axes of the corresponding first optical chain and the second optical chain using the method of the first aspect of the present invention.

[0033] According to a sixth aspect of the present invention, there is provided an additive manufacturing apparatus for manufacturing an object layer by layer by laser melting, the additive manufacturing apparatus comprising: a plurality of optical chains, each optical chain comprising a movable optical component for manipulating a corresponding laser beam on a work surface; a first coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the movable optical component of a first of the optical chains; a second coaxial melt pool sensor for detecting radiation emitted by the melt pool or another melt pool and collected by the movable optical component of a second of the optical chains; and a controller arranged to:

[0034] forming a molten pool on the work surface using one of the corresponding laser beams;

[0035] controlling the first of the optical chains so that the molten pool falls within the field of view of the first molten pool sensor, and recording a first signal with the first molten pool sensor;

[0036] controlling the second of the optical chains so that the molten pool, or another molten pool generated on the work surface using one of the corresponding laser beams having scanning parameters set to achieve the same predetermined flux at the work surface as used to form the molten pool, falls within the field of view of the second molten pool sensor, and recording a second signal using the second of the molten pool sensors; and

[0037] The first signal and the second signal are compared, and a scaling of the signal from one or both melt pool sensors is determined based on the comparison.

[0038] According to the seventh aspect of the present invention, there is provided a controller for an additive manufacturing device for manufacturing an object layer by layer by laser melting, the additive manufacturing device comprising: a first optical chain, the first optical chain comprising a first movable optical component for manipulating a first laser beam on a working surface; a second optical chain, the second optical chain comprising a second movable optical component for manipulating a second laser beam on the working surface; a first coaxial melt pool sensor, the first coaxial melt pool sensor being used to detect radiation emitted by the melt pool and collected by the first movable optical component of the first optical chain; and a second coaxial melt pool sensor, the second coaxial melt pool sensor being used to detect radiation emitted by the melt pool and collected by the second movable optical component of the second optical chain, the controller being configured to control the additive manufacturing device to perform the method according to the fifth aspect of the present invention.

[0039] According to an eighth aspect of the present invention, there is provided a method for mapping position measurements generated by a position sensor to the position of a spot on a work surface of a laser beam controlled by a movable optical component, the position sensor measuring the position of the movable optical component of an optical chain of an additive manufacturing device, the method comprising: forming marks on the work surface by scanning the laser beam on the work surface using the movable optical component; recording position measurements generated by the position sensor during the formation of these marks; measuring the positions of these marks on the work surface using an imaging device; and generating a mapping diagram that maps the position measurements from the position sensor to positions on the work surface based on the position measurements generated by the position sensor during the formation of these marks and the measured positions of these marks on the work surface.

[0040] In this way, the position of the laser beam on the working surface can be determined from these position measurements. The movable optical component may include an optical component (such as a mirror) that can rotate around an axis and a position sensor, a rotary encoder for measuring the position of the optical component around the axis. The position of the laser beam on the working surface can be indicated by the angle of the optical component around the axis, and by the deflection of the laser beam along the optical axis with respect to the position of the optical component. Therefore, in order to derive the exact position on the working surface from the position sensor that measures the position of the movable optical component, it is advantageous to calibrate the optical chain as a whole. The method provides a mapping of the position measurements of the movable optical elements to positions in a planar working surface, which takes into account the error sources in the entire optical chain.

[0041] The map may be generated by interpolating between points on the work surface at which the marks are made and for which position measurements are recorded to generate a map for other positions of the laser beam on the work surface.

[0042] The optical chain may include two movable optical components for manipulating the laser beam in a vertical direction, each movable optical component having a position sensor associated therewith, and the method includes: recording position measurements generated by each position sensor during the formation of these marks; and generating a mapping diagram that maps the position measurements from the position sensors to two-dimensional positions on the work surface based on the position measurements generated by the position sensors during the formation of these marks and the measured positions of these marks on the work surface.

[0043] According to a ninth aspect of the present invention, there is provided a method for aligning a coaxial melt pool sensor in an additive manufacturing device, the method comprising: using an optical chain to scan a laser beam along a plurality of scanning paths on a work surface to generate melt pools along the plurality of scanning paths, some of the scanning paths extending in different directions in the plane of the work surface; recording sensor values ​​using a coaxial sensor, the coaxial sensor observing the melt pools through the optical chain; and determining adjustments to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the optical chain based on changes in the sensor values ​​generated by the coaxial sensor and the directions of the scanning paths.

[0044] This method can allow alignment of the coaxial melt pool sensor without requiring another optical train for delivering another laser beam to the work plane. This may be necessary in a machine that includes only one laser scanner, although it will be understood that this aspect of the invention is not limited to a single laser machine.

[0045] The plurality of scan paths may include opposing scan path groups, each group including at least one first scan path extending in a first direction in the plane of the work surface and at least one second scan path extending in a second direction antiparallel to the first direction, the opposing scan paths between the groups extending in a transverse direction. Determining the adjustment may include determining a difference in measured values ​​for the at least one first scan path and for the at least one second scan path based on at least one of the scan path groups, and determining the adjustment based on the difference.

[0046] The controller may be arranged to determine, for each of the scan path groups, a difference in the measured values ​​for the at least one first scan path and for the at least one second scan path, and to identify the direction of the adjustment based on the difference.

[0047] The adjustment may be based on the first and second directions of the set of opposite scan paths having the largest difference between measurements determined for the at least one first scan path and the at least one second scan path. The direction of the adjustment may be determined based on the sign of the difference.

[0048] According to the tenth aspect of the present invention, there is provided an additive manufacturing device for manufacturing an object layer by layer by laser melting, the additive manufacturing device comprising: an optical chain comprising a movable optical component for manipulating a laser beam on a working surface; a coaxial melt pool sensor for detecting radiation emitted by a melt pool and collected by the movable optical component of the optical chain; and a controller arranged to: control the movable components of the optical chain to cause the laser beam to scan along a plurality of scanning paths on the working surface, thereby generating a melt pool along the plurality of scanning paths, some of the scanning paths extending in different directions in the plane of the working surface; and determine adjustments to be made to the alignment of the field of view of the coaxial sensor with the optical axis of the optical chain based on changes in sensor values ​​generated by the coaxial sensor and the directions of the scanning paths.

[0049] The controller may be arranged to determine, for each scan path direction, a measurement value derived from a plurality of sensor values ​​generated during scanning of the laser beam along that scan path direction, and to determine the adjustment to form the determined measurement value.

[0050] The multiple scanning paths may include opposite scanning path groups, each group including at least one first scanning path extending in a first direction in the plane of the working surface and at least one second scanning path extending in a second direction antiparallel to the first direction, the opposite scanning paths between the groups extending in a lateral direction, and the controller may determine the difference in measured values ​​for the at least one first scanning path and for the at least one second scanning path for each opposite scanning path group, and determine the adjustment based on the differences for these opposite scanning path groups.

[0051] The adjustment may be based on the first and second directions of the set of opposing scan paths having the largest difference between measurements determined for the at least one first scan path and the at least one second scan path and the sign of the difference (whether positive or negative).

[0052] Each group may include a plurality of first scanning paths and a plurality of second scanning paths.

[0053] According to an eleventh aspect of the present invention, a controller for an additive manufacturing device for manufacturing an object layer by layer by laser melting is provided, the additive manufacturing device comprising: an optical chain, the optical chain comprising a movable optical component for manipulating the laser beam on a working surface; and a coaxial melt pool sensor, the coaxial melt pool sensor being used to detect radiation emitted by the melt pool and collected by the movable optical component of the optical chain, the controller being configured to control the additive manufacturing device to perform the method according to the ninth aspect of the present invention.

[0054] According to the twelfth aspect of the present invention, there is provided a data carrier having instructions stored thereon, which, when executed by a controller of an additive manufacturing device for manufacturing an object layer by layer by laser melting, causes the controller to control the additive manufacturing device to perform the method according to the ninth aspect of the present invention, the additive manufacturing device comprising: an optical chain comprising a movable optical component for manipulating a laser beam on a working surface; and a coaxial melt pool sensor for detecting radiation emitted by a melt pool and collected by the movable optical component of the optical chain.

[0055] The data carrier may be a non-transitory data carrier, such as a volatile memory (e.g. RAM), a non-volatile memory (e.g. ROM), a flash memory and a data storage device (e.g. a hard disk, an optical disk); or a transient data carrier (e.g. an electronic signal or an optical signal). BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 An additive manufacturing apparatus according to an embodiment of the present invention is shown;

[0057] Figure 2 yes Figure 1 A plan view of the additive manufacturing apparatus shown;

[0058] Figure 3 yes Figure 1 and Figure 2 A perspective view of a laser beam steering system used in the apparatus shown;

[0059] Figure 4a shows the field of view of a coaxial melt pool sensor relative to the melt pool and the plume generated by forming the melt pool for different scanning directions when the field of view is not aligned with the optical axis of the laser beam;

[0060] Figure 4b A typical hatch scanning strategy used in additive manufacturing is shown;

[0061] Figure 4c shows that when the field of view is Figure 4a Figure 2 shows how the signal response from a coaxial melt pool sensor can change with scan direction when the laser beam is misaligned.

[0062] Figure 5 is a schematic diagram of the misalignment between the field of view and the laser spot in two dimensions;

[0063] Figures 6a to 6c A method for determining misalignment of a field of view with an optical axis of a laser beam for one dimension is presented;

[0064] Figure 7 A scanning strategy for determining misalignment of the field of view with the optical axis of the laser beam in two dimensions is demonstrated;

[0065] Figures 8a to 8e The expected signal response for different alignments of the field of view with the optical axis of the laser beam is shown;

[0066] Fig. 9 A scanning strategy for determining the misalignment of each detector module with its corresponding optical module is presented;

[0067] Fig.10 (a) and Fig.10 (b) shows the different signal responses of the coaxial optical sensor for different scanning directions when the field of view of the coaxial optical sensor is offset from the optical axis of the optical chain;

[0068] Fig.11 A scanning strategy for determining misalignment of an optical axis of a coaxial sensor and an optical module according to another embodiment of the present invention is presented;

[0069] Fig.12a is a plot showing coaxial sensor values ​​for a misaligned coaxial sensor, Figure 12b is the polar plot of the RMS value for each scan path direction, and Fig.12c is a plot showing the difference between the RMS sensor values ​​for each opposing scan path group; and

[0070] Fig.13a is a plot showing the coaxial sensor values ​​for a well aligned coaxial sensor, Fig.13b is the polar plot of the RMS value for each scan path direction, and Fig.13c is a plot showing the difference between the RMS sensor values ​​for each opposing scan path group. DETAILED DESCRIPTION

[0071] refer to Figure 1 and Figure 2 , an additive manufacturing apparatus according to an embodiment of the present invention includes a build chamber 101 having partitions 115, 116 defining a build volume 117 therein. A build platform 102 can be lowered in the build volume 117. When a workpiece is built by selective laser melting of powder, the build platform 102 supports a build substrate 102a, a powder bed 104, and a workpiece 103. As successive layers of the workpiece 103 are formed, the platform 102 is lowered within the build volume 117 under the control of a motor (not shown).

[0072] When the workpiece 103 is built by the dispensing device 108 and the wiper 109, a powder layer 104 is formed. For example, the dispensing device 108 can be a device as described in WO 2010 / 007396. The dispensing device 108 dispenses the powder onto an upper surface 115a defined by the partition 115, and spreads it on the powder bed by the wiper 109. The position of the lower edge of the wiper 109 defines a working surface 110, where the powder is consolidated.

[0073] A plurality of laser modules 105a, 105b, 105c and 105d generate laser beams 118a, 118b, 118c, 118d for melting the powder 104, which are steered as needed by corresponding optical modules 106a, 106b, 106c, 106d. The laser beams 118a, 118b, 118c, 118d enter through a common laser window 107. Each optical module includes a steering optical device 121 (such as two mirrors 141a, 141b (see FIG. 1 ) mounted on galvanometers 124a, 124b) for steering the laser beam 118 in the vertical direction (X and Y) across the working plane 110. Figure 3 )) and focusing optics 120 (such as two movable lenses for changing the focus of the laser beam 118). The scanner is controlled so that the focal position of the laser beam 118 remains in the same plane 110 as the laser beam 118 moves on the working plane 110. Instead of using a dynamic focusing element to maintain the focal position of the laser beam in a plane, an f-theta lens can be used. An angular position sensor 125a, 125b is integrated into each galvanometer 124a, 124b for measuring the angular position of the corresponding mirror 121a, 121b.

[0074] Each optical module 106a, 106b, 106c, 106d includes a beam splitter 122 that reflects the laser beam 118 and transmits the wavelength of radiation from the working plane 110 of the powder bed 104. The beam splitter 122 can be arranged to transmit a wavelength different from the wavelength of the laser beam. The radiation passing through the beam splitter 122 is delivered to the detector module 126 and is imaged by a coaxial molten pool sensor 123 in the form of one or more photodetector elements. The molten pool sensor 123 can be a single photodetector that integrates the intensity over the field of view, or a position sensitive device (such as a CCD). The optical system may include additional filters to filter out wavelengths of no interest before the radiation is incident on the molten pool sensor 123. For example, only visible light (such as emitted by a plasma) or light in the infrared spectrum generated by thermal emissions from the molten pool plasma / .

[0075] The detector module 126 is mounted to the optical module 106 by a flexure 127, which allows the position of the optics of the detector module to be adjusted relative to the optical axis of the optical module 106, and thus the position of the image plane relative to the sensor 123. In this embodiment, the flexure 126 is as described in International Patent Application WO 2019 / 092414, which is incorporated herein by reference.

[0076] A controller 140 including a processor 161 and a memory 162 communicates with the modules of the additive manufacturing device (i.e., laser modules 105a, 105b, 105c, 105d, optical modules 106a, 106b, 16c, 106d, build platform 102, dispensing device 108, wiper 109, and sensors 123a, 123b, 123c, 123d, 125a, 125b). Sensor data is reported to the controller 140 in the manner described in WO 2018 / 087556. This allows sensor data from different detector modules 126a to 126d to be correlated within a common time reference frame. As described below, the controller 140 controls these modules based on software stored in the memory 162.

[0077] For a melt pool sensor 123 having photodetector elements that image an area smaller than or equal to the area of ​​interest (such as the melt pool or the plasma plume generated during melt pool formation), the alignment of the area imaged by the sensor 123 with the laser spot will have an impact on the strength of the signal generated by the sensor 123. Misalignment may cause the sensor signal to be dependent on the scanning direction, as shown in FIG. Figure 4a , Figure 4b and Figure 4c What is shown.

[0078] Figure 4a The laser beam 118 is shown striking the powder bed 104 to form a molten pool 130. The formation of the molten pool 130 also results in a plume 131 formed above the powder bed 104 by the plasma and particles ejected from the bed. The field of view 132 of the sensor 123 is misaligned with the laser beam 118 (in the figure, the laser beam 118 is slightly to the left of the center of the field of view 132). For a scan direction from left to right, the plume is only partially within the field of view 132, while for a scan direction from right to left, all of the plume is within the field of view. This may result in a lower signal strength generated by the molten pool sensor for one scan direction as compared to another scan direction under the same melting conditions, even though the conditions at the bed are substantially the same.

[0079] This is further Figure 4b and Figure 4c exhibit. Figure 4bA series of hatch lines 133a, 133b, 133c are shown to be scanned by the laser beam 118. During the scanning of the hatch lines 133a, 133b, 133c, the melt pool sensor 123 generates a signal based on the melt pool and the plume. Figure 4c 1 shows a typical signal that may be generated by sensor 123 in the presence of flying particles along hatch lines 133a, 133b, and 133c. Hatch line 133a is scanned until time t 1 , hatch line 133b at time t 1 With t 2 is scanned between, and the hatch line 133c is scanned between, and the hatch line 133c is scanned between, and 2 134a, 134b, and 134c. However, due to the misalignment of the field of view 132 with the laser beam 118, the intensity of the signal from the molten pool sensor for hatch line 133b is less than the intensity of the signal from the molten pool sensor for hatch lines 133a and 133c, even though these hatch lines were formed under the same melting conditions. In addition, the intensity reduction during the scanning of hatch line 133b may cause the decrease in signal intensity due to the spatter particles to be masked by noise in the signal.

[0080] Figure 5 It is shown how the field of view 132 may be misaligned with the laser beam 118 such that the center of the field of view (as indicated by "X") is offset from the laser beam 118 in both the x-direction and the y-direction.

[0081] refer to Figures 6a to 6c , in order to determine a correction for the misalignment of the field of view 132 with the laser beam 118, a first (different) laser beam is scanned over the work surface along a first scanning path L1 using a first optical module of the optical modules 106a to 106d to generate a molten pool along the first scanning path. A field of view 132 of a coaxial sensor 135 is scanned over the work surface along a second scanning path L2 using a second optical train of the optical trains 106a to 106d for steering the second laser beam. The first scanning path L1 and the second scanning path L2 intersect at a point P. During the scanning of the first laser beam and the field of view 132, a signal from the molten pool sensor 123 is recorded.

[0082] Figure 6a A typical output signal from the melt pool sensor 123 is shown when the field of view 132 is centered around the second laser beam in a direction perpendicular to the scanning direction. In this case, the signal from the melt pool sensor 123 peaks near the intersection point P.

[0083] Figure 6b and Figure 6c1 shows a typical output signal from the melt pool sensor 123 in the case where the field of view 132 is positioned eccentrically relative to the second laser beam in a direction perpendicular to the scanning direction. In this case, the peak is offset to one side of the intersection point P. The direction in which the peak is offset can be related to the direction in which the center of the field of view 132 is offset from the second laser beam in a direction perpendicular to the scanning direction.

[0084] The misalignment of the peak with the intersection point P is used to determine the adjustment to be made to the alignment of the sensor 123 with the optical axis of the corresponding optical module 106. For example, the required adjustment can be output to a display (not shown). The operator can then use the flexure 127 to adjust the position of the sensor 123 relative to the optical axis of the optical module 106 based on the displayed adjustment.

[0085] This process can be performed iteratively until the field of view is centered around the second laser beam within acceptable tolerances (any deviation of the peak from the intersection point is small enough to fall within acceptable tolerances / noise).

[0086] refer to Figure 7 , in order to determine a two-dimensional adjustment of the alignment of the sensor 123 with the optical axis of the optical module 106, the first scanning path L1 and the second scanning path L2 include oscillating scanning paths that intersect at multiple locations (indicated by dashed lines). In this embodiment, the scanning paths L1 and L2 define a "gear" shape, where the two scanning paths oscillate around a circle and are π out of phase.

[0087] If the field of view 132 is aligned with the corresponding laser beam in both the x-direction and the y-direction, the sensor signal will be indicated by the dashed line (and as shown in FIG. Figure 8a If the field of view 132 is negatively x-offset, the brightest signal will be as shown Figure 8b If the field of view 132 is positively x-offset, the brightest signal will be Figure 8c If the field of view 132 is negatively y-offset, the brightest signal will be Figure 8d If the field of view 132 is positively y-offset, the brightest signal will be Figure 8e The combination of x-offset and y-offset can also be determined based on the pattern of maximum intensity signals generated by the weld pool sensor 123. When scanning the L2 path, the adjustment to be made to the flexure 127 is determined based on the signal from the weld pool sensor 123, and the determined adjustment is output on the display.

[0088] This process may be performed for each combination of optical modules 106 to 106 d and detector modules 126 .

[0089] In one embodiment, each other laser beam is used as a reference for the laser beam used to generate the molten pool. Figure 7 and Figures 8a to 8e The method described. Fig. 9 In the embodiment, each optical module 106a to 106d is used to scan along the corresponding scanning path L1. 1 To L1 4 The laser beam 118 is steered to form a molten pool. Each optical module 106a to 106d not used to steer the laser beam 118 is operated to follow the corresponding scanning path L2 1 To L2 4 The field of view of the corresponding coaxial molten pool sensor 123 is controlled so that when the field of view is along the scanning path L2 1 To L2 4 When moving, the field of view intersects with the molten pool. In other words, when the optical module 106a makes the laser 118a move along the scanning path L1 1 During scanning, each optical module 106b, 106c, 106d moves along the scanning path L2. 1 The optical module 106b controls the field of view 132 of the corresponding coaxial molten pool sensor 123. 2 During scanning, each optical module 106a, 106c, 106d moves along the scanning path L2. 2 The optical module 106c controls the field of view 132 of the corresponding coaxial molten pool sensor 123. 3 During scanning, each optical module 106a, 106b, 106d moves along the scanning path L2. 3 The optical module 106d controls the field of view 132 of the corresponding coaxial molten pool sensor 123. 4 During scanning, each optical module 106a, 106b, 106c moves along the scanning path L2. 4 Control the field of view 132 of the corresponding coaxial molten pool sensor 123. Scanning path L1 1 To L1 4 Centered around the “quadrant home” of the optical module 106, the optical module follows a scan path L1 corresponding to the position of the laser beam 118 perpendicular to the working surface of the powder bed 104. 1 To L1 4 The laser beam 118 is steered. In this way, when the laser beam follows the path L1 1 To L1 4 While scanning, the changes in the melt pool and the corresponding plume are minimized due to the changing angle of the laser beam relative to the working surface of the powder bed.

[0090] In order to make the scanning of the field of view 132 along the scanning path L2 consistent with the scanning of the laser beam along the scanning path L1, for each pair of paths L1 1 、L21 ; L1 2 、L2 2 ; L1 3 、L2 3 ; L1 4 、L2 4 Synchronously, for the optical module that is scanning the laser beam 118 along the scanning path L1, each scanning module 106a, 106b, 106c, 106d in the initial position positions the laser beam 118 / field of view 132 at the quadrant origin. Then, all optical modules 106a, 106b, 106c, 106d move synchronously to scan the path L1 or L2. In this way, the synchronization of the scanning along the paths L1 and L2 is maintained because the optical modules 106a, 106b, 106c, 106d scan the same distance from the common starting position in the quadrant origin. However, in another embodiment, a "deterministic" machine process can be used to generate a request signal for manipulating the optical devices 141a, 141b based on a predetermined timing sequence, as described in WO 2017 / 085469.

[0091] Based on the signals generated by the melt pool sensors 123 of the detector modules 126a to 126d, adjustments to the alignment of the sensors 123 of the detector modules 126a to 126d relative to the optical axes of the corresponding optical modules 106a to 106d can be determined. The output is displayed on the display to indicate to the operator the adjustments that should be made to the flexures of the detector modules 126a to 126d.

[0092] Once the sensors 123 of the detector modules 126a to 126d have been properly aligned, the signals from the melt pool sensors 123 may be normalized. This is performed by aligning all of the fields of view 132 of the melt pool sensors 123a to 123d at the work surface and exciting one of the laser beams 118a to 118d to form a melt pool and plume at the work surface within the field of view 132. Each melt pool sensor 123a to 123d generates a signal for the event, and the signals are compared. A scaling factor applied to the signal from the melt pool sensors 123a to 123d is determined for each melt pool sensor 123a to 123d so that the melt pool sensors 123a to 123d give the same output for the same event on the powder bed. In this way, the signals may be displayed and / or processed together without artifacts due to the differential responses of the melt pool sensors 123a to 123d.

[0093] When collecting signals, the signals from the molten pool sensors 123a to 123d can be displayed in a two-dimensional or three-dimensional representation based on the measured positions of the galvanometers 124a, 124b. For example, the two-dimensional or three-dimensional representation can be as described in US 2013 / 0168902 A1 or WO 2019 / 092415, which is incorporated herein by reference. The coordinate values ​​to be associated with the sensor signals can be determined from the signals of the position sensors 125a, 125b that measure the angular positions of the galvanometers 124a, 124b. In order to convert the signals from the position sensors 125a, 125b into Cartesian coordinates corresponding to the work surface 110, a calibration routine is performed. A markable plate such as a polyester sheet is placed at the work plane 110, and each optical module 106a to 106d is operated to manipulate the corresponding laser beam 118, thereby marking the plate at multiple positions on the plate. When each mark is formed, the signals from the position sensors 125a, 125b are recorded. An image of the marked board is then captured and the position of the marks on the board is correlated with the output from the position sensors 125a, 125b.

[0094] A map is generated that maps the output of the position sensor 125a, 125b from each optical module 106a to 106d into Cartesian coordinates at the work surface according to the correlation of the mark positions and by interpolating between the mark positions for which data has been collected. This method can be performed while calibrating the optical modules 106a to 106d for steering the laser beams 118a to 118d, for example, using a method as described in WO 94 / 15265. However, the method can also be performed as a separate process to calibrate the optical modules 106a to 106d for steering the laser beams 118a to 118d.

[0095] During operation of the additive manufacturing apparatus, coordinates are determined from the signals from the position sensors 125a, 125b using mapping to correlate with the sensor values ​​recorded by the melt pool sensors 123a to 123d. These coordinates can then be used to form a representation of the sensor values, as described above, and / or for further processing / analysis.

[0096] The above embodiments are applicable to additive manufacturing apparatuses including multiple optical chains for delivering multiple laser beams, but cannot be used to align (multiple) coaxial sensors in an additive manufacturing apparatus including a single optical chain. Figure 10 to Figure 1 3 describes another embodiment of the present invention for aligning a coaxial sensor with the optical axis of an optical train. This embodiment is suitable for (but not limited to) use in an additive manufacturing device including a single optical train.

[0097] Fig.10(a) shows the field of view 232 of the coaxial sensor that is not centered around the laser beam 218 delivered using the optical axis. For antiparallel scan directions (upward on the left page and downward on the right page in the figure), the plasma plume 231 generated by the melting of the surface extends from the molten pool in different directions. Since the misalignment of the field of view with the optical axis has components in both scan directions at the work surface, for each antiparallel scan direction, a different proportion of the plasma plume 231 falls within the field of view 232 of the coaxial sensor. This results in the following Fig.10 (a) Shown are the differences in sensor values ​​generated by a coaxial sensor for two anti-parallel scan directions, despite producing common melting conditions when scanning in each of these directions.

[0098] Fig.10 (b) shows a similar plot, but this time for antiparallel scan directions from left to right and vice versa. Because the misalignment in the field of view and the optical axis at the work surface is orthogonal to these scan directions, there is no or little difference in the sensor values ​​generated by the coaxial sensor for each antiparallel scan direction.

[0099] In order to determine the misalignment of the coaxial sensor with the optical axis of the optical chain for delivering the laser beam, a plurality of opposing scanning path groups 1 to 16 are formed on an anodized aluminum sheet located on the build platform of the additive manufacturing device. Each group 1 to 16 of opposing scanning paths includes a plurality of first scanning paths 18 extending in a first direction in the plane of the work surface and a plurality of second scanning paths 19 extending in a second direction antiparallel to the first direction. The scanning paths 18, 19 of each group extend in a direction different from the scanning paths of the other groups. In this embodiment, sixteen scanning path groups are formed to provide one scanning path every 11.25 degrees (e.g., Fig.11 ). However, it will be appreciated that more or fewer sets of opposing scan paths may be used. If only two sets of opposing scan paths are used, it is advantageous that the scan paths of each set extend in orthogonal directions. In this embodiment, the scan paths of each set are of equal length and are grouped as Fig.11 1 to 16 in the figure (for clarity, only some of the scan paths for each square are shown). These opposing sets of scan paths are formed when the gas flow used in the additive build process is generated by the build chamber and is typically in an inert atmosphere in the build chamber, such as an atmosphere of argon or hydrogen.

[0100] During the formation of these scan paths, the coaxial sensor generates sensor values ​​which are recorded. Fig.12a The sensor values ​​generated for squares 1 to 16 are shown when the coaxial sensor is misaligned with the optical axis of the optical chain. The sensor values ​​for each square include sensor values ​​for the two anti-parallel scan directions of the group. Figure 12bis a polar plot showing the measured values ​​obtained from the sensor values ​​(RMS values ​​in this embodiment) generated for each scanning direction. Figure 12b It can be seen that there are significant differences in the strength of the signal generated for different scan path directions. The direction of misalignment of the coaxial sensor with the optical axis of the optical chain can be determined based on these RMS values. Figure 12b In the graph, the scan direction at 270 degrees has the highest intensity, and the scan direction between 90 and 150 has the lowest RMS value, indicating that the misalignment is along this axis.

[0101] However, in this embodiment, the direction of the misalignment is determined based on the sign of the difference in the RMS values ​​of each anti-parallel scan path direction. Fig.12c Shown Fig.12a and Figure 12b From these difference values, it can be clearly seen that square 8 shows the smallest difference in the RMS values ​​of the two antiparallel scan paths of this square, while square 1 shows the largest difference. Based on these differences, the direction of adjustment is determined. Figures 12a to 12c In the example shown, the adjustment is a movement of the field of view in the 90 degree direction because the antiparallel scan path for this direction shows the largest difference, and the RMS value for the scan path in the 90 degree direction is less than the RMS value for the scan path in the 270 degree direction (i.e., the difference is negative for the 90 degree direction). This direction is associated with the adjustment of the flexure on the detector module. The magnitude of the required adjustment can be determined based on the magnitude of the difference. A lookup table or function can be used to determine the magnitude in the adjustment of the flexure that corresponds to a certain magnitude in the difference in RMS value. Such a functional lookup table can be determined empirically.

[0102] Figures 13a to 13c The sensor values ​​after the coaxial sensor realignment are shown. As can be seen, the variation in the difference of the coaxial sensor values ​​is now smaller.

[0103] In an alternative embodiment, the adjustments to be made to the position of the coaxial sensor relative to the optical axis of the optical chain are determined based on the magnitude of the differences between multiple opposing scan path groups. For example, a curve can be fit to these differences to eliminate noise and / or fluctuations in the difference signal that are not caused by misalignment of the coaxial sensor. In the case where only two opposing scan path groups are used, the scan paths of each group can be formed in orthogonal directions, and the differences in the RMS sensor values ​​of each group can give adjustments to the position of the coaxial sensor for the orthogonal directions.

[0104] Furthermore, multiple antiparallel scan path groups with the same scan direction may be used. For example, these groups may be distributed across the entire working plane to reduce local position effects on the RMS value. This may be achieved by repeating a pattern of antiparallel scan path groups across the entire working plane, such as Fig.11 shown.

[0105] It will be appreciated that modifications and variations may be made to the above described embodiments without departing from the invention as defined herein.For example, a scan path of a different shape may be used to determine misalignment of the melt pool sensor.

[0106] In order to determine the scaling factor for normalizing the signal from the melt pool sensor, rather than determining the scaling factor by observing the same melt pool, different melt pools may be observed by one or more melt pool sensors, each melt pool being formed using laser parameters set to achieve the same flux conditions at the powder bed. This may be necessary in the case where the scan area of ​​each optical module 106a to 106d does not overlap with the scan area of ​​each other. In order to determine the scaling factor for normalizing the signal from the melt pool sensor, multiple melt pools observed by multiple melt pool sensors may be formed at multiple locations in the work plane to account for positional dependence of the differences between the signals from the melt pool sensors. For example, the positional dependence may be due to different angles of the laser beam to the work surface and / or different viewing angles at different locations on the work surface.

[0107] The additive manufacturing device may include position sensors (such as encoders) each for measuring the position of the coaxial melt pool sensor 123 / detector module 126 relative to the optical axis of the corresponding optical chain. The controller 140 can control the display to provide feedback about the position of the coaxial sensor 123 / detector module 126 and / or control the motor based on the signal from the position sensor.

Claims

1. A method for aligning a coaxial melt pool sensor in an additive manufacturing device, the additive manufacturing device comprising a first optical train for manipulating a first laser beam, a second optical train for manipulating a second laser beam, and the coaxial melt pool sensor of the second optical train, the method comprising: The first laser beam is scanned on a work surface along a first scanning path using the first optical chain to generate a molten pool along the first scanning path, and the field of view of the coaxial molten pool sensor is scanned on the work surface along a second scanning path using the second optical chain for manipulating the second laser beam, wherein the first scanning path and the second scanning path intersect, and adjustments to be made to the alignment of the field of view of the coaxial molten pool sensor with the optical axis of the second optical chain are determined based on changes in a signal generated by the coaxial molten pool sensor caused by the molten pool passing through the field of view.

2. The method of claim 1 , comprising adjusting the alignment of the field of view of the coaxial melt pool sensor with the optical axis of the second optical chain.

3. The method according to claim 1, wherein: The second scan path includes a change in scan direction over the work surface.

4. The method according to claim 1, wherein: The second scanning path includes scanning in a first direction and scanning in a second direction orthogonal to the first direction.

5. The method according to claim 3, wherein: The second scan path includes an oscillating scan path.

6. The method according to claim 3, wherein: The second scan path starts and ends at the same location.

7. The method according to claim 1, wherein: The first scanning path is symmetrical to the second scanning path.

8. The method according to claim 1, wherein: The first scanning path and the second scanning path are oscillating scanning paths that are π out of phase.

9. A method of aligning a coaxial melt pool sensor in an additive manufacturing apparatus, comprising: For each of a plurality of optical chains of the additive manufacturing apparatus, the field of view of the corresponding coaxial melt pool sensor is aligned with the optical chain using the method according to any one of claims 1 to 8.

10. The method according to claim 9, wherein: For two or more of the optical chains, the first laser beam used to determine the alignment of the field of view of the coaxial melt pool sensor is the same laser beam.

11. An additive manufacturing apparatus for manufacturing an object layer by layer by laser melting, the additive manufacturing apparatus comprising: a first optical train comprising a first movable optical component for manipulating a first laser beam on a work surface; a second optical train comprising a second movable optical component for manipulating a second laser beam on the working surface; a coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the second movable optical component of the second optical chain; as well as a controller arranged to control the first movable part and the second movable part of the first optical chain and the second optical chain, so as to use the first optical chain to scan the first laser beam on the work surface along a first scanning path to generate a molten pool along the first scanning path, and use the second optical chain to scan the field of view of the coaxial molten pool sensor on the work surface along a second scanning path, wherein the first scanning path and the second scanning path intersect and adjustments to be made to the alignment of the field of view of the coaxial melt pool sensor with the optical axis of the second optical chain are determined based on changes in a signal generated by the coaxial melt pool sensor attributable to the melt pool passing through the field of view.

12. The additive manufacturing apparatus of claim 11, comprising a third optical train comprising a third movable optical component for manipulating a third laser beam on the work surface, The controller is arranged to: use the third optical chain to scan the third laser beam along a third scanning path on the working surface to generate a molten pool along the third scanning path, and use the second optical chain in the optical chain to scan the field of view of the coaxial molten pool sensor on the working surface along a fourth scanning path, wherein, The third scanning path and the fourth scanning path intersect, and when the field of view is scanned along the second scanning path and the fourth scanning path, the adjustment to be made to the alignment of the field of view of the coaxial melt pool sensor with the optical axis of the second optical chain is determined based on the change of the signal generated by the coaxial melt pool sensor.

13. The additive manufacturing device according to claim 11 or claim 12 comprises an adjustment mechanism for adjusting the position of the field of view of the coaxial melt pool sensor relative to the optical axis of the second optical chain.

14. A controller for an additive manufacturing device for manufacturing an object layer by layer by laser melting, the additive manufacturing device comprising: a first optical train comprising a first movable optical component for manipulating a first laser beam on a work surface; a second optical train comprising a second movable optical component for manipulating a second laser beam on the working surface; and a coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the second movable optical component of the second optical chain, the controller being configured to control the additive manufacturing device to perform a method according to any one of claims 1 to 10.

15. A data carrier having instructions stored thereon, which, when executed by a controller of an additive manufacturing device for manufacturing an object layer by layer by laser melting, causes the controller to control the additive manufacturing device to perform the method according to any one of claims 1 to 10, the additive manufacturing device comprising: a first optical train comprising a first movable optical component for manipulating a first laser beam on a work surface; a second optical train comprising a second movable optical component for manipulating a second laser beam on the working surface; and a coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the second movable optical component of the second optical chain.

16. A method of aligning a coaxial melt pool sensor in an additive manufacturing apparatus, the method comprising: Scanning a laser beam along a plurality of scanning paths over a work surface using an optical chain to generate molten pools along the plurality of scanning paths, some of the plurality of scanning paths extending in different directions in the plane of the work surface; recording sensor values ​​using a coaxial molten pool sensor that observes the molten pool through the optical chain; And determining the adjustments to be made to the alignment of the field of view of the coaxial melt pool sensor and the optical axis of the optical chain based on the changes in the sensor values ​​generated by the coaxial melt pool sensor and the direction of the scanning path.

17. The method according to claim 16, wherein: Determining the adjustment includes determining a measurement value for each scan path direction, the measurement value being derived from a plurality of sensor values ​​generated during a scan of the laser beam along the scan path direction.

18. The method according to claim 17, wherein: The measured value is an average of the plurality of sensor values.

19. The method according to claim 17 or 18, wherein: The plurality of scanning paths include a plurality of opposite scanning path groups, each group including at least one first scanning path extending in a first direction in a plane of the working surface and at least one second scanning path extending in a second direction antiparallel to the first direction, the opposite scanning paths between the groups extending in a lateral direction, wherein determining the adjustment includes determining a difference in measured values ​​for the at least one first scanning path and for the at least one second scanning path based on at least one scanning path group in the scanning path groups, and determining the adjustment based on the difference.

20. The method of claim 19, comprising determining, for each of the scan path groups, a difference in measurements for the at least one first scan path and for the at least one second scan path, and identifying a direction of the adjustment based on the difference.

21. The method according to claim 20, wherein: The adjustment is based on the first direction and the second direction of the set of opposing scan paths having the largest difference between the measurements determined for the at least one first scan path and the at least one second scan path.

22. The method according to claim 20, wherein: The direction of the adjustment is determined based on the sign of the difference.

23. The method according to claim 19, wherein: Each group includes a plurality of first scanning paths and a plurality of second scanning paths.

24. An additive manufacturing apparatus for manufacturing an object layer by layer by laser melting, the additive manufacturing apparatus comprising: an optical train including movable optical components for steering the laser beam over the work surface; a coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the movable optical component of the optical chain; and a controller, the controller being arranged to: control the movable part of the optical chain to make the laser beam scan along a plurality of scanning paths on the working surface, thereby generating a molten pool along the plurality of scanning paths, some of the scanning paths extending in different directions in the plane of the working surface, and determine the adjustment to be made to the alignment of the field of view of the coaxial molten pool sensor with the optical axis of the optical chain based on the change of the sensor value generated by the coaxial molten pool sensor and the direction of the scanning path.

25. The apparatus of claim 24, wherein: The controller is arranged to determine, for each scan path direction, a measurement value derived from a plurality of sensor values ​​generated during scanning of the laser beam along the scan path direction, and to determine the adjustment to form the determined measurement value.

26. The apparatus of claim 25, wherein: The measured value is an average of the plurality of sensor values.

27. Apparatus according to claim 25 or 26, wherein The plurality of scanning paths include a plurality of opposite scanning path groups, each group including at least one first scanning path extending in a first direction in a plane of the working surface and at least one second scanning path extending in a second direction antiparallel to the first direction, the opposite scanning paths between the groups extending in a lateral direction, wherein determining the adjustment includes determining a difference in measured values ​​for the at least one first scanning path and for the at least one second scanning path based on at least one scanning path group in the scanning path groups, and determining the adjustment based on the difference.

28. The apparatus of claim 27, wherein: The controller is arranged to determine, for each of the scan path groups, a difference in the measured values ​​for the at least one first scan path and for the at least one second scan path, and to identify a direction of the adjustment based on the difference.

29. The apparatus of claim 28, wherein: The adjustment is based on the first direction and the second direction of the set of opposite scan paths having the largest difference between the measurements determined for the at least one first scan path and the at least one second scan path.

30. The apparatus of claim 28, wherein: The direction of the adjustment is determined based on the sign of the difference.

31. The apparatus of claim 27, wherein: Each group includes a plurality of first scanning paths and a plurality of second scanning paths.

32. A controller for an additive manufacturing apparatus for manufacturing an object layer by layer by laser melting, the additive manufacturing apparatus comprising: an optical train including movable optical components for steering the laser beam over the work surface; and a coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the movable optical component of the optical chain, the controller being configured to control the additive manufacturing device to perform a method according to any one of claims 16 to 23.

33. A data carrier having instructions stored thereon, which, when executed by a controller of an additive manufacturing device for manufacturing an object layer by layer by laser melting, causes the controller to control the additive manufacturing device to perform the method according to any one of claims 16 to 23, the additive manufacturing device comprising: an optical train including movable optical components for steering the laser beam over the work surface; and a coaxial melt pool sensor for detecting radiation emitted by the melt pool and collected by the movable optical component of the optical chain.

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