CALIBRATION OF ADDITIVE MANUFACTURING APPARATUS

Abstract

A method of calibrating a scanner of an additive manufacturing apparatus, in which an energy beam is directed with the scanner to consolidate material in a working plane to build up a workpiece in a layer-by-layer manner. The method includes directing the energy beam with the scanner across a test surface in the working plane to form a test pattern, the test pattern having at least one periodic feature, capturing an image of the test pattern, determining from the image a periodic property of the test pattern and determining correction data for control of the scanner based upon the periodic property.

Claims

1.-34. (canceled)

35. A method of calibrating a scanner of an additive manufacturing apparatus, in which an energy beam is directed with the scanner to consolidate material in a working plane to build up a workpiece in a layer-by-layer manner, the method comprising directing the energy beam with the scanner across a test surface in the working plane to form a test pattern, the test pattern comprising at least one periodic feature, capturing an image of the test pattern, determining from the image a periodic property of the test pattern and determining correction data for control of the scanner based upon the periodic property.

36. A method according to claim 35, wherein the periodic property is a phase shift of the test pattern relative to a reference phase.

37. A method according to claim 36, wherein the phase shift is determined through Fourier analysis of the image.

38. A method according to claim 37, wherein the phase shift is determined by carrying out a discrete Fourier transform of the image of the test pattern at a reference frequency and determining the phase shift of a resultant frequency component from the reference phase.

39. A method according to claim 36, wherein a value for the phase shift is determined for each region of plurality of different regions of the test pattern.

40. A method according to claim 36, wherein correction data is determined by fitting a mathematical model of the scanner to the determined phase shifts.

41. A method according to claim 36, comprising locating a reference surface of a calibration artefact in a working plane of the additive manufacturing apparatus, the reference surface having a reference pattern thereon, capturing an image of the reference pattern and determining the phase shift between the test pattern and the reference pattern.

42. A method according to claim 41 wherein the image of the reference pattern is captured using the same image capture device used to capture the image of the test pattern.

43. A method according to claim 42, wherein the image capture device is located in the same location in the additive manufacturing apparatus for the capture of the images of the test pattern and the reference pattern.

44. A method according to claim 41, wherein the reference surface is located in the same location in the additive manufacturing apparatus as a surface on which the test pattern is formed.

45. A method according to claim 35, wherein the test pattern comprises a first pattern comprising a first geometric feature repeated in a first direction and a second pattern comprising a second geometric feature repeated in a second direction, perpendicular to the first direction.

46. A method according to claim 45, wherein the first and second geometric feature is the same but rotated to align with the corresponding first and second directions.

47. A method according to claim 45, wherein each of the first and second directions correspond to a spatial direction in which the energy beam is moved by a different steering element of the scanner.

48. A method according to claim 45, wherein the first pattern and second pattern are interspersed without overlap between the geometric features of each pattern.

49. A method according to claim 35, wherein the test pattern comprises a series of parallel lines.

50. A method according to claim 41, wherein the repeated geometric feature of the test pattern correlates with the regular spatial intervals of geometric features of the reference pattern and the phase shift is determined by comparing a phase of the repeated geometric feature of the test pattern to a phase of the corresponding repeated geometric feature of the reference pattern.

51. A method according to claim 35, wherein the periodic property comprises summed intensities across each of a plurality of regions of the test pattern in the image, each region comprising at least one period of the test pattern.

52. A method according to claim 35, comprising forming different periodic features of the test pattern with different focal positions of the energy beam relative to the working plane, wherein the periodic property is determined for each region of the test pattern formed with the energy beam at one of the different focal positions, and determining correction data for calibrating focussing optics of the scanner based upon the periodic property.

53. A method according to claim 52, wherein the test pattern comprises a recurring geometric feature, wherein each occurrence of the geometric feature is formed with the energy beam at a different focal position relative to the working plane.

54. A method of calibrating a scanner of an additive manufacturing apparatus, in which an energy beam is directed and focussed with the scanner to consolidate material in a working plane to build up a workpiece in a layer-by-layer manner, the method comprising directing the energy beam across a test surface in the working plane with the scanner to form geometric features on the surface, wherein a focal position of the energy beam relative to the working plane is altered for the formation of different ones of the geometric features, capturing an image of the geometric features, determining an intensity per unit area for each region formed with a different focal position of the energy beam and determining from the variation in intensity per unit area, correction data for correcting control of the focal position of the scanner.

55. A controller for controlling an additive manufacturing apparatus, wherein the controller is arranged to carry out the method of claim 35.

56. An additive manufacturing apparatus for building up a workpiece in a layer-by-layer manner comprising a scanner for directing an energy beam to consolidate material in a working plane and a controller according to claim 55.

57. An additive manufacturing apparatus according to claim 56, comprising a camera, wherein the camera is located in the additive manufacturing apparatus at a location fixed relative to a datum used to locate the reference surface in the working plane.

58. An additive manufacturing apparatus according to claim 57, comprising a wiper arranged to be positioned relative to the datum to form material layers in the working plane.

59. A data carrier having instructions thereon, which, when executed by a controller for controlling an additive manufacturing apparatus, cause the controller to carry out the method of claim 35.

60. A controller for controlling an additive manufacturing apparatus, wherein the controller is arranged to carry out the method of claim 54.

61. An additive manufacturing apparatus for building up a workpiece in a layer-by-layer manner comprising a scanner for directing an energy beam to consolidate material in a working plane and a controller according to claim 60.

62. A data carrier having instructions thereon, which, when executed by a controller for controlling an additive manufacturing apparatus, cause the controller to carry out the method of claim 54.

Description

DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 shows an additive manufacturing apparatus according to an embodiment of the invention;

[0041] FIG. 2 is a plan view of a test pattern according to an embodiment of the invention for calibrating steering optics of a scanner;

[0042] FIG. 3 schematically shows a method of calibrating steering optics of a scanner of an additive manufacturing apparatus according to an embodiment of the invention;

[0043] FIG. 4 is a schematic view of typical pixel intensities in an image of the test pattern;

[0044] FIG. 5 is a plan view of a test pattern formed on a plate for calibrating focussing optics of the scanner;

[0045] FIG. 6 is a schematic view of an intensity graph generated from an image of the test pattern shown in FIG. 5;

[0046] FIG. 7 is a perspective view of fixture for mounting the calibration artefact and the test plate in the additive manufacturing apparatus shown from below;

[0047] FIG. 8 is a perspective view of the fixture from above; and

[0048] FIG. 9 schematically shows a method of forming a hybrid workpiece according to an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

[0049] Referring to FIG. 1, an additive manufacturing apparatus according to an embodiment of the invention comprises a main chamber 101 having therein partitions 115, 116 that define a build chamber 117. A build platform 102 is lowerable in the build chamber 117. The build platform 102 supports a powder bed 104 and workpiece 103 as the workpiece is built by selective laser melting of the powder. The platform 102 is lowered within the build chamber 117 under the control of motor as successive layers of the workpiece 103 are formed.

[0050] Layers of powder 104 are formed as the workpiece 103 is built by dispensing apparatus 108 and a wiper 109. For example, the dispensing apparatus 108 may be apparatus as described in WO2010/007396. The dispensing apparatus 108 dispenses powder onto an upper surface 115a defined by partition 115 and is spread across the powder bed by wiper 109. A position of a lower edge of the wiper 109 defines a working plane 110 at which powder is consolidated and is adjustable.

[0051] A laser module 105 generates a laser beam 118 for melting the powder 104, the laser beam 118 directed as required by corresponding scanner, in this embodiment optical module 106. The optical module comprises steering optics 106a, such a two mirrors mounted on galvanometers, for steering the laser beam 118 in perpendicular directions across the working plane and focussing optics 106b, such as two movable lenses for changing the focus of the laser beam 118. The scanner is controlled such that the focal position of the laser beam 118 remains in the same plane as the laser beam 118 is moved across the working plane. Rather than maintaining the focal position of the laser beam in a plane using dynamic focusing elements, an f-theta lens may be used.

[0052] A camera 191 is located in the main chamber 101 for capturing images of the working plane.

[0053] A controller 140, comprising processor 161 and memory 162, is in communication with modules of the additive manufacturing apparatus, namely the laser module 105, optical module 106, build platform 102, dispensing apparatus 108, wiper 109 and camera 191. The controller 140 controls the modules based upon software stored in memory 162 as described below.

[0054] Referring to FIGS. 2 to 4, to calibrate the scanner 106 the user places 301 a calibration artefact 350 comprising a reference pattern 351 in the additive manufacturing apparatus such that the reference pattern 351 is located in the working plane 110. The reference pattern 351 may be located in the additive manufacturing apparatus using the fixture 400 described below with reference to

[0055] FIGS. 7 and 8. The reference pattern 351 is the same as the test pattern 251 shown in FIG. 2, with a plurality of regions 203a and 203b comprising a series of equally spaced parallel lines. Regions 203a comprise a plurality of parallel lines spaced apart in the x-direction and regions 203b comprise a plurality of parallel lines spaced apart in the y-direction. Regions 203a alternative with regions 203b in both the x- and y-directions.

[0056] A period of the parallel lines is close to a period given by the Nyquist frequency for the camera 191, i.e. the period is close to four times the spatial resolution of a pixel of the camera 191 at the working plane. FIG. 4 shows how an intensity of pixels 1-9 may vary in an image of a portion of a region 203a, 203b of pattern 351, 251.

[0057] As can be appreciated from FIG. 4, determination of a position of an individual line from such an image will be of the order of the spatial resolution of the image.

[0058] The reference pattern 351 may be printed on a sheet using a suitable technique that is capable of printing patterns to a required accuracy, in this embodiment to an accuracy of a micron or less.

[0059] An image 302 of the reference pattern 351 in the working plane is captured 303 using the camera 191.

[0060] A series of discrete Fourier transforms (DFTs) are determined 304 at the known reference frequency, k.sub.ref, of the parallel lines of the reference pattern 351, each using a basic sinusoid shifted to a different position. In this embodiment, the DFT is carried out by multiplying the image 302 of the reference pattern 351 by digitally generated sine and cosine representations. The sine and cosine representations are generated such that non-zero sine and cosine regions are spaced apart by zero value regions corresponding to the spaces between regions 203a, 203b. To determine a correct alignment of the digitally generated sine and cosine representations with the image of the reference pattern, DFTs are determined using the sine and cosine representations positioned at different positions, S, relative to the image of the reference pattern. A magnitude for the DFT is determined for each region and the magnitudes for all regions averaged. The position of the sine and cosine representation that results in the highest average magnitude for the DFT is deemed to be the position, S.sub.ref, that most closely matches the position of the reference pattern 351 in the image 302.

[0061] A phase X.sub.ref, Y.sub.ref of the reference pattern in each region, denoted by position (x,y) corresponding to the centre of the region, relative to the basic sinusoid is determined 305 from the DFT and identifying the reference phase for the region. For regions having a pattern with a feature that recurs in the x-direction, a phase shift X.sub.ref in the x-direction is determined and, for regions having a pattern with a feature that recurs in the y-direction, a phase shift Y.sub.ref in the y-direction is determined. The phase shift is determined from the arctan of the quotient of the two values obtained by multiplying the image by the sine and cosine representations.

[0062] The reference artefact 350 is then removed from the additive manufacturing apparatus and replaced 306 with an aluminium plate 250 that is also located in the working plane, for example using the fixture 400 of FIGS. 7 and 8 that is locatable in a repeatable position in the build chamber 117. The test pattern 251 is then marked onto the aluminium plate 250 using the laser beam 118 and the scanner 106.

[0063] An image 307 of the test pattern 251 is captured 308.

[0064] A discrete Fourier transform of the image 308 of the test pattern 251 is determined 307 at the reference frequency, k.sub.ref, and a phase X.sub.tst, Y.sub.tst of the test pattern 251 from the basic sinusoid in each region 203a, 203b is determined 309.

[0065] A phase shift X.sub.error, Y.sub.error of the test pattern 251 from the reference pattern 351 for each region 203a, 203b is determined 310. A mathematical model, as is known in the art, of the scanner 106 is then fitted to the determined phase shifts X.sub.error, Y.sub.error for each region 203a, 203b to determine 311 correction data, in terms of values for calibration tables, for modifying control of the steering optics 106a of the scanner 106.

[0066] It is possible for such a method to provide accuracy of measurement to a resolution of 1/100.sup.th of a pixel. Accordingly, if each pixel has a spatial resolution at the working plane of 150 m, the method can provide a measurement accuracy of 1 or 2 m.

[0067] Referring to FIGS. 5 and 6 the focussing optics of the scanner 106 are calibrated by forming a test pattern 251 as shown in FIG. 2 on an aluminium sheet located in the working plane wherein the scanner 106 is controlled to vary a focal point of the laser beam for each line of the pattern of a region, for example, from 10 mm below the working plane to +10 mm above the working plane. This may result in a pattern on the aluminium sheet as shown in FIG. 5.

[0068] An intensity in an image of the pattern may vary as shown in graph A of FIG. 6, with thicker light lines formed at the edge of the pattern where the laser beam is not focussed in the working plane 110 to thinner light lines at the centre of a pattern where the laser beam is focussed in the working plane 110. The total intensity over each period of the pattern is summed to produce graph B. As the focal point of the laser beam is moved from being out of focus to in focus on the working plane, the total intensity for a period of the pattern reduces as a thickness of the line reduces. Fitting a curve to the summed intensities can be used to correct control of the focusing optics 106b of the scanner 106.

[0069] A fixture 400 for mounting the calibration artefact 350 and aluminium plate 250 is shown in FIGS. 7 and 8. The fixture comprises a support 401 for supporting the calibration artefact/aluminium plate and wings 402, 403 for mounting the support 401 in place in the build chamber 117. The wings 402, 403 are offset relative to a supporting surface of the support 401 such that the wings 402, 403 are located above and to the side of the support 401 when the fixture 400 is located in the additive manufacturing apparatus. The wings 402, 403 comprise handles 404, 405 for manipulation of the fixture 400 and mounting elements 406, 407 and 408 for kinematically locating a calibration artefact/aluminium plate supported by the fixture 400 in a repeatable vertical position in the build chamber 117. In this embodiment, the elements 406, 407 and 408 comprise three balls that provide point surfaces for contacting surface 115a at three spaced apart positions.

[0070] The fixture 400 comprises two further positioning elements 409 and 410 for locating the support 401 in a fixed position in the x and y directions. The elements 409 and 410 each comprise a ball mounted in a recess in the support 401 and biased outwardly from the support 401 by springs (not shown) such that, upon insertion of the support into the build chamber 117, the balls engage a wall of the build chamber 117 and are deflected against the biasing of the springs, the biasing holding the fixture 400 in place.

[0071] Both the calibration artefact 350 and aluminium plate 250 have a suitable shape for mounting on support 401.

[0072] The method may further comprise aligning a lower edge of the wiper 109 with surface 115a that is used for alignment of the fixture 400, and therefore, the calibration artefact 350, such that the wiper 109 forms powder layers in the working plane 110. Alignment of the wiper 109 with the surface 115a may be carried out using known methods. Using the same datum for alignment of the wiper 109 and positioning of the calibration artefact 350 ensures that the powder layer is aligned with the working plane for which the scanner 106 is calibrated. Choosing a fixed surface 115a for the datum rather than a movable surface, such as the build platform 102, ensures that errors in alignment do not arise from lack of repeatability/inaccuracy in the positioning of the movable surface, such as the build platform 102.

[0073] An absolute position of an x, y coordinate system of the scanner 106 in the x- and y-directions relative to the build volume defined by build chamber 117 may be unknown because a position of the reference pattern 351 in the x- and y-direction may be unknown. However, the method calibrates the scanner 106 to correct for distortions in a coordinate system of the scanner 106. Accordingly, the above calibration method calibrates the scanner 106 based upon a position of the reference pattern 351 in the additive manufacturing apparatus.

[0074] The calibration method as described above could be used to calibrate each scanner in a multi-laser additive manufacturing apparatus. Each scanner could be used to mark patterns on one or more test plates and the phase shifts in the patterns formed by each scanner relative to the reference pattern used to calibrate the scanner.

[0075] A position of the coordinate system of the scanner 106 may not be known with sufficient accuracy if the additive built workpiece is to be aligned with non-additively built features, for example on substrate 501. For example, it is known to build hybrid additive parts in which a first portion of the part comprises a preformed substrate and a second portion of the part is additively built. On example of such a hybrid additive part is a mould insert in which cooling liquid channels are machined into the substrate prior to building the remainder of the mould insert using an additive process. The mould insert is formed with conformal cooling channels that connect with the cooling liquid channels in the substrate. Such a workpiece is described in U.S. Pat. No. 7,261,550.

[0076] In processes, wherein a substrate on which an additively built workpiece is built is premachined with features to be aligned with the additively built workpiece, it is important that a position of the machined features in a coordinate system of the scanner 106 are known such that the desired alignment can be achieved.

[0077] In accordance with an embodiment of the invention as shown in FIG. 9, the method of forming a hybrid workpiece may comprise locating a build substrate 501 that is to form part of the hybrid workpiece, but without the preformed features, on the build platform 102 of the additive manufacturing apparatus. The build substrate 501 and build platform 102 may comprise mounting formations for kinematically locating the build substrate 501 in a repeatable position on the build platform 102, for example as described in WO2015/092442.

[0078] The laser 105 and calibrated scanner 106 are controlled to mark 502 indicia 507 on the build substrate 501 that can be used to identify locations on the build substrate 501 in which features 506 are to be preformed. For example, in the case of preformed cooling channels, the locations of openings of the channels in a top surface of the build substrate 501 may be marked 507a. In a further embodiment, rather than marking the build substrate 501 with indicia 507a corresponding to a shape of a feature to be formed, indicia 507b may be formed that can be identified by a machine tool used to form the features 506 and used to align a coordinate system of the machine tool 510 with a coordinate system of the scanner 106. The indicia 507b may be selected for the ease of recognition and determination of position using a camera 591. For example, the indicia 507b may comprise a pattern similar to that described with reference to FIG. 2, wherein a position of the pattern is resolved by determining a phase of the pattern formed on the build substrate 501 with the laser beam 118.

[0079] The build substrate 501 is then removed from the additive manufacturing apparatus and mounted on a machine tool 510 for formation of the features 506. The features 506 are formed 503 by the machine tool at a location in the build substrate 501 based upon the location of the indicia 507 on the build substrate 501. For example, a location of the indicia on the build substrate 510 may be identified using camera 591, such as a video probe or the like, mounted in the machine tool 510. The position of the indicia 507 relative to the features 506 to be machined is known and the machine tool 510 can adjust its coordinate system or machine tool instructions to align the formation of the features to the indicia. In this example, the features 506 are channels formed in the substrate 501.

[0080] The substrate 501 is then remounted on the build platform 102, the kinematic mounting elements ensuring that the build substrate 501 is mounted in the same position as the position it was in when marked with the indicia 507. The additively built portion 505 of the hybrid workpiece is then built 504 using the additive manufacturing apparatus. Alignment of the preformed features 506 with the additively built portion 505 is ensured as a result of the indicia being formed by the calibrated scanner 106 used to form the subsequent additively built portion 505.

[0081] In an alternative embodiment, the indicia 507 are formed using the machine tool 510 and a location of the indicia on the build substrate 510 may be identified using camera 591, such as a video probe or the like, mounted in the additive manufacturing apparatus. Once the additive manufacturing apparatus has detected a position of the indicia, the additive manufacturing apparatus may build the portion 505 in a position based upon the position of the indicia. As the machined features 506 are built in a specified position relative to the indicia and the additively built portion 505 is built in a specified position relative to the indicia, the relative positions of the machined features 506 and the additively built potion 505 should also be correct. There is no need to detect the machined features 506, which may be difficult to detect with the required accuracy as they are not specifically built for recognition by the camera.

[0082] It will be understood that modification and alterations to the above described embodiments may be made without departing from the scope of the invention as defined herein. For example, the pattern may not comprise separate regions 203a, 203b from which correction data (a phase) for the x- and y-directions is calculated but may comprise a single region from which periodic components can be calculated for both perpendicular directions.