Powder bed fusion method and related apparatus

Abstract

A method of determining instructions to be executed by a powder bed fusion apparatus in which an object is built in a layer-by-layer manner by selectively irradiating regions of successively formed powder layers with an energy beam. The method includes determining exposure parameters for each location within a layer to be irradiated with the energy beam from primary exposure parameters, the exposure parameters varying from location to location. An amount by which each exposure parameter varies from the primary exposure parameter is determined at least partly in dependence on a geometric quantity of the object derived from the irradiated location.

Description

TECHNICAL FIELD

[0001] The present invention relates to powder bed fusion methods and to apparatuses for performing these methods, such as powder fusion apparatuses, build preparation systems, such as build preparation software, and instructions stored on a data carrier for controlling a powder bed fusion apparatus, in which methods an object is built in a layer-by-layer manner by selectively irradiating areas of successively formed powder layers with an energy beam. BACKGROUND

[0002] A powder bed fusion apparatus produces an object by layer-wise consolidation of a material, such as a metallic powder material, using a high-energy beam, such as a laser beam. A powder layer is formed in a build plane on a powder bed contained in a build sleeve by lowering a build platform in the build sleeve to lower the powder bed, depositing a heap of powder near the lowered powder bed, and spreading the heap of powder (from one side of the powder bed to the other) over the powder bed with a recoater to form the powder layer. Portions of the powder layer corresponding to a cross-section of a workpiece to be formed are then consolidated by irradiating these areas with the beam. The beam melts or sinters the powder to form a consolidated layer. After selective consolidation of the layer, the powder bed is lowered by the thickness of the newly consolidated layer, and another layer of powder is spread over the surface and consolidated as needed.

[0003] The surface finish of an object can vary with the angle of a downward facing surface. In particular, lower build angles can exhibit more pronounced effects due to material sag and excessive heat being input into the area. In an attempt to reduce these effects, (overhanging) zones of the area of a layer to be consolidated have been identified which are consolidated using overhang exposure parameters that are different from those (body exposure parameters) used for other (core / body) zones of the area. Overhang zones are typically associated with downwardly oriented surfaces of the object which are below a threshold angle, such as below 45 degrees, from the build plane.

[0004] However, it has been found that even with the use of overhang parameters within overhang zones, deformation and / or failure of the part can still occur at these overhang zones. Furthermore, the use of a threshold angle introduces a change in surface finish from below the threshold to above the threshold which can be considered a defect.

[0005] US 2018 / 0311757 Al discloses a powder bed fusion apparatus comprising a sensor with a camera that senses shape information, e.g. dimensional measurements, of a build piece. A comparator obtains an object model from a memory and can perform a comparison of the object model with the shape information to determine a difference from the object model. A compensator modifies print instructions based on the difference, e.g. by increasing energy application in areas of thicker powder in the next scan. In other embodiments, a physics-based model is used to predict the shape of the build piece.

[0006] US 2015 / 0174658 Al discloses a method of forming a three-dimensional article by successive fusion of portions of a powder bed, the method comprising detecting a local thickness at at least two locations in at least a second powder layer, and varying an energy beam parameter depending on the detected local thickness of the second powder layer.

[0007] DE 102016218951 Al discloses a method and apparatus for additive manufacturing, wherein exposure parameters for exposure of at least one first material layer are selectively adjusted depending on a measured surface topology of the substrate, so that the first layer is completely attached to the substrate in the exposed area.

[0008] WO 2019 / 091621 Al discloses a method for additive manufacturing of a three-dimensional component from powder, wherein each second or third layer of a shell region does not contact a laser, but exposure of other layers of the shell region is performed at substantially the same power as the core region.

[0009] WO 2018 / 182596 Al discloses energy dosing for additive manufacturing. The method comprises varying the intensity, spot size and / or overlap of a laser beam at a plurality of different voxel locations depending on an energy value associated with each voxel of an object being manufactured.

[0010] DE 102011087374 Al discloses a method for producing a shaped body by building up layers of a powder material. The energy density of a laser beam at an irradiation point on a respective layer is modulated depending on the thermal conductivity of a defined direct surrounding region of the irradiation point. The measure of thermal conductivity is the number of volume elements that have been solidified in the defined surrounding region. SUMMARY

[0011] According to a first aspect of the present invention, there is provided a method of determining powder bed fusion instructions to be executed by a powder bed fusion apparatus, wherein an object is built up in a layer-by-layer manner by selectively irradiating regions of successively formed powder layers with an energy beam, the method comprising determining exposure parameters for each location within a layer to be irradiated with the energy beam, the exposure parameters varying with location.

[0012] The exposure parameters for each location within the layer to be irradiated with the energy beam can be determined from the primary exposure parameters. The amount by which each exposure parameter varies from the primary exposure parameter can be determined at least partly from a geometric quantity of the object derived from the irradiation location.

[0013] In this way, a smaller set of primary exposure parameters can be provided, and how the primary exposure parameters should vary with location to avoid defects in the resulting part can be determined, for example, by a processor of a computer using the method of the present application. This can reduce parameter development time, as multiple exposure parameters for different zone types, such as core and overhang zones, can not need to be determined empirically.

[0014] The method can comprise determining the exposure parameters such that the exposure parameters vary gradually, for example, in a direction towards a perimeter of the zone to be irradiated. As used herein, “vary gradually” means that the change in the exposure parameter occurs over a plurality of successive locations to be irradiated by the energy beam, rather than changing in a single step. By using exposure parameters that vary gradually, defects caused by step changes in the exposure parameters are avoided.

[0015] The exposure parameters can be determined using a continuous function that relates values of the geometric quantity to values of the exposure parameters (although the exposure parameters can be quantized such that the final value of each exposure parameter is given by the allowed discrete value that is closest to the value determined using the continuous function, for example, changes in exposure time can only be allowed in steps of 2 ps). Reducing or eliminating step changes in the exposure parameters is desirable as it can reduce or eliminate defects in the object.

[0016] The geometric quantity of the object can be derived from a geometric model that describes the object to be built using the powder bed fusion apparatus. For example, the geometric model can be a CAD model or an STL model.

[0017] The geometric quantity can be a size measurement that includes the irradiation location. The size measurement can be determined (e.g., calculated) from the geometric model. The size measurement can be a measure of distance, for example, a distance between two points within the geometric model. The size measurement can be a length from the irradiation location to a surface of the object (as defined in the geometric model), such as a nearest point on the surface, a length of a line segment (such as a shortest line segment) between two points on a surface of the object (as defined in the geometric model) and passing through the irradiation location (e.g., the exposure parameter can vary for narrow cross sections), an area of a zone of the object that includes the irradiation location, and / or a volume of the object that includes the irradiation location. The size measurement calculated from the geometric model can comprise a plurality of distances, each distance being calculated from the irradiation point to a different point on the surface of the object.

[0018] It will be appreciated that the term “irradiation location” refers to a location in the geometric model that corresponds to an intended irradiation location during the build process.

[0019] The dimension measurement value can be determined (e.g., calculated) from information contained within the geometric model of the object alone. For example, the dimension measurement value can be determined without comparing the object defined in the geometric model to another definition of an object defined in another geometric model, a measurement of the object, a simulation of how the object defined by the geometric model can be distorted or differ from an intended thermal model of the build or other simulation. In this way, the determination of the dimension measurement value does not require additional input beyond the definition of the object provided by the geometric model defining the design intent. Thus, the present invention does not rely on the accuracy of measurements or simulations in determining the amount by which each exposure parameter is varied from the primary exposure parameter.

[0020] The dimension measurement value can be determined from a single definition of the object given by the geometric model. The measurement of the distance can be between two points defined within the single definition of the object given by the geometric model. The dimension measurement value can be a length from the irradiation location to a surface of the object as defined by the single definition of the object given by the geometric model (such as a nearest point on the surface), a length of a line segment (such as a shortest line segment) between two points on a surface of the object as defined by the single definition of the object given by the geometric model and passing through the irradiation location (e.g., the exposure parameter can be varied for a narrow cross section), an area of a region of the object as defined by the single definition of the object given by the geometric model that includes the irradiation location, and / or a volume of the object that includes the irradiation location.

[0021] Determining the amount by which each exposure parameter is varied from the primary exposure parameter can include using a function or mapping, such as a formula, that defines a relationship between the primary exposure parameter and the (modified) exposure parameter based on a geometric characteristic of the object defined in the geometric model. The function or mapping can include a multiplier (coefficient) of the primary exposure value, where a value of the multiplier is determined from the determined geometric quantity. The value of the multiplier can be determined in part from a ratio (fraction) of the geometric quantity to a threshold quantity of the geometric characteristic. The threshold quantity can be a value above which the primary exposure parameter is used.

[0022] The geometric quantity can be a distance from the irradiation location to a surface of the object (to be built) above the irradiation location.

[0023] Alternatively or additionally, the geometric quantity can be a distance in the plane of the layer from the irradiation position to the surface of the object. For example, the geometric quantity can be a distance in the plane of the layer from the irradiation position to a nearest point on the surface of the object. Whether the exposure parameter is varied from the primary exposure parameter as a function of the distance in the plane of the layer from the irradiation position to the surface of the object can be based on whether the irradiation point is within a threshold distance of the surface. If the irradiation point is within the threshold distance, the amount of exposure point variation can be at least partially a function of the distance of the irradiation point from the nearest point on the surface. If the irradiation point is outside the threshold distance, the amount of exposure point variation can not be a function of the distance of the irradiation point from the nearest point on the surface (e.g., the amount of exposure point variation can be a function of only other geometric quantities, such as the thickness of the consolidated material below the irradiation position). The threshold distance itself can be a function of the thickness of the consolidated material below the irradiation position. For example, the threshold distance can increase as the thickness of the consolidated material decreases. The threshold distance can have a maximum value and a minimum value.

[0024] Alternatively or additionally, the geometric quantity can be a thickness of the consolidated material below the irradiation position.

[0025] The measured value of the distance can be related to a threshold distance below which the exposure parameter is to be varied, e.g., from the primary scan parameter.

[0026] The measured value of the distance can be a number of consolidated layers (possibly including partial layers) above or below the irradiation position. The threshold distance can be a threshold number of consolidated layers. The primary exposure parameter can be selected, e.g., by empirical testing, for a specified layer thickness. Thus, selecting when to vary the exposure parameter from the primary exposure parameter based on the number of layers rather than an absolute measure of the distance means that the algorithm is applicable to primary exposure parameters determined for a plurality of different layer thicknesses. The exposure parameter can be determined by scaling the primary exposure parameter by a ratio of the measured value of the distance to the threshold distance.

[0027] The threshold thickness can be greater than 10 layers, 20 layers, 30 layers, 40 layers, or 50 layers.

[0028] The primary exposure parameter can include a maximum value of the exposure parameter (e.g., for use at a core / body of the object) and a minimum value of the exposure parameter (e.g., for use at an edge region of the object), and the exposure parameter can be determined to be a value between (preferably including) the maximum value and the minimum value based on the geometric quantity. The maximum value can be used for the exposure parameter when the measured value of the distance is equal to or greater than the threshold distance, and a value between the maximum value and the minimum value can be used for the exposure parameter when the measured value of the distance is less than the threshold distance.

[0029] The exposure parameters can be selected from the group of: energy beam parameters such as energy beam power, and / or exposure time; scanning parameters such as dot spacing, scan speed, spot size / focus position and / or spot shape; scan path parameters such as scan spacing (distance between adjacent scan paths), and / or scan path length; and time between exposure of adjacent points / areas. Adjacent points / areas can be adjacent exposure points or adjacent areas comprising multiple scan paths and / or exposure points, such as squares or stripes. It will be appreciated that the term “exposure parameters” as used herein does not include properties of the scan paths such as the position or shape of the scan paths, although the determination of the position or shape of the scan paths can be influenced by the exposure parameters (for example by variation of the scan spacing between a set of scan paths).

[0030] The minimum power can be between one quarter and three quarters of the maximum power. The minimum exposure time can be between one half and seven eighths of the maximum exposure time. The minimum dot spacing can be between one half and seven eighths of the maximum dot spacing. The minimum scan spacing can be between six tenths and nine tenths of the maximum scan spacing. In some embodiments, some beam parameters can not vary (i.e. have the same maximum and minimum values).

[0031] The method can comprise determining the amount for each exposure parameter before the build of the object commences. The amount for each exposure parameter can be determined independently of the scan direction, and possibly even independently of the scan path of the energy beam. In other words, the amount for each exposure parameter can be determined independently of the order in which the layers are irradiated during the build. This can be advantageous because other factors can be important when scanning in an order, such as the direction of gas flow (as disclosed in WO 2014 / 125258, WO 2014 / 125280 and WO 2019 / 211587, the disclosures of which are incorporated herein in their entirety by reference) or wiper position (as disclosed in WO 2015 / 140547, the disclosure of which is incorporated herein in its entirety by reference). Thus, when arranging the exposure using such principles in combination with the determination of the exposure parameters as disclosed herein, it can be useful for the determination of the exposure parameters to be independent of the order of the scanning.

[0032] The additive manufacturing apparatus can comprise a plurality of scanners, each scanner being capable of scanning an energy beam independently of the other scanners. One or more of the scanners can be used to irradiate areas in the layer in accordance with the determined exposure parameters. Thus, the time between exposure of adjacent points / areas can be applied to the exposure of adjacent points / areas by any of the energy beams. Thus, the scanners can need to be controlled so that the required timing between exposures is achieved, for example using deterministic control of the scanners as described in WO 2017 / 085469, the document of which is incorporated herein in its entirety by reference.

[0033] The method can comprise determining a scan path for the energy beam, e.g. the energy beam can travel along the scan path as a continuous scan or as a series of discrete exposures, and the exposure parameters are determined to vary as the energy beam travels along the scan path (e.g. the energy beam power, scan speed, exposure time, spot spacing, time between exposure of adjacent points / areas, spot size / focus position and / or spot shape can vary as the energy beam travels along the scan path). In particular, it can be advantageous to vary the scan parameters as the energy beam moves towards or away from the perimeter of the object.

[0034] Alternatively, the method can comprise determining the exposure parameters to be used for different parts of the area to be irradiated from the geometric quantities of the object measured from the positions, and the scan path is determined to follow an isosurface of equal exposure parameters (e.g. the exposure parameters then the scan path determination scheme). This can be advantageous because the speed at which the energy beam travels along such a scan path can not be limited by the dynamic ability of the scanner of the additive manufacturing apparatus to change the scan parameters. Furthermore, the scan spacing of a scan path following an isosurface can be changed seamlessly.

[0035] The method can comprise determining the exposure parameters for each position based on heat (e.g. the temperature of the consolidated material or powder bed measured during the build and / or the predicted temperature of the consolidated material or powder bed (e.g. at the irradiation position) determined from a thermal model).

[0036] The method can comprise outputting the powder bed fusion instructions to, e.g., a powder bed fusion apparatus.

[0037] The powder can be a metal powder, such as a steel (e.g. a martensitic precipitation hardening steel, a 316 grade steel or a tool steel), an aluminium or aluminium alloy, a titanium or titanium alloy (e.g. Ti-6AL-4V), a nickel superalloy (e.g. Hastelloy, Rene 41 or CM247) or a cobalt-chromium alloy.

[0038] According to a second aspect of the application, there is provided a data carrier having instructions stored thereon, which instructions, when executed by a processor, cause the processor to carry out the method of the first aspect of the application.

[0039] According to a third aspect of the application, there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating regions of successively formed powder layers with an energy beam, the method comprising irradiating a layer with the energy beam in accordance with a set of exposure parameters, the exposure parameters in the set varying with the irradiation position.

[0040] Each exposure parameter at each location can be determined from a primary exposure parameter, wherein the amount by which each exposure parameter varies from the primary exposure parameter is determined at least partly from a geometric quantity of the object derived from the irradiation position. The exposure parameters in the set can vary gradually as the thickness of the consolidated material under the irradiation position changes.

[0041] The irradiation energy can gradually decrease as the thickness of the consolidated material under the different irradiation positions decreases.

[0042] According to a fourth aspect of the application, there is provided a powder bed fusion apparatus comprising: irradiation means for directing an energy beam to a selected area of a working plane; layer formation means for forming a powder layer in the working plane; and a controller arranged to control the irradiation means to direct the energy beam onto successively formed powder layers to melt and / or sinter the powder material, thereby to build an object in a layer-by-layer manner, wherein the irradiation means is controlled to perform the method of the third aspect of the application.

[0043] According to a fifth aspect of the application, there is provided a data carrier having stored thereon instructions which, when executed by a controller of a powder bed fusion apparatus, cause the controller to control the powder bed fusion apparatus to perform the method of the third aspect of the application.

[0044] The data carrier can be a suitable medium for providing instructions to a machine, such as a non-transitory data carrier, for example a floppy disk, a CD ROM, a DVD ROM / RAM (including -R / -RW and +R / +RW), a HD DVD, a Blu Ray (TM) disc, a memory (such as a Memory Stick (TM), an SD card, a compact flash card, etc.), a disk drive (such as a hard disk drive), a magnetic tape, any magnetic / optical storage; or a transitory data carrier, such as a signal on a wire or fibre, or a wireless signal, for example a signal transmitted over a wired or wireless network (such as an internet download, an FTP transfer, etc.). BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 is a schematic diagram of a powder bed fusion additive manufacturing apparatus according to an embodiment of the application;

[0046] Figure 2 is a schematic diagram illustrating various exposure parameters used in this embodiment of the application;

[0047] Figure 3 is a graph illustrating the calculation of scaling factors for determining energy beam parameters from primary energy beam parameters;

[0048] Figure 4 illustrates the positioning of scan paths along contours of exposure parameters and the variation of scan spacing between scan paths;

[0049] Figure 5a and Figure 5b Fig. 1 shows an image of the component geometry and the arrangement of the component on the build plate determined from the geometric model of the component according to example 1 ;

[0050] Figure 6a and Figure 6b is a table showing the exposure parameters for the component of example 1 ;

[0051] Figure 7 is a table showing the build results for the component of example 1.

[0052] Figure 8a , Figure 8b and Figure 8c shows different scanning strategies tested in example 2;

[0053] Figure 9a and Figure 9b is a table showing the exposure parameters for the component of example 2;

[0054] Figure 10 is a table showing the build results for the component of example 2.

[0055] Figure 11a and Figure 11b is a table showing the exposure parameters for the component of example 3;

[0056] Figure 12 is a table showing the build results for the component of example 3.

[0057] Figure 13a and Figure 13b is a table showing the exposure parameters for the component of example 4;

[0058] Figure 14 is a table showing the build results for the component of example 4.

[0059] Figure 15 is a table showing the angles, thicknesses and lengths of the components according to example 5;

[0060] Figure 16 is a table showing the exposure parameters for the component of example 5;

[0061] Figure 17 is a picture of the component of example 5; and

[0062] Figure 18 is a flowchart illustrating a method of determining exposure parameters according to an embodiment of the present invention. DETAILED DESCRIPTION

[0063] Reference is made to Figure 1A powder bed fusion additive manufacturing apparatus according to an embodiment of the application comprises a build chamber 101 which can be sealed off from the external environment so that an inert atmosphere (in this embodiment argon, but other inert gases such as nitrogen or helium can be used) can be maintained therein. Within the build chamber 101 is a process plate 115 and a build sleeve 116. A build platform 102 is lowerable in the build sleeve 116 to define a build volume 117. The build platform 102 supports a powder bed 104 and a workpiece (object) 103 as the workpiece 103 is built by selective laser melting of powder. The platform 102 is lowered within the build sleeve 117 under the control of a drive (not shown) as successive layers of the workpiece 103 are formed.

[0064] As the workpiece 103 is built by a layer formation device, in this embodiment by a dispensing apparatus and a wiper (not shown), a powder layer 104 is formed. For example, the dispensing apparatus can be an apparatus as described in WO 2010 / 007396. The dispensing apparatus dispenses powder onto an upper surface defined by the process plate 115 and spreads the powder bed by a wiper (not shown). The position of the lower edge of the wiper defines a working plane 190 at which the powder is consolidated. In another embodiment, the powder layer can be formed in the working plane 190 by a non-contact recoater. A build direction BD is perpendicular to the working plane 190.

[0065] A plurality of laser modules 105a, 105c generate laser beams 118a, 118c for melting the powder 104, which are directed as required by corresponding optical modules (scanners) 106a, 106c. The laser beams 118a, 118c enter through a common laser window 107. Each optical module comprises steering optics 121 (such as two mirrors mounted on galvanometers) for steering the laser beam 118 in a vertical direction across the working plane and focusing optics 120 (such as two movable lenses for changing the focal point of the corresponding laser beam 118). The scanners are controlled so that the focal point position of the laser beam 118 is maintained in the working plane 190 as the laser beam 118 is moved across the working plane. Instead of using dynamic focusing elements to maintain the focal point position of the laser beam in the plane, f-theta lenses can be used.

[0066] An inlet and an outlet (not shown) are arranged for generating a gas flow across the powder bed formed on the build platform 102. The inlet and the outlet are arranged to generate a laminar flow having a flow direction from the inlet to the outlet. The gas is recirculated from the outlet to the inlet by a gas recirculation loop (not shown).

[0067] The controller 140 (including the processor 161 and the memory 162) is in communication with the modules of the additive manufacturing apparatus (i.e. the laser modules 105a, 105b, 105c, 105d, the optical modules 106a, 106b, 106c, 106d, the build platform 102, the dispensing apparatus 108 and the wiper). The controller 140 controls these modules based on software stored in the memory 162, as described below.

[0068] The controller 140 uses deterministic control to control the plurality of lasers 106a, 106c and scanners 105a, 105c, in which the command signals specify execution times (rather than simply being queued), as described in WO 2017 / 085469, which is incorporated herein by reference. In this way, precise control over the timing between the irradiation of the powder by each laser beam 118a, 118c can be achieved.

[0069] In use, the computer 170 receives a geometric model describing a three-dimensional object to be built using the powder bed fusion additive manufacturing apparatus, such as an STL file. The computer 170 slices the geometric model into a plurality of slices based on a defined layer thickness for building in the powder bed fusion additive manufacturing apparatus.

[0070] The computer can include an interface arranged to provide user input for selecting a material for building the object. The computer determines exposure parameters based on the material identified by the user. For example, the computer 170 can include a memory 172 having stored therein a database of primary (bulk) exposure parameters suitable for the identified material. The laser exposure pattern of the fusing region of each layer is determined, as well as the exposure parameters for different locations in the exposure pattern, to form a respective cross-section (slice) of the object. Based on these determinations, the computer 170 generates powder bed fusion instructions which are sent to the controller 140 (such as over the network connection 171) to cause the additive manufacturing apparatus to perform the build in accordance with the powder bed fusion instructions.

[0071] Reference Figure 2 According to a first embodiment, the exposure parameters for each location to be irradiated with a laser beam are determined based on a geometric quantity of the object derived from the irradiation location (in this embodiment, the thickness of the solidified material underneath the location). This can result in the exposure parameters varying with location.

[0072] Exposure parameters can include energy beam source / laser parameters such as laser beam power and exposure time, achieved by controlling the energy beam source / laser 105a, 105c; scanning parameters such as scan speed, point distance (PD), time between exposure of adjacent points / areas, spot size / focus position, and spot shape, achieved by controlling the scanners 106a, 106c and specifying how the energy beam / laser beam travels along the scan paths; and scan path parameters such as distance between scan paths (hop distance (HD)) and maximum or minimum length of a set of scan paths, specifying properties of a set of scan paths. In the present embodiment, the scan paths are scanned in both directions, however, it will be appreciated that the scan paths can be scanned in a single direction. The scan directions can be the same for all layers, or the directions can be rotated (e.g. by a set angle) between layers, such that the scan paths of successive layers are not parallel.

[0073] Upon receiving the user's selection of material, the computer 170 retrieves a primary set of exposure parameters from the database. For each exposure parameter that can vary gradually across the layer (variable exposure parameter), a maximum value E max and a minimum value E min are provided in the database. For exposure parameters that remain constant when applying the specific set of parameters (fixed exposure parameters), a single value of the exposure parameter is provided, or E max and E min are set to the same value. In systems where the scan paths are determined prior to determining the gradual variation of the variable exposure parameter along the scan path, the scan path parameters are fixed. In systems where the scan paths are determined after or iteratively with the determination of the gradual variation of the variable exposure parameter with position, the scan path parameters can be variable.

[0074] In the present embodiment, the variation of the variable exposure parameter is determined based on the thickness of the consolidated material underneath the irradiation position. The exposure parameter E dyn at a certain position is determined according to the following equation:

[0075]

[0076] where N x is the number of layers of consolidated material underneath the irradiation position (measured in the build direction BD), N c is a preset threshold number of layers below which the exposure parameter varies from the maximum value E max of the exposure parameter, and the order is a positive real number and typically an integer. In the present embodiment, the order has a value from 1 to 5, preferably 3. When N x is equal to or greater than N c , E dyn is equal to E maxWhen N x equals 0, E dyn equals E min . In N x between these values, E dyn is a value between E max and E min , and E dyn will gradually change if the thickness of the consolidated material under the irradiation position gradually changes. Typically, the object to be additively manufactured will be designed to not have a step change in thickness in the build direction BD.

[0077] N x may be determined by:

[0078]

[0079] where T x is the thickness of the consolidated material under the irradiation position at position x, and t is the layer thickness. If T x is calculated from the geometric model of the object before determining the layer, N x may have non-integer values. N c is typically greater than 10, preferably greater than 30, most preferably greater than 40.

[0080] It will be appreciated that in other embodiments other functions of N x and N c may be used. For at least part of the function, the exposure parameter gradually changes with changes in N x , typically the energy density decreases with decreasing N x .

[0081] Figure 3 A wedge-shaped overhang section of an object is shown, where the dashed line marked t*N c indicates the transition in thickness in the build direction BD from greater than the critical thickness t*N c to less than the critical thickness t*N c .the exposure parameter does not change. The size of the dynamic region w2 will vary with the angle Θ of the overhang segment. Unlike the prior art method of making a step change in the exposure parameter when the overhang angle is below a threshold angle, the present invention dynamically changes the exposure parameter for all overhang angles, only changing the length w2 of the dynamic region over which the exposure parameter varies. In addition, the change in the exposure parameter is made by a gradual change determined by (1), rather than a step change in the exposure parameter. A third region (not shown) can provide for low angle overhangs, such as overhangs at an angle of less than 20 degrees from horizontal, where exposure is skipped for every even layer. In the third region, odd layers are contacted with a laser beam having an exposure parameter that produces a higher energy density than the energy density used for the second dynamic region w2. This increase in energy density can compensate for the increase in the thickness of the powder to be melted in this third region due to the underlying unmelted even layer. The third region can be defined by a threshold value of N x , such as N x being less than 5. For even layers, exposure is stopped for irradiation points having a value of N x below the threshold value. For odd layers, exposure with a higher energy density is used for irradiation points having a value of N x below the threshold value.

[0082] In a first embodiment, the scan path parameters are fixed exposure parameters, and for each layer, the scan path is determined based on the fixed scan path parameters. Once the scan path is determined, the exposure parameters for each irradiation position along the scan path are determined. The variable exposure parameters can be laser power, exposure time, and spot spacing.

[0083] Such an embodiment has the advantage of decoupling the determination of the scan path from the determination of the variable exposure parameters. However, it limits the freedom available to vary the irradiation energy input into a portion of the layer. For example, the lasers 106a, 106c can not be able to stably operate at a lower laser power, and thus, it can not be possible to reduce the laser power below a particular minimum value in order to achieve a lower irradiation energy. In order to achieve the desired result, it can be necessary to change other exposure parameters, such as the scan spacing.

[0084] In a second embodiment, the variable exposure parameters on the layer are determined prior to the scan path or are determined in conjunction with the constant thickness N xThe associated and thus to be followed according to (1) isosurfaces of at least illumination locations with the same exposure parameters. The scan path can then be determined based on the determined exposure parameters / isosurfaces. Such embodiments can allow for variations of scan path parameters like scan pitch. To avoid discontinuities in the scan path for varying scan path parameters, it can be desirable that the scan path coincides with the isosurfaces of constant exposure parameters. This way, the scan pitch between adjacent scan paths can vary while avoiding discontinuities in the scan path.

[0085] Figure 4 An illustrative example of such a method applied to a wedge-shaped object 200 is shown. It can be seen that for the non-dynamic zone wl where the thickness of the consolidated material under the illumination locations is higher than a threshold thickness N c , the scan paths are equidistant (equal scan pitch) and the exposure parameters are the same. For the dynamic zone wl where the thickness of the consolidated material under the illumination locations is lower than a threshold thickness N c , the scan pitch between scan paths decreases as the wedge gets thinner and there is a corresponding change in exposure parameters between scan paths of this zone, but the exposure parameters remain the same along each scan path. For this example, the scan paths are shown as straight, however for objects of different shapes, the scan paths can have different curved shapes.

[0086] WO 2014 / 006094 discloses scan paths based on the geometric profile of the zone to be fused within a layer. The present invention differs in that the scan paths do not necessarily follow the profile of the zone, but rather the position of the scan paths is defined by lines of the same exposure parameters, in this embodiment, these exposure parameters are defined by the thickness of the consolidated material underneath.

[0087] Example 1

[0088] Eighteen parts with the geometry shown were built on a build plate in the arrangement shown in Figure 5a The parts formed a 20° slope with the working plane 190 and had a thickness of 2 mm. The layer thickness was 30 pm. The parameters used to build the parts are shown in Figure 5b , the dynamic laser power and scan pitch (HD) parameters (between a maximum HD of 100 pm and a minimum HD as shown in Figure 6a ) for all parts, and the dynamic point pitch parameter for some parts. The beam size was fixed, although some parts were built with a fixed beam diameter of 67 pm, while others were built with a fixed beam diameter of 89 pm. The exposure time was fixed at 80 ps. The laser beam was unidirectionally travelled along the scan paths. Figure 6a

[0089] ​Equation (1) was used to determine the dynamic change of exposure parameters for the objects. Nc values of 12 and 20 were used with orders of 1 and 5.

[0090] As can be seen from Figure 7 all parts eventually failed, but those using a Nc value of 12 to determine the dynamic parameters failed earlier than those using 20. The subject parameters with lower energy density also appear to have produced better results.

[0091] The subject exposure parameters were changed from the following initial exposure parameters:

[0092] Power = 200 W; beam distance = 67 pm; exposure time = 50 ps; dot spacing = 65 pm; scan spacing = 65 pm; resulting in an energy density of 2.37 J / mm 2 .

[0093] to the following new subject exposure parameters:

[0094] Power = 195 W; beam distance = 67 pm; exposure time = 80 ps; dot spacing = 80 pm;

[0095] scan spacing = 100 pm; resulting in an energy density of 1.95 J / mm 2 .

[0096] Example 2

[0097] Sixteen parts with the geometry shown in Figure 5a were built on a build plate in a similar arrangement to that shown in Figure 5b . The layer thickness was 30 pm and the new subject exposure parameters were used. The exposure parameters of power, scan spacing and dot spacing were all dynamically changed according to equation (1). The order used was 5 and Nc values of 10, 25 and 30 were used for different parts. Various scan strategies were used as illustrated in Figure 8a , Figure 8b and Figure 8c . Figure 9a and Figure 9b The tables of

[0098] Figure 10 illustrate the exposure parameters and Nc values used for each part and the scan path strategy. The minimum energy density was 1.56 J / mm 2 .

[0099] Example 3

[0100] The geometry of eighteen parts was similar to that shown in Figure 5aThe geometries shown are similar, but built at different angles relative to the build plane on the build plate. The parts are built with similar arrangements to those shown in Figure 5b The layer thickness is 30 pm and new base exposure parameters are used. The exposure parameters for power, scan spacing, and dot spacing are all dynamically varied according to equation (1). Different parts use orders of 3, 5, and 10 and Nc values between 40 and 56. Scan path strategies 1 and 2 are used. Figure 11a and Figure 11b The table shows the exposure parameters, Nc values, and scan path strategies used for each part.

[0101] Figure 12 The order in which the parts failed during the build is shown. Scan path strategy 2 (bidirectional scanning) appears to have produced better results. Successful builds were achieved with both order 3 and order 5.

[0102] Example 4

[0103] The geometries of the eighteen parts are similar to those shown in Figure 5a The geometries shown are similar, but built at different angles relative to the build plane on the build plate. The parts are built with similar arrangements to those shown in Figure 5b The layer thickness is 30 pm and new base exposure parameters are used. The exposure parameters for power, scan spacing, dot spacing, and exposure distance are dynamically varied according to equation (1) for different parts in different combinations. Different parts use orders of 3, 5, and 10 and Nc values between 40 and 56. The variation in exposure time is according to equation (1) in steps of 2 ps, rather than as a smooth function. Scan path strategies 1 and 2 are used. Figure 13a and Figure 13b The table shows the exposure parameters, Nc values, and scan path strategies used for each part.

[0104] As can be seen from Figure 14 most of the parts were successfully built. In comparison to Example 3, a significant improvement in the quality of the upper and lower surface regions was achieved. According to optical analysis of the parts, part 17 was considered to be the highest quality. For part 17, the exposure time, scan spacing, and dot spacing were all fixed, with only the power dynamically varied according to equation (1). However, as can be seen from the other successful builds in this example, it is possible to dynamically vary these other exposure parameters without causing a build failure.

[0105] Example 5

[0106] The geometries of the sixteen parts are similar to those shown in Figure 5a The geometries shown are similar, but built at different angles relative to the build plane on the build plate. The parts are built with similar arrangements to those shown in Figure 15The layer thickness is 30 pm and new bulk exposure parameters are used. For all parts, the laser power is dynamically varied according to equation (1) and for parts 4, 8 and 16, the exposure time is dynamically varied. Step 3 is used. For different parts, Nc values between 14 and 56 are used. Figure 16 The table below shows the angles, thicknesses, exposure parameters and N c values used for each part.

[0107] With reference to Figure 17 , samples 5 and 8 failed due to high bulk energy density. The remaining parts were successfully built. The exposure parameters for parts 13 and 14 were calculated using lower Nc values, which showed signs of powder combustion at higher layers. The walls from 25° to 40° were built without powder combustion and material loss. The problem of powder combustion was solved by printing the 15° wall and 20° wall with a thickness of 0.5 mm using lower bulk power. Eliminating powder combustion solved the problem of material loss.

[0108] A third embodiment of the present invention considers, in addition to the vertical distance / thickness N x , also the horizontal distance from the illumination point defined in the geometric model to the nearest point on the surface of the object. In this embodiment, the exposure parameter E dyn as determined using equation (1) is further adjusted based on the distance to the nearest point on the surface of the object to determine the final value of the exposure parameter E dynR . In this embodiment, the final exposure parameter E edge for the illumination point within a distance d dynR from the surface of the object is determined by:

[0109] E dynR = kE dyn (2)

[0110] where the value of the reduction factor k is determined based on the distance between the illumination point and the nearest point on the surface of the object as determined by the geometric model. In this embodiment, k is given by:

[0111]

[0112] where d max , d min , k max and k min are constants that are preset, for example, based on the material being machined, and d is the distance to the nearest point on the surface of the object as defined in the geometric model.

[0113] For illumination points that are more than a distance d edge from the surface of the object, E dynR = E dyn (4).

[0114] d edge is given by:

[0115]

[0116] Thus, a threshold distance d edge from the surface of the object is set, beyond which distance / thickness / number of layers of material underneath the irradiation point c , equation (2) is applied. In other words, the thicker the amount of consolidated material underneath the irradiation point, the closer the irradiation point can be to the surface of the object in the horizontal direction before taking the irradiation point near the surface of the object in the horizontal direction into account for adjusting the exposure parameters.d min A horizontal distance from the surface of the object is set, beyond which distance the exposure parameters will be modified from the primary exposure parameters regardless of the thickness of the consolidated material underneath the irradiation point. In other words, when the irradiation is close to the surface of the object, the exposure parameters will be down-regulated, the distance by which the exposure parameters are down-regulated being given by d dyn . min A horizontal distance from the surface of the object is set, beyond which distance the exposure parameters will be modified from the primary exposure parameters regardless of the thickness of the consolidated material underneath the irradiation point. In other words, when the irradiation is close to the surface of the object, the exposure parameters will be down-regulated, the distance by which the exposure parameters are down-regulated being given by d max . dyn

[0117] Typically, k max equals 1 and k min is less than 1.d max and d min may be set based on a fixed exposure parameter such as a fixed spot pitch or a fixed scan pitch.

[0118] For example, d min may be set equal to the fixed spot pitch or the fixed scan pitch.d max may be based on a scan length such as a minimum scan length, a strip width or a width of a square of a chessboard scan pattern. For example, d max may be set equal to or less than the minimum scan length, the strip width or the width of a square of a chessboard scan pattern.

[0119] The above embodiments of the invention are illustrated in the flowchart of Figure 18 Fig. 1. In step 201, a geometric model of an object is received by a processing unit (such as a computer) running build preparation software. The geometric model can be a CAD model, an STL model, etc. defining the surface of the object. In step 202, for each point in the object defined by the geometric model corresponding to an irradiation position in the build process, one or more relevant dimension measurements given by the definition of the object in the geometric model are determined. In the first and second embodiments, the dimension measurement is the thickness N c ​The third embodiment additionally requires the distance d from the closest point on the surface of the object in the layer plane.

[0120] In step 203, for each point in the geometric model corresponding to a position of the illumination in the build process, an exposure parameter is determined from the scaling of the primary exposure parameter. The scaling is determined from one or more relevant dimension measurements. In the first and second embodiments, the scaling is given by equation (1). In the third embodiment, the scaling is given by equations (1) to (5). In step 204, instructions for the build including the exposure parameters determined in 203 are generated and sent to the powder bed fusion additive manufacturing apparatus. The build is then performed by the powder bed fusion additive manufacturing apparatus using the exposure parameters determined in 203 according to the instructions. Typically, the exposure parameters will remain unchanged from those determined in step 203 during the build process.

[0121] It will be appreciated that modifications and alterations can be made to the above described embodiments without departing from the present application as defined herein. For example, other geometric properties of the object can be considered when determining the exposure parameter for the variable exposure parameter. Additionally, the equations used to determine the exposure parameter can be different, for example, the equations can include multiple terms of different orders based on the thickness measurement and / or use a combination of different geometric measurements of the object to determine the exposure parameter.

[0122] Other variable exposure parameters can be used, such as the time between exposures of adjacent exposure points / areas.

Claims

1. A method for determining instructions to be executed by a powder bed fusion apparatus, wherein, The method of constructing an object layer by layer by selectively irradiating regions of successively formed powder layers with an energy beam includes: determining exposure parameters for each location within a layer to be irradiated by the energy beam based on primary exposure parameters, the exposure parameters varying with location, and the amount by which each exposure parameter varies from the primary exposure parameters is determined at least in part based on a geometric quantity of the object derived from the irradiation location, characterized in that the geometric quantity of the object is a distance between two points defined in the geometric model of the object to be constructed using the powder bed fusion apparatus, from the irradiation location to the surface of the object.

2. The method according to claim 1, comprising: The exposure parameters are determined such that they are gradually varied at multiple irradiation positions.

3. The method according to claim 1 or 2, wherein, The geometric quantity is the length from the irradiation position to the surface of the object.

4. The method according to claim 1 or 2, wherein, The geometric quantity is the distance from the irradiation position to the surface of the object in the plane of the layer.

5. The method according to claim 1 or 2, wherein, The geometric quantity is a measurement of the thickness of the solidified material below the irradiation position.

6. The method according to claim 5, wherein, The thickness measurement is the thickness of the solidified material below the irradiation position relative to the threshold thickness.

7. The method according to claim 5, wherein, The thickness measurement is the amount of the consolidation layer below the irradiation position.

8. The method according to claim 6, wherein, The threshold thickness is the threshold number of the consolidation layer.

9. The method according to claim 6 or 8, wherein, The exposure parameters are determined by scaling the primary exposure parameters according to the ratio of the thickness of the solidified material below the irradiation position to the threshold thickness.

10. The method according to claim 1 or 2, wherein, The primary exposure parameters include the maximum value and the minimum value of the exposure parameters, and the exposure parameters are determined based on the geometric quantities to be a value between the maximum value and the minimum value.

11. The method according to claim 1 or 2, comprising: The scanning path of the energy beam is determined, and the exposure parameters are determined to change as the energy beam travels along the scanning path.

12. The method according to claim 1 or 2, comprising: Exposure parameters to be used for different parts of the area to be irradiated are determined based on the geometric quantities of the object measured from the location, and the scan path is determined to follow contour lines with equal exposure parameters.

13. The method according to claim 1 or 2, wherein, The exposure parameters are energy beam power and / or exposure time.

14. A data carrier storing instructions that, when executed by a processor, cause the processor to perform the method as described in any one of claims 1 to 13.

15. A powder bed fusion method, wherein, The method of constructing an object layer by layer by selectively irradiating regions of successively formed powder layers with an energy beam includes: irradiating layers with the energy beam according to a set of exposure parameters, the exposure parameters in the set varying with the irradiation position, and determining each exposure parameter at each position according to primary exposure parameters, wherein the amount by which each exposure parameter varies from the primary exposure parameters is determined at least in part based on a geometric quantity of the object derived from the irradiation position, characterized in that the geometric quantity of the object is a distance between two points from the irradiation position to the surface of the object, defined in the geometric model describing the object to be constructed using the powder bed fusion method.

16. A powder bed fusion method, wherein, The method of constructing an object layer by layer by selectively irradiating regions of successively formed powder layers with an energy beam includes: irradiating the layers with the energy beam according to a set of exposure parameters, wherein the exposure parameters in the set gradually change with the thickness of the solidified material below the irradiation location as defined in a geometric model describing the object to be constructed using the powder bed fusion method.

17. A powder bed fusion method, wherein, The method of constructing an object layer by layer by selectively irradiating regions of successively formed powder layers with an energy beam includes: irradiating the layers with the energy beam such that the irradiation energy delivered by the energy beam to different locations within the regions of the powder gradually varies with the thickness of the consolidation material beneath the different irradiation locations as defined in a geometric model describing the object to be constructed using the powder bed fusion method.

18. The powder bed fusion method according to claim 17, wherein, The irradiation energy gradually decreases as the thickness of the solidified material at the different irradiation positions decreases.

19. A powder bed fusion apparatus, comprising: An irradiation device used to direct an energy beam to a selected area of ​​a working plane; A layer forming apparatus for forming a powder layer in the working plane; And a controller, arranged to control the irradiation device to direct the energy beam onto successively formed powder layers to melt and / or sinter the powder material, thereby constructing an object in a layer-by-layer manner, wherein the irradiation device is controlled to perform the method according to any one of claims 15 to 18.

20. A data carrier storing instructions that, when executed by a controller of a powder bed fusion apparatus, cause the controller to control the powder bed fusion apparatus to perform the method according to any one of claims 15 to 18.

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