Powder bed fusion methods 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 each of a plurality of successively formed powder layers with an energy beam. The method comprises determining, for at least one of the powder layers, a scan path for the energy beam to scan to melt powder of the powder layer, the scan path including bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted by irradiation of previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a bulk threshold, and non- bulk irradiation locations, wherein, for each non-bulk irradiation location, powder is melted to form a thickness of consolidated material from the non-bulk irradiation location to a surface of the object less than the bulk threshold. The method comprises allocating a bulk value for an exposure parameter to the bulk irradiation locations, and determining a non-bulk value for the exposure parameter for each non-bulk irradiation location dependent on, at least in part, the thickness of the consolidated material from the non-bulk irradiation location.

Description

[0001]

[0002] POWDER BED FUSION METHODS AND RELATED APPARATUS

[0003] Field of Invention

[0004] This invention concerns powder bed fusion methods in which an object is built in a layer-by-layer manner by selectively irradiating areas of successively formed powder layers with an energy beam and apparatus for carrying out the methods, such as powder fusion apparatus, build preparation systems, such as build preparation software, and instructions stored on a data carrier for controlling powder bed fusion apparatus.

[0005] Background

[0006] Powder bed fusion apparatus produce objects through layer-by-layer solidification of a material, such as a metal powder material, using a high-energy beam, such as a laser beam. A powder layer is formed in a build plane across a powder bed contained in a build sleeve by lowering a build platform in the build sleeve to lower the powder bed, dosing a heap of powder adjacent to the lowered powder bed and spreading the heap of powder with a recoater across (from one side to another side of) the powder bed to form the layer. Portions of the powder layer corresponding to a cross-section of the workpiece to be formed are then solidified through irradiating these areas with the beam. The beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered and a further layer of powder is spread over the surface and solidified, as required.

[0007] A surface finish of the object can vary with angle of the downwardly facing surface. In particular, lower build angles can exhibit more prominent effects due to droop of material and excessive heat being input into the area. In an attempt to reduce these effects, it is known to identify (overhanging) regions of an area of a layer to be solidified, which are solidified using different, overhang exposure parameters to those (bulk exposure parameters) used for other (core / bulk) regions of the area. The overhang regions are typically associated with downwardly oriented surfaces of the object that are below a threshold angle, such as below 45 degrees, to the build plane.

[0008] However, it has been found that, even with the use of overhang parameters within the overhang regions, deformations and / or failure of the part can still occur at these overhang regions. Furthermore, the use of a threshold angle introduces a change in the surface finish where the geometry transitions from below the threshold to above, which can be considered to be a defect.

[0009] WO2022 / 123224 Al discloses a method of determining exposure parameters based upon a number of layers of solidified material below a location of irradiation. The exposure parameter is varied from a maximum exposure parameter for irradiation points where the number of layers below is below a threshold number. This method may result in a non-dynamic region, where the exposure parameter is not varied, and a dynamic region, where the exposure parameter is varied. The exposure parameter may be determined to vary as the energy beam progresses along a scan path. In particular, it may be advantageous to vary the scan parameter as the energy beam moves towards or away from a perimeter of the object.

[0010] US2020 / 0269352 Al discloses carrying out exposure of a downskin region as a continuous exposure with inskin parameters up to a planned contour of the component in every second, third or nthlayer. The exposure is carried out with substantially the same (full) power of the laser radiation as that irradiated in the core region. In the layers in between, the downskin region is not exposed, but simply omitted in the irradiation planning.

[0011] Summary of Invention

[0012] According to a first aspect of the invention there is provided 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 each of a plurality of successively formed powder layers with an energy beam, the method comprises determining, for at least one of the powder layers, a scan path for the energy beam to scan to melt powder of the powder layer. The scan path may include bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted by irradiation of previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a bulk threshold, and non-bulk irradiation locations, wherein, for each non-bulk irradiation location, powder is melted to form a thickness of consolidated material from the non-bulk irradiation location to a surface of the object less than the bulk threshold. The method may comprise allocating a bulk value for an exposure parameter to the bulk irradiation locations. The method may comprise determining a non-bulk value for the exposure parameter for each non-bulk irradiation location based (in dependence) on, at least in part, the thickness of the consolidated material from the non-bulk irradiation location.

[0013] In this way, exposure of the bulk and non-bulk regions is achieved through a continuous scan of the energy beam along the scan path with appropriate changes, for example through control of an energy beam scanner and / or an energy beam generator such as a laser, to achieve the required values for the exposure parameter. Scan path parameters, such as hatch spacing, may be fixed as the scan paths may be determined before determining locations along the scan paths that correspond to bulk and non-bulk irradiation locations. In this way, determination of the scan paths may be separated from determination of exposure parameters and, optionally, determination of the scan paths may be separated from the geometry of underlying consolidated material. For example, the scan paths for melting a powder layer may be determined independently from whether a region to be melted is a bulk or non- bulk region. The bulk threshold is set, for example predetermined, such that, for bulk irradiation locations, the use of a fixed bulk value for the exposure parameter achieves a desired outcome irrespective of differences in thickness above the bulk threshold. For thicknesses below the bulk threshold, the value for the exposure parameter is adjusted to take account of the effects of differing thicknesses on the melting process.

[0014] A thickness of the consolidated material may differ from a powder layer thickness because the consolidated material formed by melting powder through irradiating the irradiation location with the energy beam may join to consolidated material formed by irradiating earlier powder layers. Preferably, the thickness is from the irradiation location to a point on a surface of the object directly below the irradiation location in a build direction.

[0015] The scan path may extend to an edge of an area to be consolidated in the powder layer. The scan path may be a straight line. The scan path may be a straight line extending between opposed edges of the area (e.g. in a meander scan strategy). Alternatively, the scan path may be a straight line extending from a side of a stripe of a plurality of parallel stripes to another side of the stripe or the edge of the area (e.g. in a stripe scan strategy). In another embodiment, the scan path may be a straight line extending from a side of a square of a plurality of tessellated squares to another side of the square or the edge of the area (e.g. in a chequerboard scan strategy).

[0016] The method may comprise determining a plurality of the scan paths for an area to be consolidated in the or each powder layer. The scan paths may be parallel scan paths. The scan paths may be straight lines. Each scan path of the parallel scan paths may extend to an edge of an area to be consolidated in the powder layer. The parallel scan paths may be scan paths of a meander, stripe or chequerboard scan strategy as defined above.

[0017] The non-bulk values vary for different thicknesses of the consolidated material from the non-bulk irradiation location. The irradiation of each non-bulk irradiation location by the energy beam may melt powder of the powder layer to form together with powder melted in previous powder layers consolidated material having the thickness less than the bulk threshold or may melt powder of the powder layer above unconsolidated powder of an immediately preceding powder layer (hence the bulk threshold is for a thickness greater than 1 powder layer thickness). The bulk threshold may be defined as a number of layers for a specified powder layer thickness. Typically, the bulk threshold will between 10 and 75 layers. The bulk threshold may between 0.5mm to 3mm and preferably, 1.25mm to 1.5mm.

[0018] Different bulk thresholds may be defined for different powder layer thicknesses and / or different bulk values for the exposure parameter. For example, a user or build preparation software may select a layer thickness from a set / range of acceptable layer thicknesses and a bulk threshold may be selected or determined based on the selected layer thickness. In addition, a user or build preparation software may select a bulk value from a set / range of acceptable bulk values for the exposure parameter and a bulk threshold may be selected or determined based on the selected bulk value.

[0019] The method may comprise identifying core irradiation locations, wherein powder of two or more layers is melted by irradiation of each core irradiation location with the energy beam, and shell irradiation locations, wherein powder of a single layer alone is melted by irradiation of each non-bulk irradiation location with the energy beam. For shell irradiation locations, powder is melted by the energy beam to form, together with powder melted by irradiation of previous powder layers, a thickness of consolidated material from the shell irradiation location to a surface of the object below a shell threshold. For core irradiation locations, powder is melted to form a thickness of consolidated material from the core irradiation location to a surface of the object above the shell threshold. This may reduce build time; wherein thicker powder thicknesses are melted in core portions that correspond to the core irradiation locations.

[0020] The method may comprise identifying core irradiation locations for non- consecutive powder layers, wherein powder is melted to form a thickness of consolidated material from the core irradiation location to a surface of the object above the shell threshold. The method may comprise identifying first shell irradiation locations for the non-consecutive powder layers, wherein for each first shell irradiation location, powder is melted to form a thickness of consolidated material from the first shell irradiation location to a surface of the object below the shell threshold. The method may comprise identifying second shell irradiation locations for powder layers in-between the non-consecutive powder layers, wherein for each second shell irradiation location, powder is melted to form a thickness of consolidated material from the second shell irradiation location to a surface of the object below a modified shell threshold. The modified shell threshold may correspond to a greater number of powder layers than the shell threshold. The modified shell threshold may correspond to a number of powder layers that is greater than the shell threshold by at least the number of powder layers in-between the non-consecutive powder layers. In the case of an overhanging portion of the object, the use of a modified shell threshold for in-between powder layers increases an overlap between a shell region comprising shell irradiation locations for the inbetween powder layers and a core region comprising core irradiation locations for the next non-consecutive powder layer.

[0021] The core irradiation locations may comprise bulk irradiation locations. The core irradiation locations may comprise non-bulk irradiation locations. Accordingly, the bulk threshold (for a thickness of powder of the two or more layers melted by irradiation of each core irradiation location) may be greater than the shell threshold. The bulk threshold for the melting of a single powder layer thickness may also be equal to or greater than the shell threshold. Therefore, the shell irradiation locations may only comprise non-bulk irradiation locations. The shell irradiation locations may comprise downskin irradiation locations.

[0022] The scan path may include downskin irradiation locations, wherein, for each downskin irradiation location, powder is melted to form consolidated material having a thickness from the downskin irradiation location to a surface of the object less than a downskin threshold. The downskin threshold is less than the bulk threshold. The downskin threshold may be less than the shell threshold. The method may comprise determining a downskin value for the exposure parameter for downskin irradiation locations, wherein the downskin value results in a higher energy density than the bulk value. It has been found that using a higher energy density for the downskin irradiation locations results in higher mechanical strength and improved surface quality for the resulting object. The method may comprise determining the downskin value in dependence on, at least in part, on the thickness of consolidated material below the downskin irradiation location.

[0023] The downskin threshold may be between 3 and 10 powder layers. The downskin threshold may be between 100 and 250pm and preferably, 150 and 200pm.

[0024] The non-bulk value and / or the downskin value may be lower for lower thicknesses of consolidated material below the non-bulk / skin irradiation location.

[0025] The exposure parameter may be energy beam density, energy beam power, spot size, scan speed or (for pulsed scanning) exposure time and point distance.

[0026] Scanning of the energy beam along the scan path(s) may comprise traversing the energy beam along the scan path with the energy beam constantly on (continuousmode scanning) or with pulsing of the energy beam power (point scanning). Preferably, scanning of the energy beam along the scan path(s) comprises exposing each irradiation location on the scan path during a high-power interval of the pulsed energy beam and adjusting steering elements, such as steering optics, to direct the energy beam to the next irradiation location during a low-power interval of the pulsed energy beam, such as when the energy beam is switched off. The time an irradiation location is exposed (e.g. a length of the high-power interval) may be defined by an exposure time. The exposure time may be longer for the downskin irradiation locations than the bulk irradiation locations. Accordingly, the method may comprise selecting an exposure time for an irradiation location based on whether the irradiation location is a bulk irradiation location or a downskin irradiation location. By using a longer exposure time for the downskin irradiation locations, the higher energy density (power per unit area) may be delivered to the downskin irradiation location over a longer period of time. It is theorised that using a longer exposure time leads to better attachment between hatch lines, which is believed to be because of larger melt pool dimensions. A distance between irradiation locations may be defined by a point distance. The point distance may be the same for all irradiation locations along the scan path.

[0027] The exposure parameters may be determined using a continuous function associating values of the thickness of the consolidated material to values for the non-bulk exposure parameter (although the exposure parameters may be quantised such that a final value for each exposure parameter is given by the nearest allowable discrete value (of a plurality of allowable discrete vales) to the value determined using the continuous function, e.g. changes in exposure time may be limited to 2ps steps or changes in energy beam power may be limited to 5W steps).

[0028] According to a second aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the first aspect of the invention.

[0029] According to a third aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising carrying out a build of an object based on instructions determined in accordance with the first aspect of the invention.

[0030] According to a fourth aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising, for at least one of the powder layers, scanning an energy beam along a scan path to melt powder of the powder layer. The scan path may include bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted in previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a bulk threshold, and non-bulk irradiation locations, wherein, for each non-bulk irradiation location, powder is melted to form a thickness of consolidated material from the non-bulk irradiation location to a surface of the object less than the bulk threshold. A non-bulk value for an exposure parameter for each non-bulk irradiation location may be dependent on, at least in part, on the thickness of consolidated material from the non-bulk irradiation location. A bulk value for the exposure parameter of the energy beam when scanning to the bulk irradiation locations may be constant.

[0031] According to a fifth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a controller of a powder bed fusion apparatus, causes the controller to control the powder bed fusion apparatus to carry out the method of the third or fourth aspects of the invention.

[0032] According to a sixth aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam across selected areas of each powder layer to form the object, wherein the energy beam is controlled to achieve a higher energy density for downskin irradiation locations than for bulk irradiation locations.

[0033] Downskin irradiation locations are locations irradiated by the energy beam to melt powder to form a thickness of consolidated material from the downskin irradiation location to a surface of the object less than a downskin threshold.

[0034] The term “energy density” used herein means an energy per unit area. A value for energy density may be calculated from exposure parameters. The energy density may be calculated from:- energy beam power Energy Density = - - - - - — — scan speed X track width

[0035] The track width may be equivalent to a width of the energy beam spot, such as a 1 / e spot size, on the powder bed perpendicular to a direction that the energy beam is traversed along a scan path. The scan speed is an average speed the energy beam spot advances along a scan path. It is known to superimpose high frequency oscillations of an energy beam spot on to an underlying movement of a spot along a scan path. In such a scenario, the scan speed is the speed of the underlying movement (averaged across a plurality of oscillations) rather than the speed at any point during an oscillation. The scan speed may be determined from an exposure time and a point distance.

[0036] The method may comprise scanning the energy beam across the downskin irradiation locations, wherein the energy density is reduced as a thickness of consolidated material underlying the downskin irradiation locations reduces. The energy density may be reduced by altering the scan speed. The scan speed may be altered by changing one or both of exposure time and point distance in a pulsed scan strategy.

[0037] According to a seventh aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a controller of a powder bed fusion apparatus, causes the controller to control the powder bed fusion apparatus to carry out the method of the sixth aspect of the invention.

[0038] According to an eighth aspect of the invention there is provided 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 each of a plurality of successively formed powder layers with an energy beam, the method comprises determining bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted in previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a downskin threshold, and downskin irradiation locations, wherein, for each downskin irradiation location, powder is melted to form a thickness of consolidated material from the downskin irradiation location to a surface of the object less than the downskin threshold. The method may comprise allocating scan parameters to the bulk irradiation locations and to the downskin irradiation locations such that a higher energy density is achieved for downskin irradiation locations than for bulk irradiation locations.

[0039] According to a ninth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the eighth aspect of the invention.

[0040] According to a tenth aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising carrying out a build of an object based on instructions determined in accordance with the ninth aspect of the invention.

[0041] According to an eleventh aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam along a scan path comprising core irradiation locations and shell irradiation locations, wherein powder of two or more layers is melted by irradiation of the core irradiation locations with the energy beam whereas powder of a single layer alone is melted by irradiation of the shell irradiation locations with the energy beam.

[0042] In this way, defects formed at a transition between a core region and a shell region can be avoided as a continuum of consolidated material is formed along the scan path as it traverses the core and shell regions, whilst multiple layers of powder are melted in the core region(s) reducing irradiation time for the build compared to melting powder layers one at a time in the core region. In particular, separate scan paths for the core and shell can be avoided and the consequential interface between such scan paths. Melting of single powder layer thicknesses in the shell region is preferred as such thinner powder thicknesses facilitate the building of low angled overhanging regions. In particular, it may be desirable to reduce the energy density when irradiating at least some of the shell irradiation locations in order to avoid adverse effects in the shell regions, such as those caused by overheating. In general, thinner powder thicknesses may be melted using an energy beam having a lower energy density, although for downskin irradiation locations, a higher energy density may be used. As the energy beam advances along the scan path, exposure parameters of the energy beam may be altered as the energy beam goes from irradiating core irradiation locations to irradiating shell irradiation location or vice versa.

[0043] For shell irradiation locations, powder is melted by the energy beam to form, together with powder melted by irradiation of previous powder layers, a thickness of consolidated material from the shell irradiation location to a surface of the object below a shell threshold. For core irradiation locations, powder is melted to form a thickness of consolidated material from the core irradiation location to a surface of the object above the shell threshold.

[0044] Core irradiation locations may be located on non-consecutive powder layers. For each (first) shell irradiation location for the non-consecutive powder layers, powder may be melted to form a thickness of consolidated material from the shell irradiation location to a surface of the object below the shell threshold. For (second) shell irradiation locations for powder layers in-between the non-consecutive powder layers, powder may be melted to form a thickness of consolidated material from the shell irradiation location to a surface of the object below a modified shell threshold but greater than the shell threshold. For other (third) shell irradiation locations for powder layers in-between the non-consecutive powder layers, powder may be melted to form a thickness of consolidated material from the shell irradiation location to a surface of the object below the shell threshold. The modified shell threshold may correspond to a greater number of powder layers than the shell threshold. The modified shell threshold may correspond to a number of powder layers that is greater than the shell threshold by at least the number of powder layers in-between the non-consecutive powder layers.

[0045] The method comprises irradiating ones of the scan paths on a first powder layer to consolidate powder in a shell region, wherein portions of a core region to be consolidated in the first powder layer remain unconsolidated, and then irradiating ones of the scan paths on a second-subsequent powder layer to consolidate powder in a shell region and a core region, wherein irradiation of the core region melts powder of the second-subsequent powder layer and of the core region of the first powder layer that remained unconsolidated after irradiation of the first powder layer.

[0046] According to a twelfth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a controller of a powder bed fusion apparatus, causes the controller to control the powder bed fusion apparatus to carry out the method of the eleventh aspect of the invention.

[0047] According to a thirteenth aspect of the invention there is provided 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 each of a plurality of successively formed powder layers with an energy beam, the method comprising determining a first area to be consolidated in a first powder layer and a second area to be consolidated in a second, subsequent powder layer, dividing the first area into a first core region and a first shell region and dividing the second area into a second core region and a second shell region, determining scan paths for the energy beam to take in irradiating the first and second powder layers, wherein the scan paths are arranged such that the first powder layer is irradiated to melt the first shell region whilst at least a portion of the first core region remains unmelted and the second powder layer is irradiated to melt the second shell region and to melt powder of the second powder layer corresponding to the second core region en masse with powder of the first powder layer corresponding to the unmelted portion of first core region. The scan paths may include a second powder layer scan path for irradiating the second powder layer, the second powder layer scan path extending into the second core region and the second shell region. The second powder layer scan path may extend into the second core region such that irradiation of the second powder layer scan path melts powder of the second powder layer corresponding to a portion of the second core region en masse with powder of the first powder layer corresponding to an unmelted portion of first core region.

[0048] Dividing the first area into the first core region and the first shell region may be dependent on a thickness of consolidated material below the first area during the build. Dividing the second area into the second core region and the second shell region may be dependent on a thickness of consolidated material below the second area during the build. Irradiation locations of the second area may be identified as being in the second shell region if a thickness of consolidated material below the irradiation locations is below a shell threshold. Irradiation locations of the first area may be identified as being in the first shell region if a thickness of consolidated material below the irradiation locations is below the shell threshold or a modified shell threshold. The modified shell threshold may correspond to a greater number of powder layers than the shell threshold. The modified shell threshold may correspond to a number of powder layers that is greater than the shell threshold by at least the number of first powder layers between the second powder layer and an immediately preceding second powder layer below the second powder layer. In this way, an extent of a shell region in the first area and the second area is dependent on a thickness of consolidated material below the irradiation location. Thus, for powder layers consolidated to form lower angled (to the build plane) overhanging portions of the object of the object, the shell region will be larger avoiding or mitigating overheating in these portions whilst, for powder layers consolidated to form higher angled overhanging portions of the object, the core region will be larger reducing build time.

[0049] According to a fourteenth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the twelfth aspect of the invention.

[0050] According to a fifteenth aspect of the invention there is provided 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 each of a plurality of successively formed powder layers with an energy beam, the method comprising determining, from geometric data describing the object, a geometric characteristic of a surface of the object at different locations in a build direction, selecting different powder layer thicknesses for powder layers based on the geometric characteristic, determining at least one threshold value for each powder layer based on the selected different powder layer thicknesses, selecting values for an exposure parameters for different irradiation locations within the layer dependent on, at least in part, whether a thickness of consolidated material below the irradiation location is above or below the threshold value.

[0051] In the applicant’s European patent application No. 23306411.2 and International patent application No. PCT / GB2024 / 052212, which are incorporated herein in their entirety by reference, it is taught to select different powder layer thicknesses for powder layers based on the geometric characteristic of the object to be built. In this aspect of the invention, different values for a bulk exposure parameter are used for powder layers of different thicknesses, for example, values that provide a lower energy density may be used for thinner powder layers and / or slower scanning speeds or longer exposure times may be used for thicker powder layers to avoid adverse effects, such as vaporisation of material, for thicker powder layers where higher energy densities may be used. As a result, a boundary between different regions, as defined by the threshold value, such as bulk and non-bulk and / or nonbulk and downskin, may be different for different powder layers. In particular, bulk values of the exposure parameters may be used for irradiation locations above thinner regions of consolidated material if the bulk values provide a lower energy density, as may be the case for thinner powder layers. In other words, the energy density used for bulk regions in thinner layers may be lower allowing the transition to bulk parameters to be located above thinner thicknesses of consolidated material compared to thicker powder layers that require a higher energy density for melting irradiation locations within the bulk.

[0052] The threshold value may be a bulk threshold value or a downskin threshold value.

[0053] According to a sixteenth aspect of the invention there is provided 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 each of a plurality of successively formed powder layers with an energy beam, the method comprising determining a bulk value for an exposure parameter for bulk irradiation locations of a powder layer, determining a threshold value for the powder layer based on the bulk value, and determining, for an area to be consolidated in the powder layer, bulk irradiation locations and / or regions and non-bulk irradiation locations and / or regions dependent on whether a thickness of consolidated material below the irradiation location and / or region is above or below the threshold value, wherein a non-bulk value for the exposure parameter for use when irradiating non- bulk irradiation locations and / or regions differs from the bulk value.

[0054] In this way, a boundary between the bulk irradiation locations / regions and non- bulk irradiation locations / regions is adapted to the bulk value used for the exposure parameter. This may be advantageous as a location at which the exposure parameter should be varied from the bulk value may depend on the magnitude of the bulk value. For example, a bulk value that provides higher energy density may only achieve satisfactory results for greater thicknesses of consolidated material (that formed by melting powder by the irradiation of the bulk irradiation location together with powder melted in previous powder layers) compared to a bulk value that provides lower energy density. In one embodiment, the bulk value is selected based on a powder layer thickness. The non-bulk values for the exposure parameters may be selected in the manner described with respect to the first aspect of the invention.

[0055] According to a seventeenth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the fifteenth and / or sixteenth aspect of the invention.

[0056] The method of generating instructions according to any of the aspects of the invention may comprise generating an output of the instructions. For example, the instructions may be in the form of a data file for execution by a powder bed fusion apparatus. The method of any of the aspects of the invention for determining instructions to be executed by a powder bed fusion apparatus may be a computer- implemented method. Such methods may result in output of a build file including the instructions. For example, the method may be carried out by build preparation software. The build file may be executable by a powder bed fusion apparatus to carry out a powder bed fusion method in accordance with the instructions.

[0057] According to an eighteenth aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam over the powder layers in accordance with instructions generated in accordance with the fifteenth and / or sixteenth aspects of the invention.

[0058] According to a nineteenth aspect of the invention there is provided 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 each of a plurality of successively formed powder layers with an energy beam, the method comprising determining a plurality of irradiation locations for one or more energy beams to irradiate to consolidate material of a powder layer to form an unified area of consolidated material, and determining a starting irradiation location from the plurality of irradiation location for a one of the energy beams based on a thickness of consolidated material below the starting irradiation location.

[0059] Starting irradiation at a location where there is no or little surrounding consolidated material, whether that be below the irradiation location or to the sides of the irradiation location, can have an impact on properties of the object, because heat is conducted away relatively slowly at these irradiation locations (powder being a thermal insulator). Accordingly, it is desirable to commence irradiation at an irradiation location having a sufficient thickness of consolidated material therebelow such that heat is conducted away relatively quickly by the consolidated material.

[0060] The starting irradiation location may be an irradiation location wherein the thickness of consolidated material below the starting irradiation location is greater than a predetermined thickness. The starting irradiation location may be an irradiation location wherein the thickness of consolidated material below the starting irradiation location is a maximum for the unified area. The method may comprise setting the starting irradiation location to be an irradiation location having a thickness of consolidated material below the irradiation location greater than a predetermined thickness if present and, if not present, setting the starting irradiation location to be the irradiation location having the maximum thickness of consolidated material below the irradiation location (even though it may be below the predetermined threshold). Above the predetermined threshold, other factors such as progressing in a direction opposed to a gas flow direction may take precedence over a thickness of consolidated material below the starting irradiation location.

[0061] The plurality of irradiation locations may comprise a further irradiation location, wherein a thickness of consolidated material below the further irradiation location is less than the predetermined thickness. A scanning sequence may be determined such that scanning of the energy beam progresses from the starting irradiation location to the further irradiation location along at least one scan path. The powder that has already been consolidated along the at least one scan path by the time the further irradiation location is irradiated provides a thermal conduction path for conducting heat away from the further irradiation location.

[0062] The at least one scan paths may comprise a set of, optionally parallel, scan paths scanned by the same energy beam. The energy beam may scan the set of scan paths in a scanning sequence such that material melted by scanning a subsequent scan path, scanned after a first scan path of the set, on solidifying joins to material consolidated by scanning an earlier scanned scan path of the set. The first scan path includes the starting irradiation location. The first scan path may include the further irradiation location. Alternatively, the subsequent scan path may include the further irradiation location. A scanning direction may be determined such that scanning of the first scan path by the energy beam progresses from the starting irradiation location to the further irradiation location. The thickness of consolidated material below all irradiation locations of the subsequent scan path may be less than the predetermined thickness.

[0063] Each scan path of the set of parallel scan paths may comprise a hatch line, e.g. a straight path. A sequence in which the hatch lines are scanned defines a hatch order direction. The hatch order direction may be perpendicular to the hatch lines. The method may comprise selecting a hatch order direction for ones of the scan paths upstream of the first scan path in a gas flow direction to be opposed to the gas flow direction, for example as described in International patent application WO2014 / 125280 A2, which is incorporated herein in its entirety by reference, whereas a hatch order direction for ones of the scan paths downstream of the first scan path is with the gas flow direction. This divergence from the preferred hatch order direction opposed to gas flow may be desirable as adverse effects of first scanning a hatch line that is not above consolidated material thicker than the predetermined threshold may be greater than adverse effects of scanning in a direction with the gas flow.

[0064] A scanning direction the energy beam progresses along a subsequent scan path may be based on the scanning direction along the first scan path. The scanning of the set of scan paths may be bidirectional or unidirectional. Whether the scanning is bidirectional or unidirectional may be a preset scanning setting, for example, in a user interface, and the scanning direction along subsequent scan path(s) may be based on the preset scanning setting once the scanning direction along the first scan path has been determined.

[0065] The predetermined threshold may be the bulk threshold or the shell threshold as described above in respect of other aspects of the invention.

[0066] The starting irradiation location may be between end irradiation locations at each end of the first scan path. Scanning of the first scan path may be split such that a first section of the first scan path is scanned in a first direction and a second section of the scan path is scanned in an opposite, second direction. The splitting point may be located at the starting irradiation location. The starting irradiation location may be selected to be between the end irradiation locations when a thickness of consolidated material below the end irradiation locations is less than the predetermined threshold but a thickness of consolidated material below an irradiation location of the first scan path between the end irradiation locations is greater than the predetermined thickness. The thickness is in a build direction from the irradiation location to a surface of the object.

[0067] According to a twentieth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the nineteenth aspect of the invention.

[0068] According to an twenty -first aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam over the powder layers in accordance with instructions generated in accordance with the nineteenth aspect of the invention.

[0069] According to a twenty-second aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each powder layers with an energy beam, the method comprising, for each of a plurality of successively formed powder layers, scanning an energy beam over the powder layer along one or more scan paths to consolidate material of the powder layer to form an unified area of consolidated material, wherein a starting irradiation location on the one or more scan paths for the energy beam is an irradiation location having a thickness of consolidated material greater than a predetermined thickness and the one or more scan paths comprise a further irradiation location thickness for which consolidated material below the further irradiation location is less than the predetermined thickness.

[0070] The one or more scan paths may comprise a plurality of parallel scan paths (hatch lines), wherein a direction of the scan paths is rotated between the successively formed layers. According to a twenty -third aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the twenty-second aspect of the invention.

[0071] According to a twenty-fourth aspect of the invention, there is provided 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 each of a plurality of powder layers with one or more energy beams, the method comprising determining a plurality of scan paths for the one or more energy beams to irradiate to consolidate material of a powder layer and identifying a first section of at least one scan path as not divisible and a second section of the at least one scan path as divisible.

[0072] The method may further comprise sequencing irradiation of the at least one scan path by identifying a splitting point on the scan path to interrupt or cease irradiation of the scan path with a one of the energy beams, wherein the splitting point is selected to be in the second section of the at least one scan path.

[0073] The method may comprise determining a thickness of consolidated material below the scan path and the second section is identified dependent on the determined thickness, for example the second section corresponds to a section of the at least one scan path where the thickness of consolidated material is greater than the predetermined thickness (for example, as described above in the nineteenth aspect of the invention). The predetermined thickness may be the bulk threshold, such as described above in the first aspect of the invention The method may comprise determining exposure parameters for irradiation locations on the at least one scan path, and the second section is identified dependent on the determined exposure parameters, for example, the second section corresponds to a section of the at least one scan path where the exposure parameters are the same. The method may comprise determining a sequence for irradiating the plurality of scan paths including, if the at least one scan path crosses a boundary between two regions of the powder bed, locating the splitting point at the point the at least one scan path crosses the boundary if the section of the scan path that crosses the boundary is the second section. If the section of the scan path that crosses the boundary is the first section, the splitting point may be located in the second section away from the boundary or the at least one scan path may not be divided.

[0074] A powder bed fusion apparatus may comprise a plurality of scanners, each for independently steering a different energy beam across the powder layer(s). The powder bed may be notionally divided into scanner regions, each scanner region allocated to a different one of the scanners. In one example, the powder bed fusion apparatus may comprise a gas flow system for generating a gas flow across the powder layer from a first side to a second side and the scanner regions comprises lanes across the powder layer (often referred to as “swim lanes”) in a direction of the gas flow. Examples of such lanes are disclosed in W02025 / 040919, which is incorporated herein in its entirety by reference. By limiting each scanner to scanning the scan paths that fall within the allocated region / lane, the effect of gas borne debris on simultaneously irradiated locations is reduced. Such a division of the powder layer may comprise splitting of scan paths at the boundary between two regions / lanes. However, for sections of a scan path, such as the first sections, an effect of interrupting the irradiation of the scan path can have a greater detrimental effect than scanning within gas-borne debris generated by a simultaneously irradiated upwind point. In particular, within a section of the scan path irradiated using a lower energy density, such as a non-bulk section of the scan path, a required melt pool may not be formed on recommencing irradiation of the section of the scan path. However, this problem may be absent or at least less severe in sections of the scan path irradiated at higher energy densities, such as bulk sections of the scan path Accordingly, selecting to split the scan path at a splitting point away from the boundary or to not to divide the scan path at all despite the fact this may result in one scanner encroaching on a region allocated to another scanner may result in improved properties of the object.

[0075] A powder bed fusion apparatus may be arranged to irradiate the powder layer as the and / or a subsequent powder layer is being formed by a recoater. Examples of such methods are described in WO2015 / 140547 and W02025 / 040919, which are incorporated herein in their entirety by reference. In such methods, irradiation of scan paths may be scheduled by notionally dividing the powder layer into regions and sequencing irradiation of the scan paths dependent on a position of the recoater and the consequential availability of the region containing the scan paths for irradiating. For scan paths that extend into multiple regions, the scan paths may be divided to facilitate the sequencing of irradiation of the scan paths. WO2015 / 140547 describes with reference to Figure 10, the dividing of scan paths of a meander / raster scan. In the invention, the splitting point along a scan path that crosses the notional boundary between such regions may be located in a section of the scan path identified as a second section.

[0076] According to a twenty-fifth aspect of the invention there is provided a non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of the twenty-fourth aspect of the invention.

[0077] According to an twenty-sixth aspect of the invention there is provided a powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam over the powder layers in accordance with instructions generated in accordance with the twenty-fourth aspect of the invention.

[0078] The transient data carrier may be a suitable medium for providing a machine with instructions, for example a floppy disk, a CD ROM, a DVD ROM / RAM (including - R / -RW and +R / + RW), an HD DVD, a Blu Ray(TM) disc, a memory (such as a Memory Stick(TM), an SD card, a compact flash card, or the like), a disc drive (such as a hard disc drive), a tape, any magneto / optical storage.

[0079] Description of the Drawings

[0080] FIGURE 1 is a schematic of a powder bed fusion apparatus according to an embodiment of the invention;

[0081] FIGURE 2 is a plan view of the powder bed fusion apparatus showing the plurality of scanners;

[0082] FIGURE 3a illustrates an area of a powder layer to be consolidated, scan paths for an energy beam and the division of the scan paths into bulk and non-bulk regions;

[0083] Figure 3b illustrates a point scanning technique for advancing the energy beam along the scan paths;

[0084] Figure 4 illustrates a division of irradiation locations along a scan path into bulk, inner non-bulk and outer non-bulk (skin) irradiation locations;

[0085] Figure 5 illustrates the energy density achieved by the selected values for the exposure parameters for each of the bulk, inner non-bulk and outer non- bulk (skin) irradiation locations;

[0086] Figure 6 illustrates a shell and core scanning strategy, in which powder in core regions is consolidated in multiple layer thicknesses, whereas powder in shell regions is consolidated in single layer thicknesses;

[0087] Figure 7 is a cross-section of an object having overhanging regions at different angles to the powder plane;

[0088] Figure 8 illustrates different powder layer thicknesses selected for the manufacture of different portions of the object, the powder layer thicknesses selected based on an angle of overhang to the powder plane;

[0089] Figure 9 illustrates variations in threshold values that set the transition from bulk to non-bulk regions and inner bulk to downskin regions as the powder layer thickness varies;

[0090] Figures 10a and 10b illustrate a starting irradiation location and hatch order directions for a plurality of scan paths including hatch lines (Figure 10a) and a border scan path (Figure 10b) for a slice having irradiation locations for which the thickness of consolidated material below the irradiation locations is greater than a predetermined threshold Ncthreshoid and irradiation locations for which the thickness of consolidated material below the irradiation locations is less than a predetermined threshold Ncthreshoid;

[0091] Figures Ila and 11b illustrate a starting irradiation location and hatch order directions for a plurality of scan paths including hatch lines (Figure I la) and a border scan path (Figure 1 lb) for a slice having irradiation locations for which the thickness of consolidated material below the irradiation locations varies but is less than a predetermined threshold Ncthreshoid;

[0092] Figure 12 illustrates a starting irradiation location and hatch order directions for a plurality of scan paths, wherein ones of the scan paths include both irradiation locations for which the thickness of consolidated material below the irradiation locations is greater than a predetermined threshold Ncthreshoid and irradiation locations for which the thickness of consolidated material below the irradiation locations is less than a predetermined threshold Ncthreshoid whilst other scan paths only include irradiation locations for which the thickness of consolidated material below the irradiation locations is less than a predetermined threshold Ncthreshoid;

[0093] Figure 13 illustrates a starting irradiation location, hatch order directions and the splitting of a scan path for a plurality of scan paths, wherein the thickness of consolidated material below end irradiation locations of each scan path is less than a predetermined threshold Ncthreshoid whilst, for some of the scan paths, the thickness of consolidated material below irradiation locations between the end irradiation locations is greater than a predetermined threshold Ncthreshoid; and

[0094] Figure 14 illustrates the splitting of the scan paths shown in Figure 13 based on notional scanner regions allocated to different scanners of the powder bed fusion apparatus.

[0095] Description of Embodiments

[0096] Referring to Figures 1 and 2, a powder bed fusion apparatus according to an embodiment of the invention comprises a build chamber 101 having therein a processing plate 115 defining an aperture and a build sleeve 116 extending down from the aperture. A build platform 102 is lowerable in the build sleeve 116 such that the build sleeve 116 and the build platform 102 together define a build volume 117. The build platform 102 supports a build substrate plate 102a, a powder bed 104 and workpiece (object) 103 as the workpiece is built by selective laser melting of the powder. The platform 102 is lowered within the build volume 117 under the control of a drive mechanism (not shown) as successive layers of the workpiece / object 103 are formed.

[0097] Layers of powder 104 are formed as the workpiece / object 103 is built by dispensing apparatus 108 and a recoater 109. The recoater 109 comprises a wiper blade, brush or roller for spreading powder across the powder bed 104 in a powder spreading direction. The dispensing apparatus 108 dispenses powder 104 in front of the wiper blade, brush or roller when the recoater 109 is located below the powder dispenser 108. Movement of the recoater 109 spreads the dispensed powder into powder layers of the powder bed. A position of a lower edge of the blade, brush or roller of the recoater 109 defines a working plane 110 at which powder is melted. A thickness of a powder layer is defined by a distance between a surface of the powder bed and the lower edge of the blade, brush or roller when the recoater 109 is spreading powder. In this embodiment, the lower edge of the blade, brush or roller is maintained at a fixed vertical location and thus, a powder layer thickness is defined by an amount the build platform 102 is displaced between strokes of the recoater 109 to spread powder.

[0098] A plurality of laser modules generate laser beams 118a, 118b, 118c, 118d for melting the powder 104. Each laser module comprises a fibre optic (not shown) and beam delivery optic (BDO) 105a, 105b, 105c and 105d for delivering the laser beam 118a, 118b, 118c, 118d to the corresponding optical scanner 106a, 106b, 106c, 106d. The optical scanner 106a, 106b, 106c, 106d steers the laser beams 118a, 118b, 118c, 188d on to selected areas of the powder bed 104 in order to build the object. The laser beams 118a, 118b, 118c, 118d enter through a common laser window 107. However, it will be understood that separate windows may be provided for each laser beam 118a, 118b, 118c, 118d.

[0099] Each optical scanner 106a, 106b, 106c, 106d comprises movable steering optics 121, such as two mirrors mounted on galvanometers, for steering the laser beam 118 in perpendicular directions, X and Y, across working plane 110 and focussing optics 120, such as two movable lenses for changing the focus of the laser beam 118. The optical scanner is controlled such that the focal position of the laser beam 118 remains in the same plane 110 as the laser beam 118 is moved across the working plane 110. In this embodiment, each scanner can steer the corresponding laser beam 118 across a working field which covers the whole of the powder bed 104. However, in other embodiments, each scanner 106 has a working field that only partially covers the powder bed 104.

[0100] The powder bed fusion apparatus further comprises a gas flow system comprising a gas nozzle I l la and gas exhaust 111b for generating a gas flow across the powder bed in a gas flow direction G. Such gas flow systems are well known in the art.

[0101] The powder bed fusion apparatus comprises a controller 160 for controlling modules of the apparatus, including the lasers, the scanners 106a, 106b, 106c, 106d, movement of the build platform 102 and movement of the recoater 109. The controller comprises a processor 161 and memory 162 that has stored therein a computer programme, which, when executed by the processor causes the powder bed fusion apparatus to be controlled in the manner described below. The computer programme may comprise instructions generated by build preparation software. The build preparation software may be located on the apparatus or remote from the apparatus, such as on a general -purpose computer. The build preparation software generates instructions, such as scan paths, exposure parameters and / or scanning sequences, based on geometry of the object 103 to be built and these instructions are sent to the controller for execution.

[0102] The build preparation software may receive geometric data defining geometry of the object, for example, in the form of a CAD or STL format, as is known in the art. The build preparation software divides the object into a plurality of slices corresponding to powder layers to be formed during the powder bed fusion process. Each slice represents an area to be consolidated by melting powder of the powder layer. A layer thickness is conventionally the same for all powder layers and set by a user. However, for some of the embodiments described below, the powder layer thickness is a variable to be selected based on a geometry of the object 103 to be built.

[0103] Scan paths (hatches) are determined for the laser beam to take in order to melt powder of the areas. These scan paths are determined based on a predetermined hatch distance, HD, between adjacent scan paths. In this embodiment, the scan paths are determined for the area as a whole before the area to be consolidated is divided into regions based on underlying geometry of the object. The scan paths may be determined based on a selected scanning strategy such as meander, chequerboard or stripe scan strategies.

[0104] The exposure parameters may be laser power, scanning speed (or point distance and exposure time), spot size, and / or other parameters that affect a melt pool shape. The values for the exposure parameters may be determined in the manner described with reference to Figures 3a to 9.

[0105] After determining the slices (areas to be scanned in each powder layer), scan paths and values for the exposure parameters, the build preparation software generates instructions for a powder bed fusion additive apparatus output in a build file. The build file can be loaded onto the powder bed fusion apparatus for execution. Execution of the build file by the controller 160 of the powder bed fusion apparatus causes the powder bed fusion apparatus to carry out a build in accordance with the instructions.

[0106] Figures 3a and 4 illustrate an area 205 corresponding to a slice of an object 200 to be manufactured in the powder bed fusion apparatus. The area 205 is formed by consolidating powder of one powder layer by scanning an energy beam, in this embodiment a one (but not necessarily the same one) of the laser beam 118a, 118b, 118c, 118d, along each of a plurality of scan paths S. Each scan path S extends from a first region 201 (hereinafter referred to as a “bulk region”), for which the same set of exposure parameters (e.g. laser power, point distance and exposure time) are used for each irradiation location 203a along the scan path S, to a second region 202 (hereinafter referred to as a “non-bulk region”), for which the exposure parameters are varied between irradiation locations 203b, 203c (so called “dynamic scan parameters”). The laser beam 118a, 118b, 118c, 118d is advanced along a scan path S through a series of point exposures. Figure 3b illustrates an idealistic representation of such a scan. The scan is defined by exposure parameters: a point distance PD between each irradiation location, an exposure time for each irradiation location and a laser power. The centres of adjacent scan paths are separated by a hatch distance, HD. Adjacent scan paths are typically scanned in alternating directions, but some scan strategies may comprise scanning each scan path in the same direction. In practice, the steering optics of the optical scanners 106a, 106b, 106c, 106d have some inertia and exact spot-like irradiation locations may not be formed but instead, the spot may be elongated in the scanning direction.

[0107] Referring to Figure 4, a bulk value for one or more exposure parameters (laser power, exposure time and point distance) is set for bulk irradiation locations 203a. The bulk value may be set by a user or preprogrammed into the build preparation software. The bulk irradiation locations are irradiation locations in a bulk region 201. The bulk region 201 is defined by regions of the area 205 wherein a thickness Nx, in this embodiment in units of a number of layers, of the consolidated material directly below an irradiation point 203a (including material consolidated by the irradiation of the irradiation location 203a) is above a bulk threshold, Ncmax. The bulk value does not vary between bulk irradiation locations 203a even with variations in a thickness of consolidated material below the bulk irradiation locations (above the bulk threshold Ncmax). The build preparation software determines, for each scan path S, a portion or portions of the scan path S that correspond to the bulk region 201 and assigns the bulk value to this portion or these portions of the scan path. S.

[0108] For the non-bulk irradiation locations 203b, 203c, the build preparation software assigns non-bulk values for the exposure parameters in dependence on a geometric characteristic of the object 200 as the laser beam moves along the scan path S. In this embodiment, the geometric characteristic is a thickness Nx of consolidated material from the non-bulk irradiation location 203b, 203c to a point on a surface of the object 200 directly below the point of irradiation. The non-bulk irradiation locations are irradiation locations in a non-bulk region 202a, 202b. The non-bulk region 202 is defined by regions of the area 205 wherein a thickness Nx of the consolidated material directly below an irradiation point 203b, 203c (including material consolidated by the irradiation of the irradiation location 203b, 203c) is below the bulk threshold, Ncmax. In this embodiment, the non-bulk region 202 may be further divided into two regions, an inner non-bulk region 202a and an outer non- bulk (downskin) region 202b. The downskin region is defined by regions of the area 205 wherein a thickness Nx of the consolidated material directly below the irradiation location 203c (including material consolidated by the irradiation of the irradiation location 203c) is below a downskin threshold, Nctr. The downskin threshold may be selected to be a thickness below which irradiation of the irradiation locations 203c forms or affects the formation of a surface (downskin) 204 of the object 200 below the downskin irradiation location. It will be understood that the bulk or inner non-bulk regions can extend to an edge of an area if the thickness of consolidated material below a region is not below the bulk-threshold Ncmax or the downskin threshold Nctr.

[0109] The build preparation software determines non-bulk values Edynfor an exposure parameter for the non-bulk irradiation locations 203b, 203c using the equation:- wherein Nx is the number of layers of consolidated material below the irradiation location, NCnreshoid is a preset threshold number of layers below which the exposure parameter is varied from the maximum value Emax for the exposure parameter and order is a positive real number, and typically an integer. NCnreshoid is NCmax for the inner non-bulk and NCtr for the outer non-bulk (skin). In this embodiment, order has a value from 1 to 5, and preferably is 2. For the inner bulk region 202a, Emax is equal to the bulk value for the exposure parameter. For the downskin region 202b, Emax is a value that achieves a higher energy density than that set for the bulk irradiation locations, such as a high laser power, longer exposure time and / or shorter point distance (slower scan velocity). In a preferred embodiment, the values for exposure time and point distance are the same for the bulk and non-bulk regions 201, 202a, 202b and only the laser power is changed in the non-bulk region 202a, 202b. However, it will be understood that in other embodiments, the values for the exposure parameter in the downskin region 202b remains constant, i.e. all values for the exposure parameter are equal to Emax (which is higher than the bulk value) for the downskin region 202b.

[0110] Edyn will vary as the thickness of consolidated material below the inner non-bulk irradiation locations reduces tending towards Emin. A sudden jump occurs in the value Edyn for the exposure parameter at the boundary between the inner non-bulk region 202a and the downskin region 202b as a different, higher Emax is used for the downskin regions 202b. In this embodiment, the value Edyn then reduces with reductions in the thickness of consolidated material below the downskin irradiation locations.

[0111] Nx may be determined by:

[0112] Nx= 7 (2) where Txis the thickness of the solidified material below an irradiation location at position x and t is the layer thickness. If Tx is calculated from a geometric model of the object before the layers are determined, then Nx may have a non-integer value. NCmax is typically greater than 20 layers and less than 100 layers. NCtr is typically between 5 and 30 layers. The values for NCmax and NCtr will normally change for different powder layer thicknesses and for different bulk values for the exposure parameter. Smaller layer thicknesses would require an increase in NCmax and NCtr in order to maintain the threshold between bulk and non-bulk and between inner non-bulk and outer non-bulk at the same layer thickness as that for thicker powder layers, but thinner powder layers may allow use of a bulk value for the exposure parameter that achieves a lower energy density, such as lower laser power, compared to thicker powder layers, allowing an extension of the bulk value and inner non-bulk values to thinner regions of the part. Accordingly, a halving of the powder layer thickness may not result in a doubling of the values for NCmax and NCtr.

[0113] The allocation of values for the exposure parameter, in this embodiment values for laser power, to different regions 201, 202a, 202b, results in an energy density ED for the different regions 201, 202a, 202b like that shown in Figure 5. In the bulk region 201 the energy density EDB is constant along the portions of the scan path S that fall within the bulk region 201. In the inner-bulk region 202a, the energy density EDB, NX decreases as the thickness Nx of the consolidated material below the irradiation locations reduces. In the downskin region 202b, a sudden jump in the energy density EDDS occurs as the value Edyn for the exposure parameter at the boundary between the inner non-bulk region 202a and the downskin region 202b increases to a higher value than that used for bulk irradiation locations. This energy density EDDS.NX then decreases from the higher value EDDS as the thickness Nx above the downskin irradiation locations reduces from the threshold value NCtr. It has been found that using a higher energy density for the downskin irradiation locations results in higher mechanical strength and improved surface quality for the resulting object. In another embodiment, the exposure parameters for the downskin region 202b are fixed so as to produce a fixed energy density EDDS across the downskin region 202b (as shown by the dashed line).

[0114] It may also be desirable to advance the laser beam along the scan path in a direction from the bulk region 201 to the non-bulk region 202a, 220b, as shown in Figures 4. For a scan strategy which divides the scan paths into smaller, equal lengths, such as stripe scanning or chequerboard scanning, only the scan paths that span bulk and non-bulk regions or solely fall within non-bulk regions 202a, 202b may be scanned in the preferred direction shown in Figure 4, whereas scan paths that solely fall within the bulk region 201 may be scanned in alternating directions.

[0115] A further embodiment is shown in Figure 6. In this embodiment, “core” portions of the object shaded in grid lines, such as bulk regions 301 and non-bulk regions 302a, comprising multiple powder layers Li and L2; L3 and L4; or L5 and Le are consolidated by scanning every nth powder layer where n is greater than 1, such as every other powder layer for example every even powder layer L2, L4, Le, whereas “shell” portions shaded in dots, such as non-bulk regions 303a, and downskin regions 303b are scanned every powder layer Li, L2, L3, L4, L5, Le. For powder layers in which both core portions and shell portions are scanned the scan paths S extend across both the core and shell portions and the exposure parameter is changed from the constant bulk value for bulk region 301 to a variable non-bulk value as the scan transitions from the bulk region 301 to the non-bulk regions 302a and 303a and the downskin region 303b. Advantageously, there is no interruption in the scanning along the scan path as the laser beam transitions from core portions to shell portions and bulk to non-bulk regions or vice versa. For other powder layers, such as odd powder layers, only non-bulk regions 303a, 303b of the shell portion are scanned.

[0116] For powder layers where the core region is consolidated by exposure of that powder layer, in this embodiment even layers L2, L4, Le, the shell portion is defined by regions wherein a thickness Nx of the consolidated material directly below an irradiation point (including material consolidated by the irradiation of the irradiation point) is below a shell threshold, NCsh. For powder layers where the core region is not consolidated by exposure of that powder layer, in this embodiment odd layers Li, L3, L5, the shell portion is defined by regions wherein a thickness Nx of the consolidated material directly below an irradiation point (including material consolidated by the irradiation of the irradiation point) is below a modified shell threshold equal to the shell threshold, Ncsh, plus an overlap value, A. The overlap value, A, is set such that the modified shell threshold is greater than the shell threshold by at least the thickness of one layer. Accordingly, where the shell threshold is defined as a number of powder layers, the overlap value, A is equal to or greater than 1. This ensures that, for an overhang, the shell portion for the layers where the core region is not consolidated by exposure of that powder layer, in this embodiment odd layers Li, L3, L5, overlaps sufficiently with the core region of the next powder layer L2, L4, Le above where the core region is consolidated by exposure of that powder layer L2, L4, Le, in this embodiment the next even layer. In this way, the core portion ties (fuses) together with the shell region for both powder layers to avoid or mitigate the formation of pores. An extent of the shell portion is dependent on an angle of the overhanging surfaces of the part, with a width of the shell portion in a layer being greater for overhangs at a smaller angle to the build plane. With such a definition of the core and shell portions it is possible for a core portion to extend to an outer surface of the workpiece / object without being surrounded by a shell portion. For example, for a vertical surface of the workpiece / object. Furthermore, it is possible for a powder layer not to comprise a core portion, for example, if an overhang is at an angle to the build plane such that none of the irradiation points of an area to be consolidated have an Nx value greater than Ncsh or Ncsh + A depending on whether it is a powder layer in which the core region is consolidated by exposure of that powder layer or in which the core region is consolidated by exposure of a powder layer above.

[0117] The core region includes a bulk-region 301, in which the exposure parameter EDB2 is kept constant, and a non-bulk region 302a, in which the exposure parameter EDB2,NX is varied based on the thickness of material below the irradiation point. The shell region includes a non-bulk region 303a, 303a’ and a downskin region 303b, 303b’, in which the exposure parameter EDBI.NX or EDDS.NX is varied based on the thickness of material below the irradiation point. The non-bulk value for the exposure parameter for each non-bulk irradiation location may be determined in a similar manner to that described above. However, the value used for Emax is selected to be appropriate for the number of powder layers melted by the exposure. Accordingly, for the core region, Emax is set at a higher value EDB2 suitable for thicker powder layers, in this embodiment two times the powder layer thickness. For the shell region, Emax is set at a lower value EDBI suitable for a single powder layer thickness. The exposure parameters EDDS for the downskin region is set for a value that is suitable for the thickness of a single powder layer. NCihreshoid (NCmax and NCtr) may be altered to take into account the differing thickness of the powder that is melted in the core and shell regions, for example as explained with reference to Figures 7 and 8 for an embodiment in which the thickness of each powder layer is varied (in this embodiment, each powder layer is formed with the same powder layer thickness but a number of powder layers melted in the core portion by an exposure is different to the number of powder layers melted in the shell portion). In this embodiment, at least for odd powder layers, an extent of at least some of the scan paths for the odd powder layers is defined by the extent of the shell portion (there being no scan paths in the core portion), the extent of the shell portion dependent on the geometry of underlying consolidated material. However, determination of the scan paths may still be separated from determination of the exposure parameters.

[0118] In a further embodiment, one or more inner core portions are provided in addition to the core and shell portions, wherein, for each inner core portion, multiple powder layers are melted greater than the number of powder layers melted to form the core portion and different to any other inner core portion. For example, the threshold number of layers increases as one melts a greater number of powder layers through a single irradiation. Each inner core portion may comprise a bulk region and a nonbulk region.

[0119] In a further embodiment illustrated in Figures 7 and 8, a thickness of the powder layers formed by the recoater 109 is not constant throughout the build but is altered during the build. In this embodiment, a thickness for each powder layer is selected based on an angle to a working plane of the surface of the object 400. Thinner powder layers are selected for regions of the object having lower angle surfaces than regions of the object in which all surfaces are at a higher angle to the working plane. In this embodiment, three powder layer thicknesses can be selected, a first powder layer thickness for regions of the object 400 wherein all surfaces are at an angle to a working plane above a first threshold, in this embodiment above 10°, a second powder layer thickness for regions of the object 400 having one or more surfaces at an angle to a working plane below the first threshold but all surfaces are at an angle to a working plane above a second threshold, in this embodiment above 5°, and a third powder layer thickness for regions of the object 400 having one or more surfaces at an angle to a working plane below the second threshold. These thresholds may be different for different powder materials. In this embodiment, the first powder layer thickness is 60pm, the second powder layer thickness is 30pm and the third powder layer thickness is 15pm. When transitioning from the third powder layer thickness to the first powder layer thickness, one or more powder layers, in this embodiment, two powder layers, of the second powder layer thickness may be used to avoid a sudden step in powder layer thickness.

[0120] Figure 7 illustrates an object having an arch shape. The object includes regions 401 in which all surfaces are at an angle of above 10° to the working plane, regions 402 having a surface that is at an angle of below 10° to the working plane but all surfaces for that region 402 are at an angle of above 5° to the working plane and region 403 having a surface that is at an angle of below 5° to the working plane. Referring to Figure 8, a powder layer thickness of 60pm is selected for regions 401 (and for those above the powder layers forming regions 403), a powder layer thickness of 30pm is selected for regions 402 and a powder layer thickness of 15pm is selected for regions 403. To achieve the different powder layer thicknesses during the build, the controller 160 controls movement of the build platform 102 based on the selected powder layer thickness required at each stage of the build. Other powder layer thicknesses could be used, for example just two powder layer thicknesses, such as 30pm and 60pm, or four of more powder layer thicknesses. Furthermore, different steps in powder layer thicknesses may be used, such as 20pm, 40pm and 80pm. Preferably, the smaller powder layer thicknesses are factors of the larger powder layer thicknesses. The controller may be arranged such that a user can identify a minimum layer thickness and / or the steps in layer thicknesses.

[0121] In one embodiment, to carry out the methods described with reference to Figures 3 to 6 together with changing a powder layer thickness during the build, the values for thresholds NCmax and NCtr are set for each powder layer dependent on the powder layer thicknesses of the powder layer and underlying powder layers. Figure 9 illustrates an example of changes in powder layer thickness for a build and consequential changes in the values for thresholds NCmax and NCtr. Initially, the values for NCmax and NCtr are constant as the powder layer thickness is constant. In this example, NCmax is 30 layers (equivalent to 1800pm) and NCtr is 3 layers (equivalent to 180pm) for a powder layer thickness of 60pm. When the powder layer thickness reduces, in this example to 30pm, the value for NCmax initially reduces before rising steadily until reaching a constant target value used for the thinner powder layers once a sufficient number of the thinner powder layers have been formed to complete the transition in the value for NCmax. For NCtr, the value rapidly rises to the new target value with only a few steps in-between. In this example, the target value for NCmax is 50 layers (equivalent to 1500pm) and the target value for NCtr is 5 layers (equivalent to 150pm) for a powder layer thickness of 30pm. These transitions in the values for NCmax and NCtr can be understood when considering a minimal thickness of consolidated material required for the bulk value of the exposure parameters for each powder layer thickness. For thinner powder layers, a lower bulk value can be selected for the exposure parameter as a lower energy density may be sufficient to melt powder in the bulk region for the thinner powder layers. This lower energy density allows the bulk region to be extended out to irradiation locations having a lower thickness of consolidated material below the irradiation location than for thicker powder layers (in the example a reduction from 1800pm for 60pm powder layers to 1500pm for 30pm powder layers). Such lower thicknesses of consolidated material are achieved with fewer layers if the number of layers being counted includes thicker (e.g. 60pm) powder layers. As the value of NCmax and NCtr is a number of powder layers, when the powder layer thickness changes, the value of NCmax and NCtr takes into account that some of the powder layers being counted are of a different thickness to the current powder layer being irradiated. In the example shown in Figure 8, for the first 30pm powder layer, all underlying powder layers are 60pm powder layers hence the number of powder layers required to achieve the desired thickness for the bulk region (set by NCmax) for a 30pm powder layer is less than that required to achieve the greater thickness required for a bulk region for a 60pm powder layer. However, as more 30pm powder layers are formed, the number of powder layers required to achieve the desired thickness for the bulk region increases and hence, NCmax increases until only 30pm powder layers are being counted. For NCtr that transition to a constant value occurs over fewer layers because the value for NCtr is lower than for NCmax.

[0122] At layer number 615, the powder layer thickness drops to 15 pm for 10 layers before rising briefly back to 30pm and then to 60pm. These changes result in consequential changes in NCmax and NCtr. Thus, a location at which an area to be consolidated in a powder layer changes from a bulk region to a non-bulk region and an inner-bulk region to a downskin region is dependent on a geometry for the object to be built but also changes in a powder layer thickness as the object is built.

[0123] Figures 10a and 10b illustrate a slice 501 (unified area) of an object to be built by consolidating powder of a powder layer. The method comprises determining scan paths for an energy beam to scan to consolidate powder. The energy beam may scan each scan path as a continuous scan or as a series of discrete exposures (point scanning) to consolidate material as a continuous line along the scan path. In this embodiment, the scan paths include a set of parallel hatch lines (shown in Figure 10a) and at least one border scan path (as shown in Figure 10b) The slice 501 comprises a bulk region 502, in the drawings indicated by the shaded area, wherein a thickness of consolidated material below each irradiation location in the bulk region 502 is greater than the threshold thickness NCrhreshoid. The slice 501 also comprises a non-bulk region 503, in the drawings indicated by the non-shaded area, wherein a thickness of consolidated material below each irradiation location in the non-bulk region 503 is less than the threshold thickness NCrhreshoid. In this example, each scan path of the set of hatch lines is either entirely within bulk region 502 or entirely within the non-bulk region 503. The or each border scan path spans both regions 502, 503. The exposure parameters for irradiation locations on the scan paths are determined as described above.

[0124] In conventional methods, a starting location for scanning the scan paths is determined such that a hatch order direction is in a direction Foppopposed to a gas flow direction G. This may comprise the starting location being in the top left of the slice both for the set of hatches and the border scan path(s). However, when using variable exposure parameters as described above, this can result in undesirable properties of the object if the start location is in a region having poor thermal conductivity. Accordingly, this embodiment of the invention determines a starting irradiation location based on a thickness of consolidated material below the starting irradiation location to avoid starting in a region having poor thermal conductivity when regions of higher thermal conductivity are available for selecting as a starting location.

[0125] In the example shown in Figures 10a and 10b, there is a region having a thickness of consolidated material therebelow above a predetermined threshold, in this embodiment above the bulk threshold NCrhreshoid. Accordingly, a starting location 504 for the hatch lines and starting location 510 for the border scan path is selected to be in the region 502.

[0126] In this embodiment, the starting irradiation location 504 is selected to be an irradiation location at a start of a hatch line within region 502 that is furthest downstream in the gas flow direction G, if such a location is present. Where there is more than irradiation location to choose from, a selection between these options may be based on other factors. In this embodiment, the order of preference is left to right (opposite the gas flow direction) and top to bottom (although bottom to top could also be used or another criteria). The starting irradiation location sets the first hatch line to be scanned. After scanning the first hatch line, hatch lines are scanned in an order, wherein, first the hatch lines in a hatch order direction Fwith with the gas flow direction G are scanned (as indicated by the solid arrows) and then the hatch lines in a hatch order direction Foppopposed to the gas flow direction G are scanned (as indicated by the dashed arrows). The hatch order direction is a direction perpendicular to the direction of the hatches (e.g. it is the direction of progression as the laser beam moves from one hatch to another). The hatch lines are scanned in an order in each hatch order direction Fwith, Foppsuch that, after completing scanning of a hatch line, the next hatch line to be scanned is the adjacent hatch line in the hatch order direction Fwith, Fopp. In this way, the scanning progresses from a starting irradiation location having a thickness of consolidated material therebelow greater than the predetermined thickness to a further irradiation location in the other hatches in the hatch order direction Fwith having a thickness of consolidated material therebelow less than the predetermined thickness. The already consolidated material of earlier hatch lines connected to the first hatch line provides a heat conduction path for heat generated during consolidation of the powder.

[0127] On completing all hatch lines in the hatch order direction Fwith with the gas flow direction G, the scanning progresses to scan the adjacent hatch line to the first hatch line in the hatch order direction Foppopposed to the gas flow direction G.

[0128] The same requirements are used for the border scan path. The border scan path is scanned in a clockwise direction.

[0129] The starting irradiation location may be an irradiation location wherein the thickness of consolidated material below the starting irradiation location is a maximum for the unified area.

[0130] “Bidirectional” scanning of the hatch lines is used in this embodiment, wherein the scan directions for adjacent hatch lines are in opposite directions. However, it will be understood that alternatively unidirectional scanning of the hatch lines may be used, wherein all the hatch lines are scanning in the same direction. Alternatively, the hatch lines may be scanned bidirectionally with the exception of one or both hatch lines adjacent to the first hatch line. In particular, one or both of these hatch lines may be scanned in the same direction as the first hatch line. The border scan path may also be scanned in an anticlockwise direction rather than a clockwise direction.

[0131] Figures I la and 11b are an example of a further scenario, wherein a slice (unified area) 601 does not comprise a region having a thickness of consolidated material therebelow greater than the threshold thickness NCThreshoid. For slice 601 a thickness Nc of consolidated material below irradiation locations varies (but is always less than the threshold thickness NCThreshoid). This is indicated by the values 1, 2 and 3. In this scenario, the starting irradiation location 604 is selected to be an irradiation location at a start of a hatch line having the greatest thickness of consolidated material therebelow. As with the example described with reference to Figures 10a and 10b, scanning then proceeds to scan the hatch lines in an order from the first hatch line, first, in a hatch order direction Fwith with the gas flow direction G and then in a hatch order direction Fwith opposed to the gas flow direction G.

[0132] The starting irradiation location 610 on the border scan is also selected based on an irradiation location of the border scan having the greatest thickness of consolidated material therebelow.

[0133] Figure 12 illustrates a slice (unified area) 701, wherein the first scan path extends from a bulk region 702 to a non-bulk region 703. The starting location 704 is determined using the criteria described above and hence, is in the bulk region 702. However, Figure 12 illustrates how the scanning direction along the first scan path may be such that scanning progresses from the starting location 704 of a further irradiation location along the scan path having a thickness of consolidated material therebelow less than the predetermined threshold NCThreshoid. The powder that has already been consolidated along the scan path by the time the further irradiation location is irradiated provides a thermal conduction path for heat away from the further irradiation location. Figure 12 also illustrates how the hatch order directions Fwith and Foppmay not be parallel to the gas flow direction G.

[0134] Figure 13 illustrates an example of a slice 801 wherein none of the end irradiation locations of the hatch lines are in a region 802 of the slice 801 having a thickness of consolidated material therebelow greater than the predetermined threshold NCrhreshoid, but one or more of the scan paths cross the region 802. In this scenario, the starting location 804 is selected to be an irradiation location in a scan path, in this embodiment, the scan path furthest in the hatch order direction Fwith, that crosses the region 802 and is located in region 802. Thus, the starting location 804 is between the end irradiation locations of the first scan path.

[0135] The first scan path is split into sections at a splitting point (illustrated by the line bisecting the scan path) by the starting irradiation location 804, wherein the laser beam is scanned in opposite directions along the sections of the scan path either side of the splitting point. The laser beam is first scanned along one of the sections of the scan path from the starting location 804 to one of the end irradiation locations within the region 803 and then progresses to an adjacent scan path in a hatch order direction Fwith with the gas flow direction G. On completion of the hatch lines in the hatch order direction Fwith from the first hatch line / scan path, the laser beam scans the other section of the first hatch line / scan path and then progresses to an adjacent scan path in a hatch order direction Foppopposed to the gas flow direction G.

[0136] Referring to Figure 14, the scanning of a slice (unified area) 901 is shared between multiple scanners 106a, 106b, 106c, 106d of the powder bed fusion apparatus. It is known is divide a working surface 110 of a powder bed 104 into regions 905, 906, 907, 908, each region 905, 906, 907, 908 allocated to a different one of the scanners 106a, 106b, 106c, 106d. These regions 905, 906, 907, 908 may extend across the working surface 110 in a gas flow direction G to form “lanes” for each laser beam 118a, 118b, 118c, 118d. The use of such lanes 905, 906, 907, 908 reduces the chance that one scanner irradiates a location within the gas-borne debris generated by another scanner irradiating an upstream irradiation location. To achieve, this scan paths may be split at the boundary of the lanes 905, 906, 907, 908. This is illustrated for some of the hatch lines in Figure 14 by the bisecting lines that coincide with the boundaries to the lanes 905, 906, 907, 908. Scanning of the sections of the scan paths within each lane 905, 906, 907, 908 is then allocated to the scanner 106a, 106b, 106c, 106d allocated to that lane 905, 906, 907, 908. However, in this embodiment, if the scan path crosses a boundary of the lanes 905, 906, 907, 908 in a region that has a thickness below the irradiation locations below the predetermined threshold NCrhreshoid, the scan path is not split at this point but the scanner 106a, 106b, 106c, 106d scanning the scan path as it crosses the boundary continues to scan the scan path. In other words, this section of the scan path is identified as not to be divided. In this way, scanning of a section of the scan path in which the exposure parameters are varied between irradiation locations is not interrupted.

[0137] With multiple laser beams 118a, 118b, 118c, 118d, multiple starting irradiation locations 904a, 904b, 904c, 904d are determined. These starting irradiation locations are determined using the principles described above with reference to Figures lOd to 12 but separately for each region 905, 906, 907, 908. In the example shown in Figure 14, only one of the laser beams 118a (allocated region 905) scans ones of the hatch lines in the hatch order direction Fwith. All the other laser beams order the scanning of hatch lines in the hatch order direction Fopp. Sections of one or more scan paths 911, 912, 913 between first scan paths of different laser beams 118b, 118c, 118d in the hatch order direction Foppmay be scanned by the laser beam 118b, 118c that has scanned the first scan path earlier in the hatch order direction Fopp. This may be the case, even if scanning of the sections of the scan path starts in a region 907, 908 not allocated to the laser beam 118b, 118c. In the example, a section 911 of a scan path in scanner region 907 is scanned by the laser beam 118b allocated to region 907 and sections of scan paths 912 and 913 in scanner region 908 are scanned by the laser beam 118c allocated to region 907.

[0138] It will be understood that modifications and alterations may be made to the above- described embodiment without departing from the invention as defined herein. For example, NCmax and NCtr may be defined as a thickness of consolidated material rather than as a number of layers and Nx as a thickness of consolidated material below the irradiation location rather than a number of powder layers. Such a definition may avoid the requirement to gradually change the values of NCmax and NCtr as the powder layer thicknesses change. Instead, a constant value for NCmax and NCtr for a particular powder layer thickness may be used, e.g., for NCmax, 1800pm for 60pm powder layers, 1500pm for 30pm powder layers and 1050pm for 15pm powder layers, and for NCtr, 180pm for 60pm powder layers, 150pm for 30pm powder layers and 105 pm for 15pm powder layers. In the above-described embodiments, NCtr is a tenth of NCmax, however, it will be understood that other ratios could be used in which NCtr is less than NCmax.

Claims

CLAIMS1. 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 each of a plurality of successively formed powder layers with an energy beam, the method comprises determining, for at least one of the powder layers, a scan path for the energy beam to scan to melt powder of the powder layer, the scan path including bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted by irradiation of previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a bulk threshold, and nonbulk irradiation locations, wherein, for each non-bulk irradiation location, powder is melted to form a thickness of consolidated material from the non-bulk irradiation location to a surface of the object less than the bulk threshold, allocating a bulk value for an exposure parameter to the bulk irradiation locations, and determining a non-bulk value for the exposure parameter for each non-bulk irradiation location dependent on, at least in part, the thickness of the consolidated material from the non-bulk irradiation location.

2. A method according to claim 1, wherein the scan paths are determined before determining locations along the scan paths that correspond to bulk and non- bulk irradiation locations.

3. A method according to claim 1 or claim 2, wherein the thickness is from the irradiation location to a point on a surface of the object directly below the irradiation location in a build direction.

4. A method according to any one of the preceding claims, wherein the scan path extends to an edge of an area to be consolidated in the powder layer.

5. A method according to any one of the preceding claims, wherein the non- bulk values vary for different thicknesses of the consolidated material from the non-bulk irradiation location.

6. A method according to any one of the preceding claims, wherein the bulk threshold is between 10 and 75 layers.

7. A method according to any one of the preceding claims, wherein the bulk threshold is between 0.5mm to 3mm.

8. A method according to any one of the preceding claims comprising identifying bulk irradiation locations, wherein powder of two or more layers is melted by irradiation of each bulk irradiation location with the energy beam, and non-bulk irradiation locations, wherein powder of a single layer alone is melted by irradiation of each non-bulk irradiation location with the energy beam.

9. A method according to any one of the preceding claims, wherein the scan path includes downskin irradiation locations, wherein, for each downskin irradiation location, powder is melted to form consolidated material having a thickness from the downskin irradiation location to a surface of the object less than a downskin threshold, the downskin threshold being less than the bulk threshold, and determining a downskin value for the exposure parameter for downskin irradiation locations, wherein the downskin value results in a higher energy density than the bulk value.

10. A method according to claim 9, comprising determining the downskin value dependent on, at least in part, the thickness of consolidated material below the downskin irradiation location.

11. A method according to claim 9 or claim 10, wherein the downskin threshold is between 3 and 10 powder layers.

12. A method according to claim 9 or claim 10, wherein the downskin threshold is between 100 and 250pm.

13. A method according to any one of claims 9 to 12, wherein scanning of theenergy beam along the scan path(s) comprises exposing each irradiation location on the scan path during a high-power interval of the pulsed energy beam and adjusting steering elements, such as steering optics, to direct the energy beam to the next irradiation location during a low-power interval of the pulsed energy beam, such as when the energy beam is switched off, the method comprising selecting an exposure time for an irradiation location based on whether the irradiation location is a bulk irradiation location or a downskin irradiation location.

14. A method according to claim 13, wherein the exposure time is longer for the downskin irradiation locations than the bulk irradiation locations.

15. A method according to any one of the proceeding claims, wherein the exposure parameter is energy beam power.

16. A powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising carrying out a build of an object based on instructions determined in accordance with the method of any one of claim 1 to 15.

17. A powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising, for at least one of the powder layers, scanning an energy beam along a scan path to melt powder of the powder layer, the scan path including bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted in previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a bulk threshold, and non-bulk irradiation locations, wherein, for each non-bulk irradiation location, powder is melted to form a thickness of consolidated material from the non-bulk irradiation location to a surface of the object less than the bulk threshold, wherein a non-bulk value for an exposure parameter for each non-bulk irradiation locationis dependent on, at least in part, on the thickness of consolidated material from the non-bulk irradiation location and a bulk value for the exposure parameter of the energy beam when scanning to the bulk irradiation locations is constant.

18. A powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam across selected areas of each powder layer to form the object, wherein the energy beam is controlled to achieve a higher energy density for downskin irradiation locations than for bulk irradiation locations.

19. 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 each of a plurality of successively formed powder layers with an energy beam, the method comprises determining bulk irradiation locations, wherein, for each bulk irradiation location, powder is melted by the energy beam to form, together with powder melted in previous powder layers, a thickness of consolidated material from the bulk irradiation location to a surface of the object above a downskin threshold, and downskin irradiation locations, wherein, for each downskin irradiation location, powder is melted to form a thickness of consolidated material from the downskin irradiation location to a surface of the object less than the downskin threshold, the method comprising allocating scan parameters to the bulk irradiation locations and to the downskin irradiation locations such that a higher energy density is achieved for downskin irradiation locations than for bulk irradiation locations.

20. A powder bed fusion method, in which an object is built in a layer-by-layer manner by selectively irradiating each of a plurality of successively formed powder layers with an energy beam, the method comprising scanning an energy beam along a scan path comprising core irradiation locations and shell irradiation locations, wherein powder of two or more layers is melted by irradiation of the core irradiation locations with the energy beam whereas powder of a single layer alone is melted byirradiation of the shell irradiation locations with the energy beam.

21. 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 each of a plurality of successively formed powder layers with an energy beam, the method comprising determining a first area to be consolidated in a first powder layer and a second area to be consolidated in a second, subsequent powder layer, dividing the first area into a first core region and a first shell region and dividing the second area into a second core region and a second shell region, determining scan paths for the energy beam to take in irradiating the first and second powder layers, wherein the scan paths are arranged such that the first powder layer is irradiated to melt the first shell region whilst at least a portion of the first core region remains unmelted and the second powder layer is irradiated to melt the second shell region and to melt powder of the second powder layer corresponding to the second core region en masse with powder of the first powder layer corresponding to the unmelted portion of first core region and the scan paths include a second powder layer scan path for irradiating the second powder layer, the second powder layer scan path extending into the second core region and the second shell region.

22. 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 each of a plurality of successively formed powder layers with an energy beam, the method comprising determining, from geometric data describing the object, a geometric characteristic of a surface of the object at different locations in a build direction, selecting different powder layer thicknesses for powder layers based on the geometric characteristic, determining at least one threshold value for each powder layer based on the selected different powder layer thicknesses, selecting values for an exposure parameters for different irradiation locations within the layer dependent on, at least in part, whether a thickness of consolidated material below the irradiation location is above or below the threshold value.

23. A non-transient data carrier having instructions stored thereon, which, when executed by a controller of a powder bed fusion apparatus, causes the controller to control the powder bed fusion apparatus to carry out the method of any one of claims 16 to 18 and 20.

24. A non-transient data carrier having instructions stored thereon, which, when executed by a processor, causes the processor to carry out the method of any one of claim 1 to 15, 19, 21 and 22.

Images