Manufacturing methods and apparatus for forming objects from a nickel-based superalloy in a layer-by-layer manner

Abstract

A method of manufacturing an object comprising building the object using a powder bed fusion method, wherein an object is formed in layers by selectively solidifying powder with at least one energy beam and the powder is a nickel-based superalloy. The powder bed fusion method comprises consolidating powder to form layers of the object using a shell and core method, wherein each layer is divided into at least one shell region and a core region, and hot isostatic pressing (HIP) the object built using the powder bed fusion method.

Description

[0001] MANUFACTURING METHODS AND APPARATUS FOR FORMING OBJECTS FROM A NICKEL-BASED SUPERALLOY IN A LAYER-BY- LAYER MANNER

[0002] Field of Invention

[0003] This invention concerns manufacturing methods and apparatus for forming objects from a nickel-based superalloy in a layer-by-layer manner. The invention particularly concerns using powder bed fusion additive manufacturing and apparatus to manufacture objects from a nickel -based superalloy, such as CM 247 LC.

[0004] Background

[0005] A superalloy is a metallic alloy which can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature. Superalloys can be based on iron, cobalt or nickel, the latter being best suited for aeroengine applications.

[0006] The major alloying elements in nickel -based superalloys are aluminium and / or titanium, with a total concentration which is typically less than 10 atomic percent, (other elements, such as chromium, can be as high as 22%). This generates a two-phase equilibrium microstructure, consisting of gamma and gamma-prime phases. It is the gamma-prime phase which is largely responsible for the elevated temperature strength of the material and its increased resistance to creep deformation. Both the gamma and gamma-prime phases have a cubic lattice with similar lattice parameters and the gamma-prime precipitates in a cube-cube orientation relationship with the gamma phase. However, whereas the gamma phase is a solid-solution with a face-centred cubic lattice with a random distribution of the different species of atoms, the gamma-prime is a solid phase with a primitive cubic lattice, in which the nickel atoms are at the face-centres and the aluminium or titanium atoms at the cube comers. Due to the atomically ordered gamma-prime phase, dislocations in the gamma-phase have difficulty to cross / shear the gamma-prime phase, strengthening the alloy.

[0007] In addition to nickel, aluminium and titanium, superalloys may contain chromium for oxidation resistance, small quantities of yttrium to help the oxide scale to cohere to the substrate, for polycrystalline superalloys, grain strengthening elements, such as boron and zirconium. Carbide formers (cobalt, chromium, molybdenum, tungsten, carbon, niobium, tantalum, titanium and hafnium) may be included. The carbide formers tend to precipitate at the grain boundaries, reducing the tendency of grain boundary sliding.

[0008] Elements such as cobalt, iron, chromium, niobium, tantalum, molybdenum, tungsten, vanadium, titanium and aluminium are also solid-solution strengtheners, both in the gamma and gamma-prime phase.

[0009] Powder bed fusion additive manufacturing methods for producing objects comprise layer-by-layer solidification of a powder, such as a metal powder material, using a high energy beam, such as a laser or electron beam. A powder layer is deposited on a powder bed in a build chamber and the laser or electron beam is scanned across portions of the powder layer that correspond to a cross-section of the object being constructed. The laser or electron beam melts the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required.

[0010] Although some nickel-based superalloys have been additively manufactured substantially crack-free, such as IN625 and IN718, the processing of other nickel-based superalloys, such as CM247LC, has not been so successful.

[0011] Summary of Invention

[0012] According to a first aspect of the invention there is provided a method of manufacturing an object comprising building the object using a powder bed fusionmethod, wherein an object is formed in layers by selectively solidifying powder with at least one energy beam and the powder is a nickel-based superalloy. The powder bed fusion method may comprise consolidating powder to form layers of the object using a shell and core method, wherein each layer is divided into at least one shell region and a core region. The manufacturing method may comprise hot isostatic pressing (HIP) the object built using the powder bed fusion method.

[0013] It has surprisingly been found that, for objects built from nickel-based superalloys using a shell and core powder bed fusion building strategy, the HIP process improves the ductile properties of the object. Such an improvement has not been observed for objects built from nickel -based superalloys that do not use a shell and core powder bed fusion building strategy. In fact, the ductile properties appear to deteriorate after a HIP process for such objects.

[0014] The at least one shell region and the core region may overlap to form an intersection region in the layer. The intersection region may have width of less than 500pm, preferably less than 300 pm and optionally 200pm or less. The intersection region may have width of at least 50pm and preferably at least 100 pm. Not to be bound by any one theory, but it is believed that the intersection region acts as a barrier that prevents cracks / defects propagating from the shell into the core during hot isostatic pressing. Hot isostatic pressing can close cracks / defects in the object but can also open cracks / defects that open out to a surface of the object. Preventing or limiting propagation of the surface defect / cracks into the core along with closing of the cracks / defects in the core improves the ductility of the object. For objects without such an intersection, the hot isostatic pressing coarsens the grains within the object whilst opening surface defects / cracks, leading to a loss of ductility in the object.

[0015] The at least one shell region and the core region may be formed using a plurality of scan paths, each scan path of the plurality of scan paths extending across the at least one shell region, the core region and a boundary region between the at least one shell region and the core region. A value for at least one scan parameter (e.g. of a scan parameter set) of the energy beam may be changed as the energy beam crossesthe boundary region. The boundary region may comprise a plurality of irradiation points, and the value may be changed incrementally across the plurality of irradiation points, for example in a series of steps or as a continuous change. The change in the value of the at least one scan parameter may transition from a shell value used for the at least one scan parameter in the shell region to a core value used for the at least one scan parameter in the core region or vice versa. The at least one scan parameter may include scan velocity (which may itself be defined using a set of scan parameters, such as point distance, exposure time and, optionally, delay time). For example, a scan velocity for the core region may be greater than a scan velocity for the shell region. In this way, provision of the core region will reduce build times for certain geometries compared to consolidating powder with shell values across the entire layer. However, the provision of a shell region reduces surface cracks that would otherwise open up under the HIP process. Use of the continuous scan paths across the boundary region may reduce the chance of cracks forming at the interface between the at least one shell region and the core region.

[0016] The shell regions may be formed by consolidating powder having a first layer thickness and the core regions may be formed by consolidating powder having a second layer thickness. The first layer thickness may be thinner than the second layer thickness. Using a thinner layer thickness for the shell region reduces crack / defect formation in the shell region reducing the chance that, during the hot isostatic pressing, a surface crack / defect will open into the core.

[0017] The first layer thickness may be achieved by lowering a build platform by a first distance. The first distance may be less than 20pm, preferably less than 15pm and most preferably 10pm or less. The first thickness is typically greater than the first distance because when powder is consolidated, a thickness of consolidated material is less than a powder layer formed. Accordingly, a thickness of the next powder layer that is formed corresponds to a distance the build platform is lowered plus a shrinkage of the previous layer due to consolidation.

[0018] The first distance may be more than 5pm. Typical powder particle distributionsused in powder bed fusion have a DIO particle size around 20pm. As a thickness of the powder layers being formed reduces, more of the larger particles are removed during spreading of a powder layer. For the typical powder particle distributions, very few powder particles will remain for first distances below 5pm. Lack of powder can result in defects and porosity in the object.

[0019] The second layer thickness may be achieved after lowering a build platform by a second distance that is a multiple of the first distance. The multiple may be between 2 and 12 times the first distance and preferably 2 and 6 times the first distance and most preferably 2 or 3 times the first distance. The second distance may be less than 120pm, preferably less than 60pm and most preferably less than 40pm.

[0020] The powder bed fusion method may comprise using a shell scan parameter set for the shell regions that is different to a core scan parameter set used for the core regions. As different powder thicknesses are consolidated when forming the core regions to the shell regions, laser parameters that provide a higher energy density may be required for the core regions than the shell regions.

[0021] Using thicker powder layers for the core may allow the use of higher laser powers and faster scan speeds. This may result in shorter build times compared to consolidating the entire layer with powder at the first layer thickness. This may result in an increase in cracks in the core region after the powder bed fusion process. However, these cracks may be healed by the HIP process. Scan parameters may be used for the shell region(s), such as a lower laser power and slower scan speed, which result in fewer or no cracks. In this way, there is a reduced chance that a surface crack is present that would be opened up by the HIP process. In this way, the method may achieve faster builds without an unacceptable increase in cracks in the final object.

[0022] The nickel-based superalloy may contain 7.5-12% W, 6-23% Cr, 3-8% Al and 8-12% Co. Other common additions may be Ta, Hf, Ti, Mo, Zr, B, Si, Mn, C and Nb. In broad terms, the elemental additions in Ni-base superalloys can be categorizedas being i) y formers and strengtheners - elements that tend to partition to the y matrix, ii) y' formers and strengtheners - elements that partition to the y' precipitate, iii) carbide formers, and iv) elements that segregate to the grain boundaries. Elements which are considered y formers are Group V, VI, and VII elements such as Co, Cr, Mo, W and Fe. The atomic diameters of these alloys are only 3-13% different from that of Ni (the primary matrix element), y' formers come from group III, IV, and V elements and include Al, Ti, Nb, Ta and Hf. The atomic diameters of these elements differ from Ni by 6-18%. The main carbide formers are Cr, Mo, W, Nb, Ta and Ti. The primary grain boundary elements are B, C, Zr and Hf. Their atomic diameters are 21-27% different from that of Ni. Re and Ru and have the effect of increasing the liquidus and solidus temperature of the alloy.

[0023] The chemical composition of the nickel-based alloy by weight % may comprise: 9.3-9.7W, 9.0-9.5Co, 7.5-8.5Cr, 5.4-5.7A1, 3.1-3.3Ta, 1.4-1.6Hf, 0.6-0.9Ti, 0.4-0.6Mo, 007-.015Zr, 0.01-0.02B with a carbon concentration of around 0.07-0.09wt% and a balance of Ni. The nickel-based alloy may also comprise any one or more up to the maximum weight percentage: Si .03 max, Mn .10 max, P .005 max, Fe .2 max, Cu .05 max, Nb .10 max, and / or any one or more following up to the maximum ppm: S 20 ppm max., Mg 80 ppm max., Pb 2 ppm max., Se 1.0 ppm max., Bi .3 ppm max., Te .5 ppm max, TI .5 ppm max, [N] ppm 15 max, [O] ppm 10 max andNv3B 2.15 max.

[0024] The chemical composition of the nickel-based alloy by weight % may substantial consist of: 9.3-9.7W, 9.0-9.5Co, 7.5-8.5Cr, 5.4-5.7A1, 3.1-3.3Ta, 1.4-1.6Hf, 0.6-0.9Ti, 0.4-0.6Mo, 007-.015Zr, 0.01-0.02B with a carbon concentration of around 0.07-0.09wt% and a balance of Ni, any one or more (or none) of the following up to the maximum weight percentage: Si .03 max, Mn .10 max, P .005 max, Fe .2 max, Cu .05 max, Nb .10 max, and any one or more (or none) of following up to the maximum ppm: S 20 ppm max., Mg 80 ppm max., Pb 2 ppm max., Se 1.0 ppm max., Bi .3 ppm max., Te .5 ppm max, TI .5 ppm max, [N] ppm 15 max, [O] ppm 10 max andNv3B 2.15 max.The nickel-based alloy may be CM 247 or CM 247 LC.

[0025] The consolidation of the powder may comprise melting of the powder.

[0026] Exposure (scan) parameters and an exposure pattern may be such that melt pools are formed in a transition or conduction mode. It will be understood that “conduction mode” as used herein means that the energy of the energy beam is coupled into the powder bed primarily through heat conduction creating a melt pool having a width equal to or greater than twice its depth (a ratio of depth to width of less than 0.5). This is to be contrasted with keyhole mode in which a hole is formed in the melt pool where material is vaporised by exposure to the energy beam. A melt pool formed in keyhole mode has a deep, narrow profile with a ratio of depth to width of greater than 1.5. A transition mode exists between the conduction mode and the keyhole mode, wherein the energy does not dissipate quickly enough, and the processing temperature rises above the vaporisation temperature. A depth of the melt pool increases, and penetration of the melt pool can start. Preferably, the method comprises exposing the layer to the at least one energy beam to form melt pools in a conduction or transition mode having a depth to width ratio of less than 1.5, preferably, less than 1, more preferably less than 0.75 and most preferably less than or equal to 0.5.

[0027] A scan path direction, such as hatch line direction, on successively consolidated layers may be rotated by an angle other than 180°, for example by an angle between 10° and 170°, preferably 30° and 150° and more preferably between 60° and 120° and most preferably 67° or 90°. The scan paths of a layer may be scanned along hatch lines bidirectionally, i.e. back and forth along consecutively scan paths, or may be scanned unidirectionally. The scan path direction may be rotated for the shell regions of successively consolidated layers. Additionally or alternatively, the scan path direction may be rotated for the core regions of successively consolidated layers. If the first layer thickness is less than the second layer thickness independent rotation of the scan path direction for the shell regions and the core regions may result in the scan path direction of the shell and core regions in some layers beingaligned and, for others, misaligned (for example, for rotations of the scan path direction by 67° or 90°). The scan paths may be straight hatch lines.

[0028] The shell regions may be consolidated using a pattern of parallel hatch lines. The pattern of parallel hatch lines may be formed as a meander pattern, with each parallel hatch line extending between inner and outer borders of the shell region, or a stripe or chequerboard pattern, with a plurality of the parallel hatch lines having the same length and having start and ends points aligned in a direction perpendicular to the hatch direction to form a stripe of the stripe pattern or square of the chequerboard pattern.

[0029] The shell region may have a width in a plane of the layer greater than a hatch spacing of the hatch lines and preferably greater than 5 and preferably greater than 10 hatch spacings. The width of the shell region may be greater than 0.5mm and preferably greater than 1mm.

[0030] The core regions may be consolidated using a pattern of parallel hatch lines. The pattern may be a meander pattern, stripe pattern or chequerboard pattern. The pattern used for the core regions may be the same or different to a pattern used for the shell regions.

[0031] The method may comprise selecting to use the shell and core method for a layer and / or a shape of the core region based on a width of the layer in the hatch formation direction. An inner core region width may be a width of the core region bounded by an inner boundary of the shell region and / or the boundary region. A total width of core region may be greater than the inner core region width in the same direction because the core region may intersect / overlap with the shell region to form the intersection region. The inner core region width excludes portions of the core region that intersect / overlap with the shell region (the intersection region) and / or the boundary region. Each core region may have a core width in a direction of hatch lines greater than a minimum hatch length. Each core region may be configured such that no inner core width in a direction of hatch lines is less than the minimumhatch length. The minimum hatch length may be at least 0.5mm and preferably at least 0.8mm. The minimum hatch length may be greater than a width of the intersection and / or the boundary region. A sum of a width of the intersection and / or the boundary region and the minimum hatch length may be at least 0.6mm and preferably at least 0.9mm. In this way, a proportion of the core region is not the intersection region or boundary region and is surrounded by the intersection or boundary region. An inner core region width in a direction of hatch lines of at least the minimum hatch length may ensure that overheating due to short hatch lines is avoided, particularly in the case of higher energy density scan parameters being used for the core region (for example, due to the greater powder thickness). Overheating can cause excessive crack formation at the intersection between the shell and core regions. For these layers it may be preferable to form continuous hatch lines across the layer from the surface of the object rather than break the hatch lines with the core region. Layers having an area to be solidified having a width in the hatch line direction that is too small for an inner core region width of at least the minimum hatch length may be formed without a core region, e.g. it may all be formed as a continuous region using the shell region scan parameters. Accordingly, depending on a geometry of the object, some layers of the object may not be formed using a shell and core method.

[0032] Each core region may be configured such that all points within the core region meet the requirement that no straight line between boundary points of the layer that passes through the point within the core region has a length below a threshold length, such as the minimum hatch length plus twice a shell width. This may ensure that the high energy density core scan parameter set used for core regions is not applied to narrow sections of the layer. Use of core scan parameters in a narrow section of a layer may result in increased cracking. Thus, not using core scan parameters in these narrow sections may prevent such an increase in cracks. A possible explanation for the reduction in cracks is that the narrow sections may have poor thermal conduction properties and thus, the higher energy density core scan parameters may more readily result in the formation of deep, narrow melt pools that increase a chance of solidification cracking in these narrow sections compared towider sections that more readily dissipate the heat. Accordingly, limiting the use of the core scan parameters to the wider regions may reduce the prevalence of cracks.

[0033] An extent of the core region may be determined in a multi-step process, first, an initial core region may be determined based on shell width and the boundary of the layer and, second, a modified core region may be determined by removing points of the initial core region for which there is a straight line between boundary points of the layer that passes through the point having a length below a threshold length. The multi-step process may further comprise, third, the core region may be determined by removing portions of the modified core region having a core width in a direction of hatch lines less than a minimum hatch length. This third step may not be applicable if, for example, continuous scan paths are used that cross the boundary between the at least one shell region and the core region.

[0034] The energy beam or a one of the energy beams may be scanned continuously along each scan path (contrasted with the known point scanning technique, wherein an energy beam is progressed along a scan path by exposing a plurality of points separated by a point distance).

[0035] The exposure (scan) parameters may include power of the energy beam, scanning velocity of the energy beam, distance (referred to hereinafter as hatch distance) between the scan paths, point distance between points along the scan path and exposure time for each point (and optionally delay time between the point exposures) and / or spot size (or focal distance).

[0036] According to a second aspect of the invention there is provided a powder bed fusion additive manufacturing apparatus comprising at least one scanner for scanning an energy beam across layers of a powder bed and a controller arranged to control the at least one scanner to carry out the method according to the first aspect of the invention.According to a third aspect of the invention there is provided a data carrier having instructions stored thereon, wherein the instructions, when executed by a controller of a powder bed fusion additive manufacturing apparatus comprising at least one scanner for scanning an energy beam across powder layers of a powder bed, cause the controller to control the powder bed fusion additive manufacturing apparatus to build the object using a powder bed fusion method in accordance with the first aspect of the invention.

[0037] According to a fourth aspect of the invention there is provided a method of generating instructions for a powder bed fusion additive manufacturing apparatus, the method comprising receiving a model of an object and generating instructions for a powder bed fusion additive manufacturing apparatus, which, when executed, cause the powder bed fusion apparatus to build the object using a powder bed fusion method in accordance with the first aspect of the invention.

[0038] According to a fifth aspect of the invention there is provided a method of generating instructions for a powder bed fusion additive manufacturing apparatus, the method comprising receiving a geometric model of an object, determining from the geometric model of the object layers of the object to be consolidated during powder bed using a shell and core method based on a width of the layer in a hatch formation direction, dividing the layers identified as to be formed using a shell and core method into at least one shell region and a core region and generating instructions for a powder bed fusion additive manufacturing apparatus based on the determined layers and the shell and the core regions.

[0039] The shell and core method may only be used for a layer if the width of the layer in a hatch formation direction meets specified requirements, for example if the width is enough such that a core region can be formed having an inner core region width above a threshold core region width. The shell and core method may only be used for a layer if the widths of the layer in all direction meets specified requirements, for example if all widths are enough such that a core region can be formed having an inner core region width in all direction above a threshold core region width. Thismay mitigate overheating of a layer due to insufficiently long hatch lines if the shell and core method was used for the layer.

[0040] According to a sixth aspect of the invention there is provided a powder bed fusion additive manufacturing method for building an object, the method comprising receiving irradiating powder form the object in layers, at least one of the layers consolidated using a shell and core method comprising at least one shell region and a core region, an inner core region width being a width of the core region bounded by an inner boundary of the shell region, wherein no inner core width in a direction of hatch lines is less than the minimum hatch length. The minimum hatch length may be at least 0.5mm and preferably at least 0.8mm.

[0041] According to a seventh aspect of the invention there is provided a method of generating instructions for a powder bed fusion additive manufacturing apparatus, the method comprising receiving a geometric model of an object, determining from the geometric model of the object layers of the object to be consolidated during powder bed using a shell and core method, dividing the layers identified as to be formed using a shell and core method into at least one shell region and a core region, the core region configured such that no straight line between boundary points of the layer that passes through a point within the core region has a length below a threshold length, and generating instructions for a powder bed fusion additive manufacturing apparatus based on the determined layers and the shell and the core regions.

[0042] This may ensure that a high energy density core scan parameter set used for core regions is not applied to thin sections of the layer. Use of core scan parameters in thin sections of a layer may increase cracking. Thus, not using core scan parameters in these sections may prevent such an increase in cracks.

[0043] Configuring of the core region may comprise determining an initial core region, for example based on an offset from a boundary of the layer and then modifying the initial core region to remove points for which a straight line between boundarypoints of the layer passes having a length below a threshold length.

[0044] The instructions may be recorded on a data carrier for transmission to a powder bed fusion additive manufacturing apparatus.

[0045] According to a eighth aspect of the invention there is provided a data carrier having instructions stored thereon, wherein the instructions, when executed by a processor, cause the processor to carry out the method of the fourth, fifth and / or seventh aspect(s) of the invention.

[0046] The data carrier may be a suitable medium for providing a machine with instructions such as non-transient data carrier, 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, or a transient data carrier, such as a signal on a wire or fibre optic or a wireless signal, for example signals sent over a wired or wireless network (such as an Internet download, an FTP transfer, or the like).

[0047] A further aspect of the invention provides a powder bed fusion additive manufacturing method for building an object in accordance with instructions generated by any of the above-described aspects of the invention.

[0048] Description of the Drawings

[0049] Figure l is a schematic view of a powder bed fusion additive manufacturing apparatus according to an embodiment of the invention;

[0050] Figure 2 is a cross-sectional view of an object built using a shell and core building strategy;

[0051] Figure 3 is a flowchart illustrating a method according to one embodimentof the invention;

[0052] Figure 4 is a graph showing how optical density decreases and crack length density increases as the first- and second-layer thicknesses increase;

[0053] Figure 5 is a table showing laser parameters used to build parts from CM247 LC;

[0054] Figure 6 is a cross-sectional view of an object built using a shell and core building strategy according to another embodiment of the invention;

[0055] Figures 7 and 8 illustrate a modification to the shape of the core region based on a minimum hatch line length for the core region;

[0056] Figures 9 and 10 provide examples of modifications to the shape of a core region based on a minimum width of a section including the core region;

[0057] Figures Ila and 11b illustrate a multi-step process in determining a core region; and

[0058] Figure 12 illustrates a shell and core process in which hatch lines cross a boundary region between the shell and core regions;

[0059] Figure 13 is a table illustrating nominal weight percentage for CM247LC as set out in Harris, K., Erickson, G. L. & Schwer, R. E., 1984. MAR-M247 derivations - CM247 LC DS alloy, CMSX single crystal alloys properties and performance. 5th Int. Symp., pp. 221-230 and actual weight percentage from measurement;

[0060] Figure 14 is an illustration of aluminium and titanium percentage content for different alloys provided in C. Guo, G. Li, S. Li, X. Hu, H. Lu, X. Li, Z. Xu, Y. Chen, Q. Li, J. Lu, and Q. Zhu, "Additive manufacturing of Ni-basedsuperalloys: Residual stress, mechanisms of crack formation and strategies for crack inhibition," Nano Materials Science 5(1), 53-77 (2023);

[0061] Figures 15a and 15b are graphs of crack length density versus layer thickness for data obtained in-house and from the literature for both CM247LC and IN738;

[0062] Figure 16 is a flowchart illustrating the process of fabricating CM247LC samples;

[0063] Figure 17(a, b, c and d) are images of CM247LC powder particles obtained using a scanning electron microscope (SEM);

[0064] Figure 18 is a table of powder size distribution measured by a Malvern Mastersizer 3000;

[0065] Figure 19 is an image of notched geometry used for samples;

[0066] Figure 20 is a graph showing laser power and optical density versus scan speed for different sections of the notched geometry;

[0067] Figure 21 are images of different sections of the notched geometry sample at different magnifications and in different planes;

[0068] Figure 22 is a graph of crack length density for different sections of the notched geometry sample;

[0069] Figure 23 is a graph of optical density and crack length density versus 2D energy density;

[0070] Figure 24 is an image illustrating an electron backscatter diffraction (EBSD) grain analysis for a sample;Figure 25 is an EBSD grain analysis for CM247LC samples built using a 10 pm layer thickness and a 20 pm layer thickness;

[0071] Figure 26 shows images of a CM247LC sample having a shell built with a 10 pm layer thickness and a core built using a 20 pm layer thickness;

[0072] Figure 27 is a graph of optical density and crack length density versus hatch rotation for the shell and core samples;

[0073] Figure 28 (a, b, c, d, e, and f) are scanning electron microscope (SEM) images of the shell and core samples;

[0074] Figure 29 are scanning electron microscope (SEM) micrographs of shell and core samples after hot iso-static pressing (HIP);

[0075] Figure 30 illustrate the results of energy-dispersive X-ray spectroscopy (EDX) line scans carried out on the EUPed shell and core samples;

[0076] Figure 31a is a graph of tensile stress versus elongation for samples of CM247LC built using a 10 pm layer thickness, tested in an as-built state (sample 1) or after being HIPped (samples 2 and 3), Figure 31b is a graph of tensile stress versus elongation for samples of CM247LC built using a 20 pm layer thickness, tested in an as-built state (samples 1 and 2) or after being HIPped (samples 3 and 4) and Figure 31c is a graph of tensile stress versus elongation for a sample of CM247LC built using the shell and core strategy, tested in an as-built state (sample 1) or after being HIPped (samples 3 and 4);

[0077] Figure 32 shows images of as-built CM247LC samples built using a 20 pm layer thickness, a 10 pm layer thickness and the shell and core strategy;Figure 33 is an image showing a stripe intersection from a tensile build file; and

[0078] Figure 34 shows images of HIPed CM247LC samples built using a 20 pm layer thickness, a 10 pm layer thickness and the shell and core strategy.

[0079] Description of Embodiments

[0080] Referring to Figure 1, a powder bed fusion additive manufacturing apparatus according to an embodiment of the invention comprises a build chamber 101 sealable from the external environment such that an inert atmosphere (in this embodiment, argon) can be maintained therein. Within the build chamber 101 are partitions 115, 116 that define a build sleeve 117. A build platform 102 is lowerable in the build sleeve 117. The build platform 102 supports a powder bed 104 and workpiece (part) 103 as the workpiece is built by selective laser melting of the powder. The platform 102 is lowered within the build sleeve 117 under the control of a drive (not shown) as successive layers of the workpiece 103 are formed.

[0081] Layers of powder 104 are formed as the workpiece 103 is built by a layer formation device, in this embodiment a dispensing apparatus and a wiper (not shown). For example, the dispensing apparatus may be apparatus as described in W02010 / 007396 or W020026 / 041849. The dispensing apparatus dispenses powder onto an upper surface defined by partition 115 and is spread across the powder bed by the wiper. A position of a lower edge of the wiper defines a working plane 190 at which powder is consolidated. A build direction is perpendicular to the working plane 190.

[0082] A plurality of laser modules 105a, 105c generate laser beams 118a, 118c, for melting the powder 104, the laser beams 118a, 118c directed as required by a corresponding optical module (scanner) 106a, 106c. The laser beams 118a, 118c, enter through a common laser window 107. Each optical module comprises steering optics 121, such as two mirrors mounted on galvanometers, for steering the laserbeam 118 in perpendicular directions across the working plane and focussing optics 120, such as two movable lenses for changing the focus of the corresponding laser beam 118. The scanner is controlled such that the focal position of the laser beam 118 remains in the working plane 190 as the laser beam 118 is moved across the working plane. Rather than maintaining the focal position of the laser beam in a plane using dynamic focusing elements, an f-theta lens may be used.

[0083] An inlet and outlet (not shown) are arranged for generating a gas flow across the powder bed formed on the build platform 102. The inlet and outlet are arranged to produce a laminar flow having a flow direction from the inlet to the outlet. Gas is re-circulated from the outlet to the inlet through a gas recirculation loop (not shown).

[0084] A controller 140, comprising processor 161 and memory 162, is in communication with modules of the additive manufacturing apparatus, namely the laser modules 105a, 105b, 105c, 105d, optical modules 106a, 106b, 106c, 106d, build platform 102, dispensing apparatus 108 and wiper 109. The controller 140 controls the modules based upon software stored in memory 162 as described below.

[0085] In use, a computer receives a geometric model, such as an STL file, describing a three-dimensional object to be built using the powder bed fusion additive manufacturing apparatus. The computer slices the geometric model into a plurality of slices to be built as layers in the powder bed fusion additive manufacturing apparatus based upon a defined layer thickness.

[0086] The computer may comprise an interface arranged to provide a user input for selecting the material from which the obj ect is to be built. The computer then selects exposure parameters and build strategies from a database that are suitable for the identified material. A laser exposure pattern is then determined for melting areas of each layer to form the corresponding cross-section (slice) of the object. Based upon these calculations, the computer generates instructions that are sent to controller 140 to cause the additive manufacturing apparatus to carry out a build inaccordance with a desired exposure strategy. For nickel-based superalloys, such as CM247 LC, the following build strategy is used.

[0087] Referring to Figures 2 and 3, in a first step 301, an object is built from a nickel superalloy, in this embodiment CM247 LC, using laser powder bed fusion. The laser beams 118a, 118c melt the powder to form the obj ect 103. The dotted lines in Figure 2 represent the powder layers that are formed.

[0088] The object 103 is built using a shell and core build strategy. In such a strategy, an object is divided in slices / areas to the consolidated to form layers of the object. The slices / areas may have a defined thickness corresponding to an intended layer thickness of each core region 201, 202, 203. Each slice / area to be consolidated in each layer is divided into a core region 201, 202, 203 and shell regions 204’, 204”, 205’, 205”, 206’, 206”. The shell regions 204’, 204”, 205’, 205”, 206’, 206” have a first thickness Li and the core regions 201, 202, 203 have a second thickness L2 that is a multiple of Li, in this embodiment twice Li. Each shell region 204’, 204”, 205’, 205”, 206’, 206” extends to a periphery of the slice / area and overlaps with the corresponding core region 201, 202, 203 formed by melting a common powder layer (indicated by the dotted lines) to form intersection regions 210 (only one of which is labelled). In this embodiment, the intersection region 210 has a width of 100 pm, however, the width of the intersection regions may be different such as a width between 50pm and 200pm. Each core region 201, 202, 203 overlaps with a number of shell regions 204’ and 204”, 205’ and 205”, 206’ and 206” corresponding to how many multiples L2 is bigger than Li. The shell region will typically have a width of 1 or a few millimetres.

[0089] In this embodiment, both the shell and core regions are melted by the laser beam progressing along a set of parallel hatch lines arranged in a pattern, such as a meander, chequerboard or stripe scan pattern. The scan pattern for the shell region may be the same or different to the core region. The hatch spacing (hatch distance) may between ten or two hundred micrometres. A direction of the hatch lines is rotated between each layer of the shell region 204’, 204”, 205’, 205”, 206’, 206”and the core region 201, 202, 203. As there are more layers for the shell region 204’, 204”, 205’, 205”, 206’, 206” than the core region 201, 202, 203, the hatch line direction for the shell regions 204”, 205”, 206” and core region 201, 202, 203 consolidated by exposure of the same powder layer may be different. In this embodiment, the rotation angle is 67° or 90°.

[0090] In another embodiment, the shell is consolidated using a plurality of contour scans that follow a contour of the slice / area.

[0091] Laser parameters used for embodiments using stripes are shown in Figure 5. The layer thickness as set out in Figure 5 is a distance the build platform 102 is lowered between the formation of each shell region and each core region. The thickness of powder melted by the laser beam will be thicker than the distance the build platform 102 is lowered because of shrinkage of the material during melting and solidification compared to the powder thickness before melting.

[0092] The shell and core regions may be melted using a laser beam operating in modulated mode wherein the laser beam is hopped between exposure points, the laser beam switched off or reduced in power during each hop, or in continuous mode wherein the laser beam is scanned continuously along each hatch line. In Renishaw’s RenAM 500 E machine, continuous mode is achieved by reducing the exposure time and point distance to small enough values such that the laser beam is effectively not switched off. For CM 247 LC, higher optical density and lower crack length density has been achieved for shell / core layer thicknesses of 15 / 30pm, 13 / 26 pm and 10 / 20pm in continuous mode compared to modulated mode.

[0093] After the object has been built using powder bed fusion, the object is transferred to a furnace for hot isostatic pressing (HIP) 302. In this embodiment the parameters used for how isostatic pressing are:

[0094] Temperature: 1240°C

[0095] Pressure: 140 MPaHeating and cooling rate: 5°C / min

[0096] Duration: 4 hrs

[0097] It will be understood that other HIP parameters could be used that achieve the purpose of closing the cracks in the object. HIP reduces internal defects, lack of porosity and micro-cracks by compressing the material and allowing atoms to diffuse and fill voids. The y’ phase strengthening in CM247LC, does not directly diffuse to fill voids. Instead, the diffusion of alloying elements such as nickel, aluminium, and other elements helps reform and stabilise the y' phase. Additionally, soak cycle time (duration) allows sufficient diffusion of alloying element and the slow cooling minimises thermal stresses and aids to form uniform microstructure and gradual y' phase precipitation. The temperature may be between 1240°C and 1260°C and the pressure between 140 MPa and 150MPa.

[0098] Referring to Figure 4, an improvement in optical density and crack length density is achieved as the layer thicknesses for the core regions and shell regions are reduced. However, such reduction in layer thicknesses results in a consequential increase in the build time. Accordingly, the selected layer thicknesses should be balanced against the fixing of any cracks achievable using HIP. It is believed that good results can be achieved using layer thicknesses of 10pm for the shell regions and 20pm for the core regions.

[0099] Example 1

[0100] Figures 13 to 34 is an example of experiments carried out in processing CM247 LC with a powder bed fusion apparatus.

[0101] The Nickle Superalloy CM247LC data shown in Figures 20 to 34 was fabricated using a Renishaw 500E machine coupled with a Reduced Build Volume (RBV).

[0102] In a 500E:

[0103] A laser can address the entire bed.• Improved gas flow across the bed ensures the efficient removal of process emissions and extends filter life.

[0104] Additional information:

[0105] • A 500E machine has a build platform of 250 mm x 250mm x 300 mm, but in this work a RBV with a melting area of 80 mm x 80 mm x 64 mm was employed.

[0106] • vacuum cycle prior to melting was used.

[0107] • Laser was used in modulated and continuous mode.

[0108] Introduction to Nickel Superalloy CM247LC

[0109] • is a gamma-prime (y') strengthened nickel base superalloy. It is a refined version of MAR M 247 cast alloy, which was designed for Directional Solidification casting.

[0110] • In comparison to MAR M 247, CM247LC was developed to reduce grain boundary cracking and improve carbide microstructure with tighter elemental control.

[0111] • Due to higher y’, Ni3 (Al / Ti), it is very difficult to process.

[0112] • CM247LC is commonly used for high-temperature components like turbine blades and vanes due to its high temperature creep properties and oxidation resistance.

[0113] Literature data of CM247LC and Inconel 738

[0114] Researchers have studied layer thicknesses and crack length density. This study considered 20 pm and 10 pm layer thickness processing. Figures 15a and 15b illustrate data on crack length density and layer thickness obtained from literature and in-house experiments. The references are:

[0115] Crack length density A: Ravi G. A., Kemakolam N.C, and Pickard L.K., Additive Manufacturing of gamma prime and gamma double prime Nickel Super alloys by Renishaw Laser Melting Machines: Renishaw Pic, UK.Crack length density _B: R. Engeli, Selective Laser Melting & Heat Treatment of y’ Strengthened Ni-Base Superalloys for High Temperature Applications: Doctoral Thesis (ETH Zurich) (2017).

[0116] Crack length density _C: J. Risse, Jeroen Risse Additive Manufacturing of Nickel -Base Superalloy IN738LC by Laser Powder Bed Fusion: (Doctoral Dissertation, Rwthaachen University) (2019).

[0117] Processing of CM247LC

[0118] Referring to Figure 16, all CM247LC samples were fabricated using a reduced build volume (RBV). The hatch styles were limited to stripes. For initial builds, layer thickness of 20 pm was selected. To minimise the cracks further, 10 pm layer thickness was considered. As 10 pm layer thickness processing increases the build time, a 10 pm shell and 20 pm core style strategy was adopted. To identify the best process parameters, LPBF samples were characterised for optical density, crack length density and tensile properties.

[0119] CM247LC Powder Analysis

[0120] CM247LC powder particles were analysed using a JEOL 6010LA scanning electron microscope (SEM). Apart from few irregular particles, the majority were spherical (Figure 17 a and b). The cross sectioned SEM micrographs of polished powder particles displayed fine equiaxed grain structure, with carbides and precipitates decorating the grain boundaries (Figure 17 c and d). The powder size distribution, measured by a Malvern Mastersizer 3000 showed that the powder particles are in the 22 pm to 54 pm size range (Figure 18).

[0121] 20 pm Layer Thickness Processing of CM247LC (Effect of Power and Speed) A combination of samples was fabricated for few laser powers and scan speeds. To see the sensitivity of crack on varying profile, a notched geometry was considered (Figure 19). For which crack length density was measured in the bottom section (inward growing, a) and top section (outward growing, b) and, in the plane freefrom notch geometry (c). Reference is made to Figure 20.

[0122] 20 jim Layer Thickness Processing - CM247LC (Effect of Profile)

[0123] For a chosen parameter, crack length density varied within the geometry due to variation in heat dissipation. For example, in location c, where heat dissipation is not changing, the smallest crack length density of 0.26 mm / mm2 was observed. In contrast, in location a (inward growing) and in location b (outward growing), an increase of 2x crack length density was observed. This increase was due to the changes in the geometry profile. Reference is made to Figures 21 and 22.

[0124] 10 pm Layer Processing of CM247LC

[0125] To reduce the cracks further, the layer thickness was reduced from 20 pm to 10 pm. For the 10 pm layer, a narrow energy density range of 1.8 J / mm2 and 2.1 J / mm2 was used. For which a minimum optical density of 99.9% was obtained. Referring to Figures 23 and 24, the sample fabricated with 2.0 J / mm2 exhibited fewer cracks in both core and border sections, as shown in image (b). In comparison to the crack length density reported for the 20 pm layer along plane c in Figure 21, the 10 pm layer exhibited a 50% reduction in crack length density. The EBSD grain analysis shown higher number of epitaxial grains for 10 pm layer thickness compared to the 20 pm layer thickness.

[0126] Laser Power X Exposure

[0127] 2D ED = - - - Point Distance X Hatch Distance

[0128] EBSD Data of 10 pm and 20 pm Layer CM247LC

[0129] EBSD data for 10 pm and 20 pm layer thickness samples are compared in Figure 25. The 10 pm layer sample exhibited longer epitaxial grains compared to the 20 pm layer sample due to increased remelting. For both layer thicknesses, a stronger texture was observed in the build direction. To make good assessment of variations in these layer thicknesses, it would be preferrable to generate the texture data at multiple locations.10 jim and 20 pin Shell and Core Layer Processing of CM247LC

[0130] Since the 10 m layer thickness is slower, a shell and core style strategy was considered to improve the respective build rate. With this arrangement, the goal was to balance build rate and the crack length density to an acceptable limit. The experimental data supported this theory. No variation in optical densities was observed between the standard melting and shell and core builds. Crack length density data generated for the shell and core samples indicated that a selection of hatch rotation is also important. Reference is made to Figures 26 and 27.

[0131] Characterisation of As-built Shell and Core Processed CM247LC

[0132] 10 pm shell and 20 pm core samples were characterised under SEM for cracks. The XZ plane samples (Figure 28 a, b and c) showed fine columnar grain structure aligned in the build direction. Since samples were in the as-built state, melt pool boundaries were visible. In both XY and XZ orientation, predominantly solidification cracks were observed. They were jagged in nature with visible dendrites (red arrows) in them. Some grain boundary cracks had smooth clean edges (orange arrows). High density inclusion like hafnium-based carbide was observed in the as-built samples. XZ samples were etched using Kalling 2 etchant.

[0133] Hot Iso-static Pressing of Shell and Core Processed CM247LC

[0134] To minimise cracks, Hot Iso-static pressing (HIP) was considered for shell and core CM247LC samples. For a chosen process parameter, post-HIP SEM micrographs (Figure 29) showed very few cracks in the shell area. The few cracks that were visible had smooth crack profile, indicating ductility dip cracking. This highlighted the limitation of HIPing in crack healing. Post HIPing, grain size increased, and grain boundaries were heavily decorated with y’ and carbides.

[0135] Grain Boundary Analysis of HIPed Shell and Core CM247LC

[0136] Referring to Figure 30, EDX line scans were performed on the HIPed samples, at grain boundaries and at the crack. Crack free grain boundaries exhibited Cr-rich carbides (a), whereas analysis over a crack showed Hf-rich precipitates (b). AlongXY plane, randomly distributed globular Hf and Ta-rich precipitates were observed in the grain boundaries.

[0137] Room Temperature Tensile Properties Comparison of CM247LC

[0138] For the 10 pm layer and 20 pm layer thicknesses, tensile data was generated individually as well as in the integrated mode, shell and core. Since a reduced build volume (RBV) was used in this study, only a small number of tensile samples were built for each condition. These samples were tested in the as-built and HIPed state. In all three scenarios, post HIP, yield and tensile strength was lower because no solution treatment and ageing was not performed. For the shell and core sample, higher ductility was observed in the post HIP state, this is very likely due to the reduction in defects like cracks and an improvement in packing density between exposures. Reference is made to Figures 3 la, 3 lb and 31c.

[0139] Fracture Surface Analysis of As-built CM247LC Samples

[0140] Fewer cracks were observed for 10 pm layer samples. The fractured surface show dendritic or cell like features, characteristics of hot cracks. The defect running through the middle of the shell and core section aligns with the stripe intersection region, initiated at the edge and extending along the potential defect line. Reference is made to Figures 32 and 33.

[0141] Fracture Surface Analysis of HIPed CM247LC Samples

[0142] Like the polished samples, HIPed fractured surfaces also exhibited fewer cracks. Higher magnification images of the fractured surface showed dimple features, indicative of ductile fracture. Reference is made to Figure 34.

[0143] Conclusions

[0144] In this study, following trends were identified:

[0145] • Laser power and speed had little effect, in the chosen test range.

[0146] • Reduced cracking was observed with lower layer thicknesses, highlighting the importance of layer thickness.Varying degrees of cracks within the notch sample highlighted the importance of the geometry profile.

[0147] Shell and Core benefits:

[0148] • A small increase in the build rate was noticed.

[0149] • A minor reduction in defects like porosity and cracks were observed in the as-built samples.

[0150] • HIPing reduced the crack further.

[0151] • The mechanical properties of as-built and HIPed 10 pm and 20 pm layer samples data align with the literature. Whereas shell and core ductility data showed the effect of strategy.

[0152] As can be seen from Figures 3 la, 3 lb and 31c, carrying out a HIP process on a part manufactured using 10pm and 20pm layer thicknesses throughout the part reduced a percentage of elongation at break. However, carrying out a HIP process on a part manufactured using 10pm layer thicknesses for a shell region and 20pm layer thicknesses for a core region resulted in an improvement in elongation at break.

[0153] Referring to Figure 6, in a further embodiment, the shell and core method is modified such that a core region 601, 602, 602 is formed only if an inner core region width ICW in a direction of the hatch lines for the core region is at least a minimum hatch line length, in this embodiment at least 800pm. The inner core region width ICW is a width of core region excluding the intersection region 610. If a width of a layer to be formed into the object is insufficient to form a shell region and a core region wherein an inner core region width ICW in a direction of the hatch lines is at least the minimum hatch line length then the shell and core method is not used for that layer or at least not in that section of the layer. This is illustrated in Figure 6 by layers 607’ and 607”. Layers 607’, 607” may have a width W in a direction of the hatch lines for the core region that is less than a sum of the shell width and the minimum hatch line length. In this embodiment, the sum of the shell width (1mm) and the minimum hatch line length (0.8mm) is 1 ,8mm. The entire layer 607’, 607’ ’ may be formed with the first layer thicknessused for shell regions. All hatch lines for layers 607’, 607” may be formed using the scan parameters used for shell regions.

[0154] Referring to Figure 7, a layer having a rectangular shape is shown. As illustrated, a geometric shape of an initial core region 701 and a direction of the hatch lines can result in hatch lines 711, 712, 713 of different lengths. In this embodiment, hatch lines 712 and 713 have a length less than the minimum hatch line length, whilst other hatch lines, such as hatch line 711, have a length equal to or greater than the minimum hatch line length. In Figure 7 the scan paths (hatch lines or contour paths) of the shell region 704 are not shown for clarity. In such a scenario, a shape of the initial core region 701 may be modified to that shown in Figure 8 to provide a modified core region 701b. The modified core region 701b is modified to remove section(s) having a width in the hatch line direction less than the minimum hatch line length whilst retaining sections having a width in the hatch line direction equal to or greater than the minimum hatch line length. This may result in a modified core region 701b having a different outer shape (not just size) to an outer shape of the shell region 704’. The change in shape of the modified core region 701b extends the shell region 704’ to the areas of the layer occupied by the initial core region 701 before modification. In this way, hatch lines in the core region below a minimum length are avoided.

[0155] The modification will depend on the hatch line direction and therefore, may differ for different layers as the hatch line direction changes, even for the same initial shape for the core region. An example of this is illustrated in Figures 10, Ila and 1 lb, wherein a modified core region 901a is further modified differently depending on the hatch direction, as shown by the core regions 901bi and 901b2 in Figures Ila and 1 lb. This also results in differently shaped shell regions 904bi and 904b2.

[0156] Referring to Figure 9 and 10, in a further embodiment, a core region is only used for a section of the layer if the core region 801a, 801a’, 901a can be configured such that no straight line between boundary points of the layer that passes through thepoint within the core region 801a, 801a’, 901a has a length below a threshold length W. The intersecting region of the core region and the shell region is not shown in Figures 9 and 10 for clarity. The layer illustrated in Figure 9 has a central constriction having a width that is less than the threshold length T. As a result, no core region is provided in this constricted section of the layer. In this example, this results in two separate core regions 801a and 801a’. This requirement can also result in modification to a shape of the core region(s) from corners of the layer, as illustrated in Figure 10. In this case, the triangular layer comprises corners such that straight lines of the threshold length T extending between boundary points on the sides that meet to form the corner would pass through an initial core region defined simply by offsetting the boundary of the initial core region from the boundary of the layer. This initial core region is further modified to remove sections of the initial core region including points that are intersected by such straight lines. This modified core region 901a is illustrated in Figure 10, wherein corners of the otherwise triangular initial core region have been removed, in this embodiment rounded, to remove such points. Some the straight lines of the threshold length T are illustrated to help understand the resultant shape of the modified core region 901a. Determination of modified core region 901 is independent of hatch line direction. This modified core region 901a may then be modified further based on a hatch line direction, as described above and illustrated in Figures Ila and 1 lb.

[0157] To consolidate powder to form the layer, first the scan paths (not shown) of the shell region are irradiated using the laser beam(s) and then the scan paths (hatch lines) of the core region 901bi, 901b2 are irradiated using the laser beam(s). This order for scanning the region is preferred because, at the time of scanning the core region, a good thermal connection to underlying solid material is likely to be provided by the shell region 904b, reducing the chance of an undesirable build-up of heat, which could result in unwanted solidification cracking. For alternate layers having no core region scan paths, just an area corresponding to the shell region is scanned to consolidate powder of the first (thinner) layer thickness.

[0158] The modification of a shape of the initial core region 801, 901 to a modified coreregion 801a, 801a’, 901a, 901bi, 901b2 may ensure that heat does not build-up in regions of the layer that would otherwise increase the number of cracks. For the modified core regions 801a, 801a’, 901a, the modification prevents the high energy density core scan parameter set being used in thin sections of the layer. For the modified core regions 901bi, 901b2, the modification prevents hatch lengths below a minimum hatch length that would otherwise result in a build-up of heat because the laser beam returns too quickly to a previously scanned region (with the consolidated material of adjacent hatch lines overlapping).

[0159] In another embodiment, the modified core region 901 is not further modified based on hatch line direction. An example of such an embodiment is shown in Figure 12. In this embodiment, rather than determining hatch lines separately for each of the shell and the core regions, hatch lines 1011 are determined that extend across both the shell region 1004 and core region 1001. Shell scan parameters are used for scanning the laser beam across the shell region 1004 and core scan parameters are used for scanning the laser beam across the core region 1001. A boundary region 1010 is provided between the shell region 1004 and core region 1001 in which the scan parameters progressively change from the shell scan parameters to core scan parameters or vice versa. “Progressively change” will be understood to mean that there is a planned increase or decrease of the value for at least one of the scan parameters across a plurality of irradiation points of the boundary region that is more than merely driven by limited dynamics of the scanning system. In this sense, the progressive change is gradual compared to a maximum rate at which the scan parameter can be changed. In one arrangement, the irradiation of each hatch line is defined by a point distance between successively irradiated points, an exposure time for each point, a delay time between the exposure of successive points and a laser beam power. Whether the apparatus can achieve the defined irradiation parameters will depend on the dynamics of the scanner and the laser, but, within the boundary region 1010, the scan parameters are set to change progressively across a plurality of irradiation points.

[0160] Some hatch lines may enter and exit the boundary region 1010 from one of the shellregion 1004 and core region 1001 without entering the other of the core region 1004 and shell region 1001. The scan parameters for these hatch lines may still be altered within the boundary region 1010, for example depending on a proximity to the other of the core region 1004 and shell region 1001.

[0161] In one embodiment, the same shell scan parameters are used for each irradiation point in the shell region 1004 and / or the same core scan parameters are used for each irradiation point in the core region 1001. The core scan parameters may comprise at least a higher laser power and faster scan velocity than the shell scan parameters. However, in another embedment, the shell scan parameters and / or the core scan parameters may be changed dynamically, for example based on an underlying thickness of consolidated material. Examples of such methods are disclosed in WO2022 / 123224 and unpublished application PCT / GB2025 / 052038. However, even with such dynamic shell and / or core scan parameters, the scan parameters are still changed across the boundary region to transition between scan parameters appropriate for the powder layer thicknesses of the shell and core regions respectively.

[0162] Modifications and alterations may be made to the above-described embodiments without departing from the invention as defined herein. For example, rather than removing sections of the initial core region that result in hatch line lengths below the minimum hatch line length, a delay may be introduced between the irradiation of successive (adjacent) hatch lines to allow heat to dissipate rather than build-up to levels that could cause an unacceptable number of cracks.

Claims

CLAIMS1. A method of manufacturing an object comprising:- building the object using a powder bed fusion method, wherein an object is formed in layers by selectively solidifying powder with at least one energy beam and the powder is a nickel-based superalloy, the powder bed fusion method comprising consolidating powder to form layers of the object using a shell and core method, wherein each layer is divided into at least one shell region and a core region, andhot isostatic pressing (HIP) the object built using the powder bed fusion method.

2. A method of manufacturing an object according to claim 1, wherein the at least one shell region and the core region overlap to form an intersection region in the object.

3. A method of manufacturing an object according to claim 2, wherein the intersection region has a width of less than 500pm.

4. A method of manufacturing an object according to claim 3, wherein the intersection region has a width of 200pm or less.

5. A method of manufacturing an object according to any one of the preceding claims, wherein the shell regions are formed by consolidating powder having a first layer thickness and the core regions are formed by consolidating powder having a second layer thickness, the first layer thickness being thinner than the second layer thickness.

6. A method of manufacturing an obj ect according to claim 5, wherein the first layer thickness is achieved by lowering a build platform by a first distance, the first distance being less than 20pm.

7. A method of manufacturing an obj ect according to claim 6, wherein the first distance is less than 15pm.

8. A method of manufacturing an object according to claim 7, wherein the first distance is 10pm or less.

9. A method of manufacturing an object according to any one of claims 6 to 8, wherein the first distance is more than 5pm.

10. A method of manufacturing an object according to any one of claims 5 to 9, wherein the second layer thickness is achieved after lowering a build platform by a second distance that is a multiple of the first distance.

11. A method of manufacturing an object according to claim 10, wherein the multiple is between 2 and 12 times the first distance.

12. A method of manufacturing an object according to claim 11, wherein the multiple is between 2 and 6 times the first distance.

13. A method of manufacturing an object according to claim 12, wherein the multiple is 2 or 3 times the first distance.

14. A method of manufacturing an object according to any one of claims 5 to 13, wherein the second distance is less than 120pm.

15. A method of manufacturing an object according to claim 14, wherein the second distance is less than 60pm.

16. A method of manufacturing an object according to claim 15, wherein the second distance is less than 40pm.

17. A method of manufacturing an object according to any one of the preceding claims, wherein the powder bed fusion method comprises using a shell scan parameter set for the shell regions that is different to a core scan parameter set usedfor the core regions.

18. A method of manufacturing an object according to any one of the preceding claims, wherein the nickel-based superalloy contains 7.5-12% W, 6-23% Cr, 3-8% Al and 8-12% Co.

19. A method of manufacturing an object according to any one of the preceding claims, wherein a chemical composition of the nickel-based alloy by weight % comprises: 9.3-9.7W, 9.0-9.5Co, 7.5-8.5Cr, 5.4-5.7A1, 3.1-3.3Ta, 1.4-1.6Hf, 0.6-0.9Ti, 0.4-0.6Mo, 007-.015Zr, 0.01-0.02B with a carbon concentration of around 0.07-0.09wt% and a balance of Ni.

20. A method of manufacturing an object according to any one of the preceding claims, wherein the nickel-based alloy is CM 247 or CM 247 LC.

21. A method of manufacturing an object according to any one of the preceding claims, wherein a hatch line direction on successively consolidated layers is rotated by an angle other than 180°.

22. A method of manufacturing an object according to any one of the preceding claims, wherein the shell region has a width in a plane of the layer greater than a hatch spacing of hatch lines.

23. A method of manufacturing an object according to any one of the preceding claims, wherein the shell region has a width greater than 0.5mm.

24. A method of manufacturing an object according to any one of the preceding claims, wherein the energy beam is scanned continuously along each scan path.

25. A method of manufacturing an object according to any one of the preceding claims, wherein an inner core region width is a width of the core region bounded by an inner boundary of the shell region and each core region is configured such that no core width in a direction of hatch lines is less than a minimum hatch length.

26. A method of manufacturing an object according to claim 25, wherein theminimum hatch length is at least 0.5mm.

27. A method of manufacturing an object according to any one of the preceding claims, wherein each core region is configured such that all points within the core region meet the requirement that no straight line between boundary points of the layer that passes through the point within the core region has a length below a threshold length.

28. A powder bed fusion additive manufacturing apparatus comprising at least one scanner for scanning an energy beam across layers of a powder bed and a controller arranged to control the at least one scanner to carry out the method of powder bed fusion according to any one of claims 1 to 27.

29. A data carrier having instructions stored thereon, wherein the instructions, when executed by a controller of a powder bed fusion additive manufacturing apparatus comprising at least one scanner for scanning an energy beam across layers of a powder bed, cause the controller to control the powder bed fusion additive manufacturing apparatus to build the object using a powder bed fusion method in accordance with any one of claims 1 to 27.

30. A method of generating instructions for an additive manufacturing apparatus, the method comprising receiving a model of an object and generating instructions for a powder bed fusion apparatus, which, when executed, cause the powder bed fusion apparatus to build the object using a powder bed fusion method in accordance with any one of claims 1 to 27.

31. A method of generating instructions for a powder bed fusion apparatus, the method comprising receiving a geometric model of an object, determining from the geometric model of the object layers of the object to be consolidated during powder bed using a shell and core method based on a width of the layer in a hatch formation direction, dividing the layers identified as to be formed using a shell and core method into at least one shell region and a core region and generating instructions for a powder bed fusion apparatus based on the determined layers and shell and thecore regions.

32. A method of generating instructions for a powder bed fusion apparatus according to claim 31, wherein the shell and core method is only used for a layer if the width of layer in a hatch formation direction is enough such that a core region can be formed having an inner core region width above a threshold core region width.

33. A method of generating instructions for a powder bed fusion additive manufacturing apparatus, the method comprising receiving a geometric model of an object, determining from the geometric model of the object layers of the object to be consolidated during powder bed using a shell and core method, dividing the layers identified as to be formed using a shell and core method into at least one shell region and a core region, the core region configured such that no straight line between boundary points of the layer that passes through a point within the core region has a length below a threshold length, and generating instructions for a powder bed fusion additive manufacturing apparatus based on the determined layers and the shell and the core regions.

34. A data carrier having instructions stored thereon, wherein the instructions, when executed by a processor, cause the processor to carry out the method of any one of claims 30 to 33.

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