Powder Bed Fusion Additive Manufacturing Method

Abstract

A powder bed fusion additive manufacturing method in which an object is built in a layer-by-layer manner. The method comprises: for each of a plurality of successively fused layers, melting material of the layer by irradiating the layer with one or more energy beams for a first time using a first set of irradiation parameters, allowing the molten material to solidify to define a fused region of the layer, and reheating the fused region by irradiating the layer with one or more of the energy beams for a subsequent time using a second set of irradiation parameters. The first set of irradiation parameters includes at least one irradiation parameter that is different from the second set of irradiation parameters.

Description

Technical Field

[0001] The present invention relates to a powder bed fusion additive manufacturing method and apparatus, and more particularly to a method and apparatus for forming an object in a layer-by-layer manner by melting powder of a powder bed. Background Art

[0002] The powder bed fusion additive manufacturing method for producing an object comprises: using a high energy beam (such as a laser beam or an electron beam) to solidify a powder (such as a metal powder material) layer by layer. The powder layer is deposited on a powder bed in a build chamber, and the laser beam or electron beam is swept over the portion of the powder layer corresponding to the cross section of the object being constructed. The laser beam or electron beam melts the powder to form a solidified layer. After the selective solidification of the layer, the powder bed is reduced by the thickness of the newly solidified layer, and another layer of powder is spread on the surface as needed and solidified.

[0003] US 5393482 discloses a multiple laser sintering device, comprising a sintering beam having a focus at a powder bed and at least one defocused laser beam incident on a region near the focus of the focused beam. The defocused beam raises the temperature of the material surrounding the sintering beam to a level below the sintering temperature, thereby reducing the temperature gradient between the sintering location and the surrounding material.

[0004] US2013 / 0064706 A1 discloses a scanner unit with a coupling mirror, wherein, via the coupling mirror, a diode laser beam supplied from the outside is introduced into the beam path of an SLM or SLS laser beam. The diode laser beam is coupled into the SLM or SLS laser beam via the coupling mirror so that the diode laser beam hits the powder layer at the same point as the SLM or SLS laser beam and is guided onto the powder layer along the same path. Because the coupling mirror is arranged behind the focusing unit, only the SLM or SLS laser beam is focused, while the diode laser hits the powder bed unfocused around the focus of the SLM or SLS laser beam. As a result, points on the powder bed are preheated by the diode laser beam in a position-selective manner. As an alternative to preheating at the focus of the SLM or SLS laser beam using a diode laser beam, it is also conceivable to focus the diode laser beam at the focus. The focused diode laser beam can then be guided before or after the SLM or SLS laser beam for irradiating (and correspondingly melting or sintering) the powder layer.

[0005] US2011 / 0221099 A1 discloses a device for manufacturing a three-dimensional object, wherein a laser is controlled in two different operating modes by a control unit. The first operating mode results in a first pulse radiation of a laser having a first frequency. The second operating mode results in a second radiation, which is a pulse radiation having a higher frequency than the first pulse radiation or a continuous radiation. First, a first pulse electromagnetic radiation is irradiated onto a first area of ​​a layer of a building material, and then a second pulse radiation or a continuous electromagnetic radiation is irradiated onto a second area of ​​the layer of the building material. The first area and the second area of ​​the layer overlap at least partially, and may overlap completely. A time period is provided between irradiating the first pulse electromagnetic radiation onto the building material and irradiating the second pulse or the continuous electromagnetic radiation onto the building material, during which the building material irradiated by the first pulse electromagnetic radiation is cured.

[0006] WO 2013 / 092994 A1 discloses an apparatus for manufacturing a three-dimensional object by continuously consolidating selected regions of a powder layer layer by layer, including at least partially preheating the powder layer using an electron beam power source, and melting the powder using a laser beam energy source.

[0007] US2015 / 0283612 A1 discloses a three-dimensional molding device including multiple light beams or electron beam scanning devices. In one embodiment, the multiple light beams or electron beam scanning devices are synchronized so that the radiation positions of the multiple light beams or electron beams move along the same current scanning line with a predetermined interval between them. In this way, sintering is gradually promoted, and the thermal shock is small compared with the case of using a high-energy single light beam or electron beam.

[0008] WO 2016 / 079496 A2 discloses an additive manufacturing apparatus, comprising a laser source for generating a plurality of laser beams and a processor, the processor being arranged to control a scanning unit to direct the laser beams to solidify selected areas of a material by successively advancing a plurality of the laser beams along a scanning path. As each of the laser beams passes along the scanning path, the laser beam solidifies spaced apart portions of the scanning path, and one of the laser beams passes along the scanning path to solidify portions of the scanning path between portions solidified by another of the laser beams.

[0009] US2016 / 0236299 A1 discloses a device for manufacturing a three-dimensional object by layer-by-layer consolidation of a powdered building material, comprising a first radiation source (such as a laser), which generates a laser beam, which is directed to an unconsolidated building material layer via a deflection device. A selective heating device is provided, which is formed by a second radiation source together with another deflection device. The second radiation source can generate electromagnetic radiation, that is, the second radiation source can be a laser, or the second radiation source can generate particle radiation (such as electrons).

[0010] US2016 / 0250717 A1 discloses a method for producing a component in layers by laser melting. A molten pool is generated in the powder layer by a working laser beam. Further, an auxiliary laser beam is set to a power density that only slows down the cooling of the material in one zone, but does not cause any remelting.

[0011] US2018 / 0141276 A1 discloses a method for additively manufacturing a three-dimensional object by successively and selectively solidifying layers of a building material layer by layer by at least one energy beam. The heating of the (multiple) building material portions is carried out by at least one heating beam, which can be combined with a main laser beam to achieve a sintering or melting process of the building material, so that the two beams can be directed together and synchronously on the surface portion to be heated or solidified. The focus of the main beam is directed within the diameter of the heating beam, or is directed close to or directly adjacent to the diameter of the heating beam.

[0012] US2013 / 0233846 A1 discloses a method for generative production or repair of at least one region of a component, wherein a zone arranged downstream of a molten pool is post-heated to a post-heating temperature. The powder is heated to the melting temperature in a front zone or first zone by a first laser beam. The rear zone or second zone is heated to the post-heating temperature by a second laser beam.

[0013] US2018 / 0257140 A1 discloses a device for additively producing three-dimensional objects by sequential, layered, selective irradiation and concomitant sequential, layered, selective curing of building materials. The device includes a plurality of irradiation devices, and enables selective "dispatching" of different irradiation devices with different functions based on corresponding control information generated by a control device. At least one irradiation device can be defined as a first irradiation device or operated as a first irradiation device, so that the first irradiation device generates an energy beam for selective curing of a building material layer, and at least one other irradiation device can be defined as a second irradiation device or operated as a second irradiation device, so that the second irradiation device generates an energy beam for thermal pretreatment or post-treatment of the building layer. The energy input of the second energy beam is so low that it is impossible to melt the building material with the second energy beam. The second energy beam can be guided following the first energy beam to influence or control the cooling or curing behavior of the building material.

[0014] US2018 / 0250744 A1 discloses a method for printing a three-dimensional object, comprising (a) converting a first pre-converted material into a first converted material to print a porous hardened material layer. The method further comprises using an energy beam to re-convert the converted material to reduce or eliminate pores. The distance between two adjacent paths of the conversion energy beam (e.g., hatching) can be specified based on the requested porosity level.

[0015] La Metallurgia Italiana, No. 3, 2018, pp. 12-19, L. Wu, T Klaas, S Leuders, F Brenne, T Niendorf, “Impact of Process Conditions on the Properties of Additively Manufactured Tool Steel H13 processed by LBM” discloses that cracks are often observed in parts formed from tool steel H13. These cracks are believed to be solidification cracks, which are mainly formed at the boundaries between unit cell grains of different orientations. Summary of the invention

[0016] According to a first aspect of the present invention, a powder bed fusion additive manufacturing method is provided, wherein an object is built in a layer-by-layer manner, the method comprising: for each layer of a plurality of successively fused layers, melting the material of the layer by irradiating the layer for a first time with one or more energy beams using a first set of irradiation parameters, allowing the molten material to solidify to define a fusion region of the layer, and reheating the fusion region by subsequently irradiating the layer with one or more of the energy beams using a second set of irradiation parameters, wherein the first set of irradiation parameters includes at least one irradiation parameter different from the second set of irradiation parameters.

[0017] It may be difficult to obtain a desired fused microstructure when melting the material in a powder bed fusion additive manufacturing process. For example, the melting of the material may result in thermal conditions within and near the molten pool that lead to undesirable results, such as undesirable microstructures and / or solidification cracks. The amount of energy delivered to a region in a set period of time and the rate at which heat is conducted away from the molten pool may depend on the geometry of the object being built and whether the molten pool is surrounded by powder or fused material. Due to this complex thermal environment, attempts to adjust cooling by adding one or more additional energy beams to achieve a uniform cooling rate are complex, if not impossible. The present invention alleviates these problems by reheating the fused material to refine the microstructure initially formed when the molten material solidifies. Reheating raises the temperature of the fused region to above the temperature at which grain refinement occurs. For example, the temperature may be a tempering temperature at which tempering of the fused material occurs, an annealing temperature (recrystallization temperature) at which annealing of the fused material occurs, a solution heat treatment temperature at which solution heat treatment of the fused material occurs, a sintering temperature at which sintering of the fused material occurs, or a melting temperature at which melting of the fused material occurs. Reheating may raise the temperature of the fusion zone by at least 100° C., 200° C., 300° C., 400° C., and preferably 500° C. Reheating may raise the temperature of the fusion zone to above the martensite temperature of the material (350° C. for H13 tool steel). Reheating of the fusion zone may be performed after the fusion zone has cooled to below 350° C.

[0018] In one embodiment, reheating of the fused region heats the fused material to a temperature above the temperature at which grain refinement occurs, but below the melting temperature of the material, and preferably below the sintering temperature of the material. In this manner, a first irradiation of the layer defines the shape of the fused material, and a subsequent irradiation of the layer refines the grain structure without changing the shape.

[0019] In another embodiment, reheating of the fused region heats the fused material above the sintering temperature or melting temperature.If the melt pool is substantially surrounded by fused material connected to underlying fused material, it may be easier to determine a set of irradiation parameters that achieve a desired melt pool condition / fused microstructure.

[0020] The applicant believes that refining the grain structure by reheating the fused material shortly after the material has melted reduces or eliminates solidification cracks. Solidification cracks are cracks formed in the fused material during the solidification of the molten pool. These cracks are typically formed at grain boundaries. It is believed that refining the grain structure to reduce the number of grain boundaries between columnar grains or epitaxial grains of different orientations will reduce solidification cracks. The present invention can be particularly suitable for metal materials, such as nickel-titanium alloys, nickel-aluminum alloys, nickel-titanium-aluminum alloys, or tool steels. For example, the alloy can be a superalloy, such as Hastelloy, Inconel, Waspaloy, Renealloy, Incoloy, CM247 or CMSX single crystal alloy. The tool steel can be a hot work tool steel, such as H13 or W360 tool steel, or a high-speed steel, such as M2. The alloy can be a titanium aluminum alloy.

[0021] The fused region may include the core of an object formed within a layer using a powder bed fusion additive manufacturing method. It will be understood that the term "core of an object" as used herein refers to a fused region of a layer formed (directly) on the fused material of a previous layer and having fused material formed (directly) thereon by irradiation of the next layer in a shell and core irradiation strategy, and the fused region does not include a volume melted using a boundary scan used to form the shell. Accordingly, the core does not include fused regions forming surface (upskin or downskin) regions of the object. The core of an object is typically formed using a "fill scan pattern", which typically includes a grid pattern in which parallel predefined irradiation paths (also called "hatch lines") are progressively irradiated with an energy beam.

[0022] In addition to the core, the fused region may also include an upper skin region and / or a lower skin region.

[0023] It will be understood that the term "each of a plurality of successively fused layers" refers to each powder layer that is successively fused and reheated by first and subsequent irradiation of the material of the layer, and may correspond to each independently formed powder layer (e.g., a powder layer spread by a powder re-coater), or a powder layer comprising a plurality of independently formed powder layers that are fused together at the same time (i.e., fused together into a thicker powder layer). For example, in a shell and core scanning strategy, it is known that a shell is formed on each independently formed powder layer, but the core is fused by irradiation of an energy beam so that a thicker powder layer comprising a plurality of independently formed powder layers is fused.

[0024] The same layer is (directly) irradiated for the first and subsequent times to melt and reheat the fused area. "Irradiating the layer" as used herein means directing a spot of an energy beam to the exposed surface of the layer before it is covered by a subsequent layer.

[0025] The method may include subsequently reheating the fused region more than once. The method may include subsequently melting the fused material one or more times, wherein the melted material is allowed to solidify between each subsequent melting of the material. Each subsequent reheating of the material may use the same set or a different set of second irradiation parameters.

[0026] The fused region may be the entire cross section of the object to be formed. The entire cross section of the object to be formed may be fused by melting the material of the layer before the entire cross section is reheated. Alternatively, reheating of the fused region may begin before the entire cross section to be solidified is melted. This alternative may shorten the build time.

[0027] The at least one irradiation parameter that is different is a user-definable irradiation parameter of the energy beam and can be the power of the energy beam, the energy beam spot size (set by changing the focal position of the energy beam relative to the powder layer), the energy beam spot shape, the scanning speed of the energy beam continuously scanning on the powder layer or the point spacing and exposure time of the energy beam modulated scanning on the powder bed, the modulation frequency and / or the spacing between adjacent parallel irradiation paths (filling distance). For multiple layers, each of the first irradiation parameter and the second irradiation parameter can be the same.

[0028] The interval between the first and subsequent times (first subsequent times) may be greater than a predetermined minimum delay. Another interval between each subsequent time may be greater than a predetermined minimum delay. The predetermined minimum delay may be at least 250 microseconds, and more preferably at least 500 microseconds. The predetermined minimum delay may allow the molten material to solidify before the fused material is reheated by the subsequent irradiation of one or more energy beams. The interval between the first and subsequent times (first subsequent times) may be less than a predetermined maximum delay. Another interval between each subsequent time may be less than a predetermined maximum delay. The maximum delay may be greater than 2500 microseconds, greater than 3000 microseconds, or greater than 5000 microseconds. In one embodiment, no maximum delay is set for the interval between the first and subsequent times and / or another interval between the subsequent multiple times. The maximum delay may be set so that the desired grain refinement (e.g., remelting) of the material is achieved with the follow-up laser beam, which would not have occurred if a larger delay was used, for example because the energy density of the follow-up laser beam alone is insufficient to cause such grain refinement / remelting.

[0029] The same illumination pattern, for example the same illumination path and / or spot exposure, may be used for the first and subsequent or each subsequent illumination of the layer.

[0030] The irradiation pattern may include a filling scan strategy that includes an arrangement of multiple, typically parallel, predefined irradiation paths. Melting the material of the powder layer may include progressively irradiating the predefined irradiation paths with these energy beams or one of these energy beams. The irradiation paths may be spaced apart from each other so that irradiating these irradiation paths with the one or more energy beams for the first time results in a fusion region without unmelted powder between adjacent irradiation paths. Accordingly, any porosity that appears in the fused material when the fused material is first melted is not caused by incomplete melting of the powder material between the irradiation paths, but is caused, for example, by solidification cracks that appear during solidification of the molten pool. Accordingly, any porosity that appears in the fused region when the fused region is first melted is not "designed" by the spacing of the irradiation paths or the spacing of the exposure points along the irradiation paths (these irradiation paths are selected to achieve a completely fused region), but is an observation of the solidification process. A subsequent reheating of the fused material is performed to close or prevent the formation of porous regions formed by solidification cracks.

[0031] Irradiating the layer for the first time may include progressively irradiating a predefined irradiation path with a first leading energy beam, and subsequently irradiating the fused region once or each subsequent time may include progressively irradiating the predefined irradiation path with a following energy beam. The leading energy beam and (multiple) following energy beams may advance along the irradiation path at the same speed (for example, for continuous or modulated scanning, by setting the scanning speeds of the leading energy beam and the following energy beam to be the same, or for modulated scanning, by setting the point spacing and exposure time of the leading energy beam and the following energy beam so that the same scanning speed is achieved for the two energy beams (preferably, but not necessarily, by setting the leading energy beam and the following energy beam to have the same point spacing and exposure time)).

[0032] The leading energy beam may have a different power than the following energy beam(s). For example, the following energy beam(s) may have a power less than three quarters of the power of the leading energy beam, preferably less than half the power of the leading energy beam. The following energy beam(s) may have a spot size that is the same as or larger than the leading energy beam. The following energy beam(s) may be defocused relative to the leading energy beam. The energy density of the following energy beam at the layer may be less than the energy density of the leading energy beam.

[0033] The second irradiation parameters and / or the spacing between the leading energy beam and the following energy beam can cause the following energy beam to reheat the fused region and the powder surrounding the fused region. For example, the following energy beam can reheat the powder that will be melted by the leading energy beam as the leading energy beam advances along the adjacent irradiation path. This can be beneficial because preheating of the powder can help reduce thermal shock when the powder is melted.

[0034] In another embodiment, the reheating of the fused zone comprises progressively irradiating with the energy beams or one of the energy beams another predefined irradiation path different from the predetermined irradiation path.

[0035] Another illumination pattern may be arranged to form preferential directions for grain formation, such as described in EP19179230.8, which is incorporated herein by reference in its entirety.

[0036] In another embodiment, another irradiation pattern includes a distributed scanning strategy, wherein the portions are irradiated discontinuously and in an order such that the portions irradiated continuously are spaced apart, such as described in WO 2016 / 079496, which is incorporated herein by reference in its entirety. The portions may be irradiated in a conduction or transition mode rather than a keyhole mode to form a molten pool.

[0037] The method may include performing a first scanning strategy (such as a filling scanning strategy) to melt the material for a first time, and performing a different second scanning strategy (such as a distributed scanning strategy) to melt the material one or each subsequent time. The first scanning strategy may result in an undesirable microstructure, but provides a quick way to form a basis for using a different scanning strategy (such as a distributed scanning strategy) to form a desired microstructure.

[0038] The method may include: melting the fused region once or each time thereafter so that the formed molten pool(s) extend shallower than the molten pool(s) formed when the material of the layer was melted for the first time to form the fused region. In this way, melting the fused region once may repair cracks formed in the layer when the material was melted without initiating the formation of other porous regions by high power irradiation that may cause vaporization of the material. For example, the molten pool formed by melting the fused material once or each time thereafter may be formed in a transition mode or a conduction mode.

[0039] 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 by thermal conduction, thereby producing a molten pool that is wider than it is deep. This is in contrast to keyhole mode, in which a hole is formed in the molten pool in which the material is vaporized by exposure to the energy beam. The molten pool formed in keyhole mode has a deep and narrow profile with an aspect ratio greater than 1.5. There is a transition mode between the conduction mode and the keyhole mode, in which the energy is not dissipated quickly enough and the processing temperature is raised above the vaporization temperature. The depth of the molten pool increases and penetration of the molten pool can begin. The molten pool formed in the conduction mode or the transition mode has an aspect ratio less than 1.5, preferably less than 1, more preferably less than 0.75 and most preferably less than or equal to 0.5.

[0040] Alternatively, the method may include: melting the fused region once or each time thereafter so that the formed molten pool(s) extend deeper than the molten pool(s) formed when the material of the layer is melted for the first time to form the fused region. In this way, the subsequent or each time thereafter melting the fused region "covers" the microstructure formed when the material of the fused region is melted for the first time, so that the final microstructure of the object is defined by the scanning parameters and / or scanning strategy used for the subsequent time to melt the fused material. The depth of the molten pool(s) formed by the first time melting the material may be less than the thickness of five, four or three layers, and preferably less than or about equal to the thickness of two layers, but preferably greater than or equal to the thickness of one layer. In this way, the first time melting the material forms a strong connection with the solidified material of the underlying layer, but is shallow enough to be covered by the subsequent or each time thereafter exposure. The energy beam parameters used for the subsequent time (and optionally both the first and subsequent times) to fuse the material may be configured so that the molten pool is formed in a conduction mode or a transition mode.

[0041] According to a second aspect of the present invention, a powder bed fusion additive manufacturing method is provided, wherein an object is built in a layer-by-layer manner, the method comprising: for each layer of a plurality of successively fused layers, melting the material with a leading energy beam by advancing a leading energy beam along an irradiation path on the material of the layer, allowing the melted material to solidify, and reheating the solidified material by advancing a following energy beam along the irradiation path.

[0042] The leading energy beam may advance along an irradiation path using a first set of irradiation parameters, and the following energy beam may advance along an irradiation path using a different second set of irradiation parameters. The second set of irradiation parameters may include different energy beam powers and / or spot sizes on the layer. The following energy beam may have a larger spot size than the leading energy beam. The following energy beam may irradiate at a lower power than the leading energy beam. The following energy beam provides a lower energy density than the leading energy beam. The second set of irradiation parameters may be selected so that the following energy beam reheats the solidified material to above a grain refining temperature, such as a tempering temperature, an annealing temperature (recrystallization temperature), a solution heat treatment temperature, a sintering temperature, or a melting temperature. The reheating may increase the temperature of the solidified material by at least 100°C, 200°C, 300°C, 400°C, and preferably by at least 500°C. The reheating of the solidified material may be performed after the solidified material has cooled to below 350°C.

[0043] The interval time between the leading energy beam and the following energy beam may be nominally fixed and / or may be greater than a predetermined minimum delay. The predetermined minimum delay may be at least 250 microseconds, and more preferably at least 500 microseconds. The predetermined minimum delay allows the melted material to solidify so that the fused material is reheated when the layer is subsequently irradiated by one or more energy beams. The interval between the leading energy beam and the following energy beam may be less than a predetermined maximum delay. The maximum delay may be greater than 2500 microseconds.

[0044] According to a third aspect of the present invention, a powder bed fusion additive manufacturing method is provided, wherein an object is built in a layer-by-layer manner, the method comprising: for each of a plurality of successively fused layers, melting the material of the layer for a first time by irradiating the layer with one or more energy beams in a first irradiation pattern, allowing the molten material of the pattern to solidify to define a fusion region of the layer, and subsequently melting the fusion region once with one or more energy beams in a second irradiation pattern.

[0045] The second irradiation pattern may be different from the first irradiation pattern. Accordingly, a subsequent melting of the fused region may change the overall microstructure of the fused material relative to the microstructure achieved by melting the material the first time. The second pattern may be arranged to achieve a directional grain structure, such as described in PCT / GB2020 / 051382, which is incorporated herein by reference in its entirety.

[0046] The fused region may include the core of the object.

[0047] According to a fourth aspect of the present invention, a powder bed fusion additive manufacturing method is provided, wherein an object is built in a layer-by-layer manner, the method comprising: for each layer of a plurality of successively fused layers, irradiating the material with a leading energy beam for a first time by advancing a leading energy beam along an irradiation path on the material of the layer so as to melt the material, and subsequently irradiating the material once by advancing a following energy beam along the irradiation path on the fused material, wherein the interval time between the leading energy beam and the following energy beam is at least 500 microseconds.

[0048] The predetermined minimum delay allows the fused material to cool sufficiently so that the fused material is reheated when irradiated by the following energy beam. The interval between the leading energy beam and the following energy beam can be less than the predetermined maximum delay. The maximum delay can be greater than 2500 microseconds. In one embodiment, there is no maximum delay set between the first time and the subsequent time.

[0049] According to a fifth aspect of the present invention, a powder bed fusion device is provided, comprising: a building platform for supporting a powder bed; a layer forming device for forming a powder layer of the powder bed; a scanner for guiding one or more energy beams to a working surface of the powder bed; and a controller arranged to control the layer forming device and the scanner to perform a method according to any one of the first, second, third and fourth aspects of the present invention.

[0050] According to a sixth aspect of the present invention, a controller for controlling a powder bed fusion device is provided, the powder bed fusion device comprising: a building platform for supporting a powder bed; a layer forming device for forming a powder layer of the powder bed; and a scanner for directing one or more energy beams to a working surface of the powder bed, the controller being arranged to control the layer forming device and the scanner to perform a method according to any one of the first, second, third and fourth aspects of the present invention.

[0051] According to a seventh aspect of the present invention, there is provided a data carrier having instructions stored thereon, which, when executed by a processor of a controller of a powder bed fusion device, causes the controller to control a layer forming device and a scanner of the powder bed fusion device to perform a method according to any one of the first, second, third and fourth aspects of the present invention, the powder bed fusion device comprising: a building platform for supporting a powder bed; a layer forming device for forming a powder layer of the powder bed; and a scanner for directing one or more energy beams to a working surface of the powder bed.

[0052] According to an eighth aspect of the present invention, a method for generating a build program for a powder bed fusion device is provided, the powder bed fusion device comprising: a build platform for supporting a powder bed; a layer forming device for forming a powder layer of the powder bed; a scanner for guiding one or more energy beams to a working surface of the powder bed; and a controller for controlling the layer forming device and the scanner, the method comprising: receiving geometric data defining an object, determining slices of the object to be formed in layers in the powder bed fusion device, determining an irradiation sequence of one or more energy beams so as to perform a powder bed fusion additive manufacturing method according to any one of the first, second, third and fourth aspects of the present invention; and generating a build program, which, when executed by the controller, causes the controller to control the layer forming device and the scanner to perform the powder bed fusion additive manufacturing method.

[0053] According to a ninth aspect of the present invention, there is provided a data carrier having instructions stored thereon, which instructions, when executed by a processor, cause the processor to perform the method according to the ninth aspect of the present invention.

[0054] The data carrier may be a suitable medium for providing instructions to the machine, such as a non-transitory data carrier, for example a floppy disk, CD ROM, DVD ROM / RAM (including -R / -RW and +R / +RW), HDDVD, Blu Ray (TM) disc, memory (such as Memory Stick (TM), SD card, Compact Flash card, etc.), disk drive (such as hard disk drive), magnetic tape, any magnetic / optical storage; or a transient data carrier, such as a signal on a wire or optical fiber or a wireless signal, such as a signal sent on a wired or wireless network (such as Internet download, FTP transfer, etc.). BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 is a schematic diagram of a powder bed additive manufacturing apparatus according to an embodiment of the present invention;

[0056] Figure 2 yes Figure 1 A plan view of a powder bed additive manufacturing apparatus as shown in;

[0057] Figure 3 is a schematic diagram of a scanning strategy according to a first embodiment of the present invention;

[0058] Figure 4 is a cross section of the fused layer, showing the core, upper skin region, and lower skin region of the object;

[0059] Figure 5 is a schematic diagram of a scanning strategy according to a second embodiment of the present invention;

[0060] Figure 6 is a schematic diagram of a scanning strategy according to a third embodiment of the present invention;

[0061] Figure 7 is a schematic diagram of a molten pool formed using the scanning strategy of the third embodiment;

[0062] Figure 8 is a backscattered electron image of a cross section of a cube formed from H13 tool steel using a single laser beam;

[0063] Fig. 9 is a backscattered electron image of a cross section of a cube formed of H13 tool steel using a leading laser beam and a following laser beam (where a 200 W following laser beam follows the leading laser beam after 2500 μs);

[0064] Fig.10is a backscattered electron image of a cross section of a cube formed of H13 tool steel using a leading laser beam and a following laser beam (where a 100 W following laser beam follows the leading laser beam after 2500 μs);

[0065] Fig.11 is a backscattered electron image of a cross section of a cube formed of H13 tool steel using a leading laser beam and three following laser beams (where each following laser beam follows the previous laser beam after 500 μs);

[0066] Fig.12 is a backscattered electron image of a cross section of a cube formed of H13 tool steel using a leading laser beam and three following laser beams (where each following laser beam follows the previous laser beam after 1000 μs);

[0067] Fig.13 is a backscattered electron image of a cross section of a cube formed of H13 tool steel using a leading laser beam and three following laser beams (where each following laser beam follows the previous laser beam after 1500 μs);

[0068] Figures 14a to 14d is a table showing scanning parameters of the laser beam used for Example 4;

[0069] Fig.15a and Fig.15b is a histogram showing the number of cracks of different sizes found in a cube constructed in the manner described in Example 4;

[0070] Fig.16a and Fig.16b is a backscattered electron image of a cross section of a cube formed from H13 tool steel using a single laser beam (where the powder bed has been heated to 500°C);

[0071] Fig.17a and Fig.17b is a backscattered electron image of a cross section of a cube formed from H13 tool steel using a leading laser beam and a following laser beam (without heating the powder bed to 500°C);

[0072] Fig.18a and Fig.18b is a backscattered electron image of a cross section of a cube formed from H13 tool steel using a single laser beam (without heating the powder bed to 500°C);

[0073] Fig.19 is a table showing scanning parameters of a laser beam for Example 6 in which a cube and a track are formed from M2 high speed steel;

[0074] Figures 20a to 20g Shown with Fig.19A solidified filling line formed by a laser beam with the scanning parameters described in;

[0075] Fig.21a and Figure 21b is to use a leading laser beam and a following laser beam ( Fig.21a ) and a single laser beam ( Figure 21b ) a backscattered electron image of the top surface of a cube formed of M2 high speed steel; and

[0076] Fig. 22 is a backscattered electron image of a cross section of a cube formed from M2 high-speed steel using a leading laser beam and a following laser beam parallel to the build direction, where the fill build direction is perpendicular to the page. DETAILED DESCRIPTION

[0077] refer to Figure 1 and Figure 2 , a powder bed fusion additive manufacturing apparatus according to an embodiment of the present invention includes a build chamber 101 having partitions 115, 116 defining a build volume 117 therein. A build platform 102 can be lowered in the build volume 117. When a workpiece is built by selective laser melting of powder, the build platform 102 supports a build substrate 102a, a powder bed 104, and a workpiece 103. As successive powder layers are formed, the platform 102 is lowered within the build volume 117 under the control of a motor (not shown).

[0078] When the workpiece 103 is built by the dispensing device 108 and the wiper 109, a powder layer 104 is formed. For example, the dispensing device 108 can be a device as described in WO 2010 / 007396. The dispensing device 108 distributes the powder onto an upper surface 115a defined by the partition 115 and spreads it on the powder bed by a re-coater (in this embodiment, in the form of a wiper 109). The position of the lower edge of the wiper 109 defines a working surface / plane 110, where the powder is consolidated.

[0079] A plurality of laser modules 105a, 105b, 105c, and 105d generate laser beams 118a, 118b, 118c, 118d for irradiating the powder bed 104, which are steered as needed by corresponding optical modules 106a, 106b, 106c, 106d. The laser beams 118a, 118b, 118c, 118d are steered by the corresponding optical modules to enter the build chamber 101 through a common laser window 107. Each optical module includes steering optics 121 (e.g., two mirrors 141a, 141b (see FIG. 1 ) mounted on galvanometers 124a, 124b) for steering the laser beam 118 in the vertical direction (X and Y) across the work plane 110. Figure 3)) and focusing optics 120 (e.g., two movable lenses for changing the focus of the laser beam 118). The optical modules 106a, 106b, 106c, 106d are shown here as separate, but they can be combined together into a single-piece housing, as is the case in Renishaw's RenAM 500Q additive manufacturing machine. Each scanner is controlled so that the focal position of the laser beam 118 remains in the same plane 110 as the laser beam 118 moves on the work plane 110. Instead of using a dynamic focusing element to maintain the focal position of the laser beam in a plane, an f-theta lens can be used.

[0080] The controller 140 (including a processor 161 and a memory 162) communicates with the modules of the additive manufacturing device (i.e., laser modules 105a, 105b, 105c, 105d, optical modules 106a, 106b, 106c, 106d, build platform 102, dispensing device 108 and wiper 109). The controller 140 receives build instructions from an external computer (on which there is build preparation software) and controls each of the modules based on the build instructions so as to build one or more objects using an additive manufacturing process. The build preparation software can be as described in WO 2014 / 207454, but is further adapted to generate build instructions that implement the scanning strategy described below.

[0081] refer to Figure 3 , a scanning strategy for consolidating powder material to form a cross section 200 of an object includes: melting the material of the powder layer L5 by progressively irradiating a predefined irradiation path 201 with a first leading laser beam 203a, and reheating the fused region 205 by progressively irradiating the predefined irradiation path 201 with a following laser beam 203b. In this embodiment, the leading laser beam 203a and the following laser beam 203b are scanned continuously along parallel irradiation paths 201 (also called fill lines) at the same speed. The fill lines are spaced apart by a fill distance HD so that adjacent solidification lines form a continuous fused region without powder between them. The parallel fill lines 201 are typically used as a filling scan to solidify the core of the cross section, and one or more boundary scans 202 are performed to provide a shell around the core.

[0082] Figure 4, an example of the core of the fused material layer L2 is shown with shaded areas 510a and 510b. The shaded areas 510a and 510b are formed directly on the fused material of the layer L1 below, and have fused material formed thereon when the layer L3 above is fused. Accordingly, area 511 is not part of the core of layer L2 because it is a lower skin area that is not formed directly on the fused material (but is fused to the powder), area 512 is not part of the core of layer L2 because it is an upper skin area on which the fused material of layer L3 is not formed, and area 513 is a boundary area that is fused by irradiating this area using boundary scanning.

[0083] The filling scan may include: a zigzag scan pattern, in which all the filling lines are in the same direction (but they are scanned bidirectionally or unidirectionally); a checkerboard scan pattern, in which the cross section to be cured is divided into a plurality of squares, each square comprising a plurality of filling lines, wherein the directions of the filling lines between the squares may be different, for example, 90° apart; or a stripe scan pattern, in which the cross section to be cured is divided into a plurality of parallel stripes, each stripe comprising a plurality of filling lines. The checkerboard scan pattern and the stripe scan pattern may provide filling lines of equal length, except for the case where the squares or stripes are shortened due to the borders of the cross section 200 to be cured.

[0084] The boundary scan 202 may use a different set of parameters than the fill scan, for example, so that a desired surface finish is achieved. The boundary scan may also scan with a leading laser beam and a following laser beam, or may scan with only a single laser beam.

[0085] The leading laser beam 203a and the following laser beam 203b are separated by a delay d so that the material 204 melted by the leading laser beam 203a is allowed to solidify before the fused material 205 is reheated by the laser beam 203b. In this embodiment, the delay d is greater than 250 microseconds, and preferably 2500 microseconds. The group irradiation parameters for the leading laser beam and the following laser beam are also different. In this embodiment, the leading laser has a smaller (1 / e) than the following laser beam 203a. 2 ) Spot size S1 and high power. A larger spot size S2 following the laser beam 203b can be achieved by defocusing the laser beam, for example by focusing the laser beam to a plane above or below the working surface of the powder bed 104.

[0086] The energy density provided by the follow-up laser beam 203a may not be sufficient to melt the material, but may heat the material to a temperature above which grain refinement occurs.

[0087] Alternatively, the energy density provided by the follower laser beam 203a may be sufficient to melt the material, resulting in grain refinement. In this alternative embodiment, the energy density of the follower laser beam alone may not be sufficient to melt the material to form a continuous fill line of solidified material. However, in combination with the lead laser beam, the delay between the lead laser beam and the follower laser beam is long enough to solidify the material melted by the lead laser beam, but short enough to retain enough heat locally so that the follower laser beam re-melts the material.

[0088] It has been found that this scanning strategy reduces the number of cracks in the resulting object compared to using only a single laser beam. It is believed that the reheating of the fused material following the laser beam 203b refines the grains to reduce the number of epitaxial grains and / or columnar grains, thereby reducing the boundaries between epitaxial grains and columnar grains of different orientations. Reducing such grain boundaries reduces the number of solidification cracks in the metal material. It has been found that this scanning strategy can reduce cracks to the extent that a build that previously failed due to cracks can now be built.

[0089] refer to Figure 5 , showing another scanning strategy. This embodiment is similar to the reference Figure 3 The described embodiment differs in that an additional follow-up laser beam 303c is provided. The additional follow-up laser beam 303c reheats the fused region 205 a third time by progressively irradiating the predefined irradiation path 201 with a set delay d2 after the follow-up laser beam 303b. The delay d2 may be the same as or different from the delay d1.

[0090] It has been found that adding another follower laser beam further reduces cracks in the resulting object compared to using a leading laser beam and one follower laser beam.In this embodiment, the set of irradiation parameters for the additional follower laser beam 303c is different from the set of irradiation parameters for the follower laser beam 303b.

[0091] refer to Figure 6 and Figure 7 , the scanning strategy for consolidating the powder material to form the cross section 400 of the object includes melting the material for the first time with a first scanning pattern including a first set of exposures (in this embodiment, the hatching lines 401a), allowing the melted material to solidify to form the entire fused cross section 400 of the object, and then melting the material of the fused cross section 400 for the second time with a second scanning pattern including a second set of exposures (in this embodiment, the second set of hatching lines 401b). Figure 6 As shown in , the fill line of the second set of exposures can be in the same direction as the first set of exposures, or can be different from the direction of the first set of exposures.

[0092] The first set of exposures forms melt pools 404a', 404a", 404a'" and 404a"" that are deep enough to allow the fused material to consolidate with the fused material of the underlying layer, but are shallower than the melt pools 404b', 404b", 404b'" and 404b"" formed subsequently by the second set of exposures. In this way, the second set of exposures "overwrites" the first set of exposures so that the resulting grain structure is primarily the result of the solidification rate and geometry of the melt pools formed by the second set of exposures. The fused material formed by the first set of exposures provides a uniform environment so that the second set of exposures forms melt pools having the desired shape and relationship to each other to achieve a directional grain structure. This can be important when forming an object having a preferred grain orientation, as described in EP 19179230.8.

[0093] In this embodiment, the first set of exposures and the second set of exposures both form the molten pool in the conduction mode or the transition mode.

[0094] Example 1

[0095] Eight 10mm×10mm×10mm cubes were built from tool steel HS13 powder in a RenAM 500Q additive manufacturing machine. The cubes were built using a zigzag scanning strategy, where the filling direction was rotated 67 degrees per layer. No preheating of the powder was performed using a heater in the build platform. Seven of these cubes were built using a leading laser beam and a following laser beam, and one of these cubes was built using a single laser beam. The scanning parameters are set out below.

[0096] Single laser

[0097] The cube was constructed using a single laser beam with the following scan parameters:

[0098] Laser power: 200W

[0099] Focus relative to the working plane: 0mm

[0100] Dot pitch: 20μm

[0101] Exposure time: 20μs

[0102] Filling distance: 0.08mm

[0103] Figure 8 A backscattered electron image of a cross section of a cube formed using these scanning parameters is shown. As can be seen from the image, a large number of large cracks are present in the fused material.

[0104] 200W follower laser

[0105] A cube was constructed using a leading laser beam and a following laser beam (where the following laser beam follows 2500 μs later). The following scan parameters were used:

[0106] Leading laser beam:

[0107] Laser power: 200W

[0108] Focus relative to the working plane: 0mm

[0109] Dot pitch: 20μm

[0110] Exposure time: 20μs

[0111] Filling distance: 0.08mm

[0112] Follow the laser beam:

[0113] Laser power: 200W

[0114] Focus relative to the working plane: +5mm

[0115] Dot pitch: 20μm

[0116] Exposure time: 20μs

[0117] Filling distance: 0.08mm

[0118] Fig. 9 A backscattered electron image of a cross section of a cube formed using these scanning parameters is shown. As can be seen from the image, the number and size of cracks present in the fused material have been reduced compared to the cube formed using a single laser. A refinement of the grain structure can be observed in the image compared to building the cube using a single laser beam.

[0119] 100W follower laser beam

[0120] A cube was constructed using a leading laser beam and a following laser beam (where the following laser beam follows 2500 μs later). The following scan parameters were used:

[0121] Leading laser beam:

[0122] Laser power: 200W

[0123] Focus relative to the working plane: 0mm

[0124] Dot pitch: 20μm

[0125] Exposure time: 20μs

[0126] Filling distance: 0.08mm

[0127] Follow the laser beam:

[0128] Laser power: 100W

[0129] Focus relative to the working plane: +5mm

[0130] Dot pitch: 20μm

[0131] Exposure time: 20μs

[0132] Filling distance: 0.08mm

[0133] Fig.10 Backscattered electron images of cross sections of cubes formed using these scanning parameters are shown. As can be seen from the images, the number and size of cracks present in the fused material have been reduced compared to cubes formed using a single laser beam and cubes formed using a 200W follower laser beam. Also, the refinement of the grain structure is evident compared to cubes formed using a single laser beam.

[0134] It was also observed that the hardness of the material formed using the leading laser beam and the following laser beam was improved compared to the hardness of the material formed using a single laser beam, which also indicates that the refinement of the grain structure occurred due to the heat treatment using the following laser beam.

[0135] Example 2

[0136] Three 10 mm × 10 mm × 3 mm cubes were built from tool steel HS13 powder in a RenAM 500Q additive manufacturing machine using a leading laser beam and three following laser beams. For the first of these cubes, the delay time between each laser beam was 500 μs, for the second of these cubes, the delay time between each laser beam was 1000 μs, and for the third of these cubes, the delay time between each laser beam was 1500 μs. These cubes were built using a zigzag scanning strategy, where the filling direction was rotated 67 degrees per layer. No preheating of the powder was performed using a heater in the build platform. The scanning parameters are set out below.

[0137] Leading laser beam:

[0138] Laser power: 240W

[0139] Focus relative to the working plane: 0mm

[0140] Dot pitch: 20μm

[0141] Exposure time: 20μs

[0142] Filling distance: 0.08mm

[0143] 1st follower laser beam:

[0144] Laser power: 150W

[0145] Focus relative to the working plane: +15mm

[0146] Dot pitch: 20μm

[0147] Exposure time: 20μs

[0148] Filling distance: 0.08mm

[0149] 2nd follower laser beam:

[0150] Laser power: 100W

[0151] Focus relative to the working plane: +15mm

[0152] Dot pitch: 20μm

[0153] Exposure time: 20μs

[0154] Filling distance: 0.08mm

[0155] 3rd follower laser beam:

[0156] Laser power: 50W

[0157] Focus relative to the working plane: +15mm

[0158] Dot pitch: 20μm

[0159] Exposure time: 20μs

[0160] Filling distance: 0.08mm

[0161] For a delay of 500 μs, a volume density of 99.92% of the theoretical volume density was achieved. For a delay of 1000 μs, a volume density of 99.95% of the theoretical volume density was achieved. For a delay of 1500 μs, a volume density of 99.97% of the theoretical volume density was achieved. Fig.11 , Fig.12 and Fig.13 It can be seen that a reduction in cracks can be observed compared to the cube formed using a single laser beam.

[0162] Also, an increase in the hardness of these samples was observed compared to the cubes formed using a single laser beam.

[0163] Example 3

[0164] Eight 10mm×10mm×10mm cubes were built from tool steel W360 powder in a RenAM 500Q additive manufacturing machine. The cubes were built using a leading laser beam and a following laser beam that traversed the cross section in either a zigzag scanning strategy or a stripe scanning strategy. The infill direction was rotated 67 degrees per layer. No preheating of the powder was performed using a heater in the build platform. Two cubes were built using a single laser beam. Figures 14a to 14d The scanning parameters used for each cube are described in .

[0165] The number of cracks in each cube was counted using a computer program that automatically identified the cracks from optical images of cross-sections of the cube. FIG15 is a histogram showing the number of cracks identified as having a particular perimeter size for cubes built using a single laser beam, and Fig.15b is a histogram showing the number of cracks identified as having a particular perimeter size for cubes built using a leading laser beam and a following laser beam. As is apparent from these histograms, the number of cracks in cubes built using a leading laser beam and a following laser beam is lower than in cubes built using a single laser beam.

[0166] Example 4

[0167] 10mm x 10mm x 10mm cubes were built from tool steel HS13 powder in a Renishaw RenAM500Q HT additive manufacturing machine, with the powder bed heated to 500°C. The cubes were built using a single laser using a zigzag scanning strategy, where the filling direction was rotated 67 degrees per layer.

[0168] Fig.16a and Fig.16b These are electron backscattered images of two cross sections of the cube. It can be seen that there are many cracks in the cube.

[0169] It is believed that Example 4 demonstrates that reducing the cooling rate does not prevent crack formation in H13 tool steel.

[0170] Example 5

[0171] Cubes were built from H13 tool steel, one cube using a leading laser beam and a following laser beam, where the following laser beam followed after 2500 μs, and the other cube using only a single laser beam. The powder bed was not preheated. The following scan parameters were used:

[0172] Leading laser beam:

[0173] Laser power: 240W

[0174] Focus relative to the working plane: 0mm

[0175] Dot pitch: 20μm

[0176] Exposure time: 20μs

[0177] Filling distance: 0.08mm

[0178] Follow the laser beam:

[0179] Laser power: 100W

[0180] Focus relative to the working plane: +5mm

[0181] Dot pitch: 20μm

[0182] Exposure time: 20μs

[0183] Filling distance: 0.08mm

[0184] For a single laser beam, the same scanning parameters used are those listed above for the leading laser beam.

[0185] Fig.17a is a backscattered electron image of a cross section of a cube formed using the leading and following laser beams. Fig.18a is a backscattered electron image of a cross section of a cube formed using a single laser beam. As can be seen from the image, the number and size of cracks present in the fused material have been reduced using the leading and following laser beams compared to the cube formed using a single laser beam. Fig.17b and Fig.18b Images of cross sections of cubes that have been crack analyzed using the software are shown at higher resolution. The crack area percentage for the cross section formed using the leading and trailing laser beams is 0.05%, the average crack length is 16+ / -12μm, and the average crack width is 0.5+ / -0.5μm. The crack area percentage for the cross section formed using a single laser beam is 0.26%, the average crack length is 25+ / -15μm, and the average crack width is 2+ / -1μm.

[0186] Example 6

[0187] Cubes were built from M2 high speed steel, four cubes were built using a leading laser beam and a following laser beam, and three cubes were built using only a single laser beam. Fig.19 The scan parameters used are explained in . With these scan parameters a fill line was also formed on top of each cube to allow investigation of the melt region.

[0188] Figures 20a to 20g Corresponds to Fig.19 The sample numbers are described in the table. Figures 20a to 20dAs can be seen from the fill line visible in FIG, the smoothness of the fill line increases as the laser power following the laser increases from 50 W to 150 W, indicating that remelting increases with increasing power. Fig.20e and Fig.20f The balling effect is shown when using a single laser beam at a lower power, demonstrating that these scanning parameters of the follower laser beam are not sufficient to melt a continuous line of material without a leading laser beam.

[0189] Fig.21a The top surface of a cube built using a leading laser beam and a following laser beam is shown, showing the refined microstructure. Figure 21b The top surface of a cube built using a single laser beam is shown, showing rough dendrites and significantly more cracks.

[0190] Fig. 22 The dashed box marked in the figure encloses a single layer melted by the leading laser beam and the following laser beam in M2 high-speed steel. The filling direction is perpendicular to the page. It can be seen that the layer is formed by a deeper molten pool with a keyhole mode shape generated by the leading laser beam and a shallower molten pool with a conductive mode shape generated by the following laser beam. This shows that the remelting is performed by the following laser beam, although the energy density provided by the scanning parameters is too low to melt the material without the heating performed by the leading laser beam.

[0191] It will be appreciated that the above embodiments may be varied and modified without departing from the scope of the invention as defined herein. For example, instead of a continuous scanning of the irradiation paths 201, 401a, 401b by a laser beam, each of the laser beams or one or more of the laser beams may be modulated to irradiate a series of points or portions along the irradiation path. The scanning strategy may be applied only to boundary scanning, rather than applying the scanning strategy only to filling scanning, or to both filling scanning and boundary scanning. Cracks during part failure tend to start from microcracks at the surface of the part. Accordingly, reducing or eliminating cracks at the boundary may be sufficient to provide a part with desired mechanical properties. In addition, it may be more difficult to ensure that the melting of the material at the boundary is desired, while the desired conditions may be easier to maintain within the core of the cross section. Accordingly, when the material is melted without grain refinement, the desired mechanical properties, such as grain orientation, may be achieved within the core, but grain refinement may be required for the fused material at the boundary.

Claims

1. A powder bed fusion additive manufacturing method, wherein: A method for building an object in a layer-by-layer manner, the method comprising: for each layer of a plurality of successively fused layers, melting the material of the layer by first irradiating the layer with one or more energy beams using a first set of irradiation parameters, and allowing the molten material to solidify to define a fusion region of the layer, and reheating the fusion region by subsequently irradiating the layer with one or more of the energy beams using a second set of irradiation parameters, wherein the first set of irradiation parameters includes at least one irradiation parameter different from the second set of irradiation parameters, characterized in that the reheating raises the temperature of the fusion region to above the melting temperature at which melting of the fused material occurs therein, and the interval between the first and subsequent times can be greater than 250 microseconds.

2. The powder bed fusion additive manufacturing method according to claim 1, wherein: The reheating increases the temperature of the fused region by at least 100°C, 200°C, 300°C, 400°C, or 500°C.

3. The powder bed fusion additive manufacturing method according to claim 1 or 2, wherein: After the fused region has cooled to below 350° C., reheating of the fused region is performed.

4. The powder bed fusion additive manufacturing method according to claim 1, comprising: The fused region is subsequently reheated one or more times.

5. The powder bed fusion additive manufacturing method according to claim 1, wherein: The interval between the first time and the subsequent time is at least 500 microseconds.

6. The powder bed fusion additive manufacturing method according to claim 1, wherein: The same irradiation pattern is used for the first and the one or each subsequent irradiation of the material.

7. The powder bed fusion additive manufacturing method according to claim 1, wherein: The first irradiation of the material of each layer comprises progressively irradiating a predefined irradiation path with a first leading energy beam, and the subsequent one or each subsequent irradiation of the fused region comprises progressively irradiating the predefined irradiation path with a following energy beam.

8. The powder bed fusion additive manufacturing method according to claim 7, wherein: The leading energy beam has a different power from the following energy beam and / or has a spot size that is the same as or larger than the leading energy beam, and / or the area irradiated by the following energy beam is the same as or wider than the width of the material fusion line formed by the advancement of the leading energy beam along the irradiation path.

9. The powder bed fusion additive manufacturing method according to claim 1, wherein: Different illumination patterns are used for the first and the or each subsequent illumination of the layer.

10. The powder bed fusion additive manufacturing method according to claim 9, wherein: Another illumination pattern is arranged to produce a preferential direction of grain formation.

11. A powder bed fusion additive manufacturing method according to claim 1, wherein the reheating includes melting the fusion region once or each time thereafter so that the formed molten pool extends deeper than the molten pool formed when the material of the layer is melted for the first time to form the fusion region. 12 . The powder bed fusion additive manufacturing method according to claim 1 , comprising subsequently melting the fused region a second time so that the formed molten pool is shallower than the molten pool formed when the material of the layer is melted for the first time to form the fused region.

13. The powder bed fusion additive manufacturing method of claim 1, wherein an interval between the first time and the subsequent time is less than a predetermined maximum delay.

14. The powder bed fusion additive manufacturing method of claim 13, wherein the maximum delay is greater than 2500 microseconds.

15. The powder bed fusion additive manufacturing method of claim 1, wherein the subsequent reheating of the fused region is performed to close porous regions formed by solidification cracks.

16. A powder bed fusion device, comprising a building platform for supporting a powder bed, a layer forming device for forming a powder layer of the powder bed, a scanner for directing one or more energy beams to a working surface of the powder bed, and a controller for controlling the layer forming device and the scanner to perform a method according to any one of claims 1 to 15.

17. A data carrier having instructions stored thereon, which, when executed by a processor of a controller of a powder bed fusion device, causes the controller to control a layer forming device and a scanner to perform a method according to any one of claims 1 to 15, wherein the powder bed fusion device comprises a building platform for supporting a powder bed, the layer forming device for forming a powder layer of the powder bed, and the scanner for directing one or more energy beams to a working surface of the powder bed.

18. A method for generating a build program for a powder bed fusion device, the powder bed fusion device comprising a build platform for supporting a powder bed, a layer forming device for forming powder layers of the powder bed, a scanner for directing one or more energy beams to a working surface of the powder bed, and a controller for controlling the layer forming device and the scanner, the method comprising receiving geometric data defining an object, determining slices of the object to be layered in the powder bed fusion device, determining an irradiation order of one or more energy beams so as to perform a powder bed fusion additive manufacturing method according to any one of claims 1 to 15, and generating a build program, wherein when the controller executes the build program, the controller controls the layer forming device and the scanner to perform the powder bed fusion additive manufacturing method.

19. A powder bed fusion additive manufacturing method, wherein: The method comprises: for each layer of a plurality of successively fused layers, melting the material of the layer with a leading energy beam by advancing the leading energy beam along an irradiation path, allowing the melted material to solidify, and reheating the solidified material by advancing a following energy beam along the irradiation path, characterized in that the reheating raises the temperature of the solidified material to above the melting temperature at which melting of the solidified material occurs, and the interval between the leading energy beam and the following energy beam can be greater than 250 microseconds.

20. A powder bed fusion additive manufacturing method, wherein: Building an object in a layer-by-layer manner, the method comprising: for each layer of a plurality of successively fused layers, melting the material of the layer for a first time by irradiating the layer with one or more energy beams in a first irradiation pattern, allowing the molten material of the pattern to solidify to define a fused region of the layer, and melting the fused region a subsequent time with one or more energy beams in a second irradiation pattern, characterized in that the melting raises the temperature of the fused region to above the melting temperature at which melting of the fused material therein occurs, and the interval between the first time and the subsequent time can be greater than 250 microseconds.

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