ADDITIVE MANUFACTURE

Abstract

A method of powder bed fusion additive manufacture includes forming a component in a powder bed in a layer-by-layer process. The method may include sintering, without melting, selected regions of powder with an energy beam to form at least one support adjacent to the component; and melting further selected regions of the powder bed with an energy beam to form a component by layer-by-layer melting of material. The method may include directing an energy beam at selected regions of powder to form a friable support, the friable support including bonded powder which act as a solid and provide compressive support; and melting further regions of the powder bed with an energy beam to form a component by layer-by-layer melting of material.

Claims

1. A method of powder bed fusion additive manufacture comprising forming a component in a powder bed in a layer-by-layer process wherein the method comprises: sintering, without melting, selected regions of powder with an energy beam to form at least one support adjacent to the component; and melting further selected regions of the powder bed with an energy beam to form a component by layer-by-layer melting of material.

2. The method of powder bed fusion additive manufacture of claim 1, wherein the support formed by sintering selected regions of the powder is friable.

3. A method of powder bed fusion additive manufacture comprising forming a component in a powder bed in a layer-by-layer process wherein the method comprises: directing an energy beam at selected regions of powder to form a friable support, the friable support comprising bonded powder which act as a solid and provide compressive support; and melting further regions of the powder bed with an energy beam to form a component by layer-by-layer melting of material.

4. The method of claim 1, wherein the support is partially sintered.

5. The method of powder bed fusion additive manufacture of claim 1, wherein the method further comprises bulk heating the powder bed during the layer-by-layer process, wherein the method may further comprise monitoring and/or modelling the temperature of the powder bed to maintain the powder bed at a temperature within the stress relieving temperature range of the powder material.

6. The method of powder bed fusion additive manufacture of claim 1, comprising controlling the energy beam when melting selected regions of the powder by directing the beam to solidify a selected area of a layer of material by advancing the laser beam to melt spaced apart sections, wherein each melted section is allowed to solidify before an adjacent section is melted by irradiating the layer with the or another laser beam, wherein each section may be sized such that a melt pool extends across the entire section.

7. The method of powder bed fusion additive manufacture of claim 1, wherein the selectively melting uses an energy beam having a first energy density and the selectively at least partially sintering, without melting, uses an energy beam having a second, reduced, energy density.

8. The method of powder bed fusion additive manufacture of claim 7, wherein the second, reduced, density is a two-dimensional energy density of less than 0.75 Joules/mm.sup.2.

9. The method of powder bed fusion additive manufacture of claim 1, wherein the support comprises a region extending to a layer immediately beneath a downward facing portion of the component.

10. The method of powder bed fusion additive manufacture of claim 1, wherein the support is a floating support for the component.

11. The method of powder bed fusion additive manufacture of claim 10, wherein the process comprises providing at least one region of unfused powder between the substrate or base and the support.

12. The method of powder bed fusion additive manufacture of claim 1, wherein the partially sintered further regions of powder are formed immediately adjacent to external surfaces of the component.

13. The method of powder bed fusion additive manufacture of claim 1, wherein a plurality of components are formed in the powder bed, the components being separated by the partially sintered further regions of powder.

14. A method of powder bed fusion additive manufacture comprising the steps of: a. providing a powder bed on a substrate; b. heating the powder bed; c. selectively forming at least one sintered support region of powder above the substrate by selectively scanning the powder bed; d. selectively forming a component by selectively melting powder above the semi-sintered region; wherein step (d), and optionally step (c), are repeated on a layer-by-layer basis.

15. An additive manufacture apparatus comprising: a process chamber containing a powder bed; a radiation source for providing an energy beam; a scanner for directing the energy beam across the powder bed; and a controller configured to control the apparatus in accordance with the method of powder bed fusion additive manufacture in accordance with claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0054] Embodiments of the invention may be performed in various ways, and embodiments thereof will now be described by way of example only, reference being made to the accompanying drawings, in which:

[0055] FIG. 1 is a schematic representation of a powder bed fusion apparatus;

[0056] FIG. 1b is a schematic representation of a powder bed fusion apparatus showing a method according to an embodiment of the invention; and

[0057] FIGS. 2a to 2c are photographs showing test components and supports made in accordance with methods of an embodiment of the invention.

DETAIL DESCRIPTION OF EMBODIMENTS

[0058] It may be appreciated that references herein to vertical or horizontal are with reference to the axis of the additive manufacture process. In particular, as powder bed fusion is a layer by layer process the horizontal axis corresponds to the plane of the layers (which is in turn defined by the powder bed and support). The corresponding alignment of a component being manufactured is selected during optimisation of the process and is not therefore limited to any specific direction. Any other references to directions such as above/below or upward/downward are likewise non-limiting with respect to the component per se and should be understood to generally refer to orientation during the additive manufacturing process.

[0059] A metallic powder bed laser fusion additive manufacture apparatus 10 for use in embodiments of the present invention is shown in FIG. 1a. The apparatus may for example be a commercially available apparatus (possibly with some modification to enable embodiments of the invention) such as the Applicant's commercially available “Renishaw AM” systems. The apparatus comprises a process chamber 12 which encloses a powder bed 14. The powder bed 14 is supported on a platform 16 which, as is known in the art may also support a substrate of the same metal as the powder. The platform 16 is moveable in the vertical axis such that it may be lowered as each successive layer of the additive manufacture process is carried out. A supply 18 for providing powder to the bed 14 after the platform 16 is lowered and may include a roller (as shown in the present example) or scraper/wiper which travels across the powder bed 14 in the horizontal axis for distributing an even layer on the powder bed. The skilled person may appreciate that when implementing embodiments of the invention the movement of the supply across the powder bed 14 may need to be adjusted or optimised to ensure than friable sintered supports (particularly floating supports) are not displaced or removed.

[0060] A radiation source 20, typically a laser (although some embodiments could, for example, use an electron beam emitter), is provided for heating and fusing the powder in the bed 14. The radiation source is directed to the powder bed by a scanner 22, typically comprising a moveable mirror arrangement. A controller 30 is provided for controlling the radiation source 20, the scanner 22 and the process chamber 12 (including for example the platform 16, supply 18 and environmental systems such as heating and gas supply). In use the scanner 22 is used to move the energy beam across the surface of the powder bed 14.

[0061] In accordance with preferred embodiments the process chamber 12 includes a heating arrangement (not shown) for raising the temperature of the powder bed 14 prior to and during the layer-by-layer process. Additionally, it is highly desirable to provide a low oxygen atmosphere within the process chamber 12. The process chamber 12 is, therefore, hermetically sealed (and as the source 20 and scanner 22 are typically external to the chamber, the chamber may include a window through which the laser beam may pass into the chamber). An outlet 24 is provided which is in communication with a vacuum pump to remove air from the chamber 22. An inlet 26 is also provided and may be connected to a supply of inert gas such as argon. Typically, the chamber 22 will be evacuated first by the outlet 24 to purge the chamber 22 before the inlet 26 is opened to draw inert gas into the chamber 12.

[0062] The skilled person in the art will be aware of the general operation of a powder bed fusion additive manufacture processes. A component 50 to be built is first prepared using a file preparation software, such as the applicants QuantAM software, to optimise the process and the component. The preparation stage requires the component geometry to be appropriately orientated and support structures added where required. Scan parameter may also be optimised, for example optimisation may include factors such as the layer thickness, beam size and dwell time of the beam. The component must then be divided into a series of slices (along the vertical axis of the additive manufacturing apparatus) and a scanning strategy for each slice prepared. The software then provides an output in the form of layer-by-layer computer instructions for the additive manufacture machine. It will be understood that the methods of the present invention would be implemented by incorporating them into the preparation software such that the layer-by-layer computer instructions.

[0063] The instructions from the preparation software are uploaded to the controller 30 so that the additive manufacture process can commence. An initial layer of powder is provided in the powder bed 14 supported by the platform 16 which will initially be in an upper position. The powder supply 18 may pass a roller or the like across the powder to ensure it is evenly filled and suitably compacted. The chamber is evacuated by the outlet 24 before being filled with inert gas by the inlet 26. The laser 20 is then used to selectively scan the powder bed 14 in a two-dimensional scan pattern to melt powder so that it will solidify and form a first layer of the component 50 on the platform. In a powder bed fusion process, it is essential that the scan parameters (for example laser power, spot size and scan speed) are selected to achieve a full melt of the powder in each part of the component. This ensures that a fully dense part is formed with a homogenous mass and low porosity.

[0064] After the first layer of the powder has been fully selectively scanned, the platform 16 is moved downward and a subsequent layer of powder is added to the powder bed 14 by the supply 18. The scanning for the subsequent layer is then carried out with melted regions fusing not only with adjacent parts of the new layer but also with those of the immediately underlying layer. This process is then repeated until sufficient layers have been stacked in the vertical direction to form the full geometry of the part 50.

[0065] As discussed above, in existing methods the first layer of powder may be fused to a substrate both to support any overhand features and to anchor the component against residual stresses formed by the heating and cooling of the additive process. The components will generally be removed from the process chamber with the substrate (which may need to be of considerable bulk) and post-processed to reduce or remove the residual stress and then subsequently to detach the component(s) from the substrate and to remove any support structures from the component. This post processing may add considerable time and cost to the overall process of forming the component and is therefore undesirable.

[0066] In accordance with embodiments of the invention, a modified additive manufacture process is used. The powder bed 14 (and process chamber 12) is heated to an elevated temperature, for example 500° C. It will be appreciated that this bulk heating of the powder bed must be sufficiently below the melting point of the material that it will not interfere with the normal additive manufacture process (for example preventing correct flow of the powder during re-supply). However, the applicants have found that heating to this degree at least reduces the residual stresses formed due to the thermal effects of the additive manufacture process. The methods of the invention may, therefore, take advantage of this reduction in residual stress to utilise a modified or reduced support structure. Whilst the specific support structure will depend upon the component being formed, ideally it would be desirable to form a component with little or no physical attachment to a substrate. In other words, it is an aim of embodiments of the invention to remove the need for anchoring the component to resist residual stress related issues such as cracking or deformation and to only include support for part accuracy such as preventing sinking of overhanging features.

[0067] In accordance with embodiments of the invention supports are formed by a region 40 of “semi-sintered” powder. Such regions are formed beneath layers of the component 50 and may extend fully to the base plate or substrate or may have a few layers of separation by un processed powder. Importantly, the semi-sintered supports are not fully melted. The semi-sintered supports may generally have insufficient bonding of the powder to perform an anchoring between the part 50 and base or substrate but may be sufficiently stiff to support the position of the part within the powder bed 14. In particular, the semi-sintered region 40 may provide support for overhang features 50a to prevent them from sinking into the powder bed during the layer by layer process (which would otherwise for example result in poor geometric accuracy). The semi-sintered region is at least partially sintered, which may be understood to mean that the powder in this region has been heated sufficiently to bond to surrounding powder but is not fully sintered since it has not formed a true solid under the application of pressure and heat. There may, for example be minimal change in the grain structure of the semi-sintered powder. A semi-sintered region should be sufficiently bonded to provide support during the layer-by-layer process. For example, the powder should be sufficiently bonded from the semi-sintering that it will act as a solid support rather than as a flowable powder. However, the “sintering” should be moderate enough that the component 50 and support 40 are easily removable. For example, only moderate physical pressure may be required to separate the component 50 and support 40. Ideally, the support may be sufficiently friable that it can simply broken away or crumble by hand.

[0068] FIG. 1(b) shows how embodiments of the invention may utilise “floating supports” 42 which do not extend through the full depth of the powder bed 14. Thus, each floating support 42a, 42b, 42c and 42d may be separated from the base by at least one layer of unfused powder 15. This layer of unfused powder 15 may further ensure ease of removal of the final components 52. A further advantage of the sintered supports of embodiments of the invention is that they may be utilised to increase usage of the full three-dimensional extent of the powder bed. Thus as shown in FIG. 1(b) components 52a and 52b or 52c and 52d may be “stacked” but separated by sintered regions 42 and/or unfused regions 15.

[0069] In order to verify embodiments invention, and provide an initial optimisation of the process, a simple test structure 50 in the form of an open-sided inverted box was built using a Renishaw AM laser powder bed fusion machine. Such a structure is a useful test structure due to having an unsupported overhanging span. Within the region enclosed by the span the powder was scanned, in accordance with embodiments of the invention, to provide a semi-sintered region 40. The results of the testing are shown in the photographs of FIG. 2 and represented in tables 1 and 2 below. By variation of process parameters, it is possible to empirically identify a “Goldilocks” zone for a particular material in which the support 50 is neither too soft (i.e. insufficiently semi-sintered) or too hard (i.e. over semi-sintered)

[0070] The test structures were formed using Titanium 6AL4V a common alloy for the use in laser powder bed melting additive manufacture. The build volume, with an inert atmosphere chamber was heated to 500° C. A series of test structures were then formed in a single additive manufacturing process (i.e., on a single substrate). All the semi-sintered support regions 40 were formed with the same laser beam exposure time, 40 μsecs and point distance 300 μm. The laser output power was varied in steps between 100 W and 200 W and the focus offset and spot size were varied in different tests. The results are shown in tabular form in Table 1 and Table 2 below.

TABLE-US-00001 TABLE 1 Laser Power-Focus Offset Semi-Sintered Support ‘Goldilocks’ Zones for Heated Build Volume @ 500° C. - Titanium 6AI4V Laser Power (W) 100 125 150 175 200 Focus −20 S S X X H Offset −25 S S X X H (mm) −30 S S X X H −35 S S X X H Point Distance = 300 μm Exposure Time = 40 μS Key: S = too soft H = too hard X = just right

TABLE-US-00002 TABLE 2 Laser Power - Spot Size (W/mm.sup.2 applied at powder bed) Semi-Sintered Support ‘Goldilocks’ Zones for Heated Build Volume @ 500° C. - Titanium 6AI4V Laser Power (W) 100 125 150 175 200 Spot 0.502 200(S) 250(S) 300(X) 350(X) 400(H) Size 0.610 164(S) 205(S) 240(X) 287(X) 328(H) (mm) 0.720 139(S) 174(S) 208(X) 249(X) 278(H) 0.828 121(S) 151(S) 181(X) 211(X) 241(H) Point Distance = 300 μm Exposure Time = 40 μS W .Math. mm.sup.−2 S = too soft H = too hard X = just right Calculated assuming 600 mm focal length lens and a 0.07 μm focal spot

[0071] It may be immediately noted from the test case that the key parameter for providing a semi-sintered support was the laser power output. The ideal support consistency was found with the laser output at 150 to 175 W. This corresponded to a two-dimensional energy density, or fluence, of approximately 0.2 to 0.25 J/mm.sup.2.

[0072] As shown in FIG. 2(A), when the laser output was too low (this example being 125 W, corresponding to a two-dimensional energy density of less than 0.2 J/mm.sup.2) the semi-sintered powder 40′ was insufficiently bonded to prevent it from flowing out of the cavity beneath the test structure 50′. Thus, these settings were not providing the required support function.

[0073] As shown in FIGS. 2(B) and 2(C), when the laser output was too high (this example being at 200 W, corresponding to a two-dimensional energy density of more than 0.25 J/mm.sup.2) the bonding of the semi-sintered powder 40″ was such that the support and the test structure 50″ could not easily be removed. In fact, it was necessary to chip/chisel away powder with hand tools. Thus, these settings did not provide a support which removed the need for post processing or allowed a part to be immediately removed from the powder bed.

[0074] FIGS. 2(D) and 2(E) show the ideal semi-sintered consistency (this example being at 175 W, corresponding to a two-dimensional energy density of approximately 0.25 J/mm.sup.2). In this case the semi-sintered support 50′″ is sufficiently bonded to provide support to the overhang portion of the test structure 40′″ as it will not simply flow under loading. However, part removal is simple and does not require significant effort to remove the part and break away the semi-sintered region 50′″.

[0075] The applicants have also recognised some additional benefits which may be achieved or enhanced by using the process in accordance with embodiments of the invention. For example, the method may make more efficient use of material since the semi-sintered supports essentially comprise loose powder. As such, the powder from the support regions may be reused with little additional processing. For example, the powder may only require passing through a sieve or mesh to ensure it is ready to be re-used in a future powder bed process.

[0076] It has also been noted that the surface of test pieces adjacent to semi-sintered regions (for example the underside of overhangs on test structures) has an improved surface finish. This is believed to be a result of a reduction in un-melted powder bonding to the melted surface of the component. This advantage may be utilised to improve the finish of even component surfaces that do not require any support. Thus, the semi-sintered regions in accordance with the invention may additionally be formed adjacent to surfaces that are not requiring support. For example, a semi sintered region may be formed between parts of the component (in a single layer) or on the layer immediately above an exterior part of the component.

[0077] The semi-sintered regions may also alter the thermal properties of the powder bed. This may help to mitigate differences in thermal inertia of areas of the component. This may be a further factor in selecting regions of the powder bed to be semi-sintered. For example, it may be advantageous to provide additional semi-sintered powder regions around relatively fine component features to provide more thermal mass. Additionally, less semi-sintered powder may be provided around relatively bulky component features so that the difference in thermal inertia between such features and finer features is reduced.

[0078] Although the invention has been described above with reference to preferred embodiments, it will be appreciated that various changes or modification may be made without departing from the scope of the invention as defined in the appended claims. For example, the skilled person may appreciate that whilst the examples provided above use a semi-sintered support as an alternative to a fully fused support embodiments of the invention may in practice be used in combination with existing techniques to provide the optimum process for any given component. Thus, the skilled person may use a combination of techniques or strategies to build a particular geometry in order to provide the best combination of various factors such as geometric accuracy, build quality and process throughput. For example, in some geometries (for example a significant overhang) it may be desirable to build a first support portion having fully fused powder and dispose a semi-sintered region between the fully fused support and the surface of the component. This may provide sufficient support and thermal transfer but still provide the advantage of a “floating” construction in accordance with embodiments of the invention.

[0079] It will also be appreciated that embodiments of the invention may be used in combination with other methods of reducing residual stress. For example, methods of the invention may be used in combination with revised scan strategies such as those which perform a selective scan which melts powder in a non-raster scan sequence such that adjacent portions of the layer are not melted at the same time. Such methods may be incorporated alongside the teaching of the invention within the additive manufacture preparation software.

[0080] It may be appreciated that some commercially available machines, such as the applicants RenAM 500Q, may include multiple lasers to increase productivity. The RenAM 500Q for example includes four 500 W lasers which are each able to access the whole powder bed surface simultaneously to ensure maximum flexibility in use. Thus, in some embodiments of the invention different energy sources may be used for the sintering and melting steps of the process. Whilst one or more lasers could be dedicated to the sintering process it may be preferable to have all laser suitable for preforming both melting and sintering such that the laser use and scan pattern can be optimised specifically for a particular component build.