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

Abstract

An additive manufacturing method wherein an object is formed by selectively solidifying layers of powder with at least one energy beam. The method includes forming the object from a nickel-based superalloy, wherein exposure parameters and an exposure pattern for the at least one energy beam result in the object having a directionally solidified microstructure with columnar grains aligned with a build direction, perpendicular to the layers. A composition of the nickel-based alloy by weight % may include: 9.3-9.7W, 9.0-9.5Co, 7.5-8.5Cr, 5.4-5.7Al, 3.1-3.3Ta, 1.4-1.6Hf, 0.6-0.9Ti, Mo 0.4-0.6, 007-0.015Zr, 0.01-0.02B with a carbon concentration of around 0.07-0.09 wt % and a balance of Ni.

Claims

1. An additive manufacturing method wherein an object is formed by selectively solidifying layers of powder with at least one energy beam, the method comprising forming the object from a nickel-based superalloy, wherein exposure parameters and an exposure pattern for the at least one energy beam result in the object having a directionally solidified microstructure with columnar grains aligned with a build direction, perpendicular to the layers, and a composition of the nickel-based alloy by weight % comprises: 9.3-9.7W, 9.0-9.5Co, 7.5-8.5Cr, 5.4-5.7A1, 3.1-3.3Ta, 1.4-1.6Hf, 0.6-0.9Ti, Mo 0.4-0.6, 007-0.015Zr, 0.01-0.02B with a carbon concentration of around 0.07-0.09 wt % and a balance of Ni.

2. An additive manufacturing method according to claim 1, wherein the nickel-based alloy also comprises any one or more of the following by weight percentage: Si 0.03 max, Mn 0.10 max, P 0.005 max, Fe 0.2 max, Cu 0.05 max, Nb 0.10 max, and/or any one or more following up to the maximum ppm: S 20 ppm max., Mg 80 ppm max., Pb 2 ppm max., Se 1.0 ppm max., Bi 0.3 ppm max., Te 0.5 ppm max, Tl 0.5 ppm max, [N] ppm 15 max, [O] ppm 10 max and N.sub.v3B 2.15 max.

3. An additive manufacturing method according to claim 1, wherein the nickel-based alloy is CM 247 or CM 247 LC.

4. An additive manufacturing method according to claim 1, wherein a crystallographic orientation of the columnar grains is predominantly <100>.

5. An additive manufacturing method according to claim 4, wherein the exposure parameters and the exposure pattern for the at least one energy beam result in a percentage of the object having columnar grains with a <100> crystallographic orientation that deviates from the build direction by more than 20° of less than 30%.

6. An additive manufacturing method according to claim 1, wherein the exposure parameters and exposure pattern are such that melt pools are formed in transition or conduction mode.

7. An additive manufacturing method according to claim 1, wherein the exposure parameters and exposure pattern of the at least one energy beam are such that a cooling rate of the melt pool is above a predetermined threshold, such as above 1.4×10.sup.6K/s.

8. An additive manufacturing method according to claim 1, wherein the exposure pattern includes a geometrical arrangement of scan paths of the at least one energy beam between successive layers, wherein the same geometrical arrangement of scan paths is maintained between a plurality of pairs of successive layers.

9. An additive manufacturing method according to claim 1, wherein, the geometrical arrangement is such that the scan paths between successive layers are aligned.

10. An additive manufacturing method according to claim 1, wherein the geometrical arrangement is such that the melt pools formed by scanning the at least one energy beam along the scan paths of successive layers stack directly above each other in the build direction facilitating the formation of the columnar grains in the build direction.

11. An additive manufacturing method according to claim 1, wherein the scan paths on successively melted layers are parallel.

12. An additive manufacturing method according to claim 1, wherein each layer has a layer thickness less than half a mean melt pool depth.

13. An additive manufacturing method according to claim 1, wherein the scan paths are straight hatch lines.

14. An additive manufacturing method according to claim 1, wherein the at least one energy beam is scanned continuously along each scan path.

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

16. A data carrier having instructions stored thereon, wherein the instructions, when executed by a controller of a powder bed fusion additive manufacturing apparatus comprising at least one scanner for scanning an energy beam across layers of a powder bed, cause the controller to control the powder bed fusion additive manufacturing apparatus to carry out the method of claim 1.

17. A method of generating instructions for an additive manufacturing apparatus, the method comprising receiving a model of an object and generating instructions and generating scanning parameters for at least one energy beam to solidify layers of powder in a layer-by-layer manner, wherein the exposure parameters and exposure pattern of the at least one energy beam result in the object having a directionally solidified microstructure with columnar grains aligned with a build direction, perpendicular to the layers, when the object is formed from a nickel-based alloy having a composition by weight % comprising: 9.3-9.7W, 9.0-9.5Co, 7.5-8.5Cr, 5.4-5.7Al, 3.1-3.3Ta, 1.4-1.6Hf, 0.6-0.9Ti, Mo 0.4-0.6, 007-0.015Zr, 0.01-0.02B with a carbon concentration of around 0.07-0.09 wt % and a balance of Ni.

18. A data carrier having instructions stored thereon, wherein the instructions, when executed by a processor, cause the processor to carry out the method of claim 17.

Description

DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1 is a schematic view of a powder bed fusion additive manufacturing apparatus according to an embodiment of the invention;

[0044] FIG. 2 is a hatch exposure pattern used in a method according to an embodiment of the invention;

[0045] FIG. 3a shows a cross-section in a plane parallel to a build direction for a part built in CM 247 LC using an exposure pattern according to an embodiment of the invention, the image marked up to indicate the location of the melt pools; and FIG. 3b is an image obtained using electron backscatter diffraction (EBSD) showing the directional microstructure of the solidified material;

[0046] FIG. 4 is a schematic diagram of a melt pool arrangement that may be formed by an exposure pattern of the invention and the resultant grain directions;

[0047] FIG. 5 is a schematic diagram of a melt pool arrangement that may be formed by the exposure pattern that forms melt pools in a keyhole mode and the resultant grain directions;

[0048] FIG. 6 shows an image of a cross-section of a part in a plane parallel to a build direction for a part built in CM 247 LC using an exposure pattern according to an embodiment of the invention;

[0049] FIG. 7 shows an image of a cross-section of a part in a plane parallel to a build direction for a part built in CM 247 LC using an isolated point exposure pattern;

[0050] FIG. 8 is a processed EBSD image of a part built using a method of the invention showing the deviation angle of the columnar grains with respect to the build direction; and

[0051] FIG. 9 is a processed EBSD image of the part of FIG. 8 showing the grains whose <100> crystallographic directions deviate by less than 20° from the build direction.

DESCRIPTION OF EMBODIMENTS

[0052] Referring to FIG. 1, a powder bed fusion additive manufacturing apparatus according to an embodiment of the invention comprises a build chamber 101 sealable from the external environment such that an inert atmosphere (in this embodiment, argon) can be maintained therein. Within the build chamber 101 are partitions 115, 116 that define a build sleeve 117. A build platform 102 is lowerable in the build sleeve 117. The build platform 102 supports a powder bed 104 and workpiece (part) 103 as the workpiece is built by selective laser melting of the powder. The platform 102 is lowered within the build sleeve 117 under the control of a drive (not shown) as successive layers of the workpiece 103 are formed.

[0053] Layers of powder 104 are formed as the workpiece 103 is built by a layer formation device, in this embodiment a dispensing apparatus and a wiper (not shown). For example, the dispensing apparatus may be apparatus as described in WO2010/007396. The dispensing apparatus dispenses powder onto an upper surface defined by partition 115 and is spread across the powder bed by the wiper. A position of a lower edge of the wiper defines a working plane 190 at which powder is consolidated. A build direction BD is perpendicular to the working plane 190.

[0054] A plurality of laser modules 105a, 105c generate laser beams 118a, 118c, for melting the powder 104, the laser beams 118a, 118c directed as required by a corresponding optical module (scanner) 106a, 106c. The laser beams 118a, 118c, enter through a common laser window 107. Each optical module comprises steering optics 121, such as two mirrors mounted on galvanometers, for steering the laser beam 118 in perpendicular directions across the working plane and focussing optics 120, such as two movable lenses for changing the focus of the corresponding laser beam 118. The scanner is controlled such that the focal position of the laser beam 118 remains in the working plane 190 as the laser beam 118 is moved across the working plane. Rather than maintaining the focal position of the laser beam in a plane using dynamic focusing elements, an f-theta lens may be used.

[0055] An inlet and outlet (not shown) are arranged for generating a gas flow across the powder bed formed on the build platform 102. The inlet and outlet are arranged to produce a laminar flow having a flow direction from the inlet to the outlet. Gas is re-circulated from the outlet to the inlet through a gas recirculation loop (not shown).

[0056] A controller 140, comprising processor 161 and memory 162, is in communication with modules of the additive manufacturing apparatus, namely the laser modules 105a, 105b, 105c, 105d, optical modules 106a, 106b, 106c, 106d, build platform 102, dispensing apparatus 108 and wiper 109. The controller 140 controls the modules based upon software stored in memory 162 as described below.

[0057] In use, a computer receives a geometric model, such as an STL file, describing a three-dimensional object to be built using the powder bed fusion additive manufacturing apparatus. The computer slices the geometric model into a plurality of slices to be built as layers in the powder bed fusion additive manufacturing apparatus based upon a defined layer thickness. In this embodiment, the defined layer thickness, L, is less than 30 micrometres and, preferably 20 micrometres.

[0058] The computer may comprise an interface arranged to provide a user input for selecting the material from which the object is to be built. The computer then selects exposure parameters from a database that are suitable for the identified material. A laser exposure pattern is then determined for melting areas of each layer to form the corresponding cross-section (slice) of the object. Based upon these calculations, the computer generates instructions that are sent to controller 140 to cause the additive manufacturing apparatus to carry out a build in accordance with a desired exposure strategy. For nickel-based superalloys, such as CM247 LC, the following exposure strategy is used.

[0059] Referring to FIGS. 2 and 3, the laser beams are scanned along hatch lines 201 in each layer L.sub.1, L.sub.2 to form the object. The hatch lines 201a, 201b for every layer L1, L.sub.2 are parallel and aligned in the z-direction such that a melt pool 301 in a layer L.sub.2 is formed directly above the melt pool 300 of the immediately preceding layer L.sub.1. This can be seen in FIGS. 4a and 5. The hatch lines 201a, 201b for a layer L.sub.1, L.sub.2 may all be scanned in the same direction (unidirectional scanning), or in alternate directions (bidirectionally) as shown in FIG. 2. An overlying hatch line 201b of a successive layer L.sub.2 may be scanned in the same or opposite direction to the underlying hatch line 201a of the previous layer, L.sub.1. An order in which the hatch lines 201a, 201b are scanned by the laser beam(s) may be the same or different between layers L.sub.1, L.sub.2. Furthermore, different ones of the hatch lines 201a, 201b may be scanned by different ones of the laser beams 118a, 118c.

[0060] The laser beam parameters, such as laser power, spot size on the powder (focal distance) and scan speed (for a point scanning regime, point distance, exposure time and delay time between exposures), are selected such that the melt pools formed by scanning the hatch lines 201a, 201b are formed in transition or conduction mode. The wider and shallower the melt pools 300, 301, whilst still melting through an entire layer L.sub.1, L.sub.2 of powder, the better. A hatch distance, H, between hatch lines 201a or 201b within a layer L.sub.1, L.sub.2 is selected such that solidified material formed by steeply inclined boundary regions (with respect to a plane of the powder layer) of the melt pools 300 are remelted by less steeply inclined boundary regions of melt pools 301 of the next layer L.sub.2. For melt pools 300, 301 having a width, W, to depth, d, ratio of around 2:1, as shown in FIGS. 3a, 3b and 4, the hatch distance H may be less than 50% of the width, W, of the melt pool. For melt pools with higher width to depth ratios, the hatch distance, H, may be larger. Hatch distance may be selected based upon a function relating hatch distance to the width to depth ratio. A typical width to depth ratio of the melt pool for a particular set of laser beam parameters may be determined empirically. As can be seen from FIG. 3, the melt pools 300 width and depth will vary during processing within an expected range of values, but sufficient control can be maintained such that a desired geometrical relationship is achieved.

[0061] FIGS. 3a, 3b and 4 show images of cross-sections of the material perpendicular to the hatch line direction. Referring to FIG. 4, grains 303a to 303d grow in a direction of heat flow from the melt pool 300, 301 to surrounding material.

[0062] Accordingly, a direction of grain growth is influenced by a shape of the melt pool 300, 301. Grains 303a, 303b formed at a centre of the melt pool (a less steeply inclined boundary region) will grow in the build direction, BD, whereas grains 303c, 303d formed within steeply inclined boundary regions of the melt pool form in a direction inclined to the build direction BD. By forming the melt pools 301 of the next layer L.sub.2 directly above the (now solidified) melt pools 300 of the previous layer L.sub.1 and through an appropriate selection of the overlap between adjacent melt pools, the material forming grains 303c, 303d is remelted and the resultant solidification of the material replaces grains 303c, 303d with grains more closely inclined to the build direction, BD. Accordingly, a directionally solidified microstructure is produced.

[0063] This geometrical arrangement of the melt pools formed in conduction mode can be contrasted with melt pools formed in keyhole mode, as shown in FIG. 5, wherein the steeply inclined boundaries of the melt pools 400 result in the formation of grains 403a, 403b at a centre of the melt pool 400 at an angle to the build direction BD.

EXAMPLE 1

[0064] A CM247 LC cube was formed using the above-mentioned exposure strategy in a Renishaw AM400 additive manufacturing apparatus. The following laser beam parameters were used:

[0065] Laser power: 140 W

[0066] Diameter of spot: 70 μm

[0067] Hatch distance: 50 μm

[0068] Point distance: 70 μm

[0069] Exposure time: 70 μs

[0070] Delay time: 0 s

[0071] The 0s delay time between each point exposure effectively causes the laser beam to be continuously scanned (i.e. without the laser beam being turned off) along the hatch lines.

[0072] FIG. 6 is an image of a cross-sections of the cube in a working plane, parallel to the build direction. As can be seen, the part has a low crack density.

[0073] FIG. 8 shows the orientation of a major axis of the columnar grains with respect to the build direction. As can be seen, substantially all, if not all, of the columnar grains have their major axis oriented within 15° of the build direction.

[0074] FIG. 9 shows the crystallographic orientation of these grains. Less than 30% of the grains have their <100> crystallographic directions deviated by more than 20° from the build direction.

EXAMPLE 2

[0075] In example 2 the same laser beam parameters were used but with single isolated exposures. FIG. 7 is an image of a cross-section of the cube in a working plane, parallel to the build direction. As can be seen, the part has a significantly higher crack density compared to the part of example 1.

[0076] It will be understood that alterations and modifications may be made to the embodiments without departing from the invention as described herein. For example, in thin sectioned regions of a cross-section to the solidified, the scan paths and/or laser parameters may be modified to ensure that heat does not accumulate, which otherwise could result in the melt pool profile diverging from the required shape.