POWDER BED FUSION ADDITIVE MANUFACTURING METHODS AND APPARATUS

Abstract

A powder bed fusion additive manufacturing method includes forming layers of powder of a powder bed and exposing the layers to one or more energy beams to melt the powder to form an object. The exposure of each layer to the or each energy beam forms melt pools in a conduction or transition mode with an exposure distance between adjacent exposures within the layer being 40% to 60% of a width of the melt pools generated by the exposures and an offset of exposures between successively melted layers in a direction in which the exposure distance is measured being 40% to 60% of the exposure distance.

Claims

1. A powder bed fusion additive manufacturing method comprising forming layers of powder of a powder bed and exposing the layers to one or more energy beams to melt the powder to form an object, wherein the exposure of each layer to the or each energy beam forms melt pools in a conduction or transition mode with an exposure distance between adjacent exposures within the layer being 40% to 60% of a width of the melt pools generated by the exposures and an offset of exposures between successively melted layers, in a direction in which the exposure distance is measured, being 40% to 60% of the exposure distance.

2. A powder bed fusion additive manufacturing method according to claim 1, comprising scanning the energy beam or at least one of the energy beams along, hatch lines on each layer, wherein the exposure distance is a hatch distance between adjacent hatch lines.

3. A powder bed fusion additive manufacturing method according to claim 2, wherein the hatch distance between adjacent hatch lines is 40% to 60% of a width of the melt pools generated by exposure of the adjacent hatch lines to the one or more energy beams.

4. A powder bed fusion additive manufacturing method according to claim 2, comprising scanning the adjacent hatch lines one after the other.

5. A powder bed fusion additive manufacturing method according to claim 2, wherein the hatch lines on successively melted layers are parallel.

6. A powder bed fusion additive manufacturing method according to claim 1, wherein each layer has a layer thickness less than half an average melt pool depth.

7. A powder bed fusion additive manufacturing method according to claim 1, wherein each layer has a layer thickness of less than 50 micrometres.

8. A powder bed fusion additive manufacturing method comprising forming layers of powder of a powder bed and exposing the layers to one or more energy beams to melt the powder to form an object, wherein the exposure of each layer to the or each energy beam forms melt pools with a depth to width ratio of less than 1.5, wherein adjacent melt pools overlap such that a spacing between centres of the adjacent melt pools is 40% and 60% of a width of the melt pools and a centre of each melt pool of a next layer is offset, in a direction parallel to the layers, from the centres of melt pools of the immediately preceding layer by 40% to 60% of the spacing between the centres of the adjacent melt pools.

9. A powder bed fusion additive manufacturing method according to claim 8, wherein the exposure of each layer to the or each energy beam forms melt pools across the layers arranged relative to each other such that, for a plurality of pairs of adjacent first melt pools formed in a one of layers, columnar grains formed at substantially 45 degrees to a build direction during the solidification of the pair of adjacent melt pools coincide with a bottom of a corresponding second melt pool formed in the next layer, which partially remelts material melted during formation of the pair of adjacent melt pools.

10. A powder bed fusion additive manufacturing method according to claim 8, wherein the exposure of each layer to the or each energy beam forms melt pools across the layers arranged relative to each other such that a lowest point on melt pools in successively solidified layers are offset relative to each by an average of approximately 45 degrees to the build direction.

11. A powder bed fusion additive manufacturing apparatus comprising a build platform for supporting a powder bed, and layer formation device for forming layers of powder to form the powder bed, a scanner of directing one or more energy beams to each layer to melt the powder in selected regions of each layer and a controller configured to control the scanner and the layer formation device to carry out the method of claim 1.

12. Build instructions stored on a data carrier, which, when executed by a controller of a powder bed fusion additive manufacturing apparatus cause the powder bed fusion additive manufacturing apparatus to build an object in accordance with the method of claim 1.

13. Instructions stored on a data carrier, which, when executed by a processor, cause the processor to generate build instructions for building an object using a powder bed fusion additive manufacturing apparatus in accordance with claim 12.

Description

DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic view of a powder bed fusion additive manufacturing apparatus according to an embodiment of the invention;

[0023] FIG. 2 is a hatch exposure strategy used in a method according to an embodiment of the invention;

[0024] FIG. 3 is a diagrammatic representation of melt pools formed using a method according to an embodiment of the invention;

[0025] FIG. 4 shows a cross-section of a part in a plane parallel with a build direction for a part built in IN625 using a prior art exposure strategy;

[0026] FIG. 5 shows a cross-section of a part in a plane parallel to a build direction for a part built in IN625 using an exposure strategy according to an embodiment of the invention; and

[0027] FIG. 6 is an image obtained using electron backscatter diffraction (EBSD) showing the single-crystalline-like structure of a cube built in SS316L using a method according to an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

[0028] 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 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 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.

[0029] Layers of powder 104 are formed as the workpiece 103 is built by a layer formation device, in this embodiment a dispensing apparatus 108 and a wiper 109. For example, the dispensing apparatus 108 may be apparatus as described in WO2010/007396. The dispensing apparatus 108 dispenses powder onto an upper surface defined by partition 115 and is spread across the powder bed by the wiper 109. A position of a lower edge of the wiper 109 defines a working plane 190 at which powder is consolidated.

[0030] 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 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 focusing optics 120, such as two movable lenses for changing the focus of the laser beam 118. The scanner is controlled such that the focal position of the laser beam 118 remains in the working plane 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.

[0031] 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).

[0032] 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.

[0033] 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.

[0034] A laser exposure sequence is then determined for melting areas of each layer to form the corresponding cross-section (slice) of the object. Referring to FIG. 2, in this embodiment, a plurality of parallel hatch lines 201a to 201e; 202a to 202e are determined for each layer, the hatch lines defining paths for the laser(s) to follow in the melting of an area of the powder layer to form the object. In FIG. 2, the dark hatch lines 201a to 201e show the hatch lines for a first layer and the light grey hatch lines 202a to 202e show the hatch lines for a second, immediately succeeding layer. It will be understood that the hatch lines for every other layer will be substantially aligned in the build direction, BD (see, for example, FIG. 3).

[0035] The hatch lines are spaced within a layer a predetermined exposure (hatch) distance, HD, apart. The hatch distance, HD, is set at 40% to 60% of the melt pool width, w (the width of the melt pool perpendicular to the hatch lines). A melt pool width, w, is predetermined for the set of laser parameters selected to be used, such as laser power, scan velocity (or in a point scanning regime as used in Renishaw's AM400 machine, point distance, PD, and exposure time) and laser spot size (1/e) (focal length). Typically, the laser parameters are selected to form a melt pool having a width of 80 to 200 micrometres and more typically around 130 to 170 micrometres. The laser parameters (and scanning regime) are also selected to ensure that the melt pool is formed in a conduction or transition mode with the depth, d, to width, w, ratio of the melt pool less than unity. In this embodiment, the laser parameters are selected such that the depth to width ratio for the melt pools is around 0.5 (given fluctuations will occur due to process variability).

[0036] For a single laser exposing a plurality of neighbouring hatch lines having lengths above a predetermined threshold, such as longer than 0.5 mm, the laser beam scans the hatch lines successively in a bidirectional manner (an adjacent hatch line being scanned in an opposite direction to the immediately preceding hatch line). For shorter hatch lines, the laser beam may expose the hatch lines in order but unidirectionally or the laser beam may expose the hatch lines out of order such that adjacent hatch lines are not sequentially processed. For such small sections, an order in which the hatch lines are irradiated may be randomized with a specified minimum number of hatches between consecutively irradiated hatches and/or a specified minimum time delay between the processing of adjacent hatch lines. This may give time for the energy to dissipate such that melt pools continue to be formed having the required dimensions. In a multi-laser system, allocation of the hatch lines to different lasers allows for more complex exposure strategies but the overall aim of any such exposure strategy is for the melt pools to be formed having the required depth to width ratio. Furthermore, exposure strategies not comprising scanning of hatch lines may be used, such as spaced apart point scanning as disclosed in WO2016/079496, which is incorporated herein by reference.

[0037] An orientation of the object relative to a build direction may be selected based upon a preferred hatch length to be used for forming the desired micro-structure. For example, an object comprising a rod-like- or bar-like-structure may be orientated such that hatch lines are not parallel, such as at greater than 45 degrees, to a longitudinal direction of the rod-like- or bar-like-structure. It may be preferable for the hatch lines to be substantially perpendicular to the longitudinal direction.

[0038] In an alternative embodiment, the hatch lines are aligned with a longest direction of a cross-section of the part to be printed.

[0039] Hatch lines 201a to 201e and 202a to 202e (or in the case of spaced apart point scanning, discrete exposures) between successively melt layers are offset by an offset distance, OD, that is 40% to 60% of the exposure/hatch distance, HD and, most preferably 50% of the exposure/hatch distance, HD. The hatch lines across all the layers are parallel.

[0040] Once computed the instructions are sent to a powder bed fusion additive manufacturing apparatus for execution.

[0041] It has been found that parts formed using such an exposure strategy have a substantially single crystalline micro-structure. Not to be bound by any one theory but the applicant believes a single crystalline micro-structure is formed for the reasons as now explained with reference to FIG. 3. FIG. 3 illustrates a typical melt pool arrangement that may be achieved when using such an exposure strategy. FIG. 3 shows the melt pool shapes perpendicular to the hatch lines. A first layer, L1, is exposed to the laser beam(s) when scanned in hatch lines to form melt pools, 300a to 300c. Each melt pool overlaps by 50% of their width, w, with the or each adjacent melt pool. Columnar grains 301a to 301i are formed in each melt pool along the steepest thermal gradient, which is typically normal to a surface of the melt pool. Parts of the melt pool that are remelted after solidification due to an overlapping melt pool will be reformed with grains based upon the geometry of this subsequently formed melt pool. For the first layer, L1, columnar grains 301a, 301b may be formed at a bottom of each melt pool aligned in a build direction, BD. Columnar grains 301c, 301d, 301e, 301f formed at sides of the melt pool (away from a centre of the melt pool) will typically be aligned more closely to 45 degrees to the build direction, BD, due to the shape of the melt pool. Accordingly, towards a centre of melt pools 300a, 300b, 300c, <100> oriented grains are formed. Away from the centre of the melt pools <110> oriented grains are formed.

[0042] For the next layer, L2, (and layers thereafter), the melt pools are offset by an offset distance, OD, from the centres of melt pools of the immediately preceding layer by 50% of the hatch spacing, HD between the centres of the adjacent melt pools in the immediately preceding layer. As a result, a bottom of each melt pool coincides with columnar grains 301c, 301d, 301e, 301f formed during solidification of a melt pool in the immediately preceding layer oriented at 45 degrees to the build direction, BD. These columnar grains 301c, 301d, 301e, 301f “seed” the melt pools resulting in the continuation of grains in this orientation in the melt pool for the next layer, even at the bottoms/centres of the melt pools. This “seeding” and continuation of the columnar grains 301c, 301d, 301e, 301f oriented at 45 degrees to the build direction, BD, results in a single crystalline micro-structure with a <110> orientation substantially parallel to the build direction.

[0043] As shown in FIG. 3, in order to maintain this directional growth of the columnar grains, a line between corresponding points, such as the bottoms, of the melt pools of successively melted layers should form an angle, A, of approximately 45 degrees to the build direction, BD. This will be governed by the melt pool shape and the layer thickness.

[0044] A direction in which the hatch lines are scanned may be selected based upon a gas flow direction of the gas across the powder bed. For example, an order in which the hatch lines are scanned may be selected such that a direction the energy beam(s) progressively scan the hatch lines (in a direction perpendicular to the hatch lines) is at least partially opposed to a gas flow direction. In addition, for unidirectionally scanned hatch lines, the hatch lines themselves may be scanned in a direction at least partially opposed to the gas flow direction.

[0045] An orientation in which an object is built relative to a build direction may be selected such that the single crystalline micro-structure extends in a desired direction through the object.

Example 1

[0046] 1 cm×1 cm×1 cm cubes were built in IN625 using a Renishaw AM400 additive manufacturing machine and a prior art exposure strategy. A layer thickness of 20 micrometres was used and the following laser parameters: —

[0047] Laser power=190 W

[0048] Point distance=120 micrometres

[0049] Exposure time=120 microseconds

[0050] Hatch distance=95 micrometres

[0051] FIG. 4 is in image of a cross-section of one of the cubes. The solidified melt pools can be clearly seen with the overlapping melt pools within a layer. The melt pools of each layer are substantially aligned in the build direction (a layer offset of 0). The grain orientation of the cube was analysed using EBSD and it was found to have a poly-crystalline micro-structure.

Example 2

[0052] 1 cm×1 cm×1 cm cubes were built in IN625 using a Renishaw AM400 additive manufacturing machine and an exposure strategy in accordance with the invention as described above. A layer thickness of 20 micrometres was used and the following laser parameters: —

[0053] Laser power=160 W

[0054] Point distance=45 micrometres

[0055] Exposure time=60 microseconds

[0056] Hatch distance=55 micrometres

[0057] Layer offset=27.5 micrometres

[0058] FIG. 5 is in image of a cross-section of one of the cubes. The solidified melt pools can be clearly seen with the overlapping melt pools within a layer and the melt pools of one layer offset perpendicular to the build direction relative to the melt pools of the preceding layer by around 50%. The grain orientation of the cube was analysed using EBSD and it was found to have a single-crystalline-like micro-structure.

Example 3

[0059] 1 cm×1 cm×1 cm cubes were built in SS316L using a Renishaw AM250 additive manufacturing machine and an exposure strategy in accordance with the invention as described above. A layer thickness of 20 micrometres was used and the following laser parameters: —

[0060] Laser power=170 W

[0061] Point distance=80 micrometres

[0062] Exposure time=80 microseconds

[0063] Hatch distance=80 micrometres

[0064] Layer offset=40 micrometres

[0065] The grain orientation of one of the cubes was analysed using EBSD and it was found to have a single-crystalline-like micro-structure with its <110> orientation parallel to the build direction. An image obtained using EBSD is shown in FIG. 6.

Example 4

[0066] Parts were built in Hastelloy X using a Renishaw AM400 additive manufacturing machine and an exposure strategy in accordance with the invention as described above. A layer thickness of 20 micrometres was used and the following laser parameters: —

[0067] Laser power=160 W

[0068] Point distance=44 micrometres

[0069] Exposure time=60 microseconds

[0070] Hatch distance=66 micrometres

[0071] Layer offset=33 micrometres

[0072] The grain orientation of the parts was analysed using EBSD and the parts were found to have a single-crystalline-like micro-structure

[0073] It will be understood that alterations and modifications can be made to the above described embodiments without departing from the invention as described herein.

[0074] For example, other materials may be used, such as other difficult to weld alloys. For example, the invention may be used to form parts with a single crystalline or single-crystalline-like microstructure from superalloys, such as other Inconels, Waspaloy, other Rene alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys. Examples of such alloys are RENE 41, RENE77, IN738 and CM247.