POWDER BED FUSION APPARATUS AND METHODS

Abstract

A powder bed fusion apparatus in which an object is built in a layer-by-layer manner. The apparatus has a build sleeve and a build platform for supporting a powder bed, the build platform lowerable in the build sleeve. A processing plate is coupled to an upper end of the build sleeve. The apparatus further has a doser for dosing powder and a recoater for spreading the dosed powder across the processing plate to the powder bed. A heater is provided for heating the powder bed. An active cooling device having a cooling element and/or cooling channel is to form, in use, an active thermal barrier to conduction of heat from the build sleeve through the processing plate.

Claims

1. A powder bed fusion apparatus in which an object is built in a layer-by-layer manner, the apparatus comprising a build sleeve, a build platform for supporting a powder bed, the build platform lowerable in the build sleeve, a processing plate coupled to an upper end of the build sleeve, the processing plate being a separate component to the build sleeve, a doser for dosing powder, a recoater for spreading the dosed powder across the processing plate to the powder bed, a heater for heating the powder bed and an active cooling device comprising a cooling element and/or cooling channel to, in use, form an active thermal barrier to conduction of heat from the build sleeve through the processing plate.

2. A powder bed fusion apparatus according to claim 1, wherein the cooling element and/or cooling channel at least partially surrounds the upper end of the build sleeve.

3. A powder bed fusion apparatus according to claim 1, wherein the cooling element contacts the processing plate.

4. A powder bed fusion apparatus according to claim 3, wherein the cooling element contacts a bottom surface of the processing plate.

5. A powder bed fusion apparatus according to claim 1, wherein the cooling element and/or cooling channel is located between the processing plate and the upper end of the build sleeve.

6. A powder bed fusion apparatus according to claim 1, wherein the cooling element is embedded within the processing plate.

7. A powder bed fusion apparatus according to claim 1, wherein the cooling channel is formed by holes within the processing plate.

8. A powder bed fusion apparatus according to claim 1, wherein the cooling element comprises a thermally conductive element cooled by a cooling source which removes heat from the cooling element.

9. A powder bed fusion apparatus according to claim 1, wherein the cooling element comprises a conduit for carrying coolant and/or the cooling channel is arranged to carry a coolant.

10. A powder bed fusion apparatus according to claim 9, wherein the cooling device comprises a chiller for cooling the coolant, the conduit and/or cooling channel forming part of a circuit for recirculating the coolant through the chiller.

11. A powder bed fusion apparatus according to claim 1, wherein the cooling element and/or cooling channel comprises a loop that extends around the build sleeve.

12. A powder bed fusion apparatus according to claim 11, wherein the cooling element and/or cooling channel comprises a plurality of loops.

13. A powder bed fusion apparatus according to claim 1, wherein the processing plate has a powder opening for receiving excess powder that remains after spreading of one or more layers by the recoater and the cooling element and/or cooling channel extends between the build sleeve and the powder opening, wherein the powder opening may be a powder overflow or an opening of a doser for dosing powder.

14. A powder bed fusion apparatus according to claim 1, wherein the cooling element and/or cooling channel is arranged such that powder can be spread by the recoater between a dosing position and the powder bed without passing over the cooling element and/or cooling channel.

15. A powder bed fusion apparatus according to claim 1, wherein the processing plate is built using an additive manufacturing method, with the cooling channels contained therein.

Description

DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a schematic cross-sectional view of a powder bed fusion apparatus according to a first embodiment of the invention;

[0024] FIG. 2 is a plan view of a processing plane of the powder bed fusion apparatus shown in FIG. 1;

[0025] FIG. 3 is a schematic cross-sectional view of a powder bed fusion apparatus according to a second embodiment of the invention;

[0026] FIG. 4 is a plan view of a processing plane of the powder bed fusion apparatus shown in FIG. 3; and

[0027] FIG. 5. is a plan view of a processing plane of the powder bed fusion apparatus according to another embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

[0028] With reference to FIGS. 1 and 2, a powder bed fusion apparatus 100 according to an embodiment of the invention includes a build chamber 101 that can be sealed from the external environment. The build chamber 101 is divided into an upper processing chamber 120 and a lower chamber 140 by a processing plate 115 and a build platform 102 reciprocally movable within a bore 116 of a build sleeve 117.

[0029] The build platform 102 is moved by an elevator mechanism 118, 119 located in the lower chamber 140. In this embodiment, the build platform 102 is connected to a lead screw 118 of a drive mechanism by an A-shaped frame 114, which acts to insulate the lead screw 118 from the build platform 102. The A-frame 114 may be cooled, for example by a coolant flowing in a coolant line (not shown). Surrounding the A-frame 114 is insulation 172, for example carbon hardboard, for insulating the A-frame 114 and lead screw 118 from the hot walls of the build sleeve 117. The lead screw 118 is driven by drive 119.

[0030] The build platform 102 is sealably engaged with the bore 116 of the build sleeve 117 to prevent egress of powder into the lower chamber 140. This is achieved by seal 130, associated with an edge of the build platform 102, which physically engages with the bore 116 of the build sleeve 117. The processing plate 115, build sleeve 117, build platform 102 and associated seal 130 function to form a barrier for the powder such that the powder remains in the processing chamber 120 and does not travel to the lower chamber 140.

[0031] A rim of the build sleeve 117 comprises a stepped section including a shoulder and an inset collar that form a surface and abutment against which the processing plate 115 is engaged for coupling the processing plate 115 to the build sleeve 117. In this embodiment, the build sleeve 117 has a square horizontal cross-section formed of four walls secured together. Embedded within each wall is a resistive heater 161, 162 for heating a powder bed 104. A further resistive heater 160, for heating an object 103 as it is built and the powder bed 104, is provided in the build platform 102. The mounting of the processing plate 115 onto the build sleeve 117 allows differential thermal expansion between the two components. In this embodiment, both the build sleeve 117 and the processing plate 115 are made of steel sheets.

[0032] The powder bed fusion apparatus comprises a cooling device 150 for cooling the processing plate 115. The cooling device 150 comprises a cooling element 153 inserted into a channel in the processing plate 115 that is open to an underside of the processing plate 115. The cooling element is actively cooled by a chiller 151. In this embodiment, the cooling element is in the form of a conduit 153 for carrying a coolant, which flows under the control of a pump (not shown) in a circuit through the conduit 153 and a chiller 151. The coolant flows between the conduit 153 and the chiller 151 via supply and return lines 152. In this embodiment, the conduit forms a loop that substantially surrounds the build sleeve 117 to form, in use, an active thermal barrier to conduction of heat from the build sleeve 117 through the processing plate 115. The conduit 153 is copper piping, which provides good thermal conductivity for the conduction of heat to the coolant.

[0033] In another embodiment, the cooling channels are formed directly into the processing plate 115. For example, cooling channels may be formed by cross-drilling holes into the processing plate 115 and then sealing openings to form a single coolant circuit. Forming the cooling channels directly into the processing plate 115 may reduce the risk of coolant leaks compared to the use of a separate conduit.

[0034] In this embodiment the coolant is water. However, for higher temperature applications it may be desirable to use a coolant such as oil or a gas, such as air, nitrogen, argon and/or a cryogenic gas.

[0035] The processing chamber 120 encloses a working plane 135 to which one or more energy beams, in this embodiment a laser beam, is directed to fuse powder of the powder bed 104 to form a three-dimensional object 103. The processing chamber 120 is arranged to maintain an inert atmosphere around the working plane 125. The processing chamber 120 houses a doser 108 for dosing powder to be formed into layers to a dosing position 129 on the processing plate 115 and a recoater 109 for spreading each dose of powder from the dosing position 129 into one or more layers of powder over the build platform 102/powder bed 104. The doser may comprise a dosing mechanism as described in WO2010/007396. In this embodiment, the recoater 108 spreads the dose of powder in a single direction. Openings 135, 136 are provided in the processing plate 115 on opposite sides of the build sleeve 117 in a powder spreading direction. Each opening 135, 136 is connected to a powder collection hopper 137, 138 for capturing excess powder that remains in front of the recoater 109 after the spreading of a powder layer or is picked up on a return stroke of the recoater 109. Preferably, both openings 135, 136 lead to the same collection hopper.

[0036] In the first embodiment, the dosing position 129 is on a portion of the processing plate 115 that falls outside of an area surrounded by the conduit 153. Accordingly, the dosing position is on a “cold” portion of the processing plate 115. The openings 135 and 136 are also located outside of the area surrounded by the conduit 153 and thus, are also in a “cold” portion of the processing plate 115. The flow of coolant through the conduit 153 is in a direction such that the coolant passes through a thinner section of the processing plate 115 between the build sleeve 117 and the opening 135 before passing through other wider sections of the processing plate 115. Such narrower sections of the processing plate 115 are likely to dissipate heat more slowly than wider areas of the processing plate 115 because of the narrow routes for the conduction of heat away from these sections (the opening 135, in particular, providing a thermal barrier to conduction from the portion of the processing plate 115 between the opening 135 and the build sleeve 117). By passing the coolant first through such narrower section of the processing plate 117 before passing the coolant through wider section of the processing plate 117 the cooling of these potentially hotter narrower regions is prioritised.

[0037] Insulation 170, 171, such as carbon hardboard, is provided around the build sleeve 117 for maintaining the heat within the build volume.

[0038] Optical access to the processing chamber 120 for a high-powered laser beam is provided via window 107. A high-powered laser beam can be directed from laser 105 through the window 107 by a scanner 106 for scanning the laser beam over the working plane 125 to consolidate successive layers of powder. The scanner 106 comprises two steering mirrors (only one 110 of which is shown) and focussing optics 111.

[0039] In this embodiment, the lower chamber 140 is sealable from the external environment as part of the internal volume of the build chamber 101. However, it will be understood that in another embodiment, a region below the processing plate 115 and the build platform 102 may not be sealable from the external environment and, in such an embodiment, the seal 130 and or another seal about the build platform 102 acts as a gas seal to maintain the desired (inert) atmosphere in the processing chamber 120.

[0040] The upper and lower chambers 120, 140 are coupled to each other via an opening (not shown), which allows the pressure in the chambers 120, 140 to be equalised. Preferably there is a filter (not shown) within the opening to prevent powder and soot from entering the lower chamber. This arrangement provides the advantage that the pressure immediately above and below the build platform 102 may be maintained at the same level such that powder is not forced past the seal 130.

[0041] The powder bed fusion apparatus comprises a gas flow circuit for forming the inert atmosphere and a gas knife as described in WO2016/079494, which is incorporated herein by reference. The gas knife is formed between a gas nozzle 112 and a gas exhaust 113 in a direction (as shown by the arrows over the powder bed 104) perpendicular to the direction of movement of the recoater 109. The gas flow circuit may cool the gas recirculated through the gas flow circuit and inject the cooled gas into the processing chamber 120, for example as described in WO2016/102970.

[0042] In use, during heating of the powder bed 104 by heaters 160, 161, 162, coolant is circulated through conduit 153 to cool the processing plate 115 in the region of the conduit 153. This cooled region acts as a thermal barrier to the conduction of heat from the build sleeve 117 to other components of the powder bed fusion apparatus. In this way, damage to these other components is avoided.

[0043] Referring to FIGS. 3 and 4 a further embodiment of the invention is shown. Features of this embodiment corresponding to similar or like features of the embodiment described above with reference to FIGS. 1 and 2 have been given the same reference numerals but in the series 200. Differences between this embodiment and the previously described embodiment are described below. For description of other features that are the same in both embodiments, reference is made to be above description of these features with reference to FIGS. 1 and 2.

[0044] The second embodiment differs from the first embodiment in that the recoater 209 comprises a dual wiper system in which powder can be spread across the powder bed in both directions. For example, the dual recoater may be as described in EP1189716. As powder can now be spread in both directions, only a single opening 236 is provided in the processing plate 215.

[0045] The powder fusion apparatus further comprises a heater 264, such as a resistive heater, embedded within the processing plate 215 for heating the powder in the dosing position 229 before it is spread across the powder bed 204. For example, the recoater powder heating system may be as described in WO2017/008890. In another embodiment, the heater is provided in the recoater for heating the powder as it is spread across the powder bed 104. The conduit 253 is arranged in the processing plate 115 to surround both the build sleeve 217 and the dosing position 229. In this way, the conduit 253 carrying coolant cools the processing plate 215 to provide a thermal barrier to conduction of heat from the build sleeve and the “hot” region of the processing plate 215 heated by heater 264.

[0046] Preheating of the powder before the powder is spread into a layer may facilitate the building process as it ensures that the newly spread powder is closer to or at the required temperature for fusion immediately after the layer has been spread.

[0047] Referring to FIG. 5, a further embodiment of the invention is shown. Features of this embodiment corresponding to similar or like features of the embodiments described above with reference to FIGS. 1 to 4 have been given the same reference numerals but in the series 300. Differences between this embodiment and the previously described embodiment are described below. For description of other features that are the same in both embodiments, reference is made to be above description of these features with reference to FIGS. 1 to 4.

[0048] In this embodiment, the processing plate 315 is built with the cooling channels 353 integrally formed therein. For example, the processing plate 353 may be built by additive manufacturing or vacuum brazing. In this way, more complex shaped cooling channels 353 can be formed without significantly compromising the mechanical stability of the processing plate 315. In this embodiment, the cooling channels are formed as a series of coils formed in a common plane, the coils arranged in a loop around the build sleeve 317. Such an arrangement may be used to cool a greater surface area of the processing plate 315 to provide a wider thermal barrier to heat conduction from the build sleeve 317. In a further embodiment, an areal cooling channel is provided, wherein a cooling channel in the processing plate defines a cooling chamber having a width greater than its height. Fins or columns may be provided between the floor and ceiling of the cooling chamber to provide sufficient structural integrity to the processing plate 115.

[0049] It will be understood that modifications and alterations can be made to the above described embodiments without departing from the invention as defined herein.

[0050] For example, heating of the powder bed may be carried out by means other than resistive heaters, for example by induction heating as described in US2013/0309420 or microwave heating as described in WO2016/051163.

[0051] Furthermore, apparatus for thermally protecting components from heat generated at the powder bed may be used with powder bed fusion apparatus without means for preheating the powder bed. For example, in a multi-laser powder bed fusion apparatus, an amount of energy can be delivered to the powder bed in a short period of time causing significant heating of the powder bed/build volume. Accordingly, means, such as those described above, may be required for protecting components within the build chamber from this heat.

[0052] Rather than using a top doser 108, 208 as described above, a bottom doser may be used, which doses powder through an opening in the processing plate, such as a piston dosing mechanism. The cooling element/cooling channel may extend between the bottom doser and the build sleeve or surround both the build sleeve and the bottom doser. The latter may be desirable if the powder is to be preheated in the dosing piston. The powder may be preheated in the dosing piston to a lower temperature than the temperature to which it is heated in the build sleeve to avoid agglomeration or bonding of the powder in the dosing piston.