FABRICATING HOLLOW COMPONENTS
A method of fabricating a component having an internal void, the method comprising the steps of: i) providing a core having a shape corresponding to the internal void of the component; ii) compacting a powder around the core to form a compacted powder body around the core; iii) removing the core; and iv) heating the compacted powder body to consolidate the compacted powder body.
This specification is based upon and claims the benefit of priority from UK Patent Application Number 1811430.6 filed on 12 Jul. 2018, the entire contents of which are incorporated herein by reference.
TECHNOLOGICAL FIELDThe present disclosure relates to methods of fabricating components having an internal void using powder compaction and consolidation.
BACKGROUNDHollow components that need to withstand substantial and repeated stresses, such as bearing cages, journal bearings or torque carriers, are conventionally fabricated from solid blocks of material, typically metal. This involves substantial machining to form the external and internal features of the component. For larger sized components particularly, extensive machining to create a large void within the component can add substantial cost and complexity to the manufacturing process, as well as being wasteful of material. In some applications, such as epicyclic gearbox torque carriers for geared turbofan aircraft engines, the hollow component may need to have dimensions of the order of 1 m or more, making machining processes extensive and costly. Improved and more efficient processes for making such components would therefore be beneficial.
Applications such as in automotive, marine or aerospace tend to require high performance materials with a uniform structure and minimum defects, which is preferably achieved by creating the component as a unitary body with no joints. For high performance material applications where uniformity is of greater importance, powder metallurgy may be used to form the component, followed by machining. A general problem therefore is how to reduce any such machining to a minimum.
SUMMARYAccording to a first aspect there is provided a method of fabricating a component having an internal void, the method comprising the steps of:
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- i) providing a core having a shape corresponding to the internal void of the component;
- ii) compacting a powder around the core to form a compacted powder body around the core;
- iii) removing the core; and
- iv) heating the compacted powder body to consolidate the compacted powder body.
The use of a core to define the shape of the internal void of the component substantially reduces the amount of machining that would otherwise be required for fabricating such a component.
The core may be removed from the compacted powder body before or during the process of consolidating the powder body through heating. The core may for example be volatilised during an initial heating stage prior to full consolidation of the powder body. The core may alternatively be removed prior to the heating stage, for example by dissolving or disintegrating the core.
The method may comprise assembling one or more pre-compacted pieces of the component in or around the core prior to compacting the powder around the core and the pre-compacted pieces to form the compacted powder body. This has the advantage of allowing for more complex geometries to be created that would otherwise not be possible using compaction of a loose powder alone due to limited flow possible during powder compaction.
The core may comprise tooling configured to hold the core in position relative to the compacted powder body during the step of compacting the powder around the core. This tooling may be removable prior to removing the core, for example by mechanically removing the tooling rather than by melting or volatilising.
Step iii) may comprise heating the compacted powder body to melt or volatilise the core.
Step ii) may comprise applying isostatic pressure to form the compacted powder body. For example, the compacted powder body may be formed by cold isostatic pressing of the powder.
The method may comprise a subsequent step v) of hot isostatic pressing the consolidated powder body to densify the component.
The core may comprise an internal volume and a passage extending from the internal volume to an external surface of the compacted powder body, wherein step ii) comprises applying isostatic pressure to the powder via a fluid surrounding the powder body and extending within the internal volume. This reduces the amount of material used to form the core, and allows isostatic pressure to be applied from within the internal void as well as from external to the component, thereby improving the uniformity of the compacted powder body. The core may be defined by an inner elastomeric container, with the powder contained within an outer container. The passage may be provided between an outer surface of the outer container and an inner surface of the inner container to allow for pressure applied to the fluid to be transmitted through to the internal volume within the core. The inner and outer containers may be composed of an elastomeric material such as a rubber, and may be formed by a three dimensional printing technique such as stereolithography. An advantage of using a flexible bag for defining the shape of the compacted powder body is that draft angles are not required to allow for removal of the core after compaction, which would otherwise be the case when using hard tooling for example. A further advantage is that pre-compacted portions of the powder body may be placed in position within the internal volume between the inner and outer containers, or bags, prior to introduction of the remaining powder before compaction.
The component may be a torque carrier for an epicyclic gearbox.
The powder may be a metallic powder, for example a steel.
The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.
Embodiments will now be described by way of example only, with reference to the Figures, in which:
The core 101 will require supporting within the powder body before and during the process of introducing and compacting powder around the core 101. In the example illustrated in
To compact the powder to make a compacted powder body, isostatic pressing is preferably used (cold isostatic pressing for example).
The core may comprise further components designed to extend into or through the powder body to form pockets or passages through the compacted powder body, as illustrated in
Depending on the complexity of the shape of the core, one or more pre-compacted powder components may need to be added prior to compaction of powder around the core 401. This is because compaction under isostatic pressure may not allow for powder flow sufficient to achieve uniform compaction around a shape of the kind shown in
Following compaction of the powder body, the compacted powder body is removed from the isostatic pressing bag and subjected to an initial heating cycle to remove the core material, together with any binder present compacted powder body. The resulting form is fragile and may require mechanical support during sintering to prevent slumping. Such support may for example be in the form of a coarse ceramic powder, a foam structure or a latticework support, which may be provided within the core and remains within the body during the subsequent sintering stage.
Following removal of the core, the powder body is subjected to heating to cause the powder body to consolidate through sintering. For a metallic component, this may be carried out under vacuum to prevent oxidation of the metal and to allow pores between the metallic powder particles to be removed. In an example process, electrical field assisted sintering may be used, under a pressure of less than 10 mPa. A further hot isostatic pressing (HIP) process may be added to further densify the resulting component and reduce or remove any residual porosity.
It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.
Claims
1. A method of fabricating a component having an internal void, the method comprising the steps of:
- i) providing a core having a shape corresponding to the internal void of the component;
- ii) compacting a powder around the core to form a compacted powder body around the core;
- iii) removing the core; and
- iv) heating the compacted powder body to consolidate the compacted powder body.
2. The method of claim 1 comprising assembling one or more pre-compacted pieces of the component in or around the core prior to compacting the powder around the core and the pre-compacted pieces to form the compacted powder body.
3. The method of claim 1 wherein the core comprises tooling configured to hold the core in position relative to the compacted powder body during the step of compacting the powder around the core.
4. The method of claim 3 comprising removing the tooling prior to removing the core.
5. The method of claim 1 wherein step (iii) comprises heating the compacted powder body to melt or volatilise the core.
6. The method of claim 1 wherein step (ii) comprises applying isostatic pressure to form the compacted powder body.
7. The method of claim 6, wherein the compacted powder body is formed by cold isostatic pressing of the powder.
8. The method of claim 1 comprising a subsequent step v) of hot isostatic pressing the consolidated powder body to densify the component.
9. The method of claim 1 wherein the core comprises an internal volume and a passage extending from the internal volume to an external surface of the powder body, wherein step ii) comprises applying isostatic pressure to the powder via a fluid surrounding the powder body and extending within the internal volume.
10. The method of claim 9 wherein the core is defined by an inner elastomeric container and the powder is contained within an outer container.
11. The method of claim 1 wherein the component is a torque carrier for an epicyclic gearbox.
12. The method of claim 1 wherein the powder is metallic.
Type: Application
Filed: Jul 11, 2019
Publication Date: Jan 16, 2020
Inventors: Daniel Clark (Derby), Martin J. Rawson (Derby), Alan A. Pardoe (Derby)
Application Number: 16/509,047