Patent Application
20160332259
The invention relates to the manufacture of components using additive layer manufacturing methods. In particular, the invention provides methods which result in improved fracture resistance of the finished component.
Additive layer manufacturing (ALM) methods are known. In these methods a component is built up layer by layer until the 3D component is defined. In some ALM methods, the layers are laid down from a continuous extrusion of material. In other methods, layers are created by selective treatment of layers within a mass of particulate material, the treatment causing cohesion of selected regions of particulates into a solid mass. In other methods, a liquid mass is selectively treated to produce solid layers. Specific examples of ALM methods include (without limitation); electron beam melting (EBM), direct laser deposition (DLD), laser engineered net shaping (LNS), selective laser melting (SLM), direct metal laser sintering (DMLS) and selective laser sintering (SLS).
As will be appreciated, one of the advantages with ALM manufacturing techniques is that it can provide near net-shape components resulting in little waste which require subsequent additional machining. One exception to this may be the inclusion of supporting features or geometries which enable the components to be made. One particular application of ALM methods is in the formation of components for use in a gas turbine engine. It will be appreciated that, as well as accurate dimensional tolerances, such components must have excellent and consistent mechanical properties to prevent potentially catastrophic failure of the engine. Mechanical deficiencies in an ALM manufactured component can arise when residual stresses arising from the layering process result in a susceptibility to crack propagation from a surface of the component. The last laid layers of the component are most susceptible to fracture. If the last layer fails this impacts on the stress field in the underlying layer and the crack propagates. Surfaces which are inclined to the plane in which layers are laid down are also susceptible. Uneven distribution of stress between adjacent layers can result in delamination effects, cracks propagating between layers. These failures often only appear during a post ALM heat treatment step in the manufacture of the component.
Processes such as peening and shot blasting of the component surface can be used to induce compressive stresses in to the ALM manufactured component to discourage crack propagation during the subsequent heating operation.
The present invention provides a method for providing a component by additive layer manufacturing, the method comprising;
defining the geometry of the component,
defining a second geometry which incorporates a component geometry portion and a sacrificial geometry portion,
using an ALM method, manufacturing an intermediate having the second geometry,
applying a heat treatment to the intermediate, and
removing the sacrificial geometry portion.
In the second geometry, the sacrificial geometry portion is provided adjacent a surface of the component geometry portion which might otherwise be susceptible to crack propagation during the heat treatment. For example, the sacrificial geometry portion extends from the top (last laid) layer of the component geometry portion. In another example, the sacrificial geometry portion squares off a surface of the component geometry which is inclined to the plane in which layers are laid. In yet another example, the sacrificial geometry portion rounds off a tight radius of the component geometry portion.
The sacrificial geometry portion may take any of a wide range of forms each configured in general to spread residual stresses accruing in the component geometry to the sacrificial geometry.
The sacrificial geometry portion may be further configured with a weakness such that any failure arising in the intermediate as a consequence of the heat treatment step occurs in the sacrificial geometry portion in preference to the component geometry portion. For example (but without limitation) the weakness may, for example, comprise a notch, groove, sharp angle or rapidly changing cross sectional area.
The sacrificial geometry may be produced with an internal geometry designed to accommodate large plastic strains that might otherwise occur near the susceptible surface of the component geometry portion causing high residual stresses (and a susceptibility to cracking) in the component. For example, the internal geometry may have a lattice, honeycomb or wafer geometry or be porous. Such internal geometries will tend to deform or crack under residual stresses in preference to the component geometry.
The sacrificial geometry portion of the intermediate can be selectively engineered to have a more flawed structure by adjustment of the ALM process parameters. For example, when a DLD process is used, the material can be intentionally deposited to create a structure which is porous or contains a large number of micro cracks. This can be achieved by altering parameters which are otherwise optimised for a less flawed structure in the component geometry portion.
In ALM methods where the layers are created by selective treatment of layers within a mass of particulate material (for example, EBM), the sacrificial geometry portion can comprise an open structure such that the un-melted powder can be removed from the intermediate prior to any subsequent heat treatment. Alternatively, the sacrificial geometry portion comprises a closed structure such that a volume of un-melted powder is retained within its structure. In such embodiments, the low thermal conductivity un-melted powder acts as a thermal insulator, reducing cooling rates during deposition which should reduce the residual stress in the intermediate. These closed embodiments have further use in tuning heating rates within the intermediate during subsequent heat treatment processes to balance the relaxation of residual stresses in the component geometry portion with the microstructural changes that, if not so balanced, may cause ductility in the component portion to drop and increase the risk of cracking in the component geometry portion.
In other embodiments where the ALM process used permits the deposition of multiple materials in the intermediate (typically powder or wire feed systems), the sacrificial geometry portion may incorporate a material which is relatively more ductile than the material used to build the component geometry portion. As with other embodiments already described, the more ductile, sacrificial geometry portion of the intermediate can accommodate the large plastic strains that would otherwise occur near the susceptible surface of the component geometry portion and further mitigate the risk of crack initiation in the component geometry portion.
Where a component for ALM manufacture has a more complex geometry with multiple surfaces susceptible to crack propagation after a heating operation, a plurality of sacrificial geometry portions can be incorporated into an intermediate with the component geometry portion and all machined away after the heat treatment step.
Optionally, the component geometry portion can be built by a method other than an ALM method and an ALM method used to add the sacrificial geometry to the component geometry portion to form an intermediate prior to subsequent heat treating steps on the intermediate.
The method may further comprise a shot blasting, peening or pressing step performed after the manufacture of the intermediate and before the heat treatment step.
Direct laser deposition (DLD) is one useful ALM method to be used as the ALM process in the described methods of the invention. This process involves use of a laser as a heat source to deposit a metallic powder layer by layer for surface modification and fabrication of three-dimensional parts. The properties of the deposited layer, such as microstructure, hardness and dimensional accuracy determine the usage properties of the processed parts. The temperature and size of the melt pool, the cooling rate and the solidification conditions dominate the microstructure, hardness and dimensional accuracy. Many of these parameters can be controlled to influence the mechanical properties of the end product.
Embodiments of the invention will now be described with the aid of the following drawings of which:
As can be seen in
This “flare” geometry spreads residual stress and increases the surface area of the top surface (relative to that of the component geometry portion), emulating the baseplate (ie stabilising the part). This gives a larger area which can be optionally peened and put into compressive stress. Residual compressive stresses below the top of the intermediate should arrest any cracks before they can propagate down into the component geometry portion. After a subsequent heating step, the sacrificial geometry portion 32 can be removed.
In
The claimed invention does not preclude the use of other sacrificial elements required to manufacture the intermediate. Other sacrificial material commonly added to intermediate components such as support structures are utilised in various ALM processes to support the partly-built component structure during the manufacturing process and provide dimensional stability. These support structures need not infer any crack resisting properties to the component. The sacrificial geometry portion of the invention may combine or intersect with support structures as part of the design of the intermediate to enable over-hanging features to be added in a stable manner.
The flat surface 123 of the sacrificial geometry portion permits a full final pass of an ALM extrusion laser in building the sacrificial geometry portion 122, reducing the susceptibility of this surface to cracking. The flared portion 123 provides an increased surface area to which a peening, pressing or blasting operation can be applied with the effect of putting the intermediate into compressive stress. This peening, pressing or blasting operation further reduces the susceptibility of surfaces of the final component to cracking. For example a pressing operation might comprise hot isostatic pressing (HIPing). It will appreciated that HIPing can serve both to induce compressive stress and simultaneously perform the heating step of the method.
As can be seen from a comparison of
As can be seen from a comparison of
| Number | Date | Country | Kind |
|---|---|---|---|
| 1507984.1 | May 2015 | GB | national |
