STRESS RELIEF FOR ADDITIVE LAYER MANUFACTURING

Abstract

The present disclosure relates to techniques for stress relief in additive layer manufacturing (ALM). Example embodiments include a method for additive layer manufacturing of a metallic component, comprising the steps of: providing a substrate (20); depositing a first layer (22) of material on the substrate (20); depositing a plurality of second layers of material on the first layer (22) to form the metallic component (21), wherein the first layer (22) forms a stress relieving layer between the plurality of second layers and the substrate (20), the stress relieving layer having a lower shear stiffness compared to the metallic component (21).

Claims

1. A method for additive layer manufacturing of a metallic component, comprising the steps of: providing a substrate; depositing a first layer of material on the substrate; depositing a plurality of second layers of material on the first layer to form the metallic component, wherein the first layer forms a stress relieving layer between the plurality of second layers and the substrate, the stress relieving layer having a lower shear stiffness compared to the metallic component.

2. The method of claim 1 wherein the stress relieving layer has a lower density compared with the metallic component.

3. The method of claim 1 wherein the shear stiffness of the stress relieving layer is defined between a first plane joining the substrate to the stress relieving layer and a second plane joining the stress relieving layer to the component.

4. The method of claim 3 wherein the stress-relieving layer has a first shear stiffness in a first direction along the first plane that is different to a second shear stiffness in a second direction along the first plane orthogonal to the first direction.

5. The method of claim 4 wherein the first shear stiffness is reduced relative to the second shear stiffness where the first direction is aligned with a longer dimension of the component along the first plane.

6. The method of claim 1 wherein the stress relieving layer is formed by a plurality of first layers being partially fused from a powdered form of the material.

7. The method of claim 1 wherein the stress relieving layer comprises a porous structure.

8. The method of claim 7 wherein the stress relieving layer comprises a foam structure.

9. The method of claim 1 wherein the stress relieving layer comprises an array of columns connecting the substrate to the component.

10. The method of claim 9 wherein the columns form a lattice structure.

11. The method of claim 1 wherein the first layer is formed from a plurality of layers on the substrate by additive layer manufacturing.

12. A component assembly formed by additive layer manufacturing, the assembly comprising: a substrate; a metallic component; and a stress relieving layer between the metallic component and the substrate, wherein the stress relieving layer has a lower shear stiffness compared to the metallic component.

13. The component assembly of claim 12 wherein the stress relieving layer comprises a porous structure.

14. The component assembly of claim 13 wherein the stress relieving layer comprises a foam structure.

15. The component assembly of claim 12 wherein the stress-relieving layer has a first shear stiffness in a first direction along the first plane that is different to a second shear stiffness in a second direction along the first plane orthogonal to the first direction.

16. The component assembly of claim 15 wherein the first shear stiffness is reduced relative to the second shear stiffness where the first direction is aligned with a longer dimension of the component along the first plane.

17. The component assembly of claim 12 wherein the stress relieving layer comprises an array of columns connecting the substrate to the component.

18. The component assembly of claim 17 wherein the columns form a lattice structure.

Description

DETAILED DESCRIPTION

[0035] 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 described 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.

[0036] FIG. 1 illustrates an example component 11 fabricated on a substrate 10 by ALM. The component 11 may be made up of a plurality of layers formed over one another by fusing a powder feedstock with a laser. As each successive layer is fused, an increasing stress is built up between the substrate 10 and the component 11 due to thermal contraction as the layers cool down from being fused at high temperatures. This may eventually cause the substrate 10 to deform in the general way indicated by arrows 12, which may result in distortion of the component 11. This effect may be reduced to some extent by optimising the method used to fuse each successive layer, and by optimising the substrate, but these techniques are not capable of fully resolving the issue. A heated powder bed may in some cases by used, which can reduce the thermally-caused distortion effects but tends to result in detrimental effects on the residual powder surrounding the component after fabrication.

[0037] FIG. 2 illustrates the general principle of fabricating a component 21 on a substrate 20 using an intervening stress-relieving layer 22. The layer 22 is formed to have a reduced shear stiffness, or shear modulus, compared with that of the component 21 or of the substrate 20. The shear stiffness is defined between a first plane 23 joining the substrate 20 to the stress relieving layer 22 and a second plane 24 joining the stress relieving layer 22 to the component 21. The shear modulus of the stress-relieving layer may be engineered to be different in the direction of the planes 23, 24 compared with other directions, such as orthogonal to the planes 23, 24. In the example illustrated in FIG. 2, the stress-relieving layer 22 may be a partially fused layer of the same material as that making up the component 21, forming a porous structure having a reduced shear stiffness.

[0038] The density of the stress-relieving layer 22 may be a fraction of that of the component 21, for example within a range of 0.2 to 0.9 of the density of the component 21. If the stress-relieving layer 22 is formed via ALM from a powder feedstock, the volume fraction of fused to unfused material throughout the layer 22 may be between 0.2 and 0.9. The material forming the stress-relieving layer 22 may be the same or similar to that forming the component 21, or in some cases may be different, such as when the layer is formed by a different technique. The shear stiffness of the stress-relieving layer 22 may for example be less than 0.7 that of the component, and may be between 0.1 and 0.7 of that of the component 21.

[0039] FIG. 3 illustrates an alternative form of stress-relieving layer 32 connecting a component 31 to a substrate 30. In this example, the stress-relieving layer 32 comprises an array of columns aligned substantially orthogonally to the planes 33, 34 connecting the layer 32 to the substrate 30 and the component 31 respectively. The columns may be of any suitable cross-sectional shape, such as rectangular, square or circular, and may be arranged to provide a reduced shear stiffness in the direction where the maximum thermal contraction is expected. Rectangular section columns may, for example, be aligned such that their longer axis is orthogonal to the direction of maximum strain expected in the component, so that the columns can more easily deform as the component contracts. This is illustrated further in FIGS. 4 and 5. In a general aspect therefore, the stress-relieving layer may have a first shear stiffness in a first direction along the plane of the substrate that is different to a second shear stiffness in a second direction along the plane of the substrate orthogonal to the first direction. The first shear stiffness may be reduced relative to the second shear stiffness where the first direction is aligned with a longer dimension of the component in the plane of the substrate.

[0040] FIG. 4 illustrates schematically an edge portion of a component 41 formed on a substrate 40, with a stress-relieving layer 42 formed from an array of columns 45. The columns 45 are aligned orthogonal to planes 43, 44 connecting the stress-relieving layer 42 with the substrate 40 and component 41 respectively.

[0041] FIG. 5 illustrates schematically, with displacements exaggerated, the effect on the columns 45 as the component 41 thermally contracts during fabrication. The thermal contraction 46 of the component 41 relative to the substrate 40 causes the columns 45 to distort to accommodate the contraction without causing distortion of the substrate 40. The columns 45 may be structured to flex elastically or plastically depending on the material they are made from.

[0042] FIG. 6 illustrates schematically a further alternative example of a stress-relieving layer 62 between a substrate 60 and a component 61 being fabricated by ALM. The stress-relieving layer 62 in this case is in the form of a foam structure, which will have a reduced shear stiffness compared to that of either the substrate 60 or the component 61. The foam structure may be formed in layers by ALM as for the component 61 or may alternatively be formed separately and bonded to the substrate 60 by other means before the component 61 is fabricated by ALM over the structure.

[0043] FIG. 7 illustrates schematically a further alternative example of a stress-relieving layer 72 between a substrate 70 and a component 71 being fabricated by ALM. The stress-relieving layer 72 in this case is in the form of a latticework of columns. The shear stiffness of such a structure will tend to be higher than an equivalent density structure formed of orthogonal columns as in FIG. 4, so can be fabricated to have a lower density than the columnar structure.

[0044] FIG. 8 illustrates an example flow diagram of a method of fabricating a component by ALM. In a first step 81, a substrate is provided for the component to be fabricated upon. In a second step 82, a first layer is deposited on the substrate, which may be done by ALM or by affixing a prefabricated layer on the substrate. In a third step 83, a component is fabricated over the first layer by depositing a plurality of layers by ALM. After the component is fabricated, and the stress-relieving layer has achieved its function of allowing the component to thermally contract without distorting the substrate, in a fourth step 84 the component may be separated from the substrate. Further finishing steps may then be carried out on the component, such as final machining and/or annealing or other heat treatment steps.