MOULD FOR CASTING A MONOCRYSTALLINE COMPONENT

Abstract

A mould for casting a component in a directional solidification casting process having a preferred direction of grain growth (non-axial <001>) comprises a shell defining a cavity for receiving molten material. The cavity defines a three dimensional shape made up of a finished component geometry portion (42, 43, 44) and a sacrificial geometry portion (45) wherein the sacrificial geometry portion (45) includes a notch (48) which is shaped and positioned so as to, in use, contain high angle grain boundaries between dendritic growth in the preferred direction (non-axial <001>) and dendritic growth in a competing direction to the preferred direction (axial <001>) within the sacrificial geometry portion of a casting solidifying in the mould.

Claims

1. A mould for casting a component in a directional solidification casting process having a preferred direction of dendritic growth <001> which is inclined to a direction Y of a thermal gradient, the mould comprising; a shell defining a cavity for receiving molten material, the cavity defining a three dimensional shape made up of a finished component geometry portion and a sacrificial geometry portion, wherein the sacrificial geometry portion includes a notch which is shaped and positioned so as to, in use, contain dendritic growth in an axial direction <001>, different from the off-axial primary <001> direction, within the sacrificial geometry portion of a casting solidifying in the mould and wherein.

2. A mould as claimed in claim 1 wherein the finished geometry portion defines the shape of a turbine blade or a compressor blade for a gas turbine engine.

3. A mould as claimed in claim 1 wherein the notch extends through an entire depth of the sacrificial geometry.

4. A mould as claimed in claim 1 wherein the notch has an apex A whose position in the direction Y corresponds to a maximum acceptable distance L up to which it is acceptable for the axial <001> dendrites to overgrow the primary off-axial <001> grain.

5. A mould as claimed in claim 1 wherein the position of an Apex A of the notch in a direction orthogonal to direction Y is equal to the extent to which the axial <001> dendrites are present along a bounding wall of the sacrificial geometry.

6. A mould as claimed in claim 4 wherein the notch meets a wall of the sacrificial geometry at two positions B and C, lines adjoining A to B and A to C defining the orientation of walls of the notch and the position of at least one of B and C in the direction Y is larger than L.

7. A mould as claimed in claim 6 wherein the position of both B and C in the direction Y is larger than L.

8. A mould as claimed in claim 6 wherein the position of one of B or C in a direction L is zero, that is, a wall of the notch extends from the base wall (45c) of the sacrificial geometry portion (45) to the apex A.

9. A mould as claimed in claim 4 wherein the apex A comprises a sharp angle.

10. A mould as claimed in claim 6 wherein one or both of the walls AB, AC are straight.

11. A mould as claimed in claim 6 wherein one of the walls AB, BC is curved.

12. A mould as claimed in claim 1 wherein the notch is integrally formed into the mould.

13. A method for casting a component in a directional solidification casting process having a preferred direction of dendritic growth <001> which is inclined to a direction Y of a thermal gradient, comprising; providing a mould having a shell defining a cavity for receiving molten material, the cavity defining a three dimensional shape made up of a finished component geometry portion and a sacrificial geometry portion, wherein the sacrificial geometry portion includes a notch which is shaped and positioned so as to, in use, contain dendritic growth in an axial direction <001>, different from the off-axial primary <001>, within the sacrificial geometry portion (45) of a casting solidifying in the mould, introducing into the sacrificial geometry portion a seed crystal having a dominant growth direction of <001>, arranging the mould in a thermal gradient, the coolest end of the gradient being adjacent the sacrificial geometry portion, introducing molten material into the mould and gradually withdrawing the mould in a direction towards the coolest end whereby to control solidification of the material.

14. A method as claimed in claim 13 wherein the notch is provided in the form of an insert secured to an internal wall of the mould.

15. A method as claimed in claim 14 wherein the insert has an apex A and the apex A is positioned in the sacrificial geometry portion (45) in the direction Y at a maximum acceptable distance L up to which it is acceptable for the axial <001> dendrites to overgrow the primary off-axial <001> grain in the sacrificial geometry portion (45).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0021] An embodiment of the invention will now be further described with reference to the accompanying Figures in which:

[0022] FIG. 1 shows a schematic of apparatus known to be used in the directional solidification casting of gas turbine engine parts;

[0023] FIG. 2 shows a typical mould for the apparatus of FIG. 1;

[0024] FIG. 3 shows a mould pattern in accordance with one embodiment of the invention;

[0025] FIG. 4 illustrates how the geometry of the notch in accordance with the invention might be defined for a given investment casting process;

[0026] FIG. 5 illustrates how a bent cluster of axial <001> dendrites can overgrow the offs axial <001> dendrites comprising the primary grain;

[0027] FIG. 6 illustrates an insert positioned in a mould as a means of defining a notch in accordance with the invention.

DETAILED DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0028] FIG. 1 has already been described above. FIG. 2 shows a (partial) mould pattern 21 of a configuration known to be used in apparatus such as that of FIG. 1. The mould pattern 21 is for a turbine blade. For simplification the tip and shroud end of the mould are omitted. As can be seen the pattern comprises a blade aerofoil portion 22 extending from a hub portion 23 which sits on a root portion 24. Extending from the root portion 24 is a sacrificial geometry 25. The sacrificial portion 25 has two oppositely facing walls 25a and 25b both of which are planar and inclined to a planar base wall 25c. The sacrificial portion includes a slot 27 which may be used for holding a component cast from a mould made from the pattern 21. An interface between the sacrificial geometry 25 and the finished component geometry 24 is represented by dashed line 26, A mould made from the pattern 21 is positioned in the apparatus described in relation to FIG. 1 with the base wall corresponding to base wall 25c adjacent the chill plate 5 and substantially orthogonal to the direction of cooling. Thus dendrite growth is encouraged from the base wall towards the blade portion of the case component.

[0029] FIG. 3 illustrates a pattern 41 for a mould for making a turbine blade, the blade having a substantially identical geometry as is intended by the pattern of FIG. 2. For simplification the tip and shroud end of the mould are omitted. As can be seen the pattern comprises a blade aerofoil portion 42 extending from a hub portion 43 which sits on a root portion 44. The hub portion has an upper facing wall 43a and the root portion has oppositely facing inclined walls 44a and 44b. Extending from the root portion 44 is a sacrificial geometry portion 45. The sacrificial geometry portion 45 has two oppositely facing walls 45a and 45b both of which are planar and inclined to a planar base wall 45c. The sacrificial portion includes a slot 48 which may be used for holding a component cast from a mould made from the pattern 41. An interface between the sacrificial geometry 45 and the finished component geometry 44 is represented by witness line 46. A mould made from the pattern 41 is positioned in the apparatus described in relation to FIG. 1 with the base wall corresponding to base wall 45c adjacent the chill plate 5 and substantially orthogonal to the direction of cooling, Thus dendrite growth is encouraged from the base wall towards the blade portion of the case component.

[0030] FIG. 4 shows a sacrificial portion of a casting similar to that of FIG. 3. The mould has a base wall 45c which extends from a grain selector 50. Above the grain selector 50, the sacrificial portion is defined by walls 45a, 45b and witness line 46 along which the finished casting will be machined to remove the sacrificial portion. The width of the base wall 45c is 2R and the witness line 46 is at a height H from the base wall 45c. The primary grain <001> has inclined dendrites growing from left to right in the picture at an inclination to the direction of cooling Y. For example the inclination is 15°.

[0031] In a worst case scenario dendritic bending in the grain selector 50 produces axial <001> dendrites which are perfectly aligned with respect to the direction of cooling Y. Since these axial <001> dendrites are aligned with Y, their growth will accelerate more quickly than the off-axial <001> dendrites which may quickly become overgrown (see FIG. 5).

[0032] By the time the axial <001> dendrites from the bent cluster reach the base wall 45c, they extend a distance d as shown. An acceptable maximum ingress of the secondary grain with axial <001>, D can be defined for a casting of known material, geometry and cooling conditions, Desirably to begin with, d<˜0.2 (2R), that is, approximately 20% of the width of the base wall 45c (or less). An acceptable height L up to which it is acceptable for the axial <001> dendrites to overgrow the off-axial primary grain may also be identified, this height occurs when the ingress=D, that is when the axial <001> dendrites have grown a distance (D−d) into the off-axial <001> primary grain.

[0033] (D−d) can is dependent on L. For a given axial and off-axial <001> primary orientation, this value can be obtained either analytically from the dendrite tip growth kinetics using deterministic equations such, as in the KGT model (Kurz Giovanola Trivedi) or by a stochastic approach using the cellular automata approach finite element (CAFÉ) model or experimentally by conducting simple bi-crystal experiments (performed in Bridgman furnaces) as illustrated in FIG. 5. Here we begin with two seed crystals having the two chosen orientations corresponding to FIG. 4. By taking sections along the height for values of L; L.sub.1, L.sub.2, L.sub.3 we can determine values for d; d.sub.0, d.sub.1, d.sub.2, d.sub.3 as shown in the figure.

[0034] In FIG. 4, line AB represents a wall of a notch configured to contain dendritic growth in the axial <001> direction. The positions of the points A and B between which a line defining the position and orientation of the wall may be determined from the identified values of D and L.

[0035] For any given value of L, say L=H/3 and an off-axial <001> orientation growing competitively with respect to an axial <001> orientation, we obtain (D−d), which is the extent to which the off-axial <001> primary grain is overgrown by the secondary <001> axial grain (from an analytical or bi-crystal method, as mentioned above). Knowing the initial spatial extent of the secondary grain at the base (45c), d and the subsequent ingress, (D−d), consequently D can be obtained. Thus knowing L (=H/3) and D, the position of the notch A can be obtained. The same argument can be used to determine AB at the opposite end, where in this case the dendrites of the primary grain form a diverging disposition. A similar bi-crystal experiment can be conducted, but in this case the dendrites diverge at the boundary. The rate of overgrowth will be different, i.e. for a given L˜H/3, the corresponding (D−d) will be different compared with the converging case because of the orientation dependence on grain over growth (as demonstrated by D'Souza et al, Mater. Trans. B (2005) and Journal of Materials Science (2002)) for converging and diverging dispositions and accordingly this will give a different notch length AB.

[0036] Once the positions of AB are established, the length and orientation of AC can be arbitrary, so long as the wall AC meets the wall of mould 45b in FIG. 4.

[0037] As can be seen in FIG. 5, a sacrificial geometry portion of a mould is defined by walls 55a, 55b, 55c and witness line 56. A grain selector 60 extends beneath the sacrificial portion. A wedge shaped insert 61 is positioned in the sacrificial geometry portion with its apex A located at the desired position for an apex A of a notch to be formed in the sacrificial geometry portion. The insert 61 may, for example be secured to wall 55c by mechanical fasteners which might pass through the wall 55c, Alternatively, a bonding agent may be used to hold the insert in position against the wall 55c.

[0038] It will be understood that the invention is not strictly limited to the embodiments above-described. 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.