GB2106021A - Apparatus and method for producing a metal casting which contains a recess - Google Patents

Abstract

The grain structure of metal in the region of a recess in a casting is controlled by the disposition of a removable, expandable and contractable chill assembly (17) at an opening at the recess forming section (11) of the mould wall (10a & 15). An insert (14) whose outer surface forms at least part of the casting forming surface may be disposed at the opening exterior to the chill assembly (17) and may subsequently be removed from or remain part of the casting. The cam/spring assembly (24-28) permits selective expansion and contraction of the chill assembly (17). Also disclosed is a turbine disc with a centrally disposed recess with equiaxed grain structure in the region of the recess and columnar grain structure in the peripheral outer portion. <IMAGE>

Description

SPECiFICATION Apparatus and method for producing a metal casting This invention relates to metal castings, and, more particularly to an improved apparatus and method for producing precision castings including a recess therein and having a controlled grain structure.

It has been recognized that grain size and grain structure control can result in significantly improved properties for certain metal castings. However, production of relatively highly stressed castings, for example the disk-structures used in producing gas turbine engine turbine wheels, with or without integral blades, and including a centrally disposed recess such as a bore, channel, hole, etc. presents grain structure and size control problems. Such problems are based, at least in part, on the volume of metal poured during casting of such a structure and the difficulty in controlling the heat dissipation and casting solidification pattern. In the production of gas turbine engine disks with a substantially centrally positioned bore and with blading members radiating outwardly from the peripheral portion of the disk, the problem becomes more complex.This problem is particularly critical where one type of grain structure, for example equiaxed, is desired at a portion about the recess, such as the bore, and another type, for example columnar, is desired in the blading members about the disk periphery and perhaps in a majority of the intermediate portion of the disk between the recess and the periphery.

Current practice of casting turbine wheels or disks which are monolithic structures of wheels and turbine blades (sometimes called blisks) includes first making a plastic or wax pattern of the part. To this is attached a pattern of a gating system. Then a ceramic investment shell mold is made around the pattern in a conventional manner. However, a disadvantage results from use of such a mold system: solidification begins at the outer periphery of the mold while molten metal at the inner portions of the mold remains hot and solidifies very slowly. This solidification pattern results in a large, coarse, columnar grain structure in the casting recess or central portion which is subjected to the highest stress during operation of such an article. However, such a grain structure is characterized by low mechanical properties in the bore tangential direction.

Chills have been used in the casting art for many years. However, use of a chill in a recess, such as the bore of a disk-like member, presents problems in respect to subsequent removal of the chill: cooling and associated contraction of the casting would cause a chill plug to become part of or held tightly within the recess of a casting.

The present invention attempts to provide a metal casting and, a method for producing such a metal casting, having a recess therein and a fine grain structure within the casting about the periphery of, and for a small distance radially outwardly from, such recess.

The present invention also attempts to provide apparatus for producing such a metal casting, such apparatus including a removable chill assembly for the recess in the casting.

According to the present invention there is provided apparatus for the production of a metal casting which includes a recess, from a shell mold having wails defining an interior casting chamber including the shape of the casting to be produced and an opening through a wall at the recess, between the recess and the casting chamber: an expandable and contractable segmented chill assembly having a chill outer portion adapted to be disposed at the opening through the mold wall for control, at the recess, of cooling of molten metal poured into the mold; and means for selectively expanding and contracting the segmented chill assembly to control disposition of the chill outer portion in respect to the opening through the mold wall.

The present invention also provides a method for making a metal casting which includes a recess, the method of controlling the grain structure at the recess comprising the steps of: providing a shell mold having walls defining an interior casting chamber including the shape of the casting to be produced and an opening through a wall at the recess, between the recess and the casting chamber; disposing at the opening a removable chill assembly; depositing molten metal into the mold; and removing heat from the molten metal in the mold through the chill assembly to control the metal grain structure originating at the recess during solidification of the casting.

In another aspect, the invention provides a disk cast of a high temperature superalloy selected from the group consisting of iron-base, cobalt-base and nickel-base superalloys, the disk including a centrally disposed recess, a portion about the recess, a peripheral outer portion and an intermediate portion between the outer portion and the portion about the recess, the disk having a generally equiaxed grain structure in the portion about the recess, and a generally columnar grain structure in the peripheral outer portion.

Briefly, one form of the present invention for the production of a metal casting which includes a recess provides apparatus for use with a ceramic shell mold having an opening through a wall at the recess into an interior casting chamber. An expandable and contractable segmented chill assembly is disposed at the opening for control at the recess surface for cooling of molten metal poured into the mold. In one form of the invention, a metallic insert having a hollow interior is disposed in and closes the opening in the mold wall. The chill assembly, in such form, is disposed within the insert hollow interior for contact with the insert for controlling the cooling of the insert and molten metal poured into the mold at the recess. Also provided are means for selectively expanding and contracting the segmented chill assembly for placement in and removal from the recess.

In its method form, the present invention, for producing a metal casting including a recess, providing a mold within an opening through a wall at the recess and then disposing, in the recess, the remov able chill assembly. In one form, a metallic insert having a hollow interior is positioned in the recess and then the removable chill assembly is inserted into the insert hollow interior, for example by reason of its being expandable and contractable. With the chill assembly in position, for example in intimate contact with the insert, molten metal is poured into the mold in which it is allowed to cool and solidify while the metal grain structure originating at the recess is controlled through the chill assembly. After casting solidification, the chill is removed. If necessary, the insert can be removed by metal removal processes such as machining.In another form of the method of the present invention, the chill surface, or the insert portion, which faces the interior of the mold is treated to prevent it from bonding secureiy with the solidified casting to facilitate removal.

Another form of the present invention defines a cast disk of a high temperature superalloy, the disk including a generally equiaxed grain structure in a portion about the recess and a generally columnar grain structure in the peripheral outer portion, such as would include radially outwardly extending blading members.

Another embodiment of the invention comprises a metal insert of the same or different alloy as is being cast which has had its surface prepared to enhance bonding for the purpose of its becoming an integral part of the disk.

The present invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a vertical, partially diagrammatic, cross-sectional view showing one form of the apparatus of the present invention in the expanded condition for producing a gas turbine engine turbine blisk casting.

Figure 2 is a horizontal cross-sectional view taken along the lines 2-2 of the apparatus in Figure 1 with the apparatus in the contracted condition.

The present invention in its various forms will be mor fully understood from the following detailed description and specific embodiments, taken with the drawings, all of which are intended to be representative of, rather than in any way limiting, on the scope of the present invention.

The apparatus of the present invention in the form shown in Figure 1, is associated with a composite shell mold, shown generally at 10. The mold includes a ceramic outer wall 10a, a portion of which is substantially in the form of a gas turbine engine bladed disc (blisk) and an inner wall comprised of members 14 and 15. When fully assembled, the mold includes a disk cavity 12 communicating with additional, peripherally disposed cavities 13 substantially in the shape of blading members projecting radially outwardly from disk cavity 12. The inner wall of mold 10, in the form shown in Figure 1, consists of a metallic insert 14 and a pour director 15.

Pour director 15 along with a portion of outer shell 10a define a pour cup 16 into which molten metal is poured and then directed into mold cavities 12 and 13.

The principal portion of mold 10 can be made by any of a variety of widely used methods for making precision casting molds. In making large molds, for example of the type made in the evaluation of the present invention, it is convenient to make the mold in segments which are later joined prior to casting.

Segmented mold manufacture has been reported in the art, for example, in U.S. Patents 4,066,166 and 4,224,976. This method facilitates cleaning of the mold cavities after dewaxing and prior to joining of the segments into a mold. Pour director 15 can be made by similar precision casting mold manufacturing methods. Metallic insert 14, conveniently in the form of a metal sleeve as shown in Figure 1,can be made by casting, forging, machining, etc. When assembled, outer shell 1 Oa, insert 14 and pour director 15 define the walls of mold 10.

One form of the apparatus of the present invention, for use with a shell mold having a recess such as 11 in Figure 1, provides an expandable and contractable, segmented chill assembly shown generally at 17. Referring to Figures 1 and 2, chill assembly 17 includes segmented cooling jacket 18 such as of copper, wedge segments 11 and cam 24.

The jacket segments include cooling fluid channels 19 through which coolant, such as water, can flow, for example as fed through conduit 20 from a source, not shown. Coolants other than water, such as cooled antifreeze, can be used to control or influence the degree of chilling produced. Such coolants could enhance the cooling effects of copper or other metal materials. In Figure 2, a four segment cooling jacket, shown as 18a, 18b, 18c and 18d, comprises a cylindrical, adjustable shape. Although copper is conveniently used for such a cooling member, other well-known materials having relatively high heat conductivities can be used to transfer heat away from molten metal poured into the mold, for example transferred through metallic insert 14.

Cooling jacket 18 forms an outer element of the adjustable chill assembly 17 which is segmented into four portions 18a, 18b, 18c and 18d as discrete movable segments. In the embodiment shown in Figure 1, they can be moved toward and away from metallic insert 14. As used herein, such movement, such as for the purpose of controlling contact intimacy with insert 14, compensates for expansion and contraction. Coolant is passed through some and preferably all of such segments, for example as shown in Figure 1 in connection with segments 18b and 18d. Coolant flow, and hence rate of cooling, can be regulated to produce desired grain size. When such coolant is fed into the segments individually, each will include inlet and outlet conduits. The outer portion of each segment, when in the expanded condition, is in direct contact with the insert inner surface 21.

Included in the expandable and contractable segmented chill assembly, in the embodiment shown in the drawings, is a segmented collar 22 which cooperates with the segments of the cooling jacket 18 for the expansion and contraction of the cooling jacket into and out of contact with insert 14. Each segment 22a, 22b, 22c and 22d of collar 22 includes an internal cam surface 23. Although it is convenient to manufacture the segments of the cooling jacket and the segments of the collar separately, it will be understood that such two elements could be included in a single cooling jacket segment each of which includes a cam surface.

Positioned within and cooparating with cam surfaces 23 on the collar segments is frustoconical cam 24 for axial movement within the segmented chill assembly in cooperation with cam surfaces 23. In this way, pressure is applied or relaxed in orderto enable expansion or contraction of the segments of the chill assembly.

In the embodiment shown in the drawings, cam 24 is secured on the upper end of a shaft 25 the lower end of which is provided with a biasing means such as high temperature coil spring 26. In Figure 1, the lower end of shaft 25 extends through an aperture in a support frame 27 on which the mold 10 and the adjustable chill assembly 17 rests. Although the present invention is described in connection with a particular embodiment involving relative movement of cam surfaces for expansion and contraction, it will be understood by those skilled in the art that movement of the segments toward and away from one another in the manner described could be accomplished in a variety of ways.

As shown in Figure 1, the lower end of shaft 25 carries a coil spring 26 the upper end of which engages support frame 27 and the lower end of which is secured by adjustable nut and washer assembly 28. Assembly 28 in cooperation with biasing means such as coil spring 26 allows movement of cam 24 vertically up or down to expand or contract the segments of the chill assembly as described above. For example ciockwise rotation of nut and washer assembly 28 compresses spring 26 moving cam 24 downwardly. The downward shift of cam 24 on shaft 25 causes cam 24 to cooperate and move along cam surfaces 23 on collar 22 for the exertion of an outward expansive force on cooling jacket 18.The resultant expansion of the jacket 18 results in compressing of jacket 18 against insert inner surface 21 to facilitate cooling of the insert and the metal poured into the mold adjacent the insert.

When adjustment nut and washer assembly 28 is turned counterclockwise, the tension in spring 26 is relieved enabling cam 24to move upward along surface 23. In this way, pressure is relieved on the segments of collar 22 and cooling jacket 18 enabling the segments to contract or move radially inwardly within recess 11. Relief of such pressure enables removal of mold 10 and the casting contained therein from about the segmented chill assembly 17.

If difficulty is encountered in removing insert 14 from the casting solidified within the mold, such insert can be carried with the casting and subsequently be removed by metal removal processes such as machining, grinding, etc. Removal of the insert may be facilitated in another way by treating insert outer surface 29 to inhibit bonding and promote parting of the insert from the casting solidified within the mold. Examples of such surface treatment include coating with a ceramic spray, slurry or wash, such as zirconia or alumina, and providing a metallic coating, such as one including aluminum which can be oxidized to provide alumina at the surface. As was mentioned before, the present invention can be practiced using cooling jacket 18 as the interface with the metal poured into the mold, without the use of an insert 14.In such an instance, surface treatment of the type described would facilitate parting and removal of cooling jacket 18 from the solidified casting upon relief of the radially outward pressure exerted by cam 24 on the peripherally disposed segmented members.

If it is desired to provide an integral, nonremovable insert, the surface of such insert may be treated to promote bonding by plating techniques such as nickel plating and/or mechanical means such as knurling.

Associated with the apparatus of the present invention is conventional metal casting equipment.

For example, surrounding the shell mold 10, which generally is preheated in a separate furnace, is means for heating the mold to the desired tempera tureforcasting. If the article is to be a vacuum casting of a high superalloy, such as an iron-base, cobalt-base or nickel-base superalloy, the mold is heated in a vacuum furnace with a vacuum of at least 50 microns to a temperature in the range of about 1800"-2000"F prior to the pouring of the metal into the mold. When the desired temperature has been attained, the molten metal, heated to the proper temperature for casting is poured into the mold pour cup to fill the mold 10.

The method of the present invention for making a metal casting which includes a recess shown generally at 11 such as a centrally disposed bore in a disk member, includes providing a mold 10 including a casting chamber of the shape of the casting to be produced. Large molds can be manufactured in segments as described in the above-identified patents. Such a mold includes an outer wall such as 10a in Figure 1 and generally of a ceramic. Enclosing the central portion of the mold about the recess is an inner wall, for example defined by a pour director 15 preferably of a ceramic and a metallic portion disposed about the recess. The metallic portion together with pour director 15 and outer walls 1 0a provide a substantially continuous surface defining the bounds of the casting chamber.As was mentioned above, such a metallic portion can be a metallic insert such as 14 in Figure 1 or can bean expanded cooling jacket such as 18.

According to the present invention, for use in the above-described apparatus, an adjustable segmented chill assembly is disposed in recess 11 and expanded to provide a cooling means or chill about the peripheral walls of recess 11. With the chill or cooling jacket in operation to establish a heat sink in the mold, molten metal is poured into the casting chamber through a pour cup such as 16 to fill the casting chamber, for example 12 and 13 in Figure 1, to a point at which excess metal is provided for the casting. Typically, for the type of blisk casting mold shown in Figure 1, metal is poured approximately to broken line 30 as above, Because heat is conducted rapidly away from the molten metal at the opening or area between pourdirectorl5and outer wall 10a, closed either by insert 14 or segmented cooling jacket 18, nucleation and grain growth begins from such surface. As is well known in the art, such grain growth can be controlled to provide a variety of grain structures. For example, if the metallic surface which faces the molten metal is treated with a nucleating means such as cobalt oxide or is serrated to promote nucleation, a generally fine, equiaxed grain structure is initiated in the solidifying casting.

Similarly, slower cooling and little or no nucleation can result in a larger, more columnar grained structure emanating from the periphery of the recess. The present invention provides an additional means (cooling) to control the grain structure at a very critical area of a casting.

It is preferable in the practice of the present invention to use a metallic insert 14to close the walls of the casting chamber between pour director 15 and outer walls 10a. Such an insert can be a sacrificial insert which is later removed from the casting. In such a case, its grain structure and mechanical properties are not critical. However, an insert can be used to provide specific properties about the periphery of a recess, remaining bonded with the casting to become an integral part of the casting. In such a case, where high quality and precise dimensions are required, the shape and grain structure of the insert can be selected to provide the desired mechanical properties.For example, such an insert can be a fine grained forging of a compatible material, having improved medium temperature mechanical strength properties at the bore of a blisk where such mechanical properties are required during operation of such an article. When the insert remains with the casting, grains grow into the casting chamber from the insert as a function of the surface, for example 29, which faces the casting. If a fine grained structure is desired, the surface of the insert which remains with the casting can be serrated to promote nucleation and bonding. Bonding between the insert 14 and the solidifying molten metal can be promoted by coating surface 29 of the insert, which will be in contact with the solidifying molten metal, with an alloy or a metal, for example nickel, which will also reduce the oxidation of such sleeve during mold heating. If desired, a flange can be provided on the integral insert 14 for strength and simplicity.

After solidification of the casting within the mold 10, the adjustable segmented chill assembly is removed, as described above, and the metal casting is processed further in accordance with practices well known in the art. Generally, after appropriate cooling, the mold is broken away from the casting and the casting is machined to a desired configuration.

The present invention has been described in connection with specific examples and embodiments. However, it will be understood by those skilled in the art that the invention is capable of many variations and modifications within the scope of the appended claims.

Claims (19)

1. Apparatus for the production of a metal casting which includes a recess, from a shell mold having walls defining an interior casting chamber including the shape of the casting to be produced and an opening through a wall at the recess, between the recess and the casting chamber: an expandable and contractable segmented chill assembly having a chill outer portion adapted to be disposed at the opening through the mold wall for control, atthe recess, of cooling of molten metal poured into the mold; and means for selectively expanding and contracting the segmented chill assembly to control disposition of the chill outer portion in respect to the opening through the mold wall.

2. Apparatus as claimed in claim 1 in which a metallic insert is disposed at and closes the opening through the mold wall, the insert being disposed between the opening and the chill assembly and having an outer surface defining at least a part of the hollow interior of the mold and an inner surface defining an insert interior in which the chill assembly is disposed, the chill assembly outer portion being in juxtaposition and for contact upon chill assembly expansion with the insert inner surface for controlling cooling of the insert and molten metal poured into the mold.

3. Apparatus as claimed in claim 1 or claim 2, wherein the means for selectively expanding and contracting the segmented chill assembly comprises: an expandable and contractable collar consisting of a plurality of cooperating segments having internal cam surfaces; a frustoconical cam positioned within the collar; a shaft secured to the frustoconical cam; and biasing means which engages the shaft.

4. Apparatus as claimed in claim 3, wherein said biasing means comprises: an adjustment nut on the shaft; a coil spring about the shaft, the upper end of which cooperates with a support frame which carries the mold and the lower end of which engages the adjustment nut.

5. Apparatus for producing a metal casting which includes a recess, comprising: a shell mold having an interior casting chamber including the shape of the casting to be produced, the mold comprising outer and inner walls, each wall having an inner surface which together define the casting chamber; the inner surface of the inner wall defining at least the shape of the recess in the casting, the inner wall having a metallic insert positioned at least at the recess, the metallic insert having an insert outer surface which is, at least, a part of the inner surface of the inner wall, and an insert inner surface which defines an insert interior; an expandable and contractable segmented chill assembly having a chill outer portion disposed in the insert interior in juxtaposition and for contact upon chill assembly expansion with the insert inner surface for cooling the insert and molten metal poured into the mold; and means for selectively expanding and contracting the segmented chill assembly to control disposition of the chill outer portion in respect to the insert inner surface.

6. Apparatus as claimed in claim 5 in which the shell mold is a composite mold including a ceramic outer wall and a composite inner wall, the composite inner wall comprising the metallic insert positioned at the recess and a ceramic inner wall portion disposed on the metallic insert.

7. A method for making a metal casting which includes a recess, the method of controlling the grain structure at the recess comprising the steps of: providing a shell mold having walls defining an interior casting chamber including the shape of the casting to be produced and an opening through a wall at the recess, between the recess and the casting chamber; disposing at the opening a removable chill assembly; depositing molten metal into the mold; and removing heat from the molten metal in the mold through the chill assembly to control the metal grain structure originating at the recess during solidification of the casting.

8. A method as claimed in claim 7 in which a metal insert, including an outer surface which defines at least a part of the casting chamber, is disposed at the opening between the casting chamber and the removable chill assembly to close the opening.

9. A method as claimed in claim 8 in which the outer surface of the metal insert is treated to promote bonding of the insert with the metal solidified in the mold.

10. A method as claimed in claim 8 in which the outer surface of the metal insert is coated with a material which facilitates parting of the insert from the metal solidified in the mold.

11. A disk cast of high temperature superalloy selected from the group consisting of iron-base, cobalt-base and nickei-base superalloys, the disk including a centrally disposed recess, a portion about the recess, a peripheral outer portion and an intermediate portion between the outer portion and the portion about the recess, the disk having a generally equiaxed grain structure in the portion about the recess, and a generally columnar grain structure in the peripheral outer portion.

12. A disk as claimed in claim 11 wherein the portion about the recess includes a fine grained insert about the recess and bonded with the disk.

13. A disk as claimed in claim 11 in the form of a turbine disk having blading members cast integrally with the disk and radiating outwardly at the peripheral outer portion, the blading members having a columnar grain structure.

14. A disk as claimed in claim 13 in which the columnar grain structure extends from the peripheral outer portion into the intermediate portion.

15. Apparatusforthe production of a metal casting as claimed in claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. A metal casting when produced by apparatus as claimed in any one of claims 1 to 6 and 15.

17. A method for making a metal casting as claimed in claim 7, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

18. A casting when made by a method as claimed in any one of claims 7 to 10 and 17.

19. A disk as claimed in claim 11, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.