Cast Composite Structure With Metallic Rods

Abstract

Macrocomposite structures composed of a low tensile strength refractory mass surrounding a plurality of single crystal wires, the refractory mass being prestressed in compression by the single crystal wires or rods, and the structure made by such process.

Parent Case Text

BACKGROUND OF THE INVENTION

This is a continuation of Ser. No. 714,736 filed Mar. 20, 1968, now abandoned having the same inventors.

Description

This invention relates to a process for casting a macrocomposite structure and to a novel macrocomposite structure. More specifically, the invention relates to a casting process which provides a refractory mass which is reinforced with a plurality of single crystal wires of a high strength alloy, the resultant macrocomposite structure fabricated thereby having exceptional high temperature strength and corrosion/erosion properties.

The use of metal-refractory composites has long been recognized as offering special advantages as structural materials for high temperature applications. These composite structures have particular application and provide distinct advantages in gas turbine engines and the like. In a gas turbine engine the gas contacting elements of the engine are continuously subjected to a very severe and complex environment, such as high temperatures, severe thermal gradients, erosion, and forces which tend to deform the gas contacting elements. These forces have a particularly adverse effect on the leading and trailing edges of these gas contacting elements. As a result of the severe temperature gradients and other thermal conditions encountered by the gas contacting elements presently employed in gas turbines, the permissible operational periods and operating temperatures of these gas contacting elements are limited considerably.

Composite structures are known in the art, the composite structure described in U.S. Pat. No. 3,215,511 being a typical example of these structures. The composite structure described therein and the structures described in similar prior art generally employ a structure wherein a ceramic or other similar material is used in conjunction with a high temperature alloy. The composition of the structure is such that the high temperature alloy is the predominant constituent rather than the ceramic or other type material. While this structure is somewhat of an improvement over the metal structures generally used, it is still not the ideal in that the permissible operational periods and operational temperatures of the structure are still limited, the controlling factor being the metal alloy and its properties.

The prior art also discloses composite structures which have application and use in the field of construction; for example, prestressed concrete. While there is some similarity between these structures and the present invention these former structures are not capable of use in high temperature applications, nor do they have the characteristics of providing high temperature creep strength and resistance to oxidation, sulfidation and erosion.

SUMMARY OF THE INVENTION

One object of this invention is to provide a novel casting method for the fabrication of a macrocomposite structure which has high temperature creep strength, is resistant to oxidation, sulfidation, and erosion and has favorable impact loading characteristics at low temperatures.

One feature of the invention is a macrocomposite blade or vane for a gas turbine in which the airfoil portion of the vane that is in contact with the hot gas powering the turbine is a non-metallic heat resistant material reinforced by metallic single crystal rods extending therethrough and integral with end shrouds on the vane. More broadly, the macrocomposite structure may have more general application where the refractory non-metallic structure is exposed to a very hot environment and is held under compression during use by the reinforcing single crystal rods extending therethrough and connected to the metallic end elements on the cast article.

In practicing the process of this invention, molten metal is poured into a preheated mold and allowed to infiltrate the configuration of longitudinal holes in a shaped refractory mass, which is embedded in the mold. After pouring, the temperature gradient in the mold is controlled so as to promote directional solidification of the entire melt at a rate consistent with the maintenance of a cube oriented columnar grain structure. The coarseness of the columnar grain structure at the base of the refractory part insures that only single crystals will develop in each of the many relatively small openings in the refractory mass. The individual crystals grow together again at the top of the ingot to form once again the columnar grain structure, as previously described. In removing the metal refractory composite from the mold it is extremely important to retain at least a part of the columnar grain structure at each end, that is, at the top and bottom of the shaped macrocomposite structure. This latter step is important because during cooling of the melt, the refractory mass is prestressed in compression due to a different thermal contraction of the two materials. Maintenance of the unidirectional columnar structures at the top and bottom of the refractory metal mass maintains this beneficial compressive stress developed in the refractory mass owing to the greater thermal contraction of the metal wires. The process described herein will be directed at the production or manufacture of gas turbine parts or gas contacting elements. However, it is expressly to be understood that the process is not limited to this type application, macrocomposite structures fabricated by this process having application in prestressed pressure vessels, integral gas turbine rotors and other such applications.

The macrocomposite structure of the present invention is one which is comprised of a refractory material preferably a ceramic and a plurality of single crystal metal wires, the wires being preferably of a nickel or cobalt base. The macrocomposite structure of the present invention is generally fabricated by the novel casting process hereinbefore described. As previously noted, during cooling of the melt, the ceramic or refractory material is prestressed in compression due to the different thermal contractions of the two materials. The primary function of the metal wires is load distribution and protection of the refractory material from catastrophic failure by thermal shock, or impact loading at low temperatures, while the primary function of the refractory or ceramic material is to provide high temperature creep strength and resistance to oxidation, sulfidation and erosion.

In the novel macrocomposite structure of the present invention, it is pointed out that there is no bonding between the metal and the refractory mass, the refractory mass being held in place by either the flared ends of the metal wires or by the columnar grain structure which is maintained at the top and bottom of the macrocomposite structure on removing the macrocomposite structure from the mold. This latter portion of the macrocomposite structure, that is, the cube oriented columnar grained section at the bottom and top of the composite structure, also provides a secondary effect in that it maintains the beneficial compressive stress formed in the refractory mass.

The positioning and the numbers of longitudinal holes contained in the refractory mass is also of significance in the macrocomposite structure. In certain instances, it would be desirable to have an inhomogeneous distribution of metal wires within the refractory mass, this thereby providing the correct number of wires for any desired stress distribution and stress level. Near the outer surface a uniformly high level of compressive stress is required to retard crack initiation and growth in the refractory mass, and therefore ideally the metal wires should be very fine and closely spaced. There are limitations on wire size and inter-wire spacing imposed by both the metal casting and the refractory mass forming processes. The main limitation on this type construction is that the component must be designed to have no bending moments. This can be accomplished by maintaining a relatively constant volume fraction of metal from the leading to trailing edges. Similarly, there are applications of the present invention where it would be desirable to have a homogeneous distribution of metal wires within the ceramic. Another configuration would be one which has direct use in a gas turbine engine, this being macrocomposite structure which has a plurality of cooling air passageways positioned within the refractory mass. This type configuration provides distinct advantages in that the level of compressive stress on the refraction mass would be relatively high even at operating temperatures. Stress relief within the metal wires would occur at much lower rates due to the higher creep resistance at lower metal temperatures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an elevation of a cast vane with parts broken away.

FIG. 2 is a vertical section of a preferred mold construction for use with the novel process of the present invention and with a cast vane therein.

FIG. 3 is a sectional view through the cast article removed from the mold substantially along the line 2--2 illustrating a macrocomposite structure with inhomogeneously distributed wires.

FIG. 4 is a schematic view similar to FIG. 2 illustrating a macrocomposite structure with homogeneously distributed metal wires.

FIG. 5 is a schematic view similar to FIG. 2, showing a macrocomposite structure illustrating cooling air passages within the macrocomposite structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the present preferred embodiments of the present invention, a preferred form of mold geometry is illustrated in FIG. 2. The mold construction described herein is particularly suited for use with any of the so-called superalloys as described for example in the VerSnyder patent, U.S. Pat. No. 3,260,505. As therein noted, these alloys are generally adapted for the process known as directional solidification, the most preferred alloy being either a nickel base or a cobalt bast alloy. The mold construction herein described, in addition to the disclosure contained in the VerSnyder patent, employs the technique of forming Monocrystaloys.sub.TM as described in Piearcey, U.S. Pat. No. 3,494,709.

As herein illustrated, one end of a tubular mold 4 compatible with the procedure described in the VerSnyder patent, is placed on a relatively cool heat conductive and preferably water-cooled plate 6. Water for the chill plate 6 is carried through conduits 8. Tubular mold 6 is preferably made from a ceramic material from a conventional slurry of alumina or other high melting point refractory material in accordance with standard shell-molding techniques. As illustrated, one end of mold 4 rests on chill plate 6 and cooperates to form an inclosed cavity 10. In communication with cavity 10 is a passageway 12 through which molten metal is fed to cavity 10.

Surrounding cavity 10 are the means for heating the mold to the desired temperature for casting. Preferably the cavity is surrounded by an electrical resistance heating coil 14 supplied with variable electrical energy. Alternatively, the cavity may be surrounded by a graphite susceptor, not shown, and this in turn is surrounded by an induction coil supplied with high frequency electrical energy as is usual in an induction furnace. Prior to casting, the mold is heated to the desired temperature by supplying energy to coil 14 and when the desired temperature has been attained, molten metal heated to the proper temperature for casting is poured into cavity 10. The chill plate 6 is maintained at a relatively cool temperature by means of water circulating through conduits 8 so as to establish and maintain a temperature gradient within the molten metal filling cavity 10 as the metal solidified.

The cavity 10 within the mold has a lower cavity portion 15 forming the growth zone for the casting, a shroud portion 22 directly above the growth zone and directly below the portion 16 of the cavity 10 in which the refractory mass 18 is positioned. Above the mass 18 is another shroud forming portion 30 and at the upper end of cavity 10 is the filling portion 31.

In the present embodiment, a shaped refractory mass 18 is positioned in the portion 16 of cavity 10. The shaped vane-forming mass 18 illustrated the shape of and becomes a part of a nozzle stator vane for a gas turbine, this being more clearly illustrated in FIG. 2. The mass 18 is formed of a relatively low tensile strength refractory mass, the tensile stress being in the range of 20-40 ksi. The casting of the single crystal wires through the refractory mass permits the use of this relatively low tensile strength mass; however, it is to be understood that in some applications a high strength tensile mass may be desired and used with the invention herein disclosed. The refractory mass has excellent heat resistant characteristics and is able to withstand temperatures up to about 1,900.degree.C (3,452.degree.F). It has been found that an ideal refractory mass would be a ceramic such as an alumina base material. The refractory mass or shaped element 18 has a plurality of longitudinally extending holes 20 extending upward from the base portion 22 at the bottom of element 18 to the top portion 30 at the upper end of the element 18. In order to promote single crystal growth within the longitudinal openings, these openings may have a restriction at the bottom or base portion 22 of part 18, or they may just have a relatively small opening at the base portion. Therefore, after molten metal has been poured into cavity 10 and it begins to solidify, a controlled columnar structure forms in the growth zone and forms a columnar grained shroud 23 in the shroud portion 22 of the cavity 10. By providing a small opening such as 20 or a restriction, not shown, single crystal growth is promoted within openings 20 forming single crystal wires 28, see FIG. 2, which grow within the longitudinal openings 20. The crystalline growth again becomes columnar in the shroud 33 formed in the portion 30 of this columnar grain structure is oriented in the same direction as the columnar grain structure in the shroud formed at the base of mass 18.

Once the novel casting process described herein has been employed to permit the growth in the alloy being cast of the shroud 23 with a cube oriented columnar structure, an intermediate portion of single crystal wires or rods 28 and a top shroud 33 with substantially the same columnar structure as the base shroud 23, the solidified alloy is permitted to cool. As a result of this cooling, the ceramic or refractory mass 18 is prestressed in compression due to a differential in the rates of thermal contraction of the two materials. In other words, the ceramic element 16 is prestressed in compression due to the greater thermal contraction of the single crystal metal wires 28. This compressive stress which is developed in the ceramic vane element 18 is extremely beneficial in that it inhibits crack nucleation, causes rehealing of cracks at elevated temperatures, and impairs the propagation of cracks.

FIGS. 3, 4 and 5 illustrate different embodiments of macrocomposite nozzle vane structures. FIG. 4 illustrates a vane macrocomposite structure which positions the single crystal wires 28 substantially homogeneously within the refractory mass 18. FIG. 3 illustrates an embodiment where the single crystal wires 28 are positioned inhomogeneously within the refractory mass 18. This has been found to be extremely desirable in that it permits a design of a nozzle vane for the desired stress distribution and stress level. It is known that near the outer surface a uniformly high level of compressive stress is required to retard crack initiation and growth in the ceramic. Ideally the metallic wires should be very fine and closely spaced; however, there are limitations on wire size and in the wire spacing imposed by both the metal casting and the ceramic forming processes. By maintaining a relatively constant volume fraction of metal from the leading edge to the trailing edges we have found that a vane may be designed which has no bending moments, this being a preferred configuration in certain installations. FIG. 5 illustrates a macrocomposite vane structure in which, when the vane is incorporated in a gas turbine, the hollow single crystal wires 28 are cooled by a cooling medium circulating through central openings 32, this embodiment offering some distinct advantages. The level of compressive stress on the ceramic would be relatively high even at operating temperatures. Stress relief within the metal wires would occur at much lower rates due to the higher creep resistance at lower metal temperatures. It is to be understood that where internal cooling of the metal reinforcing wires is impracticable, the edges of the airfoil section where the metal wires are of small diameter being an example, external cooling may be accomplished by forcing air through small passages around the wires, or in fact, through passages within the refractory mass itself.

It will be understood that the completed vane as installed in a gas turbine is the refractory mass 18 together with the metallic columnar-grained shroud ends 23 and 33 integrally connected by the single crystal rods or wires. The unused portions of the casting are removed along the dotted lines 34 and 36. The refractory mass, being airfoil in shape, forms one wall of a turbine nozzle.

In all of the foregoing embodiments it is to be understood that the refractory mass 18 is to comprise at least the continuous part of the macrocomposite structure between the two shroud forming ends cast in the base portion 22 and top portion 30. More specifically, the refractory mass in the embodiment illustrated herein would be continuous or of a one-piece construction of alumina from the leading edge to the trailing edge of the vane. The constituents of a preferred macrocomposite structure are illustrated by the following example:

EXAMPLE I

A macrocomposite structure wherein the refractory mass is a 99 percent alumina (McDanel AP 35, McDanel Refractory Porcelain Co., Beaver Falls, Penna.) and the single crystal wires passing through longitudinal openings within the mass and the shroud elements are the nickel base superalloy Mar-M-200 (0.15 C, 9 Cr, 10 Co, 2 Ti, 5 Al, 12.5 W, 1.0 Cb, 0.05 Zr, 0.015 B, 1.5 Fe, Bal. Ni). Longitudinal holes, 0.030 inch diameter, are spaced in a hexagonal close packed array within the refractory mass. With a center-to-center spacing of holes equal to 0.053 inch, the refractory mass will contain 30 volume percent single crystal wires after casting. The stress in the refractory mass and the wires are:

.sigma..sub.33 = [(V.sub.2 E.sub.1 E.sub.2)/(E.sub.1 V.sub.1 + E.sub.2 V.sub.2)].DELTA..alpha..DELTA.T, (1)

and

.sigma..sub.33 = [(V.sub.1 E.sub.1 E.sub.2)/(E.sub.1 V.sub.1 + E.sub.2 V.sub.2)].DELTA..alpha..DELTA.T (2)

respectively, where

E.sub.1 = average Young's modulus of alumina

E.sub.2 = average Young's modulus of Mar-M-200

v.sub.1 = volume fraction alumina

V.sub.2 = Volume fraction Mar-M-200 wires

.DELTA..alpha. = .alpha..sub.1 - .alpha..sub.2

.alpha..sub.1 = Average linear thermal expansion coefficient of alumina

.alpha..sub.2 = Average linear thermal expansion coefficient of Mar-M-200

-.DELTA.t = operational temperature - ambient temperature

It is assumed that the stress in the ceramic and the metal is zero at the operational temperature.

Taking E.sub.1 = 50 .times. 10.sup.6 psi, E.sub.2 = 16 .times. 10.sup.6 psi, V.sub.1 = 70 v/o, V.sub.2 = 30 v/o, .alpha..sub.1 = 4.3 .times. 10.sup.- .sup.6 .degree.F, .alpha..sub.2 = 7.2 .times. 10.sup.-.sup.6 .degree.F, and -.DELTA.T = 1,800.degree.F,

-.sigma..sub.33 = 32,400 psi

.sigma..sub.33 = 73,800 psi

are the stresses calculated in the ceramic and metal wires respectively.

It is to be understood that the invention is not limited to the embodiments herein illustrated and described but may be used in other ways without departure from the spirit as defined by the following claims.

Claims

1. A cast composite structure comprising

a central body of refractory ceramic material having a plurality of longitudinally extending passages therethrough,

metallic end elements at opposite ends of the central body, and

metallic rods extending through the passages in said central body and integral with said end elements, said metallic rods having a higher rate of thermal contraction than the refractory ceramic material, and said rods

2. A cast composite structure as in claim 1 in which the rods are cast

3. A cast composite structure as in claim 2 in which the end elements are a

4. A cast composite structure as in claim 3 in which the rods are

5. A cast composite structure as in claim 3 in which the rods are

6. A cast turbine vane including

a central portion that is airfoil shape in cross-section and of a refractory ceramic material, said portion having a plurality of parallel passages extending therethrough from end to end,

metallic end shrouds at opposite ends of and in contact with the ends of the central portion, and

metallic cast rods extending through said passages and integral with said end shrouds, said rods having a higher rate of thermal contraction than the refractory material and thereby placing the latter in compression as the cast structure cools below the solidification temperature of the

7. A cast turbine vane as in claim 6 in which each of the rods is a single

8. A cast turbine vane as in claim 7 in which the shrouds are columnar grain structure and are integral with the rods.