CA1068876A - Method and apparatus for bimetallic casting without metallurgical bond - Google Patents

Abstract

ABSTRACT

Method and apparatus for bimetallic casting in which a shape such as an airfoil which has anisotropic metallurgical properties is bonded to a second shape by casting. A
disposable pattern with a configuration desired in the second shape is attached to the first shape and an investment mold is formed about the pattern in the usual investment mold making procedure. A barrier layer is provided at least about those portions of the first shape which are to be bonded to the second shape to prevent interfacial melting or diffusion bonding at the union between the first and second shapes.
The pattern material is then removed, leaving an investment casting mold having a casting cavity conforming to the second shape. Molten metal is then poured into the casting cavity and solidifies to form the second shape without any significant metallurgical bonding occurring between the two shapes.

Description

SPECIFICATION
This invention is in the field of bimetallic castings wherein an investment casting mold is built around a preform and the solidification of the metal in the mold serves to anchor the two components together.
There are numerous disclosures in the prior art of the advantages of anisotropic metallurgical properties in components intended for high temperature use such as turbine vanes and wheels. By "anisotropic metallurgical properties"
we mean that the part has enhanced strength properties parallel to the major stress axis. In the case of an airfoil shape, this type of structure has been produced by directional solidification of a casting to provide columnar grains aligned parallel to the airfoil major axis. This grain orientation greatly improves ` ~ resistance to intergranular ~acture at elevated temperatures, lS and thus improves creep strength, ductility, and particularly thermal-fatigue resistance.
Another material which evidences anisotropic metallurgical properties is a fiber reinforced metal matrix ` composite. Fibers such as boron, silicon-carbide or graphite are embedded in a metal matrix such as aluminum in the form of thin plies and the plies are laid up into the desired airfoil -shape and then diffusion bonded together with the fibers running in the direction of the major stress axis. These composites exhibit highly directional, i.e, anisotropic properties.
Still another example of structures evidencing -anisotropic metallurgical properties is directionally solidificd ., - ' ' .
' , , -, 1 .
., ~

. "' '' ' ~

eutectic alloys. These eutectic alloys solidify into lamellar or rod-like structures which resemble fiber reinforced composites in that a relatively strong rod or plate-like material reinforces the weaker matrix.
S There are yet other examples of metallurgical structures which exhibit marked microstructural directionality and anisotropy of mechanical properties. Highly elongated and longitudinally aligned grains characteristic of high temperature alloys produced from consolidated metal powder by the so-called mechanical alloying process or by a process of directional recrystallization of wrought material are further examples of ; anisotropic metallurgical structures in the sense used in this invention.
There are inherent problems in producing articles having such anisotropic structures which are generally associated with changes in cross-section. For example, in cast high temperature blades or vanes used in gas turbines considerable ; difficulties can occur in producing the necessary article by the - ~ directional solidification process when large changes in geometry -~
o-cur. The most abrupt and troublesome changes occur at the junctures of the airfoil portion and more massive attachment ` or root regions of blades, or so-called shrouds of vanes.
These areas often display a propensity for internal defects ~; referred to in the casting industry as shrinkage porosity and/or composition changes arising from changes in solidification rates.
In addition, the ledges formed by the root areas or shrouds can serve as traps for nonmetallic impurities such as inclusions

-2-., .

, . . .

or dross.
In the case of blades and vanes the directionally aligned, anisotropic, cast structure is desired generally in the airfoil portion of the article which is the region exposed to the most severe temperature and stress environment. The nature of the casting process is such, however, that the ` entire article is cast by the directional solidification process which for complex parts unduly adds to the difficulties of ` producing the article needed to meet the functional requirements dictated by service conditions.
Still other configurations exist in which the geometrical limitations are such that production of directionally solidified airfoil portions is difficult. One example is found in unitary . - - - .
cast turbine wheels consisting of a disc or hub portion supporting a plurality of airfoils on the rim. Such wheels can .; -- be produced in cast form with an equiaxed cast grain structure -using the investment casting process. The resulting product will exhibit essentially the same type of cast grain micro~
structure in the airfoil and disc portion and the properties will be more or less isotropic. Although the size of grains Illay -`~ vary somewhat, no preferred alignment or anisotropy will exist in the longitudinal direction of the airfoils. In commercial ~i practice the problem of achieving a wheel with directionally ;
solidified airfoils is approached by assembling the wheel from " 25 separately cast blades mechanically attached to the rim of a ~ -separately produced disc having equiaxed grains, usually formed by a forgin~ process. Slots machined in the rim of the disc

-3-,, :, .A~

serve to anchor the roots of the individual blades. This type of assembly is extremely expensive in comparison with a unitary casting. However, its commercial use serves to illustrate the desirability of selectively employing anisotropic metallurgical structures and combining these with other metallur-gical structures in an overall article such as a turbine wheel. The above example also illustrates the practicality of a purely mechanical attachment to transmit loads in a highly stressed component. In this case the mechani-cal attachment serves to hold the blades in proper orientation to.and proxi-mity with the disc.
According to the present invention there is provided the method of mechanically bonding a first metallic shape to a second metallic shape which comprises providing a barrier layer at least about the portions of said first shape which are to be mechanically bonded to said second shape, attaching a disposable pattern in the configuration of said second shape to said first shape, forming an investment mold over said pattern, removing said pattern to leave a casting cavity of the configuration desired in said second shape, pouring molten metal into said casting cavity, and cooling the cast ` metal to provide a casting of said second shape mechanically bonded to said first shape without significant metallurgical bonding.
According to a preferred feature of the process of the present invention, which is a bimetallic casting process, shapes with anisotropic . metallurgical properties can be readily combined with other metals to pre-serve the unique properties of the anisotropic metallurgical structure at one portion of the resultant bimetallic article and take advantage of the unique properties constituent in another portion of said article.
Bimetallic casting processes, per se, have been described in prior art literature and patents. By and large, however, these process are direct-ed to the production of a metallurgical bond between the preform and the metal which is cast about it. Reference lS invited to Schwartz et al. U.S.
Patents Nos. 3j279,006 and 3,342,564 as examples of such disclosures. These patents describe the production of composite metallic objects by melting a metallic material having a specific property desired in the poured portion thereof under '~ .

, ~

vacuum, hea$ing a refractory mold having a cavity therein adaptecl to receive melted metallic material and having a solid metallic object or element positioned therein with at least a portion of the surface thereof exposed within the cavity, under vacuum and pouring the molten metallic material while maintaining an inert a~nosphere. The bonding which èxists between the solidified molten metal and the preform rèsults from the interalloying of the preform with the poured metal to produce a metallurgically bonded zone.
While metallurgical bonding is an effective means for joining the two portions together into a bimetallic article, such bonds are difficult to achieve on a reliable and reproducible basis. In practice very high vacuum levels or other extremely inert atmospheres must be used to prevent formation of contaiminants at the interfacial region which can reduce the - level of bonding. The temperatures of the pre-existing portionand the molten metal must be such that neither too rapid - cooling of the cast-on metal occurs, which could reduce bond strength by preventing sufficient interalloying, or excessively slow cooling occurs, which could lead to gross melting oi the initial solid portion. The physical contact occurring between the two materials being joined is characterized by extreme ` proximity being aided not only by the capability of molten metal to fill even microscopic recesses in the initial solid portion but by the relatively greater contraction occurring in the cast on material by virtue of its solidifying and subsequently . .

~-.. . .

cooling from a greater temperature than the initial solid material.
The resulting physical contact precludes, except in the case of gross separation, the nondestructive inspection of the article for bond quality.
It has been found that mold preheating and casting conditions utilized to produce a bi-cast article can be such that regions showing metallurgical bonding and areas free of a metallurgical bond can both exist in the attachment region.
If the design of the article for satisfactory operation in service relies exclusively on a metallurgical bond, undetectable areas of inadequate bonding can lead to premature failure.
Reference is made to the prior technical information published in the literature, (Article by U. Okapuu and G.S.
Calvert entitled "An Experimental Cooled Radial Turbine"
appearing in AGARD C~onference Proceedings No. 73 on High Temperature Turbines, Agard-CP-73-71, Paper No. 10, January, 1971) in which a gas turbine rotor was produced by " bicasting a nickel-base superalloy hub around root areas of ` previously cast nickel-base superalloy blades. The design was based on achieving a metallurgical bond although a few small recesses were provided to yield some mechanical support. The i root areas were tapered in a manner which, in the absence of ~ the recesses and any metallurgical bonding, would permit g unrestricted removal of the blades from the hub portion. In practice use of vacuum preheating and pouring conditions based on prior con trolled tests using castings which modeled the , ~ , , attachment resulted in the metallurgical bonding being limited to a single region of the root area. The resultant performance of the bimetallic part was not wholly adequate by virtue of failures from debonding at areas not fully bonded metallurgically.
The degree of bonding necessary to achieve the necessary degree of integrity in a metallurgical sense is such that a zone of alloying due to local intermelting or diffusion occurs without a discrete interface contain-ing weakening constituents. Even the presence of a thin film of a weakening constituent having a thickness of only 0.00001 inch or even less can be sufficient to prevent bonding. In some metals and alloys, severe reductions in mechanical strength and ductility are known to occur from films only a few atomic layers thick between grains. These can arise from compositional impurities, improper metal working or casting procedures, heat treatments or various combinations of these.
A preferred embodiment of the process of the present invention con-sists in providing a preform having anisotropic metallurgical properties and combining the preform with a disposable pattern in the shape of the piece which is to be joined to it. An investment mold is built up around the pattern in the usual way by the so-called dip and stucco process resulting in the formation of an investment casting mold in which the preform is embedded.
To prevent metallurgical bonding, a barrier layer is provided on the preform either before the mold is formed around it or, preferably, afterwards by heating the mold in '' ' :
.`' :

:. , : , , 8~76 air to cause a surface film of metal oxide to form at least in those areas at which the preform is to be attached to the solidified metal. The molten metal then is poured into the casting cavity produced by elimination of the pattern. The molten metal is at a pouring temperature considerably higher than the preheated mold so that a steep thérmal gradient exists between the molten metal and the preform. ` This favors both the avoidance of casting porosity in the solidifying metal in the vicinity of the joint and the formation of fine columnar "chill" grains in the solidified metal generally perpendicular to the boundary of the two portions.
THE DRAWINGS
_ .
Other objects, features and advantages of the inventlon will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:
Figures la and lb illustrate a properly bonded and improperly metallurgically bondèd structure, respectively;
-~ Figure 2 is a fragmentary view of a turbine blade and slot arrangement; -~
Figure 3 is a view in perspective of a preform in the shape of an airfoil which is to be joined to two shrouds in the process of the present invention; -Figure 4 is a view in elevation showing how the platform of Figure 3 is received in a wax pattern assembly for the ..1 , , . : , . . ::
~. . ,', .

-production of the investment casting mold; and Figure 5 is a view of the assembly after the investment casting mold has been formed around the pattern and the pattern has been removed to produce a casting cavity for the reception of molten metal.
It is apparent that the achievement of a metallurgically bonded zone in bimetallic cast articles by means of techniques taught in the prior art is not a wholly adequate process in terms of reliability and amenability to inspection techniques. Further-more the presence of a debonded region, regardless of the proximity of the surfaces, can exert a weakening effect in certain articles. One type of effect occurs if the part is designed for a 100~ metallurgical bond; bonding over less than ; the complete interface surface will clearly represent an inferior condition. Another type of effect arises by virtue of the juncture ` along the interface surface of the area containing a metallurgical bond and a region evidencing debonding. The principle pertaining . ..
to the latter effect is shown in Figure 1 using as an example a tapered knob in the attachment region of the initial solid portion.
For purposes of illustration no specific article is implied by the geometries shown. Reference numeral 10 identifies the initial article which would pertain to the portion containing an anisotropic metallurgical structure while 11 is the cast-on material. For ;
purposes of comparison in Figure la, the pouring parameters were controlled in such a fashion that complete metallurgical bonding was achleved and the resulting article Wa6 monolithic 9~ ~-~
., , .
,. ', . ' . . .~

in a load bearing, structural sense. The boundary between the two portions is a region of interalloying possessing the ability to transfer loads across the interface denoted by a dashed line 12. A departure from this ideal condition is shown in Figure lb wherein a small region of poor metallurgical bonding appears, as denoted by 13.The material on either side of this interface is free to separate under load and the junctures of this separated area 13 and the fully bonded region 12 constitute the tips of a notch 14. Thus, the debonded region is a so-called stress raiser, commonly referred to as a crack. Under the action of an applied load, such as a tension applied along the longitudinal axis of 10, particularly if the load application is of a repetitive nature typical of numerous service applications, the notch is likely to result in the formation of cracks which ; ~ 15 can eventually grow to the stage of failure. The reduction in strength arising from the notch effect of the poorly bonded ; region is in addition to the first effect cited, where in the debonded zone simply reduced the area across which load is transmitted.
We have previously discussed the common method of constructing a turbine wheel in which individual blades are mechanically held in place in slots machined into the rim of a / separately produced turbine disc. The type of attachment s geometry employed is in some cases very similar to that illustraled in Figure 1, being generally a so-called dovetail design involving a single lobe as shown in Figure 1 or a multiplicity of lobes, in the latter case referred to as a `~

,, firtree dovetail. In such attachments the boundary between the two materials, i.e, blades and disc in the specific case discussed, is continuous. We illustrate this for a single lobe dovetail, of the type described in reference to Figure 1, in Figure 2 where 10 denotes the portion pertaining to the blade inserted into a slot in a part of a disc 15. Because the boundary between the blade 10 and the disc 15 denoted by 16 is purely mechanical and continuous, the deleterious type of notch illustrated in Figure lb is absent.
The two extremes of bonding, 100~ metallurgical and 100% mechanical, are generally to be regarded as superior to a mixture of the two, particulary when loading is severe such as would exist in a turbine wheel. Now we have described how a joint design based on a metallurgical bond using methods detailed in prior art, i.e., based on interalloying through melting or diffusion, tends to produce bonds that are difficult to inspect and which could be unreliable and inferior. On the other hand, a purely mechanical joint formed by using machined geometrical locks or interfering surfaces is an acceptable and proven design technique. The present inv~ntion ;
proceeds on the basis of securing adequate mechanical bonding - ~ ~;
rather than relying upon metallurgical bonding and, in fact, -`, takes positive steps to avoid any metallurgical bonding . .-; occurring between the solidified molten metal and the preform.
In Figure 3, reference numeral 20 has been applied generally to a preform having anisotropic metallurgical properties, -.. . .

...... ....
, ' ' ' , ~. ., . ' 10~;8876 in this case, an airfoil having columnar grains generally indicated at reference numeral 21, the grains 21 extending along the major stress axis of the airfoil. At opposite ends of the airfoil 10 are a pair of extensions 22 and 23 having spaced apertures 24 and 25, respectively, therein. The extensions 22 and 23 are arranged to anchor the airfoil 20 in the subsequently applied metal casting, with the apertures 24 and 25 facilitating mechanical bonding between the solidified molten metal and the airfoil preform. Other types of devices for facilitating mechanical locking can, of course, be used such as slots, grooves, tapers or threads. This mechanical bond is enhanced `; by the compressive stresses exerted by the solidifying metal. -~
A disposable pattern assembly is then built up as illustrated in Figure 4. The airfoil 20 is supported between ` 15 two replicas 26 and 27 of the shrouds between which the airfoil 20 is to be bonded. The pattern may be made of wax, polystyrene ;
; or mixtures of the two. The shroud patterns 26 and 27 are connected to riser forming portions 28 and 29, respectively, which are fed from a sprue forming portion 30 all composed of tne disposable pattern material.
As previously noted, the airfoil portion 20 may be made of any suitable material having anisotropic metallurgical properties.
Directionally solifified alloys of nickel and cobalt are particularly useful for this purpose. The chemistry of these alloys has been ~-well developed over the years and does not form a specific ~-feature of the present invention. For a disclosure of such chemistry and other properties of nickcl and cobalt base .' ~

- 106~876 superalloys, reference is invited to Table 1 appearing in the appendix of the work entitled "The Superalloys" edited by Sims et al., and published by John Wiley ~ Sons. Table 1, appearing at pages 596 to 597 lists many commercial nickel and cobalt base superalloys~
The assembly shown in Figure 4 is then subjected to the usual investment mold making process. While there are a number of ways to produce shell molds of this type, we particularly prefer to use the method described in Mellen et al.
United States PatentNo. 2,932,864 issued April 19, 1960. In the method described in that patent, a destructible pattern of the article to be reproduced is coated at room temperature by dip-ping it in an aqueous slurry containing refractory particles and a binder. This coating is then dried isothermally so that the .
temperature of the pattern remains constant. The drying is achieved by passing air of controlled humidity past the coated pattern, the air containing sufficient moisture to maintain a substantially constant wet bulb temperature which is substantial- --~, .
ly the same as the initial temperature of the pattern and having a dry bulb temperature which is at least 10F higher than the wet bulb temperature. The pattern is then dipped in additional aqueous refractory slurries to form successive layers on the pattern. Each successive layer is isothermally dried in the same manner as described previously while maintaining the temperature of the pattern substantially constant. Finally, j the pattern is removed by melting it out either in a furnace ,~
or in an autoclave. ~
'. .

~ ~ . .

' ~ - 13 -~ A
~,.
. .. .. ` ` ~ . .. . . . . ..
. . .. . . ,. . , . . ". . .

The mold which results is illustrated in Figure 5 of the drawings. It contains a sprue portion 31 fceding a pair of riser portions 32 and 33 which feed a pair of casting cavities 34 and 35 which are to form the shrouds of the airfoil assembly.
In the preferred embodiment of the present invention, the mold is fired at a temperature of about 1600~ to oxidize at least those portions, such as the extensions 22 and 23 which are to form the mechanical bond with the subsequently cast alloy. Alternatively, the airfoil portion may be precoated with a nonmetallic barrier material by spray, dipping or painting deposition using suitable binders. These can include various ceramics such as SiO2 or ZrO2 held in place with refractory cements, or fused glassy coatings . Still other techniqu es of applying such barrier materials are the so-called flame spraying or plasma arc spraying processes to form a ceramic surface layer. The barrier layer can also be achieved by first -depositing a layer of metallic material particularly suited for subsequent conversion to a ceramic form by heating in an oxidizing atmosphere.
The molten metal is then poured into the mold with a superheat above the melting point at a level sufficient to fill .
all recesses of the mold but not so great that melting of the preform would occur. For nickel or cobalt base superalloys ~ .3 a superheat of 150 to 350~F above the liquidus, the temperature :
of complete melting, is generally acceptable. Such temperatures . . .
are in the range of 2600F for common alloys and result in a - substantia1 tcmpcrature differential between the mold and the . . .
.

~ - , ` 1068876 superheated metal. Consequently, those portions of the molten metal which embrace the extensions 22 and 23 will chill rapidly and almost immediately with the production of very fine columnar grains perpendicular to the interface. This fine grained structure is highly desirable, being relatively superior in mechanical properties to coarser grained cast structures. In addition, the steep temperature differential, as evidenced by the chill grains, is indicative of solidification occurring in such a fashion that the rapidly cooled metal in the vicinity of interface will generally be much less prone to shrinkage porosity which, to those familiar with castings, is clèarly a superior condition in terms of metallurgical quality and m~ochanical properties.
The particular molten metal which is to form the shroud portions is essentially a matter of choice. For example, the --` 15 metal may be a nickel or cobalt base superalloy, or it may be a high temperature iron alloy. Surprisingly, we can actually " use higher melting alloys for the shroud portion than for the ~ - .
~ airfoil because of the substantial heat transfer which exists. ~ -:, :
The barrier layer at the interface between the preform and the solidifying metal should have a lower thermal coriductivity than either of the two metals being joinèd. Such a barrier ~ layer prevents intermixing or interalloying of the two metals.
-~ The thickness of the barrier layer may be quite small, on the order of one-thousandth inch or less.
J 25 While the prior art teaches the use of high vacuum preheating conditions in making bimetallic castings, such high vacuum conditions are not necessary in accordance with the -15- - .

1061~876 present invention. As noted, the preheating of the mold may take place in an oxidizing atmosphere, and the pouring may take place at moderate vacuum conditions. The actual vacuum level is not dictated by the bimetal~c casting process in the S present invention because the bond desired is mechanical and not metallurgical. Hence, the vacuum environment used in melting need be adequate for the specific material being cast.
Special precautions to assure very clean interface surfaces are not required under this invention. In the case of alloys which are usually melted in air, such as many cobalt base superalloys, the melting and casting could be performed without the use of any vacuum in marked departure from the techniques required by the prior art to achieve a metallurgical bond.
The process of the present invention can be used to produce a wide variety of complex shapes, taking advantage of unique physical properties in each portion of the shape being produced. The process may be used, for example, in the casting of entire turbine wheels about airfoil portions having different ` physical and chemical composition. The process may be used in casting of shrouds onto airfoils singly or as multiple assemblies to produce vanes or vane segments combining alloys with properties tailored to match the different operating conditions experienced in the airfoil and shroud portions. Articles which as unitary castings tend to exhibit shrinkage porosity at those regions where airfoils meet more massive attachment regions ~ - -would exhibit less sllrinkage porosity. Because such bimeta~lic castings would be based on mechanical rathcr than me~allurgical .
., ~

bonding, the element of uncertainty and performance risk arising from inspectability problems and dangers of loacl bcaring degradation in the presence of even small areas of poor ;;
metallurgical bonding is avoided. The procedures needed to S obtain a mechanical bond require far less rigorous control and result in a more readily achievable bond. Thermal conditions are such that very high preheat and pouring temperatures are not needed and the possibility of erosion or melting of ; the preform avoided. ~onsequently, the results of the process . lO are highly predictable.
Although the specific process is described herein for alloy combinations involving anisotropic metallurgical structures in the preform, these are not a prerequisite for the successful ~
utilization of the proceQs to achieve a mechanical joint by ` `
deliberately avoiding metallurgical bonding.
., . . .

. ,... ~

` , .

., '.
~, : .

... .

.~, .
. - . .; .

,",,.' .':

:' ",`~

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of mechanically bonding a first metallic shape to a second metallic shape which comprises providing a barrier layer at least about the portions of said first shape which are to be mechanically bonded to said second shape, attaching a disposable pattern in the configuration of said second shape to said first shape, forming an investment mold over said pattern, removing said pattern to leave a casting cavity of the configuration desired in said second shape, pouring molten metal into said casting cavity, and cooling the cast metal to provide a casting of said second shape mechanically bonded to said first shape without significant metallurgical bonding.

2. The method of claim 1 in which said first shape has anisotropic metallurgical properties.

3. The method of claim 1 in which said first shape has an elongated grain structure.

4. The method of claim 1 in which said first shape consists of directionally solidified casting having a columnar grained structure.

5. The method of claim 1 in which said first shape is a directionally solidified casting of substantially eutectic composition.

6. The method of claim 1 in which said first shape consists of a composite having a fibre reinforced metal matrix.

7. The method of claim 1 in which said first shape is substantially of an airfoil geometry.

8. The method of claim 1 in which said barrier layer is provided by heating the mold with the first shape positioned therein in air to a temperature sufficient to provide an oxide layer over said first shape.

9. The method of claim 1 in which said first shape is at a temperature substantially below the temperature of the poured molten metal to thereby provide a chill for the molten metal poured against said first shape.

10. The method of claim 1 in which said first shape is composed of a nickel base superalloy.

11. The method of claim 1 in which said first shape is composed of a cobalt base superalloy.

12. The method of claim 1 in which said first shape is provided with apertures for mechanical interlocking with the solidified metal of said second shape.

13. The method of claim 1 in which said barrier layer is substantially of a ceramic composition.

14. The method of claim 1 in which said barrier layer is a fused glass.

15. The method of claim 1 in which said barrier layer is applied by molten metal spraying.

16. The method of claim 1 in which said barrier layer is formed by oxidizing a metallic coating previously applied to the first shape.