JPS6132109B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in the method of manufacturing bicast products, which are a single casting made by two separate casting operations, and more particularly, A method of producing a duplicative cast product by positioning a preform formed into a predetermined shape in a mold and casting metal to be bonded to the preform around selected portions of the preform. It is related to.

Conventionally, bimetal casting methods themselves have been described in various documents and patents and are well known. Broadly speaking, this method aims to form a metallurgical bond between a preform and the metal being cast around it. Prior art examples of this method are described in US Pat. Nos. 3,279,005 and 3,342,564. These patents involve melting a metallic material having desired properties in the injected part under vacuum, having a cavity to receive the molten metallic material, and inserting a solid metallic object into the cavity. Describes the production of composite metal bodies by heating a refractory mold positioned under vacuum with at least a portion of the surface of the body exposed in a cavity and injecting molten metal while maintaining an inert atmosphere has been done. The bond that exists between the solidified molten metal and the preform is due to the preform and the injected metal alloying together to form a metallurgically bonded region. However, while metallurgical bonding is an effective means of joining two parts together to create a bimetallic product, the disadvantage is that the bonds formed in this way are not easy to form in a reliable and reproducible manner. It was hot. In fact, it is necessary to use very high vacuum or other inert atmospheres to prevent the formation of contaminants in the interfacial region that would reduce the degree of bonding. The temperature of the existing part and the molten metal should be adjusted to avoid excessively rapid cooling of the injected metal which would reduce the strength of the joint by preventing sufficient alloying or too slow cooling which would melt the entire initially solid part. Stay. The physical contact that occurs between the two materials being bonded not only allows the molten metal to fill even the microscopic recesses in the originally solid part, but also allows the injected molten metal to solidify and form a larger area than the original solid material. It is characterized by close proximity, aided by the relatively large shrinkage caused by subsequent cooling from a fairly high temperature. The resulting physical contact has the disadvantage that the quality of the joints in bimetallic products cannot be inspected without destroying the joints, unless the parts are completely separated.

It has been found that the preheating of the mold and the casting conditions for making a bicast product result in both areas exhibiting metallurgical bonding and areas not being metallurgically bonded at the attachment location. If the construction of the product to function satisfactorily in use is particularly dependent on metallurgical joints, undetectable locations of poor joints are even more susceptible to failure. Referring to the prior art described in the literature (Proceedings of the Agard Conference on High Temperature Turbines No. 73 Agard CP-73-71, January 1971, Title: Experimental Cooled Radial Turbine), this document states that It is described that a gas turbine rotor is manufactured by bicasting a nickel-based superalloy hub around the cutting edge of a previously cast nickel-based superalloy blade. This construction was based on making a metallurgical bond with several recesses to form a certain mechanical support. The blade base is tapered to allow unrestricted removal of the blade from the hub portion in the absence of recesses or metallurgical joints. However, in practice, the use of vacuum preheating and pouring conditions based on prior art controlled testing using molded attachments has shown that the metallurgical bond is limited to one area at the root. The result was that I put it away. For this reason, bimetallic parts have the disadvantage that they are easily damaged when the bond separates at a location where the bond is not metallurgically sufficient.

The degree of bonding necessary to obtain the necessary integrity in a metallurgical sense is such that the alloy regions, by local intermelting or diffusion, occur without discontinuous interfaces containing components that weaken the bond. slightly approx.
The presence of a thin film of weakening component having a thickness of 0.00001 inch or less is sufficient to prevent bonding. For some metals and alloys, significant reductions in mechanical strength and ductility result from thick films of only a few atomic layers between grains. These result from impurities in the composition, improper metal processing or casting procedures, heat treatment, or various combinations thereof.

It is an object of the present invention to provide a method for manufacturing double cast products that overcomes the drawbacks of the prior art, such as upper air.

In the method of the present invention, a solid metal preform, such as an airfoil, having anisotropic metallurgical properties has a lower melting point due to the presence of one or more minor elements, such as boron. Coated with alloy. The alloy used for the coating has a melting point below the melting point of either the preform or the cast metal to which it is bonded. The reason for this is to effectively melt the alloy for bonding without melting the preform or cast metal. For example, if the alloy has the same melting point as the preform, it will destroy the desired metallurgical properties of the preform. That is, the grain structure of the preform becomes disjointed. The coating is applied under a vacuum or other suitable protective environment at a temperature just to melt the preform and for a minimum amount of time so as not to mutually alloy with the alloy of the preform.

The coated foam is then positioned in a mold having a casting cavity of the desired shape in the cast metal to which it will be bonded. The mold is heated in preparation for pouring in a vacuum environment where the surface of the coating is not contaminated. Molten metal is injected into the casting cavity under vacuum and the joint solidifies around the desired preform portions. During casting of the cladding, the cladding undergoes some melting as the relatively thick cast metal transfers heat to the preform. Therefore, the bond between these two materials is improved due to the presence of the relatively low melting point coating. Following casting, a bicast product consisting of a preform, metal cast thereon, and a cladding therebetween diffuses a portion of the remelting point reduction element in the cladding area from the cladding into the adjacent alloy being bonded. This is combined with the diffusion of other elements into or out of the coating to improve the homogeneity of the composite, thereby providing further bonding. In this step, the melting point of the coated area is raised significantly by reducing the amount of melting point lowering elements included. The coating with the intermediate region serves as a filler material that can be selected to be compositionally very similar to the surrounding metal such that the bond after heat treatment has mechanical properties similar to those of the surrounding metal.

The method of the invention is particularly applicable to gas
It has application in the production of parts for turbine engines, especially when these parts have anisotropic metallurgical properties.

The term "anisotropic metallurgical properties" means that the part has high strength properties parallel to the principal stress axis. In the case of airfoils, such structures have been manufactured by directionally solidifying the casting to form cylindrical grains aligned parallel to the main axis of the airfoil. Orienting the grains in this manner increases resistance to interparticle cracking at high temperatures and thus increases creep strength and ductility and especially thermal fatigue resistance.

Another material that exhibits anisotropic metallurgical properties is fiber reinforced metal matrix composites. Fibers such as boron, boron-silicon or graphite are embedded in a metal matrix such as aluminum in the form of a thin layer and this layer is placed in the desired wing shape to extend the fibers in the direction of the principal stress axis and bond by diffusion. Ru. The composite exhibits highly directional, ie, anisotropic, properties and can also be fabricated using a combination of thermally resistive materials.

Yet another example exhibiting anisotropic metallurgical properties are directionally solidified eutectic alloys. Eutectic alloys solidify into laminar or rod-like structures similar to composites in which a relatively strong rod or plate material reinforces a weaker matrix with fibers. There are other examples of metallurgical structures that exhibit significant microstructure and anisotropic mechanical properties. Highly elongated longitudinally aligned particles of high temperature alloys made of metal powders strengthened by so-called mechanical alloying methods or processed material directional recrystallization methods are also used in the anisotropic metallurgy used in the method of the invention. This is another example of a structure.

There are inherent problems associated with changes in cross section in manufacturing products having such anisotropic structures. For example, when casting hot blades used in gas turbines, large variations in geometry create considerable difficulties in producing the required product by directional solidification methods. The most rapid and troublesome changes occur at the junction of the wing section with a more rigid attachment, namely at the root of the blade, ie the so-called vane cover.
These locations exhibit a tendency for internal defects, referred to in the foundry industry as holes due to solidification shrinkage, or change in composition due to changes in solidification rate, or both. Furthermore, the ridge formed by the canopy at the base acts as a trap for non-metallic impurities such as packaging or dross.

In the case of blades and vanes, directionally aligned anisotropically cast structures are generally desirable for product parts that are exposed to the most severe temperature and stress environments. However, casting methods are not suitable for composite parts, where the entire product is cast by a directional solidification method that greatly increases the difficulty of producing the product needed to meet the functional requirements demanded by the conditions of use. It has this property.

Furthermore, the production of solidified wing sections with directionality in other shapes may be constrained by geometries that are not easy to manufacture. One example is found in a one-piece cast turbine wheel consisting of a disk or hub supporting a plurality of blades on the rim. Such wheels can be manufactured in a casting process using a coated casting process and having an equiaxed cast granular structure. The resulting product necessarily exhibits a cast grain structure in the wing and disk sections and is somewhat anisotropic in properties. Although the grain size may vary somewhat, there is no preferred alignment or anisotropy along the length of the wing.
In commercial practice, the problem in manufacturing wheels with directional solidified wings is generally the mechanical process of molding the rims of individually manufactured discs with equiaxed grains formed by casting methods. The solution is to assemble individual cast blades that are mounted on the blade. Slots machined into the rim of the disc serve to lock the roots of the individual blades. This type of combination is very expensive compared to monolithic castings. However, its commercial use serves to illustrate the desirability of selectively using anisotropic metallurgical structures and combining them with other metallurgical structures into a complete product such as a turbine wheel. .

In one preferred version of the invention, the preform is made with anisotropic metallurgical properties such that the portion of the preform to be bonded to the cast metal has a melting point relative to the preform and the alloy cast thereon. It is coated with a boron-containing alloy coating that is compatible with the preform and the solidified molten metal to reduce the molten metal.
Depending on the strength required for the metallurgical bond made by the method of the present invention, the composition of the coating alloy will be related to the composition of the preform and cast metal, except for the amount of boron that is significantly greater than that in the preform or solidified metal. can be changed.

For example, if two substantially similar materials of substantially superalloy composition are used in a preform and a cast metal part, respectively, with the high degree of strength required for the joint, the coating alloy composition It is necessary to choose one with a composition very similar to that of the alloy. However, if the strength of the metallurgical joint does not need to be very close to that of the preform or cast metal, e.g. in the case of components that are subjected to relatively light loads during use, a slight Relatively simple alloys similar to alloys that are bonded with major chemical components of the type are preferred. In either case, the main component of the coating is at least one of the metals to be bonded.
The two main components must be the same.

As an example, if the preform and the solidified metal to which it is bonded are both nickel-based alloys, a preform with a predominant nickel content and a
It is preferred to use an alloy consisting of 25% chromium, 0.05 to 0.2% carbon, and about 1 to 4% boron to lower the melting point. Making the ingredients the same makes it easier for the alloy to blend in with the preform, etc.
This is because the binding ability is effectively performed. A particularly preferred boron-containing alloy has about 15% chromium and about 3.5% chromium.
% boron, approximately 0.1% carbon, and the balance nickel. This alloy is approximately 1070°C (1930°C) lower than either of the two alloys being combined.
It has a melting point of 〓). For example, nickel-based superalloys have temperatures of about 1232 to 1652°C.
It has a melting point of (2250 to 3000〓).

Boron-containing alloys can be deposited by electrodeposition, vapor deposition or spraying,
The preforms can be coated in a variety of ways, either in powder or plastic form. If either of the last two methods is used, the alloy must be fused to the area to be coated to form a continuous film of boron-containing alloy. The coating is preferably applied under vacuum or other protective environment to aid in bonding it to the preform. This particular environment can be varied to meet the requirements of the preform alloy and coating composition, and can include, for example, an atmosphere of hydrogen or argon gas in some cases in addition to vacuum. Those familiar with the art of furnace brazing will know that choosing an unsuitable environment for melting the coating will result in easily visible insufficient flow and wetting of the preform, suggesting that the procedure be modified accordingly. It becomes clear.

Similarly, the melting temperature of the coating can be varied depending on the type of coating alloy. For the particular coating alloy described above, it has been found that a temperature of about 1066°C (1950°C) in conjunction with a vacuum environment is sufficient to cause the coating to melt and adhere to the preform to the desired extent in 5 seconds. In general, the coating alloy, regardless of its composition, must be coated onto the preform at as low a temperature as possible and in as short a time as possible to reduce its boron content by fusion to the preform without increasing the melting temperature of the coating. The alloy is melted into the surface of the preform, and this melting is done slightly. The reason for this is to maintain the low melting point state of the alloy. If the alloy is significantly dissolved into the preform, the boron within the alloy will diffuse into the preform and the melting point of the alloy will reduce its bonding ability. That is, the alloy on the surface of the preform reaches a very high melting temperature, destroying the metallurgical properties of the preform and making it impossible to bond the preform to the cast metal. The dimensions of the coated zone after melting are likely to be small due to the nature of the deposition technique. Preferably, the coating thickness is, for example, 0.005 inches (0.12 mm).

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

In FIG. 1, reference numeral 10 designates a preform having generally anisotropic metallurgical properties, which in this example is derived from an airfoil having cylindrical grains, generally referenced 11, extending along the principal stress axis. It is completed. A pair of extensions 1 at each end of the wing
2, 13 are provided, these extensions being arranged to lock the wing 10 to a subsequently applied metal casting by means of a metallurgical connection. The locations where the boron-containing coating is pre-applied to the blade 10 are the extensions 12, 13.
These are the parts indicated by numerals 14 and 15 surrounding the area.

The erasable master assembly is then assembled as shown in FIG. The wing 10 is supported between two replicas 16, 17 of the enclosure. The prototype can be made of wax, polystyrene or a mixture thereof. The enclosure masters 16, 17 are connected to riser forming parts 18, 19, respectively, which are fed from a runner forming part 20 made entirely of erasable master material.

As previously mentioned, the airfoil 10 may be made of any suitable material having anisotropic metallurgical properties. Orientally solidified alloys of nickel and cobalt are particularly useful for this purpose. The chemistry of these alloys has been studied for many years and does not form a specific feature of the present invention. For the chemical properties and other properties of superalloys based on nickel and cobalt, see the article entitled ``Superalloys'' in Appendix Table 1 of the work published by Georgem Willkie and Son. I want to be

Pages 596-597 of this Table 1 tabulates a number of commercial nickel and cobalt based superalloys and this article is incorporated herein by reference.

The prototype assembly shown in FIG. 2 is then subjected to a normal flow mold manufacturing process. There are several ways to make this type of shell mold, but with particular reference to U.S. Pat. The (die) is coated at room temperature by immersing it in an aqueous chiller containing refractory particles and a binder. This coating is then dried isothermally so that the temperature of the mold remains constant. This drying is accomplished by passing moisture-controlled air through the coated mold, the air containing sufficient moisture to maintain a nearly constant wet bulb temperature approximately equal to the original temperature of the mold. It has a dry bulb temperature that is at least 10 mm higher than the wet bulb temperature. The mold is then sequentially immersed in additional aqueous refractory chiller to form successive layers thereon. Each successive layer is dried isothermally in the same manner as described above while maintaining the temperature of the original approximately constant. Finally, the master is removed by melting it either in a furnace or autoclave.

The resulting mold is shown in FIG. This mold includes a runner section 21 feeding a pair of riser sections 22, 23 which in turn form a pair of casting cavities 24, 2 which form the enclosure of the blades.
5.

In a preferred embodiment of the invention, the mold containing the coated preform is placed under vacuum prior to casting.
Preheated to a temperature of 871°C (1600°C), the mold is ready to contain the melted and injected cast metal under vacuum. One of the features of the invention is that the molten metal is injected at relatively low temperatures, conditions that would normally result in mechanical bonding, since the melting of the boron-containing coating is This is because it is not necessary at this stage.

After the cast metal has solidified, the bicast product is removed from the mold and subjected to a sufficient heat treatment to transfer boron from the intermediate layer between the preform and the solidified molten metal. Generally, the heat treatment is carried out at a temperature in the range of about 1010 to 1232°C (1850 to 2250°C). Preferably about 1121 to 1212°C
(2050 to 2175〓) temperature range. The reason for performing the heat treatment at a temperature in this range is that boron can be effectively transferred (diffused) between the preform and the solidified metal. Thermophilic treatment is
It is carried out under inert conditions of about 10 to 15 microns. The heat treatment time can vary over a fairly wide range and can vary from 1/
The duration may be as short as 2 hours to as long as 20 hours or longer. During this treatment time, boron migrates from the interlayer, thereby increasing the melting temperature of the interlayer. When boron diffuses, it also acts to enhance bonding.

Analysis of the interlayer shows that no abrupt compositional changes occur across the interlayer, and therefore the interlayer has physical properties very similar to the composition of the surrounding zone, especially for boron-containing coatings. This is true if the composition is chosen to be very similar to that of the alloy to be combined.

It is also observed that casting near the middle of the interface with the intermediate layer exhibits relatively small columnar grains perpendicular to the interface between the two superalloys as heat is extracted by the relatively cold preform. Ta.

Although boron is a particularly preferred diffusing agent to improve bonding, significant amounts of silicon, ranging from about 2 to 8%, can also be incorporated into the interlayer. However, silicon diffuses into surrounding metals rather slowly. 1 to 4% phosphorus can also be used as a component of the tie layer, but is not as effective as boron.

As described above, the method of the present invention provides improved metallurgical bonding without the need to rely on interfacial melting during casting to effect metallurgical bonding. Diffusion of boron under heat treatment conditions also results in a stable compositional state at the interface resulting in significant similarity between the bonding zone and the adjacent solid preform and the overlying cast portion, thus A practical benefit is that the resulting joint can exhibit metallurgical properties similar to the superalloy to which it is joined.

[Brief explanation of the drawing]

1 is a perspective view of a preform in the shape of an airfoil which is joined to two enclosures by the method of the invention, and FIG. 2 shows a prototype assembly of the preform of FIG. 1 in wax for producing a mold for investment casting. 1 is an elevation view showing how the preform of FIG. 1 is accommodated, and FIG. 3 is a view of the assembly with an investment casting mold formed around the mold and the master removed to accommodate molten metal. 10... preform, 11... cylindrical particles,
12, 13...extension part.

Claims (1)

[Scope of Claims] 1. A nickel-based metal preform is positioned inside a mold containing molten cast metal, and when the cast metal solidifies, a portion of the preform joins the cast metal. A method of manufacturing a double cast product in which a portion of the preform is exposed from the mold for bonding, wherein the exposed portion of the preform is treated with 1-4% boron, 5-15% chromium, and 0.05-0.2% prior to the casting step. % carbon and the balance nickel, coated with an alloy having a melting point below the melting point of the preform or cast metal,
A molten cast metal is poured into a mold so as to fix the coating alloy to the preform, the cast metal is solidified into a preform, and the preform is joined by the solidified cast metal so as to become one piece.
A method of manufacturing a double cast article comprising heating at a temperature of 1010 DEG to 1232 DEG C. (1850 DEG to 2250 DEG C.) to diffuse the boron of the fixed coating alloy into the preform and solidified cast metal. 2 The coating of the coating alloy is approximately 0.13 mm (0.005 inch)
Claim 1 made with a thickness smaller than
Section method.