Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
2000-07-06
2001-12-04
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S555000, C148S562000, C148S675000
Reexamination Certificate
active
06325871
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to bonding of cast superalloys, and more particularly relates to a method of bonding cast superalloy components for turbines and the like.
BACKGROUND INFORMATION
State-of-the-art blades and vanes that are employed in modern, high efficiency power generation combustion turbine engines rely on high quality materials such as single crystal alloys and precise control of the part's internal and external dimensions. Because of the large size of these parts, cost-effective manufacturing is being pursued by several routes.
Land-based gas turbines, such as the advanced turbine system (ATS) which is under development, require cost-effective high performance components fabricated from advanced materials. First and second row turbine blades and vanes include complex internal and external geometries, and should be fabricated from defect-free materials. Although components with such features have been developed for aircraft engines, the larger size of power generation turbine components provides a crucial challenge. To date, casting trials have been unable to produce defect-free large components in any significant yields.
An alternative manufacturing approach would be to cast essentially defect-free smaller subcomponents, and to subsequently join them using a high quality bonding process. Currently, however, the required bonding technology for advanced single crystal-containing superalloys, such as CMSX-4, that are targeted for use in ATS-class engines is not available.
SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention, bonding of single crystal nickel base superalloys employs a bonding foil that is similar in composition to the base material but contains an additional melting point depressant such as from about 1 to about 3 weight percent boron to depress the melting temperature of the foil. The major element composition of the bonding foil is close to that of the base material in order to provide approximately uniform chemical distribution across the bond region after solidification. The bonding process occurs isothermally at a temperature that is above the melting point of the foil but below the macro-melting point of the alloy, e.g., by about 100 to 150° F. The bonding thermal cycle is sufficient to cause solid state diffusion to disperse the boron away from the bonded interface, thereby raising the local melting point to make the material suitable for conventional heat treatment of the single crystal. The method may be used to bond single crystal alloys such as CMSX-4 and the like.
Parts preparation for bonding large parts such as the blades of land based gas turbines requires very good bond surface matching or fit-up, on the order of about 0.0025 cm (0.001 inch) between the two surfaces. This precision can be produced in parts after casting by low stress grinding/machining of the surfaces or by co-electrodischarge machining of the mating parts. These procedures produce surface profiles that lie within about 0.0025 cm (0.001 inch). The method also produces surfaces that are sufficiently undeformed that they are not vulnerable to recrystallization during subsequent bonding and heat treatment cycling, including the high temperature solution treatment of SC alloys, e.g., the solution heat treatment of CMSX-4 at 2,408° F.
Single crystal nickel base superalloys can be bonded to polycrystalline superalloys using transient liquid phase bonding. The chemistry of the bonding medium, either paste or foil, and the thermal cycle required to effect bonding can be controlled so that the resultant joints display a continuous gradation of chemistry and microstructure, and the properties produced in the joint region are generally between those of the base single crystal or the polycrystalline material, e.g., at least about 80 percent of the properties of the weaker base material component.
By reducing the section size of the castings, improved quality can be induced in the finished part, i.e., the production of grain boundaries, slivers and freckles may be reduced as the section size of the casting is reduced. Moreover, because the cast section can be selected to be a solid section, casting problems associated with casting around relatively sharp features of internal cores can be avoided. By using these approaches to reduce the tendency of producing defective castings, casting yields on the order of 80 to 90 percent may be possible.
The present manufacturing method based upon the assembly of segmented sections such as turbine blade subcomponents. The segmented sections are specifically defined as being bound by low vulnerability surfaces that can be used as bonding planes. This segmentation divides the component into smaller segments that can be easily cast, that are suitable for easy assembly, and that position the bonding plane(s) in minimally stressed locations. Effectively, the design segmentation process attempts to identify continuous slowly curving surfaces that will not be subject to significant loading across the bond plane. Eliminating sharp curvatures and intruding and protruding features from the surface of the subcomponents not only enhances casting yields, but also facilitates the application of the bonding medium and the fixturing of the subcomponents during bonding.
The transient liquid phase bonding process presents an opportunity for bonding large blades of advanced single crystals. The bond foil chemistry can be tailored to provide continuous structures across bond regions even in single crystal structures provided that post bonding thermal processing can be adapted to provide the optimum &ggr;/&ggr;′ structure in the bond region as well as in the base metal.
Computer aided design (CAD) coupled with state-of-the-art finite element modeling (FEM) greatly facilitates the development and mechanical analysis of segmented subcomponents. These techniques permit the definition of the blade geometry with segmentation surfaces dividing the solid model into distinct domains. Starting from the original blade, segmentation proceeds by selecting potential segmentation surfaces and assessing them quantitatively from the point of view of the anticipated loads across the surface. The surfaces are then considered qualitatively from the point of view of providing smooth continuous surfaces to facilitate casting and bonding. The selected surfaces(s) can then be modified to eliminate features such as sharp corners that will impair the casting quality and inhibit bonding. The modified surface can then be reanalyzed using FEM to reassess the potential loads across the bond line.
Current blade design requirements include high cycle fatigue, low cycle fatigue (LCF), creep, plasticity, and thermo-mechanical fatigue (TMF). The FEM analyses of the potential bond surface indicate whether the mechanical properties of the bonded metal can meet these requirements. Effectively, the bond region properties must surpass those defined by the requirements. Even though the present bonding process preferably targets 80 to 90 percent of the base metal performance, because the resulting material properties are reduced at the bond, the bond surface is placed in a location where the operating stresses are minimized.
While casting quality needs call for thinner section castings and more bonds, bonding efficiency calls for fewer bonds and thicker section castings. For example, the controlling cross-section of a single crystal blade may be approximately 102 mm (4 inches) thick. By designing the blade in two or more parts, this cross section can be reduced to as little as approximately 25 mm (1 inch) at its widest location. For most of the height of the blade, the casting thickness is actually less than approximately 13 mm (0.5 inches). An additional reduction in section width, for instance, from approximately 13 mm (0.5 inches) to 6 mm (0.25 inches), would markedly further improve the casting quality and yield.
The section of the primary bonding plane for a standard cored blade is chosen relative to the original core location. The p
Burke Michael A.
Freyer Paula D.
Hebbar Mohan A.
Seth Brij B.
Swartzbeck Gary W.
Eckert Seamans Cherin & Mellott , LLC
Siemens Westinghouse Power Corporation
Wyszomierski George
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