Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
2000-07-06
2001-12-18
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S555000, C148S562000, C148S675000, C148S410000, C416S24100B
Reexamination Certificate
active
06331217
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to power generation combustion gas turbines, and more particularly relates to blades for such turbines made from multiple segments of cast superalloys.
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 defect-free smaller subcomponents and to subsequently join them using a high quality bonding process. Currently, however, the required bonding technology for advanced superalloys, including single crystal materials such as CMSX-4 that are targeted for use in ATS-class engines, is not available.
SUMMARY OF THE INVENTION
Hot section gas turbine blades are fabricated from single crystal superalloy castings by bonding high quality cast sections or parts. The present method allows the production of large, high quality turbine blades by joining small, high quality sections, in comparison with prior attempts to cast turbine blades as single pieces which have produced very low yields with concomitant high individual component costs.
The present invention provides high yield production of large sized single crystal components for gas turbines. The method brings the costs of turbine blades into a regime that is affordable for commercial implementation. It also allows for the simultaneous attainment of precise parts profile and optimum material quality and performance, which cannot be accomplished with conventional casting of single crystal materials. By eliminating the casting core the present process provides for control of internal component geometry and features. Furthermore, by allowing access to the internal cooling passages during production, the capability for precise quality control of internal cooling features and wall dimensions is provided. The existence of internal grain structure and defects may also be determined. The invention provides more precisely controlled single crystal turbine blades at greatly reduced costs.
The blade is designed to allow the placement of the bond lines in low stress regions of the blade. The parts of the blade may be cast with specifically incorporated excess stock to provide for improved fit up for bonding. Deformation methods may be used to shape parts for profile and fit up. The turbine blade parts may be prepared to very precise fit up of the order of 0.0025 cm (0.001 inch) by machining processes such as co-EDM or the like. The bond gaps between the parts of the blade are then filled by foil or paste. Bonding foils and thermal processes are selected in order to provide high quality and strength bond joints. In one embodiment, single crystal sections may be joined to other single crystal sections. In another embodiment, single crystal sections may be joined to polycrystalline sections, including directionally solidified sections, to provide for the fabrication of cost effective hybrid blades.
A turbine blade design is sectioned along low stress regions into two or more pieces. In one embodiment, sectioning along a single surface that is approximately along the blade camber-line allows for the efficient joining of high quality castings to produce essentially defect-free blades of single crystal cast superalloys that are not capable of being conventionally produced in high yield without defects. In another embodiment, a blade design is sectioned into four pieces by sectioning along two further surfaces within the root section in addition to the original section of a single surface that is approximately along the blade airfoil camber-line. Thus, the four pieces are defined by further sectioning of the two original sections into two more sections. These extra two sections are preferably located in the root of the blade. They are provided over low stress surfaces, and are contoured to be intermediate between the surface contour of the outer surface of the blade root and the inner bonding surface contour. Subsequently casting and joining the multiple pieces into a single structure using transient liquid phase bonding allows for the efficient joining of high quality castings to produce essentially defect-free blades that are capable of performing at very high temperatures.
In a further embodiment, the airfoil section of the turbine blade is cast as a single crystal alloy, and the outer portions of the root are cast as a polycrystalline alloy. High quality individual pieces are cast in high yield and are subsequently joined by a bonding process such as transient liquid phase bonding to produce essentially defect-free, high quality turbine blades with a cost effective yield.
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 method based upon the assembly of subcomponent segments of the blade structure incorporates low vulnerability bond planes into subcomponents that are designed to meet overall thermal, aerodynamic and mechanical needs. 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. The design segmentation process preferably identifies 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 preferred transient liquid phase bonding process provides for bonding of large blades of advanced single crystal alloys. 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 provides the desired &ggr;/&ggr;′ structure in the bond region as well as in the base metal. In addition to matching the microstructure in the bond region with the microstructure of the base material, the bond foil is selected such that it is compatible with the heat treatment process used for the base material.
Computer aided design coupled with finite element modeling may be used to facilitate 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 cont
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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