Metal fusion bonding – Process – With pretreating other than heating or cooling of work part...
Reexamination Certificate
2000-08-31
2002-04-09
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
With pretreating other than heating or cooling of work part...
C228S248100, C228S205000, C148S023000, C148S024000
Reexamination Certificate
active
06367686
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a braze material, in either paste, paint or tape form, which is useful for repairing gas turbine engine parts and a method for repairing braze joints using said braze material.
Routine maintenance of gas turbine engines involves disassembling the engines, inspecting parts to determine whether they are reusable or require repair or replacement, and ultimately rebuilding the engines with reused, repaired, or replacement parts. Various methods are used to repair gas turbine engine parts including brazing. For example, high pressure compressor stator assemblies, which comprise inner and outer shrouds that support a number of brazed in compressor vanes, are often repaired by debrazing the vanes, cleaning and repairing the shrouds and reusable vanes, and brazing reusable and replacement vanes to the shrouds to rebuild the stator assembly.
The step of cleaning the shrouds to remove oxides that form during engine operation is important to obtain a clean, sound braze when rebuilding a stator assembly. Current cleaning methods include grit blasting and aqueous degreasing. While these methods can satisfactorily clean many shrouds, they typically need either a line of sight to the oxides (for grit blasting) or strong chemical compositions (for aqueous degreasing). Due to the complicated geometry of the shrouds, it is often difficult or impossible to get the line of sight needed for effective grit blasting. The option of using strong chemical compositions is becoming less desirable as repair shops look for ways to lessen the environmental impact of their operations. Even when grit blasting or aqueous degreasing are options, the time and effort required for satisfactory cleaning may be unacceptable to customers. Moreover, grit blasting and/or aqueous degreasing operations can be so aggressive that they damage shrouds to the extent that an entire stator assembly may need to be scrapped. Scrapping a stator assembly is costly to the customer and can delay engine reassembly while a replacement assembly is obtained.
U.S. Pat. No. 5,735,448 relates to a method of repairing surface and near surface defects in superalloy articles such as gas turbine engine components. In this method, after the base metal surface of the article has been cleaned, a repair coating is applied to the base metal surface of the article. The repair coating comprises a composition selected from the group consisting of between about 20 wt % and about 60 wt % volatile organic carrier and between about 20 wt % and about 60 wt % water-based carrier. The repair coating further comprises up to about 8 wt % of a fluxing agent including a halide compound and up to about 5 wt % of a thickening agent. The balance of the coating is metallic filler mix comprising a first finely-divided homogenous particulate component substantially corresponding in composition to that of the article and a second finely-divided homogenous particulate component having as its base the same base metal as that of the article and containing a melting point depressant in a quantity substantially exceeding that present in the article. The volatile organic carrier may be methyl alcohol, ethyl alcohol, or ether. The water-based carrier may be a gel binder such as Nicrobraz Cement-S. The thickening may be any substance capable of congealing the repair coating such as starches, gums, casein, gelatin, and phycocolloids, semisynthetic cellulose derivatives, and polyvinyl alcohol and carboxyvinylates. The fluxing agent includes halide compounds such as fluorides, bromides, chlorides, or mixtures thereof.
The engine repair industry needs a quicker, more reliable method of cleaning gas turbine engine parts required to support brazements.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved braze material which may be used to clean and repair engine parts.
It is a further object of the present invention to provide a quicker, more reliable method of cleaning gas turbine engine parts required to support brazements.
It is yet another object of the present invention to provide a more reliable method for repairing gas turbine engine parts.
The foregoing objects are attained by the braze material and the method of the present invention.
In accordance with the present invention, a braze material which cleans and repairs cracks and voids in braze joints in engine parts is provided. The braze material broadly comprises lithium fluoride in an amount sufficient to act as a flux which allows the braze material to flow into a crack or void in a part to be repaired up to an amount where there is no residual lithium or fluoride in the crack or void, and preferably up to about 20% volume, a gel binder in an amount up to about 15% by volume, and the balance comprising at least one of a nickel braze alloy and a cobalt braze alloy. In a most preferred embodiment, lithium fluoride is present in an amount from about 10% to about 15% by volume.
The braze material of the present invention may be applied in a paste form or, alternatively, in a paint or tape form. When the braze material is to be applied in paint form, up to 50% of its volume may comprise a suitable solvent. When the braze material is to be applied in tape form, the gel binder is replaced by a flexible binder of the type usually used to form tape products.
A method for cleaning and repairing a braze joint is also disclosed. The method broadly comprises the steps of forming a braze material containing lithium fluoride in an amount effective to act as a flux which allows the braze material to flow into a braze joint to be repaired up to an amount where there is no residual lithium or fluoride in the crack or void, and preferably up to about 20% by volume, a gel binder in an amount up to about 15% by volume, and the balance comprising at least one of a nickel braze alloy and a cobalt braze alloy, applying the braze material to the braze joint, and heating the braze material to a temperature in the range of from about a temperature sufficient to cause the lithium fluoride to volatize up to about 2300° F., preferably up to about 2200° F., and most preferably up to about 1950° F., for a time period from about 1.0 minute to about 30.0 minutes. As previously mentioned, the braze material may be applied in paste form, in paint form, or in tape form.
Other details of the braze material and the cleaning and repair method of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
It is not uncommon for gas turbine engine repair units to receive engine components, such as high pressure compressor stators, with braze joints that exhibits voids, service induced cracks, and/or lack of braze in joints that are difficult to adequately clean prior to braze repair. It is also known that cleaning methods such as grit blasting and aqueous degreasing may not successfully remove engine oxides. Thus, a braze material that contains a self-cleaning component is highly desirable.
In accordance with the present invention, a self cleaning braze material is provided. The material contains lithium fluoride, a nickel braze alloy, and a gel binder. The lithium fluoride is present in an amount sufficient to act as a flux and enhance the flow of the braze material into the contaminated joint up to an amount where there is no residual lithium or fluoride in the crack or void, and preferably up to about 20% by volume. In a preferred braze material, lithium fluoride is present in an amount from about 10% to about 15% by volume. Lithium fluoride has been found to be a particularly desirable material because it volatizes during the thermal cycle used during the brazing operation. Further, neither the lithium nor the fluoride diffuses into the base material. Instead, the lithium and the fluoride are released to the atmosphere.
Still further, it has been found that the fluoride component in the lithium fluoride bonds with the su
Abriles Beth Kwiatkowski
Manis Bryan W.
Rutz David A.
Bachman & LaPointe
Dunn Tom
Pittman Zidia
United Technologies Corporation
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