CA1202768A

CA1202768A - Method for forming braze-bonded abrasive turbine blade tip

Info

Publication number: CA1202768A
Application number: CA000413020A
Authority: CA
Inventors: Kenneth R. Cross
Original assignee: General Motors Corp
Current assignee: Motors Liquidation Co
Priority date: 1981-11-05
Filing date: 1982-10-07
Publication date: 1986-04-08
Also published as: GB2108534A; GB2108534B

Abstract

METHOD FOR FORMING
BRAZE-BONDED ABRASIVE TURBINE BLADE TIP
Abstract of the Disclosure In a preferred embodiment, a turbine blade tip is coated with an abrasive for wear forming a seal track in a gas turbine engine by gluing a self-sustaining preform comprising abrasive alumina grit and nickel-base superalloy braze powder onto the tip and heating to braze the grit to the tip.
The preferred grit is titanium coated to strengthen bonding between the alumina and the braze alloy.

Description

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METHOD FOR FORMING
BRAZE-BONDED ABRASIVE TURBINE BI~DE TIP
Background of the Invention This invention relates to an abrasive coating applied to a gas turbine blade tip for wear forming a seal track in a sta-tionary shroud. More particularly, this invention relates to a method for coating a turbine blade tip by brazing abrasive grit to the tip using a composite preform.
In a gas turbine engine~ gas flow between a tip of a rotating blade and a cooperating stationary shroud reduces efficiency and power. It is therefore desired to minimi~e clearance between the tip and the shroud to seal against gas flow. One method for accomplishing this is to employ an abrasive grit coating on one or more blade tips of a rotor to wear form a seal track in the shroud. The coatings comprise grit mechanically held, for example, in an electroplated metal matrix. A major problem with this method is that the grit tends to spall during turbine operations. Also, nickel-base superalloy is preferred for blade use but contains a plurality of minor constituents and is not suited for plating.
Thus, the coating and the blade are composed of different metals. Furthermore, a method is desired that is more suitable for mass production and provides more uniform thickness and grit dis-tribution.
Therefore, it is an object of this :invention to provide an improved method for coating a turbine blade with an abrasive for wear forming a seal track in a cooperating shroud within a gas turbine engine, which abrasive comprises refractory grit tightly bonded within a braze metal matrix to resist spalling.
More particularly, it is an object of this invention to provide a method for coating a superalloy turbine blade tip with abrasive grit braze bonded ~L2~

within a superalloy matrix haviny substantially the composition of the blad~ superalloy. The method not only tightly bonds the grit within the superalloy matrix, but also forms an integral braze bond between the coating superalloy and the blade superalloy. In one aspect of this invention, the grit is coated prior to brazing with a metallic braze-wetting agent, perferably titanium, that forms a strong metallurgical bond between the grit and the coating superalloy.
It is a further object of this invention to provide an impro~ed method for readily applying a spall-resistant, abrasive grit coating to a turbine blade tip, which method is adapted for commercial production. The method comprises brazing a grit-cont~;ning preform to the tip. The grit concentration and thickness is readily controlled in the preform to produce a uniform composition and thickness in the coating. An additional feature of this invention is that the pre~orm is readily sized and shaped to produce a coating having a desired pattern.
Summary of the Invention In a preferred embodiment, these and other objects are accomplished by forming a sheet comprising abrasive alumina grit and nickel-base superalloy braze powder and employing the preform sheet to braze the yrit to a tip surface of a nickel-base superalloy turbine blade. The alumina grit particles have a titanium coating that improves braze-wetting and substantially strengthens bonding between the alumina and braze alloy. Approximately equal volume portions of the grit and a nickel-base superalloy braze powder are slurried with a vaporizable liquid organic cement and spread into a uniform layer. After the cement sets, the layer forms a self-sustaining sheet that is suitably cut and glued onto a turbine blade tip.
The blade tip and attached preform are slowly heated to vaporize the cement and thereafter to melt the braze alloy, whereupon the melt forms a continuous phase surrounding the grit and wetting the blade tip.
Upon solidification, the grit is entrapped in a superalloy matrix that is integrally braze bonded to the blade tip, thereby forming a spall-resistant abrasive coating.
In a preferred coating of the invention, the metal matrix not only mechanically holds the grit, but it is also believed that the titanium on the grit interacts with the alumina and the braze superalloy to produce a strong metallurgical bond therebetween. The superalloy portion of the coating is integrally bonded to the superalloy of the blade and has substantially the same composition. The use of a braze-grit compo-site preform results in uniform grit distribution and thickness in the coating. In addition, the preform is readily shaped to produce a desired coating pattern.
Furthermore, the use of the preform and operations such as gluing and brazing are well suited for production.
Detailed Description of the Preferred Embodiment In accordance with this invention, an abrasive coating is applied to a gas turbine engine blade cast of a nickel~base superalloy. The preferred alloy consists of, by weight, 10 percent tungsten, 10 percent cobalt, 9 percent chromium, 5.5 percent aluminum, 1.5 percent tantalum, 1.5 percent titanium, 1.0 percent hafnium, 0.02 percent boron, 2.5 percent molybdenum, 0.15 percent carbon, 0.05 percenk zirconium and the balance nickel and is commercially available under the trade designation MAR M247* A preferred blade extends radially from a hub having a center * trademark 3 , 1q e~

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axis about which it is adapted to rotate within a gas turbine engine~ The blade comprises a tip remote from the hub and havlng a substantially flat, axially parallel surface that is intended to lie proximate to a stationary shroud surface in the engine and to be coated with abrasive to wear form a seal track in the shroud. The coating is suitably applied to an individually cast blade tip prior to assembly in a hub. Alternately, the coating may be applied to one or more blade tips of an integrally cast rotor. ~n a surprising aspect of this invention, it has been found that coatings may be simultaneously brazed to blades diametrically arranged on a hub without sagging.
A preferred abrasive grit consists of alumina A1203 particles sized to about 100 mesh~
The particles are coated with a metallic titanium film by vapor deposition. About 100 ~arts by weight alumina grit are blended with about 10 parts fine titanium hydride powder. The titanium hydride powder is sized to -325 mesh. The mixture is placed in a refractory alumina crucible and covered with a refractory perforated blanket suitable to contain the po~ders ~hile allowing gases to escape.
The covered crucible is then slowly heated in a ; vacuum furnace evacuated to a pressure less than about 5 x 10 torr. At about 400C, the titanium hydride decomposes to form hydrogen gas and metallic titanium. ~ slow heating rate is desired to allow the hydrogen to escape from the crucible without disrupting the powder. After the hydride is decomposed, the temperature is increased to about 1250C, whereat titanium vapor pressure increases and titanium is deposited onto nearby grit surfaces. The titanium film improves wetting of the surface by a nickel-base ,., -~ ~
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superalloy braze melt. It is also believed that the titanium interacts with the braze alloy and the alumina to form a strong metallurgical bondO
The tltanium-coated grit is then mixed with approximately an e~ual volume of finer braze alloy powder sized to -1~0 mesh. The preEerred braze alloy consists of, by weight, 10.8 percent tungsten, 10 percent chromium, 9.7 percent cobalt, 5.5 percent aluminum, 3.1 percent tantalum, 2.5 percent silicon, 1.6 percent titanium, 1.4 percent hafnium, 1.1 percent boron, 0.6 percent molybdenum, 0.17 percent carbon, 0.05 percent zirconium and the balance nickel. This alloy is essentially similar to MAR M247* but is modified to adapt the alloy for brazing, particularly by relatively high additions of silicon and boron that reduce the liquidus temperature and improve melt flow properties.
The mixture of grit and braze alloy powder is then formed into a self-sustaining sheet. The mixture is slurried with solvent-base vaporizable acrylic cement commercially available from Wall Colmonoy Corporation under the trade designation Nicrobraz~ The cement is of a type particularly formulated as a vehicle for bonding braze powder to metal surfaces in typical metal-metal brazing operations. A minimum volume of cement sufficient to form a pourable slurry is added to the powder.
The slurry is poured onto a sheet formed of a non-stick polytetrafluoroethylene material and covered with a similar sheet. The sandwich assembly is rolled -to spread the slurry into a layer having a uniform thickness of about 1.5 millimeters.
After rolling, the cover sheet is peeled away to expose the slurry layer to the air for drying. After the cement has sufficiently set so that the layer is * trademark 5 , f~s ~Z~Z71~13 self-sustaining, the botkom sheet is peeled away to further aid drying. Drying for about one hour at room temperature is generally adequate. The dried sheet is self-sustaining and sui~able for forming an abrasive coating. However, an acrylic lac~uer may be sprayed onto the sheet surfaces to prevent cracking and to stiffen the sheet for convenient handling.
The grit-braze sheet is cut using an abrasive silicon carbide wheel into a size and shape corresponding to a desired pattern of the abrasive coating on the blade tip. The preform is then glued onto the blade tip with a cyanoacrylate adhesive. Any overhang is trimmed by grinding. A
liquid stop-off alumina oxide slurry is painted onto the sides of the blade adjacent the tip to prevent brazing there.
Brazing is carried out in a vacuum furnace.
An individual blade is typically vertically oriented with the preform-bearing tip up. However, it has been found that brazing is also suitably carried out when a blade is oriented horizontally, without the preform falling off upon heating or the coating sagging, even though the tip surface is vertically oriented. Thus, two or more blades of an assembled or integrally cast rotor may be simultaneously coated by this invention.
The furnace is evacuated to less than 10 5 torr and initially slowly heated to vaporize the acrylic cement and cyanoacrylate glue. Thereafter, the temperature is raised to about 1230C and held for about 10 to 20 minutes to melt the braze alloy~
Upon melting, the braze alloy forms a liquid phase surrounding the alumina grit and wetting the blade tip. The melt wets and reacts with the titanium-coated '.,~

alumina particle surfaces in a manner that produces a strong bond upon cooling. Af-ter the braze powder has wet the grit surfaces and the blade tip, -the temperature is cooled to abou-t 1090C for five to ten minutes to set -the alloy to prevent sagging or flowing. Thereafter, the temperature is raised to about 1150C for 20 to 60 minutes. During this time, elements added to modify the superalloy for brazing, particularly boron and silicon, di~fuse into the blade and become substantially uniformly distributed in the coating and the cast blade near the tip.
This diffusion raises the liquidus tempexature of the coating to a level suitable for turbine engine operations.
The product coated tip compxises alumina grit tightly held within a nickel-base superalloy matrix, which matrix is tightly brazed to the cast superalloy of the blade. Superalloy in the coating and in -the blade has substantially the same composi-tion, particularly after diffusion during brazing.
Within the coating, the proportion of grit to alloy is essentially the same as formulated in the preform.
Also, uniform coating thickness is obtained, particu-larly for coatings on different bladesof a rotor.
If necessary, the coating may be ground to true up the blade dimensions or provide a desired contour.
In addition, the surface of the coating may be chemically ox electrochemically etched to better expose the grit to improve initial abrasiveness.
In the preferred embodiment, titanium is coated onto the grit to produce a strong metallurgical bond between the alumina and the braze alloy. It is believed that titanium deposited in immediate contact with the alumina surface bonds to oxygen in the alumina, essentially becoming part of the oxide 7~;~

stxucture. Titanium that is deposited further away remains substantially metallic. secause of its metallic nature, the titanium coming in contact with the braze melt improves wetting and itself alloys with the nickel-base superalloy. Thus, both the alumina titanium interface and the titanium-superalloy interface are strengthened by chemical bonding so that the overall grit-superalloy bond is stronger than mere mechanical joining. While titanium is preferably deposited onto the grit prior to mixing with the braze powder, improved bonding may also be obtained by includlng titanium hydride powder with uncoated grit and braze powder in a preform sheet, although titanium deposition is less efficient. Other metals, such as hafnium and zirconium, are also suitable for forming hydride-derived coatings that produce similar alumina-superalloy metallurgical bonds.
Other known abrasive oxides, carbides or nitrides may be substituted for the preferred alumina grit, including, for example, titania, zirconia or chromium carbide~ The coating preferably comprlses about equal portions of alumina grit and superalloy. At least about 30 percent by volume superalloy is believ~d necessary to adequately bond the grit particles into a cohesive coating. Coatings having greater than about 70 percent by volume alloy do not have adequate abrasive properties.
Although this invention has heen described in terms of certain emboaiments thereof, i-t is not in-tended to be limited to the above description, but rather only to the extent set forth in the claims that follow.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for coating a superalloy substrate with an abrasive comprising attaching to the substrate a self-sustaining preform comprising metal-coated abrasive refractory grit and superalloy-base braze powder, said grit comprising refractory particles coated by a metal selected from the group consisting of titanium, hafnium and zirconium and effective to enhance wetting by a braze liquid, said preform further comprising a vaporizable organic binder bonding the grit and the braze powder, heating the preform to vaporize the organic binder and to melt the superalloy braze to form a continuous superalloy liquid phase surrounding and wetting the grit particles and wetting the substrate, and cooling the braze phase to entrap the grit in a superalloy matrix and to braze-bond the matrix to the substrate.

2. A method for applying an abrasive coating to a nickel-base superalloy substrate comprising gluing to the substrate a self-sustaining preform comprising titanium-coated abrasive alumina grit and superalloy-base braze powder, said grit comprising alumina particles having a braze-wettable titanium coating suitable for metallurgically bonding alumina to a superalloy, said preform further comprising a vaporizable organic binder bonding the grit and the braze powder together, slowly heating the preform to vaporize the organic binder and thereafter to melt the braze superalloy to form a continuous superalloy liquid phase wetting the substrate and surrounding the grit particles and wetting titanium-coated surfaces thereof, and cooling to solidify the superalloy braze phase to form an abrasive coating comprising alumina grit enclosed within and metallurgically bonded to a superalloy matrix and to braze-bond the matrix to the substrate.

3. A method for applying an abrasive coating to a nickel-base superalloy substrate comprising heating a mixture comprising abrasive alumina grit and a metal hydride in a vacuum to decompose the metal hydride and to deposit nascent metal onto the grit, the metal hydride being selected from the group consisting of titanium hydride, hafnium hydride and zirconium hydride, mixing the metal-coated grit and a nickel-base superalloy braze powder with a liquid comprising a relatively high temperature vaporizable organic cement dissolved in a relatively low temperature vaporizable solvent, said liquid being in an amount sufficient to form a spreadable slurry and containing sufficient cement to cohesively bond the grit and the braze powder together, spreading the braze-grit slurry to a desired thickness, evaporating the solvent from the slurry to bond the grit and the braze powder into a self-sustaining sheet, gluing the sheet to the substrate, heating the sheet and substrate to vaporize the cement and to melt the braze alloy to form a braze liquid phase that wets the substrate and surrounds the grit, wetting metal coated surfaces thereof, and cooling the substrate and braze liquid overlayer to solidify the braze liquid to form an abrasive coating comprising grit within a nickel-base superalloy matrix tightly bonded to the substrate.

4. A method for applying to a nickel-base superalloy gas turbine blade tip an abrasive coating of the type employed for wear-forming a seal track within a gas turbine engine, said method comprising heating a mixture comprising abrasive alumina grit and titanium hydride in a vacuum for a time and at a temperature sufficient to decompose the hydride and to deposit nascent titanium onto the grit surfaces to form a braze-wettable film adapted for metallurgi-cally bonding alumina and nickel-base superalloy, mixing the titanium-coated grit with approximately an equal volume portion of a braze powder consisting essentially of a nickel-base superalloy similar to the blade tip but containing boron and silicon in sufficient amounts to reduce the liquidus temperature for brazing, slurrying the grit-braze mixture in a liquid comprising an organic cement dissolved in a solvent, said solvent being vaporizable at ordinary temperatures to set the cement, said cement being vaporizable at elevated temperatures and being present in an amount sufficient to cohesively bond the grit-braze mixture together, spreading the slurry to a desired thickness, evaporating the solvent therefrom to set the cement to form a self-sustaining sheet composed of the grit and the braze powder, cutting the sheet to form a preform having a desired size and shape conforming to the blade tip, gluing the preform to the blade tip, heating the glued preform and blade tip to vaporize the cement and thereafter to melt the braze alloy to form a braze liquid wetting the blade tip and surrounding the grit, wetting the titanium-coated surfaces, cooling the blade tip and braze liquid to solidify the braze superalloy to metallurgically bond the grit within a nickel-base superalloy matrix and to braze-bond the superalloy matrix to the turbine blade tip, thereby forming an abrasive coating on the blade tip, and heating the coated blade at a temperature and for a time sufficient to diffuse boron and silicon from the superalloy matrix into the blade so as to produce a substantially uniform superalloy composition.