BACKGROUND OF THE INVENTION The performance of a gas turbine engine is sometimes limited by the maximum temperature to which certain components can be expose. These components, such as turbine blades and vanes, various seals and spacers, combustors, and other components, are sometimes actively cooled with air from the engine's compressor, so that the component metal temperatures can be maintained at acceptable levels while being exposed to gas temperatures that are in excess of permissible metal temperatures.

This active cooling comes with an inherent trade off.

Although higher gas temperatures tend to improve the performance of the engine, the use of air from the

compressor for active cooling tends to lower the performance of the engine. It is imperative that the scheme for active cooling use as little air as possible, so that performance gains are maximized.

In order to cool the various components efficiently, numerus component internal cooling flowpath designs have been used to optimally transfer heat from the component into the cooling air. Examples of such cooling flowpath designs can be found in U. S. Patents No. 4,930,980 issued to North et al.; 4,236,870 issued to Hucul, Jr. et al.; 4,946,346 issued to Ito; 5,062,768 issued to Mariage; and British Patent No. 2,246,174A issued to Lings et al.

These flowpath designs provide cooling air to various regions of the component, and often use small features to enhance heat transfer. Examples of cooling flowpath designs with small features include U. S. Patents No. 4,770,608 issued to Anderson et al.; 4,962,640 issued to Tobery; 5,281,084 issued to Noe et al.; and 5,193,975 issued to Bird et al. Small sized features such as pins, pedestals, walls, and passageways permit an increased number of such features to be included in the cooling flowpath. A large number of cooling pedestals, for example, increases the surface area of the component exposed to the cooling air. A small size for cooling passageways permits not only a larger number of passageways, but may also increase the convective heat transfer coefficient of the passageway.

However, use of small features such as pins, pedestals, walls, and passageways poses a problem during manufacturing. These features are often cast or etched onto one workpiece which must then be joined to another workpiece. Joining methods often include welding, brazing, or diffusion bonding. Examples of cooling flowpath designs in which portions of the workpieces are brazed include U. S.

Patents No. 5,263,820 issued to Tubbs; 4,786,234 issued to Readnour; 4,505,639 issued to Groess et al.; 4,482,295

issued to North et al.; 5,419,039,5,392,515, and 5,405,242 issued to Auxier et al.; 5,383,766 issued to Przirembel et al.; 5,193,980 issued to Kaincz et al.; and British Patent Specifications No. 1,285,369 issued to Steel et al. and 1,299,904 to Curtiss Wright Corporation.

Brazing of flowpaths with small geometric features presents the problem of excess brazing material interfering with the intended use of the feature. For example, excessive brazing material within a cooling air passageway can reduce or completely eliminate airflow through the passageway, with subsequent local or general overheating of the gas turbine component. It is important that excessive brazing material be kept from areas adjacent to various cooling flowpath features. U. S. Patent No. 4,507,051 issued to Lesgourgues et al. discusses brazing of actively cooled gas turbine components.

One manner of depositing brazing material is by electrophoresis. U. S. Patents No. 5,336,382 issued to Bodin, and 5,411,582 issued to Bodine, describe use of electrophoresis with a mucilage remarkable for its high viscosity, the muciliage containing brazing powder with grains as large as 53 microns. Outer areas in which brazing material is not desired are masked by a suitable adhesive masking tape, and inner cavities are masked with silicone plugs.

What is needed is a method of brazing components in which brazing material can be accurately deposited near small features. The present invention provides this method in a novel and unobvious way.

SUMMARY OF THE INVENTION A method for brazing, comprising preparing a bath comprise of a solvent solution, a binder, and a compound for ionically interacting with the binder; mixing into the bath a powder suitable for brazing, the particle size of the powder being less than about 37 microns; applying a nonconductive coating to a passageway defined on a first workpiece; depositing brazing material by electrophoresis onto a portion of the first workpiece adjacent the passageway; placing the portion of the first workpiece in contact with a second workpiece; and heating the assembly of the first workpiece and second workpiece such that a braze joint is formed between the second workpiece and the first workpiece.

It is an object of the present invention to provide an improved method for brazing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side diagrammatic view of a gas turbine engine with which the present invention is useful.

FIG. 2 is a front perspective view of an actively cooled turbine blade brazed according to the present invention.

FIG. 3 is a cross sectional view of the turbine blade of FIG. 2 taken along plane 3 of FIG. 2.

FIG. 4 is a view of a portion of the turbine blade of FIG. 3 taken along line 4-4, with the second workpiece removed, and showing a portion of the first workpiece.

FIG. 5 is a cross sectional view of the portion of the first workpiece of FIG. 4 taken through line 5-5.

FIG. 6 is a cross sectional view of the portion of the first workpiece of FIG. 4 taken through line 5-5 with nonconductive coating applied according to the present invention.

FIG. 7 shows a 200X enlargement of an etched micro-structure of a braze joint produced with brazing material which included residue characteristic of an electrophoretic bath of one embodiment of the present invention.

FIG. 8 shows an 8X enlargement of a portion of a coupon which inclues pedestal and passageway features useful in the internal structure of an actively cooled turbine component, on to which a nonconductive coating has been applied and brazing material has been electrophoretically deposited according to one embodiment of the present invention.

FIG. 9 shows an 8X enlargement of a portion of a coupon which inclues pedestal and passageway features useful in the internal structure of an actively cooled turbine component on to which a nonconductive coating has been applied and brazing material has been electrophoretically

deposited according to one embodiment of the present invention.

FIG. 10 shows an 8X enlargement of the coupon of FIG. 9 with the nonconductive coating removed and after exposure to a prewetting furnace cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENT For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific langage will be used to describe the same. Vit bill nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

The present invention provides a method for depositing brazing material onto a first workpiece which is ultimately brazed to a second workpiece, forming one or several braze joints. It is useful for depositing brazing material onto portions of the first workpiece that are adjacent passageways, the passageways being either in the first workpiece or in the assembly of workpieces, in which it is desirable to keep the brazing material out of the passageway after the braze joint is formed. It is especially useful in components used in hot sections of a gas turbine engine, including blades, vanes, combustors, and seals, in which the passageways ultimately flow cooling air. Often such cooling passageways are small, so that a relatively large surface area is exposed to the cooling air, and shaped to provide cooling air where needed.

The present invention inclues the discoveries that a low viscosity, low surface tension electrophoretic bath is useful for depositing a controlled amount of brazing material onto small features such that the brazing material does not interfere with passageways adjacent to the small features after the braze joints are formed. Prior to the

electrophoretic deposition, the passageways have applied to them a nonconductive coating which both inhibits deposition within the passageway, and also inhibits excess deposition on the edges adjacent the passageways. This invention also inclues the discovery that a binder material used within the electrophoretic process, such as the protein zein, does not appreciably interfere or degrade the quality of the final braze joint.

FIG. 1 is a side diagrammatic view of a gas turbine engine 20 with which the present invention is useful. A compressor assembly 22 is attache to a hot section 24. Hot section 24 inclues various internal components that are actively cooled, such as turbine blades 26 and turbine vanes 28 as shown underneath the removed section of FIG. 1.

FIG. 2 is a front perspective view of an actively cooled turbine blade 26 brazed according to the present invention.

Cooling air from compressor 22 enters inlet hole 30, cools turbine blade 26 through various internal passageways, and exits through exit holes 32.

FIG. 3 is a cross sectional view of turbine blade 26 taken along plane 3 of FIG. 2. Blade 26 is constructed from an inner core or first workpiece 34 which is joined to an outer covering or second workpiece 36. Cooling air entering hole 30 passes into plenums 37 and into a plurality of blade inlet holes 42. Air from holes 42 flow into a plurality of passageways 39 and around a plurality of pedestals 40, ultimately flowing through exit holes 32. Passageways 39 are formed around pedestals 40 and between workpieces 34 and 36. First workpiece 34 is joined to second workpiece 36 at a plurality of braze joints 38.

FIG. 4 is a view of first workpiece 34 prior to brazing, as viewed along line 4-4 of FIG. 3. A plurality of projecting pedestals 40 can be seen. A plurality of air inlet holes 42 are shown between some of the pedestals 40.

Although the present invention is shown as used with an

actively cooled turbine blade with a cooling flowpath of passageways surrounding pedestals, the present invention is also useful with other cooling flowpath configurations.

FIG. 5 is a cross sectional view of first workpiece 34 of FIG. 4 taken through line 5-5. The present invention contemplates passageways 39 in which the distance between walls 39b and 39c is greater than about 0.010 inch. The present invention contemplates a height from passageway floor 39a to pedestal top 40a greater than about 0.010 inch. Holes 42, as well as holes 32, are generally between about 0.008 and 0.050 inch diameter.

FIG. 6 shows the portion of first workpiece 34 from FIG.

5 after applying nonconductive coating 44. Nonconductive coating 44 is applied within passageways 39 and holes 32.

Those of ordinary skill in the art will recognize that holes 32 can be regarde as either inlet holes or exit holes for cooling air. It is preferable that coating 44 be applied close to corner 40c of pedestal 40.

The present invention inclues electrophoretic deposition of brazing material onto features such as pedestals 40 after nearby passageways 39 have been nonconductively coated, with later brazing of the features to a second workpiece. A related art method of applying brazing material by use of a tape preform is not suitable for cooling flowpath designs where the tape preform would need to span a passageway, because of subsequent deposition of brazing material into the passageway which would interfere with the usage of the passageway. Another related art method of applying brazing inclues adhering a two-sided tape to the areas to be brazed, and then manually applying brazing material to the exposed side of the tape. This manual method is not desirable for cooling flowpath designs with a plurality of small features, because of expense and nonuniform application that is inherent in a manual method.

Generally, one embodiment of the present invention comprises:

preparing a bath comprise of a solvent solution, a binder, and a compound for ionically interacting with the binder; mixing into the bath a powder suitable for brazing; applying a nonconductive coating to a passageway of the first workpiece; depositing brazing material in the bath by electrophoresis onto a portion of the first workpiece adjacent the passageway; placing the portion of the first workpiece in contact with a second workpiece; and heating the assembly of the first workpiece and second workpiece such that a braze joint is formed between the second workpiece and the first workpiece.

The electrophoretic deposition occurs in a bath comprise of organic solvents with low viscosity and low surface tension. Generally, the viscosity of the bath is less than about 1 centipoises at room temperature, and the surface tension of the bath is less than about 30 dynes per centimeter. The low viscosity and low surface tension of the bath permit it to readily flow into small features that might otherwise inhibit the flow of high viscosity or high surface tension fluids. The organic solvents also maintain relatively constant properties of low viscosity and low surface tension over a range of temperatures suitable for use in an ambient working environment. Bath specific gravit, prior to addition of brazing material, is about. 88 to. 92. A sample electrophoretic bath of one embodiment of the present invention inclues: (a) solvent: 60 +/-5% by weight alcool, and 40 +/- 5% by weight of a nitroalkane, such as by way of example only nitromethane; (b) a protein binder: 2-3 grams per liter of solvent; and (c) a complex ionic solute for ionically interacting

with the binder, such as by way of example only cobalt nitrate hexahydrate (CNH), 0.1-0.2 grams per liter of solvent.

It is necessary that the alcohol be compatible with the protein, such that the protein is able to act as a surfactant and binder for the brazing material, and such that the protein does not decompose within the alcool. A preferred combination of alcohol and protein is the combination of isopropanol and zein.

Added to this bath is a brazing material in powder form. A quantity of about 0.02 to 0.03 kilograms of brazing material is added for each liter of bath. The present invention is useful with any type of brazing material. One type of such brazing materials are the nickel alloys, of which NI-405-1 brazing material powder manufactured by Praxair, Inc. is an example. It is desirable for the brazing material to be of a fine powder size for example smaller than about 37 microns. It is preferable that the brazing material powder be of a size less than ten microns.

A size less than 10 microns is preferable because of the low viscosity and low surface tension of the solvent. It is also preferable that the powder of brazing material be of a size larger than 1 micron, so as to not make the bath colloidal in nature.

The bath of the present invention thus described is useful for electrophoretically depositing brazing material on conductive surfaces. Therefore, it is necessary to apply a nonconductive coating to those areas of the workpiece in which brazing material should not be deposited. This coating is retained on the workpiece during the deposition of brazing material, and is removed afterwards preferably prior to brazing. Suitable materials for this coating include silicone rubber; AC 818 chemical resistant maskant, manufactured by AC Products, Inc.; a mixture of Nicrobraz white stopoff powder, manufactured by Wall Colmonoy, Inc.,

and Adsol 1290R cement, manufactured by Amdry, Inc.; and other similar compound and mixtures. The mixture of Nicrobraz stopoff powder and Adsol 1290R cement is advantageous as a nonconductive coating, in that the mixture can be left on the workpiece during a prewetting furnace cycle at which time it inhibits flow of the brazing material into the passageways and holes. The coating materials should be resistant to the electrophoretic bath.

The nonconductive coating is applied wherever it is preferable to not deposit brazing material. For example, when the present invention is applied to a first workpiece of an actively cooled turbine blade or vane, the maskant is applied within the cooling passageways. It is preferable that the coating be applied throughout the passageway, including in cooling holes and near corners, edges, and other features that act as boundaries between the cooling passageways and adjacent areas that are ultimately part of the finished braze joint. As shown in FIG. 6, nonconductive coating 44 has been applied near corner 40c of pedestal 40. Proximity of coating 44 to corner 40c is preferable because of the increased attraction of brazed powder in the electrophoretic bath to an external corner.

If the coating material is not applied sufficiently close to a feature such as corner 40c or sufficiently thick to prevent voltage leakage, then brazing material is deposited within the passageway along corner 40c which can interfere with the function of the passageway.

After the bath has been prepared and the nonconductive coating applied to the workpiece, the first workpiece is inserted into the bath as a cathode. The bath also contains an anode, preferably one that is complementary in shape to the workpiece. A voltage is applied between the cathode and anode such that an initial current density of approximately . 1-. 2 amps per square decimeter is achieved. This voltage is maintained constant throughout the deposition process.

The voltage is maintained for a period of time necessary to achieve the required deposition of brazing material. The amount of brazing material required depends upon factors such as the type of brazing material, type of material the workpieces are constructed from, and characteristics desired for the final braze joints. The amount of brazed material deposited is established by routine experimentation by those of ordinary skill in the art. It is preferable to stir the mixture moderately and continuously during the deposition process. It is preferable to maintain the temperature of the bath near room temperature, from about 60 deg. F to about 90 deg. F.

After the proper amount of brazing material has been deposited, the workpiece is removed from the bath and permitted to dry. The workpiece has on it a deposit that inclues not only brazing material, but also zein, cobalt nitrate hexahydrate, and solvent residue. Because of the manner in which this deposit is created, the bath constituents are distributed throughout the deposit. The present invention inclues the discovery that the zein, CNH, and solvent residues can be burned off prior to brazing with no apparent effect on the desirable characteristics of the final braze joint. As an example of a suitable method for burning off the residue, the workpiece can be heated at a relatively slow rate of about 15 degrees F. per minute up to about 1,000 degrees F., and then maintained at about 1,000 degrees F. for a period of about 10 minutes.

After the burnoff of bath residue, it may be advantageous for certain applications of the present invention to prewet the brazing material. Prewetting heats the brazing material and first workpiece to a temperature close to, but lower than, the final brazing temperature.

During this prewetting, the brazing material partially melts and becomes adherent to the workpiece. Is is desirable to maintain the nonconductive coating in place during

prewetting. Prior to prewetting, the deposited brazing material may not have sufficient mechanical strength to withstand subsequent handling. Removal of the nonconductive coating prior to prewetting can result in removal of brazing material. This prewetting is advantageous for those applications in which the workpiece with brazed material is handled subsequently, such that it would otherwise become loose and fall off of the first workpiece prior to final brazing. This prewetting is also advantageous for brazing small features, since the loss of even a small amount of brazing material on a small feature can significantly affect the integrity of the final braze joint.

The first workpiece, with deposits of prewetted brazing material thereon, is placed against a second workpiece to which it is to be brazed. The manner of brazing, including the time and temperature cycle of the furnace, has been found to be unaffected by the electrophoretic process.

Brazing can be accomplished in any manner suitable for the particular alloys, as known to those of ordinary skill in the art.

Reference will now be made to specific examples using the processes described above. It is to be understood that the examples are provided to more completely describe preferred embodiments, and that no limitation to the scope of the invention is intended thereby.

EXAMPLE 1 DETERMINATION OF THE EFFECTS OF BINDER AND BATH RESIDUE ON THE BRAZE JOINT A slurry of brazing powder, CNH, and small amounts of isopropanol and nitromethane was prepared. The brazing powder was NI-405-1, screened to less than about 37 microns. The slurry was applied by brush to a first coupon of PWA 1480 material. The coupon with slurry was air

dried. A second coupon of CMSX-4 material was then put in contact with the brazed material. The assembly was placed in a furnace which was programme with the following thermal cycle: Heating Rate Dwell Temperature Dwell Time Degrees F/Minute Degrees F 15 1000 10 Minutes 15 1900 10 Minutes 25 2250 2 Hours The assembly of the two coupons was removed from the furnace and sectioned. FIG. 7 shows a typical, chemically etched cross-section showing portions of the two coupons and the braze joint therebetween. The brazing material is shown to have melted, wet the surface of both coupons, and formed a braze joint. There were no oxide stringers or other obvious contamination found within the braze joint. The braze joint was unaffected by the binder system or other bath residue. The thickness of the braze joint was about 0.001 inch which is thinner than a typical braze joint but preferable for minimizing filleting and retaining channel shapes of small features.

EXAMPLE 2 ELECTROPHORETIC DEPOSITION WITH NONCONDUCTIVE COATING APPLIED TO PASSAGEWAYS This trial was performed with a portion of material suitable for use in an actively cooled component in a gas turbine engine. Etched onto this material was a pattern of

round pedestals separated by passageways. The pedestals were approximately 0.040 inch in diameter. A series of passageways surround each pedestal, with the distance between pedestals being about 0.025 inch, and the tops of the pedestals being about 0.015 inch above the floor of the passageways. A nonconductive coating of silicone rubber was applied to all passageways of the coupon. The coating was applied to within about 0.005 inch of the corners of the pedestals. An electrophoretic bath of the present invention was prepared as described previously herein. Brazing powder of the type NI-405-1, screened to less than 37 microns, was added to the bath in the proportion of about 0.03 kilograms per liter. The test coupon was inserted into the bath as a cathode, and voltage was applied as previously described for a period of about 5 minutes. FIG. 8 shows a photograph of the test coupon after the brazed material had been deposited, and with the nonconductive coating still intact.

The results of this trial indicate that additional brazing powder was deposited near the corners. External corners, such as those existing around the pedestals, have a relatively high current density initially during the electrophoretic process. Thus, there was preferential deposition of brazing material onto the external corners during the process.

This additional deposition of brazing material near the edges of the passageways may be useful in certain applications of the present invention. For example, if a thicker braze joint were desired, then the coating material could be applied so that there is a space of about. 002 inch from the coating to the corners of the features to be brazed.

EXAMPLE 3 EFFECT OF INCREASED AMOUNT OF OF NONCONDUCTIVE COATING Another coupon was prepared with pedestal and passageway

geometry as described for Example 2. A nonconductive coating of Nicrobraz white stopoff powder mixed with Adsol 1290R cement was applied. Nonconductive coating was applied to this coupon such that the coating was about flush with the corners of the pedestals. This coupon was exposed to the electrically charged electrophoretic bath for a period of about 3 minutes. FIG. 9 shows the coupon after the electrophoretic deposition, with the nonconductive coating intact. Note that there appears to be less déposition of brazing material around the corners of the pedestals. The brazing material appears to be deposited uniformly.

This coupon was air dryed, the bath residue was burned off, and then exposed to 2200 degrees F. for a period of about 1 hour. The purpose of this high temperature exposure was to prewet the deposited brazed material, so that it would be adherent to the top of the pedestal and remain intact during subsequent handling. FIG. 10 shows a portion of the coupon after prewetting, and also after removal of the nonconductive coating. The brazed material on the pedestals appears slightly melted, but still substantially granular in texture. The passageways between pedestals are generally clear and unobstructed by either brazing material or nonconductive coating. The holes in the coupon are also unobstructed with either brazing material or nonconductive coating. These holes are useful for permitting flow of cooling air through the passageways and around the pedestals.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.