Vane lug repair technique.

This invention relates to the repair of retaining lugs on turbine engine vanes, with specific application to jet engines.

A gas turbine engine includes a compressor section, a combustion section, and a turbine section. Disposed within the turbine section are alternating rows of rotatable blades and static vanes. The stationary vanes, disposed between rows of rotating blades, stabilize and direct the gas flow from one row of rotating blades to the next row. Such gas flow stabilization optimizes the flow through the turbine section, thereby maximizing the amount of work extracted.

As hot combustion gases pass through the turbine section, the blades are rotatably driven, turning a shaft which drives the compressor and other auxiliary systems. The higher the gas temperature, the more energy which can be extracted in the turbine section, and the greater the overall efficiency. In order to increase the turbine section operating temperature capability, nickel-base superalloy materials are commonly used to produce the turbine airfoil blades and vanes, since such materials retain mechanical properties at elevated temperatures.

The stator, or stationary vane, assembly normally mounts in the engine case. While rows of rotor blades extend outwardly from the rotor across the gas flow path, in both the turbine and compressor sections, an array of stator vanes extends inwardly from the engine case across the gas flow path at the downstream end of most blade rows. Such vanes are frequently held in place at the engine case by feet, or lugs, which are engaged by flanges or retaining rings extending inwardly from the outer case. Frequently such lugs are damaged, mis-machined during manufacture, or broken during installation or use. Vanes must operate close to their temperature limits, and if they should be subjected to even brief exposure to higher temperatures, the strength and fatigue capability of the material used may be decreased. Turbine vanes may also undergo distortion, stretching, and elongation during service. This condition, otherwise known as metal creep, may reduce service life, and in combination with reduced strength and fatigue capability, result in vane retaining lug failure. The present invention is directed to a method for the repair of such lugs, or for the reconfiguration thereof in the event of a design modification.

In the past, such repair or modification has been difficult due to the nature of the material involved. The vane assembly is frequently made from a cast material, and welding is not a suitable repair technique due to microcracking or strain age cracking. Attempts to build up the broken area by such techniques as plasma spraying additional material onto the broken base of the lug do not typically meet structural creep and fatigue property requirements at elevated temperatures.

Since many cast, hardenable nickel-base materials are not suitable for welding, means were sought to achieve a solid state bond of a replacement blank to the vane assembly. It was found that cast replacement lugs could not effectively be forge joined to a cast vane assembly, due to high joining load requirements and the inability to achieve adequate deformation to avoid continuous interfacial carbide precipitation at the bond interface. Wrought materials, on the other hand, are generally more suitable for forge joining, and a number of techniques are known for the bonding of wrought alloys. An exemplary development is the Gatorizing@ isothermal forging method useful with high temperature alloys, as described in commonly owned U. S. Patent 3,519,503.

In addition, commonly owned U. S. Patent 4,873,751 teaches a fabrication or repair technique for integral bladed rotors. However, prior to the present invention, no technique was available for the successful repair of the retaining lugs or feet of stator vane assemblies.

In both the compressor and turbine sections of modern jet engines, components are operated at or near the outer limits of their capabilities with respect to both temperature and stress. Accordingly, any repair technique with respect to such components must provide repairs which have the strength and temperature capabilities of said components. Other requirements of the components, such as fatigue resistance, resistance to rupture, and life span must also be met.

The need for repair may arise both in service and in the initial fabrication of the engine. Further, subsequent design change or modification may make it desirable to modify or replace a given component to achieve greater efficiencies, greater reliability, or longer life.

In a broad aspect, the present invention provides a method for replacing a vane assembly retaining lug or the like wherein said vane assembly comprises a cast nickel-base or cobalt-base superalloy or a titanium alloy, including the steps of: <ul id="ul0001" list-style="none"><li>a) removing the original retaining lug in such a fashion so as to leave a bonding surface;</li><li>b) providing a replacement wrought alloy lug with a bonding surface;</li><li>c) placing said bonding surfaces adjacent one another;</li><li>d) locally heating said bonding surfaces under vacuum and applying pressure to the interface of said bonding surfaces so as to forge join said replacement lug to said vane assembly to form an integral assembly;</li><li>e) heat treating said integral assembly; and</li><li>f) machining said integral assembly to a desired configuration.</li></ul>

In a preferred embodiment the present invention relates to a method for the repair or replacement of retaining lugs or feet on gas turbine engine vanes, where such vanes are comprised of nickel-base or cobalt-base superalloys or titanium alloys. A wrought material replacement lug blank, preferably but not necessarily of a chemical composition similar to the vane material, is forge joined in place by locally heating the interface surfaces of the replacement lug blank and the vane platform to a temperature within the solution temperature range of both alloys, applying a load sufficient to produce deformation in the wrought material at the interfacial surface, thereby forming a solid state bond between the replacement lug blank and the vane. After joining, the bonded assembly is isothermally vacuum solution heat treated and aged at temperatures compatible with the solution and aging temperatures of the two alloys. Thermal distortion of the vane is not a problem due to the inherently high thermal stability of the cast material of the vane. While the bonded replacement lug may distort, the lug is sized so as to be sufficiently oversized to accept such distortion. After heat treatment, the bonded lug may be machined to the final configuration.

Thus a vane assembly having a mis-machined lug may be repaired by replacement of the lug with a new piece of material, from which the new lug may be machined properly. Thus, in its preferred embodiments at least the present invention permits the replacement of improperly machined lugs with correctly machined lugs. It also permits the replacement of a damaged or broken lug with a lug of similar configuration or, if desired, with a lug of differing dimension or configuration, and the replacement of vane lugs in the event of a design modification or improvement. Although this invention is described in terms of repairing the feet or lugs of vanes of gas turbine engines, the technique is also applicable to repair of attachment means of other superalloy components of like nature.

Some preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings in which: <ul id="ul0002" list-style="none"><li>Figure 1 illustrates a dual airfoil vane of the type for which the present invention is suitable.</li><li>Figure 2 illustrates a vane assembly demonstrating a broken retaining lug.</li><li>Figure 3 illustrates a vane assembly from which a broken or damaged lug has been removed, as by machining.</li><li>Figure 4 illustrates a vane assembly to which a replacement lug has been bonded, prior to machining to final configuration.</li><li>Figure 5 illustrates a vane assembly having a new retaining lug, of different size, bonded thereto.</li><li>Figures 6 and 7 are photomicrographs of a repaired vane microstructure at magnifications of 50X and 500X, with Kallings' etchant.</li><li>Figure 8 is a composite photomicrograph illustrating breakage of a bonded vane and lug replacement under stress.</li></ul>

The present invention provides a method for the repair of broken, damaged or improperly machined attachment lugs on vanes for gas turbine engines, where such lugs are made of high strength, high temperature materials such as nickel-base or cobalt-base superalloys, or titanium alloys, and also provides a method for the replacement of such lugs when appropriate. Cast materials afford economic, performance and producibility advances over use of wrought materials for turbine vane applications. Consequently, most turbine vanes are cast, commonly using the lost wax technique. As is well known, structural welds in many hardenable cast superalloys are not feasible, due to microcrack formation. Surprisingly, we have discovered that a wrought replacement lug may be joined to the cast vane assembly, by a forge joining technique. By use of a wrought replacement piece in a re-designed configuration, superior high temperature properties may be attained in the repaired component, resulting in a vane assembly having a stronger retaining lug than the original.

A cast vane assembly, 2, such as shown in Figure 1, comprises a vane outside diameter platform, 3, vanes 4 and 5, and an annulus inside diameter platform, 6. The outer diameter platform has projecting lugs, 7, adapted to fit circumferential flanges or retaining rings on the engine case, not shown. Frequently, as a result of damage during installation or in use, lugs on such vane assembly, 2, are broken off as in Figure 2, which illustrates a vane assembly having stubs, 8, where retaining lugs were formerly located. These stubs, 8, or damaged areas of the vane assembly, may be machined away, as shown in Figure 3, to leave a vane assembly having a uniform repair site, 9, in the location of the failed lug.

An oversized retaining lug blank, 10, having the requisite dimensions to replace the failed lug, is then positioned over the uniform repair site, in contact therewith. This replacement lug blank is of the same general composition as the cast vane assembly composition, but preferably of a wrought alloy corresponding thereto. The replacement blank is accurately clamped in position in relation to the repair site, and is forge joined thereto by heating the interface surfaces of the blank and the repair site to within the solution temperature range of both alloys, but not exceeding the solution heat treatment temperature of either alloy, and without causing local melting or significant grain growth in the vane assembly and the replacement lug blank. The forge joined vane and replacement lug assembly, as shown in Figure 4, is then subjected to an appropriate isothermal vacuum solution and age heat treatment step to achieve the desired mechanical properties, after which the replacement lug blank may be machined to the desired final configuration, as illustrated in Figure 5, which demonstrates that a lug of a differing geometry and configuration, 11, may be put in place of the failed retaining lug, if so desired. Figures 6 and 7 illustrate the solid state bonding of cast IN-1 00 with a wrought modified IN-100.

Figure 6 is at a magnification of 50X, while Figure 7 is at 500X. Both samples were treated with Kallings' etchant. Figure 8 is a composite photomicrograph, at 20X magnification, of a failed tensile specimen of bonded cast IN-100 and wrought Modified IN-100, after being stressed to the breaking point at 1425°F. It is to be noted that the locus of the break occurred not at the bond line, but at a point within the cast material. Tensile properties for this piece, at 1425°F, were Y.S. 122.3 ksi, U.T.S. 148.9 ksi, and El. 13.3%, and RA 24.1.

As previously indicated, the present invention is specifically useful in the repair of damaged gas turbine engine vane assemblies, which assemblies are normally made of a cast high temperature material, such as either non-hardenable or precipitation-hardenable nickel-base superalloys, or titanium. Representative of the alloys are those nickel-base or cobalt-base alloys designated in the industry as IN-100, Inconel, Hastelloy, Mar-M, Udimet, Waspaloy, Haynes, Stellite, etc.

It has now been found that a wrought replacement lug blank of suitable high temperature alloy may be forge joined to the cast material of the vane assembly. The replacement lug blank is fashioned of a material which has been preconditioned to give it low strength and high ductility, as described in commonly owned U. S. Patent 3,519,503. This reference teaches that a preferred preconditioning method consists of extruding a compacted powder billet through a die to produce a reduction in cross-sectional area of at least 4:1, and preferably at least 6:1, at a temperature below but within about 450°F of the normal recrystallization temperature of the material. Exemplary materials include the wrought formulations of those alloys cited above as suitable for cast vane preparation. The materials suitable for use as the replacement lug blank are easily deformed in the solution temperature range of the vane material to which they are to be joined. The preferred alloy is that having the closest chemistry to the material of the vane assembly. Thus, if the vane assembly comprises cast IN-1 00, the preferred replacement lug blank material is wrought IN-100. An example of a suitable pairing of cast vane material and wrought lug material is the combination of IN-100 (9.5% Cr, 15.0% Co, 0.17% C, 4.75% Ti, 5.5% Al, 3.0% Mo, 0.015% B, 1.0% V, 0.06% Zr, Bal Ni) with a modified IN-100 (12.4% Cr, 18.5% Co, 0.07% C, 4.3% Ti, 5.0% Al, 3.2% Mo, 0.02% B, 0.8% V, 0.06% Zr, Bal Ni). While similar chemistry is desirable, the limiting factors appear to be avoidance of local chemical segregation, control of interfacial carbide formation, and creep and fatigue strength of the wrought material. It is considered within the skill of one familiarwith cast and wrought superalloys, and/or titanium, to determine the feasibility of forge joining any specific cast/wrought material pairing, and to determine a suitable wrought material replacement lug blank for any given cast vane composition.

Preparation for the lug repair includes machining away the broken area of the failed lug, or in the case of a replacement, removing the lug itself, to provide a uniform surface for attachment of the replacement lug blank. Any conventional metal removal technique is suitable, so long as a relatively smooth (20-30 microinches Ra) and uniform surface is achieved. The intended bonding surface of both the vane assembly and the replacement lug blank should then be treated to encourage bonding. This generally entails removal of all foreign materials, including dirt, grease, machining residue, etc., and surface oxidation. Such surface preparation may be accomplished by a variety of readily apparent techniques, such as mechanical surface grinding and cleaning, and selective chemical etching.

The replacement lug blank is sized to provide the desired lug size and configuration after machining, and is configured so as to provide a bonding surface having a close tolerance fit to the bonding surface of the vane assembly. After the replacement lug blank is positioned in close proximity to the bonding surface of the vane assembly, as in a tooling fixture or jig, a forge joining pressure is applied to the blank. While this pressure is applied, the interface between the blank and the vane assembly bonding surface is locally heated to a temperature within about 200°F of, but not exceeding the solution heat temperature of the material of the replacement lug blank, without causing local melting thereof. The tooling for this operation should avoid any scratching or loading of the vane assembly airfoils, and must be supported only against the inside diameter of the outer platform. The lug replacement blank and the vane assembly must be maintained in true alignment during the heat-up and upset cycles, and the lug blank and vane assembly clamping systems should preferably be of copper alloy material to provide for proper water cooling and high electrical current carrying capability. The hydraulic pressure, for upset, and the electrical current, for heating, must be individually controlled to obtain a gradual, predetermined upset of the wrought material. The entire forge joining operation must be conducted in a high vacuum environment of below 4x 10-^s Torr, and preferably below 10-⁵ Torr.

Bonding is conducted under conditions of temperature and pressure sufficient to cause metal flow. These conditions will obviously vary depending upon the exact materials involved. However, for typical superalloys the temperatures will be on the order of from about 1900°F - 2100°F, and the pressures will be on the order of about 20 to about 30 ksi. In the bonding of typical replacement lugs, it would be appropriate to have an upset or shortening of the replacement lug blank on the order of 0.025-0.075 inch, and preferably from 0.025-0.030 inch, during bonding to insure sufficient metal flow, and to minimize any cracking tendency of the replacement lug. Any upset material expelled from the bond area may be subsequently removed during machining to the final configuration.

Example 1

A number of broken vane assemblies from the fourth stage of the turbine of a military jet turbine engine were prepared for repair. The vane material was IN-100 cast nickel-base superalloy, comprising 9.5% Cr, 15.0% Co, 0.17% C, 4.75% Ti, 5.5% Al, 3.0% Mo, 0.015% B, 1.0% V, 0.06% Zr, and the balance Ni. The vane retaining lug of the selected vane assemblies had broken off in service, leaving a stub as shown in Figure 2. This stub was machined away to a smooth repair site having a uniform interface surface, which was then thoroughly cleaned in conventional manner. Wrought replacement lug blanks, machined to mate with the interface surface of each vane assembly, and having a foot section somewhat larger than the lug which had broken away, were positioned in face to face assembly with the vane interface surface. The wrought replacement lug blanks were fashioned of a modified IN-100 alloy comprising 12.4% Cr, 18.5% Co, 0.07% C, 4.3% Ti, 5.0% Al, 3.2% Mo, 0.02% B, 0.8% V, 0.06% Zr, balance Ni. The vane assemblies and replacement lug blanks were placed in a forge joining apparatus, and subjected to a forging pressure of 30 ksi under a vacuum of 4 x 10-⁵ Torr, while being resistance heated to a temperature of from 1910° to 1965°F. An upset of 0.030 inches was achieved before release of the forging pressure. The bonded assemblies were then isothermally heat treated, still under vacuum, at 1975±25°F for 8 hours, cooled at a rate equal to or faster than air cooling to room temperature, and then age heat treated at 1600±15°F for an additional 12 hours. The pieces were then cooled to ambient at a rate equal to orfaster than air cooling. The bonds formed between the vane assemblies and the replacement lugs were inspected and found to be uniform. The replacement lug blanks were then machined to lugs of a larger base dimension than those which they replaced, as illustrated in Figures 1 and 5. The repaired vane assemblies were subjected to a variety of tests and found to have properties comparable or superior to those of the average vane assembly used in the military jet engine from which they were taken. When the repaired vane was subjected to tensile testing, breakage occurred in the original cast IN-100 material, rather than in the bonding zone. Photomicrographs of repaired vane assemblies show (Figures 6 and 7) a very strong bond line, and (Figure 8) breakage under tension in the cast IN-1 00 vane. Figure 8 is a photomicrograph of a longitudinal section of a failed tensile test specimen, tested at 1425°F. Shown are wrought IN-1 00, to the left of the bond line, and cast IN-100, to the right of the bond line. The tensile test failure occurred outside the bond interface zone, in the weaker cast IN-100 material. When a vane assembly has been repaired, as in this example, and is utilized in a jet engine, the enlarged lug successfully results in greater strength, and higher resistance to the working stresses which caused the original retaining lug to fail, due to increased loading surface area.

Example 2

Elevated temperature stress-rupture testing was performed on bonded samples prepared as in Example 1. Two individual bonded samples were exposed to 1550°F/22 ksi and 1425°F/22 ksi, respectively. These temperatures and stresses are representative of the maximum typical military engine operating temperatures and stresses which the vane repair area would be exposed to in service, and are thus a measure of life. The sample test at 1550°F/22 ksi went 93 hours before failure, and the sample tested at 1425°F/22 ksi went 543 hours, before the test was discontinued without specimen failure. These test results indicated suitable life for engine service, with the 1425°F/22 ksi temperature/stress condition being the maximum designed operating conditions. Thus, the bonded repair area is provided with the required stress-rupture capability to meet the high temperature and stress conditions encountered during typical and maximum stress and temperature in-service conditions.

It is to be understood that the above description of a preferred embodiment is subject to considerable modification, change, and adaptation by those skilled in the art, and that such modifications, changes, and adaptations are to be considered to be within the scope of the present invention, which is set forth by the appended claims.