Vane lug repair technique

VANE LUG REPAIR TECHNIQUE ίΰΜϋ tpP Tip’ll lip’330

VANE LUG REPAIR TECHNIQUE

Technical Field

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

Background 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, shaft which drives the compressor and 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 Zi 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 -1- 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 -2- 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, the teachings of which are incorporated herein by reference. 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.

SUMMARY OF THE INVENTION

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 -3- reliability, or longer life.

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. According to the present invention the 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. - 4 -

According to the invention a method is taught for replacement or repair of damaged or broken lugs of component rotor lugs of gas turbine assemblies. It is an object of the invention to permit 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. It is a further object of the present invention to permit 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.

These and other objects and advantages will become more readily understood through reference to the following description of the drawings and preferred embodiments,

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a vane assembly demonstrating a broken retaining lug.

Figure 2 illustrates a vane assembly from which a broken or damaged lug has been removed, as by machining.

Figure 3 illustrates a vane assembly to which a replacement lug has been bonded, prior to machining to final configuration.

Figure 4 illustrates a vane assembly having anew retaining lug of different size, bonded thereto. 5 -

Figure 5 is a composite photomicrograph illustrating breakage of a bonded vane and lug replacement under stress.

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 3, 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 4, 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 3 and 4 illustrate the solid state bonding of cast IN-100 with a wrought modified IN-100, in accordance with the present invention. Figure 3 is at a magnification of 50X, while Figure 4 is at 500X. Both samples were treated with Railings' etchant. Figure 5 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. - 6 -

As already indicated, the parent invention Application No. 99354 is specifically directed to 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 in accordance with this invention are easily deformed in the solution temperature range of the vane material to which they are to be joined. The preferred alloy is 7 - that having the closest chemistry to the material of the vane assembly. Thus, if the vane assembly comprises cast IN-100, 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 familiar with 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 - 8 - 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 below a pressure of 4 x IO"5 Torr and preferably below 10 Torr.

Bonding is conducted under conditions of teirfperature and pressure sufficient to cause metal flow. These conditions will obviously vary - 9 - 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. 10 -

Example

Elevated temperature stress-rupture testing was performed on bonded samples prepared as the Example of the parent application.

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. Tlius, the present invention also provides the bonded repair area 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 the present invention 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. 11