Method for repairing titanium alloy components

Abstract

A method for repairing a titanium alloy surface of a turbine component includes the step of cold gas-dynamic spraying a powder material comprising at least one titanium alloy directly on the titanium alloy surface. The method may further include the steps of hot isostatic pressing the cold gas-dynamic sprayed turbine component, and performing a separate heat treating step after the hot isostatic pressing. Thus, the cold gas-dynamic spray process and post-spray processing can be employed to effectively repair degraded areas on compressor turbine components.

Description

TECHNICAL FIELD

The present invention relates to repair and overhaul of turbine engine components. More particularly, the present invention relates to methods for repairing turbine engine components made from titanium alloys.

BACKGROUND

Turbine engines are used as the primary power source for many types of aircrafts. The engines are also auxiliary power sources that drive air compressors, hydraulic pumps, and industrial gas turbine (IGT) power generation. Further, the power from turbine engines is used for stationary power supplies such as backup electrical generators for hospitals and the like.

Most turbine engines generally follow the same basic power generation procedure. Compressed air generated by axial and/or radial compressors is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a rotatable turbine disk. The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use the power created by the rotating turbine disk to draw more air into the engine and the high velocity combustion gas is passed out of the gas turbine aft end to create forward thrust. Other engines use this power to turn one or more propellers, fans, electrical generators, or other devices.

Low and high pressure compressor (LPC/HPC) components such as compressor blades and impellers are primary components in the cold section for any turbine engine, and they must be well maintained. The LPC/HPC components are subjected to stress loadings during turbine engine operation, and also are impacted by foreign objects such as sand, dirt, and other such debris. The LPC/HPC components can degrade over time due to wear, erosion and foreign object damage. Sometimes LPC/HPC components are degraded to a point at which they must be repaired or replaced, which that can result in significant operating expense and time out of service.

There are several traditional methods for repairing damaged turbine engine components, and each method has some limitations in terms of success. One primary reason for the lack of success is that the materials used to make LPC/HPC components do not lend themselves to efficient repair techniques. For example, titanium alloys are commonly used to make fan and compressor blades because the alloys are strong, light weight, and highly corrosion resistant. However, repairing the compressor blade with conventional welding techniques subjects the compressor blade to high temperatures at which the welding areas are oxidation-prone. For this reason, welding conventionally is performed in a well-shielded atmosphere such as an inert gas chamber or a chamber that is under vacuum. Maintaining such a controlled environment is inefficient in terms of both time and expense.

Also, conventional techniques for repairing titanium alloy components which are made of alpha-beta alloys with high beta stabilizers such as Ti-6Al-2Sn-4Zr-6Mo possibly cause the components to crack while in a welding zone and/or a heat-affected zone because the alloy components have limited weldability. Resistance to cracking can be improved by preheating the components before welding and then stress relieving immediately after welding. However, combining preheating and welding is inefficient in terms of both time and expense.

Hence, there is a need for new repair methods for titanium alloy components. There is a particular need for new and more efficient repair methods that improve the reliability and performance of the repaired components.

BRIEF SUMMARY

The present invention provides a method for repairing a titanium alloy surface of a turbine component. The method comprises the step of cold gas-dynamic spraying a powder material comprising at least one titanium alloy directly on the titanium alloy surface.

In one embodiment, and by way of example only, the method further comprises hot isostatic pressing the cold gas-dynamic sprayed turbine component, and a separate heat treating step is performed after the hot isostatic pressing

Other independent features and advantages of the preferred methods will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an exemplary cold gas-dynamic spray apparatus in accordance with an exemplary embodiment;

FIG. 2 is a perspective view of an exemplary compressor turbine blade in accordance with an exemplary embodiment; and

FIG. 3 is a flow diagram of a repair method in accordance with an exemplary embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.

The present invention provides an improved method for repairing LPC/HPC components. The method utilizes a cold gas-dynamic spray technique to apply high-strength titanium alloy materials to worn LPC/HPC component surfaces. These materials can be used to repair components such as compressor and fan blades and vanes, including impeller and blisk blades, which have been degraded due to erosion and foreign object damage, to name several examples.

Turning now to FIG. 1, an exemplary cold gas-dynamic spray system 100 is illustrated diagrammatically. The system 100 is illustrated as a general scheme, and additional features and components can be implemented into the system 100 as necessary. The main components of the cold-gas-dynamic spray system 100 includes a powder feeder for providing repair powder materials, a carrier gas supply (typically including a heater), a mixing chamber and a convergent-divergent nozzle. In general, the system 100 mixes the repair particles with a suitable pressurized gas in the mixing chamber. The particles are accelerated through the specially designed nozzle and directed toward a target surface on the turbine component. When the particles strike the target surface, converted kinetic energy causes plastic deformation of the particles, which in turn causes the particle to form a bond with the target surface. Thus, the cold gas-dynamic spray system 100 can bond repair powder materials to an LPC/HPC component surface and thereby restore degraded LPC/HPC component geometry and dimensions.

The cold gas dynamic spray process is referred to as a “cold gas” process because the particles are mixed and applied at a temperature that is far below the melting point of the particles. The kinetic energy of the particles on impact with the target surface, rather than particle temperature, causes the particles to plastically deform and bond with the target surface. Therefore, bonding to the LPC/HPC component surface takes place as a solid state process with insufficient thermal energy to transition the solid powders to molten droplets.

According to the present invention, the cold gas-dynamic spray system 100 applies high-strength titanium alloy materials that are difficult to weld or otherwise apply to LPC/HPC component surfaces and other titanium alloy substrates. For example, titanium alloy welding processes are conventionally performed in a well-shielded atmosphere such as an inert gas chamber or a chamber that is under vacuum. Maintaining such a controlled environment is inefficient in terms of both time and expense. In contrast, the cold gas-dynamic spray system 100 can be operated at ambient temperature and pressure environment.

The cold gas-dynamic spray system 100 is also useful to spray a wide variety of titanium alloys. Near alpha titanium alloys, alpha-plus-beta titanium alloys, and near beta titanium alloys are classes that the system 100 can cold spray. Examples of the type of titanium alloys that can be cold sprayed using the system 100 include Ti-6Al-4V, Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1Mo-1V, Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si, as well as specially formulated and tailored alloys. In an exemplary embodiment of the invention, the cold sprayed titanium alloy is selected to be the same material that forms the LPC/HPC component to be repaired, although it is clearly within the scope of the present invention to select a titanium alloy that is different from the LPC/HPC component material.

As previously mentioned, the cold gas-dynamic spray process can be used to repair a variety of different turbine engine components. For example, the turbine blades in the high pressure stages of a turbine engine are particularly susceptible to wear, erosion and other degradation. Turning now to FIG. 2, a compressor blade 150 that is exemplary of the types that are used in turbine engines is illustrated, although compressor blades commonly have different shapes, dimensions and sizes depending on gas turbine engine models and applications. The blade 150 includes several components that are particularly susceptible to wear, erosion and foreign object damage, and the process of the present invention can be tailored to repair different blade components. Among such blade components is an airfoil 152, which is a smooth, curved structure. The airfoil 152 includes one concave face and one convex face. In operation, air is drawn into the compressor where multiple stages of compressor airfoils act to compress the air in preparation for combustion with some type of fuel. The airfoil 152 includes a leading edge 162 and a trailing edge 164 that encounter air streaming around the airfoil 152. The compressor blade 150 also includes a tip 160. In some applications the tip may include features commonly known as squealers. The compressor blade 150 is mounted on a non-illustrated compressor hub or rotor disk by way of a dovetail 154 that extends downwardly from the airfoil 152 and engages with a slot on the compressor hub. A platform 156 extends longitudinally outwardly from the area where the airfoil 152 is joined to the dovetail 154. Common features on some compressor and fan blades are midspan dampers or snubbers 158, which are typically centrally located on each side of the airfoil 152. The dampers or snubbers 158 extend outwardly to engage with mating features of adjacent compressor or fan blades within the rotor. This engagement makes the dampers or snubbers 158 common wear features that can be repaired according to the method of the present invention. Other compressor configurations include blisks or integrally bladed rotors (IBRs) and impellers or centrifugal compressors, which have blades that are integral to the rotor hub.

As mentioned previously, the process of the present invention can be tailored to fit the blade's specific needs, which depend in part on the blade component where degradation has occurred. For example, the airfoil tip 160 is particularly subject to degradation due to rubbing and other contact with the static shroud, in addition to foreign particle impacts, and the cold gas dynamic spray process of the present invention is used to apply materials to the blade tip 160 by filling any material defects with titanium alloy material. Following the cold spraying process, the tip 160 is machined to restore the tip 160 to the original design dimensions.

As another example, degradation on the leading edge 162 and trailing edge 164 of the airfoil 152 can be repaired using the cold gas-dynamic spray process. The leading edge 162 and trailing edge 164 are both subject to degradation, again typically due to tip rubs and foreign particle impacts. In this application, the cold gas dynamic spray process is used to apply materials that return the edges of the compressor blade back to the required dimensions. Again, this can be done by filling the worn surface and other defects with cold gas-dynamic sprayed repair material followed by dimensional restoration and post-spray processing.

As another example, degradation on the platform 156 can be repaired using the cold gas-dynamic spray process. In some applications, wear on the platform 156 occurs at the contact surfaces 166 between adjacent compressor blades as well as the dovetail contact surface 154. At those locations, the friction can cause fretting and other wear. The cold gas-dynamic spray process can be used to fill the worn surface, cracks and other defects on the platform and dovetail to restore the desired dimensions.

Again, the above repair processes are just examples of how a typical titanium alloy compressor blade can be repaired by cold gas-dynamic spraying according to the present invention. It is also emphasized again that compressor blades are just one example of the type of titanium alloy components that can be repaired using a cold gas-dynamic spray process. For example, many gas turbine engines include a shroud structure that surrounds a row of compressor blades at the outer radial end of the blades. The shroud, like the blade tips, can be subject erosion and repaired using the cold gas-dynamic spray process. Other turbine engine components that can be repaired in such a manner include compressor stator vanes, vane support structures, rotor nozzles and other LPC/HPC components.

A variety of different systems and implementations can be used to perform the cold gas-dynamic spraying process. For example, U.S. Pat. No 5,302,414, entitled “Gas-Dynamic Spraying Method for Applying a Coating” and incorporated herein by reference, describes an apparatus designed to accelerate materials having a particle size of between 5 to about 50 microns, and to mix the particles with a process gas to provide the particles with a density of mass flow between 0.05 and 17 g/s-cm². Supersonic velocity is imparted to the gas flow, with the jet formed at high density and low temperature using a predetermined profile. The resulting gas and powder mixture is introduced into the supersonic jet to impart sufficient acceleration to ensure a particle velocity ranging between 300 and 1200 m/s. In this method, the particles are applied and deposited in the solid state, i.e., at a temperature which is considerably lower than the melting point of the powder material. The resulting coating is formed by the impact and kinetic energy of the particles which gets converted to high-speed plastic deformation, causing the particles to bond to the surface. The system typically uses gas pressures of between 5 and 20 atm, and at a temperature of up to 750° F. As non limiting examples, the gases can comprise air, nitrogen, helium and mixtures thereof. Again, this system is but one example of the type of system that can be adapted to cold spray powder materials to the target surface.

Turning now to FIG. 3, an exemplary method 200 for repairing turbine components is illustrated. This method includes the cold gas-dynamic spray process described above, and also includes additional optional processes to optimize the resulting repairs. As described above, cold gas-dynamic spray involves “solid state” processes to effect bonding and coating build-up, and does not rely on the application of external thermal energy for bonding to occur. However, thermal energy may be provided after bonding has occurred since thermal energy promotes formation of the desired microstructure and phase distribution for the repaired components. Also, additional processing may be necessary to optimize bonding within the material and many thermo-mechanical properties for the material such as the elastic/plastic properties, mechanical properties, thermal conductivity and thermal expansion properties. In the method 200, additional optional processing includes vacuum sintering, hot isostatic pressing and additional thermal treatments to consolidate and homogenize the cold gas-dynamic spray applied material and to restore metallurgical integrity to the repaired turbine component.

The first step 202 comprises preparing the repair surface on the turbine component. For example, the step of preparing a compressor blade can involve pre-machining, degreasing and grit blasting the surface that needs to be repaired to remove any oxidation and dirty materials.

The next step 204 comprises performing a cold gas-dynamic spray of repair materials on the turbine component. As described above, in cold gas-dynamic spraying, particles at a temperature well below their melting temperature are accelerated and directed to a target surface on the turbine component. When the particles strike the target surface, the kinetic energy of the particles is converted into plastic deformation of the particle, causing the particle to form a strong bond with the target surface. The spraying step can include the application of repair material to a variety of different components in the turbine engine. For example, material can be applied to worn surfaces on compressor blades, impellers, and vanes in general, and to blade tips, knife seals, leading/trailing edges, and platforms. In all these cases, the spraying step 204 generally returns the component to its desired dimensions.

With the repair materials deposited directly on the turbine component surfaces, the next step 206 comprises performing a vacuum sintering. In vacuum sintering, the repaired turbine component is diffusion heat treated at a desired temperature in a vacuum for a period of time. The vacuum sintering enables metallurgical bonding to occur across splat interfaces through elemental diffusion. The vacuum sintering can also remove inter-particle micro-porosity, homogenize and consolidate the cold-sprayed buildup via an atom diffusion mechanism. The thermal process parameters for the vacuum sintering depend on the titanium alloy that forms the turbine component.

The next step 208 comprises performing a hot isostatic pressing on the repaired turbine component. The hot isostatic pressing (HIP) is a high temperature, high-pressure process. The HIP process can be performed at a desired temperature that is sufficient to fully consolidate the cold-sprayed buildup and eliminate defects such as porosity. Additionally, the HIP process strengthens bonding between the repair material buildup and the underlying component, homogenizes the applied materials, and rejuvenates microstructures in the base material. Overall mechanical properties such as tensile and stress rupture strengths of repaired gas turbine components can thus be dramatically improved with the HIP process.

As one example of HIP parameters, pressing can be performed for 2 to 4 hours at temperatures of between about 1650 and about 1750° F. and at pressures of about 10 to about 15 ksi for most titanium alloys, although the procedure is carried out at up to about 30 ksi for some high-temperature titanium alloys. Of course, this is just one example of the type of hot isostatic pressing process that can be used to remove defects after the application of repair materials.

In some embodiments, it may be desirable to perform a rapid cool following the HIP process to reduce the high-temperature solution heat treatment aftermath that could otherwise exist. One advantage of the rapid cool capability is that the component alloy and the repair material are retained in “solution treated condition,” reducing the need for another solution treatment operation. In other words, the HIP followed by rapid cool can provide a combination of densification, homogenization and solution treat operation. Using this technique can thus eliminate the need for other heat treatment operations.

The next step 210 comprises performing a heat treatment on the repaired component. The heat treatment can provide a full restoration of the mechanical properties of turbine components. It should be noted that in some applications it may be desirable to delete the high temperature solution treatment if such operation can be accomplished in steps 204 and/or 206. However, some examples of heat treatments are described below for applications in which such a treatment is desired or necessary.

A two-stage heat treatment is applied in a first example, which is useful for repairing the Ti-6Al-4V alloy, among others. According to this example, a compressor blade or other component is heated for about one hour at a temperature between about 1725 and about 1775° F. After cooling the component with water, the component is heated between about two and about eight hours at a temperature between about 900 and about 1100° F.

Another two-stage heat treatment is applied in a second example, which is useful for repairing the Ti-6Al-2Sn-4Zr-6Mo alloy, among others. According to this second example, a compressor blade or other component is heated for about one hour at a temperature between about 1550 and about 1650° F. The component is air cooled, and then heated between about four and about eight hours at a temperature between about 1075 and about 1125° F.

Yet another two-stage heat treatment is applied in a third example, which is useful for repairing the Ti-8Al-1Mo-1V alloy, among others. According to this third example, a component is heated for about one hour at a temperature between about 1800 and about 1850° F. The component is then cooled with water or oil. The component is then heated between about four and about eight hours at a temperature between about 1050 and about 1100° F.

The present invention thus provides an improved method for repairing turbine engine components. The method utilizes a cold gas-dynamic spray technique to repair degradation in fan blades, compressor blades, impellers, blisks, and other turbine engine components. These methods can be used to repair a variety of defects thus can improve the overall durability, reliability and performance of the turbine engine themselves.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A method for repairing a titanium alloy surface of a turbine component, the method comprising the step of:

cold gas-dynamic spraying a powder material comprising at least one titanium alloy directly on the titanium alloy surface.

2. The method of claim 1, wherein the powder material consists of at least one titanium alloy.

3. The method of claim 1, wherein the powder material comprises at least one titanium alloy selected from the group consisting of near alpha titanium alloys, alpha-plus-beta titanium alloys, and near-beta titanium alloys.

4. The method of claim 1, wherein the powder material comprises an alloy that is the same alloy that forms the titanium alloy surface.

5. The method of claim 1, wherein the cold gas-dynamic spraying is performed in an atmosphere comprising an inert gas.

6. The method of claim 5, wherein the inert gas comprises helium.

7. The method of claim 1, further comprising the step of:

heating the turbine component at a temperature sufficiently high to consolidate the sprayed powder material.

8. The method of claim 1, further comprising the step of:

performing a vacuum sintering on the turbine component after the cold gas-dynamic spraying step.

9. The method of claim 1, further comprising the step of:

hot isostatic pressing the turbine component after the cold gas-dynamic spraying step.

10. The method of claim 9, wherein the hot isostatic pressing step is performed 2 to 4 hours at temperatures of between about 1650 and about 1750° F. and at a pressure of at least 10 ksi.

11. The method of claim 1, further comprising the step of:

heat treating the turbine component after the cold gas-dynamic spraying step, the heat treating comprising a first heating step performed for about one hour at a temperature between about 1725 and about 1775° F., followed by a second heating step performed for between about two and about eight hours at a temperature between about 900 and about 1100° F.

12. The method of claim 11, wherein the titanium alloy surface being repaired comprises Ti-6Al-4V.

13. The method of claim 1, further comprising the step of:

heat treating the turbine component after the cold gas-dynamic spraying step, the heat treating comprising a first heating step performed for about one hour at a temperature between about 1550 and about 1650° F., followed by a second heating step performed for between about four and about eight hours at a temperature between about 1075 and about 1125° F.

14. The method of claim 13, wherein the titanium alloy surface being repaired comprises Ti-6Al-2Sn-4Zr-6Mo.

15. The method of claim 1, further comprising the step of:

heat treating the turbine component after the cold gas-dynamic spraying step, the heat treating comprising a first heating step performed for about one hour at a temperature between about 1800 and about 1850° F., followed by a second heating step performed for between about four and about eight hours at a temperature between about 1050 and about 1100° F.

16. The method of claim 15, wherein the titanium alloy surface being repaired comprises Ti-8Al-1Mo-1V.

17. The method of claim 1, wherein turbine component comprises a compressor blade.

18. The method of claim 17, wherein the compressor blade comprises a tip, and wherein the cold gas-dynamic spraying is performed on the tip.

19. The method of claim 17, wherein the compressor blade comprises a leading edge, and wherein the cold gas-dynamic spraying is performed on the leading edge.

20. The method of claim 17, wherein the compressor blade comprises a platform, and wherein the cold gas-dynamic spraying is performed on the platform.

21. A method for repairing a titanium alloy surface of a turbine component, the method comprising the steps of:

cold gas-dynamic spraying a powder material comprising at least one titanium alloy directly on the titanium alloy surface;

hot isostatic pressing the cold gas-dynamic sprayed turbine component; and

heat treating the turbine component after the hot isostatic pressing.

22. The method of claim 21, wherein the powder material consists of at least one titanium alloy.

23. The method of claim 21, wherein the powder material comprises at least one titanium alloy selected from the group consisting of near alpha titanium alloys, alpha-plus-beta titanium alloys, and near-beta titanium alloys.

24. The method of claim 21, wherein the powder material comprises an alloy that is the same alloy that forms the titanium alloy surface.

25. The method of claim 21, wherein the cold gas-dynamic spraying is performed in an atmosphere comprising an inert gas.

26. The method of claim 25, wherein the inert gas comprises helium.

27. The method of claim 21, further comprising the step of:

before the hot isostatic pressing step, heating the turbine component at a temperature sufficiently high to consolidate the sprayed powder material.

28. The method of claim 21, wherein the heat treating step comprises a first heating step performed for about one hour at a temperature between about 1725 and about 1775° F., followed by a second heating step performed for between about two and about eight hours at a temperature between about 900 and about 1100° F.

29. The method of claim 28, wherein the titanium alloy surface being repaired comprises Ti-6Al-4V.

30. The method of claim 21, wherein the heat treating step comprises a first heating step performed for about one hour at a temperature between about 1550 and about 1650° F., followed by a second heating step performed for between about four and about eight hours at a temperature between about 1075 and about 1125° F.

31. The method of claim 30, wherein the titanium alloy surface being repaired comprises Ti-6Al-2Sn-4Zr-6Mo.

32. The method of claim 21, wherein the heat treating step comprises a first heating step performed for about one hour at a temperature between about 1800 and about 1850° F., followed by a second heating step performed for between about four and about eight hours at a temperature between about 1050 and about 1100° F.

33. The method of claim 32, wherein the titanium alloy surface being repaired comprises Ti-8Al-1Mo-1V.