Patent Application
20090175727
The present invention relates to dimensional restorations of aerospace components. In particular, the present invention relates to methods of dimensionally restoring stationary shroud segments used in gas turbine engines.
The turbine stage of a gas turbine engine typically include assemblies of stator vanes and rotatable blades, where each rotatable blade assembly includes multiple airfoil blades disposed radially within a stationary shroud. The stationary shroud is made up of an array of arcuate shroud segments that are circumferentially spaced to form the annular structure. Each shroud segment correspondingly includes a flowpath, which is the radially inner portion of the shroud segment that faces the tips of the airfoil blades. To increase operational efficiencies in a gas turbine engine, it is desirable to minimize the gaps between the tips of the airfoil blades and the flowpaths of the stationary shroud segments. The minimized gaps reduce the amount of combustion gases that flow radially outward of the airfoil blades, thereby directing the combustion gases to flow between the airfoil blades. However, the minimized gaps also subject the flowpaths of the stationary shroud segments to potential wear damage from contact with the rotating airfoil blades. Over extended periods of operation, this wear damage may increase the gaps between the tips of the airfoil blades and the flowpaths, thereby allowing greater amounts of the combustion gases to flow radially outward of the airfoil blades. This gap increase correspondingly reduces operational efficiencies of the gas turbine engine.
Due to economic factors, it is common practice in the aerospace industry to restore stationary shroud segments rather than replace them. However, standard restoration techniques, such as low-pressure plasma spray and thermally-densified coating techniques, typically require minimum substrate thicknesses (e.g., at least about 1.5 millimeters) for the flowpaths to maintain the structural integrity of the shroud segments, and to reduce distortions in the shroud segments. As a result, stationary shroud segments having flowpath substrate thicknesses less than the minimum required thicknesses are unsuitable for such dimensional restoration techniques.
The present invention relates to a method for dimensionally restoring a stationary shroud segment having a damaged flowpath. The method includes removing at least a portion of the damaged flowpath to provide an exposed surface, casting a replacement flowpath, positioning the replacement flowpath on the exposed surface, and diffusion bonding the replacement flowpath to the stationary shroud segment.
After the stationary shroud segment is uninstalled, the damaged flowpath material is removed to provide an exposed surface of the stationary shroud segment (step 14). Removal of the damaged flowpath material may result in a substrate thickness that is less than a minimum thickness required to restore the flowpath with standard restoration techniques (e.g., less than about 1.5 millimeters). This is particularly true if the amount of abrasive damage to the flowpath is substantial. However, as discussed below, method 10 is suitable for restoring shroud segment flowpaths regardless of the thicknesses of the substrates after the damaged flowpath material is removed. This increases the numbers of stationary shroud segments that may undergo dimensional restorations rather than requiring replacements.
After the damaged flowpath material is removed, the resulting exposed surface is then cleaned to remove any residual material and oxide layers that may have formed (step 16). A replacement flowpath is then cast to match the dimensions of the damaged flowpath material that was removed (step 18). In alternative embodiments, the replacement flowpath may be cast before the stationary shroud segment is uninstalled (pursuant to step 12), before the damaged flowpath material is removed (pursuant to step 14), and/or before the exposed surface is cleaned (pursuant to step 16). The replacement flowpath is then positioned on the exposed surface (step 20), and is diffusion bonded to the exposed surface (step 22). As discussed below, the diffusion bond process causes the alloy of the replacement flowpath to interdiffuse with the alloy of the shroud substrate. The resulting stationary shroud segment may then undergo one or more post-restoration processing before being reinstalled in the gas turbine engine. For example, the stationary shroud segment may be machined to finish the exterior surfaces, and to form cooling holes through the restored flowpath. Additionally, the stationary shroud segment may also be coated for thermal, corrosion, and abrasion resistance.
As discussed below, method 10 is suitable for dimensionally restoring stationary shroud segments having damaged flowpaths regardless of the resulting substrate thicknesses obtained after the damaged flowpath material is removed. Furthermore, the use of cast replacement flowpaths allows the replacement flowpaths to be formed from the same or similar materials used in the original shroud segment flowpaths (e.g., abradable single crystal alloys). This allows the replacement flowpaths to exhibit similar material properties to the original shroud segment flowpaths.
Flowpath 28 is disposed on substrate 26, and is the portion of shroud segment 24 that faces the rotatable airfoil blades when shroud segment 24 is mounted in the gas turbine engine. Flowpath 28 is desirably formed from one or more abradable single crystal alloys (e.g., nickel and cobalt-based alloys and superalloys). This allows flowpath 28 to withstand the extreme temperatures and pressures that occur during operation of a gas turbine engine, particularly in the HPT stage of a gas turbine engine. Additionally, the abradable nature of the alloy(s) of flowpath 28 allow flowpath 28 to function as a sacrificial layer that is slowly eroded over extended periods of operation. This correspondingly reduces the risk of damaging the tips of the rotatable airfoil blades during operation of the gas turbine engine. In alternative embodiments, flowpath 28 may be formed from one or more non-single crystal alloys, such as cast equiax alloys, cast directionally solidified alloys, and wrought alloys.
Shroud segment 24 is desirably mounted in the gas turbine engine in a manner that reduces the gap between flowpath 28 and the tips of the rotatable airfoil blades. This reduces the combustion gas flow between flowpath 28 and the tips of the rotatable airfoil blades, thereby directing the gas to flow between the rotatable airfoil blades. However, the reduced gap also subjects flowpath 28 to abrasive conditions from contact with the tips of the rotatable airfoil blades. After a given period of operation, the wear damage to flowpath 28 requires a restoration process to restore proper sealing between flowpath 28 and the tips of the rotatable airfoil blades. As a result, pursuant to method 10, shroud segment 24 is initially uninstalled from the gas turbine engine, and is furnished to a proper location to perform the restoration process (step 12).
As shown in
The removal of the damaged flowpath material correspondingly reduces the thickness of shroud segment 24 along a radial axis (referred to as radial axis 36). As discussed above, one or more portions of shroud segment 24 may have thicknesses along radial axis 36 that are below a minimum required thickness to restore shroud segment 24 with standard restoration techniques (e.g., low-pressure plasma spray and thermally-densified coating techniques). For example, as shown in
After the damaged material is removed, exposed surface 34 is then cleaned to remove any residual damaged material, and to remove any oxide layers disposed on the flowpath portion (step 16). The oxide layers may be removed by cleaning exposed surface 34 with scale-removing treatment chemicals, such as hydrogen fluoride gas. The cleaning process is desirably continued for a sufficient duration to substantially remove any oxide layer from exposed surface 34.
In one embodiment, replacement flowpath 40 is cast based on specified dimensions, and flowpath 28 is machined down to substantially match the specified dimensions of replacement flowpath 40. This embodiment allows multiple replacement flowpaths (e.g., replacement flowpath 40) to be prefabricated from a single casting mold for restoring multiple stationary shroud segments (e.g., shroud segment 24). This is beneficial for reducing the time required to dimensionally restore multiple stationary shroud segments regardless of the amount of wear damage each of the multiple stationary shroud segments incurs during operation. This reduction in the restoration time correspondingly reduces the amount time that the gas turbine engine is down for repair.
After the casting process, replacement flowpath 40 is then positioned on exposed surface 34 (step 20), and is diffusion bonded to substrate 26 (step 22). The diffusion bonding process involves subjecting substrate 26 and replacement flowpath 40 to one or more heating cycles (e.g., melt and diffusion cycles), thereby causing the alloy of replacement flowpath 40 to interdiffuse with the alloy of substrate 26. The heating cycle(s) are desirably performed under reduced pressure, such as in a vacuum furnace. Examples of suitable reduced pressures include about 1 Torr or less, with particularly suitable pressures including about 0.001 Torr or less. Alternatively, the heating cycle(s) may be performed under partial pressure in an inert-gas atmosphere (e.g., argon gas).
In one embodiment, the heating cycle(s) involve an initial melt cycle to melt the alloys at the interface between substrate 26 and replacement flowpath 40, followed by one or more diffusion cycles. Examples of suitable temperatures for the melt cycle include at least about 1170° C. (about 2140° F.), with particularly suitable temperatures ranging from about 1200° C. (about 2200° F.) to about 1260° C. (about 2300° F.). Examples of suitable durations for the melt cycle range from about fifteen minutes to about two hours, with particularly suitable durations for the melt cycle ranging from about thirty minutes to about one hour. Examples of suitable temperatures for the one or more diffusion cycles range from about 1090° C. (about 2000° F.) to about 1190° C. (about 2165° F.), with particularly suitable temperatures ranging from about 1120° C. (about 2050° F.) to about 1160° C. (about 2115° F.). Examples of suitable durations for the diffusion cycle(s) range from about one hour to about ten hours, with particularly suitable durations for the diffusion cycle(s) ranging from about two hours to about five hours.
The diffusion bonding process may alternatively be performed pursuant to the method disclosed in Minor et al., U.S. patent application Ser. No. 11/657,788, entitled “Diffusion Braze Repair Of Single Crystal Alloys”. This embodiment is beneficial for reducing the coarsening of the gamma prime (γ′) phases of the single crystal alloys of substrate 26 and replacement flowpath 40, which is measurable by increases in the average particle sizes of the γ′ phases. This correspondingly preserves the low-temperature (e.g., 815° C.-870° C./1500° F.-1600° F.) creep resistances of the single crystal alloys of substrate 26 and replacement flowpath 40.
As shown in
After the diffusion bonding process, shroud segment 24 may undergo one or more post-restoration processes. For example, cooling holes 42 may be formed through replacement flowpath 40 to provide coolant air during operation in the gas turbine engine. Restored shroud segment 24 may then be assembled with the additional shroud segments (not shown) to form an annular stationary shroud (not shown) for installation in the gas turbine engine.
As shown in
Replacement flowpath 140, however, may be used to restore shroud segment 124. Replacement flowpath 140 is cast from one or more alloys (e.g., abradable single crystal alloys, cast equiax alloys, cast directionally solidified alloys, wrought alloys, and combinations thereof) (step 18), and is then positioned on exposed surface 134 (step 20). As such, replacement flowpath 140 covers cavity region 144. Replacement flowpath 140 is then diffusion bonded to exposed surface 134 (step 22), which causes the alloy(s) of replacement flowpath 140 to interdiffuse with the alloy(s) of substrate 126. This seals off cavity region 144, and dimensionally restores shroud segment 124.
Accordingly, the restoration of shroud segment 124 shown in
As shown in
In comparison to exposed surface 134 (shown in
As discussed above, method 10 (shown in
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
