Patent Application
20080263863
The present invention relates to dimensional restorations of gas turbine engine components, such as turbine blades. In particular, the present invention relates to methods of dimensionally restoring knife edge seals disposed on turbine blades.
Gas turbine engines operate by burning a combustible fuel-air mixture in a combustor and converting the energy of combustion into a propulsive force. Combustion gases are directed axially rearward from the combustor through an annular duct, interacting with a plurality of turbine blade stages disposed within the annular duct. This transfers the combustion gas energy to the turbine blades. In a typical turbine section, there are multiple, alternating stages of stationary vanes and rotating blades disposed in the annular duct.
During the course of operation, the combustion gas temperature may reach 2000° F. or more. As such, some turbine blade stages are cooled with lower temperature cooling air for improved durability. Air for cooling the first-stage blades bypasses the combustor and is directed to an inner diameter cavity located between a first-stage vane support and a first-stage rotor assembly. The rotational force of the rotor assembly pumps the cooling air radially outward and into a series of conduits within each blade, thus providing the required cooling.
Since the outboard radius of the inner cavity is adjacent to the annular duct carrying the combustion gasses, the outboard radius is sealed to prevent leakage of the pressurized cooling air into the combustion gas stream. Knife edge seals are typically used to seal annular gaps between adjacent rotating components and non-rotating components in the engine. The knife edge seals typically include one or more seal members (e.g., knife edges) that engage with complimentary lands (e.g., honeycomb surfaces) on the non-rotating portions of the turbine section. This accordingly restricts the flow of air between the knife edge seals and the complimentary lands. However, during the course of operation, the engagements between the knife edge seals and the complimentary lands can erode the seal members of the knife edge seals, thereby reducing the effectiveness of the seals.
To increase the effectiveness of the eroded knife edge seals, the seals need to be replaced or restored. Due to economic factors, it is common practice in the aerospace industry to restore turbine engine components rather than replace them. Such restorations desirably restore damaged regions of the engine components to their original dimensions. As such, there is a need for a dimensional restoration method that is efficient and cost effective, thereby providing dimensionally restored knife edge seals for use in gas turbine engines.
The present invention relates to a turbine blade assembly having a knife edge seal that includes at least one seal member. The seal member(s) includes a stub portion and a restored portion fused to the stub portion, where the restored portion includes a plurality of fused layers of a restoration material, and where the restoration material is melted and deposited in a layer-by-layer manner to form the plurality of fused layers.
As shown, seal member 22 includes stub portion 26 and restored portion 28, and seal member 24 includes stub portion 30 and restored portion 32. Over extended periods of operation, the rotation of turbine blade assembly 10 causes seal members 22 and 24 to abrasively erode against the complementary lands, thereby respectively wearing seal members 22 and 24 down to stub portions 26 and 30. This reduces the effectiveness of knife edge seal 18, which may potentially allow the pressurized cooling air to bleed into the combustion gas stream. However, pursuant to the present invention, restored portions 28 and 32 are each formed in a layer-by-layer manner on stub portions 26 and 30, respectively. This restores seal members 22 and 24 to their original dimensions, thereby restoring the sealing efficiency of knife edge seal 18.
While the following discussion focuses on the dimensional restoration of knife edge seal 18 having a pair of knife edge seal members (i.e., seal members 22 and 24), the present invention is also suitable for use with a variety of knife edge seals secured to turbine blades, where the knife edge seals have at least one seal member configured to engage with a complementary land of a gas turbine engine. Examples of suitable knife edge seals that may be dimensionally restored pursuant to the present invention include below-platform seals secured to turbine blades, such as labyrinth seals (e.g., as shown in
Turbine blade assembly 10 is then pivoted at an angle such that at least stub portion 26 is generally aligned in a vertical direction. The pivoted orientation of turbine blade assembly 10 allows a restoration material used in the dimensional restoration to be deposited downward in a substantially vertical direction onto stub portion 26. For ease of discussion, the orientation of turbine blade assembly 10 is referenced relative to an x-y-z Cartesian coordinate system, where the lateral widths of stub portions 26 and 30 extend along the x-axis, the longitudinal length of base portion 20 extends along the y-axis, and the z-axis represents the vertical direction. Turbine blade assembly 10 may be retained in the pivoted orientation with a brace mechanism (not shown), or alternatively, may be manually held. As discussed below, restored portions 26 and 28 are then each formed in a layer-by-layer manner on stub portions 26 and 30, respectively.
Suitable restoration materials for use with the present invention include weldable metals, such as nickel, nickel-based alloys and superalloys, cobalt, cobalt-based alloys and superalloys, and combinations thereof; and may also include one or more additional materials such as carbon, titanium, chromium, niobium, hafnium, tantalum, molybdenum, tungsten, aluminum, and iron. Examples of particularly suitable restoration materials for use with the present invention include nickel-based single crystal superalloys, such as those commercially available under the trade designation “PWA 1484” alloys.
Examples of suitable laser systems for laser system 34 include solid-state laser systems and gas laser systems (e.g., Nd:YAG and CO2 lasers), which emit laser beams (i.e., laser beam 36) having intensities capable of readily melting the restoration material. Turbine blade assembly 10 is desirably positioned such that stub portion 26 is located below focal point 38 of laser beam 36. The restoration material is then fed in a continuous manner to focal point 38, and is accordingly melted. The restoration material may be supplied to focal point 38 in a variety of media, such as powders, granules, wire stock, and rod stock. Particularly suitable laser systems for use in this embodiment include laser systems that inject, or otherwise coaxially feed the restoration material to the laser beam. An example of a suitable coaxial-feed laser system is disclosed in Baker et al., U.S. Pat. No. 7,030,337. Such laser systems allow successive portions of the restoration material to be fed to the laser beam focal point to melt the restoration material.
Upon melting, the restoration material then deposits onto stub portion 26 to build layer 28a. Laser beam 36 and the restoration material feed are then moved across stub portion 26 along the x-axis to deposit successive portions of the molten restoration material. For example, laser system 34 may be a multiple-axis laser system (e.g. a 5-axis laser system), which provides a wide range of motion to provide accurate depositions. Alternatively, turbine blade assembly 10 may be moved relative to laser beam 36 to guide the deposition process along the x-axis. Suitable deposition rates of the restoration material for forming layer 28a range from about 0.1 grams/minute to about 5 grams/minute, with particularly suitable deposition rates ranging from about 1 gram/minute to about 3 grams/minute. Suitable thicknesses for layer 28a may vary depending on a variety of factors, such as the amount of restoration material, the composition of the restoration material, and the original dimensions of seal member 22. Examples of suitable thicknesses for layer 28a range from about 10 micrometers to about 200 micrometers, with particularly suitable thicknesses for layer 28a ranging from about 25 micrometers to about 100 micrometers.
The use of laser beam 36 to melt the restoration material is beneficial for providing strong welds and fast build times. Laser beam 36 rapidly melts the restoration material and reduces the formation of porosities (e.g., air bubbles) in the molten restoration material, thereby increasing the strength of the resulting weld. Upon being deposited on stub portion 26, the molten restoration material cools and substantially solidifies, thereby forming layer 28a fused to stub portion 26.
In an alternative embodiment, the restoration material is melted with a plasma arc welding system (not shown). In this embodiment, the plasma arc welding system melts the restoration material with the use of an electrode (e.g., a tungsten electrode) that emits an electric arc between the electrode and stub portion 26. The molten restoration material is then deposited and fused to stub portion 26 in the same manner as discussed above. Suitable restoration material media for use with plasma arc welding systems include wire stock and rod stock.
In one embodiment, layers 28a-28e are built by depositing the restoration material in a series of back-and-forth patterns along the x-axis. For example, layer 28a is initially formed by depositing the restoration material in a first direction along the x-axis, and providing a suitable amount of time for the restoration material to cool and fuse to stub portion 26. Layer 28b is then formed by depositing the restoration material in an opposing direction along the x-axis from the first direction used to form layer 28a. This pattern is then repeated for layers 28c-28e. In one embodiment, the back-and-forth movement is accomplished by moving laser system 34 back-and-forth along the x-axis in a vertical raster pattern, thereby reducing the amount of movement required by laser system 34 to build layers 28a-28e. The reduced amount of movement correspondingly reduces the amount of time required to build restored portion 28.
Layer 32a is then built in the same manner as discussed above for layer 28a. Accordingly, layer 32a is built by melting the restoration material with laser beam 36, depositing the molten restoration material onto stub portion 30 (represented with broken line 42), and allowing the molten restoration material to cool and fuse to stub portion 30. Suitable restoration materials, deposition rates, and layer thicknesses are the same as those discussed above for layer 28a.
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.
