Method of sealing disk slots for turbine bucket dovetails

Dovetail seals are quickly and inexpensively applied to turbine buckets or rotor disks by ultrasonically welding a piece of material onto the dovetail portion of the bucket or the disk slot of the rotor disk. The method includes placing the piece of material into contact with the appropriate turbine structure and applying a compressive force so as to press the piece of material against the turbine structure. Ultrasonic energy is then applied to the piece of material so as to ultrasonically weld it to the turbine structure.

BACKGROUND OF THE INVENTION
 This invention relates generally to blades or buckets used in gas turbine
 engines and more particularly to applying dovetail seals to turbine
 buckets.
 A gas turbine engine includes a compressor that provides pressurized air to
 a combustion section where the pressurized air is mixed with fuel and
 ignited for generating hot combustion gases. These gases flow downstream
 to one or more turbine stages that extract energy therefrom to drive the
 compressor and provide useful work such as generating electricity or
 powering an aircraft in flight. Each turbine stage includes a plurality of
 circumferentially spaced apart blades or buckets extending radially
 outwardly from a rotor disk that rotates about the centerline axis of the
 engine. Each bucket is mounted on the rotor disk through the engagement of
 a dovetail portion in a corresponding disk slot. An airfoil portion
 extends radially outward into the hot combustion gas flow.
 Because they are exposed to high temperature combustion gases, the buckets
 are ordinarily cooled to keep their temperatures within certain design
 limits. One common approach to cooling buckets is to pass a suitable
 coolant through an internal cooling circuit in the bucket. The coolant
 normally enters the internal cooling circuit through one or more inlets in
 the bottom of the bucket dovetail and exits through airfoil tip holes
 and/or cooling holes formed in the airfoil surface. Known cooling circuits
 often include a plurality of radially oriented passages that are
 series-connected to produce a serpentine path, thereby increasing cooling
 effectiveness by extending the length of the coolant flow path.
 Since the dovetail inlets are in fluid communication with the disk slot in
 which each dovetail is located, the coolant is delivered to the inlets via
 the respective disk slots. However, leakage of coolant from the disk slots
 will result in reduced coolant flow to the bucket and a corresponding
 reduction in the service life of the bucket. Thus, it is desirable to seal
 leakage paths between each dovetail and the slot in which it is mounted.
 One approach to such sealing is to apply metal stripes to specified areas
 of the dovetail. When the bucket is mounted to the rotor disk by driving
 the dovetail into the slot, excess stripe material is sheared off, leaving
 a patch of material adhered to the dovetail and filling the corresponding
 gap between the dovetail and the slot. Accordingly, the corresponding
 portion of the slot is sealed.
 Presently, the stripe material is ordinarily applied to the dovetail using
 thermal spraying techniques. This method requires extensive masking and is
 very time-consuming and expensive. Accordingly, it would be desirable to
 be able to apply dovetail seals quickly and inexpensively.
 SUMMARY OF THE INVENTION
 The above-mentioned need is met by the present invention, which provides a
 method of sealing the disk slot of a turbine rotor disk by ultrasonically
 welding a piece of material onto the dovetail portion of a turbine bucket
 or the disk slot of the rotor disk.
 The present invention and its advantages over the prior art will become
 apparent upon reading the following detailed description and the appended
 claims with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 Referring to the drawings wherein identical reference numerals denote the
 same elements throughout the various views, FIGS. 1 and 2 show an
 exemplary turbine bucket 10, which is one of a plurality of such buckets
 mounted to a turbine rotor disk 12 that rotates about the centerline axis
 of a gas turbine engine. The bucket 10 includes a dovetail portion 14 for
 mounting the bucket 10 in a corresponding disk slot 16 formed in the rotor
 disk 12. Specifically, the dovetail portion 14 includes one or more lobes
 18 that engage one or more complementary lobes 20 on the disk slot 16. The
 dovetail portion 14 and the disk slot 16 are shown to have the so-called
 fir tree shape although other suitable configurations may be utilized. The
 bucket 10 is axially loaded into the disk slot 16 and radially retained
 therein due to the complementary interlocking configurations of the
 dovetail lobes 18 and the slot lobes 20. The bucket 10 is preferably
 formed as a one-piece casting of a suitable alloy, such as a nickel-based
 superalloy, which has acceptable strength at the elevated temperatures of
 operation in the gas turbine engine.
 The bucket 10 includes an airfoil portion (not shown) that extends radially
 outward from the dovetail portion 14. As is known in the art, the airfoil
 portion has an internal cooling circuit through which a suitable coolant
 is passed to keep the bucket temperature within design limits. The coolant
 enters the internal cooling circuit through one or more inlets 22 (FIG. 2)
 formed in the bottom of the dovetail portion 14 and located in fluid
 communication with a passage 24 (FIG. 1) defined by the bottom of the disk
 slot 16. During operation of the gas turbine engine, coolant is delivered
 to the passage 24 in a conventional manner from a source that may include,
 but is not limited to, the engine's compressor. Coolant flows from the
 passage 24 into the internal cooling circuit of the bucket 10 through the
 inlets 22.
 In one preferred embodiment, the disk slot 16 is sealed by ultrasonically
 welding one or more seals 26 to either one of the bucket 10 or the rotor
 disk 12 at a location that is appropriate to prevent undesirable leakage
 of coolant from the passage 24. That is, the seals 26 can be applied to an
 appropriate location on the dovetail portion 14 or, alternatively, to an
 appropriate location in the disk slot 16. As shown in FIGS. 1 and 2, the
 seals 26 comprise pieces of material strategically placed on the dovetail
 lobes 18, at one end thereof, so as to fill corresponding gaps between the
 dovetail lobes 18 and the slot lobes 20. Thus, the seals 26 prevent
 coolant leakage from the corresponding end of the disk slot 16. It should
 be noted however that this is simply one exemplary seal arrangement used
 to illustrate the inventive concept. Other seal placements are possible
 depending on bucket design and the cooling configuration. The seals 26 are
 preferably made of a suitable metal material such as aluminum.
 Referring now to FIG. 3, an ultrasonic welding apparatus 28 used for
 welding the seals 26 onto a workpiece W is shown. As mentioned above, the
 seals 26 can be welded to either the dovetail portion 14 or the disk slot
 16. Thus, the workpiece W is intended to encompass both the bucket 10 and
 the rotor disk 12. That is, either one of the bucket 10 or the rotor disk
 12 could be the workpiece W of FIG. 3. The ultrasonic welding apparatus 28
 includes a base 30 and a welding head 32 moveably mounted to the base 30
 via a ram 34. A work holding fixture 36 is mounted on a platform 38, which
 is moveably mounted to the base 30. The platform 38 is moveable in two
 horizontal axes with respect to the base 30 so as to position the fixture
 36 (to which the workpiece W is secured) below the welding head 32. The
 ram 34 moves vertically by known means (not shown) so as to move the
 welding head 32 into and out of welding engagement with the workpiece W.
 The welding head 32 includes a frame 40 that is fixed to the ram 34 and a
 transducer system 42 attached to the frame 40. A conventional wire feeding
 mechanism 44 for advancing and cutting bonding wire 46 is also mounted on
 the frame 40. A spool 48 of wire is mounted atop the frame 40 and provides
 a supply of the bonding wire 46, which is preferably, but not necessarily,
 aluminum wire. It should be noted that the pieces of material that make up
 the seals 26 are not limited to wire, but could be in many other forms,
 such as strips of aluminum or the like. The primary components of the
 transducer system 42 are an ultrasonic transducer 50, a horn 52 and a weld
 tip 54. The feeding mechanism 44 provides a desired length of bonding wire
 to the weld tip 54, which includes a lower contact surface 56 that presses
 the bonding wire 46 against the workpiece W.
 The ultrasonic transducer 50 includes one or more piezoelectric or
 magnetostrictive transducer elements that convert high frequency
 electrical energy produced by a conventional generator (not shown) into
 mechanical energy in the form of longitudinally propagated ultrasonic
 energy waves. The horn 52 couples the longitudinally propagating
 ultrasonic waves to the weld tip 54, resulting in ultrasonic transverse
 motion of the weld tip 54 that causes the bonding wire 46 to be welded to
 the workpiece W.
 In operation, a workpiece W (i.e., either a bucket 10 or a rotor disk 12)
 is secured in the fixture 36 and then positioned with respect to the
 welding head 32 by adjustment of the platform 38. Once the workpiece W is
 properly located for the welding operation, an appropriately sized piece
 of the bonding wire 46 is produced by the wire feeding mechanism 44, which
 advances and cuts the piece of bonding wire 46 in a known manner. The
 welding head 32 is moved vertically downward via the ram 34 so as to press
 the piece of bonding wire 46 into contact with the workpiece W. The
 bonding wire 46 is strategically placed in contact with the workpiece W at
 a location thereon where a seal 26 is desired. The ram 34 moves the
 welding head 32 downwardly such that the contact surface 56 presses the
 piece of bonding wire 46 against the workpiece W with a force sufficient
 to achieve ultrasonic welding. Typically, a compressive force in the range
 of 50-1000 pounds, depending on the type of material being welded, will be
 applied.
 The ultrasonic transducer 50 is energized to produce ultrasonic transverse
 motion in the weld tip 54. Ultrasonic energy is thus applied to the piece
 of bonding wire 46, which causes the piece of bonding wire 46 to be welded
 to the workpiece W. The ultrasonic transducer 50 is energized at
 frequencies in the range of about 15-40 kHz and for a short time period,
 typically 0.05-1.0 seconds. The welding head 32 is retracted upon
 completion of the welding operation. The platform 38 can then be adjusted
 to reposition the bucket with respect to the welding head 32 for a
 subsequent welding operation, such as welding another piece of wire
 material to another one of the dovetail lobes 18.
 Once all of the pieces of material have been ultrasonically welded to the
 workpiece W (be it either a bucket 10 or a rotor disk 12), the bucket 10
 is mounted to the rotor disk 12 by axially driving the dovetail portion 14
 into the disk slot 16. As the dovetail portion 14 is driven into the disk
 slot 16, excess seal material is sheared off at the mating edges of the
 dovetail portion 14 and the disk slot 16, leaving a portion of each piece
 adhering to the dovetail portion 14 or the disk slot 16 as the case may
 be. The remaining portions fill the corresponding gaps between the
 dovetail portion 14 and the disk slot 16, thereby creating the seals 26
 and sealing the disk slot 16.
 The structure of the seal material may be intentionally weakened internally
 so as to facilitate the shearing-off process. This ensures that the
 material failure or shearing will always occur at the shearing edge and
 not at the surface of the dovetail portion 14 or the disk slot 16. Failure
 at the surface of the dovetail portion 14 or the disk slot 16 would cause
 the material to flake off. The intentional weakening can be accomplished
 by using bonding wire 46 that is formed from a plurality of wound strands
 instead of being a solid material. The subsequently welded piece of
 material would have internal defects such as pores and oxide inclusions
 and would thus be more likely to fail internally than a welded piece of
 material formed from a solid wire.
 An alternative manner for intentionally weakening the welded piece of
 material is to use a weld tip 54 having a deeply patterned contact surface
 56 as shown in FIG. 4. It is known in the art of ultrasonic welding to
 provide shallow patterns on such weld tips in order to facilitate good
 welding by causing the weld tip to "grip" the material being welded. By
 contrast, the contact surface 56 is provided with a plurality of
 indentations 58 that are substantially deeper than those normally found on
 conventional weld tips. Therefore, when the contact surface 56 is pressed
 against the piece of bonding wire 46 during the ultrasonic welding
 operation, corresponding deep indentations are impressed into the outer
 surface of the welded piece of material. These deep indentations extend
 substantially into the welded piece of material, thereby weakening it
 internally against shear forces. As shown in FIG. 4, the deep indentations
 58 are arranged in a "waffle" pattern. Other patterns such as a knurled
 pattern could alternatively be used. The deeply patterned tip could be
 used in combination with the wire strand means to provide a maximum
 weakening effect.
 The foregoing has described a method of using ultrasonic welding to quickly
 and inexpensively apply dovetail seals to turbine buckets or rotor disks.
 The method requires little or no surface preparation of the bucket and
 requires no masking. While specific embodiments of the present invention
 have been described, it will be apparent to those skilled in the art that
 various modifications thereto can be made without departing from the
 spirit and scope of the invention as defined in the appended claims.