Patent Document

BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention generally relates to machining equipment and processes. More particularly, this invention relates to a method and apparatus that combines a fluid-jet system and an electrical-discharge machining (EDM) system for use in the repair of air-cooled airfoil components of gas turbine engines. 
     2. Description of the Related Art 
     Components located in the high temperature sections of gas turbine engines are typically formed of superalloys. Such components, which include combustors and turbine nozzles (vanes) and buckets (blades), are under strenuous high temperature conditions during engine operation, which can lead to various types of damage or deterioration. For example, erosion, cracks and other surface discontinuities tend to develop at the trailing edges of airfoils (e.g., buckets and nozzles) during service due to foreign object impact (foreign object damage, or FOD). 
     Because the material and processing costs of superalloys are relatively high, repair of damaged or worn superalloy components is typically preferred over replacement. For this purpose, weld repair methods have been developed using tungsten inert gas (TIG) and other welding processes. 
     The first and second stage power nozzles of industrial gas turbine engines are notably prone to damage caused by impact with foreign objects. For purposes of discussion, a section of a nozzle segment  50  is represented in FIG. 1, in which multiple nozzle partitions  52  are supported between a pair of bands  54 . In a typical repair process, the nozzle segment  50  is removed and then undergoes repair by hand. The damaged area of the nozzle segment  50  may be a small surface region of the segment  50 , such as the trailing edge  58  of a partition  52 , or encompass a much larger area. If the former, the damaged area can be selectively removed by grinding using a high speed grinder with a burr attachment, while the latter may require removal of an entire partition  52  using a high speed grinder with an abrasive cutting disc. Each of these operations is labor-intensive, often requiring about four man-hours or more. After removal of the damaged area, the repair process is completed by welding and grinding. If a partial partition  52  has been removed, a replacement may be welded in its place. Smaller surface areas are repaired by TIG welding to build up a weldment that replaces the removed material. The welding process is followed by grinding in order to closely duplicate the original contours (e.g., suction and pressure surfaces) of the partition  52 . 
     Weld repairs of air-cooled turbine components, such as the partitions  52  of FIG. 1, are further complicated by the presence of cooling holes  60 , which are typically formed at the trailing edge  58  by such drilling techniques as electrical-discharge machining (EDM) and laser machining. During welding, cooling holes  60  in the surfaces of a nozzle partition  52  are susceptible to blockage by weld filler material that enters the holes  60 . The performance of a partition is directly related to the ability to provide uniform cooling of its surfaces with a limited amount of cooling air. In particular, the size and shape of each hole  60  determine the amount of air flow exiting the hole  60  and the distribution of the air flow across the downstream surface of the partition  52 , and also affect the overall flow distribution within the cooling circuit containing the hole  60 . Consequently, it is important that the cooling holes  60  in a weld-repaired partition are substantially restored to their original size, shape and location. Methods for reestablishing cooling holes or blocking existing cooling holes are known, such as through the use of carbon rods. However, this technique challenges the welder in retaining the integrity of the weld around the carbon rod, and often requires rework. 
     In view of the above, it can be seen that the removal and repair of a gas turbine airfoil component is labor-intensive, particularly with the added demand that the contours and cooling holes of the repair closely duplicate that of the original component. While various other approaches have been proposed for repairing nozzle partitions, such as in commonly-assigned U.S. Pat. No. 5,895,205 to Werner et al., there is an ongoing effort to develop improved repair methods. 
     SUMMARY OF INVENTION 
     The present invention provides a method and apparatus for repairing an article, and particularly an air-cooled airfoil, during which at least a portion of the airfoil must be removed and replaced. The method and apparatus make use of a combined fluid-jet system and an electrical-discharge machining (EDM) system that enables the contours and cooling holes of a repaired airfoil to closely duplicate that of the original. 
     The apparatus of this invention includes at least one workpiece holder adapted to position and secure an airfoil on the apparatus, a multi-axis head adapted for movement relative to an airfoil positioned and secured on the apparatus, a nozzle mounted to the multi-axis head and operable to remove at least a portion of the airfoil with a jet of fluid discharged therefrom, an electrical-discharge electrode mounted to the multi-axis head and operable to form surface holes in the airfoil by electrical-discharge machining, and means for controlling the movement of the multi-axis head. More particularly, the controlling means is operable to precisely position and move the nozzle relative to surface contours of the airfoil when removing the portion of the airfoil, and to precisely position and move the electrical-discharge electrode relative to surface contours of the airfoil when forming the surface holes in the airfoil. 
     The above-described apparatus makes possible a method of repairing an air-cooled airfoil by positioning the airfoil on the apparatus, operating the multi-axis head to remove at least a portion of the airfoil by cutting the airfoil with a jet of fluid discharged from the nozzle mounted to the multi-axis head, removing the airfoil from the apparatus, welding the airfoil to form a replacement section that replaces the portion removed from the airfoil, positioning and securing the airfoil to the apparatus with a workpiece holder, and then operating the multi-axis head to form surface holes in the replacement section of the airfoil by electrical-discharge machining the replacement section with an electrical-discharge electrode mounted to the multi-axis head. 
     In view of the above, the apparatus and method of the present invention are able to improve the productivity, quality and safety of the operation of repairing an air-cooled airfoil by combining equipment for two separate cutting operations on a single multi-axis head that is configured and controlled to be highly and precisely maneuverable. Use of a multi-axis head enables movement of both the fluid-jet nozzle and the electrical-discharge electrode to be controlled so as to precisely position and move the nozzle relative to surface contours of the airfoil when removing the portion of the airfoil, and later to precisely position and move the electrical-discharge electrode relative to surface contours of the airfoil when forming the surface holes in the repaired airfoil, based on contour data that can be stored by the apparatus. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 represents a section of a nozzle segment of an industrial gas turbine engine. 
     FIG. 2 is a schematic elevational view of an apparatus that combines the functionalities of an EDM and waterjet in accordance with this invention. 
     FIG. 3 is a more detailed schematic view of a multi-axis head of the apparatus of FIG. 2, on which an electrical-discharge electrode and waterjet nozzle are mounted in accordance with this invention. 
    
    
     DETAILED DESCRIPTION 
     Illustrated in FIGS. 2 and 3 is an apparatus  10  adapted to perform both waterjet and EDM machining operations on components, such as in the repair of an air-cooled airfoil to closely duplicate the contours and cooling holes of the original in accordance with a preferred aspect of this invention. While the apparatus  10  and the process performed by the apparatus  10  will be discussed in reference to repairing air-cooled nozzle partitions (such as the partitions  52  of FIG.  1 ), the apparatus  10  can be used to perform similar repair operations on other types of hardware, including various air-cooled components of other turbomachinery. 
     The apparatus  10  includes a multi-axis head  12  suspended from a gantry  14 , and an EDM unit  16  and a waterjet unit  18  mounted to the head  12 . Aside from the EDM unit  16  and its associated equipment and controls, the apparatus  10 , including the head  12  and waterjet unit  18 , can be of a type commercially available. More preferably, the apparatus  10  is a modified adaptation of a waterjet cutting system equipped with a five-axis waterjet head that is commercially available from PAR Systems under the name VectorÂ®. The PAR System waterjet cutting system provides a desirable and convenient foundation from the apparatus  10  of this invention can be built. Various features of this cutting system advantageously used in the apparatus  10  include a pressure capability of about 60,000 psi (about 4130 bar), a linear positioning accuracy of about +/−0.003 inch (about +/−75 micrometers), and the versatility of a five-axis positioning capability, which is particularly advantageous in view of the complex three-dimensional contours of airfoils. However, while the apparatus  10  is depicted in FIGS. 2 and 3 as being based on the PAR Systems waterjet cutting system, various other configurations are possible for the apparatus  10 . 
     FIG. 2 shows the apparatus  10  as including a controller  20 , which can be of a type provided with the PAR System waterjet cutting system, e.g., preferably PC-based with standard CNC programming capability to control the movement of the head  12  using absolute and relative point coordinate data. A single handheld remote pendant  44  is provided with which the movement of the multi-axis head  12  can be controlled by a single operator. The controller  20  preferably stores coordinate data for the particular airfoil (not shown) to be processed, so that the head  12  can be operated to precisely position the EDM unit  16 , and optionally the water jet unit  18 , relative to the surface contours of the airfoil. 
     The waterjet unit  18  shown in FIG. 3 includes a waterjet nozzle  22  mounted to the multi-axis head  12 . The nozzle  22  can be of any suitable type capable of discharging a jet stream capable of cutting through the material of the airfoil, e.g., nickel-base and cobalt-base superalloys commercially-known under the names GTD-222 and FSX-414, respectively. A high-pressure fluid line  24  delivers water (or another suitable fluid) to the nozzle  22 . A separate supply line  26  is provided for delivering to the nozzle  22  an abrasive media (e.g., garnet) of a type known and used to promote the cutting action of waterjets. 
     The EDM unit  16  is shown as being supported on a side of the multi-axis head  12  opposite the waterjet nozzle  22 . As with the waterjet unit  18 , the EDM unit  16  can be of a type commercially available. More preferably, the EDM unit  16  is adapted from an EDM electrode machine commercially available from Ann Arbor Machine, Inc. While a particular type and configuration for the EDM unit  16  is represented in FIG. 3, it is foreseeable that various other configurations and types could be used. In the repair of airfoils such as the nozzle segment  50  of FIG. 1, the EDM unit  16  is intended to restore the cooling holes in a weld-repaired section of the airfoil such that the contours and cooling holes of the repaired section closely duplicate that of the original airfoil. It is within the knowledge of those skilled in the art to appropriately identify operational parameters for the EDM unit  16  that render the unit  16  capable of quickly penetrating the airfoil material to consistently produce accurately-sized cooling holes without distorting the surrounding material. 
     FIG. 3 shows the EDM unit  16  as comprising an EDM head  28  modified to include a quick-position adapter plate  29 . The adapter plate  29  is secured with two quick-snap bushing and plug sets  31  to a second adapter plate  30  bolted to the multi-axis head  12 . The bushing and plug sets  31  enable the quick-position adapter plate to be quickly released from the adapter plate  30 , so that the EDM head  28  can be can be readily removed from the head  12 . As a result of the manner in which the quick-position adapter plate  29  is mounted, the head  28  generally has an inverted L-shape. An electrode guide  32  is mounted to the EDM head  28 , with the lower end of the guide  32  projecting below the lower end of the head  28 . The guide  32  can be of a conventional type for supporting one or more EDM electrodes  33 . A power source  34  is shown mounted to an upper end of the head  28 , by which voltage and current are supplied to the electrode  33 . The electrode  33  may be formed of graphite or another suitable material (e.g., brass), and preferably has a cross-sectional shape corresponding to the desired shape of the cooling holes to be machined in the airfoil. With the multi-axis head  12 , the electrode  33  can be precisely and repeatably positioned a specified distance from the surface of an airfoil to be machined, establishing a spark gap that is typically on the order of about 0.001 to about 0.003 inch (about 25 to about 75 micrometers). 
     The power source  34  is operated to cause a charge to build up on the electrode  33 , which when sufficient causes an electrical current to jump the spark gap. Charge buildup and discharge is achieved by providing a suitable dielectric electrical-discharge medium between the electrode  33  and airfoil, such that material is removed from the airfoil by a sparking discharge action while the airfoil surface is being flushed with the medium. The medium can be delivered to the electrode-to-airfoil spark gap via appropriate plumbing through the center of the electrode  33  to the cutting contact surface. While oils have been widely used for this purpose, the present invention preferably makes use of partially deionized water. As used herein, partially deionized water has an electrical resistance that is greater than that of tap water, but less than that of pure distilled water. A preferred range for the electrical resistance of the water used with the present invention is about 1000 to about 1500 ohms per centimeter. According to commonly-assigned U.S. Pat. No. 6,489,582 to Roedl et al., partially deionized water is a desirable medium for the EDM machining of cooling holes in air-cooled airfoils because, in addition to cooling the airfoil and aiding in removing the residual material machined therefrom, water is less likely to plug the cooling holes in comparison to oil-base media. Using partially deionized water as the machining medium, suitable EDM machining results can be achieved with the apparatus  10  of this invention by operating the power source  34  to supply an applied voltage of about 480 VAC to about 40 VDC operational at the tip of the electrode  33  with an applied current capable of generating about 120 amperes. 
     In addition to its airfoil surface being flushed with partially deionized water, the nozzle segment is preferably immersed in a bath of partially deionized water during machining. For this purpose, FIG. 2 shows the apparatus  10  as including a catch tank system  38  comprising an EDM tank  42  within a larger tank  40 , the latter of which collects spent water from the waterjet operation. As such, the tank  40  can generally be of a type conventionally used in waterjet cutting systems, such as the PAR System unit discussed above. The EDM tank  42  is preferably adapted to be placed within the larger tank  40  when needed for the EDM operation, so that the EDM head  28  can be positioned over the EDM tank  42 , with a workpiece holder  36  (schematically represented in FIG.  3 ), nozzle segment, and lower end of the electrode  33  submersed in the EDM tank  42  so that partially deionized water within the tank  42  is present in the spark gap between the electrode  33  and the surface of the partition being machined. The EDM tank  42  collects the partially deionized water used in the EDM operation, and then delivers the collected water to a deionizing system (not shown) that supplies the EDM operation. The EDM tank  42  is preferably equipped with a float valve (not shown) for controlling the water level within the tank  42 , and sensors (not shown) for monitoring the electrical resistance of the partially deionized water. 
     As discussed above, the apparatus  10  is particularly adapted to repair air-cooled nozzle segments of a gas turbine engine. The section of a nozzle segment  50  represented in FIG. 1 comprises multiple partitions  52  (airfoils), each of which is at last partially hollow, with cooling holes  60  present in the airfoil wall generally along the trailing edges  58  of the partitions  52 . In service, cooling air is forced into the hollow interior of the partitions  52  and exits through the cooling holes  60 , with the effect that the temperature of each partition  52  is minimized through a combination of heat transfer and film cooling. When repair of a partition  52  is necessary, the region most likely to need replacement is the airfoil trailing edge  58 , encompassing the region in which the cooling holes  60  are present, though any surface region of a partition  52  may require repair, from the trailing edge  58  forward to the leading edge  56 , and the suction and pressure surfaces therebetween. 
     Removal of a damaged portion of a partition is performed after the nozzle segment is removed from the turbomachine in which it is installed. The nozzle segment is placed on an appropriate support or fixture (e.g., a platform or a specially adapted workpiece holder similar to the holder  36  of FIG. 3) in the waterjet tank  40 . The EDM tank  42  is preferably removed from the waterjet tank  40  for this part of the operation, so as to permit relatively conventional operation of the waterjet unit  18 . The operator then controls the position of the waterjet nozzle  22  relative to the nozzle segment through the controller  20  and pendant  44 . Depending on the particular application, the waterjet nozzle  22  is typically positioned about 0.25 to about 0.30 cm from the surface of the partition (or another region of the segment that requires repair), and then traversed across the surface of the partition to cut a preselected damaged region from the partition using waterjet parameters (e.g., pressure, jet diameter and traversal rate) appropriate for the partition (e.g., based on material, thickness, etc.). During this operation, the operator can use the pendant  44  to visually position the waterjet nozzle  22  relative to the surface being cut. Alternatively, the controller  20  could be used to control the movement of the multi-axis head  12  so that the waterjet nozzle  22  is precisely positioned and moved relative to the surface contours of the partition, based on the stored coordinate data of the nozzle  22 . 
     Once the intended damaged region is removed (e.g., the trailing edge of the partition), the nozzle segment is removed from the apparatus  10  and undergoes a welding repair operation by which a replacement section is fabricated, such as by building up a weldment or welding a preformed insert in place. In either case, the welding operation preferably yields a replacement section that is as close as practical to the final aerodynamic shape desired for the partition, though additional grinding, etc., may be necessary for this purpose. However, the cooling holes having the appropriate shape and size required to achieve adequate air cooling of the partition cannot be readily produced or maintained during the welding repair operation. For this purpose, the nozzle segment is placed on the workpiece holder  36  of the EDM tank  42 , which has now been positioned within the larger waterjet tank  40 , and the multi-axis head  12  is operated with the pendant  44  and controller  20  to appropriately control the position and orientation of the electrode  33  relative to the surface of the partition before operating the EDM unit  16  to electrical-discharge machine the desired cooling holes in the replacement section of the partition. As previously noted, the EDM operation is performed while partially deionized water is present as the dielectric medium between the replacement section and the electrode  33 . As with the waterjet cutting operation, the multi-axis head  12  is controlled during this step of the operation, though at this time the movement of the multi-axis head  12  is controlled to precisely position and move the electrode  33  relative to surface contours of the partition based on the stored coordinate data that precisely locates the surface of the partition relative to the electrode  33 . 
     While the invention has been described in terms of a particular embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.

Technology Category: 4