Abstract:
A method of forming cooling holes in a gas turbine engine component. The method includes the steps of forming a hole in the component extending between a first surface of the component and a second surface of the component opposite the first surface and supplying pressurized air to the hole from the second surface of the component. The method also includes the step of sensing a parameter representative of air flow through the hole. Further, the method includes enlarging the hole until the sensed parameter represents a preselected air flow rate through the hole.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to cooling holes in gas turbine engine components, and more particularly to a method of forming such holes. 
     Cooling holes are formed in gas turbine components such as vanes, blades and shrouds for transporting film cooling air through the component to cool the component and to form a fluid barrier between the component and hot gases traveling through a main flowpath of the engine. Frequently, these cooling holes are formed by aiming a laser at a selected point on the component for a predetermined period of time to burn a hole through the component. The laser is then aimed at another selected point on the component for a predetermined period of time to burn another hole in the component. This process is repeated to form all the cooling holes in the component. The period of time during which the laser is aimed at particular points is determined by evaluating holes made previously in other components. 
     Conventional lasers produce holes having large variations in flow rate. Thus, relying on historical data to determine the period of time to burn each hole results in large variations in cooling flow rates which ultimately reduce the effectiveness of the cooling. To compensate for worst case cooling flows, more air must be directed through the cooling holes. Directing more cooling air through the holes reduces the overall engine efficiency. Thus, the variability in cooling hole flow rates adversely affects overall engine performance. 
     Further, when conventional lasers are used to form holes in some components having internal cavities (e.g., turbine vanes and blades), fillers such as wax, plastics or ceramics must be injected in the cavities to prevent the laser from burning a hole through the wall at the opposite side of the cavity. These fillers must be removed from the cavities after the holes are formed. As a result, using fillers increases the time and expense required to form the holes. 
     SUMMARY OF THE INVENTION 
     Among the several features of the present invention may be noted the provision of a method of forming cooling holes in a gas turbine engine component. The method includes the steps of forming a hole in the component extending between a first surface of the component and a second surface of the component opposite the first surface and supplying pressurized air to the hole from the second surface of the component. The method also includes the step of sensing a parameter representative of air flow through the hole. Further, the method includes enlarging the hole until the sensed parameter represents a preselected air flow rate through the hole. 
     In another aspect, a method of the present invention includes the step of aiming a laser beam at a first surface of the component to form a hole in the component extending between the first surface of the component and a second surface opposite the first surface. In addition, the method includes the steps of supplying pressurized air to the hole from the second surface of the component and sensing a pressure of the air entering the hole. The method also includes the step of stopping the laser beam after the sensed pressure meets a predetermined value representing a preselected air flow rate through the hole. 
     In yet another aspect of the present invention, the method includes the step of directing a pulsing laser at the component. The method also includes the steps of adjusting the laser to have a pulse frequency of between about 100 hertz and about 100 megahertz, adjusting the laser to have a pulse energy of between about 0.001 joules and about 5 joules, and adjusting the laser to have a pulse duration of between about 10 picoseconds and about 100 microseconds. 
     Other features of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a laser forming a hole in a turbine vane using the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and in particular to FIG. 1, a gas turbine engine component is generally designated in its entirety by the reference numeral  10 . Although the method of the present invention may be used to form holes in other components such as turbine blades and shrouds, the component shown in FIG. 1 is a turbine vane  10  having internal passages or cavities  12  separated by walls  14 . Holes  20  are formed in the vane  10 . The holes  20  extend between a first surface  22  and a second surface  24  opposite the first surface defining, in part, the internal passages  12 . Although only two holes are shown in FIG. 1, those skilled in the art will appreciate that more holes  20  are preferably formed in the component  10 . The number, sizes, positioning and orientations of these holes  20  are determined based upon the desired cooling airflow characteristics of the component  10 . As the features of the gas turbine engine component  10  described above are conventional and well known, they will not be described in further detail. 
     The cooling holes  20  of the present invention are formed in the component  10  using a laser  30 . Rather than using a conventional laser having a pulse duration of 0.1 to 4.0 milliseconds, a pulse frequency of 1 to 100 hertz and a pulse energy of 5 to 100 joules, the laser  30  used in the method of the preferred embodiment is adjusted to have a pulse duration of about 10 picoseconds to about 100 microseconds, a pulse frequency of about 100 hertz to about 100 megahertz and a pulse energy of about 1 millijoules to about 5 joules. As can be seen by comparing these parameters, the laser  30  used in the method of the present invention has a shorter pulse duration, a higher frequency and a lower energy than conventional lasers used to form holes in gas turbine engine components. Although other lasers may be used without departing from the scope of the present invention, the laser  30  of the preferred embodiment is a Coherent General M34 Nd: YAG pulsing laser available from Convergent Energy of Sturbridge, Mass. As will be appreciated by those skilled in the art, the preferred laser  30  is “q-switched” to have the parameters described above. Further, although the laser  30  may be mounted on other equipment without departing from the scope of the present invention, in the preferred embodiment the laser is mounted on a Huffman HP75 5-axis CNC machine tool available from Huffman Corp. of Clover, S.C. 
     To form the holes  20 , the component  10  is loaded in a conventional fixture (not shown) and one end of the component is attached to a pressurized air supply  32 . A calibrated airflow measurement device  34  (such as available from Flow Systems, Inc. of Berthoud, Colo.) is mounted between the component  10  and the air supply  32  for measuring a parameter representative of airflow through the holes  20 . For instance, the device may measure pressure in the internal passage  12  or it may measure air flow through the passage. The end of the pressurized internal passage  12  opposite the air supply  32  is blocked by a sealing plate  36  to prevent air from escaping from that end of the passage. Once the component  10  is in position on the fixture and the air supply  32  and plate  36  are in place, the air supply is activated to deliver air to the internal passage  12  of the component until some predetermined pressure (e.g., 16 psi to 30 psi or more) is achieved inside the passage. 
     A control  38  (such as a FANUC CNC controller frequently sold with the previously mentioned machine tool) translates and rotates the laser  30  and/or the component  10  into the correct location for forming the first hole  20 . The control  38  opens a shutter (not shown) inside the laser  30  so a laser pulse train or beam  40  is emitted from the laser. The beam  40  is aimed at the component  10  to form the hole  20 . After the hole  20  is formed, the flow measurement device  34  senses a change in the measured parameter (e.g., a drop in pressure inside the internal passage  12  or an increase in flow rate through the passage) which corresponds to the air leaking through the hole. This parameter is input to the control  40  which repositions the laser and/or the component  10  to enlarge the hole  20 . The parameter is continuously monitored and compared to a preselected value which represents a preselected air flow rate. When the parameter reaches the preselected value, the control  38  signals the laser  30  to close the shutter. 
     After the first hole  20  is formed, the control  38  translates and rotates the laser  30  and/or the component  10  into the correct location for forming the second hole  20  and the process is repeated. As will be appreciated by those skilled in the art, the laser  30  may be stopped during the formation of the second and subsequent holes after the measured parameter reaches a preselected value or after a period of time related to (e.g., equal to) the amount of time taken to form the first hole. Further, when the process is repeated to form subsequent holes, the actual air flow through the previously formed holes may be used to determine the preselected parameter for drilling the next few holes. For instance, if the actual flow through the first two holes is slightly below nominal, the preselected value of the parameter used for forming the next three holes may be adjusted slightly upward so the total flow through all five holes is nominal. 
     Using conventional lasers, one to one hundred pulses are required to drill a cooling hole. Using the laser and method of the present invention, one hundred to several million pulses are required to drill a hole. However, the total drill time is not increased significantly because the pulse frequency is increased. Further, the method of the present invention permits the laser pulses to be stopped precisely (e.g., plus or minus one pulse). Although the conventional laser drilling methods produce variations in component flow rates of as much as about ten percent from nominal and sometimes as much as thirty percent from nominal, it is anticipated that the method described above will produce flow rates which vary no more than about five percent from nominal and possibly as little as one percent from nominal because smaller amounts of material are taken during each laser pulse. 
     In addition, because smaller amounts of material are removed during each pulse, it is envisioned that the method described above may be used to form non-circular (e.g., trapezoidal) holes to vary the thickness and position of cooling air flowing over the component  10  to optimize film cooling. Further, because each pulse removes an extremely small amount of material, each pulse is incapable of appreciably damaging the component  10 . Thus, errant pulses directed toward internal walls  14  or other interior features of the component  10  will not damage the component. As a result, no fillers are needed to protect the wall  14  or other interior features against damage from errant pulses when using the method of the present invention, and consequently the method eliminates the need for steps required to clear the passages. 
     Because the pulses are at extremely high peak power (e.g., about 100 kilowatts to over 1 megawatt, compared to 10 to 70 kilowatts with conventional lasers), there is a higher efficiency in vaporization of material being removed from the hole and less melting. Consequently, the method of the present invention produces a cleaner more repeatable hole, reducing the need for cleaning pulses once the hole is formed. 
     Another advantage of the method of the present invention is that pressurizing the internal cavity of the component improves hole quality. A typical laser hole has a burr and dross at the edge of the hole opposite the laser (i.e., at the internal cavity). The burr is re-solidified material inside the hole which can affect air flow through the hole. Because the cavity is pressurized during the method of the present invention, the escaping air prevents a burr from forming internally by pushing the molten material back through the hole. This eliminates the burr and provides a smoother surface inside the hole. 
     In addition, because the method of the present invention produces holes having flow rates which are closer to nominal, the worst case flow condition (i.e., minimum flow) is closer to nominal. Frequently, nominal cooling flow rates of components are chosen to ensure sufficient flow during worst case conditions. If this condition is closer to the nominal flow rate, the nominal flow rate may be reduced. When less cooling air is required to account for worst case flow conditions, the engine uses less fuel. Alternatively, the aggregate hole area can be increased and the same amount of air may be used allowing the engine to run cooler. This enables extended component life or permits increased maximum engine temperatures and available thrust. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.