Patent Publication Number: US-2011076405-A1

Title: Hole drilling with close proximity backwall

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00019-02-C-3003. 
    
    
     BACKGROUND 
     The present invention relates generally to drilling of holes in components utilizing water jet cutting, and, more particularly, to drilling through a coating on a component with a close proximity backwall utilizing water jet cutting. 
     Various components of gas turbine engines, such as combustion liners and augmentors, often require a complex cooling scheme in which cooling air flows through the component and is then discharged through carefully configured cooling holes in the outer wall of the component and/or its associated structures. The performance of a turbine component is directly related to the ability to provide uniform cooling of its external surfaces. Consequently, control of cooling hole size and shape is critical in many turbine engine component designs, because the size and shape of the opening determines the amount of flow exiting a given opening, its distribution across the surface of the component, and the overall flow distribution within the cooling circuit that contains the opening. Other factors, such as back flow margin (the pressure differential between the cooling air exiting the cooling holes and working gases impinging on the component) are also affected by variations in opening size. 
     Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. 
     Conventional aperture creation techniques include laser machining and electrical-discharge machining (EDM). These techniques yield components with dimensionally correct openings in order to repeatably control opening size, but are expensive to obtain, operate, and maintain in manufacturing. Further, additional considerations must be accounted for when creating apertures through a part that contains a coating, such as ceramic. Care must be taken in creating apertures so as not to damage the coating. Additionally, some coatings inhibit certain cutting techniques, such as EDM that requires electrical contact for optimal operation. 
     Components that are constructed of a single substrate with a coating such as a TBC can be manufactured by processes utilizing laser machining, EDM, and water jet cutting and drilling. On these components, the drilling is done from the substrate side of the components, which allows for the cutting processes to break through the coating. However, in a part with a close proximity backwall to a coated substrate, such a process is not possible as the cutting head or tool of the machine can not be positioned to contact the substrate first in the manufacturing process. Thus, to utilize typical machining processes, the cutting head or tool contacts the coating first. 
     SUMMARY 
     In one embodiment, a method of making a coated component with a close proximity backwall is achieved by applying a coating to a pre-existing workpiece that contains a substrate with a plurality of apertures. The substrate is in close proximity to a backwall. The coating is removed from the plurality of apertures with a fluid jet cutting system. The fluid jet cutting system has a fluid jet without a particulate material added thereto. 
     In an alternate embodiment, a method of creating cooling holes in a component is disclosed. A substrate is provided and a plurality of apertures is created in the substrate. A backwall is attached adjacent to the substrate. A coating is applied to the substrate on a first surface opposite a second surface that faces the backwall. The coating is removed from the plurality of apertures. The removal process does not damage the backwall adjacent the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a water jet drill apparatus. 
         FIG. 2  is a cross-sectional view of a substrate with apertures formed therein. 
         FIG. 3  is a cross-sectional view of a backwall attached to the substrate. 
         FIG. 4  is a cross-sectional view of a coating applied to the assembly of  FIG. 5 . 
         FIGS. 5 and 6  are cross-sectional views of the coating being removed to create apertures in the coating. 
         FIG. 7  is a cross-sectional view of a completed coated component with a close proximity backwall. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a fluid jet cutting system  34  that includes cutting head  36  with nozzle  38  coupled to mount assembly  44 . Illustrated are workpiece  10 , cutter head  36 , nozzle  38  with jet J, sensor  40 , drive assembly  42 , mount assembly  44  with lower portion  48 , fluid conduit  46 , mounting arm  50 , and fixture table  52 . Mount assembly  44  is driven by a control having a drive assembly  42  that positions nozzle  38  throughout an x-y Cartesian plane that is substantially parallel to the surface of workpiece  10 . Drive assembly  42  may include a pair of drives oriented along the x and y axes and a pair of electric drive motors to provide motion along the x and y axes. Alternately, drive assembly  42  may include a multi-axis motion system. Drive assembly  42  may move either cutting heat  36  or fixture table  52  (or both) to produce relative movement of nozzle  38  with respect to workpiece  10 . 
     Cutting head  36  with nozzle  38  is connected to a fluid inlet coupled to a fluid source, such as a high-pressure pressure pump, by fluid conduit  46 . Mount assembly  44  includes mounting arm  50  having an aperture disposed therethrough for reception of nozzle  38  and the associated mounting hardware. The back side of mounting arm  50  is coupled to a lower portion  48  of the control gantry of mount assembly  44  and drive assembly  42 . Nozzle  38  is secured within a mounting aperture of mounting arm  50 . 
     In operation, the fluid, which is typically water, from fluid source enters the fluid inlet and travels through fluid conduit  46 , and exits from nozzle  38  toward the workpiece  10  as a cutting jet J. Cutting jet J pierces workpiece  10  and performs the desired cutting. Workpiece  10  is secured to a fixture on table  52 . Using drive assembly  42 , cutting head  36  with nozzle  38  is traversed across workpiece  10  in the desired direction or pattern based on the known position of workpiece  10  fixtured on table  52 . Alternatively, table  52  moves to position workpiece  10  with respect to cutting jet J from nozzle  38 . Sensor  40  is disposed adjacent nozzle  38 , which slides along or slightly above the surface of workpiece  10 . Sensor  40  indicates the relative height of workpiece  10 , as well as monitors the fluid of cutting jet J to indicate breakthrough of cutting jet J through workpiece  10 . The machine is controlled through automated programs, such as a computer numerical controlled (CNC) program. 
     Fluid jet cutting system  34  may be utilized in a process for cutting and drilling apertures in a coated component with a close proximity backwall. To start the process, a substrate layer is formed.  FIG. 2  is a cross-sectional view of substrate  62 . Substrate  62  is constructed from a sheet of metal or metal alloy. In one embodiment, substrate  62  is a light weight, high temperature alloy, while in another embodiment substrate  62  is a high temperature alloy such as a nickel cobalt or iron superalloy. The thickness of substrate  62  may vary, and in one embodiment is between 1 mm and 2 mm. 
     Apertures  60 A and  60 B are fabricated into substrate  62 , and may be cooling holes of workpiece  10 . Apertures may be made by any drilling or cutting manufacturing process known in the art, such as laser cutting, plasma cutting, water jet cutting, electrical discharge machining (EDM), mechanical drilling or machining with a bit, or formed by a punch press. Apertures  60 A and  60 B may contain differing cross-sectional profiles including circles and ovals, as well as any other profile that is demanded for adequate fluid flow for cooling of the component. For example, aperture  60 A may be a circular hole with a diameter of between 0.25 mm to 1.00 mm. As illustrated, aperture  60 A is perpendicular to substrate  62 , while aperture  60 B has been cut at an angle to substrate  62 . 
     After creating apertures in substrate  62 , backwall  64  is added.  FIG. 3  is a cross-sectional view of substrate  62  with backwall  64  to create fluid channel  66 . Fluid channel  66  acts as a pressure plenum and will receive a cooling fluid that will exit apertures  60 A and  60 B to cool the component. Backwall  64  is constructed from the same or similar materials as substrate  62 . Backwall  64  is attached to substrate  62  by methods common in the art, such as welding, brazing, fastening, or similar processes that secure backwall  64  with respect to substrate  62 . Backwall  64  is in close proximity to substrate  64 . Close proximity may be defined as a distance between backwall  64  and substrate  62  such that a cooling fluid may adequately flow through the channel between backwall  64  and substrate  62 , but cutting head  36  and nozzle  38  can not be positioned between backwall  64  and substrate  62  to obtain a desired cut from cutting jet J through substrate  62 . In one embodiment, substrate  62  is less than 5.00 cm from backwall  64 , while in another embodiment backwall  64  is spaced less than 1.00 cm from substrate  62 . Thus, apertures are pre-existing in substrate  62  prior to sealing off fluid channel  66  with backside  64 . In an alternate embodiment, backwall  64  may also contain apertures for the discharge of cooling fluid on the opposite side of the component. 
       FIG. 4  illustrates coating  68  applied to substrate  62  to create outer layer  70  of the component workpiece  10 . Coating  70  may be a thermal barrier coating or ceramic overlay on substrate  62 . In one embodiment, coating  70  includes a metallic bond coat that adheres the thermal-insulating ceramic layer to substrate  62 , forming a TBC system. Metal oxides, such as zirconia (ZrO 2 ) partially or fully stabilized by yttria (Y 2 O 3 ), magnesia (MgO) or other oxides, are commonly used as the materials for TBCs. Coating  68  is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD). Coating  68  may contain a bond coat applied prior to the ceramic coat. Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel). The thickness of coating  16  will vary with the insulating requirements of the component, and in exemplary embodiments has a thickness of between 0.25 mm and 0.50 mm thick. Larger components may require coating  68  to be thicker, such as up to 1 mm thick. 
     In certain applications, coating  68  must be applied after attaching backwall  64  to substrate  62 . For example, in a turbine engine augmentor liner, backwall  64  is brazed to substrate  62  utilizing temperatures of over 900° C. At this temperature, a ceramic coating may be damaged if the coating has been applied to the substrate prior to the joining process. Thus, coating  68  is applied after joining of backwall  64  to substrate  62 . 
       FIG. 5  is a cross-sectional view of coating  68  being removed to create aperture  60 A, and  FIG. 6  is a cross-sectional view of coating  68  being removed to create aperture  60 B in coating  68 . Aperture  60 A is being created by fluid jet J that is perpendicular to outer layer  70 , while aperture  60 B is created by fluid jet J that is oriented at an angle with respect to outer layer  70 . Cutting jet J is created by fluid jet cutting system  34  described with respect to  FIG. 3 . 
     While it is known to modify a waterjet to contain an abrasive particulates (i.e., essentially nonspherical particles with sharp corners and edges), practice has shown that the erosion and abrasion caused by abrasive particles in a water jet at pressures adequate to remove a ceramic deposit can severely damage the cooling hole and the surrounding component surface. In addition, abrasive materials in an abrasive fluid jet fracture to the point where the abrasive particulates cannot be reused or are difficult to separate from the material removed by the jet. As a result, the spent abrasive fluid must be disposed of, which adds unwanted cost to the process. Conventional water jet drilling is primarily performed on structures that do not have a shallow drop through region to a backwall. This is due to the physical limitations of being able to stop the drilling jet with particulate matter before the jet hits the opposing surface of the backwall. Cutting jet J may be water or similar fluid, and is substantially free from particulate material. That is, no particulate material is intentionally added to the fluid of cutting jet J. In some embodiments, ordinary tap water, distilled water, or de-ionized water are utilized as jet J without the addition of any other additives, making cutting jet J substantially free of particulate material. Fluid cutting jet J does not contain any abrasive particulates, but may contain other fluid additives. This prevents foreign object damage associated with adding abrasive particulates to the fluid prior to the cutting process, as well as prevents the need for extra steps to remove the particulate matter from channel  66 . 
     The cutting process may be done at a much lower pressure than would be required if cutting the aperture through the entire outer layer  70  including coating  68  and substrate  62 . The fluid cutting jet J may have a velocity sufficient to remove coating  68  overlying the cooling holes without damaging the cooling holes, substrate, or the backwall. Pressures below 70,000 kPa may be utilized. Utilizing lower fluid pressure saves energy input, as well as prevents cracking and chipping of the coating adjacent the apertures that may occur at higher pressures. 
     Cutting jet J is used to remove material until breakthrough is detected by sensor  40  of water jet cutting system  34 . Sensor  40  may be an acoustic breakthrough detection sensor that is common in the industry. During the cutting process, there is no blocking of the cutting stream. This is due in part to the close proximity of backwall  64  which inhibits the use of a backing insert, which is difficult to install and remove. Upon breakthrough, the fluid flow to the cutting jet is turned off and the jet dissipates. No damage is done to backwall  64 , which may be done if utilizing other processes such as laser drilling or mechanical machining, as cutting jet J does not contain force to damage backwall  64  absent particulate material. Due to the absence of abrasive material in fluid jet J, no damage is done to backwall  64  and there is no additional flushing of the part required to remove the abrasive material from fluid channel  66 . 
     Coating  68  is applied over substrate  62  that contains pre-existing apertures  60 A and  60 B. If coating  68  were applied to a substrate without apertures, the coating would be formed as a matrix in compression. With the apertures  60 A and  60 B in substrate  62 , coating  68  is in tension over apertures  60 A and  60 B. Coating  68  in tension is much easier to remove, and less pressure and force is required for the cutting process. Thus, if utilizing a water jet cutting process, no additional particulate material is required within cutting jet J to remove coating  68 . Elimination of the particulate additive is a great cost saver in the production of the component. 
     The removal of coating  68  is done starting with coating  68 , and not through substrate  62 . The position of backwall  64  inhibits the proper positioning of removal equipment adjacent the uncoated side of substrate  62 . In one embodiment, apertures  60 A and  60 B are created with the apparatus of fluid jet cutting system  34 . After backwall  64  and coating  68  have been secured to substrate  62 , the same apparatus is used to remove coating  68  from apertures  60 A and  60 B. This allows the same machine to be used without reprogramming the controls of the machine, and allows utilizing the same fixture for the component. 
       FIG. 7  is a completed component utilizing the above mentioned process. Workpiece  10  contains an outer layer  70  that has coating  68  on substrate  62  and backwall  64 . Outer layer  70  and close proximity backwall  64  create fluid channel  66  that will provide a pathway for cooling fluid of the component. Outer layer  70  has apertures  60 A and  60 B that allow for the discharge of the cooling fluid from fluid channel  66  to cool the component. Workpiece  10  may be any fabricated component that required film cooling apertures, contains a coating, and has a close proximity backwall. In an exemplary embodiment, workpiece  10  is a fabricated exhaust augmentor or combustion liner for a turbine engine. 
     The process disclosed herein may be utilized with any component that has cooling holes and a close proximity backwall. For example, in one embodiment, coating  68  may be a slag or recast layer remaining from another manufacturing procedure, such as a recast layer from laser drilling. The fluid jet may then be utilized to remove this recast layer. The fluid jet can be low pressure and does not require the addition of particulate material, such as garnet, to perform the cutting operation. Further, a cutting process with a low pressure fluid without particulate additives will only remove coating  68 . Absent higher pressure and abrasive materials, the cutting process will not expand the size of apertures  60 A and  60 B which may lead to a part being outside the range of specifications for the component. 
     The disclosure herein allows for the process of creating a coated component with film cooling holes. In one embodiment, a method of making a coated component with a close proximity backwall is achieved by applying a coating to a pre-existing workpiece that contains a substrate with a plurality of apertures. The substrate is in close proximity to a backwall to form a cooling channel such that the proximity of the backwall to the substrate prevents the operation of a fluid jet within the cooling channel. The coating is removed from the plurality of apertures with a fluid jet cutting system. The fluid jet cutting system has a fluid jet without a particulate material added thereto. 
     In an alternate embodiment, a method of creating cooling holes in a component is disclosed. A substrate is provided and a plurality of apertures is created in the substrate. A backwall is attached adjacent to the substrate. A coating is applied to the substrate on a first surface opposite a second surface that faces the backwall. The coating is removed from the plurality of apertures. The removal process does not damage the backwall adjacent the substrate. 
     In yet another embodiment, a method of creating cooling holes in a component starts with providing a substrate with a plurality of apertures. A backwall is attached adjacent to the substrate, and a coating is applied to the substrate on a first surface opposite a second surface that faces the backwall to create a coated component. The coated component is secured in fixture on a fluid jet apparatus with a moveable and programmable portion, which may be the mount assembly or the fixturing table, and a nozzle directs a fluid jet for material removal. The coating previously applied is removed from the plurality of apertures with a fluid jet that is essentially of water. Additionally, the breakthrough of the fluid jet through the coating may be detected, and fluid flow to the fluid jet is stopped based on the detection. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.