Abstract:
A process for producing through-holes in a sheet member to form a perforated article, such as an arcuate (non-planar) acoustic skin suitable for use in an acoustic panel of an aircraft engine nacelle. The process includes deforming a sheet member to have an arcuate shape with an arcuate surface, mounting and rotating the arcuate-shaped sheet member on a mandrel and then, while rotating the sheet member, directing an electron beam at the arcuate surface of the sheet member and deflecting the electron beam toward multiple locations on the arcuate surface to produce the through-holes through the sheet member in a defined hole pattern and thereby yield a perforated arcuate-shaped sheet member with holes having axes substantially normal to the arcuate surface. The holes are not subjected to elongation in a nonuniform manner after they are produced, and have the same transverse cross-sectional shape.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to processes for producing perforated articles, and more particularly to a process for forming numerous holes in a curved panel suitable for use in an acoustic panel, such as of the type used in nacelles of gas turbine engines. 
         [0002]    A typical construction used in aircraft engine nacelle components (for example, the engine inlet, thrust reversers, core cowl, and transcowl) and other aerostructures such as acoustic panels is a sandwich-type layered structure comprising a core material between thinner top and bottom sheets or skins. The core material is typically a lightweight material, often a foam or honeycomb metallic or composite material. A variety of metallic and composite materials can also be used for the skins, with common materials including aluminum alloys. 
         [0003]    Nacelle fan duct flow surfaces typically include acoustic panels to suppress noise. A common form of acoustic panel comprises a contoured sheet/skin (sometimes referred to as a face sheet or acoustic skin) that faces the duct airflow, a backing sheet/skin, and an open-cell foam or honeycomb core therebetween. The acoustic skin is acoustically treated by forming numerous small through-holes that help to suppress noise by channeling pressure waves associated with sound into the open cells within the core, where the energy of the waves is dissipated through friction (conversion to heat), pressure losses, and cancellation by reflection of the waves from the backing skin. For some gas turbine engine applications, perforations on the order of about 0.03 to about 0.06 inch (about 0.75 to about 1.5 mm) in diameter and hole-to-hole spacings of about 0.06 to about 0.12 inch (about 1.5 to about 3 mm) are typical, resulting in acoustic hole patterns containing seventy-five holes or more per square inch (about twelve holes or more per square centimeter) of treated surface. Given the large number of holes necessary to acoustically treat airflow surfaces of acoustic panels, rapid and economical methods for producing the holes are desirable. 
         [0004]    A process currently employed to produce acoustic skins is to perforate a flat aluminum sheet stock, such as by punching, to have the desired acoustic hole pattern, after which the sheet stock is formed to produce the arcuate shape required for the nacelle. Multiple heat treatments and forming steps are typically performed to reduce the likelihood of tearing the sheet stock during forming. During forming, the holes tend to elongate, often in a nonuniform manner such that the holes do not consistently have the same cross-sectional shape. While likely acceptable and adequate for many applications, an acoustic skin with nonuniform holes or a nonuniform hole pattern is likely to have unpredictable sound absorption performance that does not meet design requirements for more demanding applications. Other problems arise as a result of the holes often being punched in large sheet stock, which must be of sufficient size for the intended nacelle. The holes are typically punched over the full surface of the sheet, except for a small border along the edges of the sheet stock. As a result of the forming operation, holes may be present where none are desired for structural reasons. Though this problem can be solved by the use of doublers and reinforcements to maintain structural integrity, the solution comes with weight, part count, and cost penalties. 
         [0005]    Another common process for producing acoustic hole patterns is to mechanically drill the holes in the surface of the acoustic skin after it has been formed. While this method overcomes the problems associated with fabricating acoustic skins from pre-perforated sheets, it requires the use of special tooling and machinery to place the holes in the proper orientation on the contoured non-planar skin. Though special-purpose machines designed to drill specific parts may be capable of as many as twenty-five holes per second, state-of-the-art mechanical drilling machines are typically limited to drilling about four holes per second. In addition to speed limitations, mechanical drilling processes tend to be expensive due to the special tooling and machinery required. 
         [0006]    A more recent method for producing acoustic hole patterns is to employ an electron beam drilling technique, as reported in U.S. Pat. No. 6,358,590 to Blair et al. Face sheets (acoustic skins) with holes having diameters and spacings of up to 0.020 inch and 0.11 inch, respectively, are disclosed. Furthermore, Blair et al. teach that a decreasing hole diameter (in the direction toward the skin surface) is necessary. It appears unclear as to whether Blair et al. drill the face sheet before or after forming. Blair et al. also do not describe the fixturing for the face sheet during drilling, other than that a backing sheet is used. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The present invention provides a process for producing multiple through-holes in a sheet member to form a perforated article, such as an arcuate (non-planar) acoustic skin suitable for use in an acoustic panel of an aircraft engine nacelle. 
         [0008]    According to one aspect of the invention, the process includes deforming a sheet member to have an arcuate shape with an arcuate surface, mounting and rotating the arcuate-shaped sheet member on a mandrel and then, while rotating the arcuate-shaped sheet member, directing an electron beam at the arcuate surface of the rotating arcuate-shaped sheet member and deflecting the electron beam toward multiple locations on the arcuate surface to produce the multiple through-holes through the arcuate-shaped sheet member in a defined hole pattern and thereby yield a perforated arcuate-shaped sheet member with holes having axes substantially normal to the arcuate surface. 
         [0009]    Significant advantages of this invention include the ability to controllably and consistently produce holes with the same cross-sectional shape, which may be a circular, elliptical, or slot shape transverse to the axis of each hole. Holes with various longitudinal cross-sections can also be generated, such as rectilinear, tapered, bell, and hourglass shapes. In addition, very small holes of consistent size, for example, as small as about 0.001 inch (about 25 micrometers), can be economically produced. Another advantage is the ability to rapidly drill small holes in a contoured surface at very high rates, for example, rates as high as about two hundred holes per second, depending on the diameter, spacing, and geometry of the holes. 
         [0010]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a scanned image of a section of an aluminum acoustic skin for an aircraft engine nacelle, in which holes were produced in accordance with an embodiment of this invention. 
           [0012]      FIG. 2  schematically represents a cross-section through the acoustic skin of  FIG. 1 . 
           [0013]      FIG. 3  schematically represents an electron beam drilling system suitable for performing an electron beam drilling operation in accordance with an embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1  shows a region of an acoustic sheet or skin  10  of a type intended to face the airflow through an aircraft engine nacelle. The skin  10  was produced by processing steps of this invention to have a well-defined pattern  14  of small, equi-spaced perforations or holes  12  on a curved surface  16  of the skin  10 , as represented in  FIG. 2 . The skin  10  may be formed from a variety of materials, a notable example being an aluminum alloy, though the use of other materials is also foreseeable and within the scope of this invention. Particular applications of interest to the invention include, but are not limited to, thrust reverser core cowl skins and duct sidewall skins of high-bypass turbofan engines, such as the General Electric CF34-10. While the invention will be described in reference to the use of the skin  10  in an acoustic panel of an aircraft engine, it should be understood that the invention is applicable to a variety of components having contoured (non-planar) surfaces in which numerous small holes are to be formed. 
         [0015]    As represented in  FIG. 2 , the holes  12  extend entirely through the skin  10 , which has a typical thickness of about 0.038 to about 0.045 inch (about 0.95 to about 1.15 mm), though lesser and greater thicknesses are foreseeable. As noted above, the skin  10  is intended for use in an acoustic panel, and as such will be assembled with an open-cell core (not shown) disposed between the acoustic skin  10  and a backing skin (not shown). The acoustic skin  10  can be used with entirely conventional cores and backing skins, including such known materials as open-cell foam or honeycomb metallic or composite materials for the core and metallic or composite sheets for the backing skin. As such the core and backing skin to which the acoustic skin  10  is to be attached (for example, by adhesive bonding) will not be described or discussed in any further detail here. 
         [0016]    According to a preferred aspect of the invention, the holes  12  are formed by an electron beam (EB) drilling process configured for rapidly drilling large numbers of small diameter holes  12  in a well-defined hole pattern  14 . As known in the art, EB drilling entails focusing a highly-concentrated energized electron beam onto a substrate surface to vaporize the substrate material at the point of impact. Depending on the EB drilling equipment used, multiple holes  12  can be sequentially formed be deflecting the electron beam. The holes  12  can be formed to have diameters of less than 0.03 inch (about 0.75 mm) and as small as about 0.001 inch (about 25 micrometers), which is far less than the more conventional hole diameter range of about 0.03 to about 0.06 inch (about 0.75 to about 1.5 mm). However, particularly suitable hole diameters are believed to be greater than 0.020 inch (about 0.5 mm) up to about 0.045 inch (about 1.1 mm). Benefits of minimizing the diameter of the holes  12  generally include lower sensitivity to grazing flow effects, lower noise generated as a result of the skin  10  behaving as a resonator, lower skin friction drag, improved surface appearance, and impeded water ingress into the core. 
         [0017]    To fit the required arcuate contour of a nacelle, the skin  10  is likely to have a radius of curvature of, for example, about twelve to sixty inches (about 30 to about 150 cm) or more. According to a preferred aspect of the present invention, the acoustic skin  10  is formed to have its requisite curved shape prior to forming the holes  12 , and the holes  12  are not parallel to each other but instead are oriented to have an axis roughly normal to the surface  16  of the acoustic skin  10 , preferably within about five degrees of perpendicular to the skin surface  16 . As such, the holes  12  cannot be drilled with a tool whose transverse movements are limited to a simple two-dimensional pattern, but instead further require the capability of maintaining an orientation roughly normal to the skin surface  16 . Preferred EB drilling processes and systems employed by this invention utilize computer controls to not only control process parameters that determine the hole size, hole cross-section, drilling speed, etc., but also orientation of the skin  10  to the electron beam gun that generates the electron beam used in the drilling process.  FIG. 3  schematically represents an EB drilling system  20  suitable for this purpose, in which the skin  10  is loaded on a rotating mandrel  22  (such as a fixtured shaft or spindle) and positioned in an evacuated (vacuum) chamber  24  within which the EB drilling process is performed. The rotation, translation and orientation of the skin  10  within the chamber  24  is determined with a positioning drive system  32 , such as a five-axis positioning drive system controlled by a computer  34  (or other control device). A power supply  36  delivers power to an EB gun  26 , whose operation can be controlled with the computer  34  in relation to the drive system  32  to direct an electron beam  28  roughly along a radial of the curvature of the skin  10 . The path that the beam  28  travels to the skin  10  can be precisely controlled with a deflection coil  30  in accordance with known practices. In combination, the rotation and translation of the mandrel  22  and the deflection of the beam  28  are controlled to drill holes  12  through the skin  10  at desired locations. Deflection of the beam  28  can be rapidly and precisely performed to control the placement of the holes  12  at rates far exceeding mechanical drilling rates. Process parameters that are controlled to determine the hole size, hole cross-sectional shape, drilling speed, etc., include energy (pulse time and beam current) and the position and manipulation (rotational and/or translational speeds) of the skin  10  in the vacuum chamber  24 . 
         [0018]    Drilling acoustic skins  10  by the electron beam process described above provides several advantages over conventional punching and mechanical drilling processes. A general advantage is the ability to rapidly drill the holes  12  approximately normal to the contoured surface  16  of the skin  10  at very high rates, for example, up to two hundred holes per second or more. A more particular advantage is that, by forming the skin  10  prior to drilling the holes  12 , the formed skin  10  is produced with holes  12  having essentially the same cross-sectional shape. Because the skin  10  does not undergo deformation after the holes  12  are drilled, the holes  12  do not elongate in a nonuniform manner, with the result that the holes  12  consistently have the same cross-sectional shape, for example, a circular, elliptical, or slot shapes transverse to the axes of the holes  12  (and as visible at the surface  16  of the skin  10 ). Additionally, various longitudinal cross-sections can be generated with an EB beam, including the rectilinear shape shown in  FIG. 2 , as well as tapered, bell-shapes, and hourglass-shapes. However, EB drilling will typically create a slightly larger entrance area to the hole  12  as a result of the initial beam pulsing. Electron beams are also capable of drilling very small holes, for example, on the order of about 0.001 inch (about 25 micrometers), which cannot be economically produced by conventional mechanical punching and drilling methods. 
         [0019]    In investigations leading to the present invention, various aluminum alloy test specimens and acoustic skins were processed as described above to have holes with diameters in a range of about 0.035 to about 0.055 inch (about 0.9 to about 1.4 mm). One such specimen is shown in  FIG. 1  as having equi-spaced holes with diameters of about 0.055 inch (about 1.4 mm) and center-to-center spacings of about 0.128 inch (about 3.25 mm). Because the holes were formed after shaping the skin, the cross-sectional shapes of the holes are substantially circular and the same throughout. Examination of the hardware for metallurgical and mechanical properties indicated that the process is capable of producing acoustic skins whose mechanical properties may meet or exceed those of conventionally drilled skins. For example, because the holes are round instead of oblong (as would result from conventionally performing the forming operation after drilling the holes), the skins are capable of exhibiting tensile strength and edge-wise compression strength that are roughly the same in both the hoop and axial directions. The holes were also free of tiny fissures and cracks often observed in conventionally formed acoustic hardware. 
         [0020]    Notably, because an acoustic skin is preformed to the desired nacelle shape prior to EB drilling, the holes can be selectively located where required for the particular nacelle application, whereas other surface regions of the skin can be left undrilled to promote the structural integrity of the skin and the acoustic panel in which the skin is assembled. An additional benefit of this process is the ability to economically alter the acoustic treatment hole pattern ( 14  in  FIG. 1 ) with respect to acoustic waves within the nacelle structure. In other words the hole pattern  14 , as defined by the size, shape and spacing of the holes  12 , may be altered to tune a specific area or areas of a flow path surface to attenuate specific frequency bands of noise. 
         [0021]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the acoustic skin could differ from that shown, and materials other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.