Abstract:
A method for casting a cooled component includes molding a sacrificial pattern. A plurality of holes are formed through the pattern. A shell is formed over the pattern including filling the holes. The pattern is destructively removed from the shell. A metallic material is cast in the shell. The shell is destructively removed.

Description:
The invention relates to turbine engines. More particularly, the invention relates to casting of cooled thin-wall components of gas turbine engines. 
   Gas turbine engine combustor components such as heat shield and floatwall panels are commonly made of polycrystalline alloys. These components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components to provide durability. For example, to provide cooling of heat shield panels, the panels often include arrays of film cooling holes at angles off-normal to the surface facing the combustor interior. A low (shallow) angle through the panel (large off-normal angle) wall increases the surface area exposed to the air passing through the holes and, thereby, increases convective cooling. A low discharge angle provides the film cooling as the flow passes along the surface. Such cooling holes may be drilled in the cast panel (e.g., by laser drilling). 
   SUMMARY OF THE INVENTION 
   One aspect of the invention involves a method for casting including molding a sacrificial pattern. After the molding, a plurality of holes are formed through the pattern. A shell is formed over the pattern including filling the holes. The pattern is destructively removed from the shell. A metallic material is cast in the shell. The shell is destructively removed. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal sectional view of a gas turbine engine combustor. 
       FIG. 2  is a view of an inboard heat shield panel of the combustor of  FIG. 1 . 
       FIG. 3  is a view of an outboard heat shield panel of the combustor of  FIG. 1 . 
       FIG. 4  is a cross-sectional view of film cooling holes in one of the heat shield panels of  FIGS. 2 and 3 . 
       FIG. 5  is a sectional view of a pattern along with an apparatus for forming the film cooling holes. 
       FIG. 6  is a cross-sectional view of the pattern of  FIG. 5  after a first shelling stage. 
       FIG. 7  is a sectional view of a shell formed using the pattern of  FIG. 6 . 
       FIG. 8  is a sectional view of a pattern in a pattern forming die including an inserted probe array. 
       FIG. 9  is a sectional view of the pattern of  FIG. 8  with the probe array retracted. 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a gas turbine engine combustor  20 . The exemplary combustor  20  is generally annular about an engine central longitudinal axis (centerline)  500  parallel to which a forward direction  502  is illustrated. The exemplary combustor has two-layered inboard and outboard walls  22  and  24 . The walls  22  and  24  extend aft/downstream from a bulkhead  26  at an upstream inlet  27  receiving air from the compressor section (not shown) to a downstream outlet  28  delivering air to the turbine section (not shown). A circumferential array of fuel injector/swirler assemblies  29  may be mounted in the bulkhead. 
   The bulkhead includes a shell portion  30  and a heat shield  31  spaced aft/downstream thereof. The heat shield  31  may be formed by a circumferential array of bulkhead panels, at least some of which have apertures for accommodating associated ones of the injector/swirler assemblies. The combustor has an interior  34  aft/downstream of the bulkhead panel array. The inboard and outboard walls  22  and  24  respectively have an outboard shell  35  and  36  and an inner heat shield  37  and  38 . The shells may be contiguous with the bulkhead shell. Each exemplary wall heat shield is made of a longitudinal and circumferential array of panels as may be the shells. In exemplary combustors there are two to six longitudinal rings of six to twenty heat shield panels. From upstream to downstream, respective panels of the shields  37  and  38  are identified as  37 A-E and  38 A-E. With reference to the exemplary panel  37 C, each panel has a generally inner (facing the interior  34 ) surface  40  and a generally outer surface  42 . Mounting studs  44  or other features may extend from the other surface  42  to secure the panel to the adjacent shell. The panel extends between a leading edge  46  and a trailing edge  48  and between first and second lateral (circumferential) edges  50  and  52  ( FIG. 2 ). The panel may have one or more arrays of process air cooling holes  54  between the inner and outer surfaces and may have additional surface enhancements (not shown) on one or both of such surfaces as is known in the art or may be further developed. 
   The inner surface  40  is circumferentially convex and has a center  60 .  FIG. 1  further shows a surface normal  510  and a conewise direction  512  normal thereto. The exemplary panel has a conical half angle θ 1 , a longitudinal span L 1 , and a conewise span L 2  ( FIG. 2 ). A radial direction is shown as  514 . A circumferential direction is shown as  516 . An angle spanned by the panel between the lateral edges about the engine centerline is shown as θ 2 . With an exemplary eight panels per ring, θ 2  is nominally 45° (e.g., slightly smaller to provide gaps between panels). 
   Similarly, the exemplary panel  38 C has inner and outer surfaces  80  and  82 , leading and trailing edges  84  and  86 , and lateral edges  88  and  90  ( FIG. 3 ). The inner surface  80  is circumferentially concave and has a center  100 . A surface normal is shown as  520  and a conewise direction shown as  522 . The conical half angle is shown as −θ 3  (for reference, a negative angle will be associated with a rearwardly convergent cone) and the longitudinal span is shown as L 3 . A circumferential direction is shown as  524  in  FIG. 3 . A circumferential span is shown as θ 4  and the conewise span is shown as L 4 . 
     FIG. 4  shows a main body wall portion  150  of an exemplary one of the panels (e.g., of the shields  37  and  38  or the bulkhead shield  31 ). The main portion has a local thickness T between an outboard surface portion  152  and the adjacent inboard surface portion  154  (e.g., of the surfaces  40  or  80 ). An array of film cooling holes or channels  160  extend between inlets  162  in the surface  152  and outlets  164  in the surface  154 . The exemplary holes  160  are straight, having central longitudinal axes  530 . Exemplary holes  160  have circular cross-sections normal to the axis  530  and having a diameter D. The holes  160  extend off-normal to the local inboard surface portion  154  by an angle θ 5 , thus being off the surface portion  154  by θ 6 , the complement of θ 5 . The holes  160  may be grouped in regular or irregular arrays and may be distributed to provide a desired cooling profile. Exemplary θ 5  are in excess of 45° (e.g., 50-70°) so that discharged air flows  170  provide a film cooling effect. 
     FIG. 5  shows a molded wax pattern  180  having the overall form of the heat shield panel but molded without the cooling holes. For example, the pattern may be molded with portions corresponding to the panel main body, the process air cooling holes, perimeter and internal outboard reinforcement rails, and the like. After molding, features corresponding to the film cooling holes  160  may then be formed.  FIG. 5  specifically shows a heated array  182  of probes  184  inserted into the pattern in a direction  540  (parallel to the ultimate axes  530 ) to form holes  185  corresponding to the cooling holes  160 . To maintain pattern integrity, a backing element  186  may be placed along one of the faces of the pattern. The backing element  186  may be pre-formed with apertures for receiving tip portions  188  of the probes as they pass through the pattern. Alternatively, the backing element  186  may be deformable to accommodate the tip portions. After insertion, the probe array may be retracted in the opposite direction. The probe array may displace material to create the holes  185 . This may leave elevations  190  at one or both faces. The elevations  190  may be trimmed. Alternatively, the probes may be hollow and may evacuate the displaced material. 
   There may be multiple groups of the holes  185 . As noted above, the holes of the individual groups may have parallel axes. The holes of the different groups may have axes parallel to the axes of the holes of the other groups or not parallel thereto. For example, non-parallel axes may be appropriate to achieve desired flow patterns in the ultimate cast panel. Other drilling techniques for forming the holes  185  may be used including mechanical twist drilling. The holes  185  may be formed individually or simultaneously in groups as noted above. 
   After the holes  185  are formed in the pattern, the pattern may be shelled in a multi-stage stuccoing process.  FIG. 6  shows the pattern  180  after a first slurry dip in the shelling process. The initial dip is typically in a thin and fine slurry to provide a smooth final interior surface for the ultimate shell.  FIG. 6  shows a layer  200  of this slurry on both faces of the pattern main body and substantially filling the holes  185  (e.g., due to surface tension, having slight recesses  202  at the ends of the holes). Further shelling steps may involve thicker and coarser slurries. After the final shelling step, the shell may be permitted to dry. The wax may be removed such as by a steam autoclave and/or shell firing (to harden the shell). 
     FIG. 7  shows the shell  210  after wax removal. The shell has first and second sidewalls  212  and  214 . Shell features  216 , formed in the pattern holes  185  connect the sidewalls  212  and  214  by spanning the shell interior  218 . Upon introduction of cast metal to the shell interior  218 , the spanning features  216  form and define the film cooling holes  160 . After the pouring and metal solidification, the shell may be destructively removed (by mechanical and/or chemical means). An exemplary removal involves mechanically breaking away the sidewalls  212  and  214  and then chemically (e.g., by an acid or alkaline leaching) removing the spanning features  216 . 
   An alternative method of manufacture pre-forms the holes in the pattern as the wax material is molded. An array of probes or tines  250  (FIG.  8 —similarly arranged to the array  182 ) may be formed on a slider element  252  of the pattern molding die  254 . The slider  252  is inserted into one of the main elements  256  of the die during die assembly and the wax  258  is molded around the slider probes  250 . After wax cooling/hardening, the slider is then retracted ( FIG. 9 ) to disengage the probes  250  from the pattern, leaving the holes  160  and releasing a backlocking of the pattern relative to the main element  256 . 
   The present methods may have one or more of several advantageous properties and uses. Mechanical drilling of cooling holes in a casting is increasingly difficult as the off-normal angle increases. Thus, casting may be particularly useful for providing film cooling holes. Additionally, the spanning features  216  may tend to maintain the relative positions of the sidewalls  212  and  214  during casting. This may provide improved consistency of the thickness T among castings and uniformity of the thickness T within given castings. With such improved uniformity, the practicability of making a relatively thin casting is improved. 
   For a combustor heat shield, an exemplary thickness T is advantageously less than 0.08 inch (2.0 mm). More broadly, the thickness may be less than 0.12 inch (3.0 mm) or 0.10 inch (2.5 mm). In an exemplary reengineering or remanufacturing situation, the panel is engineered or manufactured as a drop-in replacement for an existing panel having drilled film cooling holes. In this reengineering/remanufacturing situation, the final thickness T may be approximately 0.06 inch (1.5 mm) compared with a baseline thickness in excess of 0.08 inch (2.0 mm). For an exemplary panel thickness in the 0.06-0.08 inch (1.5-2.0 mm) range, an exemplary diameter D is less than about 0.032 inch (0.81 mm). Although particularly fine passageways maybe more desirable, shell integrity issues may mitigate in favor of a diameter 0.018-0.030 inch (0.46-0.76 mm) range. More broadly, this diameter is advantageously less than the thickness and, more advantageously less than half the thickness. For non-circular sectioned holes, hole cross-sectional areas may be compared with the areas corresponding to these diameters. For the 0.46-0.81 mm diameter range corresponding areas are 0.16-0.52 mm 2 . A narrower range would be 0.20-0.46 mm 2 . 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the principles may be applied to manufacture of exhaust nozzle liners and other thin wall cast structures. Where applied as a reengineering of an existing component, details of the existing component may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.