Abstract:
An article may be manufactured by providing a reticulate core element in a mold shell having a shape at least partially corresponding to a shape of the article. The molten metallic material is introduced to the shell so as to at least partially infiltrate into the reticulate core element. The molten metallic material is permitted to solidify. The shell and the reticulate core element are destructively removed. The removal leaves the article with one or more gas-permeable porous regions.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to investment casting. More particularly, the invention relates to investment casting of cooled parts. 
   Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. 
   Gas turbine engines are widely used in aircraft propulsion, electric power generation, ship propulsion, and pumps. In gas turbine engine applications, efficiency is a prime objective. Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures in the turbine section exceed the melting points of the superalloy materials used in turbine components. Consequently, it is a general practice to provide air cooling. Cooling is typically provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. Such cooling comes with an associated cost in engine efficiency. Consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. This may be obtained by the use of fine, precisely located, cooling passageway sections. 
   A well developed field exists regarding the investment casting of internally-cooled turbine engine parts such as blades/vanes, seals/shrouds, and combustor components. In an exemplary process, a mold is prepared having one or more mold cavities, each having a shape generally corresponding to the part to be cast. An exemplary process for preparing the mold involves the use of one or more wax patterns of the part. The patterns are formed by molding wax over ceramic cores generally corresponding to positives of the cooling passages within the parts. In a shelling process, a ceramic shell is formed around one or more such patterns in well known fashion. The wax may be removed such as by melting in an autoclave. The shell may be fired to harden the shell. This leaves a mold comprising the shell having one or more part-defining compartments which, in turn, contain the ceramic core(s) defining the cooling passages. Molten alloy may then be introduced to the mold to cast the part(s). Upon cooling and solidifying of the alloy, the shell and core may be thermally, mechanically, and/or chemically removed from the molded part(s). The part(s) can then be machined, treated, and/or coated in one or more stages. 
   The ceramic cores themselves may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into hardened metal dies. After removal from the dies, the green cores are thermally post-processed to remove the binder and fired to sinter the ceramic powder together. The trend toward finer cooling features has taxed core manufacturing techniques. The fine features may be difficult to manufacture and/or, once manufactured, may prove fragile. Commonly-assigned co-pending U.S. Pat. No. 6,637,500 of Shah et al. discloses exemplary use of a ceramic and refractory metal core combination. Other configurations are possible. Generally, the ceramic core(s) provide the large internal features such as trunk passageways while the refractory metal core(s) provide finer features such as outlet passageways. 
   U.S. Pat. No. 4,789,140 discloses ceramic foam filtering material compatible with the casting of superalloys. U.S. Pat. No. 4,697,632 discloses use of such material in forming a core having a smooth exterior face. U.S. Pat. No. 6,648,596 discloses an airfoil having a tip region including a ceramic foam. 
   U.S. Pat. No. 6,544,003 discloses a turbine engine blisk having airfoils made at least in part of an open-cell solid ceramic foam. 
   SUMMARY OF THE INVENTION 
   One aspect of the invention involves the method for manufacturing an article. A reticulate core element is provided in a mold shell having a shape at least partially corresponding to a shape of the article. A molten metallic material is introduced to the shell so as to at least partially infiltrate into the reticulate core element. The molten metallic material is permitted to solidify. The shell and the reticulate core element are destructively removed. The removal of the reticulate core element leaves the article with one or more gas-permeable porous regions. 
   In various implementations, the porous regions may be chemically expanded. The solidified metallic material may be integrated with a metallic substrate. The method may be used to make a turbine engine blade outer air seal wherein the solidified material forms an exterior surface portion of the seal. The method may be used to make a turbine engine airfoil element wherein the solidified material forms an exterior surface portion of the airfoil or of an element platform. The reticulate core element may be formed by one or more of: coating reticulate organic material with a slurry (e.g., ceramic or metallic) and then firing; coating a reticulate organic material with a metallic layer; and coating a reticulate metallic material with a slurry and then destructively removing the reticulate metallic material. The reticulate core element may have a first region of essentially a first characteristic pore size (alternatively another porosity characteristic such as a volume fraction of porosity) and a second region of essentially a second characteristic pore size (or other porosity characteristic) smaller than the first characteristic pore size. The reticulate core element may be integrated with a non-reticulate core element. The non-reticulate core element may form one or more feed passageways. The porous regions may be outlet passageways in communication with the one or more feed passageways. The method may be used to manufacture a gas turbine engine component. 
   Another aspect of the invention involves a sacrificial investment casting core comprising a reticulate first portion and a non-reticulate second portion. 
   In various implementations, the second portion may be shaped for forming one or more feed passageways in a turbine airfoil element. The first portion may be shaped for at least partially forming one or more outlet passageways from the one or more feed passageways. The first portion may protrude from the second portion. The first portion may be secured to the second portion via a ceramic layer. The first portion may be secured to the second portion via a mechanical back-locking of the first portion relative to the second portion. The first portion may be secured to the second portion via one or more pins having portions received in recesses in each of the first and second portions. The first portion may be held spaced-apart from the second portion with a gap therebetween. 
   Such a core may be manufactured by a method including at least one of: securing the second portion to the first portion via a ceramic adhesive; welding the second portion to the first portion; joining the first and second portions via one or more pins received in associated recesses in the first and second portions; and/or other suitable methods. 
   Another aspect of the invention involves an article of manufacture having a cast metallic substrate with a cooling passageway system within the substrate. The cooling passageway system includes one or more feed passageways for receiving cooling gas. The cooling passageway system includes one or more outlet passageways for discharging the cooling gas from the one or more feed passageways and having a reticulate passageway portion. 
   In various implementations, the substrate may form a major, by weight, portion of the article. The metal of the substrate may be a single continuous piece of an alloy. The metal of the substrate may be an iron-, nickel-, or cobalt-based superalloy. The article may be a turbine element having an airfoil extending between inboard and outboard ends and having pressure and suction side surfaces. The reticulate portion may be located within a sidewall of the airfoil. The reticulate portion may form a trailing edge outlet of the airfoil. The reticulate portion may be located within a platform of the turbine element. There may be multiple such reticulate portions in multiple such locations. 
   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 view of a gas turbine engine blade according to principles of the invention. 
       FIG. 2  is a sectional view of a platform of the blade taken along line  2 — 2 . 
       FIG. 3  is a view of a tip region of the blade of  FIG. 1 . 
       FIG. 4  is a mean sectional view of the tip region of  FIG. 3 . 
       FIG. 5  is a streamwise sectional view of a trailing edge portion of an airfoil of the blade of  FIG. 1 . 
       FIG. 6  is a transverse sectional view of an alternate tip region. 
       FIG. 7  is a partial streamwise sectional view of an intermediate portion of the airfoil of a blade of  FIG. 1 . 
       FIG. 8  is a view of a blade outer air seal. 
       FIG. 9  is a sectional view of the blade outer air seal of  FIG. 8 , taken along line  9 — 9 . 
       FIG. 10  a sectional view of an alternate blade outer air seal. 
       FIG. 11  is a view of a core for forming the blade outer air seal of  FIG. 10 . 
       FIG. 12  is a view of a gas turbine engine vane. 
       FIG. 13  is a partial streamwise sectional view of the airfoil of the vane of  FIG. 12 , taken along line  13 — 13 . 
       FIG. 14  is a sectional view of a first core attachment. 
       FIG. 15  is a sectional view of a second core attachment. 
       FIG. 16  is a sectional view of a third core attachment. 
       FIG. 17  is a sectional view of a fourth core attachment. 
       FIG. 18  is a sectional view of a fifth core attachment. 
       FIG. 19  is a sectional view of a sixth core attachment. 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a gas turbine engine blade  20  (e.g., from a turbine section of the engine). The blade may comprise a unitarily-formed metallic casting, optionally coated for thermal and/or chemical protection. The general configuration of the exemplary blade  20  is considered merely illustrative. The blade includes an airfoil  22  extending from a root  24  at a platform  26  to a tip  28 . The airfoil has pressure and suction side surfaces  30  and  32  extending between leading and trailing edges  34  and  36 . A disk attachment portion (e.g., a so-called fir tree blade attachment root)  38  depends from the underside of the platform  26  (e.g., depending/inboard/underside indicating a direction generally toward rather than away from the engine centerline). As thus described, the blade may be of a variety of known or yet-developed general configurations. 
   The exemplary blade  20 , however, may include one or more of several improvements for encouraging heat transfer and/or controlling weight.  FIG. 1  shows a leading portion  40  of the platform  26  including an enhanced heat transfer region  42 . In the exemplary embodiment, the enhanced heat transfer region  42  is circumferentially elongate, extending between first and second ends  44  and  46  proximate first and second circumferential sides of the platform  26 , and between a leading side  48  ( FIG. 2 ) near a leading end  50  of the platform and a trailing side  52  near the airfoil. In the enhanced heat transfer region  42 , the metal of the casting is reticulated, providing the region with degrees of porosity and permeability. The region  42  has an outboard boundary  54  generally continuous with an adjacent outboard surface  56  of the platform  26 . The region  42  has an inboard boundary  58  within the platform  26 . 
   In the exemplary embodiment, although unitarily formed with a remainder of the casting, the region  42  appears as if captured within a compartment of non-reticulated metal having a base  60  and a lateral perimeter  62 . In alternative implementations, the reticulated metal may be separately formed (e.g., by infiltrating a reticulate preform of final or near final shape and then removing the preform or by cutting/machining from an undifferentiated metallic foam block or other piece). It may then be integrated with the unreticulated metal (e.g., a casting) such as by welding or diffusion bonding. In the exemplary embodiment, this virtual compartment is not a blind compartment and thus the region  42  is not blindly within the unreticulated metal. A passageway  64  having a perimeter surface  66  extends from the region  42  to the inboard surface or underside  68  of the platform  26 . The passageway  64  allows gas communication from the underside  68  through the passageway  64  and through the region  42  to exit the boundary  54 . Depending on pressure gradient, communication may be in a reverse direction. With communicating gas consisting essentially of cooling air, the high specific surface area of the region  42  enhances heat transfer to cool the platform. In some alternative embodiments, the region  42  may extend fully between inboard and outboard platform surfaces  68  and  56 . In other alternative embodiments, the region  42  may be blind (e.g., lacking communication with a passageway). With such a blind situation, there may still be a moderate degree of enhanced heat transfer between the region  42  and gas flowing over the surface  56  and boundary  54 . 
   As is described in further detail below, the region  42  may be formed by the use of a reticulated casting core element (e.g., a ceramic foam such as alumina, silica, zirconia, and/or zircon). With such an element incorporated into a shelled investment casting pattern (which may also include one or more substantially non-reticulated ceramic or other cores), upon ultimate casting, the metal infiltrating the reticulated ceramic core will have an essentially inverse reticulation. After solidification of the metal, the ceramic may be chemically removed, leaving the region  42  with porosity and permeability. The porosity and permeability may further be enhanced by subsequent chemical etching of the as-cast metal of the region  42 . 
   Exemplary reticulated foam for such casting cores resembles an interconnected three dimensional web interspersed with interconnected cellular voids (or pores in the foam). When the metal is cast into the foam, the metal takes the form of the voids. When the foam is removed, the pores in the metal have the elongate interconnected web structure of the foam. Exemplary foam has a pore size from fifty pores per inch (ppi) up to five ppi. A narrower exemplary range is from 30 ppi to 10 ppi. With many reticulated foams, the thickness of the foam material between pores increases or decreases with pore size. Thus a foam material with a smaller pore size (e.g., 50 ppm) will tend to cast a metal part with finer passageways, greater specific surface area (and thus heat transfer), and greater resistance to flow than material with a larger pore size. The pore/passageway size of the cast metal may be characterized in several ways. One parameter involves taking a section through the casting and measuring the linear dimensions of the pores along the section. Due to the elongate nature of the pores, one may look to minimum transverse dimensions as characteristic of dimensions perpendicular to the length of the passageway. In an exemplary inspection of a casting made from 30 ppi foam, transverse dimensions were in the vicinity of (300–1100) micrometer, averaging close to (550) micrometer. With 20 ppi foam, they were (350–1300) micrometer, avenging close to (760) micrometer. With 10 ppi foam, they were (800–1700) micrometer, averaging close to (1000) micrometer. 
   Another parameter, however, is the volume fraction of porosity. For this parameter, the properties of the cast part will vary inversely with those of the foam core. Thus, to achieve a cast part volume fraction of porosity of 10%, the foam core would have a volume fraction of porosity of essentially 90%. Exemplary as-cast volume fractions of porosity are 10–50%, more narrowly 15–30%. 
     FIG. 3  shows a further enhancement in the form of a reticulated region  80  within a portion of the airfoil adjacent the tip  28 . The region  80  has an outboard boundary  82  and a perimeter  84  circumscribed by a wall region  86  of unreticulated material. The region  80  has an inboard boundary  88  ( FIG. 4 ) adjacent internal feed passageways  90  within the blade. In operation, cooling air from the passageways  90  enters the inboard/interior boundary  88 , passing through the region  80  and exiting the outboard/exterior boundary  82 . The exemplary inboard boundary  88  may also be continuous with outboard ends of walls  92  separating the passageways  90 . 
     FIG. 4  shows a further enhancement in the form of a reticulated region  100  defining an outlet slot from a trailing one of the passageways  90 . The exemplary reticulated region  100  extends between an inboard end near the platform  26  and an outboard end near the tip  28  and has leading and trailing extremities/boundaries  102  and  104  and pressure and suction side extremities/boundaries  106  and  108  ( FIG. 5 ). 
   The regions  80  and  100  may be formed in a similar manner to the region  42  of  FIG. 1 . For example, correspondingly-shaped ceramic foam precursors may initially be manufactured. These may be joined to non-reticulated ceramic cores for forming the feed passageways and/or to additional exterior ceramic components. These exterior ceramic components may serve to position the reticulated cores and feed cores during wax overmolding for forming the pattern and may become embedded in the applied ceramic shell. 
     FIG. 6  shows an alternate tip reticulated region having subregions  120  and  122  of different porosity/permeability. In the exemplary embodiment, the reticulated region forms a tip portion of the airfoil in communication with the passageways  90 . The proximal subregion  120  spans pressure and suction sidewall portions  124  and  126  and shares an outboard boundary  130  where an inboard boundary of the distal/outboard subregion  132 . The exemplary reticulated region includes surface/boundary portions continuous with remainders of the pressure and suction side surfaces  30  and  32 . In the exemplary embodiment, the inboard/proximal subregion  120  has a higher porosity (e.g., larger and/or more numerous voids) than the outboard/distal subregion  122 . Thus, air may pass more freely from the passageway through the inboard/proximal subregion  120  than subsequently through the outboard/distal subregion  122 . A principal portion of the air may exit the pressure and suction side boundary portions of the inboard/proximal subregion  122  with a lesser portion passing through the boundary  130  and exiting from the pressure and suction side boundaries of the second subportion  122  and its outboard extremity/boundary  132 . The lower porosity of the second subportion  122  may also provide it with a greater strength and abrasion-resistance than the first subportion  120 . The porosity of the second subportion  122  may thus be optimized to provide a desired degree of destructive deformation upon contact with a rub strip (so as to avoid other damage to the engine) while providing an appropriate degree of strength to allow for continued operation (including the possibility of further rub strip contact further attritting the second subportion  122 ). Such an embodiment may be created by securing two reticulated cores to the non-reticulated cores and the additional exterior components. The core forming the first subportion  120  would have a smaller porosity than the reticulated core forming the second subportion  122 . 
     FIG. 7  shows yet alternate reticulated regions  140  and  142  within the pressure and suction side sidewalls  124  and  126 . Each of these reticulated regions have outboard boundaries contiguous with remaining portions of the associated pressure or suction side surface  30  and  32  and inboard boundaries along the associated passageway(s)  90 . In an exemplary embodiment, these reticulated regions may be spanwise elongate along the blade.  FIG. 1  shows each region  140  extending along a major portion of the blade length. Shorter regions (e.g., extending over at least 20% or 30% of the span) are also possible. 
     FIG. 8  shows a blade outer air seal (BOAS)  200  having a main body  202  with an inboard surface  204 , an outboard surface  206 , circumferentially-extending fore and aft ends/extremities  208  and  210 , and longitudinally-extending ends/extremities  212  and  214 . Groups of L-sectioned mounting brackets  216  and  218  are unitarily formed with the body and extend from the outboard surface  206  near the fore and aft ends. In the exemplary embodiment, an array of cooling holes  220  penetrates the inboard surface  204  and an array of holes  222  penetrate each of the circumferential end surfaces  212  and  214 . The exemplary holes  220  have centerlines oriented off-normal to the surface  204  to provide desired film cooling flows. The holes  220  and  222  communicate with a central plenum  230  ( FIG. 9 ). The plenum  230  separates inboard and outboard body wall portions  232  and  234 . For additional cooling, the outboard portion is provided with reticulated regions  240  having outboard boundaries continuous with remaining portions of the outboard surface  206 . The exemplary reticulated regions  240  may be of rectangular, circular, square, or other cross-section. In the exemplary embodiment, the reticulated regions  240  are open to the plenum  230  by means of one or more passageways or channels  242 . The reticulated regions  240  and passageways  242  may provide a cooling air inlet flow to the plenum for feeding outlet flows through the holes  220  and  222 . Variations and exemplary methods of manufacture may be similar to those described above for blade reticulated regions. By way of example, a non-reticulated ceramic core may form the plenum  230 . Unitarily formed therewith or secured thereto (e.g., via ceramic adhesive) may be posts for forming the passageways  242  which, in turn, are connected to reticulated ceramic cores for forming the regions  240 . Similar portions of the non-reticulated core or non-reticulated cores secured thereto may form the passageways  222  and/or  220  or these may be drilled post-casting. Optionally, the passageways  242  and/or holes  220  and/or  222  may be reticulated and formed by reticulated cores or core portions. 
     FIG. 10  shows an alternate implementation of the blade outer air seal  200  in which the plenum is replaced by a correspondingly shaped/sized reticulated region  250 . This may be combined with additional blind or open reticulated regions and/or drilled or cast inlet holes to the outboard surface. 
     FIG. 11  shows a reticulated ceramic core  260  having a body  262  for forming the reticulated region  250 . Unitarily formed or integrally secured reticulated or non-reticulated ceramic posts  264  may protrude from circumferential and/or longitudinal ends of the body  262  for forming passageways such as  222 . Additional reticulated and/or non-reticulated cores and/or core combinations may be secured to the faces of the body  262  (e.g., to the outboard face for forming inlet passageways). In manufacturing, the core  260  may be assembled to pre-molded wax pattern elements for forming inboard and outboard portions of the BOAS. For example, these two portions could meet along a common centerplane of the pins/posts  264 . Ends of one or more of the posts  264  may protrude from the assembled wax for capturing within an applied shell so as to retain the core  260  in position within the shell upon removal of the wax and during subsequent introduction of molten metal. Alternatively, the wax or a portion thereof may be directly molded to/over the core  260 . 
     FIG. 12  shows a vane  300  having an airfoil  302  extending between an inboard shroud  304  and an outboard shroud  306 . The airfoil may have cooling features similar to those of the blade airfoil above. The inboard and/or outboard shrouds may have reticulated regions similar to those of the blade platform and/or the BOAS. The exemplary reticulated regions include a streamwise and spanwise array of leading edge reticulated regions  320  and streamwise and spanwise arrays of elongate pressure and suction side regions  322 . The exemplary regions  320  are of relatively non-elongate section (e.g., circular). They may be essentially straight. The exemplary regions  322  are shown as spanwise elongate at the associated airfoil surface. However, they may have a convoluted streamwise section. For example,  FIG. 13  shows a streamwise section including a first portion  324  extending from the inboard surface of the associated airfoil wall at the cavity  90  and generally normal to such surface. A second portion  326  extends within the wall generally parallel thereto. A third portion  328  extends to the associated pressure or suction side surface. The enhanced length of the portion  326  within the wall provides enhanced heat transfer. 
   Various attachment means may be utilized to secure reticulated cores to non-reticulated cores.  FIG. 14  shows reticulated ceramic cores  400  secured to a non-reticulated ceramic core  402  by means of a ceramic adhesive  404  atop an otherwise smooth and continuous surface  406  of the ceramic core  402 . 
     FIG. 15  shows an alternate embodiment wherein the non-reticulated ceramic core  410  has features  412  for registering and retaining the reticulated core  414  in a desired position/orientation. The exemplary features  412  comprise recesses in the adjacent surface  416  of the core  410  receiving complementary features  418  of the reticulated core  414 . In the exemplary embodiment, the features  418  comprise end portions of posts  420  projecting from a main body  422  of the core  414  and spacing an adjacent surface  424  of the body away from the surface  416  to create a gap  426 . Ceramic adhesive  428  may secure the features  412  and  418  to each other. 
     FIG. 16  shows yet a further variation in which the non-reticulated core registration and retention features comprise dovetail slots  430  and the complementary portions of the reticulated core comprise dovetail projection  432 . The projections may be slid into the slots to assemble the cores. With such a mechanical back-locking effect, ceramic adhesive  434  is particularly optional. Although illustrated with respect to a reticulated core having a main body and multiple projections spacing the main body apart from the non-reticulated core, other implementations lacking the main body and/or lacking the associated gaps may be possible. 
     FIG. 17  shows the attachment of reticulated cores  440  to a non-reticulated core  442  via separately-formed pins  444  and ceramic adhesive  446 . The pins may be straight or may have back-locking features either for directly engaging complementary features of one or both cores or for more robustly engaging the adhesive. 
   In yet alternate embodiments, attachment posts may be unitarily-formed with either the non-reticulated core or the reticulated core. For example,  FIG. 18  shows an attachment post  450  protruding from a remaining portion of a unitary non-reticulated core  452  and being received by a corresponding aperture  454  in a reticulated core  456 . A layer of ceramic adhesive  458  may further join the two cores. 
     FIG. 19  shows pre-formed reticulated and non-reticulated cores  470  and  472  joined by a cast-in-place ceramic  474 . The cast-in-place ceramic  474  may form back-locking projections  476  filling complementary back-locking recesses  478  and  480  in the cores. Alternatively, back-locking projections may be formed in one or both of the cores  470  and  472  with the castable ceramic forming the associated recess(es). By way of example, the cores  470  and  472  may be placed in a die and the castable ceramic (e.g., alumina- or silica-based self-hydrolizing material) injected into a space between the cores and permitted to dry or cure to solidify. 
   The reticulated elements may be formed by a variety of techniques. For example, an organic or inorganic reticulated material (e.g., a natural sponge, synthetic (e.g., polymeric) sponge or synthetic foam) may be coated with a ceramic slurry. In one exemplary situation, the slurry may finely coat the reticulated element so that even as coated the coated element is itself reticulate. The element may be firmly decomposed or melted (e.g., as a precursor to or part of firing the slurry to harden it) leaving the ceramic core with reticulations formed both by the voids from lost organic or inorganic material and from the voids in the as-coated element. Alternatively, the slurry may fully coat the element leaving no reticulation or voids. After thermal decomposition or melting, the reticulations may entirely be those due to the lost material. For multi-porosity cores, a piece of organic or inorganic material having zones of different porosity may be used in the slurry coating processes described above. Alternatively, separate pieces of such material (each having an associated porosity) may be assembled prior to the slurry coating process. Alternatively, separately-formed reticulate ceramic elements may be secured to each other. 
   Alternatively to a reticulate ceramic material, a reticulate metallic material may be used (for example, a refractory metal-based (e.g., molybdenum) foam or a non-refractory metal (e.g., nickel)). Such foam may be formed by similar processes. Such cores may be secured to each other by processes including welding, brazing, diffusion bonding, and/or other fusing. They also may be secured by similar means as ceramic cores (particularly when secured to ceramic cores). Alternatively, the metallic foam may be an intermediate, for example, the metallic foam may be filled with ceramic slurry which is allowed to harden. The metallic foam may be decomposed (e.g., thermally decomposed via oxidation at elevated temperature as such or otherwise chemically etched) to leave reticulated ceramic. 
   Other reticulate elements including carbon and composite (e.g., intermetallics) foams may be used either directly as cores or as core precursors. 
   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, details of the particular components to which the teachings are applied may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.