Abstract:
A method including: submerging a ceramic preform ( 10 ) in a layer ( 12 ) of powdered superalloy material ( 14 ), wherein the preform defines a desired shape of a channel ( 60, 62, 64, 78 ) to be formed in a layer ( 42 ) of superalloy material; melting the powdered superalloy material around the preform without melting the preform; and cooling and re-solidifying the superalloy material around the preform to form the layer of superalloy material with the preform defining the shape of the channel therein.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to additive manufacturing, and more specifically to forming a material layer having an internal void or channel of fine detail, and in one embodiment to forming a superalloy cladding layer containing a precision detailed cooling channel using a high deposition rate cladding operation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Additive manufacturing is generally considered the buildup of three dimensional components by multiple layer processing, each layer representing a portion of the three dimensional component. The three dimensional component may be produced using energy sources of high enough power to melt a powdered metal or a powdered alloy used in the three dimensional component. For example, high power laser beams are commonly used in a manner where the laser sinters or fuses the powdered metal or powdered alloy layer by layer. These processes include selective laser sintering (SLS), selective laser melting (SLM), direct metal laser sintering (DMLS), laser engineered net shape (LENS), etc. The processes build up the component after many minute layers are processed. However, these processes have disadvantages and limitations. For example, some of the processes are exceedingly slow and cost prohibitive if many parts are required, or if a part is relatively large. 
         [0003]    High deposition laser cladding such as that described in U.S. patent publication number 2013/0140278 to Bruck et al. and incorporated in its entirety by reference herein resolves the issue of speed. However, many gas turbine engine components used to guide hot gases require cooling channels disposed in the part near the surface. These cooling passages include fine detail that has not yet been achieved using the processes noted above. Consequently, there remains room in the art for improvement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show: 
           [0005]      FIG. 1  schematically illustrates positioning a preform in a layer of powdered superalloy material. 
           [0006]      FIG. 2  schematically illustrates melting the superalloy material around the preform without melting the preform, and cooling and re-solidifying the superalloy material. 
           [0007]      FIG. 3  is a close-up view of a deposition process. 
           [0008]      FIG. 4  schematically depicts a layer having cooling passages formed via the process illustrated in  FIGS. 1-3 . 
           [0009]      FIG. 5  schematically depicts a component having cooling passages formed by repeating the process illustrated in  FIGS. 1-3 . 
           [0010]      FIG. 6  schematically depicts a preform having structural surface detail. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The inventors propose using a preform having fine detail and which doesn&#39;t melt during a high deposition rate cladding process to create finely detailed channels (a.k.a. voids) within a superalloy cladding layer. The channels may be used for cooling, diagnostic, or other purposes, depending on the application. An exemplary embodiment of cooling channels is chosen for discussion herein. However, the process is applicable to other voids, such as dead-ended channels to accommodate instrumentation such as a thermocouple etc. Once the cladding process is complete and the cladding layer thereby formed, the preform may be removed to reveal the channel. Alternately, the preform may remain in the cladding layer and may define the channel within itself. The inventors have recognized that a ceramic preform immersed within an alloy melt pool may not become fully wetted, particularly if the surface of the preform contains fine structural detail. Accordingly, the preform of the present invention may be coated with a metal or metal alloy that is compatible with the superalloy material. This metal coating may improve wetting of molten superalloy material to the geometry of the preform, thereby preserving the fine detail of the preform surface in the subsequently cooled cladding layer. 
         [0012]      FIG. 1  schematically illustrates positioning a preform  10  in a layer  12  of powdered superalloy material  14  disposed on a substrate  16  (or process bed). A layer  18  of powdered flux  20  may be placed on the layer  12  of powdered superalloy material  14 . Alternately, the powdered superalloy material  14  and the powdered flux  20  may be mixed together. In another alternate exemplary embodiment, inert shielding gas or a vacuum may be used in place of (or in addition to) the powdered flux  20  to avoid atmospheric reactions during processing. The preform  10  may be made of any material that does not melt during the cladding operation. The preform  10  may be solid (e.g. a filament) or may have a shape that defines a passage there through (e.g. a tube or foam with interconnected porosity). A size and shape of the preform corresponds to the geometric detail of desired openings or voids in the final part. Example preform materials include ceramics such as alumina and zirconia. Rods (filaments) of alumina having a diameter as small as 0.28 mm in diameter are currently available. Rods (filaments) of zirconia having a diameter as small as 1.6 mm in diameter are currently available from, e.g. Ortech Advanced Ceramics of Sacramento, Calif. Both alumina and mullite (3Al 2 O 3  2SiO 2 ) are available in tube form from, e.g. CoorsTek of Golden CO. 
         [0013]    Optionally, an outer surface  30  of the preform  10  may be coated with a coating  32  (e.g. a metal coating) to facilitate wetting during the laser cladding operation. Suitable preform  10  materials include alumina, beryllium oxide, sapphire, zirconia, silica, magnesium oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide, aluminum silicates (including mullite), magnesium silicates, and any other suitable ceramic. Suitable coating materials include molybdenum-manganese, tungsten manganese, moly tungsten manganese, titanium, hafnium, zirconium, chromium, and niobium. Any combination of the above preform materials and coating materials may be used. 
         [0014]    In an exemplary embodiment an alumina, beryllium oxide, or sapphire preform  10  may be coated with any of molybdenum-manganese, tungsten manganese, and moly tungsten manganese. The preform  10  may be coated using any of several known processes, including brushing, screen printing, spraying, dipping, plating, sputtering, and needle application. Typically the coating  32  may have a thickness of twenty-five microns, with a ten micron tolerance. When the coating  32  is a plating, the coating  32  may have a thickness of two to ten microns. Accordingly, in some cases plating to accomplish wetting may be applied directly to the preform  10  without an intermediate coating. Optionally, a second coating  34  may be applied a surface  36  of the coating  32  to further facilitate the wetting. The second coating  34  may include a metal or alloy compatible with the powdered superalloy material  14  which is applied during a plating process in which case the second coating  34  is a plating. For example, for a nickel based powdered superalloy material, nickel may be used for the second coating  34 . Here again, the plating, and hence the second coating  34 , may have a thickness of two to ten micron. An optional final step in the metallizing of the preform  10  may include a heat treatment. In an exemplary embodiment an alumina preform  10  is coated with molybdenum-manganese which is plated with nickel. 
         [0015]    In an alternate exemplary embodiment, a zirconia preform  10  may be coated (via physical vapor deposition) with titanium (commercially available through Forschungszentrum Jülich of Jülich, Germany). Other materials suitable for coating zirconia (via physical vapor deposition) include hafnium, zirconium, chromium, and niobium. 
         [0016]    The coating  32  and optional second coating  34  improve the wetting of the molten superalloy material to the preform during the cladding operation. The coating  32  and optional second coating  34  may or may not be consumed during the cladding operation. If consumed, the molten superalloy material will conform directly to the outer surface  30  of the preform  10 . If not consumed, the molten superalloy material will conform to the outer surface  36  of the coating  32  when only the coating  32  is present, or an outer surface  38  of the second coating  34  when the second coating  34  is present. Since a shape of the outer surface  30  of the preform  10  defines a shape of the outer surface  36  of the coating  32  as well as a shape of the outer surface  38  of the second coating  34 , the molten superalloy material still conforms to the shape of the outer surface  30  of the preform  10 . Solidification of the molten superalloy material around the details preserves the details in the cladding layer. 
         [0017]    If the coating  32  and/or the second coating  34  are consumed during the cladding operation, it will not have a detrimental effect on the cladding layer because the elements present in the coating  32  and the second coating  34  are preferably already found in the alloys of interest (e.g. nickel based superalloys) and the amount of material added would be so small that it would have no significant effect on the cladding layer composition or mechanical or physical properties. 
         [0018]      FIG. 2  schematically illustrates melting the powdered superalloy material  14  around the preform  10  without melting the preform  10 . The melting may be accomplished in any manner known to those in the art. In an exemplary embodiment, a laser beam  40  selectively heats the powdered superalloy material  14  to create a melt pool of melted superalloy material. The melted superalloy material cools and solidifies to form a cladding layer  42  covered by a layer of slag  44 . The preforms  10  remain intact within the cladding layer  42 . 
         [0019]      FIG. 3  is a close-up of the melting process. As the laser beam  40  moves in a direction of travel  50  it melts the powdered superalloy material  14  and flux material  20  to form the melt pool  52  covered by slag  44 . Upon reaching a preform  10 , the traveling melt pool  52  surrounds the preform  10  and retains any detail present in the preform  10  as the molten superalloy material solidifies around the preform. If the coating  32  remains, once the preform  10  is removed, the cooling channel will be defined by an inner surface  54  of the coating  32 . If the coating  32  is consumed, once the preform  10  is removed, the cooling channel will be defined by an inner surface of the cladding layer  42 . 
         [0020]    Conventional laser heating may position the laser beam  40  generally above the preform  10  during the heating process. Consequently, powdered superalloy material  14  in a shadowed region  56  (e.g. shadowed from direct laser impingement) under the preform  10  may not be directly heated by the laser beam  40 . To ensure the powdered superalloy material  14  in the shadowed region  56  is melted and the molten superalloy material reaches an underside  58  of the preform  10 , the laser beam  40  may slow its speed of travel proximate the preform  10 . Increasing heat transfer into the melt pool  52  proximate the preform  10  may increase conductive heat transfer to the powdered superalloy material  14  in the shadowed region  56 . High thermal conductivity preform materials such as alumina may enhance heating toward the shadowed region  56 . This can promote greater heating and melting of the powdered superalloy material  14  in the shadowed region  56 . Subsequent mixing of such material with a balance of the melt pool  52  will result from temperature gradient (Rayleigh-Bénard convection) as well as surface tension gradient (Marangoni convection). Agitation of the melt which promotes wetting from such convective effects may be supplemented by mechanical agitation. For example, the preform itself could be vibrated or rotated to enhance wetting. The laser beam  40  may also be angled differently as it approaches the preform  10  (e.g. proximate the preform) in a manner that permits the laser beam  40  to reach some or all of the shadowed region  56  that the laser beam  40  would not reach if an orientation of the laser beam  40  used in a balance of the laser processing were not adjusted. The improved wetting provided by the coating  32  and/or the second coating  34  will also facilitate movement of the melted material along the underside surface of the preform  10 . Single or combined effects of slowed laser travel, an angled laser, a conductive preform, convective effects, surface tension effects, and mechanical agitations can enhance the molten superalloy material fully surrounding the preform  10  and conforming to the shape of the outer surface  30  of the preform  10 . The single or combined effects may be employed when the laser beam  40  is proximate the preform  10  (i.e. close enough to the preform  10  to have an effect on the shadowed region  56 ). 
         [0021]      FIG. 4  shows the cladding layer  42  with the layer of slag  44  removed. This cladding layer  42  may be a repair on a component, or a layer of a new component. Three cooling channels  60 ,  62 , and  64  are visible. In an exemplary embodiment the preform  10  has been removed and a first cooling channel  60  is defined by the inner surface  66  of the cladding layer  42 . This may occur when there was no coating  32  or second coating  34  on the preform  10  or it was consumed during the cladding operation. In this case the molten material conformed to the outer surface  30  of the preform  10 . The preform  10  may be removed by mechanical or chemical or other means known to those in the art. For example, the preform may be cracked and shattered using ultrasonic energy and reduced to powder which is then flushed out using fluid such as pressurized air or a rinsing agent. Preforms of foam or pressed powder structure may be relatively fragile to enhance such removal. When the preform  10  comprises a hollow shape such as a tube, foam etc., the preform  10  may be cracked, shattered, and removed via thermal shock. For example, liquid nitrogen may be flushed through the perform  10 , causing it to crack, during or after which it can be removed. Alternately, or in addition, when hollow the preform  10  may be cracked, shattered, and removed with mechanical shock (e.g. ultrasonic). 
         [0022]    In an exemplary embodiment, the preform  10  has been removed and a second cooling channel  62  is defined by the inner surface  54  of the coating  32 . This may occur when the preform  10  has the coating  32  and optionally the second coating  34 , and the coating  32  is not consumed during the cladding operation of  FIG. 3  (whether or not the second coating  34  is consumed). 
         [0023]    In an exemplary embodiment the preform  10  remains and a third cooling channel  64  is defined by an inner surface  68  of the preform  10  which is, for example, a hollow tube or a foam. When the preform  10  is a foam that is left in place, the interconnected porosity of the foam may increase a cooling effect of the cooling channel  64 . A hollow preform  10  may or may not also be coated and optionally plated. 
         [0024]      FIG. 5  schematically depicts a component  70  having cooling passages formed by repeating the cladding process illustrated in  FIGS. 1-3  during an additive manufacturing process. Each iteration of the process forms a respective cladding layer  42 . The component  70  may have one cladding layer  42 , several cladding layers  42 , or may be composed entirely of cladding layers  42 . The component may be machined between formations of cladding layers  42  and/or after all cladding layers  42  are applied. In the exemplary embodiment shown, multiple cladding layers  42  include respective cooling channels, but each cladding layer  42  need not have a respective cooling channel. 
         [0025]    Each cladding layer  42  may have a respective type or types of cooling channels and these cooling channels may have any orientation, size, and cross sectional shape. A first cladding layer  72  includes cooling channels  60  defined by the inner surface  66  of the cladding layer  42  and oriented in one direction. A second cladding layer  74  includes cooling channels  64  defined by an inner surface  68  of the preform  10  and oriented in a different direction. A third cladding layer  76  includes a cooling channel  78  having a different and polygon shape that is oriented in yet another unique direction. A fourth cladding layer  80  has no cooling channel. Exemplary embodiments within the scope of the disclosure may connect cooling channels from one cladding layer  42  to another cladding layer  42 . The connection may be provided by subsequent machining, or connecting passageways may be formed as part of the cladding operation through the use of a preform including a vertically extending leg. In this manner cooling passages could extend laterally (within a respective cladding layer  42 ) as well as vertically, from one cladding layer  42  to another. While connected channels for cooling have been described, channels for other purposes may be included. For example, a channel that is ended at an important location for diagnostics could be provided by a tubular preform with a coincident end. After part completion, instrumentation such as a thermocouple could be inserted through the tubular preform or into the channel formed by the preform. 
         [0026]      FIG. 6  schematically depicts a preform  10  having details in the outer surface  30  that are replicated in the subsequently formed cooling channels. Exemplary types of such details includes a raised or recessed fin  80  (as disclosed by Kakac et al., “Heat Transfer Enhancement of Heat Exchangers”), raised or recessed trip strips  82 , raised or recessed chevrons  84 , and raised or recessed dimples  86  or any other feature known to those in the art disposed on the inner or the outer surface of the preform. These details may, for example, increase heat transfer to a cooling fluid in the cooling channel, and/or regulate a flow rate of the cooling fluid flowing through the cooling channel. 
         [0027]    Geometries other than wires/filaments and tubes may be used for the preform  10 . Flat plates may be used in, for example, semiconductor applications. Single and double curved plates may be used in, for example, body armor applications. Valves with precisely dimensioned holes may be formed, for example, for components that meter blood. Cups and specialized contours may be used in, for example, orthopedic implants. These non-limiting examples represent just a few of the possible applications for the process disclosed herein. 
         [0028]    From the foregoing it can be seen that the Inventors have developed a new and innovative method for forming a material layer containing an internal void having a highly detailed internal surface geometry using a high speed cladding process with a relatively high temperature preform. This process achieved a level of detail not before possible in such a process and can be used in an additive manufacturing process to create parts faster than before possible. Consequently, this represents an improvement in the art. 
         [0029]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.