Abstract:
A method of forming structure on a component includes: providing a component having a first surface; adhering powder to the first surface; and directing a beam from a directed energy source to fuse the powder in a pattern corresponding to a layer of the structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to additive manufacturing methods, and more particularly to methods for forming structures on two- or three-dimensional substrates. 
         [0002]    Additive manufacturing is a process in which material is built up layer-by-layer to form a component. Unlike casting processes, additive manufacturing is limited only by the position resolution of the machine and not limited by requirements for providing draft angles, avoiding overhangs, etc. as required by casting. Additive manufacturing is also referred to by terms such as “layered manufacturing,” “reverse machining,” “direct metal laser melting” (DMLM), and “3-D printing.” Such terms are treated as synonyms for purposes of the present invention. 
         [0003]    In the prior art, additive manufacturing may be carried out by laser melting of selected regions of layers of powder starting from a powder bed. The first layer is consolidated to a pattern, then powder is added, excess powder is removed (typically by scraping along a planar reference surface) to leave the next layer thickness, the powder is laser melted by pattern, then the steps are repeated. This is a planar process that builds an entire part with two-dimensional (“2-D”) planar layers only. 
         [0004]    While effective for manufacturing complete components, this process lacks the flexibility to build structures on substrates having non-planar or three-dimensional (“3-D”) surfaces. 
         [0005]    Accordingly, there remains a need for a process for additive manufacturing of structures on nonplanar surfaces. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    This need is addressed by the technology described herein, which provides a method for additive manufacturing of structures on an existing 2-D or 3-D substrate. 
         [0007]    According to one aspect of the technology, a method of forming a structure on a component includes: providing a component having a first surface; adhering powder to the first surface; and directing a beam from a directed energy source to fuse the powder in a pattern corresponding to a layer of the structure. 
         [0008]    According to another aspect of the technology, a method of forming a cooling channel on a component includes: providing a component having a first surface; adhering powder to the first surface; directing a beam from a directed energy source to fuse the powder in a predetermined pattern; and repeating in a cycle the steps of depositing and fusing to build up the structure in a layer-by layer fashion, wherein the structure includes spaced-apart walls which define an open channel therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a block diagram of an additive manufacturing process; 
           [0011]      FIG. 2  is a schematic cross-sectional view of an exemplary turbine component; 
           [0012]      FIG. 3  is a schematic view of a portion of the turbine component of  FIG. 2 , showing cooling holes being formed in a substrate; 
           [0013]      FIG. 4  is a schematic view of a portion of the turbine component of  FIG. 2 , showing adhesive being applied; 
           [0014]      FIG. 5  is a schematic view of a portion of the turbine component of  FIG. 2 , showing powder being applied; 
           [0015]      FIG. 6  is a schematic view of a portion of the turbine component of  FIG. 2 , showing powder being fused; 
           [0016]      FIG. 7  is a schematic view of a portion of the turbine component of  FIG. 2 , showing a later stage of construction; 
           [0017]      FIG. 8  is a schematic view of a portion of the turbine component of  FIG. 2 , showing an additive structure with un-fused powder therein; 
           [0018]      FIG. 9  is a schematic view of a portion of the turbine component of  FIG. 2 , showing a completed additive structure; 
           [0019]      FIG. 10  is a view taken along lines  10 - 10  of  FIG. 9 ; 
           [0020]      FIG. 11  is a perspective view of a portion of a gas turbine engine airfoil; 
           [0021]      FIG. 12  is a view taken along lines  12 - 12  of  FIG. 11 ; 
           [0022]      FIG. 13  is a view taken along lines  13 - 13  of  FIG. 11 ; 
           [0023]      FIG. 14  is a sectional view of a portion of the airfoil of  FIG. 11 , showing the layers in an additive-manufactured portion thereof; and 
           [0024]      FIG. 15  is another sectional view of the additive-manufactured portion shown in  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  is a flowchart showing the steps in an additive manufacturing process. The process begins with an existing part surface. The term “part” refers both to an otherwise-complete component as well as a part in an uncompleted state, such as a rough casting, blank, preform, or part manufactured by an additive manufacturing process. The surface is appropriately prepared (block  100 ) as required to accept bonding of a powdered material thereto. For example, contaminants may be removed and/or the surface roughened by solvents, fluoride ion cleaning, grit blasting, etc. 
         [0026]    Next, a powder is adhered to the surface, block  102 . The powder may be any suitable material for additive manufacturing. For example, the powder may be of metallic, polymeric, organic, or ceramic composition. 
         [0027]    As used herein, the term “adhere” refers to any method that causes a layer to adhere to the surface with sufficient bond strength so as to remain in place during a subsequent powder fusion process. “Adhering” implies that the powder has a bond or connection beyond simply resting in place under its own weight, as would be the case with a conventional powder-bed machine. For example, the surface may be coated with an adhesive product, which may be applied by methods such as dipping or spraying. One non-limiting example of a suitable low-cost adhesive is Repositionable 75 Spray Adhesive available from 3M Company, St. Paul, Minn. 55144 US. Alternatively, powder could be adhered by other methods such as electrostatic attraction to the part surface, or by magnetizing the powder (if the part is ferrous). As used herein, the term “layer” refers to an incremental addition of mass and does not require that the layer be planar, or cover a specific area or have a specific thickness. 
         [0028]    The powder may be applied by dropping or spraying the powder over the surface, or by dipping the part in powder. Powder application may optionally be followed by brushing, scraping, blowing, or shaking as required to remove excess powder (block  104 ), for example to obtain a uniform layer. It is noted that the powder application process does not require a conventional powder bed or planar work surface, and the part may be supported by any desired means, such as a simple worktable, clamp, or fixture. 
         [0029]    Once the powder is adhered, a directed energy source (such as a laser or electron beam) is used to melt a layer of the structure being built, bock  106 . The directed energy source emits a beam and a beam steering apparatus is used to steer the beam over the exposed powder surface in an appropriate pattern. The exposed layer of the powder is heated by the beam to a temperature allowing it to melt, flow, and consolidate. This step may be referred to as fusing the powder. 
         [0030]    The fusing step may be followed by removing any un-fused powder (e.g. by brushing, scraping, blowing, or shaking) as required, block  107 . This step is optional, meaning it may or may not be required or desired for a particular application. 
         [0031]    This cycle of adhering powder, removing excess powder, and then directed energy melting the powder is repeated until the entire component is complete (block  109 ). 
         [0032]    The general process described above may be used to form any type of additive structure desired. The process is particularly useful for forming cooling structures on gas turbine engine hot section components.  FIG. 2  shows an example of a hot section component  10  having an airfoil configuration, representative of a high pressure turbine blade or nozzle. As indicated, the component  10  comprises a substrate  12  with an outer surface  14  and an inner surface  16 . For example, the substrate  12  may be a casting. The inner surface  16  of the substrate  12  may define at least one hollow interior space or cavity  18 , in a non-limiting example for the supply of coolant. An additive structure  20  is built upon and surrounds at least a part of the outer surface  14 . The component  10  incorporates a number of surface cooling channels  22 , also referred to as micro-channels. The surface cooling channels  22  may be formed in the substrate  12 , partially in the substrate  12  and the additive structure  20 , or completely in the additive structure  20 . 
         [0033]    The component  10  may be made from a material such as a nickel- or cobalt-based alloy having good high-temperature creep resistance, known conventionally as “superalloys.” 
         [0034]    The outer surface  14  of the illustrated component  10  is non-planar or “a 3-D surface”. Stated another way, it is curved about at least one axis. The process described herein is equally applicable to 2-D and 3-D substrates. 
         [0035]      FIGS. 3 through 10  illustrate sequential steps in the process of manufacturing the additive structure  20 , using the process described above, Initially, the substrate  12  is provided and the outer surface  14  is prepared as required. 
         [0036]    Coolant feed holes  24  may be formed through the wall of the substrate  12  as needed, as shown in  FIG. 3 . The coolant feed holes  24  may be plugged or covered with an appropriate material (e.g. wax, polymer tape, etc.) to prevent powder from entering them during subsequent steps. Plugs  23  are shown as an example. If the surface cooling channels  22  are to be made partially in the substrate  12 , then these would already be incorporated therein, as part of the casting or performed by a conventional machining process. In the example of  FIGS. 3-10  the surface cooling channels  22  are formed entirely within the additive structure  20 . 
         [0037]    Powder P is then adhered to the outer surface  14 . In the illustrated example the powder P is adhered by first applying an adhesive  25  to the outer surface  14  ( FIG. 4 ), for example by dipping or spraying, and then applying the powder P over the adhesive  25 , for example by dropping or spraying powder P from a nozzle  28 . Excess powder P may be removed by mechanically brushing the surface, blowing with an air jet, or agitating the substrate  12 .  FIG. 5  shows the substrate  12  after application of a layer of powder P. 
         [0038]    In this example, the powder P may be made from a material such as a nickel- or cobalt-based alloy having good high-temperature creep resistance, known conventionally as “superalloys.” As a non-limiting example, the thickness of the powder layer may be about 10 micrometers (0.0004 in.). 
         [0039]    A directed energy source  30  (such as a laser or electron beam gun) is used to melt the layer of powder P in a pre-programmed pattern representing a desired structure, as shown in  FIG. 6 . The directed energy source  30  emits a beam “B” and a beam steering apparatus is used to steer the focal spot “S” of the beam B over the exposed powder surface in an appropriate pattern. The exposed layer of the powder P is heated by the beam B to a temperature allowing it to melt, flow, and consolidate.  FIG. 6  shows a beam B being used to form the first layers of a plurality of walls  32 . The spaces between adjacent walls  32  define the surface cooling channels  22 . Each surface cooling channel  22  communicates with one of the coolant feed holes  24 . It is noted that the surface cooling channels  22  can be of any shape, for example the bottom may be a shape other than flat, the side walls may be angled inwards or outwards, etc. 
         [0040]    The steps of adhering powder and fusing the powder are repeated to build up a structure in layer-by-layer fashion.  FIG. 7  shows a subsequent step after many layers have been applied, with the surface cooling channels  22  having reached their full radial height “H”. In this example, un-fused powder P is left in the surface cooling channels  22  to serve as a support for a subsequent cover. It is noted that the un-fused powder P shown in the surface cooling channels  22 , as well as the plugs  23 , may not be necessary if a structure will not be built over the coolant feed holes  24  or the surface cooling channels  22 , or if a structure will be built over the coolant feed holes  24  using an alternative method as described below. In such circumstances, any un-fused powder P may be cleaned out in each cycle of the steps of adhering and fusing powder. 
         [0041]    A cover may be formed over the surface cooling channels  22  by continuing the additive process described above.  FIG. 8  shows a cover  34  formed over the surface cooling channels  22 . The cover  34  defines an exterior surface  36  of the component  10 . Subsequent to forming the cover  34 , the un-fused powder P remaining in the surface cooling channels  22  may be removed (see block  111  in  FIG. 1 ), for example by air jet, vacuum extraction, chemical removal, fluid flush, and/or vibration of the component  10 , leaving the completed surface cooling channels  22  as seen in  FIGS. 9 and 10 . 
         [0042]    The cover  34  may include a plurality of exit film holes  38 . It should be noted that although the exit film holes  38  are shown in  FIG. 9  as being round, and at an angle relative to the exterior surface  39  as shown in  FIG. 10 , these are non-limiting examples. The film holes may also be non-circular shaped holes and configured substantially perpendicular to, or at any angular instance, relative to the coating surface, and may optionally have variously shaped inlet and exits, for example diffuser exits of various types are known in the art. In addition, in an embodiment, the exit film holes  38  may not be formed as discrete features that match up one film hole per surface cooling channel  22 . In such embodiment, one or more film trenches that connect more than one surface cooling channel  22  exit together into a continuous exit feature may be formed. The exit film holes  38  or similar apertures may be formed as part of the additive process, or machined afterwards by a conventional method as known in the art. 
         [0043]    As an alternative to the additive covering method described above, the surface cooling channels  22  may be completed up to the open channel stage shown in  FIG. 7 , and then a cover may be made using prior art methods. 
         [0044]    As noted above, the additive manufacturing process does not require that the layers be planar. To more clearly illustrate this point,  FIGS. 11-13  illustrate a further example of a component built up using arbitrary-shaped layers, more specifically a tip portion of a high pressure turbine airfoil  200 . The airfoil  200  includes opposed pressure and suction sidewalls  202 ,  204  respectively, extending between a leading edge  206  and a trailing edge  208 . A tip cap  210  closes off the distal end of the airfoil  200 . The airfoil  200  also includes a “squealer tip”  212  comprising a wall extending radially outward from the tip cap  210 . The squealer tip  210  incorporates a flared portion  214  that extends laterally outward from the outer surfaces of the pressure and suction sidewalls  202 ,  204  and extends around a portion of the airfoil&#39;s periphery. 
         [0045]    The squealer tip  212  is an example of a structure that may be formed using the principles described herein. Starting with an airfoil substrate comprising the tip cap  210  and pressure and suction sidewalls  202 ,  204 , the squealer tip  212  may be built up in a series of layers using the repeated steps of adhering powder and fusing the powder described above. In  FIGS. 14 and 15 , lines  216  represent generally the layers. (The thickness of the layers  216  is greatly exaggerated for illustrative purposes). It can be clearly seen that the layers  216  may take on any shape or size needed for efficient construction with a minimum of powder and processing time. For example, a core  218  of the squealer tip  212  includes a plurality of planar layers, while an intermediate portion  220  includes a plurality of three-dimensional enveloping layers, and distal portions  222  include three-dimensional layers extending over only a portion of the surface area of the squealer tip  212 . 
         [0046]    The process described herein has several advantages over the prior art. The additive manufacturing process is much simpler and requires far fewer process steps to produce a component as compared to conventional investment casting. The specific method described herein does not require the use of large powder beds and enables the building of additive structures onto existing 3-D components at low cost. 
         [0047]    The foregoing has described an apparatus and method for additive manufacturing of structures on 2-D and 3-D components. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0048]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0049]    The invention is not restricted to the details of the foregoing embodiment(s). The invention extends any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying potential points of novelty, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.