Abstract:
A method of additive manufacturing, including: placing a layer ( 10 ) of strip-cast superalloy sheet material over a subcomponent ( 12 ) leaving a gap ( 20 ) between the layer and the subcomponent; and creating a weldment ( 14 ) to the layer. Shrinkage in the layer caused by the weldment is accommodated by a decrease in the gap with reduced shrinkage stress in the weldment. The layer may be formed of more than one piece ( 16 ), and the weldment may join the pieces together with or without joining the layer to the subcomponent. The gap may again grow due to differential thermal expansion when the resulting component is placed into service, thereby functioning as a passively regulated cooling channel.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to the field of additive manufacturing, and more particularly to building-up a component with cast superalloy material by welding layers of strip-cast superalloy material with an allowance in the build to allow weld related shrinkage to occur without restraint. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas turbine engine components operate in extremely harsh environments and this often necessitates that they be made using superalloy materials. Superalloys are difficult to cast in a manner that achieves uniform properties throughout the component. This is largely related to the challenge of removing enough heat from the melt at a consistent rate throughout the part&#39;s cross section during the casting operation. Typically, the center of the part is last to solidify because heat is extracted from the periphery of the melt. A similar issue happens in welding superalloys where the weld centerline is last to solidify and where centerline segregations and shrinkage issues can lead to solidification cracking. 
         [0003]    Part specific casting is also labor-intensive, time consuming, and costly. Typical steps to generate a specific cast geometry include die fabrication, wax injection, assembly on a sprue, shell building (coating with ceramic slurry and sand stucco), drying, wax removal in an autoclave, furnace burnout, mold filling with metal, shell removal, gate removal, and final sandblasting and machining. 
         [0004]    Some recent interest has been devoted to selective laser melting (SLM) to build parts by additive manufacturing. The SLM process is, however, relatively slow, limited to buildups in a horizontal plane (e.g. no part extending above the plane), and limited to fine grain structure. SLM also results in properties that are different in the direction of building than in other directions. 
         [0005]    Consequently, there remains room in the art for uniform, predictable, and even customizable properties throughout a superalloy component, as well as a need for faster part production. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is explained in the following description in view of the drawings that show: 
           [0007]      FIG. 1  is a top view depicting the addition of a layer in an additive manufacturing process. 
           [0008]      FIG. 2  is a top view depicting the addition of a layer in an alternate exemplary embodiment of the additive manufacturing process of  FIG. 1 . 
           [0009]      FIG. 3  is a side view depicting the addition of a layer in an alternate exemplary embodiment of the additive manufacturing process. 
           [0010]      FIG. 4  is a top view depicting the addition of the layer of  FIG. 3 . 
           [0011]      FIG. 5  is a side view depicting the addition of another layer of the alternate exemplary embodiment of  FIG. 3 . 
           [0012]      FIGS. 6-8  depict an alternate exemplary embodiment of the process of forming the other layer of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The present inventor has developed a unique and innovative approach to additive manufacturing of a component using cast superalloy material that overcomes drawbacks associated with existing techniques. The inventor has recognized that thinner sections of superalloy are less prone to centerline casting issues because they solidify more consistently across their narrow section. Consequently, a process known as strip casting provides faster and more uniform cooling, refinement of microstructure, and improved uniformity of composition. The method disclosed herein takes advantage of these properties and also overcomes weld cracking associated with superalloys. The result is an additive manufacturing process that produces a superalloy component having cast alloy grain structure while avoiding problems normally associated with casting. The process utilizes relatively inexpensive, bulk strip cast superalloy substrate material. 
         [0014]    The method disclosed herein proposes to manufacture fully cast parts in an additive fashion. The method includes layering cast superalloy strip material to build up the parts in an additive process. The cast material has superior properties than wrought material. Moreover, the present invention utilizes defined gaps around the strips to accommodate subsequent weld shrinkage (e.g. mitigating restraint) when welding each strip to itself and/or to an underlying subcomponent (that may include other layers of strip cast superalloy material). The welded component is then available for final machining and heat treatment. Structural details in any given strip layer or between given strip layers can be achieved by pre-forming the strip or by an intermediate machining step. Such details include, for example, pockets, holes, channels/passageways etc. Certain such details may be so fine, intricate and complex that they could not be achieved by conventional casting practices. Incremental, additive layered construction as described herein provides a unique opportunity to introduce internal manufacturing details never before possible in cast components. Such passageways could be continuous or could be dead-ended and could serve any number of functions including cooling, temperature instrumentation, stress instrumentation, inspection, etc. 
         [0015]    Mitigating shrinkage stress (eliminating or reducing the stress compared to fully restrained welding) associated with welding the superalloy layers facilitates the avoidance of weld solidification cracking and weld reheat cracking. This may be accomplished in a variety of ways, depending on the geometry of the layer and its position in the component being formed. 
         [0016]      FIG. 1  is a cross sectional view depicting the addition of a layer  10  to a subcomponent  12  in an additive manufacturing process. As used herein, the subcomponent  12  is any unfinished part of a component to which the layer  10  is being added. The subcomponent  12  may be composed fully of other layers of strip cast superalloy material. Alternately, the subcomponent  12  may include non superalloy material, or a mix of other strip cast superalloy layers and non superalloy material. In this exemplary embodiment the subcomponent  12  is another layer of strip cast superalloy material including a weld  14 . 
         [0017]    The layer  10  being added includes two pieces  16  having an oversized pre-weld profile  18  as indicated by dashed lines. The pre-weld profile  18  forms a gap  20  between the subcomponent  12  and the layer  10 . Upon butt-welding the two pieces  16  together, weld shrinkage transverse to the joints  30  (as shown by the arrows) causes the layer  10  to become smaller, thereby reducing or eliminating the gap  20 , as shown by the solid line indicating a post-weld profile  32 . The gap  20  therefore accommodates the shrinkage because it permits the weld  14  in layer  10  to shrink without being restrained by the subcomponent  12 . Without the gap  20 , the layer  10  would begin to shrink, but would be restrained from doing so by the subcomponent  12 , which may already be in a final form. When restrained by the subcomponent  12  the weld  14  would experience additional stress which could cause weld solidification cracking and weld reheat cracking. The process may be repeated to add additional layers. 
         [0018]    While shown as a concentric wrap having two pieces butt welded together, other types of layer configurations may be used, including spiral wraps that are fillet welded, and coil winding etc. Varying the thicknesses of overlapping layers is also possible. Further, varying a thickness of the component locally by varying the size and shape of the layer is also possible. Still further, varying the material type of a layer or portion of a layer is possible to impart desired changes in properties. 
         [0019]    In an alternate exemplary embodiment, the gap  20  may accommodate enough shrinkage to prevent the weld solidification cracking and weld reheat cracking, but may permit some restraint of the shrinkage. This may be advantageous when pre-stressing is desirable. In such an exemplary embodiment, the layer  10  may experience some pre-tension, while the subcomponent  12  may experience some pre-compression. In such an exemplary embodiment, weld shrinkage may initially be unrestrained by the subcomponent  12 , after which the subcomponent  12  will restrain any remaining shrinkage. Stress in the weldment will be lower than in a weldment that is fully restrained. By way of example, pre-stressing of the innermost layer and introduction of compressive stresses could be of advantage if the interior represented a conduit for fluid that would otherwise cause stress corrosion cracking (tensile stress induced). 
         [0020]    In an exemplary embodiment where the subcomponent  12  is likewise formed by butt welding strip cast superalloy material, the pieces  16  of the subcomponent  12  may similarly be oversized pre-weld to produce a desired post-weld profile  34 . Alternately, the subcomponent  12  may be machined, cast using other casting techniques (e.g. lost wax), or forged, extruded, etc. Once the layer  10  is added to the subcomponent  12 , the layer  10  is considered part of the subcomponent to which a next layer is added. The process of adding layers repeats until the component is completed. 
         [0021]    The layer  10  may be welded to the subcomponent  12 . For example, the weld  14  may join the pieces  16  to each other and may join layer  10  to the subcomponent  12 . Alternately, the layer  10  may remain not bonded to the subcomponent  12 . This may be accomplished in any number of ways. For example, the subcomponent  12  may include a recess  36  adjacent the weld  14  in the layer  10 . In such an exemplary embodiment the weld  14  would join the pieces  16  of the layer  10 , but would not join the layer  10  to the subcomponent. In this exemplary embodiment the welds  14  in the subcomponent were staggered from the welds  14  in the layer  10 , i.e. not adjacent to each other in a through-thickness direction. The recess  36  may be formed, for example, by machining. 
         [0022]    Joining the layer  10  to the subcomponent  12  may readily be accomplished simply by foregoing the recess  36 , causing the weldment to incorporate material from the layer  10  and the subcomponent  12  and, if required, additional filler metal and metallurgically joining them together. In various embodiments the welds  14  may or may not align from one layer to the next. 
         [0023]    The layer  10  and the subcomponent  12  in  FIG. 1  may form a component wall  40  that encloses a hollow space  42 . Accordingly, the additive manufacturing method disclosed herein may be used to form a pressure vessel such as for a boiler. Similarly, the process can be used to form an airfoil of a blade or a vane of a gas turbine engine, or a hot gas path duct such as a transition duct. In a component where an outer wall  44  is exposed to hot gases, such as when the component wall  40  forms an airfoil of a blade or vane, the arrangement may be particularly beneficial. When the outer wall  44  is exposed to the hot gases it may thermally grow relative to an inner wall  46 . This relative thermal growth may form a cooling passage  50  that can carry cooling fluids. In the exemplary embodiment of  FIG. 1 , the cooling passage  50  may be naturally disposed at a leading edge  52  of the airfoil, advantageously exactly where a high need for cooling exists. Further, a size of the cooling passage  50  would vary depending on a temperature difference between the outer wall  44  and the inner wall  46 . This characteristic may be relied upon to throttle the amount of cooling fluid used, providing for a self-regulating cooling passage. 
         [0024]    A minimum amount of cooling may be provided by creating other cooling passages. A groove  54  may be machined into a surface  56  of the layer  10 , a surface  58  the subcomponent  12 , or both. When assembled together, the layer  10 , the subcomponent,  12 , and the groove  54  define a cooling passage  60 . The recess  36  may also be used for cooling. The surface  56  of the layer, the surface  58  of the subcomponent  12 , or both may be roughened to form a cooling passage  70 . An insert recess  72  may be formed between the layer  10  and the subcomponent  12  and an insert  74  placed therein. The insert  74  may include cooling passages  76  or other cooling features, such as trip strips, turbulators etc. that guide/influence cooling flow in the cooling passages  76 . 
         [0025]    When not welded to the subcomponent  12 , the layer  10  may be held in place through a mechanical interlock. For example, the outer layer  44  of an airfoil may remain free to float relative to the inner layer  46 , but the movement may be limited by a blade platform or vane shroud. 
         [0026]    The layers  10  may be selectively applied as needed. This can be seen in  FIG. 2 , where the layer  10  is applied to a portion of the subcomponent  12 . Here the layer  10  is metallurgically bonded (e.g. fillet welded) to the subcomponent  12 . Similar to that process of  FIG. 1 , the layer  10  is oversized and creates the gap  20  to accommodate the shrinkage. Selective application of layers  10  allows for more structure where needed without having unnecessary structure where it is not needed, which may save weight and cost and may facilitate balancing a component. Where the weld  14  bonds the layer  10  to the subcomponent  12  as shown in  FIG. 2 , the layer  10  is pinned at the weld  14  and will naturally expand to form the cooling passage  50  in a desired location, such as a leading edge  52  of an airfoil. 
         [0027]      FIG. 3  shows a side view of a component such as a flange for a pressure vessel, a platform for a blade, or a shroud for a vane etc. The subcomponent  12  (e.g. a pipe or airfoil) again defines a hollow space  42 , but the layer  10  is oriented transverse to the hollow space  42  and a long axis  80  of the subcomponent  12 . The layer  10  includes an oversized pre-weld profile  18  that shrinks to form the post-weld profile  32 .  FIG. 4  shows the layer  10  and subcomponent  12  of  FIG. 3  from a top view. The layer  10  includes plural pieces joined together via butt welds at edges  82  and joined via corner/t-joint welds to the subcomponent  12  at an inner periphery  84 . Upon welding, the shrinkage transverse to the joints (as shown by the arrows) causes the layer  10  to shrink from the pre-weld profile  18  to the post-weld profile  32 . In an alternate exemplary embodiment, the subcomponent  12  may include a groove (not shown) into which the inner periphery  84  may shrink, thereby creating a mechanical interference that could hold the layer  10  in place before welding. In this case the weld  14  at the inner periphery  84  may be optional. 
         [0028]    In  FIG. 5  the layer  10  and the subcomponent  12  of  FIGS. 3-4  become the subcomponent to which a new layer  10  is added. In the case of a flange for a pressure vessel, adding the layer  10  may build up the flange. In the case of a blade, adding the layer  10  may build up the platform. In the case of a vane, adding the layer  10  may build up the shroud. 
         [0029]    When adding a layer  10  to a subcomponent  12  such that the layer  10  may shrink in two different directions, e.g. a radially inward direction  86  and a transverse direction  88 , additional allowance may be necessary to accommodate the differing shrinkages. Similar to  FIG. 4 , in  FIG. 5  the layer  10  may include plural pieces  16  that are oversized. In addition, they are canted at a slight angle  90  to a transverse portion  92  of the subcomponent  12  as shown by the pre-weld profile  18 . Upon butt welding the edges  82  to each other, corner/t-joint welding the inner periphery  84  to the component wall  40 , and edge welding an outer periphery  94  of the layer  10  to an outer periphery  96  of the transverse portion  92  of the subcomponent  12 , the combined shrinkage causes the layer  10  to shrink from the pre-weld profile  18  to the post-weld profile  32 . 
         [0030]    It should be noted that although assembly gaps in the layer  10  help to avoid shrinkage restraint, there may be increasing restraint as more and more welds are performed. For example, the first weld may be completely free to shrink and freely draw the pieces  16  together. The last weld, however, may be somewhat restrained by the subcomponent  12 . In principle, this can be avoided or mitigated by using multiple energy sources to perform the welding such that all welds and all shrinkage occur at the same time. Multiple arc weld torches, multiple laser beams, time shared laser beams, multiple resistance welds, etc. could be applied to accomplish this. 
         [0031]    In more common practice where welds are performed one at a time, sequencing of the welds will be helpful in minimizing the restraint during manufacture. For example, before completely welding a given joint, other joints could be partially started as well. As the joints continue to be performed some plastic yielding of partially deposited metal is possible to reduce restraint in the last welds to be completed. 
         [0032]    The layer  10  and the subcomponent  12  of  FIG. 5  could be considered the component  98  when at least one of the component wall  40  and the transverse portion  92  include at least one layer of strip cast superalloy material. For example, if the component wall  40  were fabricated with plural layers of strip cast superalloy material using the process shown in  FIGS. 1 and/or 2 , and the layer  10  is a strip cast superalloy, a component  98  may be considered formed. It is envisioned that the final component may include plural layers of strip cast superalloy material, and may be composed entirely of layers of strip cast superalloy material. 
         [0033]      FIGS. 6-8  disclose another method for creating an allowance to accommodate shrinkage.  FIG. 6  shows the layer  10  of  FIG. 5  having all welds  14  completed except for the edges  82  of the last two pieces  100  to be joined at joint  102  and the outer periphery  94  of the layer  10  to be joined to the outer periphery  96  of the transverse portion  92  of the subcomponent  12  at joint  104 .  FIGS. 7-8  are taken along line A-A of  FIG. 6 .  FIG. 7  shows the pre-weld profile  18 , where wedges  106  angle the pieces  16  away from the transverse portion  92  of the subcomponent  12  and form an angle  108  and a gap  110  there between. As welding is performed the wedges  106  are slid out. Weld shrinkage causes the pieces  18  to rotate toward the transverse portion  92  in the transverse direction  88 , decreasing the gap  110 . Alternately, instead of gradual removal of the wedges  106 , the gap  104  may be created by springs or, for example, a substance which sublimates upon heating such as dry ice, etc. Any mechanism is permissible to create gap  110  initially but to then permit restraint free shrinkage during the weld. 
         [0034]    Various welding processes could be used to create the welds  14  used to accomplish additive manufacturing of cast components using layers of strip cast construction. Examples include arc welding, beam welding, resistance welding, and solid state welding. Brazing may be used for at least some areas to reduce shrinkage and to provide some structural joining, however, except for diffusion brazing or transient liquid phase bonding, brazing would normally result in lower structural strength of the final product than welding. 
         [0035]    In addition, various material properties are possible with layered construction. For example, subsequent layers of different cast materials could be applied to create a part of varied properties throughout, such as improved oxidation resistance for the outermost layer. 
         [0036]    Further, various cast microstructures are possible with layered construction. For example, one layer could be conventionally cast (polycrystalline), and a subsequent layer could be directionally solidified (DS). Cast and wrought materials could be layered together. One layer could be DS and the next layer could also be DS, but could be oriented at a different DS direction of the underlying layer. Limited control of the grain structure created during conventional strip casting of superalloys has been achieved. By example, Inconel® 606 has been strip cast producing fine columnar grains at the surface and equiaxed grains at the centerline. Also, alloy Ni 50 Ti 50  has been strip cast with columnar grains extending from the surfaces of the strip to the centerline. Further development of the process will likely lead to more and better controlled advanced microstructures which can be used in the process disclosed herein. 
         [0037]    From the foregoing it can be seen that the Inventor has devised an improved additive manufacturing process that uses strip cast superalloy material to create components. The strip cast superalloy material is readily available and can be cut to form any shape necessary for a layer. Therefore, it is no longer necessary to create molds etc. to form a part. All that is required is a computer model and a generic sheet of strip cast superalloy material that can be cut as necessary. Further, the assembly process is much quicker than conventional additive manufacturing processes such as SLM and utilizes welding techniques known to those in the art. 
         [0038]    The strip cast superalloy material has a more uniform grain structure than conventionally cast superalloy components where, because of practical limitations of heat extraction, the last to solidify material is of large grain size and typically occurs toward the center of large parts. Alternately, consistency associated with strip casting improves component performance. The layers can be locally tailored and/or varied layer to layer in order to meet local component requirements, such as varying the material grain size, structure and/or orientation, varying the superalloy material composition, and/or varying the layer dimensions etc. The component can include strip cast superalloy layers and layers of other materials as well. All of this leads to an improved ability to locally tailor the component to meet local component requirements. This, in turn, enables cost savings because the entire component need not be manufactured with expensive materials necessary to withstand the harshest local requirement, as it must in a conventional casting process. As such, the process saves in capital costs, saves in manufacturing time and costs, produces a superior component, and does so more quickly than conventional processing. Therefore, it represents an improvement in the art. 
         [0039]    The term “superalloy” is used herein as is understood in the art to describe a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures, as well as good surface stability. Superalloys are often used to form gas turbine engine hot gas path components. Superalloys typically include a base alloying element of nickel, cobalt or nickel-iron. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g., IN 700, IN 738, IN 792, IN 939), Rene alloys (e.g., Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C 263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g., CMSX-4, CMSX-8, CMSX-10) single crystal alloys. 
         [0040]    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.