Patent Application
20170080526
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.
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'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.
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.
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.
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.
The invention is explained in the following description in view of the drawings that show:
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.
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.
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.
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.
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.
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).
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.
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.
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.
The layer 10 and the subcomponent 12 in
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.
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.
The layers 10 may be selectively applied as needed. This can be seen in
In
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
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.
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.
The layer 10 and the subcomponent 12 of
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.
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.
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 Ni50Ti50 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.
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.
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.
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.
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.
