Patent Application
20110135446
The disclosure relates to investment casting. More particularly, it relates to the investment casting of superalloy turbine engine components.
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, and ship propulsion. Cooling of engine components (especially of the turbine section(s)) is provided by flowing relatively cool air from the compressor section(s) of the engine through passageways in the components to be cooled.
For cast components, the cooling passageways may be created using casting cores that are later leached out of the casting. Commonly-assigned U.S. Pat. Nos. 6,637,500 of Shah et al., 6,929,054 of Beals et al., 7,014,424 of Cunha et al., 7,134,475 of Snyder et al., and U.S. Pat. No. 7,438,527 of Albert et al. (the disclosures of which are incorporated by reference herein as if set forth at length) disclose use of ceramic core and refractory metal core (RMC) combinations. The refractory metal cores may be formed from sheet metal (e.g., sheet stock of molybdenum or molybdenum-based alloys).
In certain other industries, asymmetries associated with punching of sheet metal have been observed. In punching, an approximately cylindrical (although not necessarily a circular cylinder) punch may be driven through a piece of metal from a first face side. Along the opposite second face side, the metal is supported by a die having an aperture shape complementary to the punch to receive the coupon of metal punched by the punch. As the punch enters the metal, it locally shears the coupon creating a relatively cylindrical surface very close to the punch cross-section. The engagement of the punch produces high stress concentrations in the material below the shear zone. At some point in the travel of the punch, the stresses are high enough that the material fractures (breaks/ruptures) between the shear zone and the second face. The fracture causes the punched hole to diverge toward the second face. In the situation of a circular punch, the fracture zone is approximately frustoconical.
If a refractory metal core (RMC) is punched, the punching asymmetry may be reflected in an asymmetry of the cast article features cast by the punched features. The punched features may be internal holes (which may cast posts in the ultimate cast article). Alternatively, the punched features may be the lateral perimeter of the RMC (in which case the cast features may be the lateral perimeter of the associated passageway in the cast article). The shear zone of the RMC will cast a relatively narrow portion of the post near one end; whereas the fracture zone will cast a relatively broader portion near the other end. The broader portion will also have a relatively shallow transition to the adjacent face of the slot-like passageway cast by the RMC. Where there is a performance asymmetry across the slot in the cast article in-use, the punching direction may be chosen to address the asymmetry (e.g., resulting from locally high temperature, locally high temperature gradients, locally high temperature fluctuations, locally high inertial loading, locally high stress from external forces, and the like. For example, the relatively broad portions of the posts may fall along the relatively higher desired heat transfer face of the passageway. Also, the relatively broad portions may be along the higher mechanical stress face of the passageway.
Thus, one aspect of the invention involves a method for engineering a cast part. The cast part has internal passageways including a slot. The slot is cast over a metallic sheet casting core. The casting core has a first face, a second face opposite the first face, and a plurality of holes extending between the first and second faces. A slot includes a first face cast by the core first face, a second face cast by the core second face, and a plurality of posts extending between the slot first and second faces and cast by the holes. The method comprises selecting a direction for punching the holes so that an asymmetry in hole cross-section associated with said direction provides an asymmetry in post cross-section associated with one or both of a desired relative heat transfer and a desired relative mechanical strength at said slot second face relative to said slot first face.
In various implementations, a perimeter portion of the core and at least some of the holes may be punched in the same direction. The first group of the holes may be punched from an opposite direction relative to a second group of the holes. The method may be formed using a complete user model which reflects the asymmetry in post cross-section. A computer modeling may model heat transfer including a difference in thermal loading across the slot and a difference in heat transfer associated with the asymmetry in post cross-section.
Other aspects of the invention involve a metallic casting core or core substrate (e.g., prior to coating). The core or substrate has a first face, a second face opposite the first face, a lateral perimeter, and a plurality of through-holes. The through-holes are punched holes each having a shear zone and a fracture zone. The shear zone may be proximate to the first face and the fracture zone may be proximate to the second face. The perimeter may have a shear zone proximate to the first face and a fracture zone proximate to the second face. The core or substrate may be a molybdenum-based alloy. A core may include the core substrate and a coating thereover.
A core may be used in a pattern for casting a component having an airfoil. The pattern has a pattern material having an airfoil portion. A casting core combination may be at least partially embedded in the pattern material and comprises the core or core substrate and at least one additional casting core. The first and second faces may be parallel. The thickness between the first and second faces may be 0.2-2.5 mm over a majority of an area of the metallic casting core.
The pattern may be formed by forming the metallic casting core from sheetstock and molding the at least one additional core of a ceramic. The metallic core and the at least one additional core are assembled. The pattern material may be molded at least partially over the casting core combination for forming the pattern.
The pattern may be used in a method for casting. The pattern is shelled. The pattern material is removed from the shelled pattern for forming a shell. Molten alloy is introduced to the shell. The shell and casting core combination are removed. The method may be used to form a gas turbine engine component.
Another aspect involves a gas turbine engine component comprising an airflow having a leading edge, a trailing edge, and pressure and suction sides extending between the leading and trailing edges. One or more cooling passageways extend through the airfoil from an inlet and include a trailing discharge slot having a plurality of posts. The posts may have relatively broad portions proximate the pressure side and relatively narrow portions proximate the suction side. The slot may have an inboard perimeter portion having a relatively shallow transition to a pressure side face of the slot and a relatively tighter transition to a suction side face of the slot.
Another aspect involves a cast part having internal passageways including a slot being subject to differences in thermal mechanical stresses across the slot. Posts extending across the slot have a relatively broad portion against one of the first and second faces of the slot and a relatively narrow portion adjacent the other. Relatively broad portions are positioned to correspond to the difference in thermal loading such that, at each post, the relatively broad portion is along the relatively higher load one of the first and second faces and the relatively narrow portion is along the relatively lower load face. The article may have an airfoil. The passageway may be a trailing edge slot. The relatively broad portions may fall along a pressure side of the slot. All of the relatively broad portions may fall along the pressure side of the slot.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
A second support plate 54 may be positioned with a surface/face 56 along the first face and may have holes 58 respectively associated with the punches. The second support plate may help maintain sheet planarity (or other shape) during the punching. The holes 58 may be complementary to the associated adjacent punches and may have a smaller clearance than do the holes of the first support plate/die 32. In the punching operation, the actuators bring the distal ends 42 of punches 40 into initial engagement with the first face 22. Further engagement/pressure/movement punches into the sheet.
The initial further movement of the punches will slightly depress the contacted area of the first face, creating a slight rounding/roll zone 60. As stresses accumulate, the punch will penetrate further shearing the metal. This creates a shearing (shear) zone 62 just below the rounding zone 60. Further depression of the punch concentrates stress below the shearing zone. Eventually, the stress is sufficient to cause rupture/fracture/break which creates a rupture/fracture/break zone 64 between the shearing zone and the second face. The fracture zone 64 is generally divergent from the shear zone to the second face. Thus, in the case of a right circular cylindrical punch, the shear zone 62 will be a right circular cylinder of essentially the same diameter as the punch. The fracture zone 64 will be frustoconical, diverging from that diameter to a larger diameter.
Optionally, there may be one or more second punches 80 positioned to engage the second face of the sheet. This may be implemented in any of several ways. First, the second punches could be operable simultaneously with the first punches in a single punching apparatus. In such a situation, the second support and the die would, respectively, have holes complementary to the second punches (in a similar fashion to the holes of the die and second support being complementary to the first punches). In a second alternative, the second punches could be associated with a second punching apparatus (not shown) into which the sheet is moved after punching of the first holes. An intermediate third alternative involves punching the second holes in the same apparatus as the first holes are punched but in a temporally separate step.
The perimeter of the RMC may be punched in the same process and/or apparatus/station or a separate one.
The exemplary shear zone has a depth or height H1 (
The exemplary fracture surface is off-normal to the faces 22 and 24 by an angle θ (which forms the half angle of the frustum) for a circular-sectioned hole. Exemplary θ is 5-40°, more narrowly 10-25°. Exemplary hole diameter (or other traverse dimension for non-circular holes (e.g., slot width for casting a wall)) along the shear zone is 14-26 mil (0.35-0.65 mm), more broadly, 10-50 mil (0.25-1.3 mm). Dimensions of the associated cast features would essentially be similar.
The exemplary blade 20 is cast from an alloy (e.g., a nickel-based superalloy) and has an internal cooling passageway system 144. The exemplary cooling passageway system has a plurality of inlets. The exemplary inlets are along the root 140, more particularly along the inboard end/surface 42. The exemplary blade has inlets 146 (
The exemplary inlets 146 of
The exemplary trailing edge (discharge) slot 162 extends from a final/downstream (near the trailing edge) passageway leg. The exemplary discharge slot has a first face 180 and a second face 182 parallel thereto. The exemplary first face falls close/adjacent to the airfoil suction side 136 and is designated a suction face of the slot. Similarly, the second face is designated a pressure face of the slot. A plurality of posts 190 extend between the first and second faces. The slot may have perimeter portions (e.g., an outboard perimeter portion 192 near the tip and an inboard perimeter portion 194 near the platform.
In an exemplary implementation, the slot first face is cast by the RMC first face and the slot second face is cast by the RMC second face. The posts are cast by the holes in the RMC. Accordingly, the posts have a generally cylindrical first portion 200 (e.g., departing from cylindrical due largely to the effects of the roll zone, the abrasive media treatment (if any) and the smoothing effect of coating (if any)) extending from the slot first face and cast by the roll zone and shear zone of the associated RMC hole. The post dimensions will correspond to the hole dimensions (e.g., within any appropriate contraction tolerance and reflecting abrasive treatment (if any) RMC coating thickness (if any)). The post has a generally frustoconical second portion 202 between the first portion and the second face. The second portion is cast by the fracture zone of the hole and is correspondingly divergent from the first portion.
In the exemplary implementation of a trailing edge slot, the divergent post second portions 202 fall adjacent the pressure side of the airfoil. This side is subject to higher aerodynamic heating which passes higher thermal/mechanical stresses into the airfoil along the slot second face than are present at the slot first face. The relatively blunt/shallow (less tight) interface that the post second portion has with the slot second face allows better stress accommodation than does the relatively steeper (tighter) interface between the first portion and the slot first face. Similarly, at the slot perimeter portions 210 and 212 are respectively cast by the shear (and roll) zones of the RMC perimeter 68 and the fracture zone 72 of the RMC perimeter 68. This orientation of RMC produces a relatively shallower (higher radius of curvature) transition between the portion 212 and the face 182 than between the portion 210 and the face 180. This may be particularly significant near the inboard end 194 relative to the outboard end 192. To provide this similar orientation to those of the posts, the punches 90 are punched from the same direction as the punches 40.
In design/engineering, the principles may be applied in any of several ways. They may be implemented responsive to observation of actual part performance (e.g., part wear/damage/failure or in-use observation such as thermal imaging). Responsive to such observations, the hole punch directions may be changed to provide desired performance benefits (e.g., mitigate cracks that appeared near one side of the slot). It may similarly be used with computer simulations/modeling. For example, a model that does not consider the post asymmetry may simply model the post symmetrically. However, such a model could be used to determine which slot face experiences relatively high thermal and/or mechanical loads and, thereby, could explicitly or implicitly indicate to the user the appropriate punching directions for the appropriate holes. In a more sophisticated modification, the computer model could be programmed to consider the properties of the post asymmetry (e.g., particular angles θ for particular hole diameters and sheet thicknesses). This might be particularly relevant where the model is used to arbitrate between competing performance considerations of which individually might suggest a different hole orientation. The model could be used to select the preferred orientation.
In other situations, the relatively high stress areas might fall along opposite sides of the slot at different portions thereof. In such a situation, correspondingly mixed orientation of posts.
Steps in the manufacture 900 of the core assembly and casting are broadly identified in the flowchart of
In a second step 904, if appropriate, each cutting is bent to form any contouring (e.g., to more slightly bend a portion of the metallic core to more closely follow an associated external surface (if any) of the cast part). More complex forming procedures are also possible. The RMC may receive an abrasive media treatment (e.g., tumbling) 905 as discussed above.
The RMC may be coated 906 with a protective coating. Exemplary coating materials include silica, alumina, zirconia, chromia, mullite and hafnia. Coatings may be applied by any appropriate line-of sight or non-line-of sight technique (e.g., chemical or physical vapor deposition (CVD, PVD) methods, plasma spray methods, electrophoresis, and sol gel methods). Individual layers may typically be 0.1 to 1 mil (2.5 to 25 micrometer) thick. Layers of Pt, other noble metals, Cr, Si, W, and/or Al, or other non-metallic materials may be applied to the metallic core elements for oxidation protection in combination with a ceramic coating for protection from molten metal erosion and dissolution.
The RMCs may then be mated/assembled 908 to the feedcore. For example, the feedcore may be pre-molded 910 and, optionally, pre-fired. Optionally, a ceramic adhesive or other securing means may be used. An exemplary ceramic adhesive is a colloid which may be dried by a microwave process. Alternatively, the feedcore may be overmolded to the RMCs. For example, the RMCs may be placed in a die and the feedcore (e.g., silica-, zircon-, or alumina-based) molded thereover.
The overmolded core assembly (or group of assemblies) forms a casting pattern with an exterior shape largely corresponding to the exterior shape of the part to be cast. The pattern may then be assembled 932 to a shelling fixture. The pattern may then be shelled 934 (e.g., via one or more stages of slurry dipping, slurry spraying, or the like). After the shell is built up, it may be dried 936. The drying provides the shell with at least sufficient strength or other physical integrity properties to permit subsequent processing. For example, the shell containing the invested core assembly may be disassembled 938 fully or partially from the shelling fixture and then transferred 940 to a dewaxer (e.g., a steam autoclave). In the dewaxer, a steam dewax process 942 removes a major portion of the wax leaving the core assembly secured within the shell. The shell and core assembly will largely form the ultimate mold. However, the dewax process typically leaves a wax or byproduct hydrocarbon residue on the shell interior and core assembly.
After the dewax, the shell is transferred 944 to a furnace (e.g., containing air or other oxidizing atmosphere) in which it is heated 946 to strengthen the shell and remove any remaining wax residue (e.g., by vaporization) and/or converting hydrocarbon residue to carbon. Oxygen in the atmosphere reacts with the carbon to form carbon dioxide. Removal of the carbon is advantageous to reduce or eliminate the formation of detrimental carbides in the metal casting. Removing carbon offers the additional advantage of reducing the potential for clogging the vacuum pumps used in subsequent stages of operation.
The mold may be removed from the atmospheric furnace, allowed to cool, and inspected 948. The mold may be seeded 950 by placing a metallic seed in the mold to establish the ultimate crystal structure of a directionally solidified (DS) casting or a single-crystal (SX) casting. Nevertheless the present teachings may be applied to other DS and SX casting techniques (e.g., wherein the shell geometry defines a grain selector) or to casting of other microstructures. The mold may be transferred 952 to a casting furnace (e.g., placed atop a chill plate in the furnace). The casting furnace may be pumped down to vacuum 954 or charged with a non-oxidizing atmosphere (e.g., inert gas) to prevent oxidation of the casting alloy. The casting furnace is heated 956 to preheat the mold. This preheating serves two purposes: to further harden and strengthen the shell; and to preheat the shell for the introduction of molten alloy to prevent thermal shock and premature solidification of the alloy.
After preheating and while still under vacuum conditions, the molten alloy is poured 958 into the mold and the mold is allowed to cool to solidify 960 the alloy (e.g., after withdrawal from the furnace hot zone). After solidification, the vacuum may be broken 962 and the chilled mold removed 964 from the casting furnace. The shell may be removed in a deshelling process 966 (e.g., mechanical breaking of the shell).
The core assembly is removed in a decoring process 968 to leave a cast article (e.g., a metallic precursor of the ultimate part). The cast article may be machined 970, chemically and/or thermally treated 972 and coated 974 to form the ultimate part. Some or all of any machining or chemical or thermal treatment may be performed before the decoring.
One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, the principles may be implemented using modifications of various existing or yet-developed processes, apparatus, or resulting cast article structures (e.g., in a reengineering of a baseline cast article to modify cooling passageway configuration). In any such implementation, details of the baseline process, apparatus, or article may influence details of the particular implementation. Accordingly, other embodiments are within the scope of the following claims.
