Patent Description:
In typical manufacture of nickel-based superalloy disks (e.g., for gas turbine engine turbine sections or high pressure compressor (HPC) sections), manufacture is by forging of powder metallurgical (PM) or cast forms.

In distinction, only casting techniques are typically used to form blades, vanes, and combustor panels. Many blades are manufactured by single crystal casting techniques. In an exemplary single crystal casting technique, a seed of single crystal material is used to define a crystalline orientation that propagates into the cast blade alloy as it cools and solidifies.

In casting blades, etc., it is well known that removal of high angle grain boundaries (< <NUM>°) in single crystal nickel base superalloys leads to improved creep resistance and consequently enhances its temperature capability. In addition, it is also known that by properly orienting the low modulus <<NUM>> direction along the direction in which high thermal strain exists, the thermal mechanical fatigue (TMF) capability of the material can also be significantly improved.

However, direct application of nickel base superalloy single crystal to a component such as a turbine disk, has not been practical. This is so because loading of such components due to high rotation speed around an axis is axially symmetric and will lead to uneven strain distribution in a single crystal body, with cubic symmetry and anisotropic elastic and plastic properties.

One method to achieve an axially symmetric ring is to bond separately cast single crystal segments. This has been considered by <CIT>. However, this approach requires bonding between circumferential segments, in the high temperature area, which is likely to create weak points.

Another approach is described in <CIT>. A single crystal rim is cast along with single crystal blades and then diffusion bonded to a high strength conventional disk. This approach relies on casting the bladed ring using a large number of discrete single crystal seeds such that presence of high angle grain boundaries is avoided. In this approach, for example, if one wishes to limit the grain boundary misorientation to <NUM>°, then ideally <NUM>°/<NUM>°=<NUM> discrete single crystal seeds may be required.

<CIT> discloses a system for producing cast components using a precision casting system driven by a pressure differential between the molten metal delivery system and the mold.

One aspect of the disclosure involves a method for casting a component according to claim <NUM>.

A further embodiment may additionally and/or alternatively include partially melting the seed.

A further embodiment may additionally and/or alternatively include the seed having weld or braze joint.

A further embodiment may additionally and/or alternatively include the weld or braze joint being partial height.

A further embodiment may additionally and/or alternatively include forming the seed by: casting at least one precursor of the seed; and bending the at least one precursor into said arcuate form.

A further embodiment may additionally and/or alternatively include forming the seed by bending at least one precursor of the seed into said arcuate form.

A further embodiment may additionally and/or alternatively include the bending being by at least <NUM>°.

A further embodiment may additionally and/or alternatively include the assembling comprising one or more of: clamping; welding; and brazing.

A further embodiment may additionally and/or alternatively include the assembling comprising tack welding.

A further embodiment may additionally and/or alternatively include the providing the molten material comprising pouring a molten metal.

A further embodiment may additionally and/or alternatively include: forging the solidified metal; and machining the forged metal.

A further embodiment may additionally and/or alternatively include a pre-forging height to diameter ratio being not greater than <NUM>.

A further embodiment may additionally and/or alternatively include: ring rolling the solidified metal; and machining the rolled metal.

A further embodiment may additionally and/or alternatively include the component being one of: a turbine engine disk or component thereof; a blade outer air seal; a combustor panel; or an engine case or component thereof.

A further embodiment may additionally and/or alternatively include the cooling and solidifying comprising downwardly shifting a shell.

A further embodiment may additionally and/or alternatively include the cooling and solidifying comprising upwardly drawing from a melt pool.

A further embodiment may additionally and/or alternatively include the melting comprising a local melting with a heat source moving relative to the seed to melt material while leaving solid material above and below the melted material.

A further embodiment may additionally and/or alternatively include a disk manufacture method, comprising: casting a first component according to the method above, wherein the first component is a disk or a component thereof; and bonding a second component concentrically within the first component to form a disk.

A further embodiment may additionally and/or alternatively include forging and machining the cast first component prior to the bonding.

Another aspect of the invention discloses a turbine engine rotor component comprising: a continuous structure circumscribing a central aperture along a central longitudinal axis; and a crystalline structure continuously progressively varying around the central longitudinal axis wherein the structure has been formed by any of the above methods of casting.

A further embodiment may additionally and/or alternatively include the component being a disk rim in combination with a disk bore.

A further embodiment may additionally and/or alternatively include the combination wherein: the disk rim has a [<NUM>] axis within <NUM>° of axial; and the disk bore has a [<NUM>] axis within <NUM>° of axial.

A further embodiment may additionally and/or alternatively include the structure being a nickel-based superalloy.

The details of one or more non-limiting embodiments are set forth in the accompanying drawings and the description below.

As is described in detail below, a casting method uses one or more bent seeds (e.g., single crystal) to cast an arcuate (e.g., generally annular) component. Exemplary generally annular components may be a disk rim or an entire disk. A full annulus seed may be formed by bending one seed a full <NUM>° and then securing the ends. Alternatively, a plurality of arcuate segments may be assembled end-to-end to form the final seed.

Use of bent seeds to propagate their crystalline orientation into the annular component results in the annular component having a continuously progressively circumferentially changing crystalline orientation that essentially remains constant relative to the local surface. Thus, for example, the component may have one given crystalline direction extending radially outward along the entire circumferential extent of the component. In a specific example, the entire disk body is processed by casting and forging a curved single crystal ring.

As shown in the flow chart of <FIG>, in one exemplary method, single crystal plates are cast <NUM>, bent <NUM>, and assembled <NUM> into a seed. The seed is used to cast <NUM> an annular component.

Exemplary plates are initially formed as right parallelepipeds. <FIG> shows the exemplary plate <NUM> having opposed pairs of faces: 22A, 22B; 24A, 24B; and 26A, 26B. Exemplary dimensions between the opposite faces of each pair are respectively labeled S<NUM>, S<NUM>, and S<NUM>. In one example, these are in progressively increasing order: <NUM> inch (<NUM>), <NUM> inches (<NUM>), and <NUM> inches (<NUM>). It is thus seen in this example that one dimension is much longer than the other two. This longer dimension may be appropriate for bending into a seed so that this long dimension becomes the arc of a circular segment.

An exemplary seed material is a superalloy, more particularly, a nickel-based superalloy, more particularly, a superalloy with a solvus temperature greater than <NUM>°F (<NUM>). PWA1484 is an exemplary such alloy.

Such plates <NUM> may be cast in groups as is known, for example, in the casting of blades. Exemplary such casting involves a mold cluster having <NUM>-<NUM> cavities for casting respective plates (more particularly, <NUM>-<NUM> cavities). The molds (or individual mold segments for forming a cluster) may be formed using conventional techniques such as forming a ceramic stucco shell over wax patterns.

<FIG> schematically shows the cavity <NUM> of such a mold. A portion or section <NUM> of the cavity generally corresponds to the volume in which the seed <NUM> is cast. There may be a headspace <NUM> of the cavity thereabove.

<FIG> shows a casting seed <NUM> associated with the cavity <NUM>. In this example, which is typical of blade casting configurations, the seed connects to the main portion of the cavity <NUM> via a spiral (helical) grain starter passageway <NUM> extending upward from the seed to a gating region <NUM> which diverges outwardly to join the cross-section of cavity portion <NUM> corresponding to the ultimate seed. In this example, the main portion <NUM> of the cavity is vertically elongate to correspond to the long dimension S<NUM> and has a transverse footprint corresponding to the dimensions S<NUM> and S<NUM> (e.g., potentially reflecting slightly greater size to allow for machining (if any) of the plate <NUM> to final specification).

The exemplary seed <NUM> is a right parallelepiped having a bottom 50A, a top 50B, a first pair of sides 52A, 52B, and a second pair of sides 54A, 54B. <FIG> shows an exemplary vertical direction <NUM>. A surface normal of the seed face 52A is shown as <NUM> and a surface normal of the face 54A is shown as <NUM>. A direction corresponding to the surface normal of the face 22A of the plate is shown as <NUM> and a direction corresponding to the surface normal of the face 24A is shown as <NUM>.

Exemplary seeds <NUM> are of single crystal nickel-based superalloy. More particularly, they may be of the same material used to form the plates. Exemplary seeds <NUM> are square-sectioned and vertically elongate (e.g., of dimensions <NUM> inch (<NUM>) x <NUM> inch (<NUM>) x <NUM> inch (<NUM>). As shown in <FIG> and <FIG>, seeds <NUM> are oriented in an [<NUM>]-upward direction and rotated relative to the cavity <NUM> and to cast the plates such that normal <NUM> and <NUM> are parallel to [<NUM>] or [-<NUM>,<NUM>,<NUM>] directions, respectively, which are orthogonal to [<NUM>] directions. In this example, viewed downward, the direction <NUM> is rotated <NUM>° counterclockwise about the direction <NUM> relative to the direction <NUM> and the direction <NUM> is <NUM>° counterclockwise of the direction <NUM>.

As in conventional single-crystal casting, the individual seeds <NUM> may be obtained by machining out blocks from: (a) naturally cast material (cast without seeding) <<NUM>> oriented single crystal; or (b) material cast using another seed. In a conventional manner, the investment casting mold cluster (not shown) may be placed or assembled on a water-cooled copper chill plate in a commercially available directional solidification furnace in such a way that a lower portion (e.g., the bottom half) of the seeds remain solid when the mold is heated to the melt temperature of the alloy (e.g., in excess of <NUM>°F (<NUM>)). Molten metal is poured to fill the mold and then the mold is withdrawn downwardly from the hot zone of the furnace (e.g., at a rate of <NUM>-<NUM> inches (<NUM>-<NUM>), more narrowly, <NUM>-<NUM> inches (<NUM>-<NUM>) per hour). This allows the crystalline orientation of the unmelted bottom portion of the seeds which is kept solid to propagate into the solidifying metal. The helical connectors <NUM> between the seeds and the plate is a standard casting practice to eliminate any stray recrystallized or randomly nucleated grain that may have developed on the surface of the seed.

After solidification, there may be a conventional deshelling <NUM> of the castings and excess material may be removed <NUM>. For example, the casting will leave the surfaces 22A, 22B and 24A, 24B or precursors thereof. However, there may be cast gating material from the gating region <NUM> which may then be cut away to form one of the end faces 26A, 26B. Similarly, excess material cast in the headspace <NUM> to be cut away to form the other surface 26A, 26B. Additional machining as part of step <NUM> may remove any final shell material and/or true up the surfaces. Yet further machining may cant the surfaces to improve assembly upon subsequent bending. For example, the surfaces 26A, 26B may be angled to converge toward each other so as to allow close mating after assembly of bent plates.

Various inspection steps <NUM> may occur at this or other points. In one exemplary inspection of the cleaned/machined plates, the plates are macroetched to reveal any grain defects. Plates that pass said inspection stage may be further evaluated using X-ray Laue technique to determine crystal orientation. Plates that do not have crystalline orientation within a desired tolerance may be rejected. In one example, plates are accepted only if the primary axis <NUM> is within a threshold (e.g., <NUM>°) of a [<NUM>] direction and the width direction <NUM> and the thickness direction <NUM> are within a threshold (e.g., also <NUM>°) of [<NUM>] and [-<NUM>,<NUM>,<NUM>] directions respectively.

Acceptable plates may then be solution heat treated <NUM>. Exemplary solution heat treat is for thirty minutes at <NUM>°F (<NUM>). This may be followed by a slow cool <NUM> to a slightly lower temperature (e.g., at a rate of <NUM>°F/min to <NUM>°F (<NUM>/min to <NUM>)) and then an air cool <NUM> (e.g., back to ambient temperature).

Such plates are then handled carefully to avoid impact damage of the surface and slowly bent <NUM>. Exemplary bending is via standard mechanical bending techniques. Exemplary bending is performed at room temperature. Exemplary bending is about directions parallel to <NUM> so that the span S<NUM> becomes an arc span. In the situation where the final seed is to be an end-to-end assembly of segments, each plate may be bent to approximately the nominal arc span of the final segment. For example, for an assembly of four segments, the nominal arc span would be <NUM>°. The bending may be to within an exemplary <NUM>° or <NUM>° of such nominal value or within an exemplary <NUM>% or <NUM>% of that arc span. In either case, deformation to the final arc span may occur during assembly and securing of the segments. An exemplary number of segments is <NUM>-<NUM>. Alternative upper limits on such range are <NUM>, <NUM>, <NUM>, and <NUM>. Alternative lower limits with any of said upper limits are <NUM>, <NUM>, and <NUM>. Thus, for example, an exemplary eight plates each have a final arc span of <NUM>°. As noted above, the initial bending may thus be by at least <NUM>° for such example.

<FIG> shows a pair of bent plates <NUM>'. For ease of illustration, the respective features of the bent plates are referenced with the same numerals used in <FIG>. In this example, the bent plates have essentially semicircular planform viewed from above. Thus, in this example, a [<NUM>] direction corresponds with a circumferential direction at all points along the arc length of the bent plate.

A final machining <NUM>, if necessary, may precision cut the plates to form a desired full-annulus assembly. For example, the plates may be assembled around a cylindrical mandrel of a given diameter and machined to fit with their respective surfaces 22A engaging the cylinder or having a minimal gap.

A further stress relief <NUM> step may follow. An exemplary stress relief step involves strapping the two plates together around a mandrel and raising to a temperature less than the solution heat treat temperature (e.g., slightly less than the temperature at the end of the slow cool stage). In this example, the stress relief temperature is <NUM>°F (<NUM>). The bent plates may then be re-evaluated/inspected <NUM> to ensure that no recrystallized grains are formed. X-ray Laue technique may be used to verify that [<NUM>] orientation along the axis of the plate curves around the bent plate.

The bent plates may be assembled/secured <NUM> to each other. Exemplary securing involves welding or diffusion bonding. A particular welding is a tack welding along a lower portion of the annular assembly of plates. <FIG> schematically shows the weld in the form of weld zones <NUM> along lower portions of the surfaces 26A, 26B. Exemplary welds are confined to the lower half of the vertical span of the seed, more particularly, to the lower third. During subsequent casting <NUM> using the assembled plates as a seed, the tack welded portion remains solid. Thereby, any disturbance to crystalline orientation caused by the tack welding does not affect casting. Accordingly, upper portions of the plates including the portion that melts and some portion therebelow may have a mere abutting engagement between ends 26A, 26B. As noted above, the final assembly may be a full <NUM>° annulus. Alternative assemblies may involve less than a full annulus but may preferably be essentially full annulus (e.g., at least <NUM>°). In the illustrated embodiment, the ends 26A, 26B are at essentially right angles to the adjacent surfaces and therefore radial when viewed in the context of the overall seed assembly <NUM> and its central longitudinal axis. Alternative implementations may partially tangentially orient these ends. This provides greater mating surface of the abutting pairs of ends and also can create a more gradual transition in any crystalline propagation associated with the two distinct pieces (i.e., circumferentially around the seed at the joint the cross-section will progressively transition between the two pieces <NUM>'). Such exemplary angles may be from <NUM>° to an exemplary <NUM>° off-radial, more particularly, <NUM>°-<NUM>°.

<FIG> shows a shell <NUM> atop a chill plate <NUM>. The shell contains a seed <NUM> formed by the assembled bent plates. The shell contains an annular cavity <NUM> having a portion <NUM> for forming an annular disk precursor. A pour cone <NUM> is also shown above the cavity <NUM>. To provide communication with the seed <NUM>, an annular gating region <NUM> is at a lower end of the cavity portion <NUM> and communicates with the seed via an annular grain starter <NUM>. The grain starter <NUM> attempts to mimic the function of a helical grain starter. However, due to its annular nature, its cross-section shifts radially inward and outward instead of spiraling. The exemplary shift is enough to avoid line-of-sight between the lower end of the grain starter and the upper end.

The shell <NUM> may be prepared <NUM> by conventional techniques of shelling assembled wax pattern components.

The exemplary cavity portion <NUM> is sized to cast an approximately <NUM> inch (<NUM>) tall ring of <NUM> inch (<NUM>) outer diameter and <NUM> inch (<NUM>) inner diameter. After the shell is de-waxed and fired, the bent plate seed assembly <NUM> may be inserted from the bottom into the seed cavity. The seed cavity is designed taking into account the differential thermal expansion between the seed alloy and the mold material.

Once again the assembled investment casting mold is placed on a water cooled copper chill plate in a commercially available directional solidification furnace in such a way that bottom half of the seed <NUM> remains solid when the mold is heated to a melt temperature of the alloy (e.g., in excess of <NUM>°F (<NUM>).

<FIG> shows the shell <NUM> in an induction furnace after pouring of metal but during withdrawal/solidification. The furnace includes a susceptor <NUM> surrounded by an induction coil <NUM>. Molten metal <NUM> fills the shell to a surface level <NUM> with a solidification front <NUM> shown near the base of the susceptor.

Molten metal composition is poured into the pour cone to fill the mold and then the mold is withdrawn from the hot zone of the furnace (e.g., at a rate of <NUM>-<NUM> inches (<NUM>-<NUM>) per hour). This allows molten metal to copy the crystalline orientation of the bottom portion of the seed <NUM> which is kept solid. In this case, unlike the normal seeding process, the crystalline orientation of the seed <NUM> at each circumferential location is copied to the cylindrical component above. As long as the transverse temperature in the casting furnace is sufficiently uniform, this results in a single crystal ring with the axis <NUM> having the [<NUM>] orientation identical to that along the width of the seed plate and the hoop of the ring curving along the [<NUM>] direction tangentially following the circumference of the curved plates.

As is discussed further below, the molten metal may be of the same composition used for the seed <NUM> and/or seed <NUM> or of a different composition. For example, the composition of the seed <NUM> may be selected merely for its seeding properties. The composition of the seed <NUM> may also be chosen for its seeding properties. In the example above, these are both PWA <NUM>. However, the material of seed <NUM> may be modified if this would facilitate the bending, etc. Additionally, it may develop that certain materials have better relative abilities to seed when bent than others compared with unbent states. However, in a first example, the subsequent molten metal is selected for the properties of the ultimate casting.

In alternative embodiments (including other embodiments discussed below) this step may be used to cast more seeds. For example, the annular bent seed assembly <NUM> may be used to cast a tube-like structure which is then axially segmented and the individual annular segments used as seeds to cast final components. Such a process may be used if the additional step does not adversely affect final crystalline orientation and may be economically advantageous.

In the first example, after a routine deshelling <NUM> process, the casting is solution heat treated <NUM> (e.g., at <NUM>°F (<NUM>) for <NUM> minutes) and then air cooled <NUM>. After the solution heat treatment, the standard procedure currently used for single crystal turbine blades may be followed. First seed, and other extraneous material is cut off <NUM>, and the surface cleaned <NUM>, to provide the desired ring component. This is then macroetched <NUM> to reveal grain defects, if any (e.g., to become visible to the naked eye).

If grain defects are found, then Laue analysis <NUM> is needed to define grain misorientation. For the exemplary alloy PWA <NUM>, if the misorientation is less than a threshold value (e.g., <NUM>°), then the ring may be considered an acceptable single crystal. Anything with misorientation greater than that may be acceptable if the defect is limited to a small region or anticipated to be machined out in subsequent operation or is in an area not deemed critical from a structural point of view.

Beyond this though, the resulting large curved single crystal may advantageously be subjected to a more extensive X-ray Laue analysis <NUM> at multiple locations than is a cast blade. This may be achieved by taking multiple (e.g., <NUM> to <NUM> for full annulus or at least three for every <NUM>° of arc) X-ray Laue patterns along the top of the ring, parallel to the ring axis <NUM>, evenly spaced interval along the circumference. The objective of this analysis is to verify; (a) that axial orientation at each location deviates no more than a specified angle (e.g., <NUM>°) from the target (e.g., [<NUM>]) orientation; and (b) the projection of the second target (e.g., [<NUM>]) orientation in the plane normal to the axis of the disk is within a specified range (e.g., ± <NUM>°) of the local tangential direction. As has been the case with prior art single crystal blades, the specified angular tolerance is generally determined by a combined consideration of casting yield and structural requirements.

Alternatively the cast ring may be evaluated by nondestructive evaluation (NDE) technique based on sound velocity locally and/or globally. Measurement of sound velocity near the surface or through the volume allows one to determine elastic modulus, which in turn allows one to define the acceptable range of crystal orientation. This methodology is not direct but is more suitable for a large component like this.

Such a cast ring may then be given an additional heat <NUM> treatment (e.g., of <NUM>°F/4hrs (<NUM>/<NUM> hours)) and a precipitation hardening cycle (e.g., of <NUM>°F/<NUM> hours (<NUM>/<NUM> hours)) and appropriately machined to form a bladed rim or simply an outer rim to be bonded to a conventional fine grained disk which will form the high strength bore of the disk. In a bladed rim for example, the single crystal orientation may be such that blade axis <NUM> is along [<NUM>]. That does not change the rest of the method described here. In either example, the rim may be machined <NUM> to form blades or features (e.g., slots) for mounting blades.

However, a particular approach is to forge the single crystal ring at least to some degree. Such a warm working increases dislocation density and makes plastic response of cast single crystal much more uniform. Generally low temperature (<<NUM>°F (<<NUM>)) creep response to small strain (e.g., of <NUM>-<NUM>%) is improved with suppression of primary creep. To forge the ring, the cast ring is re-solutioned <NUM> (e.g., at <NUM>°F (<NUM>)) and slow cooled <NUM> (e.g., at <NUM>°F/min (<NUM>/min) to <NUM>°F (<NUM>) to coarsen the gamma prime precipitates. In this condition then the part is forged <NUM> isothermally (e.g., at <NUM>°F (<NUM>) at a strain rate of <NUM>-<NUM> inch/inch/min (cm/cm/min) or slower to reduce the height (e.g., by ~<NUM>%, more broadly, <NUM>%-<NUM>%). To achieve uniform deformation, keeping the aspect ratio of height to diameter to be less than (or at least not more than) <NUM> is desirable. The ring may be machined down to this aspect ratio. This includes the possibility of cutting multiple rings from a casting of greater height. Also it is desirable that the ring be enclosed in a sacrificial outer container (sleeve) (e.g., with wall thickness at least one third the outer radius of the ring). This serves to keep the shape circular and provide evenness of the foregoing deformation. <FIG> schematically illustrate the step with cast ring <NUM> and sleeve <NUM>. The initial length of both the ring and sleeve is shown as L<NUM>. The initial ring outer diameter and sleeve inner diameter is shown as DO1. The initial ring inner diameter is shown as DI1. The sleeve thickness is shown as TS1 and the ring thickness is shown as TR1. After forging (<FIG>) the ring and sleeve are shown as <NUM>' and <NUM>' with length L<NUM>. DO1 has expanded to DO2. The exemplary forging is such that L<NUM> is initially less than or equal to DO1 but is decreased by the amounts mentioned above to L<NUM>. After forging the sacrificial sleeve <NUM>' is cut and discarded <NUM>. The as-forged ring <NUM>' is then finally heat treated <NUM> (e.g., for <NUM> hours at <NUM>°F (<NUM>) and is ready for machining <NUM>.

After the forging, the X-ray Laue technique is generally not usable to ensure that no undesirable recrystallization has taken place. But in lieu, X-ray texture analysis <NUM> (before or after the machining) investigation can be performed to track the crystalline texture of the material. Alternatively, once again at this stage an NDE technique based on sound velocity may be employed, to achieve the same goal. In production, an array of detectors may be used to provide desired elastic modulus/ orientation information at as many points as desired for quality assurance.

Depending on the desired performance and engineering requirements, optimum balance of tensile and creep properties may be achieved by varying: (<NUM>) the cooling rate from the solution temperature; aspect ratio prior to forging; and (<NUM>) forging rate within <NUM>-10X bound of the numbers specified. For a different nickel base superalloy, the solution temperature, grain misorientation acceptance standard, and forging parameters are expected to change for optimum performance.

Three distinct classes of nickel base superalloys for casting using seed <NUM> are discussed below. First, nickel and iron base alloy IN <NUM> (and similar) is already widely used in polycrystalline cast disks. The alloy has the ability to provide strengthening via two different types of precipitates, of which one forms at lower temperature. These lower temperature precipitates may have a particularly significant advantage of increasing strength of forged single crystal. Exemplary IN-<NUM> derivative alloys have a weight percentage composition of carbon <NUM>-<NUM>, chromium <NUM>-<NUM>, molybdenum <NUM>-<NUM>, tungsten <NUM>-<NUM>, cobalt <NUM>-<NUM>, iron <NUM>-<NUM>, niobium <NUM>-<NUM>, titanium <NUM>-<NUM>, aluminum <NUM>-<NUM>, boron <NUM>-<NUM>, remainder nickel plus impurities.

Another class of alloys which has been developed specifically for cast and PM turbine disk applications have moderately high amount of grain boundary strengthening elements such as carbon, boron, zirconium and hafnium. Such alloys generally display high tensile properties at lower temperature, and greater fracture resistance and are likely to display greater tolerance to grain defects (as would be inherent in the bent seed casting). The combination of attributes may help mitigate manufacturing risk. For both class of alloys, the present bent seed approach is expected to enhance their temperature performance relative to their current use in polycrystalline form. Exemplary compositions of such gamma prime precipitation strengthened nickel-base disk alloy comprise, by weight percentage, carbon <NUM>-<NUM>, chromium <NUM>-<NUM>, molybdenum <NUM>-<NUM>, tungsten <NUM>-<NUM>, cobalt <NUM>-<NUM>, niobium <NUM>-<NUM>, titanium <NUM>-<NUM>, aluminum <NUM>-<NUM>, boron <NUM>-<NUM>, tantalum <NUM>-<NUM>, zirconium <NUM>-<NUM>, hafnium <NUM>-<NUM>, vanadium <NUM>-<NUM>, remainder nickel and impurities.

The third class of alloys was developed for single-crystal blade applications. These are most creep resistant and are most attractive for achieving the highest temperature performance. However, such alloys are likely to show lowest tolerance to grain boundary defects. Exemplary such superalloys comprise, by weight percent, carbon <NUM>-<NUM>, chromium <NUM>-<NUM>, molybdenum <NUM>-<NUM>, tungsten <NUM>-<NUM>, cobalt <NUM>-<NUM>, niobium <NUM>-. <NUM>, titanium <NUM>-<NUM>, aluminum <NUM>-<NUM>, boron <NUM>-<NUM>, tantalum <NUM>-<NUM>, zirconium <NUM>-<NUM>, hafnium <NUM>-<NUM>, rhenium <NUM>-<NUM>, ruthenium <NUM>-<NUM>, remainder nickel and impurities.

Yet other potential alloys include refractory metal-based alloys.

The foregoing example gave one particular set of crystalline orientations. However, the process can be used with any combination of axial and circumferential crystalline orientations which are orthogonal to each other. As shown in <FIG>, the single crystal ring <NUM> has axial orientation [uvw], with orthogonal orientation [pqr] curving around the circumferential direction. Of these, a few specific combination pairs are of initial interest depending on the design and end application intent. This is summarized in Table I.

Similarly, a variety of bonded combinations of rim and bore materials are possible, of which the combinations listed in Table II are of specific interest.

<FIG> shows a disk formed with a bore <NUM> bonded to an integrally bladed rim <NUM>. The rim <NUM> includes a ring portion <NUM> and a circumferential array of blades <NUM> extending from an outer diameter (OD) surface of the ring to free tips. The exemplary blades <NUM> are airfoils extending from a leading edge to a trailing edge and having a pressure side and a suction side between such edges. The bore <NUM> has an inner diameter (ID) surface <NUM> (circumscribing a central aperture along a control longitudinal axis <NUM>) and an outer diameter (OD) surface <NUM>. The ring <NUM> has an ID surface <NUM> (similarly circumscribing a central aperture along the axis <NUM>) mated to the OD surface <NUM> and bonded thereto by a bond <NUM> (e.g., via friction welding or other bonding process). From the part's point of view, the axis <NUM> is coincident with various of the aforementioned central longitudinal axes of part precursors and annular seeds.

<FIG> shows an otherwise similar disk but where a ring <NUM> replaces the ring <NUM> and has a circumferential array of blade retention slots <NUM> in its OD surface. The slots <NUM> receive the fir tree roots of blades (not shown) which may be separately formed and may be conventional cast single crystal blades. Generally the intent of bonding rim or bladed rim to bore is to achieve optimum high temperature performance of blades and rim while maintaining low temperature tensile strength of the bore.

Also in the exemplary case one method in which the partially bonded bent seed assembly being introduced into a pre-fabricated shell mold was described in details. Several modifications of this method can also be practiced. In one case the partially bonded bent seed assembly <NUM> may be fitted around a ceramic core (e.g., a molded core) <NUM> and then an external shell mold <NUM> assembled to or built around that as shown in <FIG>. Thus, for example, the assembly of seed <NUM> and core <NUM> may be inserted into a shell <NUM> that is pre-formed via shelling a wax pattern in the conventional manner. Alternatively, the assembly of seed <NUM> and core <NUM> may be over-molded with the wax for, in turn, receiving ceramic stucco in a shelling process to form the shell <NUM>. Such processes discussed relative to <FIG> may have some advantages for preserving structural integrity of the large ceramic mold.

In yet another approach a pre-fabricated ceramic crucible can be used in place of the shell <NUM>. Use of such a crucible may require modification of the geometry of the core <NUM> and may include inserting an additional core around the seed <NUM> in the base of the crucible. However, this may allow economical use of off-the-shelf crucibles instead of preparing shell molds from scratch for every casting.

In yet another alternative approach, a thick ceramic clamshell mold may be used, allowing a quick assembly and extraction of castings.

These variations in mold preparations are possible because unlike a single crystal blade with complex external shape and internal cooling passages, the casting shape requirements may be relatively simple and axially symmetric.

As is discussed above, the casting process may initially be used to make a seed precursor from which one may harvest multiple arcuate seeds for use in yet further casting stages. A mold may be designed specifically to make a long arcuate or full ring seed casting from which multiple seeds may be cut. Such an approach will help eliminate heat treatment, bending, and tack welding etc. of single crystal plates for producing seed assembly. More particularly, it may reduce the frequency of such steps as those steps might be used only to make a smaller number of master seeds <NUM>, each of which might yield ten or more seed rings or seed arcuate segments.

Although the primary example has involved a full-annulus seed and casting, less than a full annulus seed may be used to form a full annulus component. Also, components of less than full annulus may be made. Examples of such components having arcuate cross-section are blade outer air seal (BOAS) segments, combustor panels, knife edge seals, drum rotors, and engine cases. Such segments and panels may be more likely than disks or other annular components to be directly cast rather than cast and forged.

In addition to a modified conventional directionally solidified casting process, various other processes may be used. A first alternative group of processes involve a modified Czochralski method using the arcuate segment or full-annulus seed. The Czochralski process is used in single crystal semiconductor growth and involves introducing a seed at the top of a body of molten material and drawing the seed and progressively solidifying material upward from the body. This can produce a cylindrical cast body. Thus, the body cast from a full annulus seed may be a generally tubular structure. This has possible advantages in the growth of relatively long bodies as might be used for shafts or portions thereof. Also since such process is primarily containerless, risk of contaminating the material with ceramic inclusions is reduced, which is considered very critical for improving fatigue life.

Also a float zone melting process may use the bent seed <NUM>. In <FIG>, a hollow cylinder <NUM> of starting material in polycrystalline form is held around a ceramic core <NUM> touching a bent seed assembly <NUM>. Then a local heating element <NUM> (e.g., an induction coil surrounding a susceptor) is used to melt both the starting material and the seed such that liquid metal <NUM>' is held in place by high viscosity, surface tension, and small volume. The heat source can be induction, electric resistance, or optical. Then either the heating zone is traversed away from the seed or the entire seed-starting material ingot is moved out of the heating zone. This allows the liquid metal in close contact with the seed to copy its local orientation in the direction of movement. In this particular example, the heating source <NUM> is shown at an intermediate stage of movement in a vertical direction <NUM>. As noted above, the traversal starts with the source at even level with an upper portion of the seed to melt the seed upper portion. Then the traversal continues up along the height of the cylinder <NUM> leaving single crystal material <NUM>" therebehind with crystalline orientation following that of the seed <NUM>.

To increase thermal gradient, and help the solidification process, a cooling coil <NUM> may be used right below the hot zone. Thus, the cooling coil <NUM> may move vertically with the heat source <NUM> as a unit. In this type of float zone process it is customary to rotate the top and the bottom part of the solid cylinder counter to each other to help viscous liquid metal stay axially symmetric and better mixed. <FIG> shows the seed and solidified material <NUM>" rotating in the direction <NUM> about the central vertical axis of the apparatus while the yet unmelted material <NUM> rotates in an opposite direction <NUM>. This produces shearing in the melt zone <NUM>'. This process facilitates casting of a long length of cylinder.

If continuous casting of the cylinder is required, optionally a ceramic feeder <NUM> can be provided to supply additional metal to increase the length of starting material. The exemplary feeder <NUM> may move with the heat source <NUM> as a unit; or the seed <NUM>, core <NUM>, and casting material may move downward with the feeder, heat source, and cooling means remaining fixed. For example, when the upper surface of the initial material <NUM> reaches a given threshold (proximity to the melt zone) another disk of material may be stacked atop with a central bore of the disk receiving the core <NUM>. In yet further variants, sequential core extensions may mate to each other allowing essentially infinite casting.

Both the Czochralski process as well as the float zone process may also be particularly useful for bent seed formation of articles other than superalloy components. As removal grain boundaries as defects, help improve high temperature creep resistance, elimination of grain boundaries help improve memory alloy performance, electrical properties, magnetic properties, optical properties, as well as piezoelectric and many other physical properties. The resulting single crystal typically will make these properties anisotropic depending on the crystal symmetry. This generally limits application of material highly one directional. This could be limiting in many applications such as parabolic mirrors, lenses, micro machines, and actuators, where axial symmetry may be desirable. Use of bent single crystal seed(s) will open up such applications where some low angle grain boundaries can be tolerated to produce arcuate single crystal. It is recognized that with exception of memory alloys, in most of these application the material used are either intermetallics, maxphases, semi-metals such as silicon, ceramics, silicides, oxides, carbides, and other inorganic compounds, which are brittle. In such cases initially thin single crystal membrane may be used and bent elastically and then held in the elastically-bent state for use as a seed.

In the example above, case only axial forging was described as a method of warm working the solidified metal to change the shape and/or increase dislocation density. Alternatively, many other variations such as ring-rolling may be used to warm work a ring, in conjunction with forging or by itself. In such a process the ring is placed around an idle roller (or ID roller) and the ring is squeezed by a driver roller (OD roller) from the outside. The process is useful for expanding the ring diameter or for simply imparting some warm work to the metal from a different direction.

Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical's units are a conversion and should not imply a degree of precision not found in the English units.

Claim 1:
A method for casting a component, the method comprising:
providing a seed (<NUM>) having a longitudinal axis, the seed characterized by:
an arcuate form about the longitudinal axis and a crystalline orientation progressively varying along an arc of the form in a circumferential direction;
providing molten material (<NUM>; <NUM>');
cooling and solidifying the molten material so that a crystalline structure of the seed propagates into the solidifying material; and
forming the seed by:
assembling end-to-end a plurality of arcuate segments, wherein:
the segments are bent plates;
two to eight said segments combine to encircle at least <NUM>° about a central longitudinal axis of the seed; and
each segment has two ends, wherein each end extends in a radial direction from the central longitudinal axis of the seed.