Method and mold for casting articles having a predetermined crystalline orientation
BACKGROUND OF THE INVENTION 
     Cast single crystal articles such as turbine blades and vanes can be produced by several techniques. A common method involves the use of a starter zone at the bottom of the mold wherein a plurality of columnar grains are formed. A &#34;nonlinear&#34; or transversely displaced crystal selector (e.g., a helix-shaped passage) connects the starter zone to the article cavity, and this selector insures that only one columnar grain grows into the article cavity. Single crystal castings also can be produced using molds which have a vertical &#34;slender projection&#34; at the bottom of the article cavity (i.e., a linear or non-transversely displaced &#34;neck&#34;) as described in Bridgman U.S. Pat. No. 1,793,672. 
     When traditional directionally solidified (columnar-grained polycrystalline) articles are desired, the starter zone communicates directly with the article cavity (no crystal selector is present) as described in Chandley U.S. Pat. No. 3,248,764, VerSnyder U.S. Pat. No. 3,260,505, and Piearcey U.S. Pat. No. 3,494,709. 
     These techniques generally are restricted to producing articles that have the &#34;natural&#34; crystal growth directional (e.g., the &lt;001&gt; direction in face-centered cubic and body centered cubic metals) oriented along the &#34;longitudinal&#34; dimension of the article. This longitudinal dimension generally is normal to the chill plate and/or parallel with the direction of heat withdrawal. In addition, with these methods of making a single crystal, it can be difficult or impossible to simultaneously align the secondary orientation of the grain relative to a desired &#34;transverse&#34; dimension of the article i.e., to orient an orthogonal &lt;010&gt; or &lt;100&gt; direction within the article cavity). 
     These limitations can be avoided by using seed crystals as described in the aforementioned Bridgman patent. Briefly stated, one of Bridgman&#39;s methods involves use of a mold with a cavity that terminates with a vertical passageway, the end of which constitutes a mold aperture. Seed crystals of any desired primary and/or secondary orientation are inserted into the aperture, liquid metal is formed in (or preferably poured into) the mold, and solidification proceeds by epitaxial growth from the seed (in the presence of a longitudinal temperature gradient) using practices which avoid the nucleation of new grains. 
     It is well known to those skilled in the art that effective use of the Bridgman seeding methods requires that the size and shape of the mold aperture closely approximate the cross section of the seed crystal, both to preclude metal running past the seed and out of the mold, and to avoid the nucleation of new grains in interstices between the mold and the seed. In addition, it also is well known that it is generally desirable for both technical and economic reasons to use seeds of relatively small crosssectional area. These considerations can restrict the utility of the Bridgman seeding method in the following ways: 
     (1) It can be difficult or impossible to accommodate individual deviations in the longitudinal crystallographic orientation of seeds relative to their external envelopes since they must mate with a fixed mold aperture. 
     (2) It can be awkward to position small diameter seeds (e.g., 0.030&#34;) in the proper secondary orientation, as a result of ordinary handling and manipulation problems. 
     (3) When ceramic molding techniques are utilized, as is preferred in the production of directionally solidified turbine blades and vanes, the dimensional reproducibility limitations of current ceramic molding methods can limit the accuracy of seed crystal positioning. This is of particular concern when precise orientation relationships are required in the cast article. 
     (4) Also with respect to ceramic molds, it is difficult to reuse seeds, since after shell removal and cutoff, the seeds must be sorted, cleaned, usually reinspected for grain orientation, and then repositioned within another cluster. 
     (5) The use of mold passages and apertures that are small relative to the size of the article cavity can present structural rigidity problems during pattern assembly. Ancillary members (e.g., ceramic tie bars) may be needed to support the pattern, which adds cost and weight to the assembly, and may under certain circumstances, compromise technical effectiveness during solidification, such as by altering heat flow characteristics or by inducing the undesirable nucleation of crystals at points of contact with the article cavity. 
     (6) Small mold passageways and apertures can also present difficulties during pattern removal (e.g., dewaxing). Pattern materials usually expand during heating (e.g., steam dewaxing or &#34;burnout&#34;) and it is advantageous to have more than one relatively large mold opening. Although many molds can be dewaxed successfully through the top (via the metal feed), the presence of a large aperture at the bottom of the mold increases the speed and effectiveness of the operation, while minimizing the probability of shell damage. 
     (7) Small mold passageways and apertures can restrict the cross sectional area of metal which conducts heat to the chill plate. This limitation obviously can exist with small nonlinear passageways, and Erickson, et al. U.S. Pat. No. 3,724,531 
     teaches the use of a double-wall mold construction method to ameliorate that difficulty. 
     It will be obvious to those skilled in the art that many of these restrictions become more onerous when multiple cavity molds are involved or when more than one seed is used with an article cavity. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method and means for producing articles having a predetermined crystalline orientation. The system of the invention is particularly concerned with the use of at least one seed crystal positioned in a mold cavity which defines the shape of the article to be formed. As already indicated, it is known that material, such as molten metal, may be introduced to the cavity in the area of the seed with the crystalline structure being formed beginning at the location of the seed crystal and then progressively throughout the mold cavity. 
     The invention is particularly concerned with the use of molds which are made by preparing a pattern and then applying a mold-forming material, such as layers of ceramic around the pattern. When using such techniques, the pattern material is typically discharged through a mold passage which is provided during the mold-making operation. For example, where the pattern is of wax, a passage is provided in the mold for removal of molten wax after the mold is formed around the pattern. As explained, this passage is preferably in addition to the metal feed passage at the top of the mold. 
     The particular improvement of the invention involves the step of forming a mold passage which is of large cross sectional dimension relative to a corresponding cross-sectional dimension of the seed crystal or crystals to be used in conjunction with the mold. A cradle having external wall surfaces dimensioned to mate with the interior wall surfaces of this mold passage is located in the mold passage. The seed crystal is mounted in the cradle, and the cradle is located in the passage in a manner such that the seed crystal is exposed within the mold cavity whereby the desired crystalline structure can be formed by introducing an article-forming material into the mold cavity. 
     The limitations discussed above under the heading &#34;BACKGROUND OF THE INVENTION&#34; can be reduced or eliminated through the use of a seed holder or cradle of the type described. Particularly where metal castings are involved, the cradle will comprise a preformed structural member, made of either ceramic or some relatively high melting point metal or alloy, which contains one or more internal cavities that contain one or more seeds. The seed or seeds can be accurately positioned relative to the external envelope of the cradle and with respect to each other, where applicable. The mold passage forms mating surfaces with the cradle, and thus orients the cradle and the one or more seeds which it contains relative to the article cavity. 
     In accordance with one embodiment of the invention, the cradle would be inserted after completion of shell fabrication. It should be understood, however, that it could be inserted immediately after pattern removal if the seed alloy were sufficiently refractory to withstand the mold firing cycle. 
     In accordance with another embodiment, the cradle takes the form of a cylinder or tube which is associated with the pattern prior to the mold making steps. After pattern removal, the cradle provides a precisely dimensioned mounting means for a crystal or crystals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a pattern of the type typically used for the preparation of molds to be used for the casting of a turbine blade; 
     FIG. 2 is a vertical, cross-sectional view of a ceramic mold produced utilizing a pattern of the type shown in FIG. 1; 
     FIG. 3 is a vertical, elevational view of pattern, partly cut away, modified in accordance with one form of this invention; 
     FIG. 4 is a vertical, sectional view illustrating a ceramic mold modified in accordance with another form of this invention; 
     FIG. 5 is a vertical, sectional view illustrating still another modification in accordance with this invention; 
     FIGS. 6A--6D comprise perspective views illustrating alternative forms of cradles and seed crystal configurations; 
     FIG. 7 is a perspective view of a modified cradle and seed crystal assembly; 
     FIG. 8 is a vertical, sectional view taken about the line 8--8 of FIG. 7; 
     FIG. 9 is a horizontal, sectional view taken about the line 9--9 of FIG. 7; 
     FIG. 10 is a top view of a modified form of the cradle and seed crystal; 
     FIG. 11 is a top view of another modified form of the cradle and seed crystal; and, 
     FIG. 12 is a vertical, sectional view illustrating still another modification in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 illustrate typical prior art pattern and mold structures. The pattern 10 shown in FIG. 1 may be formed of wax and utilized in the production of a turbine blade. This pattern includes an extension 12 at the top which is typically provided for forming a metal feed passage in a mold. Another extension 14 at the bottom of the pattern is provided to form a passage in the mold which will ultimately be employed for removal of the pattern material after the mold has been formed. 
     FIG. 2 illustrates a mold 16 which may be formed by any conventional means. For example, the mold 16 can be produced by repeatedly dipping a pattern 10 into a ceramic slurry to build up layers of ceramic around the pattern. After firing, a mold having a metal feed passage 18, a lower passage 20, and an intermediate article forming cavity 22 will result. The passage 20 is particularly useful as a means for permitting removal of the pattern material, for example, where the material comprises wax or some other substance which can be brought to a molten state and allowed to flow out of the mold. 
     In order to provide a suitable means for removing pattern material, the passage 20 should have relatively large dimensions so that the pattern material will flow freely out of the mold. As previously indicated, however, this creates problems when the mold is to be used in conjunction with a seed crystal which must be accurately positioned relative to the mold cavity and which is preferably of relatively small diameter. The arrangement shown in the subsequent figures illustrates means for avoiding these problems and limitations. 
     FIG. 3 illustrates one embodiment of the invention wherein a cylinder or tube 24 is associated with a pattern 10. This cylinder may be made of a ceramic material or a high melting point metal, and it is held in a fixed position relative to other pattern portions. Additional wax or other material 26 may be utilized to position the cylinder 24 relative to the support upon which the pattern is mounted to insure that the cylinder is fixed relative to the remainder of the pattern. 
     A ceramic or metal stiffener 28 extends into the pattern 10 to provide additional rigidity during the pattern assembly and handling operations. The stiffener is usable as an option particularly where the diameter of the cylinder 24 is small relative to the size of the pattern 10. 
     After formation of the mold 16 around the pattern, the pattern material is removable through the feed passage 18 and also through the passage 30 defined by the interior of the cylinder 24. Where a stiffener 28 is employed, the stiffener will be automatically removed from the mold cavity along with the pattern material. The resulting assembly is shown in FIG. 4. 
     The assembly of FIG. 4 provides a mold with the cylinder 24 comprising a cradle for a seed crystal as contemplated by this invention. It will be appreciated that the ceramic or metal cylinder 24 can be preformed with high precision to a desired cross section. Accordingly, a seed crystal can be readily located in the passage 30, and by controlling the dimensions of the seed crystal with equal precision, an uncomplicated assembly operation is possible. 
     Although the cradle of FIG. 4 has been described as a &#34;cylinder&#34; 24, no limitation on the crosssectional shape of this member is intended. Various shapes are possible (and even desirable in some cases) as described, for example with reference to FIGS. 6A through 6D and 10. 
     As explained, it is often desirable that the seed crystal be oriented relative to the mold cavity in both longitudinal and transverse respects. Since the orientation of the crystal can be determined before it is associated with the cavity, it is desirable to provide means for controlling this orientation when the seed crystal is inserted into the cradle comprising the cylinder 24. 
     FIG. 11 illustrates a means for controlling this orientation wherein the seed crystal 32 is provided with a flat face 34. This face is dimensioned to correspond with a face of the cylinder 24 so that the seed crystal will always have a precise relationship with the cylinder 24. During pattern assembly, a worker need only orient the cylinder 24 properly relative to the pattern, and this will automatically result in proper orientation of the seed crystal with respect to the mold. 
     FIG. 5 illustrates a modified form of the invention wherein a cradle 36 carrying a seed crystal 38 is associated with mold 16. In this instance, the cradle 36 is dimensioned to correspond with the dimensions of passage 40 formed during mold making. Thus, the cradle 36 is not associated with the mold until after the pattern material has been completely removed. At that point, the cradle is inserted. 
     In the embodiment of FIG. 5, the pattern portion designed to form passage 40 is precisely dimensioned to provide cross sectional dimensions corresponding with the external dimensions of the cradle. This cradle can be readily manufactured with precision so as to mate precisely with the interior dimensions of the passage 40. The assembly of the seed crystal 38 with the cradle take place independently of the mold making operations, and this greatly simplifies the location of the seed crystal relative to the mold cavity. 
     The embodiments of the invention described also greatly simplify pattern removal operations since the mold passages which receive the cradles provide available avenues for removal of pattern material. This is particularly true with respect to the embodiment of FIG. 5 since the diameter of passage 40 can be large even where the seed crystal 38 is of very small diameter. 
     The embodiment of FIG. 5 can also be designed to provide automatic orientation of the seed crystal relative to the mold cavity. As shown in FIG. 10, the cradle 36 may have a flat 42 on one side and a corresponding flat can be formed in the pattern portion prior to mold formation. The result will be that the cradle 36 can only be inserted in the mold in one position, and workers can thereby control seed orientation by locating the seed 38 in a precise position relative to the cradle. Orientation of the seed relative to the cradle can be achieved automatically by producing seeds and cradles with flats as shown at 44, and as discussed with reference to FIG. 11. 
     An additional or alternative means for achieving orientation may involve the use of indicia such as arrows 46 formed on a seed and/or 48 formed on a cradle. The indicia could be lined up with each other, or with indicia such as ridges or grooves formed in a mold thereby providing visual means for an operator for achieving orientation. It will be appreciated that other means for achieving orientation are possible including the use of other indicia or the use of notches and grooves. 
     The geometry of the seed cradles and/or seeds is subject to wide variations. FIGS. 6A through 6D illustrate cradles 50, 52, 54 and 56, respectively, illustrating forms that may be assumed by cradles. It will be particularly noted with respect to FIGS. 6C and 6D, that the cradles may contain a plurality of seed crystals 58 for achieving multiple locations for initiating crystal growth within a mold. 
     As shown in FIGS. 5 and 8, seed crystals 38 and 60 may have a length in excess of the bore length of the respective cradles. Such a seed crystal protrusion will increase the choice of casting parameters which will result in controlled seed melt back and subsequent epitaxial growth. The parameters chosen must avoid the formation of undesirable equiaxed grains such as by &#34;chilling&#34; on the seed surface. 
     It is also contemplated, however, that the seed crystal will terminate short of the juncture between the cradle passage and mold cavity. With the exposed end of the seed crystal located short of this juncture, the article forming material will enter the passage for contact with the exposed end to begin the article formation. 
     Under normal circumstances, the surface of the cradle that communicates with the article cavity (the &#34;tip&#34;), would define a plane parallel to the chill plate. This configuration facilitates the reuse of seeds and seed cradles, in that they can be easily cut off after casting and simply reinserted into another mold. However, it may be advantageous in certain circumstances to taper the tip surface of the cradle so as to match the slope of, and essentially create an extension to, the adjacent ramped portion of the internal surface of the mold cavity. This is illustrated by the tapered surfaces 62 shown in FIG. 5 which match the slope of the adjacent mold surfaces 64. This geometry can facilitate simultaneous longitudinal and transverse growth of a single crystal into the mold cavity. 
     FIGS. 7 through 9 illustrate another alternative construction wherein seed cradles 66 formed of ceramic material are provided with internal cavities. These cavities are filled with a material of higher heat conductivity such as sodium or copper metal. It will be appreciated that in the formation of single crystals, it is desirable to withdraw heat longitudinally by means of a chill plate 70 of the type shown in FIG. 5. The arrangement shown in FIGS. 7 through 9 will tend to increase the longitudinal temperature gradient and will also favorably influence the solidification rate. It will be appreciated that other means may be employed for constructing the cradles to provide a &#34;heat&#34; pipe construction and to thereby improve the solidification conditions. 
     In the modification of the invention shown in FIG. 12, there is illustrated a mold 70 with the longitudinal axis of the mold cavity 72 formed at an angle to the vertical and thus tilted relative to chill plate 78. The seed cradle 74 and seed 76 are oriented with their longitudinal axes parallel to the longitudinal axis of the mold cavity 72. 
     The arrangement shown can be useful in improving the soundness of directionally solidified castings while maintaining the advantages associated with the use of a seed crystal contained in a seed cradle. More specifically, under normal circumstances, the &#34;longitudinal&#34; axis of the part will lie substantially perpendicular to the chill plate (or other means of heat extraction) and thus be parallel to the direction of heat withdrawal. In the case of face-centered cubic metal solidification using an &lt;001&gt; seed, for example, the resulting &lt;001&gt; crystal will grow parallel to the longitudinal axis of the part. 
     The arrangement of FIG. 12 contemplates situations where the longitudinal axes of the mold cavity, cradle, and seed will lie at angles other than 90° relative to the chill plate. Acute angles of inclination, for example, up to about 15° (from the perpendicular), can be an effective way to improve the soundness of cast articles, particularly in &#34;corners&#34; or otherwise &#34;blind&#34; horizontal surfaces, such as in the platforms of gas turbine engines blades and vanes, by permitting the access of &#34;feed metal&#34; during solidification. It should be noted that in this kind of situation, the orientation of the cradle need not be parallel to the longitudinal axis of the &#34;tilted&#34; article, and/or it may be desirble to select a seed crystal of slightly different orientation, in order to &#34;compensate&#34; for the tilting of the article cavity. 
     Also contemplated is the use of acute or obtuse angles of inclination, for example, up to about 75°, in order to achieve crystalline orientations in the article which are different than those of the seed. For example, a cradle containing an &lt;001&lt; seed (with a proper secondary orientation) could be used to produce an article exhibiting a &lt;111&gt; orientation (relative to its longitudinal axis) by tilting the mold cavity by about 54.7° to the chill plate. 
     The various cradles described may also be used in association with another cradle, for example of the type shown in FIG. 4. Thus, the cylinder 24 may define an opening which corresponds dimensionally with the external dimensions of a cradle holding one or more seed crystals. The latter cradle can then be positioned within cylinder 24 at any appropriate time prior to introduction of molding material into a mold cavity. 
     As set forth above and in the various prior art references discussed herein, the present invention is particularly suitable for the casting of metals particularly metals of the superalloy type typically used for the production of turbine blades and vanes. The invention is, however, also applicable to other structural transformations such as directional recrystallization and solid-tosolid phase changes. 
     More specifically, it is contemplated that crystalline or non-crystalline powder, flake, or other solid material, be placed in the mold cavity. Such material, preferably having at least one dimension of less than about 0.010 inches, can be consolidated using techniques such as hot isostatic pressure, dynamic compaction, or sintering, and can then be directionally recrystallized or solid state transformed within the mold cavity. 
     The material for forming the cradles may be selected from ceramic materials of the type used in this art, e.g., alumina or zirconia. The seed crystal composition is, of course, dependent on the composition of the article to be formed in a mold although duplication is not required. For example, use of a &#34;universal&#34; seed crystal material (such as pure nickel for all nickel-base alloys) is contemplated. 
     It will further be understood that changes and modifications may be made in the above-described system which provide the characteristics of the invention without departing from the spirit of the invention as set forth in the following claims.