US 3752221 A
Unidirectionally solidified castings are produced by inverted solidification using a chill plate at the top of the mold and controlling the temperature gradient to cause solidification to occur from the top downwardly through the mold.
Claims available in
Description (OCR text may contain errors)
 Aug. 14, 1973 United States Patent [191 Copley et al.
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FOREIGN PATENTS OR APPLICATIONS  Inventors: Stephen M. Copley, Madison; 241,758 5/1881 Anthony F. Giamei, New Haven, 3,233,292 2/1966 Kramer et both of Conn.
988,645 5/1951 France................................ 249/109  Assignee: United Aircraft Corporation, East Hartford, Conn.
July 22, 1971 App]. No.: 165,302
Primary Examiner-Robert D. Baldwin Attorney-Charles A. Warren  Filed:
 ABSTRACT Unidirectionally solidified castings are produced b Related US. Application Data y mverted solidification using a chill plate at the top of the mold and controlling the temperature gradient to cause solidification to occur from the top downwardly through the mold.
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[  Int. B22d 27/04 164/60, 120, 127, 164/352, 353, 361, 338; 249/78, 109
5 Claims, 4 Drawing Figures  Field of W mesaoeooaceeeeeeocaeoog v i w Patented Aug. 14, 1973 2 Sheets-Sheet l 4 La W Patented Aug. 14, 1973 2 Sheets-Sheet 2 MOLD APPARATUS FOR CASTING WITH DOWNWARD UNIDIRECTIONAL SOLIDIFICATION This is a division of Ser. No. 872,562 filed Oct. 30, 1969, now US. Pat. No. 3,598,172 for Process of Casting with Downward Unidirectional Soldification.
BACKGROUND OF THE INVENTION Unidirectionally solidified castings of the'columnar grained type as described in VerSnyder US. Pat. No. 3,260,505 and also of the type described in Piearcey Ser. No. 540,114, filed Feb. 17, 1966, US. Pat. No. 3,494,709 and assigned to a common assignee, are generally formed by casting in a mold resting on a chill plate with a control of the thermal gradient to cause a controlled upward movement of the liquid-solid interface from the chill plate upward to the top of the mold. Such castings are generally satisfactory although at times there is a density inversion during solidification that creates imperfections within or on the surface of the castings.
SUMMARY OF INVENTION One feature of this invention is the inverted unidirectional solidification of alloys in the mold for the purpose of eliminating the imperfections resulting from the density inversion. Another feature is a mold assembly that makes possible the directional solidification of alloys from top to bottom with a controlled thermal gradient thereby to produce the desired crystalline structure and/or grain growth.
BRIFF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical sectional view through a mold construction.
FIG. 2 is a horizontal sectional view along the line 2-2 of FIG. 1.
FIG. 3 is a fragmentary view of a modification.
FIG. 4 is a wiring diagram.
DESCRIPTION OF THE PREFERRED EMBODIMENT The shell mold apparatus includes a central vertical feeder tube connected at the bottom by lateral arms 12 having passages 14 thereon. At the outer ends of the arms are vertical mold elements 16 extending parallel and in spaced relation to the feeder tube and having an article forming cavity 18 therein. This cavity is shown as being the shape of a turbine blade with an airfoil portion 20, a shroud portion 22 at the bottom end and a root portion 24 at the top thereof. The shroud portion communicates through a vertical passage 30 with the horizontal passage 14. The root portion of the article cavity 18 communicates through a helical crystal selector passage 32 with a growth zone cavity 34. A chill plate 36 is secured to the mold at the top end of the cavity 34, the mold having a flange 38 at this point by which the chill plate is attached.
As shown in FIG. 2, the mold is generally made up of a plurality of vertical mold elements 16 so that a plurality of articles may be cast at one time. The mold may be made with a plurality of supporting feet 40. The mold is made by the usual shell mold technique using a wax or other disposable pattern over which successive layers of mold material are placed and dried, with the finished mold cured by heating at which time the pattern is melted out.
When the mold is ready for use, it is placed on a support plate 42 resting on a bed of a heat insulating powered material 44 such as aluminum oxide. The mold is surrounded by a susceptor 46 which in turn is surrounded by vertically spaced induction heating coils 48 and 49 which are selectively energized for controlling the temperature of the mold. The susceptor and heaters extend at least from a point above the chill plate to a point substantially below the shroud portion to assure an accurate control of the thermal gradient from the chill to the bottom end of the vertical passage 30.
The central tube is surrounded by a heating coil 50, preferably a resistance coil, and this coil is shielded from the surrounding article forming portions of the mold by a cylindrical shield 52. A graphite sleeve 53 may be positioned within the coil, as shown. The latter and the shield 52 may rest on the lateral arms 12 and the coil may be incorporated in this shield, as shown. A sprue 54 positioned at the top of the tube directs molten alloy into the vertical pasage defined by the tube. The entire structure is preferably mounted in a vacuum chamber so that the casting procedure may be done under vacuum or in an inert atmosphere.
The assemblage is used for example in producing parts such as turbine blades or vanes which operate in a high temperature environment under high stress and are thus cast from the so-called super alloys such as decribed in the VerSnyder US. Pat. No. 3,260,505 or the Piearcey US. Pat. No. 3,494,709. The article forming portion of the mold and the adjacent portions thereof are heated to a temperature above the melting temperature of the alloy and the central tube is also raised to this same temperature. A flow of water is maintained through the chill plate so that it is not melted during this heating process. When temperatures are stablized the molten alloy, somewhat superheated, for example, about C, is poured into the sprue to fill the mold completely. A suitable vent 56 in the chill plate permits the escape of entrapped gases within the mold to assure complete filling.
The coils 48 and 49 may be connected, as shown in FIG. 4 to a source of alternating current represented by the power leads 58. Energy to coil 49 is controlled by a switch 60, and energy to coil 48 is controlled by a rheostat 62 in order to provide a programmed rate of reduction of power as determined by the solidification rate. The coil 50 may also be connected to the power leads as by a switch 64. As stated later in the specification, this coil is cut off from the supply when the downward solidification in the article forming portion of the mold is completed.
At the time the mold is filled, solidification of the alloy begins at the water cooled copper chill plate. Up to this time both induction heating coils 48 and 49 have been energized but as the mold is filled the upper coil 49 is cut off as by the switch 60, FIG. 4 to begin the cooling of the mold from top to bottom. The escape of heat to the' chill plate causes the vertically downward growth of columnar grains of alloy in the growth cavity 34 and into the helical passage 32 where a single crystal is selected to continue its growth downwardly into the article forming cavity thereby forming a single crystal article as described and claimed in Piearcey US. Pat. No. 3,494,709.
The maintenance of heat from the resistance coil around the central tube keeps the alloy in thistube molten, and the heat from induction coil 48 keeps the alloy molten in the horizontal passage so that the metal in the central tube maintains a positive hydrostatic pressure on the liquid-solid interface where the alloy is solidifying downwardly in the article cavity. This hydrostatic pressure increases as the liquid-solid interface proceeds downwardly and thus the pressure on the mushy zone increases toward the bottom end of the article cavity.
As the liquid-solid interface moves downwardly during the casting operation, the power in the lower heater 48 is reduced at a programmed rate as by the rheostat 62, FIG. 4 determined by the rate of solidification, and is then cut off to permit completion of the solidification. Once the downward solidification in the article forming portion is complete, heat to the central tube heater is cut off so that the remainder of the alloy may solidify.
It has been found that in the usual formation of directionally solidified articles from bottom to top there are liquid jets of the alloy that flow upwardly within the mushy zone caused by instability resulting from a density inversion in this zone. These jets detrimentally affect the proper solidification of the alloy within the mushy zone. This may result in local areas of segregation trails rich in rejected solute which on the surface of the cast article are referred to as freckles. These trails often contain small randomly oriented grains and are a defect which may make the cast article unacceptable. Such freckles or trails would be eliminated by the inverted solidification above described, since the mushy zone density profile will be stable with respect to the gravitational field.
Referring now to P10. 3, the mold may be adapted for making columnar grained articles as in the VerSnyder patent. To accomplish this, the growth cavity 34' communicates directly with the root portion 24 so that the columnar growth that is started at the chill plate 36' and becomes parallel, vertically oriented columnar grains in the growth zone are propagated downwardly through the article forming portion. Casting of such articles by this inverted solidification process would be used to produce acceptable columnar grained article such as turbine blades for use in the highest temperature turbine stages of the engine.
1. Apparatus for forming directionally solidified cast articles, including a mold having a filling cavity and an article forming cavity in horizontally spaced vertically positioned relation, and an interconnecting passage between said cavities at the bottom, a chill plate closing the open upper end of the article forming cavity, heating means for said filling cavity, and other heating means for said article forming cavity, and means for selectively reducing the heating effect in portions of said other heating means between top and bottom thereof to produce a controlled thermal gradient lower at the top than at the bottom of said article cavity.
2. Apparatus as in claim 1, including a container to receive the mold with a granular material within the container and supporting the mold.
3. Apparatus as in claim 1 in which the article forming cavity of the mold has a single crystal selector adjacent the upper end directly below the chill plate.
4. A shell mold apparatus for casting of directionally solidified articles including an article mold portion having means adjacent the top for starting the crystallization of the material therein as a single crystal, said mold portion having an open top end above said means,
a chill plate extending across and closing said open a parallel filling mold portion open at the top for filling the mold, an interconnecting passage mold portion connecting said articlev and filling portions at the bottoms thereof, the top of said filling portion being higher than the open top end of the article portion,
induction heating means around the article mold portion for maintaining this portion at a temperature higher than the melting temperature of the material to be cast in the mold, and
means for causing a gradual reduction in the effectiveness ofsaid heating means to cause a reduction in temperature from the top to the bottom of said article mold portion for solidification of the material therein from the top to the bottom.
5. A shell mold as in claim 4 including other heating means for the filling mold portion to retain this portion above the melting temperature of the material to be cast during the solidification of the material in the article mold portion.
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