US3519063A - Shell mold construction with chill plate having uniform roughness - Google Patents

Description

July 7,' 1970 a. J. PIEARCEY 3,

SHELL MOLD CONSTRUCTION WITH CHILL PLATE HAVING UNIFORM ROUGHNESS Filed July 18, 1 968 2 Sheets-Sheet 1 By M B. J. PIEARCEY 3,519,@63 SHELL MOLD CONSTRUCTION WITH CHILL PLATE HAVING UNIFORM ROUGHNESS 2 Sheets-Sheet 2 Filed July 18 F/G. Z

United States Patent SHELL MOLD CONSTRUCTION WITH CHILL PLATE HAVING UNIFORM ROUGI-INESS Barry J. Piearcey, Galmpton, Brixham, Devon, England,

assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Continuation-impart of application Ser. No. 472,611,

July 16, 1965. This application July 18, 1968, Ser.

Int. Cl. B22c 9/02; B22d 15/02 U.S. Cl. 164353 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a chill plate for use in making directionally solidified castings and castings in which the crystalline structure is directionally oriented. The chill plate is used with a temperature-controlled shell mold which heats the casting material and establishes a unidirectional temperature gradient between the material and the chill plate. The surface of the chill plate in contact with the material is roughened to improve heat transfer to the plate and to promote directionally oriented crystalline growth or columnar grain growth within the material.

This is a continuation-in-part of Piearcey Ser. No. 472,611, filed July 16, 1965, now abandoned.

BACKGROUND OF THE INVENTION One feature of this invention is a chill plate construction that will facilitate the production of this type of casting. Another feature is a surface treatment of the chill plate to improve the start and continued growth of the columnar grains.

The growth of the columnar grains is promoted by rapid heat removal through the chill plate and a feature of the invention is a roughened chill plate surface thereby increasing the surface area of the plate in contact with the metal being cast. It has been found that the crystalline growth with the desired orientation can be substantially improved by more rapid heat dissipation into the chill plate.

Other features and advantages will be apparent from the specification and claims, and from the accompanying drawings which illustrate an embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view through a mold embodying the invention.

FIG. 2 is an enlarged fragmentary vertical sectional view through a chill plate having a smooth surface, and part of a casting thereon.

FIG. 3 is an enlarged view similar to FIG. 2 but in which the chill plate has a roughened surface.

FIG. 4 is an enlarged sectional view on a smaller scale of a portion of the surface of a modified form of chill plate.

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FIG. 5 is a plan view of a modified form of chill plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is for use primarily in making directionally solidified castings as described in the above-identified VerSnyder patent and in making single crystal castings as described in the above-identified Piearcey application. The purpose of this invention is to facilitate the production of such elements as turbine blades or vanes such that the orientation of the crystallization will have a preferred relation to the longitudinal axis of the cast element or such that the grain boundaries also will have a preferred relation to this same axis.

Referring first to FIG. 1, the mold is shown as a shell mold 2 positioned with the bottom opening 4 thereon resting on a chill plate 6. The shell mold may be arranged to produce a plurality of turbine blades or vanes at one time and to this end incorporates a plurality of vane-shaped'recesses 7 in parallel vertical arrangement. These recesses communicate at the lower ends with growth area recesses 8 and at their upper ends with top filling areas 10. These latter areas connect through lateral passages 12 with the central pouring sprue 14 by which all the recesses 7 can be filled simultaneously.

The mold, when completed and ready to be used for casting structural parts, is positioned within a ring or cylinder of heating elements 16 as described in the copending application of Barrow and Sink, Ser. No. 472,644, filed July 16, 1965, now Pat. No. 3,405,220 issued Oct. 8, 1868, and having the same assignee as this application. These heating elements provide for heating the mold to a temperature above the melting temperature of the alloy and are preferably arranged to be successively turned off during the casting process.

According to the present invention the chill plate 6 which has cooling means therein such as the cooling passages 17 has a roughened surface 18 contacting with the mold and with the molten metal when it is poured therein. The roughness may be obtained by forming a plurality of grooves 20 therein as in FIG. 4 or by knurling 22 as in FIG. 5 to provide a substantially larger area of contact between the chill plate and the molten metal. The grooves 20 of FIG. 4 may be rectilinear grooves parallel to one another or may be annular or spiral grooves to produce substantially the configuration shown. The most effective groove is that made with a threading tool producing a N.C. thread and the grooves are preferably about one-sixteenth inch apart. The tool cut is deep enough to leave lands or flats about .02 inch wide and the depth of the groove is about .05 inch. Grooves spaced as closely as one-thirty-second of an inch with much narrower lands produces a finer grain structure and more widely spaced grooves and thus wider lands produces a faster grain growth. Thus the range of groove spacing may be as much as between one thirty-second and one eighth of an inch and the depth of the groove may go from .03 to .10 inch, the depth being preferably about .05 inch. The lands may vary from .01 to .045 inch in width and still be effective.

As above stated these grooves may be circular or rectilinear or they may be comparable to a knurled surface with the grooves intersecting at an angle to one another varying from a 45 angle to a angle. The lands produced by the intersecting grooves are preferably from .01 to .04 inch in width, the most effective being about .02 inch. As the chill plate is used repeatedly in producing cast articles, the grooves and lands must retain their effectiveness over an extended use and for durability, when the sides of the grooves approach that of an N.C. thread the land width of from .01 to .03 inch is most effective in repeated casting operations.

The spacing, depth and shape, as above described, produces an effectively greater contact surface than a flat plate to produce more effective heat transfer to the plate. With a depth of groove as described, the mold contact with the chill plate requires no particular sealing arrangement since the immediate cooling of the molten metal as it contacts the chill plate effectively seals any of the small spaces that might exist.

FIGS. 2 and 3 show graphically the improvement in cast structures with the roughened chill plate. These views are about fifty times normal size. In FIG. 2 it is apparent that the crystalline growth is random rather than directional and no directional growth becomes established for a substantial distance from the chill plate. However, with a chill plate as in FIG. 3, the directional crystalline growth begins at the peak or top surfaces of the ridges in the chill plate and a directional growth substantially at right angles to the plate is well established almost immediately. The view of FIG. 3 was made after the grooved plate had been used for several castings and the lands had been eroded away to some extent.

In addition to the directional growth being well established it has been found that the roughened chill plate surface improves materially the heat transfer from the melted alloy to the chill plate so that solidification occurs at a substantially faster rate than is the case with a smooth plate. It has been found that the desirable properties in the finished part are best obtained with rapid solidification and the roughened plate contributes to this effect.

It is well known that the rate of thermal conductivity is much less in the well-known alloys used in high temperature environments such as gas turbine vanes and blades, than in other simpler metals that would be used in chill plates. The ability of the chill plate to remove heat faster by its roughened surface helps to maintain a higher temperature gradient in the article being cast and thus increases to an optimum the effective rate of heat transfer lengthwise of the blade or vane and thence into the chill plate.

With the nickel or cobalt base superalloys which have been unidirectionally solidified, it has been found that the l direction of growth produces the crystalline orientation for optimum properties. During solidification the dendrite crystals grow fastest in the 00l direction and under the temperature gradient resulting from a heated mold and a chill plate this growth is substantially perpendicular to the chill plate surface. This temperature gradient is effectively measured by the increase in surface area resulting from the grooves formed in the surface. The roughness also serves to provide a more effective mechanical joint so that casting and chill plate stay in better contact during the solidification process.

The grooves increase the number of dendrites per unit area by a factor of more than two. This promotes the possibility of obtaining an orientation closer to 00l Further, as shown in FIG. 3, a greater proportion of the dendrites are oriented substantially perpendicularly to the 4 chill plate than where the surface of the chill plate is smooth.

When single crystal particles are cast, the first dendritic growth occurring at the chill plate is directionally oriented in the same manner as above described and the grooved chill plate is thus equally effective in establishing the properly oriented dendritic growth in single crystal parts.

The grooved chill plate by measuring the rate of cooling has effectively reduced the size of the carbides in the alloy. These zone carbides affect the machinability of the alloy, particularly in electrochemical machining of the alloy and thus the parts produced by the use of the roughened chill plate has improved machinability substantially.

I claim:

1. In a mold construction for use in casting directionally solidified articles, a temperature-controlled shell mold having a pouring spout, a mold cavity open at the end opposite to the spout, and a chill plate closing said open end, a portion of the plate thereby contacting directly with the material poured into the mold to form the article, said plate having a controlled substantially uniform roughness forming substantially fiat surface lands from .01 to .045 inch in width over the portion adapted to contact with the poured material to improve the start and continued growth of columnar grains within the material, said plate adapted to substantially retain said uniform roughness throughout the casting operation.

2. A mold construction as in claim 1 in which the roughness is between .03 to .10 inch in depth.

3. A mold construction as in claim 1 in which the roughness is formed by substantially parallel uniform grooves having a depth of between .03 and .10 inch and spaced apart to form said lands.

4. A mold construction as in claim 1 in which the roughness is formed by intersecting grooves spaced apart to leave said lands.

5. A mold construction as in claim 4 in which the grooves are spaced about one-sixteenth inch apart.

References Cited UNITED STATES PATENTS 1,727,565 9/1929 Schall 164353 2,951,272 9/1960 Kiesler 164127 X 2,970,075 1/ 1961 Grenoble.

3,366,362 1/1968 Chandley et al. 164-60 X 3,248,764 5/1966 Chandley 164-127 FOREIGN PATENTS 416,784 7/ 1925 Germany.

OTHER REFERENCES Transactions of the Metallurgical Society of AIME, volume 224, December 1962, pages 1271-1277.

ROBERT D. BALDWIN, Primary Examiner U.S. Cl. X.R. 164-361

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