US3485291A - Method of casting by directional solidification - Google Patents

Description

2 Sheets-Sheet l B. .1. PIEARCEY Dec. 23, 1969 METHOD OF CASTING BY DIRECTIONAL SOLIDIFICATION Original Filed July 16, 1965 Maw/M gullllliii METHOD OF CASTING BY DIRECTIONAL SOLIDIFICATION 2 Sheets-Sheet 2 Original Filed July 16, 1965 United States Patent 3,485,291 METIIUD 0F CASTING BY DIRECTIONAL SOLIDIFICATION Barry I. Piearcey, Cheshire, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Original application July 16, 1965, Ser. No. 472,611. Divided and this application June 7, 1968, Ser. No. 753,308

Int. Cl. B2211 15/04 U.S. Cl. 164-127 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a chill plate for use in directionally solidified castings. 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 columnar grain growth within the material. This application is directed to the method of casting with this chill plate.

This is a division of the Piearcey application, Ser. No. 472,611, filed July 16, 1965, now abandoned.

This invention relates to a chill plate for use in making castings having a controlled grain structure and to a process and mold for use therewith.

The copending application of VerSnyder Ser. No. 361,- 323, filed Apr. 17, 1964 now Patent No. 3,260,505, is sued July 12, 1966, and assigned to the same assignee as the present application is directed to a cast turbine blade or vane having a columnar grained structure. 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 in the desired direction 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.

In the drawings:

FIGURE 1 is a sectional view through a mold embodying the invention.

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

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

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

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

The invention is for use primarily in directionally solidified casting as described in the above-identified application of VerSnyder, Ser. No. 361,323. The purpose of that invention is to produce such elements as turbine blades or vanes such that the crystallization will extend longitudinally of the element and with the grain boundaries also extending in this same direction.

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Referring first to FIGURE 1, the mold is shown as a shell mold 2 positioned with bottom opening 4 thereon on a chill plate 6. The shell mold is 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 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 Patent No. 3,405,220, issued Oct. 8, 1968, 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 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 FIGURE 4 or by knurling 22 as in FIGURE 5 to provide a substantially larger area of contact between the chill plate and the molten metal. The grooves 20 of FIGURE 4 may be rectilinear grooves parallel to one another or may be annular or spiral grooves to produce substantially the configuration shown. It has been found that the grooves should be less than one-tenth of an inch in depth and the spacing should be close enough together to produce an effectively greater contact surface. 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.

The showing of FIGURES 2 and 3 shows graphically the improvement in cast structures with the roughened chill plate. These views are about fifty times normal size. In FIGURE 2 it is apparent that the grain growth is random rather than directional and no directionally oriented grain growth becomes established for a substantial distance from the chill plate. However, with a chill plate as in FIGURE 3, the columnar grain growth begins at the peak or top surfaces of the ridges in the chill plate and a direction substantially at right angles to the plate is well established almost immediately.

In addition to the directional grain 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 001 direction of growth produces the crystalline orientation for optimum properties. During solidification the dendrite crystals grow fastest in the 001 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 001 Further, as shown in FIGURE 3, a greater proportion of the dendrites are oriented substantially perpendicularly to the chill plate than where the surface of the chill plate is smooth.

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.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

I claim:

1. In casting a high temperature nickel or cobalt base 4 alloy by directional solidification to produce columnar grains, the steps of providing a controlled substantially uniform roughness in the top surface of a chill plate member, mounting a shell mold structure having a pouring spout at its top end and an open bottom end on said plate member so that said roughness forms part of the mold cavity, heating the mold to a temperature above the melting point of the alloy while keeping the plate member cooled to a relatively low temperature which is considerably less than the melting point of the plate member material, pouring the alloy into the mold against the chill plate member thereby causing the start of growth of the grains on the high points of the roughness and continuing to cool the chill plate member as the remainder of the mold is gradually cooled.

2. In the method of casting as claimed in claim 1 in which the controlled roughness is in the form of uniformly spaced grooves providing a depth of roughness less than substantially A of an inch.

References Cited UNITED STATES PATENTS 2,951,272 9/1960 Kiesler 164-127 X 3,248,764 5/1966 Chandley 164-427 FOREIGN PATENTS 1,183,121 11/1959 France.

0 I. SPENCER OVERHOLSER, Primary Examiner R. D. BALDWIN, Assistant Examiner

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