US3598167A

US3598167A - Method and means for the production of columnar-grained castings

Info

Publication number: US3598167A
Application number: US772754A
Authority: US
Inventors: Michael H Snyderman
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1968-11-01
Filing date: 1968-11-01
Publication date: 1971-08-10
Anticipated expiration: 1988-08-10

Abstract

A method and materials for producing shell molds for casting columnar grained metal components wherein a pattern is prepared corresponding to the shape of casting to be formed. The pattern is then combined with a base formed of a material selected from the group consisting of graphite and a refractory of high thermal conductivity. A shell mold is formed around the assembly comprising the pattern and base followed by removal of the pattern material to provide the shell mold. The base operates to conduct heat away from molten metal poured therein at a rate greater than the surrounding mold walls whereby nucleation will begin in the area adjacent the base and columnar grain growth will occur. The base included in the assembly of the pattern and base can also be used for connecting supporting rods to lend stability to an arrangement which includes a plurality of the assemblies whereby dip coating for formation of the shell mold can proceed without the need for a supporting baseplate.

Description

D United States Patent 1 1 3,598,167

[72] Inventor Michael H. Snyderman 3,441,078 4/1969 Chandley 164/361 X Pardppany, NJ. 3,131,999 5/1964 Suzuki et al. 164/36 Q E 1 Primary Examiner-J. Spencer Overholser Pacmed b Assistant Examiner-V, K. Rising 1 [73] Assignee United Aircraft Corporation Auomey chafles wane East Hartford, Conn.

[54] METHOD AND MEANS FOR THE PRODUCTION OF COLUMNAIt-GRAINED CASTINGS 11 Claims, 4 Drawing Figs.

[52] [LS-Cl 164/26, 164/24, 164/34, 164/35, 164/127, 164/236, 164/246,164/361, 164/353, 164/371 [51 1 Int. Cl B22c 9/00 ABSTRACT: A method and materials for producing shell molds for casting columnar grained metal components wherein a pattern is prepared corresponding to the shape of casting to be formed. The pattern is then combined with a base formed of a material selected from the group consisting of graphite and a refractory of high thermal conductivity. A shell mold is formed around the assembly comprising the pattern and base followed by removal of the pattern material to provide the shell mold. The base operates to conduct heat away from molten metal poured therein at a rate greater than the surrounding mold walls whereby nucleation will begin in the area adjacent the base and columnar grain growth will occur.

The base included in the assembly of the pattern and base can also be used for connecting supporting rods to lend stability to an arrangement which includes a plurality of the assemblies whereby dip coating for fonnation of the shell mold can proceed without the need for a supporting baseplate.

METHOD AND MEANS FOR THE PRODUCTION OF COLUMNAR-GRAINED CASTllNGS This invention relates to an improved system for the production of components characterized by a columnargrained structure. The invention will be specifically described relative to the production of turbine blades and vanes employed in gas turbine power plants. It will be understood, however, that the invention is also applicable to the production of components which might be subjected to similar operating conditions or which are more satisfactorily characterized by a columnar-grained growth.

The provision of turbine components having a columnargrained growth has previously been described, for example in Ver Snyder Pat. No. 3,260,505, issued July 12, 1966. As explained in this patent, turbine components having columnargrained growth exhibit a longer life before rupture and have greater uniformity of stress rupture life when compared under the same conditions with components of the same alloy but having a nonoriented grain growth. The previously described procedure also provides blades having third stage creep so that predictable changes in creep" rate can be utilized in estimating time between engine overhauls. The components also have especially good tensile elongation properties, and can be employed at substantially higher temperatures than prior, con ventionally cast blades. Completely satisfactory ductility, elongation, and extended life properties are obtained along with unusually uniform strength properties and relatively high tensile strength.

In accordance with the previously described procedure, the alloy to be cast is vacuum melted and poured into a mold. The mold is preferably a ceramic or siliceous material such as a shell mold which is formed of alternate layers of finely divided siliceous material, such as silicates, zirconia, or other argillaceous or refractory material, and finely divided sand or like material. Several layers of the ceramic or siliceous material and of the sand are usually employed. The shell molds are usually formed on a wax pattern with the mold being fired during which time the wax is removed.

In order to achieve the columnar-grained growth, Ver Snyder teaches the provision of heating means for maintaining the upper portion of the mold at a temperature of about 100 F. above the melting point of the alloy being cast. The mold is characterized by an open bottom which is located on a support member maintained substantially colder than the body of the mold during casting. The chilling of the cast metal at one end causes the component being cast to crystallize in a columnar structure wherein the crystals are unidirectionally oriented and are aligned substantially parallel to the vertical axis of the component. The presence of grain boundaries normal to the stress axis of the cast component is substantially avoided.

Other methods have also been described for achieving differential cooling rates in molds to thereby promote columnargrained growth. In Chandley Pat. No. 3,248,764, reference is made to the use of metal shells which are located around the mold at different levels to provide a thermal gradient. Chandley Pat. No. 3,376,915 refers to the use of induction heating means which can be partially and selectively deenergized for providing a thermal gradient.

It is a general object of the instant invention to provide an improved system for achieving columnar-grained growth in cast components.

It is a more specific object of this invention to provide a casting method and apparatus which will enable the production of columnar-grained growth in cast components by highly efficient means in that the cast products can be produced at relatively low cost with a significant reduction in rejects due to the presence of inclusions in the cast products.

These and other objects of this invention will appear hereinafter and for purposes of illustration, but not of limitation, specific embodiments of the invention are shown in the accompanying drawings in which:

FIG. 1 is an elevational view, partly in section, illustrating the manner in which an assembly of a wax pattern and conductive base is formed for use in the practice of the invention;

FIG. 2 is a vertical sectional view of an assembly of the type shown in FIG. 1 having a shell mold formed therearound;

FIG. 3 is an elevational view of a prior art arrangement for preparing patterns for the application of a shell mold; and,

FIG. 4 is an elevational view of an improved arrangement for the formation of a shell mold around wax patterns.

The instant invention includes a method which involves the preparation of a pattern conforming in shape to the columnargrained metal component to be cast. The pattern material is associated with a base formed of a material characterized by high heat conductivity and by the ability to withstand the casting temperatures. A material consisting of graphite or of high thermal conductivity refractory will serve as a suitable material for forming the base.

A mold is invested around the assembly comprising the pattern and base and thereafter the pattern material is removed from the mold. Conventional pattern and mold materials may be employed, for example, wax may be utilized for forming the pattern and shell molding techniques employed for building up a mold around the assembly of the pattern and base. After removal of the wax, molten metal can be introduced into the mold with the base and shell mold walls defining the configuration of the component being cast. Because of the high thermal conductivity of the base, heat will be removed at a relatively high rate in this area thereby promoting nucleation of the metal adjacent the base surface and thereby promoting a columnar-grained growth in a direction extending away from the base.

The novel means of the invention comprises the assembly of the pattern material and base. In a preferred form of the invention, the base and pattern assembly are produced by forming (as by molding) one or the other of these elements in direct association with the remaining element. With this arrangement, the base will define the exact configuration of the end of the pattern and, accordingly, the cast component will have a substantially finished configuration thereby reducing machining and other finishing costs. The system of this invention also simplifies the provision of nucleating means in association with the base, and the use of the graphite or ceramic base substantially reduces the possibility of inclusions being formed in the casting.

The invention also contemplates the provision of a novel arrangement for use during investing of mold material in a shell molding process. Specifically, the base portions of a plurality of assemblies comprising the pattern and base can be interconnected through the use of rod means. This arrangement will also include pattern material for forming gates and sprues whereby the entire arrangement can be provided with a surrounding shell mold through alternate applications of dip coat and refractory material. The use of the supporting rods in conjunction with the base portions of the individual assemblies permits elimination of a supporting base plate thereby simplifyingthe shell molding procedure.

FIG. 1 illustrates a wax pattern 10 which conforms in shape to the turbine element in the cast. The wax pattern includes a base portion 12 defining recessed areas 14.

A mold wall 16 extends around the end portion 12 of the wax pattern. This mold wall thereby provides a mold cavity in conjunction with the end portion 12, and a base 18 is adapted to be formed in this cavity. A slurry of material may be employed for filling the mold cavity or other standard techniques for forming components from graphite or ceramic materials may be utilized. At any rate, the preferred form of the invention calls for the combination with the pattern of a base 18 which conforms with the end of the pattern and which thus conforms with the end of the component to be ultimately cast.

As an alternative to the arrangement shown in FIG. l, the base 18 could be formed in a mold specifically designed for forming the configuration illustrated. This base 18 could then be associated with a pattern forming mold whereby the pattern including the end portion 12 would become associated with the base 18. As a further alternative, the base could be machined to the desired configuration and then joined with the wax pattern.

The assembly comprising the base 18 and the pattern 10 may then be provided with a surrounding shell mold 20 as shown in FIG. 2. Standard techniques may be employed for forming the shell mold. Ceramic pin means 22 may be utilized for insuring a good connection between the base 18 and the shell mold 20. This arrangement can be accomplished by drilling holes in the base 18 and then press fitting the pins 22 in these holes. The shell mold material will then be invested around the pins to provide a secure connection. A layer of paraffin may be formed around the periphery of the base 18 prior to investing to allow for expansion of the base during subsequent heating.

FIG. 3 illustrates one standard arrangement which may be employed for investing a shell mold around the assembly comprising the pattern 10 and base 18. In this arrangement, an aluminum plate 24 is provided for supporting the assemblies. A connecting rod 26 is inserted through this plate and extends upwardly for connection with a handle 28. A top plate 29, ceramic sprue cup 30, wax sprue 32, and wax gates 34 are connected in conventional fashion. When supported by the rod 26 and the handle 28, this arrangement can be alternately dip coated and covered with refractory for the formation of shell molding material around all of the exposed surfaces. The pattern material is then removed and the mold fired in preparation for the introduction of molten metal. Shell molding techniques of a type suitable for use are described, for example, in Erdle and Feagin Pat. No. 3,005,244 issued Oct. 24, 1961.

Due to the relatively high thermal conductivity of the base 18, nucleation of the molten metal will occur on the base surface, and columnar-grained growth will proceed. The relatively high thermal conductivity of the base will provide solidification in this matter; however, cooling means may also be provided to insure solidification in the desired fashion. Thus, the assemblies could be located on a metal support which includes internal cooling whereby heat will be more rapidly drawn through the base portion 18 for promoting the desired solidification. Nucleating, for example, oxides of cobalt or nickel, cobalt aluminate, or cobalt silicate, may be utilized to provide additional assurances that nucleation will occur on the base surface.

FIG. 4 illustrates an alternative means for supporting the assemblies comprising a pattern 10 and base 18 prior to shell molding. In this arrangement, a ceramic cup 38,

wax sprues

40 and 42, and wax gates 44 are employed in accordance with conventional practice. The wax gates 44 extend to the pattern material 10. The base portions 18 associated with the pattern material 10 are interconnected by means of a ceramic rod 46. The extension 48 of the sprue 42 extends to a central portion of this rod primarily for purposes of supporting the rod during investing of a mold around the arrangement of FlG. 4.

The rod 46 may be press-fitted in bores defined by the respective base portions 18. The rod will, therefore, assist in holding the arrangement intact during investing and after removal of the pattern material from the invested mold, the rod will continue to provide support. Thus, the provision of the base portions 18 provides a means for anchoring a supporting rod, and with this arrangement, the use of an aluminum setup plate, such as the support plate 24 shown in FIG. 3, can be eliminated. This simplified the setup prior to dip coating as well as the eventual casting operation particularly when the base portions 18 are to be placed in contact with a cold support during the casting operation.

It has been found that the formation of shell molds around the assembly comprising pattern material 10 and base portion 18 will provide definite advantages over systems currently in use. The arrangement of this invention permits the use of patterns which have a configuration directly corresponding with the configuration of the component to be produced. This provides a distinct saving in machining cost since the end portions of the cast components do not have to be machined to any extensive degree.

The instant invention also simplifies the use of nucleating means since these means can be formed directly in conjunction with the formation of the base portions 18. Finally, the use of graphite or highly conductive refractory materials for forming the base portion has been found to substantially reduce the presence of inclusions in the finished product. This substantially reduces the number of rejects and, therefore, increases the efficiency ofthe casting operation. Where the molten material is cast into direct contact with a shell block as in conventional practice, inclusions are a continuing problem and result in a relatively high rate of rejects.

The system of the invention greatly reduces the necessity for reworking wax patterns since simpler wax assemblies can be used. The setup procedures for an arrangement suitable for the formation of shell molds are greatly simplified and the overall casting cycle time is shortened because of the simplification and because hot molds can be employed with the techniques ofthis invention.

As indicated, conventional materials and techniques can be utilized during formation of the patterns and shell molds. With respect to the mold base, the material selected may be any highly heat-conductive materials which are more highly conductive than the mold itself. Shapes made from powdered metals, metallic carbides, borides, beryllides, graphite and heat-conductive glasses and ceramics would be suitable. Pow dered metal shapes may include those formed from powdered tungsten, tantalum, copper, stainless steel and molybdenum with the specific metal shape being selected depending upon the metal being poured since the metal shape should be sufficiently stable to allow the particular alloy to be poured against it. Tungsten and tantalum carbides, titanium dibordie, tantalum boride and beryllide, titanium nitride, boron nitride and shaped beryllium oxide are examples of carbides, borides and beryllides which may also be employed.

It will be understood that various changes and modifications may be made in the above described system which provide the characteristics of this invention without departing from the spirit thereof particularly as defined in the following claims.

That which I claimed is:

1. A method for the production of a cast columnar-grained metal component comprising the steps of preparing a pattern corresponding to the shape of the casting to be formed and having a bottom surface, forming against the bottom surface of said pattern a base formed of a material selected from the group consisting of graphite and high thermal conductivity refractory, said base having a surface corresponding in shape to the base surface of the pattern and in contact therewith, building up mold walls of sufficient thickness and strength about the assembly comprising the pattern and the base to form a shell mold thereabout, leaving exposed the surface of the base opposite to the pattern and then removing the pattern material from the mold to provide a shell mold, said base forming the bottom wall of the mold and operating to remove heat from molten metal poured into the formed shell mold at a rate greater than the surrounding mold walls to promote nucleation of the metal adjacent the surface of the base.

2. A method in accordance with claim I wherein said base is formed to a configuration conforming with the bottom wall of the pattern and the surface of the base opposite to the bottom wall of the pattern is exposed by the mold for effective heat removal through said base.

3. A method in accordance with claim 2 wherein said assembly is formed by first preparing said pattern, providing mold walls around the bottom of the pattern whereby the pattern bottom serves as the bottom wall of said last mentioned mold, and introducing material for forming said base into said last mentioned mold for securing the base to the pattern.

4. A method in accordance with claim 2 including the step of providing a mold conforming to the shape of said base, molding said base, and thereafter applying said pattern material to said base for forming the assembly comprising the pattern and base.

5. A method in accordance with claim I wherein a shell molding process is followed for investing said mold around said pattern and base 6. A method in accordance with claim 1 including the step of locating nucleating agents on said base prior to the introduction of molten metal into said mold.

7. A construction for the production of cast columnargrained metal components comprising an assembly consisting of a disposable pattern corresponding to the shape of the casting to be formed and having a bottom wall and a base engaging and conforming to the bottom wall of said pattern, said base being selected from the group consisting of graphite and high thermal conductivity refractory, and a shell mold built up around said assembly, leaving exposed as the bottom wall of the mold the surface of the base opposite to the pattern.

8. A construction in accordance with claim 7 wherein said base is formed by solidification against the bottom wall of the pattern to a configuration conforming with the bottom wall of the pattern.

9. A construction in accordance with claim 7 wherein a plurality of said assemblies are secured together prior to application of the mold material, and including supporting rod means extending between the base portion of the assemblies, said rod means being formed of nondisposable material whereby the rod means operate to support the respective base portions relative to the mold after removal of the pattern material from the mold.

10. A construction in accordance with claim 7 including pin means secured to said base and extending outwardly therefrom, said pin means extending into the mold material invested around the base for thereby securing said mold relative to said base.

11. A method in accordance with claim 1 including the step of positioning the mold on a chill plate with the base in contact with the chill plate prior to introducing molten metal into the mold. t

Claims

2. A method in accordance with claim 1 wherein said base is formed to a configuration conforming with the bottom wall of the pattern and the surface of the base opposite to the bottom wall of the pattern is exposed by the mold for effective heat removal through said base.

5. A method in accordance with claim 1 wherein a shell molding process is followed for investing said mold around said pattern and base.

6. A method in accordance with claim 1 including the step of locating nucleating agents on said base prior to the introduction of molten metal into said mold.

7. A construction for the production of cast columnar-grained metal components comprising an assembly consisting of a disposable pattern corresponding to the shape of the casting to be formed and having a bottom wall and a base engaging and conforming to the bottom wall of said pattern, said base being selected from the group consisting of graphite and high thermal conductivity refractory, and a shell mold built up around said assembly, leaving exposed as the bottom wall of the mold, the surface of the base opposite to the pattern.

11. A method in accordance with claim 1 including the step of positioning the mold on a chill plate with the base in contact with the chill plate prior to introducing molten metal into the mold.