Means for supporting core in open ended shell mold

[57] ABSTRACT

An open ended refractory shell mold having a casting cavity therein, a chill block positioned to close off the open end of said mold and a ceramic core having one end received within said mold and having a free end extending into said molding cavity. The free end of said core is restrained from lateral movement within said molding cavity by engaged pin means having opposed ends anchored in the mold walls.

Claims, 4 Drawing Figures

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Patented May 2, 1972

3,659,645

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3,659,645

MEANS FOR SUPPORTING CORE IN OPEN ENDED SHELL MOLD

The present invention relates to apparatus for the casting of high temperature alloys and, more particularly, to the production of castings having controlled grain structure and exceptional soundness. In the preferred embodiment of the present invention, the apparatus is used to produce columnar castings which have been found to be particularly desirable in articles such as jet engine blades and vanes which are subject in use to extreme heat and thermal cycling.

Recent work on columnar structures indicates that for some applications, such structures are markedly superior to equiaxed structures. For example, it has been found that the high temperature properties of columnar structures are superior, particularly in fracture resistance and ductility under creep loading conditions.

Columnar structures are formed by the unidirectional growth of dendrites during solidification. The relationship between the dendritic structure and the columnar grains is not exact. Each columnar grain is usually composed of more than ²⁰one dendrite, and the number may vary from a few to several hundred. The interdendritic spacing is related to the solidification rate only. Columnar grain size, however, may be affected by factors other than the solidification process such as ordinary grain growth. Despite these differences, the most con- ^{25 ca}^ⁱⁿgvenient approach for the examination of columnar structure ~ formation is through the study of dendrites formed during solidification.

The primary requirement for the formation of a parallel dendritic structure is the presence of a unidirectional thermal gradient during initial pouring of the metal into the mold, and during the controlled cooling of the casing during solidification. When the metal first enters the mold, the initial solidification occurs at the mold wall due to a chill effect, assuming the mold wall to be below the solidification temperature of the metal. This chill zone consists of many fine dendrites having a random orientation. The initial freezing releases the heat of fusion, resulting in some temperature rise locally, arresting the chill zone formation. At the interface of the chill zone and the melt, the dendrites begin to grow into the melt at a rate dependent upon the amount and depth of the supercooling.

Initially, all dendrites at the chill zone-melt interface grow at equal rates, since equal supercooling is present. However, those oriented parallel to the thermal gradient are growing into an area of continued supercooling. Those oriented unfavorably cannot advance as rapidly in the direction of the thermal gradient, since only a component of the growth velocity is aligned with this gradient. The dendrites growing parallel to the gradient, since they have already undergrown some growth, will give off a latent heat of fusion, due to the freezing process. This heat of fusion increases the temperature at the base of the dendrites and decreases the amount of supercooling available for growth of the more unfavorably oriented neighbors. In this manner, the growth of the misoriented dendrites is stifled, and only those aligned with the thermal gradient will undergo significant growth.

The most suitable process for the production of columnar structures employs an open ended ceramic type mold of the shell mold type produced by conventional precision investment mold making processes. The open end of the mold is placed against a chill block consisting of a metal of high thermal conductivity such as copper, and the ceramic shell mold is preheated before pouring of the metal to provide a desired _ . ______________ temperature gradient. Upon pouring of the metal, the molten 65 ^{tern is} originally made. The ends of the pins 17 and 18 extend metal first contacts the relatively cold chill block and begins to solidify with the formation of the dendritic structures. By controlling the thermal gradient existing in the mold during pouring and afterwards, the major axis of the columnar grains can , ________________I____________ be preselected, and the grains grown with a desired directional 70 the shell moid from the type of pattern shown'in^FIG^l' l orientation. -- .... ·

Cooling of the metal to solidification is equally important with adequate preheating. In the best technique thus far developed, heat is continually supplied to the mold during which is lower than that which would occur if the source of heat were removed entirely. This controlled cooling can be achieved in several ways, one being a controlled de-energization of the heating source about the mold in order to establish zones of differing temperatures. Another consists in physically moving the casting during solidification relative to the heat source for the mold, thereby maintaining desired thermal gradients within the body of metal remaining to be solidified.

While the techniques of columnar casting have been reasonably well developed for completely solid castings, the use of this technique in conjunction with molds having cores therein has posed additional problems. Typically, a core used, for example, in the manufacture of a turbine blade having a hollow vane portion is composed of a ceramic which expands more than the material of the mold. It is accordingly difficult to position such a core accurately within the molding cavity, particularly since it has to be supported in the molding cavity in a depending position, with a free end extending in relatively close proximity to the highly thermally conductive chill block. Consequently, the core cannot be held in the conventional manner at the bottom of the mold because of the chill contact requirements, yet it must be structurally supported within the mold to assure proper location of the cored passage in the

One of the objects of the present invention is to provide an improved apparatus for casting, particularly for the growth of columnar grains, which provides means for supporting a core within a casting cavity in an open ended mold.

Another object of the invention is to provide a means for supporting a core in a casting cavity against lateral displacement, without interfering with grain growth or orientation during the casting and solidification phases.

Still another object of the invention is to provide a method for achieving columnar grain structures in cored parts without interfering with the normal columnar grain development.

A further description of the present invention will be made in conjunction with the attached sheet of drawings in which:

FIG. 1 is a view partly in elevation and partly in cross-section illustrating a wax pattern embodying a ceramic core, and used for the production of the shell mold of the present invention;

FIG. 2 is a cross-sectional view of the finished mold assembly disposed on a chill block, and having a core supported therein in proper alignment with the casting cavity;

FIG. 3 is a cross-sectional view taken substantially along the line ΠΙ—IHof FIG. 2; and

FIG. 4 is a view similar to FIG. 3, but illustrating somewhat modified form of the present invention.

AS SHOWN IN THE DRAWINGS

In FIG. 1, reference numeral 10 indicates generally „ disposable pattern composed of a material such as wax or the like used for the manufacture of a cored turbine vane. The particular vane shown in the drawings includes a shroud portion 11, an arcuate vane portion 12 and a root portion 13. A ceramic core 14 having a core print portion 16 is embedded in the wax during the formation of the pattern, to provide the cored passage in the finished turbine vane. The lower end of the core 14 is held in position by means of a pair of ceramic pins 17 and 18 (FIG. 3) which abut the opposed sides of the core 14, and are prepositioned in the die in which the wax patbeyond the external limits of the root portion 13 so that they can be ultimately received in the shell mold itself, as illustrated in FIG. 3 of the drawings.

While many different techniques can be used for producimz ~U~11__1J JC___- . /. . . ^r__ ®

-_Jr- — ,——... ,,, , _1VJ. j prefer to use a method of making refractory molds described in U.S. Pat. No. 2,932,864. In the process described in that patent, the temperature of the pattern material is held substantially constant from the time the pattern is formed until solidification of the metal to provide a controlled cooling rate 75 the pattern is removed from the shell mold. Typically, the tem3,659,645 perature of the pattern during this process may range from about 70° to 80° F.

Initially, the pattern at room temperature or so is dipped in an aqueous ceramic slurry having a temperature about the same as that of the pattern material to form a refractory layer of a few mils in thickness. A typical slurry may contain ceramic materials such as zirconium oxide, and a binder such as methyl cellulose. The initial layer while still wet can then be dusted with small particles (40 to 200 mesh) of a refractory glass composition such as that known as “Vycor” which is a finely divided high silicon oxide glass containing about 96 percent silica and a small amount of boric acid together with traces of aluminum, sodium, iron, and arsenic. The dusted wet refractory layer on the pattern is then suspended on a conveyor and moved through a drying oven having a controlled humidity and temperature, so the coated pattern is dried adiabatically. When using air at a wet bulb temperature of 75° F., the prime coat can safely be dried by air having a relative humidity of 45 to 55 percent.

The steps of dipping, dusting, and adiabatic drying are then 20 made to the described embodiments without departing from repeated using air at progressively lower humidities for sueceeding coats. For example, the first two coats can be dried with air having a relative humidity of 45 to 55 percent. The third and fourth coats can be dried at a relative humidity of 35 to 45 percent, the fifth and sixth coats at a relative humidity of 25 25 to 30 percent, and the seventh and final coat with a relative humidity of 15 to 25 percent.

The first layer is preferably applied to a thickness of about 0.005 to 0.020 inches, and the fine refractory particles are dusted onto the wet layer with sufficient force to embed the particles therein. It is preferred that the dusting procedure used provide a dense uniform cloud of fine particles that strike the wet coating with substantial impact force. The force should not be so great, however, as to break or knock off the wet prime layer from the pattern. This process is repeated until a plurality of integrated layers is obtained, the thickness of the layers each being about 0.005 to 0.020 inches.

After the mold is built up on the pattern material, the pattern can be removed by placing the same in conventional steam autoclave, and then the green mold is ready for firing. Generally, firing temperatures on the order of 1,500° to 1,900° F. are used. The resulting shell mold, identified at reference numeral 20 in FIG. 2, is hard, smooth, and relatively permeable, and measures on the order of one-eighth to one-fourth inch in thickness.

The ceramic mold 20 includes a casting cavity 19 therein, and an open ended portion 21 which is set on a chill block consisting of a block of copper 22 or the like. Heat transfer from the chill block 22 may be further improved by circulating a suitable coolant through the block 22. Metal is introduced into the casting cavity 19 by means of a riser 23 fed from a gate 24. While the core 14 may be dependently supported from the top of the shell mold 20 by means of the core print 16, it is not necessary to do so in the case of the present invention. Accordingly, the core print 16 may be initially coated 55 with wax during the formation of the pattern, so that there is a slight clearance space provided between the core print 16 and the material of the shell mold immediately adjacent it. The actual support function is provided by the pair of pins 17 and 18 which have their ends embedded in the shell mold 20 as illustrated in FIG. 3 of the drawings. The pins 17 and 18 firmly hold the free end of the core 14 in position as well as supporting it vertically. To further insure proper location, the pins can be firmly attached to the core 14 by means of a ceramic adhesive paste.

A modified form of the present invention is shown in FIG. 4 wherein the two pins 17 and 18 are replaced by a single pin 26 which extends through a suitable hole drilled in the core 14 to provide a tight fitting engagement therewith. The ends of the pins 26a and 26h are firmly secured within the shell mold to anchor the same and thus restrain lateral and vertical movement of the core 14. Then, upon pouring of the metal in the casting cavity 19, directional solidification resulting in the production of the columnar casting can proceed without interference with respect to directional grain growth.

The ceramic core positioning pins shown in the drawings are located in a portion of the part which is subsequently cut or machined from the finished configuration, so that it does not provide a defect in the finished casting.

From the foregoing, it will be understood that the use of the core holding pins of the present invention provides a highly effective means for locating the free end of a depending core portion within an open ended, ceramic mold. The pins are small enough so that they do not interrupt grain growth, and the columnar grain grows around them. Still, they are strong enough to maintain the core in its proper position without lateral movement.

It will be understood that various modifications can be the scope of the present invention.