Method Of Preventing Core Shift In Casting Articles

Abstract

Shell-type casting molds for producing hollow cast articles, the mold being built up by forming a low melting pattern about a ceramic core, inserting thin metal pins through the pattern and into engagement with the core, forming a shell mold about the resulting pattern so that the ends of the pins are anchored in the resulting shell mold, removing the meltable pattern, and casting the molten metal into the cavity thus produced whereby the molten metal dissolves the pins and no holes appear in the finished article.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of precision investment casting molds and, more specifically relates to a means for preventing core movement or shifting of a core in a shell mold, such means including a plurality of thin wire pins which are anchored in the shell mold and extend into engagement with the surface of the core, thereby spacing the core properly from the wall of the casting cavity.

2. Description of the Prior Art

Modern investment casting procedures are frequently used to produce castings which have complex hollow interiors. Some of the best examples of such cast articles are turbine blades and vanes containing a hollow interior for the purpose of providing cooling to the blade or vane during use. In order to provide the hollow interior in the cast article, of course, it is necessary to use a core, usually ceramic in composition. A problem arises when there is even a slight movement or shift of the core in the mold which may occur during removal of the pattern material, during preheating of the mold prior to pouring the metal therein, or during pouring of the metal into the mold.

A prior practice which has attempted to solve this problem involved drilling holes in the wax pattern until the core was reached. Then, when the shell mold was formed about the pattern, the holes would be filled with the ceramic material of the shell mold making composition. After removal of the wax, the portions of the ceramic material which had found their way into the holes formed posts which remained to hold the core in place. However, when the metal was cast around these ceramic posts, holes were left in the casting wall. These holes then had to be plugged with metal or otherwise removed.

Another method which has been tried involves the use of chaplets consisting of two discs of metal connected by a cylinder. The pattern material such as wax was injected around the chaplet, with one head of the chaplet contacting the core and the other head pressing against the mold wall. The chaplet was held in place by the pressure exerted against the two heads. The disadvantage of this method is that a relatively large mass of metal in the chaplet prevents fusion with the poured metal.

SUMMARY OF THE INVENTION

The present invention provides a mold structure for producing hollow castings. The mold is built up by first forming a low melting pattern about a ceramic core, then inserting metal wires through the resulting pattern into engagement with the ceramic core while leaving exposed end portions on the wires. A ceramic shell mold is then formed about the combination of core, pattern and wires, thereby anchoring the exposed end portions of the wires in the completed shell mold. When the pattern material is melted out, the metal wires remain anchored in the shell mold and provide lateral support for the core. Next, the molten metal is cast into the casting cavity provided by the removal of the pattern, thereby causing the portions of the wires which extended through the casting cavity to be fused within the molten metal and disappear, so that no imperfections remain in the body of the finished casting.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view in elevation of a pattern having a core therein;

FIG. 2 is a cross-sectional view taken substantially along the line II-II of FIG. 1;

FIG. 3 is a view similar to FIG. 2 but illustrating the manner in which the wire pins are positioned through the pattern and against the core;

FIG. 4 is a view similar to FIG. 3 but illustrating the complete assembly resulting after the shell mold has been formed around the core and pattern;

FIG. 5 is a view similar to FIG. 4 but illustrating the elements of the assembly after the pattern material has been removed; and

FIG. 6 is a view similar to FIG. 5 but showing the mold assembly after casting and solidification of the metal, but prior to the removal of the core.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, reference numeral 10 indicates generally a relatively low melting pattern composed of wax, polystyrene or similar low melting pattern making material. The particular pattern shown in FIG. 1 is to be used for the manufacture of a turbine blade, the blade having an arcuate vane section 11, an upper shroud 12 and a relatively massive root portion 13. A gate forming portion 14 is also included on the pattern to provide the cavity through which molten metal may be introduced into the finished mold. While FIG. 1 is concerned, for purposes of simplicity, with a single pattern it should be recognized that the present invention can be used and usually will be used with clusters of patterns.

The pattern material has an internal core 15 composed of a ceramic material such as fused silica. Typically, the material of the pattern is injected about the ceramic core 15 in a conventional pattern making mold. The core 15 may be provided with upper and lower extensions 15a and 15b which serve to anchor the top and bottom of the core when the ceramic mold is formed about the core.

As illustrated in FIG. 3, a plurality of wire pins 16 is then positioned at selected locations along the length and breadth of the pattern, each of the pins 16 having an end portion 17 which passes through the pattern material and engages the surface of the ceramic core 15. In order to expedite insertion of the pin 16 through the pattern material, it is preferable to preheat the pin 16 slightly above the melting temperature of the wax or other pattern material so that only a minimum diameter hole is provided by the insertion of the pin 16 through the pattern material. In order to facilitate removal of the pin, it is desirable that the diameter of the pin 16 be kept very small, on the order of less than 0.050 inch, although this value can be increased for larger sizes of castings.

As also illustrated in FIG. 3, the opposed end portions 18 of the pin 16 extend beyond the pattern material by a matter of one-eighth or one-fourth inch or so to serve as anchoring means in the completed shell mold.

In order to avoid contamination of the melt with the metal of the pins 18 when they are fused therein, it is preferable that the pins be composed of the same metal as will be used for forming the ultimate casting.

As seen in FIG. 4, a shell mold 19 is then built up around the combination of the core 15, the pattern 10 and the pin 16.

There are a number of ways in which to build up a ceramic shell mold about a low melting pattern. Usually, the pattern assembly is dipped in a series of ceramic slurries, with intermediate drying, to form a mold which, after firing, provides a relatively gas permeable ceramic structure of one-eighth to one-fourth inch or so in thickness.

A particularly preferred method of building up the ceramic shell mold involves dipping the pattern 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 material such as zirconium oxide, a binder such as colloidal silica and a thickener and low temperature binder such as methyl cellulose. The initial layer while still wet is then 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 pattern with the dusted wet refractory layer on it is then suspended on a conveyor and moved to a drying oven having a controlled humidity and temperature, thereby drying the coated pattern adiabatically.

The steps of dipping, dusting and adiabatic drying are then repeated using air at progressively lower humidities for succeeding 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 then be dried with a relative humidity of 35 to 45 percent, the fifth and sixth coats with a relative humidity of 25 to 30 percent, and the final coat with a relative humidity of 15 to 25 percent.

The first layer is preferably applied to a thickness of 0.005 to 0.020 inch, 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 being about 0.005 to 0.020 inch.

After the mold is built up on the pattern material, the pattern can be removed by heat, and then the green mold is ready for firing. Generally, firing temperatures on the order of 1,500.degree. to 1,900.degree. F. are used. The resulting shell molds are hard, smooth and relatively permeable.

The condition of the assembly after melt out of the pattern and firing of the mold is illustrated in FIG. 5. It will be seen that the core 15 remains laterally supported within the casting cavity 20 by virtue of the pins 16 which have become anchored in the ceramic shell mold.

FIG. 6 illustrates the assembly after the molten metal has been poured into the casting cavity 20 to provide a casting 21. The molten metal which is usually superheated by a matter of several hundred degrees above its liquidus temperature before pouring, causes the portions of the pins 16 which had previously extended into the casting cavity to become fused therein while the end portions 18 of the pins remain anchored in the walls of the shell mold 19.

After solidification of the metal, the shell mold 19 is broken off and the silica core 15 is removed as by dissolution in strong alkali solutions. The presence of the wires maintains the same distance between the core surface and the mold wall surface during wax removal, mold preheating, and during pouring of the metal into the mold. This prevents core movement and the resulting undersize wall thickness.

The process of the present invention was used to cast turbine blades from a nickel base superalloy. The pins employed were about 0.020 inch in diameter and extended for about one-eighth to one-fourth inch beyond the surface of the pattern. Consistently good results were obtained with respect to preventing core movement during the mold making and casting process as evidenced by the fact that the method was used on a two hundred piece production run and resulted in a better than 90 percent yield of good parts, whereas the same number of blades produced without the supporting wires yielded only about 10 percent good parts.

It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

Claims

We claim as our invention:

1. The method of making a hollow casting which comprises forming a low melting pattern about a ceramic core, inserting metal wires through the resulting pattern into engagement with said ceramic core while leaving exposed end portions on said wires, forming a ceramic shell mold about said pattern thereby anchoring said exposed end portions therein, melting out said pattern to leave said metal wires providing lateral support for said core, casting molten metal into the casting cavity provided by melting out said pattern to thereby melt out the portions of the wires extending through said casting cavity, and removing the ceramic core from the casting.

2. The method of claim 1 in which said wires are composed of the same metal as said molten metal.

3. The method of claim 1 in which said wires are inserted through said pattern while hot.

4. The method of claim 1 in which said wires are less than about 0.050 inch in diameter.