US3142875A - Metal casting cores - Google Patents

Description

Aug. 4, 1964 N. s. LIRONES IETAL METAL CASTING CORES Filed April 6. 1961 Illa-Ill "'1' "'6 INVENTORS Nlck G. L/r-mes y Theodore 0 M18 United States Patent 3,142,875 METAL CASTING CGRES Nick G. Lil-ones, North Muskegon, and Theodore Operhail, Whitehall, Mich assignors to Howe Sound Company, New York, N.Y., a corporation of Delaware Filed Apr. 6, 1961, Ser. No. 191,201 4 Claims. (Cl. 22-196) This invention relates to new metal casting cores useful in the casting of high melting point metals such as those contained in Group IV-B of the Periodic Table. Specifically, the invention relates to metal casting cores useful in forming ceramic shell metal casting molds which may be employed in the production of castings of Group IVB metals.

In the propulsion of high speed aircraft, which use as part of their propulsion systems turbines and similar type devices, problems have arisen in connection with the materials to be used to produce components for such systems. Such components must be capable of withstanding the extremes of high temperature and high stress which quite commonly occur in normal operation. To a lesser extent, similar problems arise in high pressure, high speed gas and steam turbines, and the production of turbine components which have sufficient thermal properties to withstand the extremes of heat and stress is thus made more critical.

In one approach to these problems, designers and manufacturers have utilized such high melting point metals as titanium, zirconium and hafnium, and alloys thereof. However, the temperatures now encountered in some of the newer aircraft and turbine equipment tend to cause these materials to fail. To overcome this barrier, a further approach has involved modifications in equipment design, particularly of turbine blade construction.

It has become common practice to place a series of vents running throughout the interior surfaces of such components in an attempt to effectuate air cooling which would keep these metal structures from overheating. When designing such air cooling type structures, it is desirable to achieve maximum surface areas about which air can conduct away heat, yet, at the same time, not structurally weaken the component.

In order to produce air cooled structures of the type described above, particularly turbine components where high melting point metals are used as materials of construction, it has frequently been the practice of the art to resort to manufacturing these products using a lost wax casting technique. In a preferred manufacturing operation, such parts are made utilizing the special type lost wax casting process generally known as a ceramic shell metal casting process.

It was soon found that the production of passages in turbine components was a difficult problem to solve using the ceramic shell molding process because of the difficulties experienced in making cores for the molds. The problem of providing cores for ceramic shell molds, when used to cast high melting point metals and alloys, includes the need for finding of a suitable core material which can withstand the rigors of the casting process and for finding a suitable method for positioning and afiixing the cores within the mold. The problem is amplified, since, in the case of producing cores for small castings, such as aircraft turbine blades or vanes, the cores used for producing openings in the components are quite frequently of small diameter, thus making their manufacture and use difficult due to the physical limitation of size.

Experiments conducted with various conventional core materials quickly proved such materials to be woefully inadequate when used in producing castings of high melting point metals and alloys by shell molding processes,

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e.g., most ceramic cores tended to slump or crack when subjected to the high temperatures required in pouring these metals. When ceramics were chosen that had the proper thermal characteristics, it was found their removal from the finished part was accomplished with extreme difiiculty. Similarly, cores formed of low melting point metals such as copper, brass, iron and the like, failed completely since they melted during the pouring operation. High melting point metals and their alloys, when capable of withstanding the pouring temperatures, tended to fuse with the metal being poured, thus producing a contaminated casting. Non-metallic materials, such as graphite, tended to produce gaseous reaction products which formed air pockets throughout the castings thus, rendering them defective.

From the above discussion, it becomes evident that to be satisfactory as a core for casting high melting point metals in a ceramic shell metal casting process, the particular core employed must possess the following characteristics: (1) It must be easily formed; (2) will be of a type which will not gasify, slump or melt out at the temperatures used during the casting cycle; (3) be capable of easy removal after the casting has been completed; and finally, (4) when used to produce the openings in air cooled parts, it should be capable of producing maximum surface areas in the finished part. These characteristics must be combined with a core capable of being accurately aflixed in a ceramic shell mold and sealed in relationship to such mold whereby it can be effectively utilized in producing finished metal parts.

It, therefore, becomes an object of this invention to produce a new type core for use in casting high melting point metals and alloys, such as the metals and alloys of Group lV-B, particularly adaptable for use in a ceramic shell mold casting process.

Another object is to provide novel casting cores of the type described above which are capable of withstanding temperatures of such casting processes without deforming or being oxidized to a gaseous state and which are easily removed from the finished piece.

A specific object of this invention is to furnish a new type casting core which is capable of producing particularly advantageous interior surfaces in finished parts such as high temperature turbine components and the like.

These and other objects of this invention will appear hereinafter and for purposes of illustration, but not of limitation, specific embodiments of this invention are shown in the accompanying drawings in which FIGURE 1 is a perspective fragmentary view of a core of this invention;

FIGURE 2 shows in perspective a ceramic shell metal casting mold containing positioned therein a plurality of cores;

FIGURE 3 is an enlarged cross-sectional view taken along the line 33 of FIGURE 2;

FIGURE 4 is a fragmentary cross-sectional view of a cast metal part containing a coating formed by the cores of the invention; and

FIGURE 5 is an enlarged cross-section taken along the line 4-4- of FIGURE 4.

In accordance with the practice of this invention, it has been found that a suitable casting .core for use in producing high melting point metal castings by the ceramic shell metal casting process may be afforded by coating onto a core type material a uniform film of a high melting point, difliculty oxidizable metal or alloy. The core type material makingup the interior portion of the core, may be composed of any type solid substance which is capable of accepting a film of the high melting point difficultly oxidizable metal. Thus, for instance, the core interior may be composed of either metals or non-metals,

although metals are preferred. Also, such non-metallic substances as waxes and the like having been found suitable as the core base. In the case of metals, any metal which has a melting point lower than the melting point of the high melting point, difficulty oxidizable metal outer coating may be used. Thus, such metals as copper, iron, steel, brass, admiralty metal, tin silver, bismuth, and the like may be used. In a preferred embodiment, it is desired to use as the inner portion of the core the metal molyb denum, which possesses special properties, when used in the invention, that make it the most satisfactory under most conditions.

The high melting point difiiculty oxidizable metal employed as the outer surface coating of the core is preferably selected from those metals and alloys thereof of this particular class which have a boiling point and/ or melting point higher than the temperature of the metal poured into the mold. Thus, with such materials as titanium, which are frequently poured at temperatures ranging from 28004 100 F., it is desirable to use as the outer coating such metals as chromium, platinum, tungsten, columbium, and tantalum, and alloys thereof. It will, of course, be understood that other metals possessing properties similar to those described above may also be employed in making the cores.

FIGURE 1 illustrates a typical core composed of an outer coat 14 of a high melting point, dih'icultly oxidizable metal which has been formed about a suitable solid substance 12 which forms the interior of the core. As previously indicated, in certain types of cores, the substance 12 will be removed from the interior of the core, thus providing a hollow-type structure.

A ceramic shell mold 16 having positioned therein a plurality of cores 10 adapted to form a series of voids in the interior of a finished metal casting is shown in FIG- URE 2. The mold 16 is shown to be in the form of a turbine blade having a leading edge 30 and a trailing edge 32. The mold is formed of a plurality of ceramic coats in accordance with shell molding pratcice, the coats being more clearly shown in FIGURE 3 by numerals 18-27. The mold is provided with a pouring opening 28 into which molten metal is introduced at the time the casting is to be made. The cores 34 are located within the mold 16 and are positioned at the end portion 40 thereof. These cores may extend substantially into the surface of the mold such as shown by cores 42 or the cores may be extended substantially beyond the end of the mold as is the case of the core designated by the numeral 46. The opposite ends of the cores may likewise pass through the other end of the mold, or locate within the mold 16.

In order to accurately and rigidly affix the cores to the interior surfaces 38 of the mold, a series of holes was first formed into the wax or plastic pattern by using a suitable die designed with extensions which corresponded to the general dimensions of the core. Portions of the die corresponding to the cores which are to be employed extend beyond the extremities of the pattern a distance sufiicient to provide openings corresponding to the core positions. After formation or injection of the wax or plastic into the die cavity, the die portions are removed and the cores inserted in place.

As shown in FIGURE 3, the cores 10 will extend into the interior surface 38 of the mold 16 throughout its entire thickness and beyond when using the above described expedient of a die to position the core within the mold. The outer surface of cores 10 are fitted where they contact the interior of the mold with expansion means 48 which are most suitably made of the same ceramic as the composition of the mold, but may be of other materials such as metal, wax, etc. Since, in most instances, the area to be fitted by the core and the mold is extremely small, the expansion means may be suitably formed by coating the end portion of the core with a thin film of a substance which thus forms the expansion means. In the case of wax, the ends of the core would be dip coated with the substance and then inserted into the wax or plastic pattern. The collar thus formed around the end of the core would then be allowed to bond to the interior of the mold.

Molybdenum which is a preferred substance from which the interior of the core is fabricated is an extremely high melting point substance which has the unique property of forming gaseous products when employed under the conditions of use in the invention. The interior molybdenum core is oxidized out after the casting has been made, leaving a thin shell of the high melting difiicultly oxidizable metal interior.

It has further been discovered, particularly in the case where chromium was used as the outer coating of the core, that a surface is produced between the metal core surface and the high melting point metal poured around the core which has special characteristics and unique properties. A microscopic inspection of the interior surface of the core showed it to appear as a roughened ridge-like surface. This surface 56 is illustrated in FIGURES 4 and 5. The chrome plating in the drawing is shown as being bonded to the inner surfaces 50 of the casting 51 at the interface 52. The roughened surfaces are extremely desirable in turbine components since they tend to produce maximum air turbulence, and increase the cooling capacity of the passages.

When it is not desired to have the surfaces of the core become bonded to the interior of the casting, suitable methods may be employed for removing the thin coating. Such methods as acid etching, mechanical burring, and the like may be used to remove the interior finish.

The cores may be formed using any suitable manufacturing technique. Thus, once the interior segment of the core has been formed either by casting, extrusion, power metallurgical techniques or by the use of adhesives, the outer film of the high melting point difiiculty oxidizable material is placed around the core by using known techniques, such as, for instance, electrical vacuum deposition, electroplating, dip coating and the like. In the case of chromium or molybdenum, it was found that the expedient of chrome plating gave excellent results. The chrome plate was applied, using conventional copper and nickel base coatings prior to the actual deposition of the chrome film.

After the core has been formed, in some cases, it may be desirable to remove the interior of the core from the outer coating of the high melting point, difiiculty oxidizable outer metal shell prior to actually pouring the mold. When this is done, removal of the inner portion of the core may be accomplished by using such removal technique as oxidation, chemical dissolution or physical removal methods such as drilling, suction and the like. When the interior of the core has been removed, what remains is essentially a hollow core which is composed of the high melting point, difiiculty oxidizable metal. Such cores are invested with loose sand or the like for a back-up and used in ceramic shell molds as previously described.

The thickness of the coat, which comprises the outer layer of the core is preferably about .005 inch but may be varied from between .0001 to .010 of an inch as long as the interior of the core has sufficient rigidity to maintain the shape and structure of the core throughout most of the casting process. Where the core is removed prior to the casting operation or is removed at an early stage of the casting process, then it is desirable that the thickness of the exterior of the core be of sufiicient magnitude to provide strength and rigidity, e.g., somewhat greater than .005 inch. Chromium sleeves which may or may not be filled can have inner diameters ranging from .030 to .050 inch and outer diameters up to .080 inch.

The instant invention has been described with reference to shell molding processes and a complete description of such processes may be found in Theodore Operhalls application Serial No. 708,628, entitled Metal Casting Process and Elements and Compositions Used in Same, filed January 13, 1958, now Patent No. 2,961,751. It is understood, however, that the application of the inventive principles to other casting procedures where the use of the inventive cores would be feasible is contemplated.

It will be understood that various modifications in the above disclosed metal casting cores may be provided without departing from the spirit of this invention, particularly as defined in the following claims.

We claim:

1. The process of forming castings of high melting point metals wherein said castings are provided with elongated internal passages, said process comprising the steps of forming cores by providing a main body portion of a material adapted to be removed from the castings after formation thereof, coating said main body portion with a metal selected from the group consisting of chromium, platinum, tungsten, columbium, tantalum, and alloys thereof to provide a coating between .0001 and .01 inch thick, imbedding said cores in an expendable pattern material whereby the ends of the cores extend outwardly of the pattern material, forming a ceramic shell mold around the pattern material whereby the said ends of the cores are at least partially imbedded in the shell mold, sealing the embedded portions of the core ends in the mold, removing the pattern material from said mold, and introducing said metal into said mold to form said castlugs.

2. The process of claim 1, wherein the core is sealed into the mold by placing a thin wax coating around a portion of the core embedded in the mold.

3. The process of claim 1, wherein the core is sealed into the mold by placing a ceramic collar around the portion of the core embedded in the mold, said collar being composed of the ceramic used to form the mold.

4. The process of claim 1, wherein the core is sealed into the mold by placing a metal collar around the core embedded in the mold, said collar having a coefficient of expansion similar to the coefiicient of expansion of the mold.

References Cited in the file of this patent UNITED STATES PATENTS 1,912,889 Couse June 6, 1933 2,609,576 Rouse et al. Sept. 9, 1952 2,679,669 Kempe June 1, 1954 2,688,781 Fahlberg et al. Sept. 14, 1954 2,844,855 Gadd et al. July 29, 1958 2,880,486 Wallace Apr. 7, 1959 2,883,724 Hancock Apr. 28, 1959 FOREIGN PATENTS 826,340 Great Britain Jan. 6, 1960 OTHER REFERENCES Modern Core Practices and Theories, by Harry W. Detert, published by American Foundrymens Association, 1942, pages 312, 313, 422-425.

Claims (1)

1. THE PROCESS OF FORMING CASTINGS OF HIGH MELTING POINT METALS WHEREIN SAID CASTINGS ARE PROVIDED WITH ELONGATED INTERNAL PASSAGES, SAID PROCESS COMPRISING THE STEPS OF FORMING CORES BY PROVIDING A MAIN BODY PORTION OF A MATERIAL ADAPTED TO BE REMOVED FROM THE CASTINGS AFTER FORMATION THEREOF, COATING SAID MAIN BODY PORTION WITH A METAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, PLATINUM, TUNGSTEN, COLUMBIUM, TANTALUM, AND ALLOYS THEREOF TO PROVIDE A COATING BETWEEN .0001 AND .01 INCH THICK, IMBEDDING SAID CORES IN AN EXPENDABLE PAT-

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