US 3643728 A
A process for producing heat-resistant metal articles from nickel base or cobalt base superalloys having at least one cavity therein comprising fabricating a ceramic core to the configuration of the cavity, the core consisting essentially of at least 50 percent calcia, the balance, if any, being a refractory ceramic material, the whole fired at a density range of 70 to 100 percent of theoretical density, supporting the core in a mold, casting the nickel alloy therearound and dissolving the core with hot water.
Description (OCR text may contain errors)
United States Patent Hulse  PROCESS OF CASTING NICKEL BASE ALLOYS USING WATER-SOLUBLE CALCIA CORES  Inventor: Charles 0. I-Iulse, Manchester, Conn.
 Assignee: United Aircraft Corporation, East Hartford, Conn.
 Filed: July 8, 1970  Appl. No.: 53,291
 U.S.Cl. ..l64/l32, 164/369, 18/DIG. 12, 249/61, 164/138, 164/36, 106/389  Int. Cl ..B22d 29/00  Field oiSearch ..l64/131, 132, 369, 36, 35,\
164/138, 34; 18/DIG. 11, DIG. 12; 249/61 Feb. 22, 1972  References Cited UNITED STATES PATENTS 2,383,812 8/1945 Navias 164/35 3,407,864 10/ 1 968 Anderko 164/41 Primary Examiner.l. Spencer Overholser Assistant Examiner-V. K. Rising Attorney-John D. Del Ponti  ABSTRACT A process for producing heat-resistant metal articles from nickel base or cobalt base superalloys having at least one cavi- 6 Claims, N0 Drawings PROCESS OF CASTING NICKEL BASE ALLOYS USING WATER-SOLUBLE CALCIA CORES BACKGROUND OF THE INVENTION This invention relates to a method for producing cast metal articles containing inner passages or cavities and more particularly relates to a process for the formation of cavities in castings of nickel base and cobalt base superalloys adapted for use in gas turbine engines for jet aircraft.
lt is known to fabricate gas turbine blades or vanes with cavities or passages for cooling purposes. With the continuing requirement for higher jet engine temperatures, the extent and complexity of the internal passages is evergrowing and has presented severe problems which cannot readily be overcome by present fabrication techniques. At present, one .of the pri- 'mary methods for forming internal cooling passages in these parts comprises casting the nickel or cobalt base alloy around a plurality of ceramic cores of zircon, zirconia, mullite or the like with a binder of silica glass or pure fused silica and subsequently removing them by long term exposure to caustic solutions at elevated temperatures in an autoclave. Unfortuconfiguration of the desired cavity, supported in a conventional mold and the alloy is cast therearound. As will be appreciated, the nickel base and cobalt base alloys typically melt at about l,380 C. When casting, the melt is usually superheated an additional several hundred degrees. After the cast- 'ing is cooled, the core is then exposed and consequently dis solved in preferably hot or warm water. The calcia cores of the present invention have a relatively high water solubility at temperatures from 50 to 200 C., with a greatly reduced solubility at lower temperatures. The rate of hydration ranges from 0.1 to 100 percent per minute on a volume basis. Investigation has indicated that satisfactory cores are those which have a composition of 50 to 100 percent CaO, the balance, if any, including a refractory ceramic inert to water such as MgO, CaO-Al O MgO-AhO and/or a refractory ceramic reactive with water such as SrO and BaO. The added refractory ceramic material must have a melting point of at least l,800 C. and must be nonreactive with the molten nickel base or cobalt base superalloysunder consideration.
It is important to note that the strength of polycrystalline CaO compares favorably to prior art materials which have been used for cores as shown in the table following.
TAB LE I nately, during removal of the cores from the casting, the caustic solutions attack the cast alloy and thus make it more susceptible to accelerated corrosion during use. This problem of corrosion is, of course, magnified by the increasing tem-- peratures of the engine environment, as indicated hereinbei fore. ln addition, strengthening of these core materials by firing, while desirable, is substantially precluded since it causes diffusion of the silica binder and thus makes the removal problem substantially more difficult. Other problems, such as hot-tearing of the metal during cooling due to a mismatch of coefficients of thermal expansion, have also indicated the real need for a readily removable core for the nickel and cobalt base alloys which does not react with the alloy during casting and which further, does not possess the shortcomings of the prior art.
SUMMARY OF THE INVENTION The present invention contemplates a process for producing nickel and cobalt base metal castings containing cavities therein adapted for usage in a gas turbine engine which comprises the casting of the article at temperatures of approximately l,600 C. or higher which is to contain the cavity while using as a cavity former a core material which is nonreactive to the alloy during casting yet which is water soluble for ready removal thereafter. It has been found that a calcia base ceramic core, consisting essentially of at least 50 percent CaO with the remainder, if any, including certain refractory ceramics selected to achieve a predetermined solubility in water, will solve the problems of the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENT and achieve the desired densities. The core is formed to the 1 -11,000-extrap.
As will be noted, the thermal expansions of the materials are included in the table and can be compared with that of nickel which is l7.l 1O C. over the same temperature range. It cam be seen that calcia has a high coefficient of thermal expansion, approximating that of nickel. Thus, upon cooling, a
core of this material will shrink almost as quickly as the metal casting around it, thereby minimizing hot tearing problems due to tensile forces set up in the casting as it contracts down upon the core.
In order to successfully utilize the calcia base core, it has been found that a density of 70 to 100 percent of theoretical density must be provided. This range applies to the core whether or not additives as mentioned hereinbefore are present. Within the range stipulated, the more dense bodies, i.e., those from approximately to percent may be used uncoated while the less dense bodies, i.e., those from 70 to approximately 85 percent, are to be provided with a protective coating to prevent hydration in air. Suitable protective coatings of impervious inorganic material such as cellulose acetate, polystyrene or Krylon (87.66 percent volatilealiphatic, aromatic and halogenated hydrocarbons, 9.18 percent nonvolatile-acrylic ester resin chlorinated rubber butye benzyl phthalate.) are burned away during heat up of the mold before or during casting,
The composition and density of the core selected depends upon a balance of factors in view of the particular problems at hand for a specific application. In general, the more porous the core, the faster will be its removal by water since the water is able to more easily penetrate and react with the calcia. if the porosity results in too much sensitivity to water vapor in the air or to water vapor present during a step in the mold forming process, the reactivity may be reduced by including an inert ceramic, as for example MgO, in the core batch composition.
Ordinarily, higher densities mean higher strengths. Where strength is a problem with a particular core shape, a high density will be desirable or necessary. Unfortunately, a high-density core is much more slowly removed by water than a lower density one. In order to offset this disadvantage, an addition of SrO and/or BaO to increase reactivity with water should be made. SrO and BaO are well suited to their task since they are both very refractory ceramic oxides, have the same crystal structure as CaO and react much more rapidly than CaO to water.
The following examples are set forth for the purpose of illustration and not limitation ofthe present invention.
EXAMPLE I Commercially available calcium carbonate was poured into carbon dies having the curvilinear configuration of the air cooling passages of a turbine blade, and was hot pressed in vacuum at I,400 C. and 5,000'lb./in. for 30 minutes. The calcia core formed in the foregoing manner was then placed in standard ceramic molds and a nickel base alloy (nominal composition, by weight; 15 Cr, 15.3 Co, 4.4 M0, 3.4 Ti, 4.3 A1, 0.02 B, Bal. Ni) was cast around it in a vacuum at a temperature of l,550 C. After casting, the cores were removed using water heated to a temperature of 1 C. Metallographic examination of sectioned portions of the casting revealed that the core material did not react with or corrode the cast metal.
EXAMPLE 1] Using the same technique as in Example 1, a calcia core was fabricated having a porosity of percent. The core was placed in a standard ceramic mold and a cobalt base alloy (MAS5382-nominal composition by weight; 25.5 Cr, l0.5 Ni, 7.5 W, 2.0 Fe, 0.5 C, Bal. Co.) was cast around it in air at a temperature of l,600 C. It was quickly removable from the casting without reacting with or corroding same.
While particular embodiments have been described, it will be understood that various modifications may be made without departing from the scope of this invention. Thus the soluble cores can be removed by means other than hot water, as for example heating under steam pressure in an autoclave with KOH or other additives to hasten the process. It will further be appreciated that while the description herein has been directed primarily to cores, it is not intended to exclude usage of the calcia base material in other modes, as for example an external casting mold.
What is claimed is:
l. A method for producing heat-resistant metal articles having at least one cavity therein from a cast nickel or cobalt base superalloy comprising:
fabricating a ceramic core to the configuration of the cavity,
said ceramic core consisting essentially of at least 50 percent by weight CaO, the remainder, if any, being at least one refractory ceramic material for controlling the water solubility of the core;
supporting said ceramic core in a mold;
casting the alloy around said ceramic core in the mold;
removing the casting from the mold; and
dissolving said ceramic core in water.
2. The method of claim 1 wherein the ceramic core is at a density range of 70 to percent of theoretical density.
3. The method of claim 2 wherein the ceramic core contains a refractory ceramic material inert to water.
4. The method of claim 3 wherein the refractory ceramic material is selected from the group consisting of MgO, ZrO CaO--Al O and MgO-Al O 5. The method of claim 2 wherein the ceramic core contains a refractory ceramic material reactive with water.
6. The method of claim 5 wherein the refractory material is selected from the group consisting of SrO and BaO.