Method for reducing metal casting porosity

Jan. 7, 1969 _G d.chandley etal

METHOD FOR REDUCING METAL CASTING POROSITY

3,420,291

Filed Dec. 29, 1965

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United States Patent Office „ ., ³&gt;⁴²⁰’²⁹¹

Patented Jan. 7, 1969

420 291

METHOD FOR’REDUCING METAL

CASTING POROSITY

George D. Chandley, Alliance, and Stuart Z. Uram, Canton, Ohio, assignors to TRW Inc., Cleveland, Ohio, a corporation of Ohio

Filed Dec. 29, 1965, Ser. No. 517,400

U.S. CL 164—66

Int. Cl. B22d 23/00

Claims

ABSTRACT OF THE DISCLOSURE

Casting process wherein molten metal is cast into a preheated, gas permeable mold and thereafter a substantial positive gas pressure is applied to the material in the mold to effect a condition of pneumatic forging, thereby substantially reducing the amount of microporosity in the casting produced.

The present invention relates to a method of casting by means of which microporosity is substantially reduced, if not eliminated.

Metal castings normally contain varying amounts of porosity as a result of solidification shrinkage. With many metals, this porosity is substantially eliminated by subsequently hot working operations. However, certain alloys are very difficult or impossible to hot work by conventional means. It is therefore necessary to reduce or eliminate porosity in such alloys during the actual casting.

Microporosity is a particularly difficult problem when attempting to produce high quality hollow castings. In the past, attempts to overcome this problem during the casting operation have required the use of elaborate gating and risering techniques which added to the cost and complexity of the casting operation. Such techniques have not succeeded.

One of the objects of the present invention is to provide a method for casting to achieve a soundness approximating that achieved in wrought products without distortion of the casting and without destroying the original cast structure.

Another object of the invention is to provide a method for eliminating microporosity in alloys which are not forgeable.

Still another object of the invention is to provide a method of casting which produces very accurate dimensions in the casting and does not require extensive finishing operations which are required when a forging operation is employed.

A further object of the invention is to provide a method which can be used to produce high quality hollow castings.

A still further object of the invention is to provide a method of casting which achieves soundness in the casting without the use of extensive gating and risering techniques.

The method of the present invention involves the use of a pneumatic forging operation under isostatic conditions during solidification of the casting. Basically, the method involves positioning of gas permeable ceramic mold within an enclosure, the mold having a temperature determined to be optimum or appropriate for the casting of a part having the particular shape and thickness and casting the metal into the mold while the mold is at this elevated temperature. After completion of the casting, a substantial positive gas pressure is established in the enclosure, using a gas which is substantially unreactive with the metal. After solidification, the resulting casting may be recovered in the usual way by the destruction of the mold.

The ceramic mold which is used in accordance with the present invention is one which has a significant degree of gas permeability. Such molds are commonly produced by precision investment casting techniques utilizing wax or wax and resin mixtures as a disposable pattern mate5 rial. One such method involves coating a disposable pattern of wax or the like by dipping it 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 sluny may contain ceramic 10 materials 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 “Vy15 cor” which is a finely divided high silicon oxide glass containing about 96% 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 20 drying oven having a controlled humidity and temperature, wherein the coated pattern is dried 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 25 be dried with air having a relative humidity of 45 to 55%. The third and fourth coats can be dried with a relative humidity of 35 to 45%, the fifth and sixth coats with a relative humidity of 25 to 30%, and the final coats with a relative humidty of 15 to 25%.

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 35 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 ⁴⁰ 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 to 1,900° F. are used. The resulting shell ⁴® molds are hard, smooth, and relatively permeable and measure on the order of Va to Ά inch in thickness.

The ceramic shell mold is preheated, either before introduction into the enclosure, or while it is so enclosed, to a temperature approaching the solidification point of ⁵⁰ the metal to be cast therein. Generally, this means heating the mold to a temperature within the range from about 600° F. below the solidification point, to a few degrees below the solidification point.

The molten metal, superheated to a temperature appro⁵ priate to the shape to be poured, is then introduced into the mold either in air or under vacuum conditions, depending upon the nature of the alloy being cast. When the casting has been completed, the enclosure is made gas tight, and then the inert gas is introduced into the enclo⁶⁰ sure under substantial pressures ranging from about 1,000 to 4,000 pounds per square inch, with 2,000 to 3,000 pounds per square inch being preferred. The gas used is “inert” in the sense that it is not reactive substantially with the molten metal. For reactive melts such as the ⁶⁵ so-called “superalloys,” an atmosphere of a completely inert gas such as helium or argon is provided.

The pressure of the gas in the enclosure exceeds the strength of the metal casting during solidification. As a _ result, the metal solidifies under isostatic conditions, ‘⁰ resulting in the production of a casting of extreme soundness. The method has been found to virtually eliminate microporosity in alloys which evidenced such micro3,420,291 ³ porosity even under the most carefully controlled casting conditions.

A further description of the present invention will be made in conjunction with the attached sheet of drawings in which: 5

FIGURE 1 is a cross-sectional view of the components of the casting assembly prior to casting of the metal therein; and

FIGURE 2 is a view similar to FIGURE 1 but illustrating the assembly after casting has been completed, ₁₀and during application of the gas pressure during solidification.

AS SHOWN IN THE DRAWINGS

In FIGURE 1, reference numeral 10 indicates generally a high pressure container which is lined with a lining 11 ¹⁵composed of an insulating refractory material or an exothermic material. A porous ceramic shell mold generally indicated at reference numeral 12 is positioned in the container 10 and is preheated, either outside the container 10 or while in the container to a temperature ²⁰approximating the solidification point of the metal to be introduced into the mold. The shell mold 12 includes a casting cavity 13, a gating cavity 14 and a runner 16, in accordance with usual practices of mold design.

An insulating cover 17 is positioned over the liner ²⁵11, and the cover 17 having a centrally disposed aperture 18 which is arranged to overlie the gating cavity 14.

A refractory cover 19 is threadedly engaged, or otherwise secured to the top of the container 10, and a funnel 21 is inserted into a central aperture 22 of the cover 19. ³⁰The funnel serves to direct molten metal through the aperture 18 and ultimately into the casting cavity 13 of the mold 12.

The pouring may occur in air or under vacuum conditions, depending upon the nature of the metal or alloy ³⁰being poured. Immediately after pouring, the funnel 21 is removed, and a cap 23 (FIGURE 2) is threadedly engaged with the threads existing along the aperture 22. The insertion of the cap 23 serves to make the entire enclosure 10 gas tight. A high pressure gas stream is introduced into the container 10 by means of a fitting 24 and a conduit 26 connected to a pump or other suitable device for introducing the gas under high pressure into the container.

The high pressure gas environment is maintained dur- ⁰ing solidification of the metal which may take from a few minutes to 30 minutes or so. At the completion of the solidification, the container is disassembled, and the mold broken away from the solidified casting. The casting can then be processed with normal manufacturing ⁰methods.

The following specific examples illustrate the method of the present invention, and the advantages achieved through its use. _r_

EXAMPLE I ⁰⁰

A number of jet engine blades were cast from a nickel base superalloy having a solidification point of about 2,350° F. A mold constructed as shown in FIGURES 1 and 2 was preheated to approximately 2,500° F. and θο placed in the container. The container was lined with exothermic material which had been preheated to a temperature of about 2,450° F. The pouring funnel was placed in position, and the entire assembly was evacuated to a pressure of approximately 20 microns of (;g mercury. The alloy was melted, superheated to 3,000° F., and poured into the mold cavity. The funnel was then removed, the cover was screwed on, and a pressure of 2,000 pounds per square inch of argon was fed into the pressure chamber. This pressure was maintained on the γθ casting for 10 minutes. The apparatus was disassembled and the resulting casting was sectioned and evaluated for microporosity. The average amount of porosity in a controlled casting without the use of the high pressure environment was 1.2%. The average amount in the root section of the casting solidified under the conditions of high gas pressure was 0.12%, indicating a reduction of microporosity of approximately 90%.

EXAMPLE II

A Vi inch diameter bar with a pour cup on one end was made in air from the alloy “Hastalloy B.” The bars were made under identical conditions, except for the use of the high pressure gas environment in some samples. Those samples which solidified in the high pressure environment exhibited an X-ray soundness which was quite satisfactory, while the bars which solidified under normal conditions contained extensive shrinkage porosity.

From the foregoing it will be understood that the method of the present invention provides a means for securing castings of a soundness heretofore unattainable in the absence of subsequent hot working. The resultant casting is very accurate dimensionally and does not require extensive finishing operations.

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