US3441078A - Method and apparatus for improving grain structures and soundness of castings - Google Patents

Description

April 29, 1969 G. D. CHANDLEY METHOD AND APPARATUS FOR IMPROVING GRAIN STRUCTURES AND SOUNDNESS OF CASTINGS Filed Feb. 1, 1967 INVENTOR. 650266 Q [AMA/0467 United States Patent ()7 3,441,078 METHOD AND APPARATUS FOR IMPROVING GRAIN STRUCTURES AND SOUNDNESS F CASTINGS George D. Chandley, Alliance, Ohio, assignor t0 TRW, Inc. Cleveland, Ohio, a corporation of Ohio Filed Feb. 1, 1967, Ser. No. 613,313 Int. Cl. B2241 27/04, 23/00; B22c 9/02 U.S. Cl. 164--53 7 Claims ABSTRACT OF THE DISCLOSURE A casting process and apparatus utilizing porous ceramic shell molds in which layers of exothermic material are positioned around the molds and then ignited to provide preheated zones of different temperature in the molds, thereby preparing the molds for the reception of the molten metal and its solidification with directional orientation.

BRIEF SUMMARY OF THE INVENTION This invention is particularly intended for use in the casting of metals such as superalloys in vacuum or in air. In the practice of the invention, the ceramic shell mold is first preferably wrapped with a liquid and vapor barrier film and then discrete layers of a moldable exothermic material are packed around the shell mold structure. The exothermic material contains a hardenable binder which, upon heating at relatively moderate temperatures, substantially below the casting temperature, hardens and provides a self-sustaining layer. Each exothermic composition has its own characteristic ignition temperature and the mass and dimensions of the exothermic layers are arranged so that the ceramic mold and their gates and runners achieve a desired temperature upon ignition of the exothermic material. The shell molds thereby reach operating temperatures far faster than they would by other means of heating. In addition, the consolidation of the exothermic material makes it possible to handle the molds more conveniently so that after ignition, it can be moved into position over a heat abstracting surface whereupon the molten metal can be poured into the mold. The Zones of differing temperature provided by the ignition of the exothermic material, coupled with the use of the highly heatconductive surface at the base of the mold provide directional grain growth resulting in the preferred form of the invention in a columnar structure being produced.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a cross-sectional view of a molding assembly prior to the introduction of molten metal into the casting cavities;

FIGURE 2 is a view similar to FIGURE 1 but illustrating the mold in position to receive the cast metal; and

FIGURE 3 is a plan view of the interior of a vacuum casting chamber illustrating the manner in which the molds can be handled during preheating and casting.

DESCRIPTION Recent studies have shown that castings consisting of an oriented columnar structure have certain desirable properties in applications such as jet turbine airfoils. The

methods heretofore employed for producing such structures, while adequate, are costly due to the fact that they usually require several hours time and employ expensive equipment. These long cycles also can cause excessive segregation in certain types of alloys, producing imperfect grain structures. These imperfect structures and misori ented grain in general have been shown to be harmful in jet engine operation.

One of the objects of the present invention is to provide an improved method for achieving directional orientation in cast structures, wherein the time involved is substantially reduced from that required by previous methods.

Another object of the invention is to provide a method for producing sound castings of airfoils and the like eliminating segregation defects.

Another object of the invention is to provide a method for producing columnar castings through the use of conventional vacuum melting and pouring equipment rather than with the use of more expensive, specially designed equipment.

A still further object of the invention is to provide an improved molding assembly which can be handled without breakage at high temperatures, thereby decreasing the time involved in the overall casting process.

The present invention is particularly applicable to the use of porous ceramic shell molds having a significant degree of gas permeability. Such molds are commonly produced by precision investment casting techniques utilizing wax or wax and resin and mixtures as a disposable pattern material. 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 slurry may contain ceramic 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 the refractory glass composition such as that known as Vycor which is a finely divided, high silicon oxide glass containing about 98% 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 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 coat with a relative humidity 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 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 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 1500 to 1900 F. are used. The resulting shell molds are hard, smooth, and relatively permeable and measure on the order of A; to A1 inch in thickness.

Referring now to the drawings, in FIGURE 1 there is illustrated a casting assembly in which a ceramictype shell mold 11 is disposed. The shell mold 11 includes a pouring basin 12 communicating with a down gate 13 which feeds runners 14 from which the molten metal is delivered to molding cavities 16. In the particular embodiment illustrated in the drawings, the molding cavities define airfoil shapes for jet engine blades. The molds have open ends which rest on a ceramic block 17. A ring support 18 is provided to facilitate lifting the mold assembly, as will be apparent from a succeeding portion of the description.

Before the various layers of exothermic material are placed in the mold assembly, it is desirable to wrap the entire mold assembly 11 in a liquid and vapor barrier film such as polyvinylidene chloride which prevents liquids from penetrating into the clean ceramic molding cavity. The wrapped mold assembly 11 is then positioned, as shown in FIGURE 1, and a first layer 19 of moldable exothermic material is applied to the desired depth. Exothermic materials are available commercially for use at specified temperatures. They usually consist of a mixture of aluminum powder and iron oxide in varying proportions, depending upon the temperature to be achieved, together with a strong oxidizer such as potassium perborate. In accordance with the present invention, this exothermic material is mixed with a hardenable binder such as linseed oil to form a moldable composition which is then rammed into place to form the layer 19. Then, a supporting sleeve 21 is placed within the ring support 18, and a second layer 22 of a moldable exothermic composition having a different characteristic burning temperature is rammed into the space between the mold assembly 11 and the sleeve 21. Finally, in the area of the pouring basin 12, a third layer 23 of a moldable exothermic material is rammed into the assembly to provide a zone of different temperature along this portion of the mold. The thickness of the exothermic materials, their composition and their geometry can be varied to secure quite accurately defined temperatures in the various zones of the mold.

Next, the pouring basin 12 is covered with a ceramic cover 24, and the composite mold assembly is baked in an oven at about 400 F. to form a low temperature bond in the rammed powders by hardening of the linseed oil binder. This produces a self-sustaining structure which can then be lifted without fracture by means of the support ring 18.

The exothermic layers are then ignited, whereupon they heat the molds very rapidly, in most cases less than onehalf hour, to provide temperatures suitable for directional solidification. When the temperatures in the various zones of the mold have become fairly uniform within the zone, the mold assembly 10 is lifted and placed on a refractory block 26 in a vacuum chamber generally indicated at numeral 27 in FIGURE 3. The refractory block 26 is supported on a turntable 28 which also carries a pair of chill plates 29 and 31 consisting of a highly heat-conductive metal such as copper and being preferably provided with a circulating coolant.

A vacuum is drawn through the vacuum chamber and the alloy to be poured into the mold is melted. Of course, for alloys which are capable of being cast in air, the vacuum chamber would be unnecessary. When the molten metal is ready for pouring, the mold assembly 10 is raised and the turntable 28 is rotated to place the chill block 29 under the mold assembly as illustrated in FIGURE 2.

Shortly after the mold assembly is placed on the chill block 29, the molten metal is poured in, and solidification begins.

The ignition of the exothermic layers 19, 22, and 23 drives out the binder and leaves the layers in sintered form. These masses of exothermic material protect the solidifying casting from air, so that the vacuum chamber may be opened shortly after pouring and a second mold assembly can be placed in the refractory block 26. Then, the process of vacuum pumpdown and pouring can be completed while the previous casting is solidifying and cooling on the chill block '29.

The following specific example illustrates the process of the invention more specifically.

Example A six cavity ceramic mold was made up using the conventional ceramic mold process. The mold was uniformly wrapped with Saran Wrap (polyvinylidene chloride) to form a low temperature seal between the exothermic materials and the ceramic mold. A moldable exothermic material providing a temperature of 3150 F. was rammed to a height of about two inches in the layer identified at reference numeral 19 of FIGURE 1. This exothermic material included linseed oil as a binder. Then, material providing a temperature of 2950 F. was rammed into the space to form a layer 22 as illustrated in FIGURE 1, and finally exothermic material having a combustion temperature of 3150 F. was rammed in the upper portion of the assembly to provide the layer 23, shown in FIG- URE 1. The mold assembly was then baked at a temperature of 350 F. for twelve hours and was ignited by placing it in the furnace at 1900 F. for three minutes. The mold assembly was then placed on a ceramic block and the exothermic material was allowed to burn completely. After complete ignition, the assembly was placed in a vacuum furnace on a ceramic block such as block 26, illustrated at FIGURE 3. A nickel base superalloy was melted and brought to pouring temperature at which time the mold assembly was raised off the refractory block and a water cooled copper chill block, such as block 29, was rotated underneath the mold assembly, as illustrated in FIGURE 2. The metal was poured into the mold at 2950" F., and the casting was allowed to solidify and cool for thirty minutes, whereupon it was removed from the vacuum furnace. A metallographic cutup revealed excellent soundness and columnar grain structure.

It has been found that the method of the present invention reduces the time cycles for the production of fully columnar castings to less than about 20% of the time required by previous methods using electrical heating. What is more, this method reduces cracking in airfoil type castings in some high temperature alloys. Furthermore, segregation defects in alloys have been found to be greatly reduced because of the short solidification time. In addition, conventional vacuum melting and pouring equipment can be used which is far cheaper than that previously in use for making directionally solidified castings. As a further advantage, since the temperatures are determined by pre-mixing exothermic materials, no constant temperature measurements are required.

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

I claim as my invention:

1. The method of casting which comprises providing a ceramic shell mold having a molding cavity therein, molding a plurality of exothermic compositions about said shell mold, each of said compositions having a characteristic burning temperature, said compositions being positioned adjacent areas of said mold which are to be preheated to different temperatures, igniting said compositions while so positioned, and thereafter pouring molten metal into said molding cavity when said temperatures have been achieved.

2. The method of claim 1 in which heat is abstracted from the base of said mold during and after pouring to provide a directionally solidified casting.

3. The method of claim 1 in which said igniting is done while said mold is positioned on a heat insulating surface and said pouring is done While said mold is positioned on a highly heat conductive surface.

4. The method of claim 1 in which said exothermic compositions contain a binding agent, and said compositions are preheated prior to ignition to form a self-sustaining mass.

5. A casting assembly comprising a porous ceramic shell mold having a casting cavity therein and a plurality of layers of exothermic material positioned about said cavity, each of said exothermic layers having a characteristic ignition temperature.

6. The casting assembly of claim 5 which also includes a liquid and vapor barrier film interposed between said shell mold and said compositions.

UNITED STATES PATENTS 3,367,393 2/1968 Lenahan et al. 164-361 X FOREIGN PATENTS 212,984 1/1961 Austria.

I. SPENCER OVERHOLSER, Primary Examiner.

V. RISING, Assistant Examiner.

US. Cl. X.R.

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