HEAT REFLECTOR
United States Patent 3680625
Method and apparatus for producing columnar castings by precision investment casting techniques, wherein a refractory reflector is positioned in the furnace behind the molds contained therein to make it possible to control the unidirectional temperature gradient more closely and provide more pieces per mold.
US Patent References:
MOLD CONSTRUCTION FORMING SINGLE CRYSTAL PIECES
Wickstrand - June 1970 - 3515205
Method for improving grain structure and soundness in castings
Chandley - May 1966 - 3248764
Method of casting directionally solidified articles
Sink - December 1968 - 3417809
COCOON CASTING OF DIRECTIONALLY SOLIDIFIED ARTICLES
Giamei et al. - December 1971 - 3627015
MOLD CONSTRUCTION FORMING SINGLE CRYSTAL PIECES
Wickstrand - June 1970 - 3515205
Method for improving grain structure and soundness in castings
Chandley - May 1966 - 3248764
Method of casting directionally solidified articles
Sink - December 1968 - 3417809
COCOON CASTING OF DIRECTIONALLY SOLIDIFIED ARTICLES
Giamei et al. - December 1971 - 3627015
Inventors:
Hein, Frank J. (Minerva, OH)
Fleck, Donald G. (Alliance, OH)
Fleck, Donald G. (Alliance, OH)
Application Number:
05/088608
Publication Date:
08/01/1972
Filing Date:
11/12/1970
View Patent Images:
Export Citation:
Assignee:
TRW Inc. (Cleveland, OH)
Primary Class:
Other Classes:
164/361, 164/353, 164/122.100, 244/111, 164/338.100, 164/129
International Classes:
B22D27/04; C30B11/00; B22D27/04; B22D25/06
Field of Search:
164/60,122,125,127,129,338,353,361 249/111
Primary Examiner:
Overholser, Spencer J.
Assistant Examiner:
Roethel, John E.
Claims:
We claim as our invention
1. An apparatus for precision casting comprising a furnace wall, an induction heating coil disposed around said wall, a susceptor disposed inside said wall and adjacent thereto, a casting mold assembly comprising a plurality of circumferentially spaced ceramic molds enclosed by said wall, a common feed means disposed centrally of said molds for introducing molten metal radially into each of said molds, and refractory reflector means separate from said ceramic molds disposed in closely spaced relation to said molds on the sides thereof opposite to said susceptor.
2. The apparatus of claim 1 in which said reflector is integral with said mold.
3. The apparatus of claim 1 which includes a surface of high heat conductivity upon which said molds rest.
4. The apparatus of claim 1 in which said reflector means is in the form of an annulus surrounding said common feed means.
5. The method of producing columnar castings which comprises positioning a plurality of spaced, open-ended ceramic molds on a highly heat conductive surface within a furnace including a susceptor which radiates heat at said molds, preheating said molds to a temperature above the solidus temperature of the metal to be cast, positioning a ceramic reflector inwardly of said molds so that such molds are disposed between said reflector and said susceptor, said reflector being separate from said ceramic molds and being positioned to reflect heat radiated from said susceptor to portions of said molds which would otherwise be screened from such radiated heat, pouring molten metal into said molds, and providing a unidirectional temperature gradient within said molds during the solidification of the metal therein.
1. An apparatus for precision casting comprising a furnace wall, an induction heating coil disposed around said wall, a susceptor disposed inside said wall and adjacent thereto, a casting mold assembly comprising a plurality of circumferentially spaced ceramic molds enclosed by said wall, a common feed means disposed centrally of said molds for introducing molten metal radially into each of said molds, and refractory reflector means separate from said ceramic molds disposed in closely spaced relation to said molds on the sides thereof opposite to said susceptor.
2. The apparatus of claim 1 in which said reflector is integral with said mold.
3. The apparatus of claim 1 which includes a surface of high heat conductivity upon which said molds rest.
4. The apparatus of claim 1 in which said reflector means is in the form of an annulus surrounding said common feed means.
5. The method of producing columnar castings which comprises positioning a plurality of spaced, open-ended ceramic molds on a highly heat conductive surface within a furnace including a susceptor which radiates heat at said molds, preheating said molds to a temperature above the solidus temperature of the metal to be cast, positioning a ceramic reflector inwardly of said molds so that such molds are disposed between said reflector and said susceptor, said reflector being separate from said ceramic molds and being positioned to reflect heat radiated from said susceptor to portions of said molds which would otherwise be screened from such radiated heat, pouring molten metal into said molds, and providing a unidirectional temperature gradient within said molds during the solidification of the metal therein.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is in the field of precision investment casting to produce columnar structures, and is particularly involved with reflector elements for mold clusters which make it possible to employ more molds on a given cluster than has heretofore been the practice.
2. Description of the Prior Art
In recent years, there has been a substantial amount of development work done on the directional solidification of castings to produce columnar grain structures. It has been found that such castings have superior elevated temperature performance in gas turbine engines than castings produced with equiaxed grain structures.
Columnar structures are generally produced by positioning a doubly open-ended ceramic mold on a chill block composed of copper or other highly heat conductive material. The mold structure is positioned within a furnace, usually heated by selectively energizable induction heating coils and provided with a susceptor which radiates the heat at the cluster of molds within the furnace. The molds are preheated to a temperature at least as high as the solidus temperature of the metal to be cast, and the molten metal is then cast into the molds. Solidification proceeds upwardly from the copper chill block and is controlled by a variety of means, including selective deenergization of the induction heating coils to produce a unidirectional temperature gradient throughout the mold during solidification.
Heretofore, the number of molds which could be utilized in a given cluster has been limited. This, of course, is undesirable since the greater the number of pieces which can be obtained per casting operation, the more economical is the process. Basically, the number of pieces which can be produced from a mold cluster is limited by the geometry of the part and the size of the induction heating coil. In practice, however, it has been found that the overall geometry of the mold and the proximity of one mold structure to another have an effect on the ability of the system to produce acceptable grain structures. It has been found necessary on some configurations, for example, to thicken the mold at the top as much as one inch to assure proper solidification characteristics. These drawbacks limit the number of molding cavities which can be fed from a common source and solidified under conditions of unidirectional cooling to produce columnar structures.
SUMMARY OF THE INVENTION
This invention provides improvements in the field of precision investment casting and, more particularly, in the specific field of producing columnar grain structures in castings. Specifically, we have now found that by positioning a ceramic reflector element within the furnace assembly, the interaction between the individual molds in the mold cluster is significantly reduced and more molds can be put on a cluster thereby increasing the yield of the process, and a higher percentage of the castings result in the desired columnar grain structure.
The reflector is positioned between the central portion of the mold and the molding cavity so that the molding cavities are disposed between the reflector and the radiating inner wall of the furnace, which is usually a graphite susceptor. The ceramic reflector may be separately introduced into the molding assembly or, more preferably, it may be an integral part of the mold produced at the same time as the remainder of the cluster by the usual precision investment mold making processes.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:
FIG. 1 is a plan view of a pattern cluster which can be used for making the mold assemblies of the present invention;
FIG. 2 is a cross-sectional view taken substantially along the line II--II of FIG. 1; and
FIG. 3 is a view partially in cross-section and partially in elevation of a mold assembly and furnace assembly of the type with which the present invention is involved.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A pattern assembly for producing shell type investment molds of the present invention is illustrated at reference numeral 10 in FIG. 1. While the particular pattern as shown in the drawings is designed for the production of four molds, it should be understood that any number of mold assemblies can be employed and one of the advantages of the present invention is that the mold assemblies can be placed closer together in the cluster than has heretofore been commonplace. Individual patterns 11 through 14 composed of wax or other pattern material are spaced about a central pouring basin-forming portion 15 is connected to the upper ends of the patterns 11 through 14 by means of upper runners 17 through 20, respectively. Similarly, the bottom ends of the patterns 11 through 14 are connected to the base of the sprue-forming portion 16 by means of radially extending runners, two of which identified at reference numerals 21 and 22 are visible in FIG. 2. It will be understood that in keeping with ordinary investment casting procedures, the patterns 11 through 14 may be made in individual pattern molds and thereupon connected to the runners and the pouring basin forming portion 15 and the sprue-forming portion 16 by means of heat welding or solvent welding.
The pattern assembly shown in FIGS. 1 and 2 also includes a reflector-forming portion 23 which may consist of a thin sheet of wax which is suitably secured to the bottom runners 21 and 22 as well as the bottom runners feeding the patterns 12 and 14. Typically, the sheet 23 may be about 0.1 inch in thickness.
The pattern cluster shown in FIGS. 1 and 2 is then used to form a mold cluster through conventional precision investment mold-making techniques. One such method involves coating the wax pattern assembly 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 material such as zirconium oxide, a binder such as colloidal silica and a thickener and low temperature binder such as methyl cellulose. The methyl 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 98 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 assembly 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 be dried with a relative humidity of 35 to 45 percent, the fifth and sixth coats with a relative humidity of 45 to 55 percent. The third and fourth coats can 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 each being about 0.005 to 0.020 inch.
After the mold has been built up around the pattern assembly, the pattern material 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 molds are hard, smooth and relatively permeable, and have a thickness on the order of one-eighth to one-fourth inch.
The resulting mold cluster produced from the pattern assembly 10 is shown at reference numeral 30 in FIG. 3. The mold assembly 30 is disposed within a furnace having a refractory outer wall 31 about which one or more induction heating coils 32 are disposed. Located within the wall 31 is a susceptor 33 composed of graphite or the like which serves to deliver radiant energy to the molds. The top of the furnace is closed by means of a top plate 34 composed of refractory material, and a funnel 35 is provided to deliver molten metal to the casting cavities.
The mold assembly 30 itself contains a pouring basin 36 and a cylindrical sprue portion 37 extending downwardly therefrom. The pouring basin 36 communicates with the interior of the casting molds, two of which have been identified at reference numerals 38 and 39 in FIG. 3. Runners 40 and 41 are used to deliver molten metal from the common source into the casting cavities of the respective molds.
The mold assemblies are open ended and their bottom ends are positioned on a chill block 42 composed of copper or other highly heat conductive material. If desired, a circulating fluid may be passed through the chill block 42 to increase the rate of heat transfer.
The mold assembly also includes a refractory reflector 43 which in the form of the invention illustrated in FIG. 3 consists of a continuous annulus of ceramic mold forming material with a hollow center. Alternatively, the reflector 43 can consists of a preformed ceramic material which is placed in the mold prior to pouring and it need not be continuous. In other words, a plurality of ceramic baffles can be positioned closely adjacent the individual molds, the width of the baffles being at least as large as the projected widths of the mold assemblies with which they are associated.
In operation, the molding assembly is operated under vacuum conditions, and the molds are heated to a temperature above the solidus temperature of the metal to be poured in the mold. The metal is melted and cast into the mold and the temperature of the mold is gradually reduced in order to obtain unidirectional solidification from the chill block 42 upwardly to the top of the casting. One convenient means of doing this is to progressively deenergize individual coils making up the induction heating coil 32 so that as solidification proceeds upwardly, a unidirectional temperature gradient exists longitudinally of the mold, and a columnar grain structure having a longitudinal orientation is produced.
It has been found that through the use of the reflector elements of the present invention, it is possible to employ more molds in the cluster than heretofore used without adverse interaction occurring between the molds. It has also been found that the grain structure of the castings is actually improved primarily we believe, because the reflecting elements help to produce a more carefully controllable temperature gradient within the mold assembly.
1. Field of the Invention
This invention is in the field of precision investment casting to produce columnar structures, and is particularly involved with reflector elements for mold clusters which make it possible to employ more molds on a given cluster than has heretofore been the practice.
2. Description of the Prior Art
In recent years, there has been a substantial amount of development work done on the directional solidification of castings to produce columnar grain structures. It has been found that such castings have superior elevated temperature performance in gas turbine engines than castings produced with equiaxed grain structures.
Columnar structures are generally produced by positioning a doubly open-ended ceramic mold on a chill block composed of copper or other highly heat conductive material. The mold structure is positioned within a furnace, usually heated by selectively energizable induction heating coils and provided with a susceptor which radiates the heat at the cluster of molds within the furnace. The molds are preheated to a temperature at least as high as the solidus temperature of the metal to be cast, and the molten metal is then cast into the molds. Solidification proceeds upwardly from the copper chill block and is controlled by a variety of means, including selective deenergization of the induction heating coils to produce a unidirectional temperature gradient throughout the mold during solidification.
Heretofore, the number of molds which could be utilized in a given cluster has been limited. This, of course, is undesirable since the greater the number of pieces which can be obtained per casting operation, the more economical is the process. Basically, the number of pieces which can be produced from a mold cluster is limited by the geometry of the part and the size of the induction heating coil. In practice, however, it has been found that the overall geometry of the mold and the proximity of one mold structure to another have an effect on the ability of the system to produce acceptable grain structures. It has been found necessary on some configurations, for example, to thicken the mold at the top as much as one inch to assure proper solidification characteristics. These drawbacks limit the number of molding cavities which can be fed from a common source and solidified under conditions of unidirectional cooling to produce columnar structures.
SUMMARY OF THE INVENTION
This invention provides improvements in the field of precision investment casting and, more particularly, in the specific field of producing columnar grain structures in castings. Specifically, we have now found that by positioning a ceramic reflector element within the furnace assembly, the interaction between the individual molds in the mold cluster is significantly reduced and more molds can be put on a cluster thereby increasing the yield of the process, and a higher percentage of the castings result in the desired columnar grain structure.
The reflector is positioned between the central portion of the mold and the molding cavity so that the molding cavities are disposed between the reflector and the radiating inner wall of the furnace, which is usually a graphite susceptor. The ceramic reflector may be separately introduced into the molding assembly or, more preferably, it may be an integral part of the mold produced at the same time as the remainder of the cluster by the usual precision investment mold making processes.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:
FIG. 1 is a plan view of a pattern cluster which can be used for making the mold assemblies of the present invention;
FIG. 2 is a cross-sectional view taken substantially along the line II--II of FIG. 1; and
FIG. 3 is a view partially in cross-section and partially in elevation of a mold assembly and furnace assembly of the type with which the present invention is involved.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A pattern assembly for producing shell type investment molds of the present invention is illustrated at reference numeral 10 in FIG. 1. While the particular pattern as shown in the drawings is designed for the production of four molds, it should be understood that any number of mold assemblies can be employed and one of the advantages of the present invention is that the mold assemblies can be placed closer together in the cluster than has heretofore been commonplace. Individual patterns 11 through 14 composed of wax or other pattern material are spaced about a central pouring basin-forming portion 15 is connected to the upper ends of the patterns 11 through 14 by means of upper runners 17 through 20, respectively. Similarly, the bottom ends of the patterns 11 through 14 are connected to the base of the sprue-forming portion 16 by means of radially extending runners, two of which identified at reference numerals 21 and 22 are visible in FIG. 2. It will be understood that in keeping with ordinary investment casting procedures, the patterns 11 through 14 may be made in individual pattern molds and thereupon connected to the runners and the pouring basin forming portion 15 and the sprue-forming portion 16 by means of heat welding or solvent welding.
The pattern assembly shown in FIGS. 1 and 2 also includes a reflector-forming portion 23 which may consist of a thin sheet of wax which is suitably secured to the bottom runners 21 and 22 as well as the bottom runners feeding the patterns 12 and 14. Typically, the sheet 23 may be about 0.1 inch in thickness.
The pattern cluster shown in FIGS. 1 and 2 is then used to form a mold cluster through conventional precision investment mold-making techniques. One such method involves coating the wax pattern assembly 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 material such as zirconium oxide, a binder such as colloidal silica and a thickener and low temperature binder such as methyl cellulose. The methyl 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 98 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 assembly 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 be dried with a relative humidity of 35 to 45 percent, the fifth and sixth coats with a relative humidity of 45 to 55 percent. The third and fourth coats can 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 each being about 0.005 to 0.020 inch.
After the mold has been built up around the pattern assembly, the pattern material 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 molds are hard, smooth and relatively permeable, and have a thickness on the order of one-eighth to one-fourth inch.
The resulting mold cluster produced from the pattern assembly 10 is shown at reference numeral 30 in FIG. 3. The mold assembly 30 is disposed within a furnace having a refractory outer wall 31 about which one or more induction heating coils 32 are disposed. Located within the wall 31 is a susceptor 33 composed of graphite or the like which serves to deliver radiant energy to the molds. The top of the furnace is closed by means of a top plate 34 composed of refractory material, and a funnel 35 is provided to deliver molten metal to the casting cavities.
The mold assembly 30 itself contains a pouring basin 36 and a cylindrical sprue portion 37 extending downwardly therefrom. The pouring basin 36 communicates with the interior of the casting molds, two of which have been identified at reference numerals 38 and 39 in FIG. 3. Runners 40 and 41 are used to deliver molten metal from the common source into the casting cavities of the respective molds.
The mold assemblies are open ended and their bottom ends are positioned on a chill block 42 composed of copper or other highly heat conductive material. If desired, a circulating fluid may be passed through the chill block 42 to increase the rate of heat transfer.
The mold assembly also includes a refractory reflector 43 which in the form of the invention illustrated in FIG. 3 consists of a continuous annulus of ceramic mold forming material with a hollow center. Alternatively, the reflector 43 can consists of a preformed ceramic material which is placed in the mold prior to pouring and it need not be continuous. In other words, a plurality of ceramic baffles can be positioned closely adjacent the individual molds, the width of the baffles being at least as large as the projected widths of the mold assemblies with which they are associated.
In operation, the molding assembly is operated under vacuum conditions, and the molds are heated to a temperature above the solidus temperature of the metal to be poured in the mold. The metal is melted and cast into the mold and the temperature of the mold is gradually reduced in order to obtain unidirectional solidification from the chill block 42 upwardly to the top of the casting. One convenient means of doing this is to progressively deenergize individual coils making up the induction heating coil 32 so that as solidification proceeds upwardly, a unidirectional temperature gradient exists longitudinally of the mold, and a columnar grain structure having a longitudinal orientation is produced.
It has been found that through the use of the reflector elements of the present invention, it is possible to employ more molds in the cluster than heretofore used without adverse interaction occurring between the molds. It has also been found that the grain structure of the castings is actually improved primarily we believe, because the reflecting elements help to produce a more carefully controllable temperature gradient within the mold assembly.