US3739835A

US3739835A - Process of making shell molds

Info

Publication number: US3739835A
Application number: US00203673A
Authority: US
Inventors: S Copley; A Giamei; M Hornbecker; B Kear
Original assignee: United Aircraft Corp
Current assignee: RTX Corp
Priority date: 1971-12-01
Filing date: 1971-12-01
Publication date: 1973-06-19
Anticipated expiration: 1990-06-19

Abstract

Apparatus for the formation of single crystal articles by directionally solidified casting techniques which substantially eliminates the formation of heterogeneous discontinuities in the casting.

Description

United States Patent 1 Copley et al.

PROCESS OF MAKING SHELL MOLDS inventors: Stephen M. Copley, Madison;

Anthony F. Giamei, Middletown; Merton F. Hornbecker, Woodbury;

Bernard H. Kear, Madison, all of Conn.

United Aircraft Corporation, East Hartford, Conn.

Filed: Dec. 1, 1971 Appl. No.: 203,673

Related U.S. Application Data Division of Ser. No. 63,738, Aug. 14, 1970, which is a continuation-in-part of Ser. No. 714,722, March 20, 1968.

[73] Assignee:

U.S. Cl. 164/35, 164/60 Int. Cl. 1322c 9/04 Primary ExaminerR. Spencer Annear AttorneyCharles A. Warren [57] ABSTRACT Apparatus for the formation of single crystal articles by directionally solidified casting techniques which substantially eliminates the formation of heterogeneous discontinuities in the casting.

5 Claims, 6 Drawing Figures same m3 PAIENIEU JUN 1 9 ms M QQ 1 PROCESS OF MAKING SHELL MOLDS This is a division of US. Pat. application Ser. No.

63,738, filed Aug. 14, 1970.

BACKGROUND OF THE INVENTION bars it is desirable to produce such complex shapes as turbine blades and vanes with the entire blade or vane including the root airfoil section and at times a shroud all of a single crystal The complex shaped castings have been limited in the past by the problem of extracting heat at the proper rate since the removal of heat from the solidifying alloy has been accomplished primarily by the conduction of heat from the solidifying alloy through a constriction adjacent to the bottom of the mold. The rate of heat removal through the relatively small cross-section of alloy in the constriction has been low and therefore the thermal gradient and the rate of growth of the crystal have been limited. One method for increasing the heat extraction rate has been described in a co-pending U.S.- Pat. application, Ser. No. 714,743, filed Mar. 20, 1968 now U.S. Pat. No. 3,543,284 for a process for, casting single crystal shapes. This application has the same assignee as the present application.

One of the problems in obtaining single crystal cast defect free articles is the occurrence of jets which resuit in the formation of trails of small equiaxed grains. Such convective jets can occur within the mushy zone during solidification. These jets lead to erosion and breakage of the dendrite structure and thus cause the formation of trails of equiaxed grains or freckles. The jets are a consequence of hydrodynamic instability within the mushy zone and become more pronounced as the liquidus and solidus interfaces become curved or as the mushy zonehe ight increases (i.e., the thermal gradient decreases). Freckle grains are a major defect since they detrimentally effect the mechanical properties of the casting.

SUMMARY OF THE INVENTION One feature of this invention is an apparatus by which to cast single crystal parts such as turbine blades and vanes without the formation of these trails of equiaxed grains. The elimination of these discontinuities in the surface is obtainedby appropriate control of the thermal gradient within the mold, a control of the rate of solidification of the alloy upwardly within the mold and a control of the wall temperature of mold surrounding the article being cast. The latter resultis ob tained by surrounding the article mold at least in part by a cavity in which a controlled solidification can take place ata selected rate thereby to more effectively control the temperature of the article mold as solidification occurs.

In accordance with the, present invention, the article mold is positioned within a surrounding control mold with the latter having one or more casting cavities therein for toreceive the control portion of the hot alloy of the control of the alloy which is poured at the same time as the alloy is poured into the article mold. These control cavities are connected through solidification passages to the chill plate and the article tnold has a crystal selecting passage in the lower end by which to cause the growth of a single crystal into the article portion of the mold. Although the mold is adapted for the production of a single cast article at one time the construction is more adapted for the gang casting of a plurality of articles at one time and the construction is such that the solidified article may be readily removed from the solidified alloy in the adjacent and partially surrounding control cavities.

The solidification process of the present invention is carried out by positioning the one or several article molds within the surrounding mold and heating the en tire assembly to a temperature above the melting point of the alloy. Once the alloy is poured in the mold assembly, directional solidification begins at the chill plate in which the mold assembly rests andthe liquidsolid interface moves upwardly from the chill plate in both the bottom of the article mold and in the connectors from the casting cavities to the chill plate. The solidification of the alloy within the casting cavities or control cavities helps to maintain the article mold at the desired temperature during; solidification and serves to control the rate of heat removal from the article molds so that the liquid-solidinterface of the alloy within the article mold will be retainedin asubstantially horizontal configuration during the s olidifi cation of the alloy through the entire mold. This mold construction favors higher thermal gradients and growth rates needed for the elimination of discontinuities such as trails of equiaxed grainsin the cast surface. The rate of heat extraction by the metal in the casting cavities is controlled to a substantial degree by the size of the connector from these casting cavities to the chill plate since area of solidified alloy in these passages is a direct control of the rate of heat transfer to the chill plate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a horizontal sectional view through amold assembly.

FIG. 2 is a vertical sectional view substantially along the line 2-2 of FIG. 1.

FIG. 3 is an elevational view of a portion of the mold assembly prior to the formation of the second moldcoating thereon.

FIG. 4 is a view similar to FIG. 1 of a modified form.

FIG. 5 is a vertical sectional view substantially along the line 5--5 of FIG. 4.

FIG. 6 is a fragmentary elevational view of the mold configuration of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT known shell molding process in which thin mold shells are formed around disposable patterns as forexample wax patterns. The apparatus shows the mold shells after the wax has been melted out in order that the cavities in which the alloy is cast may be more readily distinguished. It will be understood however that after the mold material has been completely formed around the patterns they are melted out to leave the cavities shown and the wax pattern did, in fact, occupy all of the shown cavities prior to the baking of the mold to harden it and to provide removal of the wax.

. The article mold 2 has a centrally located airfoil cavity 4 extending from a root portion 6 to a shroud portion 8. Above the shroud is the filling passage 10 and below the root 6 is the helix 12 that selects a single crystal from the columnar growth base portion 14. The function of the growth portion 14 and helix 12 in selecting a single crystal has been described in the copending application of Copley et al., U.S. Pat. Ser. No. 806,978 filed Mar. 13, 1969 now U.S. Pat. No. 3,625,275 and having the same assignee as the present application.

A plurality of these article molds are positioned in a ring with the chordwise dimension of the airfoil portion of the several article molds extending in a generally cir cumferential direction as shown in FIG. 1. The several article molds are connected together in the circumferentially arranged relation by a disk-like filler portion 16 for the several article molds. This portion 16 has a chamber 18 therein communicating with all of the sev-' eral article molds.

Between adjacent article molds isa chamber 20 defined between inner and outer mold rings 22 and 24 which are in engagement with the surfaces of the article molds as shown in FIG. 1 leaving the trailing edge and leading edge portions of the mold exposed to the adjacent chambers 20. The chambers 20 are connected by passages 26 at the top communicating with the horizontal filling chamber 18 and the same chambers communicate at the bottom by a passage 28 with the chill plate 30 on which the entire mold assemblage rests. At the top of the filler portion 16 is a pouring funnel shaped spout 32.

As above stated, the article mold 2 with the appendages thereon is formed by dip coating a wax or otherwise disposable'pattern that is the shape of the article together with the growth portion at the bottom and the filling opening at the top. When the article mold is completed the several article molds are connected to a wax pattern that is the shape of the chamber 18. Wax is positioned between the leading edge of one article mold and the trailing edge of the adjacent mold in the shape of the chamber 20 and wax rods are positioned at opposite ends which are the shape of the passages 26 and 28. With all of the wax in position, the entire assemblage is again dip coated thereby forming the inner and'outer walls 22 and 24 togetherwith the mold l6 surrounding the chamber 18 and the portions of the mold connecting the mold portion 16 to the individual article molds and also the portions of the mold extending down from the portion 16 to the top of the annular mold portions 22 and 24. Once the entire mold is created as shown, the assemblage is positioned in a suitable baking oven to harden the entire mold and to melt out the wax pattern thereby leaving a mold structure as shown in the drawings.

FIG. 3 shows an interim construction between the making of the individual article mold 2 in this figure and the finished mold. In the showing of FIG. 3 a wax pattern 34 has been positioned between adjacent article molds and wax rods 36 and 38 extend from top and bottom of the wax portion 34. At the top the rod 36 is attached to a wax disk 40 which forms the chamber 18 in the completed mold. A wax pouring spout is also attached to the top surface of the disk 40 to shape spout 32.

Accordingly when the mold is completed ready to havealloy poured therein for the formation of the several cast articles the mold assembly consists of the circumferentially spaced apart article molds 2 positioned between inner and outer rings 22 and 24 which define between'them the control chamber 20 the latter being an interrupted circumferentially extending chamber since the article molds are in contact with the inner and outer mold rings and thereby break the circumferentially defined chamber into individual segments which enclose the trailing edge of one of the article molds and the leading of the adjacent article molds.

The entire mold assembly when it is ready to have alloy poured therein is positioned within a heating device such as an induction furnace includinga cylindrical susceptor surrounded by an induction coil and the mold is heated to a temperature above the melting temperature of the alloy in readiness for pouring. With the mold assembly resting on the chill plate 30, the alloy within the mold is in contact with the chill plate through the growth portion 14 of the article mold and through the passages 28 leading to the control chamber segments 20. solidification begins at the chill plate and moves upwardly through the growth portion and through the helix where a single crystal is selected to grow into the article forming portion of the article mold. Columnargrain growth extends upwardly through the passage 28 and into the chambers 20 and it is the solidification in these chambers and the absorption of the heat from the article mold into the solidified alloy and the chambers 20 and the fence by conduction through the solidified alloy and the passage 28 to the chill plate that permits a precise control of the solidification rate and thereby the configuration of the mushy zone that is to say the liquid-solid interface of the solidifying alloy in the article molds. By suitable dimensioning of the passage 28 the rate of heat removal from the article mold may be so controlled that the liquid-solid interface remains substantially flat and horizontal during the entire solidification cycle. A larger overall heat conduction path leads to a higher thermal gradient.

Referring now to FIGS. 4, 5 and 6 the article mold 52 is similar to that described above and has the growth zone 54 at the bottom connected by a helix 56 to the article forming portion and with an extension 58 at the top of the article forming portion which serves as a place for filling the article mold. Several of these article molds are positioned in a ring with the chordwise direction of the airfoil portion of the mold extending in a generally radially position rather than circumferentially as in FIG. 1. Inner and outer rings 60 and 61 define between them a substantially annular chamber 64. The

inner and outer rings communicate and are in vertical 1 engagement with the leading edges of the article mold by outward and inwardly projecting

ribs

66 and 68 formed on the inner and outer mold rings respectively.

As in the apparatus above described, there is a horizontal mold portion 70 at the top of the mold assembly and this has a horizontal chamber 72 communicating with all of the article molds and also through vertical passages 74 in the mold with the segments of the annular chamber 64 which are located between adjacent article molds. The mold also has vertical passages 76 extending downwardly from the segments of the annular chamber 64 to engage at theirbottom ends with the chill plate 78 as do the growth zones 54 of the article mold.

The mold assemblage is made in the same manner as above described with respect to FIGS. 1, 2 and 3. A wax pattern for example is formed in the configuration of the article desired with a helical projection at the bottom and a configuration corresponding to that of the chamber in the growth zone portion of the article mold. The wax pattern has at the top an extension that serves to form the passage through the upward extension 58 on the article mold and from there upwardly to communicate with a horizontal wax disk the shape of and serving to form the chamber 72 when the second step of making the mold is performed. When the wax pattern for the article mold is completed it is dipped into appropriate mold forming slurries by the usual technique of making the shell mold thereby forming the article mold 52 with the extensions at top and bottom as shown. This article mold is then assembled together with the wax pattern to form the chamber 72 wax patterns forming the

passages

74 and 76 and a ring of wax that forms the annular chamber 64 and encloses the article molds from top to bottom of the portion of the article mold that becomes the finished cast article that is being cast for use. Suitable grooves are formed in this. wax pattern to communicate with the leading and trailing edges of the article mold thereby permitting the mold forming material to contact the first shell when the assemblage is dipped to form the secondary mold coating.

FIG. 6 shows the assemblage of the article molds together with the surrounding and associated wax patterns by which the secondary mold is produced. As shown in this figure, the pattern assembly includes the flat disk 80 having the wax rod 82 extending downwardly into the article mold and the parallel wax rod 84 extending downwardly to communicate with the heavy wax ring 86 that surrounds and embeds the individual article mold for the effective portion of the article mold. The assemblage also has the downwardly extending wax rods 88 from the wax ring 86 down to a level coincident with the bottoms of the growth portions 54. Grooves 90 are formed in the wax and extend inwardly from the outer and inner vertical surfaces of the wax to expose the leading and trailing edges of the airfoil portion of the article mold 52. It is these grooves that permit the formation of

theconnecting ribs

66 and 68.

The assemblage of FIG. 6 is then coated with mold material as by the usual shell molding process thereby forming the secondary mold which surrounds the wax pattern and assemblage of FIG. 6. This secondary mold having been completed'the device is placed in a furnace where the mold is baked to harden the material thereof to melt out the wax pattern leaving the empty casting spaces as shown in FIGS. 4.and 5.

Once the mold is hardened ready for use, it is positioned within a suitable heating means preferably an induction furnace and preferably all enclosed within a vacuum and the mold is heated to a temperature above the melting point of the alloy. The molten alloy is then poured to fill the spaces within the mold and solidification begins where the alloy contacts the chill plate in 6 the same manner as above described with FIGS. 1, 2 and 3.

With respect to both modifications of mold apparatus described, once the alloy is entirely solidified and cooled so that the mold with the alloy therein can be handled the outer or secondary mold is removed to as great an extent as possible and the molded parts of the casting are pulled apart. Because of the particular construction of the mold apparatus in both cases such that the control ring of alloy that is to say the alloy that is formed within the segments of the annular chamber 20 of FIG. 1 or the segments of the annular chamber 64 are separable one from another the cast segments may be readily withdrawn from one another thereby disengaging the cast control alloy from the article mold and the cast article within the mold. Subsequently removal of the helix of cast alloy at the bottom of the article mold and from the removal of the cast article from the upwardly extending portion of solidified alloy extending upwardly from the top of the article portion the latter is readily removable in readiness for inspection and machining for use. I

With the use of the alloy in the

annular chambers

20 or 64 and the effective control of the solidification rate as well as the temperature of the article mold during solidification of the alloy it is possible to obtain acceptable cast articles all of a single crystal and free of the equiaxed grains that tend to form on the surface when appropriate temperature controls are not provided. Although the article produced by a casting technique of this type appears to be a relatively small portion of the total amount of alloy used in making the casting it will be readily understood that the remainder of the cast alloy the so-called revert may be added to the alloy for the subsequent casting operations and is not in any sense wasted.

We claim: 1. The process of making a mold including the steps of forming a plurality of shell type article molds around disposable patterns each of the latter having a filling extension at one end of the article portion and a growth zone at the other end of the article por tion of the article mold, arranging said plurality of molds in a ring placing a wax overlay between adjacent molds in the ring with the overlay surrounding and embedding the effective portion of each of the adjacent article molds and filling the space therebetween, with a wax projection forming a filling passage extending upwardly from said overlay and another wax projection forming a downward extension on said overlay forming a second shell mold around the assemblage of article molds and wax inlay with said second shell mold in contact with the article molds where they are not encompassed by the wax overlay and heating the mold assembly to harden the mold and melt out the wax.

2. The process of making a mold as in claim 1 including the additional step of attaching a connecting wax connector element to the several upwardly extending wax projections and to the wax filling extension on the several article molds.

3. The process of claim I in which the article molds are shaped to form a turbine vane having an airfoil shaped portion with the added steps of respect to positioning the article molds in a ring with the chord positioning the article molds in a ring with the chordof the airfoil shaped portion extending in a circumwise direction of the airfoil shaped portion extendferential direction and ing in a radial direction with respect to the ring and embedding the leading and trailing edges of the airembedding the entire airfoil portion in the wax overfoil shape portion of the article mold in the wax lay except for asmall portion at leading and trailing overlay. edges of the article mold.

4. The process of making a mold as in claim 1 in 5. The process of making a mold as in claim 1 which the article molds are for the manufacture of a wherein the wax overlay extends for the entire length turbine blade or vane having an airfoil shaped portion of thearticle forming portion of the article mold. with the added steps of 10

Claims

1. The process of making a mold including the Steps of forming a plurality of shell type article molds around disposable patterns each of the latter having a filling extension at one end of the article portion and a growth zone at the other end of the article portion of the article mold, arranging said plurality of molds in a ring placing a wax overlay between adjacent molds in the ring with the overlay surrounding and embedding the effective portion of each of the adjacent article molds and filling the space therebetween, with a wax projection forming a filling passage extending upwardly from said overlay and another wax projection forming a downward extension on said overlay forming a second shell mold around the assemblage of article molds and wax inlay with said second shell mold in contact with the article molds where they are not encompassed by the wax overlay and heating the mold assembly to harden the mold and melt out the wax.

3. The process of claim 1 in which the article molds are shaped to form a turbine vane having an airfoil shaped portion with the added steps of positioning the article molds in a ring with the chord of the airfoil shaped portion extending in a circumferential direction and embedding the leading and trailing edges of the airfoil shape portion of the article mold in the wax overlay.

4. The process of making a mold as in claim 1 in which the article molds are for the manufacture of a turbine blade or vane having an airfoil shaped portion with the added steps of positioning the article molds in a ring with the chordwise direction of the airfoil shaped portion extending in a radial direction with respect to the ring and embedding the entire airfoil portion in the wax overlay except for a small portion at leading and trailing edges of the article mold.

5. The process of making a mold as in claim 1 wherein the wax overlay extends for the entire length of the article forming portion of the article mold.