Casting Of Directionally Solidified Articles

Abstract

The cooling cycle for directionally solidified castings is improved by lowering the temperature of the mold adjacent to the chill plate at the beginning of the solidification while maintaining the high temperature in the greater portion of the mold and then further cooling the mold near the chill plate during continued solidification to increase the heat removal and thereby maintain the desired high thermal gradient.

Description

BACKGROUND OF THE INVENTION

Directionally solidified articles formed as a single crystal as in Piearcey U.S. Pat. No. 3,494,709 are most readily produced in shell molds resting on a chill plate for removal of heat during solidification and surrounded by heating means to maintain the mold at a high temperature near the top so that the liquid-solid interface progresses from the chill plate to the other end of the mold in a substantially planar form to produce the desired crystalline or granular structure without spurious nucleation.

STATEMENT OF INVENTION

A feature of this invention is a mold apparatus by which to control the temperature of the mold more effectively and thereby maintain the most effective temperature gradient within the mold throughout solidification of the alloy in the entire mold.

Another feature is an arrangement for additional controlled cooling of the portion of the mold adjacent to the chill plate at the beginning of and during the solidification of the alloy for better control of the thermal gradient. Another feature is a process by which to produce directionally solidified cast articles free of such imperfections as freckles. One particular feature is the control both of the temperature gradient and also of the rate of solidification that is the rate of upward movement of the liquid-solid interface during solidification.

According to the present invention, the thermal gradient is increased by moving the mold partially out of the heating apparatus into such a position that the lower portions of the mold may radiate heat and thereby lose heat at a much faster rate than the chill plate can remove it. To accomplish this suitable openings are located below the heating apparatus so that heat may be radiated to the relatively cool wall of the vacuum chamber surrounding the heating apparatus. Appropriate control of the continued heat input together with a precise control of the extent to which the mold is withdrawn permits a high thermal gradient to develop within the mold and also permits a controlled rate of solidification of the alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view through the mold apparatus for casting single crystal articles.

FIG. 2 is a transverse sectional view along line 2--2 of FIG. 1.

FIG. 3 is a diagram showing the relationship of thermal gradient to growth rate.

DETAILED DESCRIPTION OF THE DEVICE

As shown in the drawing, the mold assembly 2 is the cocoon type of mold described and claimed in the copending application of Giamei et al., Ser. No. 42,423 filed June 1, 1970, and includes two spaced apart parallelly extending article molds 4. These article molds are shown as molds for a turbine blade and have an airfoil shaped portion 6 with a platform portion 8 at the bottom thereof, and a root forming portion 10 below the platform. Connected to the bottom of the root portion 10 is a helix portion 11 which serves to select a single crystal to grow into the article forming portion during solidification of the alloy. A control mold 12 surrounding and spaced from the article mold defines chamber 13 between the molds to be filled with molten alloy in the casting operation for controlling the rate of cooling in the article mold. This control mold has an open bottom end to rest on a chill plate 14 and this bottom end holds the open end of the helix 11 about 1 inch above the chill plate. The several control molds 12 are interconnected near their upper ends by gating 15 having a common filling spout 16. The article mold has an open end extension 17 at the top forming a riser. The article mold is filled by a flow of the molten alloy downwardly around the article mold and then upwardly through the helix 11 into the article mold. When the article being cast is a gas turbine blade or vane, the alloy used is one of the so called super alloys, examples of which are given in the Piearcey patent above identified.

The mold assembly when ready to be used for making a casting is positioned on the chill plate 14 the latter being supported by a device by which the chill plate with the mold thereon may be moved upwardly and downwardly with respect to the surrounding apparatus. The actuating device shown by way of example is a threaded supporting rod 18 the position of which is controlled by a nut, not shown that may be driven through suitable gearing by a motor, also not shown, which permits the chill plate to be moved vertically as desired and serves, as will be pointed out hereafter, to permit a precise control of the temperature existing within various portions of the mold during solidification of the alloy. The motor is preferably manually controlled for a precise control of the position and motion of the chill plate.

The mold and chill plate are surrounded by a susceptor 22 in the form of a cylindrical graphite sleeve which is insulated from a surrounding heat insulating sleeve 24 by one or more sleeves of insulation 26 such as graphite felt. Around the sleeve 24 is a tapped induction coil 28 so arranged as will be apparent that the top half of the coil may be energized separately from the lower part of the coil. The total coil surrounds the susceptor and extends for substantially the same height as the mold within the susceptor. Clips 30 may attach the coil to the outer sleeve 24 for supporting the coil in position. The sleeve 24 rests at the bottom on a base 32.

At the base of the susceptor are one or more rings of insulation 36 such as graphite felt on which the susceptor and insulating sleeve or sleeves 26 are positioned. The rings 36 rest on a cylinder 38 of insulation material extending down to the base 32 within the sleeve 24.

The insulating sleeve 24 has openings or windows 40 extending preferably down to the support 32 and the cylinder from a point about midway between the insulating rings 36 and the support and the cylinder 38 has coincident openings 42 generally taller than the openings 40 as shown. Both sets of openings are located adjacent to the article portions of the mold as shown in FIG. 2. The top levels of the openings 40 and 42 are located so that the mold cannot radiate heat through these openings when the chill plate is in the position of FIG. 1. Should the gang mold be made for casting more than two articles at one time, as for example, four or eight articles there would be a corresponding number of openings or windows 40 and 42, even to the extent of omitting this portion of the sleeve 24 and cylinder 38.

The device is normally in the position shown at the start of a casting operation. The entire unit shown is positioned within a suitable chamber, represented by the walls 39, preferably a vacuum chamber since the type of alloy used is preferably cast in a vacuum and the walls of the chamber are generally water cooled as shown by the passages 41 in the walls 39. After the chamber is evacuated, the induction coil is energized for heating the mold to a temperature above that of the melting point of the alloy to be cast. When the entire mold with the exception of the portion in direct immediate contact with the chill plate reaches the desired temperature above the melting point of the alloy, the alloy is poured into the mold and immediately before or immediately after such pouring the chill plate is lowered far enough to expose the portion of the mold immediately above the chill plate to the openings or windows 40 and 42. This permits heat to be radiated from the portion of the mold immediately above the chill plate directly to the water cooled walls 39 and thereby reduces the temperature of this part of the mold to a temperature lower than that obtainable by the cooling function of the chill plate alone.

With a more rapid chilling of this portion of the mold the temperature gradient in the mold becomes steeper and the thickness of the mushy zone, the liquid-solid zone, is thereby decreased. As the solidification of the alloy within the mold progresses the chill plate may be moved down in increments to increase the rate of heat radiation through the windows 40 and 42 thereby maintaining the high thermal gradient during the solidification of the alloy at least to a position above the article forming portion of the mold.

As best shown in FIG. 2 the openings 40 and 42 are located at points adjacent to the article mold portions 4 thereby providing a most rapid radiation of heat from these mold portions. It will be understood that the lowering of the chill plate with the mold resting thereon not only positions the lower portion of the mold into alignment with the openings 40 and 42 but also effectively withdraws this portion of the mold more or less from within the induction heating coil and thereby reduces or discontinues the heat input to this portion of the mold.

FIG. 3 shows the effect of the growth rate as compared to the thermal gradient and the respective areas in which it is possible to obtain freckle-free directional growth in the form of single crystal castings. The plot of this figure is deliberately non-dimensional since the effective thermal gradient above which freckle-free growth may be obtained is different depending upon the dimension of the article to be cast, the alloy being used and the type of mold used in producing the casting. The relative relationship is however represented in this figure and it emphasizes that a relatively high thermal gradient is desirable in producing a single crystal casting free of freckles or other imperfections in the surface.

Although a high thermal gradient is provided within the alloys as it solidifies, the solidification rate, that is, the rate of upward movement of the liquid-solid interface, should be so controlled as to prevent growth at a rate to cause the alloy to become equiaxed or randomly crystallized. The slow rate of upward movement of the liquid solid interface is controlled effectively by the amount of heat supplied by the induction heating coils to the upper end of the mold during the casting cycle. If adequate temperature sensing devices are positioned adjacent to or within the mold structure, as for example a thermocouple near the top of the mold, another thermocouple perhaps an inch from the chill plate, and preferably an intermediate thermocouple it is possible to so determine the rate of solidification or the rate of cooling of the alloy toward the solidification temperature that the desired rate of upward movement of the liquid-solid interface may be precisely controlled.

As above stated, the type of mold particularly effective for this purpose is the so-called cocoon mold described above. This particular type of mold which provides a shell of heat control alloy effectively surrounding the article being cast for use makes possible a much more precise rate of movement of the liquid-solid interface upwardly through the article portion of the mold and also permits a control of the height of the mushy zone, that is to say the volume of alloy within the mold between the level where the alloy is completely solid and the point thereabove where the alloy is completely liquid. This mushy zone represents that volume that is partially filled with solid dendritic growth but the alloy surrounding the dendrites is still unsolidified. The cocoon mold serves to maintain a substantially flat mushy zone during the solidification cycle.

It will be understood that there are frequently gang molds having more than the two mold elements as in FIG. 1 so that there may be a large number, for example, four or eight or even more article forming molds interconnected together at the top and each terminating in an open end at the bottom resting on the chill plate and all arranged in a ring for simultaneously casting a plurality of articles. When this occurs, it will be obvious that the radiation windows, that is to say the openings 40 and 42, form a greater and greater proportion of the periphery of the cylinder of insulation 38 and the periphery of insulating sleeve 24 if a radiation window is provided for each article forming portion of the mold. To this extent, it may be desirable, where the device may be used primarily in making a plurality of articles during each casting operation to extend the openings 40 and 42 to encompass substantially the entire periphery of the sleeve of insulation 24.

In casting a plurality of single crystal turbine blades at one time using a cocoon type mold and where the length of the turbine blade including the root portion was 5 inches, and the helix 12 and surrounding outer mold formed a lower end extension that was substantially 11/2 inches long, the top of the completed cast article that would be used was then located at a point 61/2 inches above the chill plate. The alloy from which these blades were cast is an experimental nickel base cast super alloy. The casting technique was established to produce a single crystal growth throughout the article portion of the mold so that the finished article was completely a single crystal from end to end.

To accomplish this with the casting apparatus set up as in FIG. 1 the mold was instrumented with a plurality of thermocouples 44 the bottom one of which is located about an inch above the chill plate, the top one is located close to the top of the article forming portion of the mold and at least one thermocouple is positioned midway therebetween. In the particular casting process carried out for producing a turbine blade of the type pictured the thermocouples were located precisely at points one inch above the chill plate, at the top end of the article forming portion and at the blade platform as indicated on the drawing.

With the thermocouples in position the mold was located within the susceptor and on the chill plate and the entire assemblage was positioned within a vacuum chamber in order that the casting might be accomplished under a vacuum. The vacuum chamber having been evacuated the induction coils were energized to raise the temperature of the mold to 2,900.degree. F as indicated by the top thermocouple. During the heating of the mold, cooling water was circulated at the normal rate, although only enough water may be circulated through the chill plate to keep the chill plate from being damaged by the heat.

When the top thermocouple reached the desired temperature, the chill plate was lowered to obtain a bottom thermocouple temperature indication of 2,400.degree. F.

The alloy was then poured into the mold, having been heated to 2,850.degree. F which is about 300.degree. above the melting point of the alloy so that a significant amount of superheat was provided in the alloy as it was poured.

The normal amount of cooling water that had been flowing through the chill plate during the heating, was continued during the solidification process. At the time of pouring the alloy, the bottom portion of the induction coil was completely de-energized. As soon as the alloy had been poured and solidification had begun the energy to the top coil was reduced by increments to reduce the temperature indicated by the top thermocouple as follows:

40.degree. reduction 10 minutes after pour

40.degree. reduction next 10 minutes after pour

35.degree. reduction next 10 minutes after pour

35.degree. reduction next 10 minutes after pour

35.degree. reduction next 10 minutes after pour

30.degree. reduction next 10 minutes after pour

to reach a temperature of 2,635.degree. F. The indicated temperature at the completion of the pouring of the alloy into the mold becomes 2,850.degree., the temperature of the poured alloy. The bottom thermocouple indicated a temperature of 2,125.degree. F, 60 min. after the alloy was poured. With heat still being supplied from the top induction coil and from the susceptor, the temperature of the top of the mold reached at 2,635.degree. F, 60 min. after pour and the temperature of the middle thermocouple was at 2,400.degree. F at this time thereby indicating that the thermal gradient was very steep in the portion of the mold where solidification was going on. With the exposure of the lower most portion of the mold to the windows so that radiation may take place the temperature was effectively lowered rapidly. The effect of the lowered temperature near at the bottom of the mold by radiation through the windows is to produce a thermal gradient within the alloy as great as 125.degree. F per inch.

After the first 60 minutes following pouring of the alloy, the chill was dropped further from the position at pour in increments of one-fourth inch and this was done at intervals of 10 min. It is essential that the downward motion of the chill be so controlled that the bottom of the dendrite field must be above the top of the radiation window by a pre-determined amount preferably about 2 inches and the timing of the downward motion of the chill must be such that the dendrite field remained in this relation to the top of the window. The bottom of the dendrite field will be understood to be the level where the complete alloy is solidified. It will be understood that in castings of this character the dendrites within the alloy grow vertically and horizontally within the mushy zone and are surrounded by unsolidified alloy within the mushy zone. By maintaining a steep thermal gradient within the mold, the height of this mushy zone is made relatively thin and this steep thermal gradient assures crystallization in the desired form.

During the solidification of the alloy, heat was continuously added from the top coil to the portions of the mold still within the susceptor and this coil was de-energized 200 min. after the alloy was poured. At this time, further heating of the mold was thus discontinued and the cycle was completed after the total downward movement of the chill plate had reached 3 inches and this occurred 180 min. after the alloy was poured. The top thermocouple reached a temperature of 2,200.degree. F 200 min. after the alloy was poured and this temperature indicated that the solidification of the alloy had reached a point above the top of the article portion of the mold.

After the mold and alloy were cooled sufficiently, the mold was removed from the rest of the apparatus, the casting was removed from the mold and the article portion of the casting was removed from the remainder. The result was a cast article all of a single crystal.

In the apparatus used in the foregoing casting procedure, each window 42 had its upper edge one-half inch below the starting position of the chill plate and each opening 40 had its upper edge 11/2 inches below this position of the chill plate. The openings 42 were 4 inches wide and the openings 40 were 5 inches wide. The mold was positioned so that the convex side of the blade portion of the mold faced outwardly so that radiation occurred more effectively on that side of the mold. It will be understood that the cooling of the mold by radiation is enhanced by the temperature of the walls of the vacuum chamber to which the mold is exposed more or less through the windows. These chamber walls are generally water cooled during the use of the chamber.

The effectiveness of the present apparatus and method is enhanced by the ability to control the steepness of the thermal gradient and to have a much steeper thermal gradient than is usually obtainable in this type of casting. Furthermore with the steep thermal gradient, the rate of upward movement of the liquid-solid interface may be controlled by the extent of the radiation provided and also by the extent of the continued heat input during solidification of the alloy. It will be apparent that the steepness of the thermal gradient may be relatively independent of the rate of upward movement of the solidification front. By the use of the above-described apparatus, the extremely steep thermal gradient is obtained and a very effective control of the rate of solidification so that the solidification will occur in such a manner as to assure that a single crystal article will be produced.

Claims

We claim:

1. In the casting of directionally solidified articles in a mold the steps of

positioning the mold on a chill plate,

heating the mold to a temperature above the melting temperature of the alloy,

reducing the temperature of the mold immediately above the chill plate by the cooling action of the chill plate,

pouring the alloy into the mold,

withdrawing the mold from the heating zone into a cooling zone having an enclosing wall, and

rapidly further lowering the temperature of certain selected peripheral areas of the mold by radiation of heat laterally from said selected areas only through openings in said enclosing wall to surrounding water cooled surfaces external to said wall thereby cooling said areas of the mold to a greater extent than the remainder of the mold to establish in said areas a higher thermal gradient than is accomplished by the chill plate alone.

2. The method of claim 1 including the steps of

surrounding the mold with a susceptor for heating the mold, and

partially withdrawing the mold from within the susceptor into a supporting structure below and supporting the susceptor to position the bottom portion of the mold below the susceptor to expose the selected areas of the mold through openings in the supporting structure to the water cooled surfaces when the alloy is poured for the rapid further cooling of the selected areas of the mold by radiation.

3. The method of claim 2 including the step of

further withdrawing the mold from within the susceptor to increase the area of the mold exposed through said openings for a greater radiation of heat therefrom.

4. The method of claim 1 including the step of continuing the heating of the top portion of the mold thereby to control the rate of solidification of the alloy in the mold.

5. The method of claim 2 including the step of

continuing the heating of the portion of the mold within the susceptor to control the rate of movement of solidification front within the mold.

6. Apparatus for casting directionally solidified articles including

a susceptor providing an enclosure for a mold and for heating the mold,

induction coil means surrounding the susceptor for heating it,

supporting means for said susceptor and coil means,

cooling means in the form of water cooled walls located around said susceptor and externally of said supporting means,

said supporting means extending downwardly from said susceptor and between said mold and said walls, said supporting means having openings therein, said openings being located adjacent those parts of the mold for which an increased rate of cooling is desired,

means for moving the mold axially of the susceptor to position it within the susceptor and to withdraw it therefrom and into said supporting means to permit radiation of heat from said parts of the mold through said openings to said water cooled walls.

7. Apparatus as in claim 6 including a chill plate on which the mold is mounted and with which the cooling means cooperates by receiving radiant heat from the mold, resulting in more rapid cooling of the mold.

8. Apparatus as in claim 6 in which

the moving means is arranged for positioning the mold with the chill plate substantially in alignment with the bottom of the susceptor and for positioning the chill plate below the bottom of the susceptor to place the lower portion of the mold in a position to radiate heat through said openings.