US 3802482 A
A process for making directionally solidified alloy castings in which the mold or core is surface coated where it contacts the molten alloy with a material which will minimize interaction between the alloy and the mold or core thereby avoiding depletion of any of the ingredients in the alloy and preventing contamination of the alloy by grains of the mold loosened by the interaction.
Description (OCR text may contain errors)
United States Patent [191 Phipps, Jr.
[111 3,802,482 Apr. 9,1974
[ PROCESS FOR MAKING DIRECTIONALLY SOLIDIFIED CASTINGS  Inventor: Charles M. Phipps, Jr., South Windsor, Conn.
 Assignee: United Aircraft Corporation, East Hartford, Conn.
 Filed: Mar. 9, 1972  Appl. No.: 233,168
 US. Cl. 164/71  Int. Cl. B22c 3/00  Field of Search 164/60, 72, 361, 25, 26; 1 17/51, 5.2
 References Cited UNITED STATES PATENTS 3,180,632 4/1965 Katz et a1 l17/5.l X 3,472,310 10/1969 Ernest et al... 164/361 X 3,373,795 3/1968 Hein 164/361 X 3,645,767 2/1972 Taylor 164/72 X 3,515,201 6/1970 Zimmerman 164/72 X 3,727,666 4/1973 van der Sluis 3,743,003 7/1973 Brown 164/72 X FORElGN PATENTS 0R APPLICATIONS 1,234,575 6/1971 Great Britain 164/26 Primary Examiner-J. Spencer Overholser Assistant Examiner.lohn E. Roethel Attorney, Agent, or FirmCharles A. Warren  ABSTRACT 12 Claims, 2 Drawing Figures PROCESS FOR MAKING DIRECTIONALLY SOLIDIFIED CASTINGS SUMMARY OF THE INVENTION The present invention relates to a process for making directionally solidified castings and is applicable to the well known superalloys having nickel or cobalt base and also to titanium base alloys, eutectics and quasieutectics and other alloys having one or more ingredients that react with the mold or core material.
BACKGROUND OF THE INVENTION In conventional castings of alloys the solidification is quite rapid and only a short time lapses between the time of pouring and the solidification of the alloy. With such solidification, the alloy has only a small reaction time within which the reactive element in the alloy may be depleted to affect the composition of the alloy adjacent to the surface or to cause loosening of some of the grains in the mold surface to contaminate the casting. However, with the advent of eutectics and quasieutectics and/or the development of directional solidification processes in making columnar grained or single crystal articles the solidification time has been lengthened and the molten alloy is in contact'with the core or mold for a much longer time before solidification is completed. This naturally extends the time for reaction between the mold or the core and the alloy with the result that depletion of one or more of the ingredients in the alloy becomes a problem and contamination of the alloy is also more frequent.
STATEMENT OF THE INVENTION One feature of the invention is the coating of the surface of the mold or core which is in contact with the molten alloy with a material which will minimize interaction between the alloy and the mold or core. Another feature is the coating of the exposed parts of the mold or core with an oxide of one of the reactive elements in the alloy for the purpose of minimizing reactions between the alloy and thematerial of the core or mold.
In accordance with the invention the process involves forming the mold which is generally a shell type mold, coating the surface of the mold that is exposed to the alloy with a material with which the reactive element of the alloy does not react, filling themold with the alloy and causing solidification of the alloy in the mold. The invention contemplates making a split mold so that the coating may be readily applied to the inner surface of the mold which contacts with the alloy when the mold is filled with the molten alloy in making the cast- BRIEF DESCRIPTION 'OF THE DRAWING FIG. 1 is a vertical sectional view through a mold for a turbine blade showing a core in position.
FIG. 2 is a horizontal sectional view through the mold of FIG. 1
DETAILED DESCRIPTION OF THE INVENTION The mold and/or core are generally made of a ceramic material such as silica, alumina, zircon or a combination of these compounds with the silica serving as a high temperature bond tohold the particles together. The inner surface of the mold may also be a finer material than the rest of the mold in order to give a smooth surface finish to the wall of the casting cavity. This surface is formed by the first dip coat used in preparing the multi-layer mold in shell molding and is generally of the same material as the rest of the mold, that is to say, silica, alumina, zircon and/or a combination of these compounds but in which the particles are more finely powdered to give the smoother surface. In this description the term mold will hereinafter be used to include the core which is essentially a part of the mold.
To avoid detrimental contact between the alloy in the mold and the surface of the mold, it has been found de sirable to apply to the mold surface a coating of an oxide which will be inert to the reactant ingredient of the alloy. In most cases, this coating will be the oxide of the refractory ingredient in the alloy. Thus, if the alloy has hafnium as an addition, the applied coating would be hafnia since this is an inert oxide. If the alloy being cast is an yttrium bearing alloy, the core and the mold would have a coating of yttria. Where the ingredient in the alloy that might be depleted is carbon, the inert oxide used for coating the mold would desirably be the most inert oxide available and useable and thus would'be yttria or hafnia. In' casting titanium or titanium alloys, zirconia was found to be sufficiently inert to provide a retarded reaction between the mold and the alloy so that compositionally good surfaces of the casting were obtained.
Where eutectic alloys were used and the carbon was depleted by interaction between the mold and the alloy, an alumina coating on the mold and core surface was found to control the reaction so that the carbon was not significantly depleted in the alloy.
The oxide coating may be applied in a number of different ways but is preferably applied by either flame spraying or plasma spraying of the oxide onto the surface of the mold. Under most circumstances the mold was at room temperature at the start of the spraying operation. More uniform coatings were found to be possible under certain circumstances if the mold were warmed somewhat, for example, to a temperature of about 600F.
The core presents no problem in being coated either by flame or plasma spraying or by other suitable means prior to positioning in the mold, or in the pattern around which the mold is formed. The mold, however, may be difficult to coat unless the mold is split or made in several parts to expose the alloy contacting surfaces when the mold is open. To this extent as shown in the drawing, the mold is made up of two opposed and cooperating parts 2 and 4 which are separableone from another and are so made during the mold forming process by positioning separators 6, for example, at leading and trailing edges of the airfoil portion 8 of the mold. These separators extend upwardly for the entire length of the mold so that the latter may be separated in two pieces and thereby expose the inner surface of the mold, the part that contacts with the alloy during the casting operation so that a spray coating operation may readily be performed thereon. The separators will obviously be so made that the material of the mold will not adhere thereto and thereby permit easy separation of the mold into the two parts. Obviously, the split is so arranged that if there is a core 10 positioned in the mold, the core may be positioned along the line of separation of the mold for support by the mold. In making the casting the mold is positioned on a chill plate 12.
The effect of the sprayed-on coating is to so completely cover the surface of the mold coming in contact with the alloy being cast that the reactive elements in the mold will not be exposed directly to the alloy itself. It is believed that the greatest reaction between the alloy and the mold occurs with the silica in the mold and the coating effectively insulates the silica within the mold from the alloy when it is poured into the mold. The coating of the mold or core is of particular advantage where the solidification time is relatively long as in the solidification of some of the eutectic alloys or the nickel or cobalt base alloys which are solidified in a directionally solidified arrangement thereby producing either the columnar grain of the VerSnyder patent above referred to, the single crystal of the Piearcey patent or the so-called plane front solidification. All of these forms of solidification require a relatively slow rate of movement of the liquid-solid interface from the bottom of the mold to the top. Such solidification is accomplished in gradient molds where the temperature above the liquid-solid interface may be kept at as high as 2,850 F while the bottom of the mold is exposed directly to a water cooled chill plate. In some of the directionally solidified techniques, the rate of solidification may be as high as 6 to 10 inches an hour but is frequently slower than that and there is accordingly a relatively long time for reaction between the mold and the molten alloy.
This concept of an oxide barrier on the surface of the mold or core has been used in connection with a nickel base alloy known as PWA 649 and having the following composition:
Carbon 0.10 max Manganese 0.35 max Silicon 0.35 max Phosphorus 0.015 max Sulfur 0.015 max Chromium 17.00 21.00
Nickel+Cobalt 50.00 55.00
Cobalt (if determined) 1.00 max Columbium+ Tantalum 4.75 5.50
Molybdenum 2.80 3.30
Titanium 0.65 1.15
Aluminum 0.40 0.80
Boron 0.006 max Copper 0.10 max Zirconium 0.05 max Iron Remainder Both zirconia and alumina sprays were used on cores that were reactive with this alloy and substantially reduced the reaction. That is to say, the alloy depletion previously occurring was reduced and the surface of the cast alloy had the same percentage of alloying elements therein as the part of the casting nearer the center of the part.
A high purity alumina coating was applied to a zircon-silica core for use with a cobalt base alloy PWA 657 having the following composition:
Carbon 0.78 0.93%
Manganese max 0.10
Silicon 0.10 0.40
Chromium 20.00 23.00
Tungsten 9.00 11.00
Tantalum 8.00 10.00
Zirconium 0.10 0.30
Iron 0.75 1.50
Nickel max 1.50
Boron max 0.010
Cobalt Remainder The presence of the alumina plasma spray minimized the surface carbon depletion in the cast alloy.
Similarly a high purity alumina coating was applied by plasma spray on zircon-silica cores for use with a eutectic type alloy UARL 73C having the following composition:
Cobalt Remainder and the surface carbon depletion in this alloy was also significantly reduced.
A similar alloy UARL 236C was used with the same type of alumina coated core and again the effect of the coating was to minimize the surface carbon depletion in the cast alloy. The composition of this alloy is as follows:
Cobalt Remainder The titanium alloy used was a cast alloy similar to AMS 4928 (6A1, 4V, Bal Ti).
Another alloy PWA-AMRDL 350 having the following composition incorporates yttrium as a modifying ingredient:
Nickel Balance To minimize the depletion of yttrium from the alloy, the split mold was plasma sprayed with yttria prior to the casting operation. The effect was almost completely to reduce the reaction between the mold and the alloy so that depletion of yttrium in the alloy was eliminated to the extent that could be determinedfrom testing the various portions of the cast alloy. This alloy was directionally solidified to produce columnar grained structure.
An alloy to which hafnium has been added as a modifying ingredient PWA 1422 has the following composition:
Carbon 0.08 0.14
Chromium 8.00 10.00
Cobalt 9.00 11.00
Tungsten 11.50 13.50
Columbium 0.75 1.25
Titanium 1.75 2.25
Aluminum 4.75 5.25
Hafnium 1.75 2.50
Boron 0.010 0.020
Nickel Remainder This alloy was cast in a mold the surface of which was coated with hafnia by a plasma spray coating process. This alloy was directionally solidified to produce a single crystal structure in the cast alloy. The use of the hafnia coating reduced the hafnium losses through reaction with the mold significantly.
In all of these cases, the mold reaction was minimized so that substantially no particles from the mold were loosened to form imperfections in the casting. The improvement over uncoated molds and cores was significant. The use of this coating did not interfere with the removal of the mold material from the casting and the core material was removable in the usual way by leaching. In the nickel or cobalt base alloys this leachant is preferably either a solution of NaOH or KOH in water. The same leachant has been used on the eutectics and core removal was not detrimentally affected.
1. In casting articles of an alloy having as at least one ingredient an element reacting with the material of a part of a ceramic shell mold such that the element is depleted in the casting, the steps of making the mold,
coating essentially the entire surface of the mold to be exposed to the alloy with an inert oxide with which said element of the alloy does not react, such that the element will not be depleted in the finished casting,
filling the mold, and
solidifying the alloy in the mold.
2. The process of claim 1 in which the part of the mold reacting with said element is the core and including the step of coating the core with the inert oxide with which the element does not react.
3. The process of claim 1 in which the coating is applied by plasma spraying.
4. The process of claim 1 in which the coating is a metallic oxide substantially inert to the said ingredient.
5. The process of claim 1 in which the coating is a metallic oxide substantially inert to said ingredient and in which the coating is accomplished by plasma spraying the metallic oxide onto the mold surface.
6. In casting articles of an alloy having as at least one alloying ingredient, an element reacting with the material of a part of a ceramic shell mold such that the element is depleted in the casting, the steps of making the mold as at least a two-piece split mold,
opening the mold after it is completed,
coating essentially the entire surfaces of the mold exposed to the alloy being cast with an inert oxide with which said element of the alloy does not react, such that the element will not be depleted in the finished casting,
reassembling the mold, and
casting said alloy in said mold.
7. The process of claim 1 in which the alloy is a titanium alloy and the coating is an oxide selected from zirconia or alumina.
8. The process of claim 1 in which the alloy has carbon therein as the reacting element and in which the coating for the surface of the mold is alumina.
9. The process of claim 1 in which the reacting alloy element is yttrium and the coating for the mold is yttria.
the use of chemically inert coatings on the core.