US3532155A - Process for producing directionally solidified castings - Google Patents

Description

United States Patent Inventors Larry 1. Kane Skokie; John E. Anderson, Chicago; Terry L. Loveless, Arlington Heights, Illinois Appl. No. 688,212 Filed Dec. 5, 1967 Patented Oct. 6, 1970 Assignee By mesne assignment to Martin Metals Company, Wheeling, 111.,

a corporation of Delaware PROCESS FOR PRODUCING DIRECTION/ALLY SOLIDIFIED CASTINGS 13 Claims, 8 Drawing Figs.

U.S. Cl 164/60, 164/65,164/251,164/258,164/361 Int. Cl 822d 25/06 Field ofSearch 164/60, 51,

[56] References Cited UNITED STATES PATENTS 3,008,855 l1/1961 Swenson 148/32 3,170,205 2/1965 Brown 164/60 3,283,377 11/1966 Chandley 165/65 3,376,915 4/1968 Chandley 164/60UX Primary Examiner- Robert D. Baldwin Attorney-John A. Crowley, Jr. and Francis .1. Mulligan, .lr.

Patented 0a. 6, 1970 Sheet J hi I'JNVENTORS LARRY KANE BY JOHN E. ANDERSON TERRY L. OVELESS ATTORNEY Patented Oct. 6, 1970 I Sheet 2 M3 QQQQ QQQQ FIG. 2A

FIG. 2

S S. H R OER NE M W wmm l .IYEY NR HR OE LJT ATTORNEY Patented Oct. 6, 1970 Sheet ]NVENTOR$ LARRY I. KANE BY JOHN E.ANDER$ON TERRY L.LOVEL ESS .AT JORNEY PROCESS FOR PRODUCING DIRECTIONALLY SOLIDIFIED CASTINGS The present invention is concerned with a casting process and, more particularly, with a process for casting turbine blades and stator blades for use in gas turbine engines.

Relatively recently, it has been realized that cast alloy articles having a columnar crystalline habitat, with the longitudinal axes of the crystals parallel to the direction of stress applied to the articles in use, can have advantageous characteristics. In a conventionally cast alloy comprising many small grains, each substantially equal in dimension in all directions (Le. an equiaxed structure), the gross mechanical characteristics of the article at best represent an average of the characteristics obtainable in various crystalline directions of the metal plus the usually detrimental effects of a multiplicity of grain boundary areas. In actuality, the mechanical characteristics of a real conventional cast article are further detrimentally affected by segregations and macro or micro pores produced in the article as a result of the meeting of solidification fronts moving in uncontrolled directions during the freezing of molten metal in a mold.

The advantageous characteristics obtainable in directionally solidifed, thermally nucleated castings made from a face-centered-cubic crystalline alloy containing about 75.5 percent nickel, about 21 percent chromium and about 3.5 percent aluminum were disclosed in 1960 by Ver Snyder and Guard in an article published in the transactions of the A.S.M.-Vol. 52, pages 485 to 493. The authors produced directionally solidified, columnar crystalline metal in an elongated sand mold having a water-cooled copper chill at one end, an exothermic hot top at the other end, and an exothermic lining adjacent to the cast metal. It was found that mechanical characteristics important in the jet engine field were substantially enhanced in a direction parallel to the longitudinal axes of the columnar crystals. Subsequently, in 1964, Ver Snyder filed an application for a U.S. patent on similar subject matter which was issued as U.S. Pat. No. 3,260,505 on July l2, 1966. In this patent, it was disclosed that many facecentered cubic alloys having a nickel-chromium or cobalt chromium base and having use in essentially the as-cast form as turbine blades are advantageously cast into turbine blade form in such fashion that columnar crystals therein are oriented parallel to the longitudinal axis of the turbine blade. Advantageously, these columnar crystals extend substantially unbroken from the lower end of the cast blade root to the tip end of the blade. Further, it is advantageous to cast under conditions conducive to the production of a large number of columnar crystals in the cross section of the blade. U.S. Pat. No. 3,260,505 does not provide many details as to the method of producing such columnar structured turbine blade castings. What is disclosed is essentially that a shell mold is supported, with its bottom open end on a support member which can be chilled and which will remain substantially colder than the body of the mold during the casting operation. The mold and its supporting chill are positioned in a vacuum furnace prior to casting. The mold is heated to above the melting temperature of the metal to be cast by means of resistance heating with molybdenum heater wire and the metal is poured therein. in describing the preparation of a casting made from an alloy, known then as SM-200 but now known as MAR M 200, Ver Snyder uses the following language to describe his steps of cooling the casting: Subsequent to pouring, the power to the mold assembly heater is reduced in a stepwise manner after a holding period of approximately 20 minutes, after the furnace and casting are allowed to cool, air or nitrogen gas is introduced into the furnace, and the castings are removed from the furnace.

Prior to the issuance of U.S. Pat. No. 3,260,505, present applicants received a substantial amount of information regard ing the state of the art as to thermally nucleated, columnar grained castings from United Aircraft Corporation, the assignee ofU.S. Pat. No. 3,260,505. Essentially, this information supplemented, rounded out and brought up to date the process disclosures of U.S. Pat. No. 3,260,505. It was learned from United Aircraft that columnar grained castings were made under ultrahigh vacuum in ceramic shell molds similar to those used in conventional precision casting. It was learned that the shell molds were open at the bottom and were designed to sit on a solid copper plate cooled by water flowing through coils brazed to the underside of said plate. It was further learned that mold heating was accomplished by means of a tubular graphite susceptor fixed in position above and concentric with said plate; that the susceptor was heated by means of an induction coil; that the water-cooled copper plate and the susceptor and heating coils were fixed in position in a vacuum chamber; and that induction melting apparatus, a

pouring device, heat sensors and various control elements were also positioned in said vacuum: chamber. The operation of the apparatus was explained to be that remelt metal stock, such as an alloy known commercially as MAR M alloy 200 and further identified hereinafter, was charged into a melting crucible in a vacuum furnace which at the time of charging was not under vacuum. A mold which was positioned on the chill plate externally to the vacuum furnace was positioned in the furnace. Vacuum connections were made around the water feed system whichfeed system also comprised the chill supporting mechanism. The furnace was closed; a vacuum was drawn; water flow was initiated; the mold was heated to about 2,500F. to 2,800F. The metal was melted and outgassed and was poured at about 2,700F. into the mold. Once pouring is complete, the power to the susceptor is reduced such that the temperature of the mold gradually reduces to below the solidus of the alloy being cast. The furnace arrangement is such that at any time during the cooling there is a temperature gradient in the mold between any two longitudinally separated points thereon such that the lower of said two points is the cooler. Once this cooling operation is completed, the entire furnace including the mold and the susceptor are allowed to cool freely to room temperature or to some intermediate temperature at which it is safe to break the vacuum with air or an inert gas and remove the casting. All told, the process as disclosed by United Aircraft takes several hours to complete and usually produces acceptable castings ofthe type required.

The manufacture of gas turbine structures comprised of a single grain or crystal is disclosed in United Aircraft Belgian Pat. No. 68l,776 and Netherlands specification No. 6,607,354. Essentially single crystal structures are made from high temperature alloys using a special type of ceramic shell mold adapted to be used in apparatus such as employed by United Aircraft in producing columnar crystalline structures. Essentially the special type of ceramic shell mold includes a restricted passage in communication on one end with a cavity abutting on a water-cooled chill plate and on the other end with a cavity in the shape of the object to be produced. In most other respects the known process of producing single crystal castings as disclosed in the Belgian and Netherlands documents is similar to the process of producing columnar crystalline castings.

lt is understandable that a casting process taking several hours to produce a limited number, e.g. up to 20, of cast articles is bound to be expensive. Furthermore, holding molten metal in a hot, ceramic shell mold for any extended period of time entails certain difficulties. The material of the shell mold is not immune to the effects of temperature and the metallostatic pressure imposed by the head of molten metal. Under these conditions the mold material can in time sag out of the critical dimensions required in turbine blade castings and deleterious metal-mold reaction can occur. Approximating equilibrium conditions considered advantageous by Ver Snyder in U.S. Pat. No. 3,260,505 at column 6, lines 57 to 64, can occasionally be deleterious. As equilibrium conditions are more closely approached, the slowly advancing solidification front can provide a zone refining effect which, in highly complex gamma prime super alloys, can result in at least partial segregation of vital alloying elements in the upper part of the casting or in feeder heads which are discarded.

It has now been discovered that by a combination of furnace design and the use of a solidification process particularly adapted to be carried out in said furnace, castings of metal freezing at temperatures above about 2,250F. (1,232C.) having columnar grained structures or single crystal structures can be made in the usual turbine blade sizes in a production cycle approximating 1 hour total time. This represents a substantial improvement over the present day technology of those persons most skilled in the particular field of producing thermally nucleated, columnar structured cast articles and cast articles comprising a single crystal.

It is an object of the present invention to provide an improved process for producing castings having an as-cast, columnar crystalline habitat.

Another object of the present invention is to provide an improved process for producing castings comprising a single crystal of metal.

A still further object of the present invention is to provide a novel apparatus for carrying out the process of the present invention.

An additional object of the present invention is to provide particular assemblages comprising parts of an apparatus for carrying out the process of the present invention.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawings in which:

FIG. I is a perspective view of an embodiment of the apparatus of the present invention less certain items which have been omitted for simplicity;

FIG. 2 is a view of the cross section of the apparatus of FIG. I cut by plane ll-ll-lI-ll'.

FIG. 2A is an enlarged view of the central portion of an apparatus similar to that depicted in FIG. 2;

FIG. 3 is an exterior view of a novel chill plate in accordance with the present invention;

FIG. 4 is a sectional view of the chill plate of FIG. 3along line IV-IV; I

FIG. Sis a sectional view olthe chill plate of FIG. 3along line V-V;

FIG. 6 is a schematic view in cross section illustrating the flow of energy during one stage of the process of the present invention; and

FIG. 7 shows in cross section a type of ceramic shell mold as disclosed in Belgian Pat. No. 681,776 used to produce a turbine blade having a structure comprising a single grain (crystal).

Generally speaking, the present invention contemplates a casting process in which a mass of molten metal is confined, advantageously in an elongated configuration, and heat is withdrawn, advantageously longitudinally, from the mass of molten metal primarily by conduction from only one end of the confined configuration to establish a rapidly advancing thermally nucleated solidification front traversing the length of the confined configuration to induce columnar crystalline habitat in the thus frozen metal. Simultaneously with the advancement of the solidification front a substantial portion of the heat in the frozen (solidified) metal is withdrawn laterally, usually primarily by means of radiation, from that region of themass situate between the advancing solidification front and that end of the elongated mass from which heat is being withdrawn primarily by conduction. It is to be noted that this process, like the prior Ver Snyder techniques, involves thermal nucleation of the solidification front as distinguished from prior laboratory processes involving nucleation by seeding.

As will be observed in connection with the description of the drawings, the lateral withdrawal of heat mentioned above is accomplished by providing for relative movement between a heat radiating means and the solidifying metal such that, when frozen, the metal is outside the radiant heating zone. By this means a rapid loss of heat from the solidified metal can occur, said heat being absorbed by a radiant heat sink. Advantageously, the aforementioned radiant heating zone is positioned above the heat sink zone and the solidifying metal is withdrawn downwardly from the radiant heating zone to the zone containing the radiant heat sink. By means of this process columnar structured and single crystal castings can be made rapidly and efficiently, particularly when such castings are made in ceramic shell molds.

In prior art processes relating to the production of columnar structured and single crystal castings by thermal nucleation, an effort was made to induce heat losses by radiation by reducing power fed to sections of the radiant heating zone, a concept which can be a useful assist in the present process. However, no substantial heat loss by radiation can occur rapidly using the reduction of power concept alone. As is well known, radiant heat transfer between two surfaces is a function of the difference between the absolute temperatures of said surfaces both taken to the fourth power, i .e.

In practice, hdiant-heat-zone materials of construction, e.g. graphite susceptors, will not lose heat very rapidly thereby making the difference between T, (the temperature of a cooling mold) and T (the temperature of a cooling susceptor) rather small. Furthermore, the usual monolithic graphite susceptor cannot mechanically withstand the thermal stresses induced by large differences of temperature along its length even if large temperature differences could be maintained. In the usual instance, the application of power to any portion ofa monolithic graphite susceptor will result in an increase in temperature even in a nonpowered area of the susceptor because graphite is a reasonably good conductor of heat, even allowing for the nonisotrophic nature of the material. When the heat conductivity of the material of construction of the radiant heat source is equal to or exceeds the heat conductivity of the solidified metal of the casting (as is the case with a graphite susceptor and nickel-base or cobalt-base turbine structure al loys), it is nearly impossible to maintain any appreciable heat gradient between the susceptor and the solidified metal merely by applying power to only a portion of the monolithic susceptor. Even if one uses a susceptor made in two or three sections with insulation therebetween, the turning off of power on one section will not transform that section into an effective heat sink. The necessary insulation on the outer surface of the section will prevent any rapid loss of heat. Thus by means of the process of the present invention which contemplates establishing and maintaining an effective radiant heat gradient by means of relative movement, one can produce the desired columnar structured or single crystal product with far greater speed and efficiency than has heretofore been possible.

In carrying the present invention into practice, one can employ a furnace such as depicted in FIGS. 1, 2 and 2A. As mentioned hereinbefore, many details have been omitted from FIGS. 1 and 2 for purposes of clarity. However, many such details will be mentioned herein in order to provide as complete a disclosure as is possible commensurate with necessary brevity. Referring now to FIGS. 1 and 2, vacuum furnace 11 comprises upper chamber 12, lower chamber 13 and water-cooled valve 14 therebetween. Both chambers are made of suitable material such as steel and are closeable by doors, not illustrated. It is to be noted that, while chambers 12 and 13 are illustrated to be of substantially cubic construction, any suitable shape, e.g. cylindrical may be employed. As illustrated, upper chamber 12 has water jacketed floor 15, side walls 16 and 17, waterjacketed top 18 and rear wall 19 having vacuum duct 20 positioned therein. Lower chamber 13 is similarly constructed, although fully water-jacketed, with floor 21, side walls 22 and 23, top 24 and rear wall 25 having vacuum duct 26 positioned therein. It is advantageous to have the entire furnace enclosure water jacketed or otherwise suitably cooled to dissipate heat caused by radiation losses from the furnace hot zones. Ducts 20 and 26 lead to a vacuum system, not illustrated. This vacuum system can be conventional in nature comprising roughing pumps and diffusion pumps in toto capable of maintaining chambers 12 and 13 at a pressure not exceeding about 1 micron of Hg. for optimum results. Another feature of the vacuum system is that, advantageously, it is mechanically isolated from furnace 11 to prevent, insofar as possible, vibration of furnace 11 during casting operations.

In the interior of chamber 12 there is positioned melting crucible 27 powered by an induction coil not visible in the illustration. Melting crucible 27 can be fed with melting stock by means of a vacuum lock (not illustrated) passing through top 18. Melting crucible 27 can be tilted into pouring position by means of axle 28 turned by a motor drive with a clutchbrake device 29. Induction coil 30 internally cooled by water is connected through wall 16 at vacuum tight ports 31 and 32. Induction coil 30 is positioned above and concentric with port 34 in floor and port 33 in top 24 and is supported by transite supports 35. Directly under induction coil 30 is positioned radiation heat sink 36 comprising, for example, a block of foamed carbon insulating material having a hole in the middle thereof significantly smaller in diameter than the diameter of ports 33 and 34. This hole fits concentrically over port 34 forming a passage between lower chamber 13 and the interior of induction coil 30 when water cooled valve slide 38 is in the open position. Concentric with and within induction coil 30 is susceptor 37 made of a high purity graphite. Susceptor 37 has the form of an open ended tube and has an inside diameter about equal to the diameter of the hole in heat sink 36 and considerably smaller than the diameter of ports 33, 34 and the opening in valve 14. Between induction coil 30 and susceptor 37. is a layer of heat insulating material 78 such as graphite felt.

Directly under port 33 and passing through floor 21 of lower chamber 13 is port 39 through which rod 40 of hydraulic piston 41 passes. Seal 42 assures vacuum tightness at the point where rod 40 passes through floor 21. One end of hydraulic cylinder 43 in which piston 41 operates is attached to floor 21 on the underside thereof. The other end of hydraulic cylinder 43 is closed. Ports 44 and 45 adjacent to each end of hydraulic cylinder 43 permit the introduction of fluid under pressure to cause movement of piston 41 and rod 40. Piston 41 advantageously is elongated and/or finned to assure vibration free movement of rod 40.

Water cooled chill plate 46 is carried atop rod 40 removably attached thereto by means of fixture 47. Metallic armored flexible water lines 48 and 49 which feed water to and from chill plate 46 pass through side wall 23 by means of vacuum tight fittings 50 and 51. Thermocouple leads in a bundle 52 lead from contact board 54 through side wall 22 by means of vacuum tight fitting 53 and thence to thermal recorders not illustrated. Thermocouple wires 55 are fixed on one end on contact board 54, the other ends are cemented on the exterior surfaces of ceramic shell mold 56 and pass around the exterior of chill plate 46 as shown on FIG. 2 and more clearly in FIG. 2A.

Details of the novel chill plate 46 of the present invention are shown in FIGS. 3 to 5. Basically, chill plate 46 comprises an upper section 57 and a lower section 58 brazed together to form a vacuum seal. Lower section 58 has vacuum tight water supply coupling fittings 59 positioned thereon. As shown in FIGS. 4 and 5, upper section 57 is unitary and comprises a thin face section 60 and a generally spiral water channel 61 with which the water channels in fittings 59 communicate. Web sections 62 which add mechanical stiffness to face 60 in addition to channeling the water integral with face section 60 abut against lower section 58 and are brazed thereto. With respect to chill plate 46, it is to be noted that the inner of the two water access fittings is eccentrically located to facilitate positioning on top rod 40. Top face 63 of chill plate 46 is advantageously concentrically grooved or otherwise patterned, a particular feature of chill plates which is not novel with present applicants. In addition, as shown in FIG. 5, top face 63 can be recessed to facilitate cementing molds thereto. Cement for sealing the mold chill plate interface can be placed in undercut 79 and thus held mechanically in place. When, as is advantageous, chill plate 46 is made of copper, face section 60 need be no thicker than about one-fourth inch inorder to provide good thermal transfer. Adequate mechanical strengthis provided by fins 62.

Ceramic shell mold 56, which is shown to rest with its open.

face downward on chill plate 46, is substantially identical to the shell molds described by Ver Snyder in U.S. Pat. No. 3,260,505. It differs from usual shell molds in several ways. First, the article portions of the mold have no bottom, the bottom being in effect surface 63 of chill plate 46. Second, the shell molds are carefully constructed to avoid cracks. Cracks which would cause only minor flash in normal precision castings produced in accordance with the usual commercial practice can be the cause of complete failure when present in shell molds used to produce columnar structured or single crystal castings. Third, the molds must be designed to provide for an initially solidifying cast portion which is discarded. In the present process. as in the processes which it supersedes, the metal initially crystallizing upon contact with chill plate 46 crystallizes in an equiaxed fashion. This equiaxed area must be cut off the final casting and thus the molds must be designed with this in mind. As a final point, molds for use in the present process have no need for the complex of gates and risers normally provided in casting molds. Since solidification proceeds from one direction only, adequate feeding of the mold is assured by the use of only rudimentary headers. Consequently, the useful portion is a major portion of the metal cast rather than otherwise which is the usual situation prevailing in conventional casting.

In carrying out the process of the present invention in the apparatus depicted in FIGS. 1 through 5, and in any apparatus equivalent thereto, one starts by tuming on all water cooling systems except that which is connected to the chill plate. Next rod 40 is lowered into chamber 13. Chamber 12, sealed from chamber 13 by valve slide 38, is evacuated by means of vacuum duct 20. Open bottom ceramic shell mold 56 (or any equivalentthereof) is preassembled on top of chill plate 46 and both are mounted atop fixture 47, while in chamber 13.

Prior to scaling chamber 13, any necessary thermocouple connections are made between thermocouple units on mold 56 and contact board 54. Once chamber 13 is sealed, water flow is initiated to chill plate 46 and maintained continuously during operations. A vacuum is then drawn through duct 26 to evacuate chamber 13 in the manner usual to vacuum furnace operation. When the atmospheric pressure in chamber 12 is sufficiently low to prevent oxidation of graphite susceptor 37, induction coil 30 is energized to initiate heating of the susceptor. This initial heating is usually done rather slowly in order to avoid breaking susceptor 37. In the meantime, the ceramic shell mold loading, as described above, can be carried out. When the atmospheric pressure in chamber 13 is sufficiently low to prevent excessive oxidation of the graphite susceptor 37 in chamber 12, the valve slide 38 is open and rod 40 bearing chill plate 46 and mold 56 is raised hydraulically to the position depicted in FIG. 2, that is to a position where chill plate 46 essentially forms a bottom for susceptor 37. In this configuration susceptor 37 constitutes a heat radiating means. It is to be observed that while susceptor 37 is illustrated in FIGS. 1 and 2 to be heated by induction, it is possible by use of high melting point resistance materials to heat the susceptor resistively. Furthermore, it is also possible to fashion high melting point materials such as molybdenum wire, strip orfoil into supported or self-supported resistance heating elements which by themselves will act as a radiant heating means. It is to be observed that in reference to FIGS. 1, 2 and 6 it is somewhat misleading to designate only block 36 as a'radiant heat sink since in fact every water-cooled furnace part below block 36 is also an effective radiant heat sink. Block 36 is constructed of an unusual material, i.e. foamed graphite, simply because of its proximity to an inductive electrical field, the necessity that the block be machinable and the fact that it must resist high temperatures. If susceptor 37 were to be heated by resistance methods, there would be no reason why water-cooled, properly surfaced, heat sinks could not be used in place of a major portion of heat sink block 36. As a practical matter, however, even in such circumstances heat insulation would have to be provided between the susceptor and the water-cooled heat sink to prevent undue thermal stresses.

While mold 56 is being heated radiantly by susceptor 37, crucible 27 is charged with alloy and activated to melt the alloy and bring it to a specified temperature above the alloy liquidus. Charging of crucible 27 can be done either prior to sealing chamber 12 or by means of a vacuum lock such as mentioned hereinbefore. When mold 56 and the molten metal are both at a suitable temperature, e.g. 2,750F. with certain nickel-base turbine blade alloys, the metal can be poured into mold 56. For purposes of clarity in FIGS. 1 and 2, full details concerning the radiant heating zone including susceptor 37 have not been illustrated. Additional details are depicted in FIG. 6 which shows susceptor 37 topped by radiation shield 64. Radiation shield 64 like radiation heat sink 36 is made of foamed graphite and has a central port 65, closeable by graphite foam plug 66. Graphite plug 66 is removed and replaced by conventional means (not illustrated) acting through the walls or top of chamber 12. Of course, when metal is poured into mold 56 it is poured through central port 65 with plug 66 removed. After pouring, plug 66 is immediately replaced to close port 65. Further reference to FIG. 6 will be made hereinafter.

When the poured liquid metal touches chill plate 46, it solidifies substantially instantaneously in an equiaxed crystalline mode. Heat flows by conduction from the solidified metal into chill plate 46 to initiate an advancing solidification front. Substantially immediately, for example within about one-half minute after completing pouring of the metal into mold 56, rod 40 carrying chill plate 46 and mold 56 is started moving downwardly at a steady rate in a substantially vibration-free manner. All during the withdrawal of rod 40, power is fed to susceptor 37 in order to prevent crystalline nucleation in the molten metal in the upper part of mold 567 The heat flow prevailing during the downward withdrawal of rod 40 is depicted schematically in FIG. 6 together with an indication of the manner in which the metal solidifies. in this figure the principle areas of heat (or energy) flow is indicated by arrows associated with subscripted fs. The significance of these designations is set forth in table l.

TABLE 1 radiate. As long as an appreciable amount of heat is withdrawn by chill plate 46, the vector direction of heat flow at the solid-liquid metal interface will be downward thus providing the directional gradient needed to continue the advancement of the solidification front in a longitudinal direction. The solidification front remains within the confines of susceptor 37. It is to be appreciated that the identical heat flow effects can be produced by raising the radiant heating zone and the heat sink and allowing the mold and the chill plate to remain stationary. It is also possible to provide for motion of both the heating-heat sink zones and the chill platemold assembly.

When the top of mold 56 carried on chill plate 46 is brought to a temperature below the solidus of the alloy being cast by means of the heat extraction process described hereinbefore, rod 40 is then rapidly dropped to bring mold 56 fully into chamber 13. Controlled speed withdrawal of rod 40 can be conveniently accomplished by passing hydraulic fluid through a needle valve in the control circuit of hydraulic cylinder 43. Rapid dropping of rod 40 is then accomplished by bypassing the needle valve and permitting full flow of hydraulic fluid to act upon piston 41. Of course, mechanical and electrical analogues of this hydraulic system can be used to activate rod 40 or any other lifting means equivalent thereto. When mold 56 is clear, valve 14 is activated to move slide 38 into closed position. The valve in the vacuum circuit, mentioned hereinbefore, is then closed to isolate chamber 12 under high vacuum. In the meantime, mold 56 is rapidly cooling by free radiation in chamber 13. After a few minutes the vacuum in chamber 13 is broken, mold 56 is removed together with its contained casting and a new mold 56, preferably cured somewhat, mounted on chill plate 46 external to chamber 13 is placed on fixture 47. Chamber 13 is rescaled, and a vacuum is drawn through duct 26. When a satisfactory vacuum is attained, valve slide 38 is opened, rod 40 is driven up to the position shown in FIG. 2 and the mold heating, pouring and withdrawing cycle is repeated. With reasonable skill and turbine parts about 4 to 8 inches long the casting cycle can be accomplished in about 1 hour with a production of a cluster of up to 20 or more turbine blade castings. It is to be noted that during a production run producing turbine blades up to about 8 inches long there is no need at any time to cool susceptor 37 to any appreciable extent except for energy conservation purposes while the molds 56 and contained castings are cooling in Forced convection heat drain to cooling, water. Conduction of heat through ceramic wall of mold an.

f0 Radiant ll'flllSltJl' of heat from mold so to heat sink an. L. Radiant and conductive losses of heat from heat sink 3U.

. Conduction of heat through solidified cast metal and copper of chill plate 415.

In attempting to account for the extraordinary success of the presently disclosed process in producing columnar grained and single crystal castings rapidly and efficiently (for example, rod 40 can be withdrawn at a rate of 25 inches per hour or faster), the heat flow symbols of H6. 6 can be enlightening. It is known that the heat conductivity of most nickel-base gamma prime alloys or cobalt-base alloys is very poor. The lower the temperature, the lower the coefficient of thermal conductivity becomes. Thus when the distance between the interface between solid and liquid metal and surface 63 of chill plate 46 becomes appreciable, this poor conductivity becomes the limiting factor of heat loss by conduction. However, in the present process, as the heat loss by conduction to chill plate 46 becomes less and less, the heat loss by radiation to the radiation heat sink becomes greater due to the maintenance of a substantially constant temperature at the liquidsolid interface and the greater wall area of mold 56 free to chamber 13, a new mold is loaded and vacuum is being drawn. Thus the useful life of susceptor 37 is extended due to absence of undue thermal stresses. When large castings, for example castings longer than about 8 inches, are to be produced, it is advantageous for speed of operation to use a split induction coil which will enable cooling of the lower portion of susceptor 37 while the upper portion remains hot. In such instances it can be advantageous to use twoor three-part susceptors to avoid undue thermally induced stresses therein.

As will be recognized by those skilled in the art, exact conditions of operation, such as metal and mold temperatures withdrawal rates, mold heating rates and the like, will be dictated by the nature of the metal being cast, the nature of the mold materials and the like. As has been recognized by Ver Snyder and his coworkers, many alloys useful in turbine engines are amenable to being cast into columnar structured castings. Nickel-base alloys and cobalt-base alloys having compositions within the ranges set forth in table 11 have been recognized by Ver Snyder as usable in columnar structured castings.

process of producing such castings. However, such castings can be made rapidly and efficiently by means of the process and apparatus of the present invention. A rather complex shell mold of the type used to make a single crystal casting is depicted in FIG. 7. Referring now thereto, complex shell mold 67 is depicted with open bottom 68 resting on the top face 63 of chill plate 46. Complex shell mold 67 substantially encloses main cavity 69 in communication with surface 63 of chill plate 46 via restricted channels A and C and cavity 68. As metal is cast into the mold of FIG. 7 columnar crystal growth initiates at face- 63 of chill plate 46. When the solidification front reaches restricted channel A only one or at most a few of the columnar crystals continue growing therein. At corner B of restricted channel A all but one crystal is eliminated such that in continuation C of restricted channel A only a single crystal exists. This single crystal is propagated throughout main cavity 69 to form a single crystal structure in said cavity. After all metal has solidified and cooled, those portions of metal below the base of main cavity 69 are cut off together with riser portions to leave only the desired structure having a single crystal.

While, at present, single crystal and columnar structured castings have their most practical uses in connection with turbine structures such as rotor blades, stator guide vanes and the like produced using nickel-base and cobalt-base alloys, the present process and apparatus is not limited to the production of such structures from such alloys. Any metal, metalloid or, more broadly, meltable solid having a solidification temperature in excess of that temperature at which radiation heat losses can be significant can be treated according to the process of the present invention. While there is no specific temperature above which radiation losses become significant (since a curve solely dependent on T is a smoothly accelerating ascending curve) and below which they are insignificant, for practical purposes, any meltable solid, stable in the molten condition, freezing at a temperature above red heat will be amenable to processing in accordance with the present invention. An example of a substance which can be produced in single crystal form by means of the present invention is doped silicon useful for electronic purposes. Other high melting semiconductive substances can also be employed.

in the present specification most of the exemplifying subject matter has concerned vacuum casting of alloys sensitive to oxidation while in the molten state. it is possible to employ the principles of the present invention with atmospheric casting either with atmospheres of inert gases or with reducing or oxidizing atmospheres as long as precautions are taken to limit TABLE 11 Nickel- Cobaltbase bas alloys, alloys, percent percent by by weight weight Element:

Chromium. 2-25 -27 Cobalt 0-30 (1) Tungsten 5-12 Molybdenum 2 14 v 15 Aluminum-" 0-1 Titanium 0-1; 0-1 Aluminum plus titan] Carbon 0.1-0.5 0. 4-12 'Boron. n ximum. r. 0. 005-01 0.01 Zirconium 0. 05-0. 2 005-15 Vanadium. maximum 1.5 Iron, maximum 5 1. 5 Manganese, maximum r 1.0 0. 2 Silicon, maximum 1.0 0.2 Tantalum 0-10 Columbium. 0-3 Nickel... 0-12 Base of alloy. About 35. 3 Base of alloy 35.

it is now recognized that nickel-base and cobalt-base alloys within the ranges set forth in table 11 and other nickel-base and cobalt-base alloys can be used in the process of the present in vention. Not all alloys within the tabulated ranges are equally benefited by the process of the present invention especially with respect to strength characteristics However, most high temperature nickel-base and cobalt-base casting alloys show substantially improved thermal fatigue characteristics when produced in columnar crystalline form or in single crystal form provided, that in any transverse cross section of an article cast in columnar crystalline form, there is a multiplicity of columnar crystals. A cast article having a transverse cross section showing only two or three crystals is usually unbelievably poor in thermal fatigue resistance.

The nominal chemical compositions in weight percent of commercially available nickel-base and cobalt-base alloys 5 recognized by Ver Snyder to be amenable to columnar crystalline processing are set forth in table 111.

TABLE I11 Alloy (1r .\[0 CI) Ti Al 1% Z1 ()0 Ni \l' Tu F0 V l lnconel is a trntlenmrk of The international Nickel Company Inc.

Mar Al is a trademark of Martin Marietta Corporation. Tin: sold under the trademark S-M.

In this alloy Girl-Tn totals 2731); weight.

TABLE IV Alloy, B1900:

Cr, 8.0; M0, 6.0; Cb, 0; T1, 1.0; A1, 6.0; B, .015; Zr, .10; Co, 10.0; C, .10; Ni, 1331.; W, 0; Ta, 4.0; Fe, 0.

'Heretofore in the present specification, mention has been made of single crystal castings. No claim is made herein that present applicants are the inventors or originators of the basic alloys sold under this trademark were formerly uncontrolled convective cooling of the castings during the critical period of solidification. It is, of course, necessary in any case to avoid conditions such as vibration, foreign particles and the like which will induce secondary nucleation and subsequent growth of equiaxed or misoriented grains. Such nucleation is particularly apt to occur in the edge portion of thin sections of turbine blades. After cooling and chemical or electrical etching, these improperly nucleated equiaxed grains have a freckled appearance. One advantage of the process of the present invention is that by virtue of the large thermal gradient at the liquid-solid interface the incidence of freckled castings is much lower than in the processes of prior workers in the field of single crystal and columnar-structured castings. While the present invention has been described in conjunction with advantageous embodiments, those skilled in the art will recognize that modifications and variations may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention.

We claim:

1. A process for producing directionally solidified castings made of a meltable material comprising establishing a first zone wherein the temperature is maintained above the liquidus of said material, establishing a heat extractive zone adjacent said first zone, confining said material at a temperature above its liquidus in said first zone, extracting heat from said material confined in said first zone in a first direction to induce the formation of a thermally nucleated solidification front advancing in a second direction opposite said first direction and away from said heat extractive zone and substantially immediately thereafter, while maintaining the extraction of heat in said first direction, providing continuous relative motion between said zones and said confined material such that said solidification front remains within said first zone and the frozen material behind said solidification front continuously enters said heat extractive zone and said frozen material continuously loses heat from a continuously increasing frozen surface area in said heat extractive zone in directions normal to said first direction.

2. A process for producing directionally solidified castings made of a meltable metal comprising establishing a first zone wherein the temperature is maintained above the liquidus of said metal, establishing a heat extractive zone adjacent said first zone, confining said metal in molten condition in said first zone to provide an elongated shape, extracting heat from only one end of said elongated shape to induce the formation of a thermally nucleated solidification front advancing along the length of said shape and away from said heat extractive zone and substantially immediately thereafter, while maintaining the extraction of heat from said one end, providing continuously relative motion between said zones and said confined metal such that said solidification front remains within said first zone and the frozen metal behind said solidification front continuously enters said heat extractive zone and loses heat in lateral directions from the continuously increasing surface area in said heat extractive zone.

3. A process as in claim 2 wherein the metal is a heat resistant alloy and the loss of heat in lateral directions is primarily by radiation.

4. A process as in claim 3 wherein heat is extracted from one end of said elongated shape primarily by conduction.

5. A process as in claim 4 wherein the elongated shape is essentially the shape of a turbine blade with the largest crosssectional area thereofbeing adjacent that end from which heat is extracted by conduction.

6. A process for producing directionally solidified castings made of a meltable material comprising establishing a first zone wherein the temperature is maintained above the liquidus of said material, establishing a heat extractive zone adjacent said first zone, confining said material at a temperature above its liquidus in said first zone, extracting heat from said material confined in said first zone in a first direction to induce the formation of a thermally nucleated solidification front advancing in the direction opposite said first direction and away from said heat extractive zone and substantially immaterial in said first direction such that said solidification front remains within said first zone and the frozen material behind said solidification front continuously enters said heat extractive zone and continuously loses heat in directions normal to said first direction from the continuously increasing surface area in said heat extractive zone.

7. A process as in claim 6 wherein the directionally solidified castings produced thereby have a columnar crystalline stnicture and said confined solid is confined in the shape ofa turbine blade.

8. A process for producing directionally solidified castings made of a meltable metal comprising establishing a first zone wherein the temperature is maintained above the liquidus of said metal, establishing a heat extractive zone adjacent said first zone, confining said metal in molten condition in said first zone to provide an elongated shape, extracting heat from only one end of said elongated shape to induce the formation of a thermally nucleated solidification front advancing along the length of said shape and away from said heat extractive zone, and substantially immediately thereafter, while maintaining the extraction of heat in said first direction, continuously moving said confined metal in said first direction such that said solidification front remains within said first zone and; the frozen metal behind said solidification front enters said heat extractive zone and continuously loses heat in lateral directions from a continuously increasing lateral surface within said heat extractive zone.

9. A process as in claim 8 wherein the metal is a heat resistant alloy and the loss of heat in lateral directions is primarily by radiation.

10. A process as in claim 8 wherein heat is extracted from one end of said elongated shape primarily by conduction.

11. A process for producing directionally solidified, elongated alloy articles comprising establishing a radiant heating zone radiating sufficient energy to maintain the alloy from which said articles are to be made in the molten condition, establishing a radiation heat sink zone below and adjacent to said radiant heating zone, confining molten alloy in an elongated shell mold in said radiant heating zone, the bottom of said shell mold comprising a chill plate, extracting heat from said confined alloy through said chill plate to initiate the formation of a thermally nucleated solidification front advancing from the bottom of said shell mold toward the top thereof and, substantially immediately thereafter, while maintaining the extraction of heat through said chill plate, gradually and continuously lowering said confined alloy into said radiation heat sink zone such that said solidification front remains within said radiant heating zone and the frozen alloy behind said solidification front continuously radiates heat from a continuously increasing lateral area to said heat sink zone.

12. A process as in claim 11 wherein the alloy articles are turbine structures and the radiant heating zone and the radiation heat sink zone are both held under a high vacuum.

13. A process as in claim 12 operated in cyclical fashion wherein after all the alloy is frozen, the radiant heating zone and a part of the radiant heat sink are isolated and vacuum is maintained thereon, the thus frozen alloy article is removed from the evacuated environment and the process is repeated by first confining a fresh batch of molten alloy in said radiant heating zone followed by the remaining steps.

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