Abstract:
A grain starter capable of nucleating a multiplicity of grains in a casting is positioned within a mold. The mold is filled with molten metal and a solidification interface is caused to pass from the grain starter through the molten metal by immersing the mold in a cooling bath to form a casting that has a multiplicity of grains nucleated by the grain starter.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for controlling grain spacing of a columnar superalloy casting. 
     A superalloy includes nickel, cobalt, nickel-iron or iron-based heat resistant alloys that have superior strength and oxidation resistance at high temperatures. The superalloy can contain chromium to impart surface stability and one or more minor constituents, such as molybdenum, tungsten, columbium, titanium or aluminum, for strengthening purposes. The physical properties of a superalloy make it particularly useful for the manufacture of a gas turbine component. 
     A grain is an individual crystal in a polycrystalline solid. A grain boundary is an interface between individual crystals. The crystal grain characteristics of a superalloy can determine superalloy properties. For example, the strength of a superalloy is determined in part by grain spacing. At low temperatures, grain boundaries impede dislocation motion. Hence, fine grain equal axial structures are preferred for low temperature applications. At high temperatures, deformation processes are diffusion controlled. Diffusion along grain boundaries is much higher than within the grains. Hence, large-grain spacing structures can be stronger than fine grain structures in high temperature applications. Generally, failure originates at grain boundaries oriented perpendicular to the direction of an induced stress. By casting a superalloy to produce an elongated columnar structure with unidirectional crystals aligned substantially parallel to the long axis of the casting, the number of grain boundaries normal to the primary stress axis can be minimized. 
     Directional solidification is a method that is used for producing turbine blades and the like with columnar crystalline structures. Generally, a crystalline growth structure is created at the base of a vertically disposed mold defining a part and a solidification front is propagated through the structure under the influence of a moving thermal gradient. During directional solidification, crystals of nickel, cobalt or iron-based superalloys are characterized by a “dendritic” morphology. Dendritic refers to a form of crystal growth where forming solid extends into still molten liquid as an array of fine-branched needles. Spacing between the needles in the solidification direction is called “primary dendrite arm spacing.” Spacing of side branches or arms along a needle&#39;s length is termed “secondary dendrite arm spacing.” Both primary and secondary dendrite arm spacing are functions of cooling rate. Cooling rate is the product of solidification rate and thermal gradient at a solid liquid interface. 
     Solidification rate kinetics vary with crystallographic orientation. For a fixed driving force, the solidification rate in nickel based superalloys is typically highest in the crystallographic unit cell edge direction (&lt;100&gt; direction). 
     One desired macrostructure of a directionally solidified superalloy consists of grains elongated along the direction of solidification so that grain boundaries are aligned in the solidification direction. It is further desired that the crystallographic unit cell edge direction &lt;100&gt; of the grains be parallel to the solidification direction to provide improved mechanical properties. If two grains are growing side-by-side into a temperature gradient, the grain with growth axis closest to a &lt;100&gt; direction grows faster. The faster growing grain also spreads laterally. Lateral spread of a grain occurs by growth of secondary arms. If two grains are growing side by side into a liquid, and one grain leads the other by secondary arm spacing, the leading grain will extend a secondary arm in front to pinch off the lagging grain. This phenomenon is termed “competitive growth.” Until competitive growth achieves a structure of only grains close to the &lt;100&gt; direction, the grain boundaries will not be parallel and along the axis of the cast part. The section of a casting where the grains are competing to establish parallel growth is unusable as a turbine part and must be discarded. 
     A need exists for a directional solidification process that can produce columnar castings characterized by prescribed grain spacings. Additionally, a need exists for a directional solidification process that provides an increased proportion of casting characterized by aligned parallel axis oriented crystals. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method of controlling grain spacing of a casting and to the product of the method. In the method, a grain starter that is capable of nucleating a multiplicity of grains, is positioned within a mold. The mold is filled with molten metal and a solidification interface is caused to pass from the grain starter through the molten metal by immersing the mold in a cooling bath to form a casting that has a multiplicity of grains nucleated by the grain starter. 
     In an embodiment, a grain spacing is determined for a columnar article. A grain starter is selected that has a grain spacing determined to provide the grain spacing in the article when the article is cast in a liquid metal cooled directional solidification process. The grain starter is positioned in a mold and the mold is filled with molten metal. A solidification interface is caused to pass from the grain starter through the molten metal by immersing the mold in a liquid metal cooling bath to form an article having a grain spacing determined by the grain starter spacing. 
     In still another embodiment, the invention relates to a method of producing a cast article with a substantially increased proportion of parallel axis grain structure. The method comprises making a fine grain superalloy multicrystalline grain starter capable of nucleating a multiplicity of grains in a solidifying casting. The grain starter is provided within a mold and the mold is filled with molten metal. A solidification interface is then caused to pass from the grain starter through the solidifying casting by immersing the mold in a cooling bath to form a casting that has a multiplicity of grains nucleated by the grain starter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a microstructure with heterogeneous grain nucleation; 
     FIG. 2 is a schematic representation of a microstructure growth from a fine-grain grain starter; and 
     FIG. 3 is a schematic sectional view of a furnace for conducting a directional solidification process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the invention, A grain starter, as embodied by the invention, is characterized by many, fine grains. The grain starter is used at the base of a directionally solidified casting to assure emergence of &lt;100&gt; grains. The grain starter nucleates fine grains of all orientations. In all locations, there will be some grains with &lt;100&gt; orientation. The &lt;100&gt; grains, by competitive growth, pinch off the other orientations. 
     FIG. 1 illustrates a casting produced by a liquid metal cooling process without use of the grain starter of the present invention. In FIG. 1, the superalloy casting is made by pouring liquid metal into a mold or melting metal in situ. A moving temperature profile is generated along the casting, for example by lowering the mold out of a furnace. At the beginning of the solidification process, a few solid crystals nucleate from the liquid metal in the coldest region of the mold. These crystals are typically of random orientation. As directional solidification progresses, the grains with nearest &lt;100&gt; orientation pinch off the other grains. This competitive growth process can result in misoriented grains that persist along the casting for considerable distance. The casting may be deemed of poor quality and not useful for some applications. 
     A grain starter, as embodied by the invention, is illustrated in FIG.  2 . The grain starter has fine, randomly oriented grains. Randomly oriented grains result in crystalline growth characterized by many grains. In all regions, there will be a few grains with &lt;100&gt; orientation. These &lt;100&gt; grains are well-oriented grains that quickly pinch off growth of other grains. A spacing of grain for the grain starter is less than the dendrite arm spacing of the casting. This permits &lt;100&gt; oriented grains to establish first dendrite side arms that quickly pinch off growth of other grains. 
     The grain starter comprises a metal casting agent that is capable of nucleating fine grains in a superalloy. The grain starter has a multiple crystalline structure comprising at least a multiplicity of grains. The grain starter can be characterized by a grain spacing at least greater than about 10 μm. Grain spacing of the grain starter can be chosen to be less than the anticipated dendritic arm spacing of the developing crystals. While dendritic arm spacing is a function of cooling rate, typically, the dendritic arm spacing is from about 100 μm up to about 500 μm. The grain spacing of the grain starter can be chosen to be less than about 500 μm, desirably less than about 200 μm, such as less than about 100 μm. 
     The required fine grain starters can be made by densification of at least one of a metal powder, a wrought metal, and a ceramic that is an efficient nucleating agent for fine grains. In one process, the grain starter is made by mechanically working a bar of superalloy metal having a cast coarse grain size. The superalloy metal is plastically deformed, then annealed, or otherwise heat treated, to recrystallize the metal structure with a fine randomly oriented grain structure. 
     In another process as embodied by the invention, the superalloy can be mechanically worked, machined, or otherwise compounded at a low temperature, and then heat treated to recrystallize the metal to a final desired grain size. 
     In further process, as embodied by the invention, a superalloy grain starter of undetermined grain structure is mechanically deformed and then placed in a directional solidification furnace mold. The grain starter is used in an “as-worked” condition. 
     The grain starter can comprise nickel, chromium, or iron based superalloys. Examples of such grain starters include Hastelloy X, René 80, IN 738, Ni—20Cr—10Ti alloy, GTD-111, GTD-222, René 41, René 125, René 77, René 95, René 80, Inconel 706, Inconel 718, Inconel 625, cobalt-based HS188, cobalt-based L-605 and stainless steels. Ceramic nucleating agents for fine grains in superalloys include, but are not limited to, cobalt oxide and cobalt aluminate. 
     The use of a grain starter is advantageous for castings made by a liquid metal cooling process. In a directional solidification process, a mold is placed on a chill plate. Grains nucleate in a cold region as liquid metal is poured into the mold. In the liquid metal cooling process, the mold is lowered into a liquid metal for cooling. The mold is above the cooling bath at the beginning of solidification. The grain starter can be used to assure proper grain nucleation as the metal cools. 
     FIG. 3 illustrates a schematic sectional view of a furnace  10  for a directional solidification process, as embodied by the invention. In FIG. 3, the directional solidification furnace  10  is heated, for example, by resistance heated graphite strips  12  within insulated furnace box  14 . A ceramic shell mold  16  is suspended in the furnace  10  by a mold positioner  18 . A grain starter  20  is disposed within the mold  16 . 
     A superalloy  22  is melted in crucible  24  and poured through a basin  26  into the mold  16 . Randomly oriented solid superalloy grains then nucleate on the grain starter  20 . Mold  16  is then lowered into a liquid cooling metal  28  in a container  30  at a prescribed lowering rate. A solid-liquid interface advances upward in the liquid metal away from the cooling metal  28  as heat is carried away from the liquid metal by the cooling metal  28 . An ingot is formed after the mold  16  is withdrawn from the furnace  10  and immersed into the cooling metal  28 . 
     These and other features will become apparent from the following examples. The examples are not intended to limit the invention in any way. 
     In the following examples, cylindrical castings were made in a Bridgman furnace using aluminum-based molds. In each example, the furnace casting temperature is about 1550° C. and withdrawal rate is about 12 inches/hour, and all dimensions and measurements are approximate. Grain starters of sintered cobalt aluminate and of hot pressed nickel alloy René 80 are used as grain starters for nickel alloy GTD-111 castings. The initial René 80 powder size is less than 500 μm. In a conventional Bridgman furnace, the casting is lowered from the furnace and is cooled by radiation. In the following examples, the furnace mold was provided with a cooling plate to simulate cooling by a cooling bath such as a liquid metal bath. The composition of the nickel alloy René 80 in weight percent is: Ni with 9.5 Co, 14 Cr. 4.0 Mo, 4.0 W, 3.0 Al, 5.0 Ti, 0.17 C, 0.015 B and 0.03 Zr. The composition of the nickel alloy GTD-111 in weight percent is: Ni, 9.5 C0, 14 Cr, 1.6 Mo, 3.8 W, 3.0 Al, 4.9 Ti, 2.8 Ta, 0.10 C, 0.12 B and 0.04 Zr. 
     EXAMPLE 1 
     A ⅞″ diameter casting of GTD-111 is grown in an aluminum oxide mold. The casting is sectioned longitudinally and etched with acid to reveal the casting grain structure. Grain boundaries at angles greater than about 20° to the growth direction were observed for 2″ above the base of the casting. 
     EXAMPLE 2 
     A casting is made in a manner similar to Example 1, except that a grain starter of hot pressed René 80 powder is placed at the bottom of the casting mold. The grain size of the pressed René 80 is less than 500 μm. The resulting casting exhibits a region of very fine grains, less than {fraction (1/16)}″ in diameter for a ½″ length of the casting. Above this region of fine grains, the grain structure is columnar, with boundaries oriented less than 20° to the growth direction. The structure above the region of fine grains is useable. The region of acceptable casting represents an increase in yield beyond the yield of Example 1. 
     EXAMPLE 3 
     A casting is made in a manner similar to that of Example 1, except that a grain starter of sintered cobalt aluminate (CoAl 2 O 4 ) is placed at the bottom of the casting. This grain starter has ⅛″ inch deep grooves machined on its surface to increase contact area of the grain starter with superalloy. A ½′ region at the base of the casting exhibits equiaxed grains. Above the region, columnar structure is present beginning at less than ½″ from the casting bottom. The useable casting extends beyond the ½″ length. The region of acceptable casting represents a substantial increase in yield beyond Example 1. 
     While embodiments of the invention have been described, the present invention is capable of variation and modification, and therefore should not be limited to the precise details of the examples. The invention includes changes and alterations that fall within the purview of the following claims.