Patent Application
20100304161
The present invention generally relates to casting equipment and processes. More particularly, the invention relates to reducing surface defects in directionally-solidified castings, including single-crystal (SX) and directionally-solidified (DS) castings.
Hot gas path components of gas turbines, such as blades (buckets), vanes (nozzles) and combustor components, are typically formed of nickel-, cobalt- or iron-based superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a demand for hot gas path components that are capable of withstanding higher temperatures. As the material requirements for gas turbine components have increased, various processing methods and alloying constituents have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys. For example, buckets, nozzles and other components employed in more demanding applications are often cast by directional casting techniques to have DS or SX microstructures, characterized by a crystal orientation or growth direction in a selected direction to produce columnar polycrystalline or single-crystal articles. As known in the art, directional casting techniques generally entail pouring a melt of the desired alloy into an investment mold held at a temperature above the liquidus temperature of the alloy, and then gradually withdrawing the mold into a cooling zone where solidification initiates at the base of the mold and the solidification front progresses upward.
Investment molds are typically formed by dipping a wax or plastic model or pattern of the desired component into a slurry comprising a binder and a refractory particulate material to form a slurry layer on the pattern. Common materials for the refractory particulate material include alumina, silica, zircon and zirconia, and common materials for the binder include silica-based materials, for example, colloidal silica. A stucco coating of a coarser refractory particulate material is typically applied to the surface of the slurry layer, after which the slurry/stucco coating is dried. The preceding steps may be repeated any number of times to form a shell mold of suitable thickness around the wax pattern. The wax pattern can then be eliminated from the mold, such as by heating, after which the mold is fired to sinter the refractory particulate materials and achieve a suitable strength. To produce hollow components, such as turbine blades and vanes having intricate air-cooling channels, one or more cores are provided within the shell mold to define the cooling channels and any other required internal features. Cores are typically prepared by baking or firing a plasticized ceramic mixture, and then positioned within a pattern die cavity into which a wax, plastic or other suitably low-melting material is introduced to form the pattern for the mold. Once solidified, the pattern with its internal cores can be used to form the shell mold as described above.
A particular known investment casting process employs a Bridgman-type furnace to create a heated zone surrounding the mold, and a chill plate at the base of the mold. Solidification of the molten alloy within the mold occurs by gradually withdrawing the mold from the heated zone and into a cooling zone beneath the heated zone, where cooling occurs by convection and/or radiation. A high thermal gradient is required at the solidification front to prevent nucleation of new grains during directional solidification processes. For example, commonly-assigned U.S. Pat. No. 6,217,286 to Huang et al. discloses a casting process that achieves a high thermal gradient at the solidification front with the use of a cooling zone that comprises a tank containing a liquid cooling bath, such as molten tin or another molten metal.
Mechanical properties of DS and SX articles depend in part on the avoidance of casting defects, including pitting and other surface defects that may result from chemical reactions with the mold during the solidification process. One potential source of surface defects is a molten metal coolant noted above for achieving high thermal gradients during solidification. An undesirable cast surface reaction may occur if the coolant penetrates the mold by infiltration of porosity or a crack in the mold prior to the completion of the casting operation. Consequently, shell molds used in investment casting processes must exhibit sufficient strength and integrity to survive the casting process.
Additional challenges are encountered when attempting to form castings of alloys that contain an appreciable amount of one or more reactive materials, including nickel-based superalloys that contain niobium, titanium, zirconium, yttrium, tantalum, tungsten, rhenium and potentially other elements that tend to readily react with oxygen when molten or at an elevated temperature. For this reason, surfaces of molds and cores used in the casting of materials containing reactive elements may be provided with protective barriers known as facecoats. Facecoats are generally applied to mold and core surfaces in the form of a slurry, which may be dried and sintered prior to the casting operation or undergo sintering during the casting operation. Typical facecoat slurries comprise a refractory particulate material in an aqueous-based inorganic binder, optionally with various additional constituents such as organic binders, surfactants, dispersants, pH adjusters, etc., to promote the processing, handling, and flow characteristics of the slurry. The refractory particulate material is chosen on the basis of being sufficiently unreactive or inert to the particular reactive material being cast. Various facecoat materials have been used and proposed, including those containing yttria (Y2O3), alumina (Al2O3), and zirconia (ZrO2) in a colloidal silica binder.
The present invention provides a casting process and apparatus for producing directionally-solidified castings, as well as castings produced with the process and apparatus.
According to a first aspect of the invention, a directional solidification process is provided that entails a facecoat slurry applied to a surface within a mold cavity to form a continuous solid facecoat on the surface. The facecoat consists essentially of at least 60 weight percent of a first phase containing yttria, and the balance of the facecoat is essentially a binder phase consisting essentially of an inorganic material. After a molten metal alloy is introduced into the mold cavity so that the molten metal alloy contacts the facecoat, the mold is immersed in a liquid coolant to cool and solidify the molten metal alloy and form a casting of the metal alloy, during which an oxide layer forms on a surface of the casting. The facecoat is sufficiently adherent to the oxide layer such that at least a portion of the facecoat detaches from the mold surface and remains tightly adhered to the casting surface in the event the casting contracts during cooling. Thereafter, the mold can be removed from the liquid coolant, and the casting with the oxide layer and remnant facecoat can be removed from the mold.
Another aspect of the invention are castings produced by the directional solidification process described above, including the oxide layer and the remnant portion of the facecoat on the casting at the conclusion of the casting operation. Following the casting operation, the oxide layer and remnant facecoat can be removed from the casting prior to carrying out further processes on the casting.
According to yet another aspect of the invention, a directional solidification casting apparatus is provided that includes a mold and a continuous solid facecoat on a surface of a cavity within the mold. The facecoat consists essentially of at least 60 weight percent of a first phase containing yttria, with the balance of the facecoat being essentially a binder phase consisting essentially of an inorganic material. The mold cavity is adapted to receive a molten quantity of a metal alloy so that the molten metal alloy contacts the facecoat. The apparatus further includes a liquid coolant adapted to immerse the mold, cool and solidify the molten quantity of the metal alloy within the mold, and form a casting of the metal alloy.
Casting materials for which this invention is particularly advantageous include superalloys, and particularly nickel-based alloys which may contain various alloying constituents capable of forming the oxide layer on the casting. A notable advantage of the invention is that the facecoat and oxide layer on the casting form a protective barrier that is capable of reducing and preferably prevents reactions that might otherwise occur between the casting alloy and the liquid coolant during the casting operation if the liquid coolant is able to infiltrate porosity or cracks in the mold. Another notable advantage is that the facecoat is very adherent to the oxide layer, such that if the casting sufficiently contracts during cooling at least the portion of the facecoat contacting the oxide layer will remain tightly adhered to the oxide layer and tend to delaminate from any portion of the facecoat that might remain bonded to the mold surface. As a result, the adherent portion of the facecoat and the oxide layer continue to define a protective barrier on the casting surface. Other advantages associated with the facecoat include a long shelf life exhibited by the facecoat slurry due to improved stability, a high solids loading for achieving desirable casting surface finishes, and strength and relatively low porosity to provide a reliable protective barrier between the molten alloy and the mold. The facecoat slurry also exhibits relatively low viscosities for achieving desirable mixing properties.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
Consistent with known investment casting processes, the cavity 14 may be defined through the use of a wax pattern (not shown) whose shape corresponds to the desired shape of the casting. The pattern is removed from the shell mold 12 prior to the casting operation, such as with conventional techniques including flash-dewaxing, microwave heating, autoclaving, and heating in a conventional oven. The cavity 14 may contain cores (not shown) for the purpose of forming internal cavities or passages within the casting.
The mold 12 is shown secured to a chill plate 16 and located within a heating zone 18 (for example, a Bridgman furnace) to heat the mold 12 to a temperature at least equal to and preferably above the melting temperature of the casting alloy. The apparatus 10 is shown as equipped for unidirectional solidification of the casting. For this purpose, a cooling zone 20 is represented as being located directly beneath the heating zone 18, and a baffle or heat shield 22 is represented as being between and separating the heating and cooling zones 18 and 20. The heat shield 22 is useful for insulating the cooling zone 20 from the heating zone 18 to promote a steep thermal gradient that will be experienced by the mold 12 as it exits the heating zone 18 and enters the cooling zone 20. The heat shield 22 may have a variable-sized opening 26 that enables the shield 22 to fit closely around the shape of the mold 12 as it is withdrawn from the heating zone 18, through the heat shield 22, and into the cooling zone 20.
According to a particular aspect of the invention, the cooling zone 20 is represented as comprising a tank that contains a liquid coolant 24, typically a molten metal though the use of other materials is foreseeable. A variety of metals can potentially be used as the liquid coolant 24, including relatively low-melting metals such as lithium, magnesium, aluminum, zinc, gallium, indium, and tin. Particularly suitable liquids for the coolant 24 are believed to be molten tin at a temperature of about 235° C. to about 350° C., or molten aluminum at a temperature of up to about 700° C. Molten tin is more commonly used and believed to be preferred because of its low melting temperature and low vapor pressure.
The casting process is preferably carried out in a vacuum or an inert atmosphere. As will be discussed in more detail below, to promote sintering of the facecoat 36 and the formation of a metal oxide layer 42 (
Various alloys can be cast using a casting apparatus of the type represented in
Heating and sintering of the facecoat slurry 32 to form the facecoat 36 can be performed by firing to about 1000° C. prior to introducing the molten alloy into the mold cavity 14. Additional sintering can occur in situ as a result of the mold 12 being preheated to above the metal melting temperature and molten alloy being introduced into the mold cavity 14 while the pre-fired facecoat slurry 32 is still present on the mold cavity surface 34. Though not shown, it should be understood that a core placed in the mold cavity 14 may also be provided with a layer of the same or similar slurry to form a facecoat.
As noted above, the facecoat 36 on the interior surface 34 of the mold 12 serves as a protective barrier to prevent the liquid coolant 24 on the exterior of the mold 12 from contacting and chemically reacting with the casting alloy during the solidification process. According to a particular aspect of the invention, preferred compositions for the facecoat 36 are also capable of reacting with the molten alloy during solidification to form the aforementioned metal oxide layer 42 on the surface 40 of the casting 38, which is capable of bonding a surface region layer of the facecoat 36 to the casting surface 40. The layer of facecoat 36 that remains bonded to the surface 40 of the casting 38 provides an additional barrier capable of protecting the casting surface 40 from chemical reactions with the liquid coolant 24.
According to a preferred aspect of the invention, the facecoat 36 is a ceramic-based composition that contains yttria (Y2O3) and a minimal amount of an inorganic binder, such that the facecoat 36 has a refractory phase in an inorganic binder phase. The facecoat 36 preferably consists essentially of the refractory and inorganic binder phases in the sense that the facecoat 36 is free of unintended phases or otherwise contains such phases at only impurity levels. The yttria refractory phase is the dominant phase of the facecoat 36 and constitutes at least 60 weight percent of the facecoat 36. The shell mold 12 may also be formed of the same or similar composition used to form the facecoat 36, though the presence of the facecoat 36 permits the use of traditional mold compositions for the mold 12.
As is generally conventional in the fabrication of facecoats for casting processes, the slurry 32 of
The slurry 32 is formed by combining the refractory powder with a particulate of the inorganic binder in an aqueous suspension, a thixotropic organic binder, a dispersant, and possibly optional constituents excluding particulate refractory materials and inorganic binders. The aqueous suspension containing the particulate inorganic binder preferably does not constitute more than 35 weight percent of the slurry 32, and more preferably constitutes about 1 to about 5 weight percent of the slurry 32, with a suitable nominal content of about 2.5 weight percent. This minimal amount of inorganic binder in the slurry 32 reduces the likelihood of potential reactions between the binder and the molten alloy placed in the mold 12. A preferred inorganic binder is believed to be entirely colloidal silica, though other inorganic binders could be used. The aqueous suspension preferably contains about 15 to about 40 weight percent inorganic solids, more preferably about 20 to about 30 weight percent inorganic solids, with a suitable nominal content of about 30 weight percent inorganic solids. The balance of the aqueous suspension is preferably water. A typical particle size for the inorganic binder particulate is generally about 14 nanometers and less. A commercial example of a suitable colloidal silica is Remasol® LP-30, available from Remet.
While additional additives, such as organic binders, surfactants, dispersants, defoaming agents, pH adjusters, etc., are known in the art as useful in facecoat slurries, slurry compositions preferred by the present invention selectively utilize certain additives in certain amounts that have been determined with this invention to compensate for the very high solids content and low inorganic binder content of the slurry 32, as described above. In particular, the slurry 32 is formulated to contain a dispersant whose composition is chosen in part on the basis of being capable of stabilizing the pH of the slurry 32 and maintaining the pH within a suitable range, preferably up to a pH of about 10 with a particular preferred example being a pH of 8.6 to 10.1. Dispersants believed to be suitable for use in the slurry 32 of this invention have the general formula Hx[N(CH2)yOH]z, where x has a value of 0 (tertiary amines), 1 (secondary amines) or 2 (primary amines), y has a value of 1 to 8, and z=3−x. A preferred dispersant is believed to be triethanol amine (N[(CH2)2OH]3), which is believed to have properties important to the slurry 32. First, triethanol amine is weakly basic and therefore capable of raising the pH of the slurry 32. Second, triethanol amine contains three alcohol functionalities that give it dispersant properties. Other compounds having the general formula Hx[N(CH2)yOH]z that could be used in the slurry 32 include monoethanol amine, diethanol amine, monopropanol amine, dipropanol amine, tripropanol amine. The dispersant constitutes at least 1 up to about 10 weight percent of the slurry 32, more preferably about 1 to about 5 weight percent of the slurry 32, with a suitable nominal content of about 2 weight percent. A commercial example of a suitable dispersant is Alfa Aesar® 22947 available from Alfa Aesar.
The slurry 32 is further formulated to contain a thixotropic organic binder that helps maintain the high solids loading of the slurry 32, while also promoting a smooth surface finish for the facecoat 36 and reducing the viscosity of the slurry 32, especially during mixing. The term thixotropic is used according to its ordinary meaning to denote certain materials whose viscosities change greatly with changes in shear (velocity). Preferred thixotropic organic binders also allow for lower mixing speeds, which are believed to promote the shelf life of the slurry 32 by reducing slurry friction and temperature during mixing. The thixotropic nature of the organic binder also allows the viscosity of the slurry 32 to be modified during mixing by adjusting the mixing speed. Thixotropic organic binders of particular interest to the invention include styrene-butadiene polymer dispersions particular suitable for use with colloidal silica binders. The organic binder constitutes at least 0.3 up to about 0.9 weight percent of the slurry 32, more preferably about 0.6 to about 0.7 weight percent of the slurry 32, with a suitable nominal content of about 0.6 weight percent. A commercial example of a suitable thixotropic organic binder is LATRIX® 6305 commercially available from the Ondeo Nalco Company.
The slurry 32 may contain other additives, such as surfactants, defoaming agents, additional organic binders, etc. For example, the slurry 32 may contain a wetting agent, such as NALCO® 8815 ionic wetting agent, and/or a defoamer such as NALCO® 2305 water-based defoamer, both commercially available from the Nalco Company. Notably, however, the slurry 32 preferably does not contain any further particulate constituents that would form any part of a solid phase in the facecoat 36. Instead, the thixotropic organic binder, dispersant, and any additional additives in the slurry 32 are preferably cleanly burned off during drying, heating and/or sintering of the slurry 32 to form the facecoat 36.
The slurry 32 can be prepared by standard techniques using conventional mixing equipment, and then undergo conventional processes to form the facecoat 36 on the mold cavity surface 34, such as by dipping, molding, or another suitable technique. Using these application methods, a suitable viscosity range for the slurry 32 is about five to about seven seconds using a standard #5 Zahn cup measurement. Suitable thicknesses for the slurry layer will depend on various factors, including the particular reactive material, mold material, and slurry composition. In general, the slurry is preferably applied to produce a facecoat 36 having a thickness of at least about 0.2 mm, for example, about 0.2 to about 0.6 mm and more preferably about 0.4 mm to produce a reliable protective barrier for the mold 12. The slurry 32 can be applied as multiple layers, for example, to promote separation by delamination so that a continuous layer of the facecoat 36 remains bonded to the casting surface 40 as the casting 38 contracts.
As previously noted, heating and sintering of the facecoat slurry 32 to form the facecoat 36 can occur prior to and during the introduction of molten alloy into the mold cavity 14. The layer of facecoat slurry 32 is preferably dried and fired prior to contact with the molten alloy in accordance with well-known practices. The organic binder, dispersant, and other additional additives of the slurry 32 preferably provide an adequate level of green strength to the slurry layer after drying, and then burn off completely and cleanly prior to or during firing, by which the particles of the refractory powder sinter. Drying can be performed at room temperature, which is then preferably followed by a pre-sintering step that entails heating at a rate of about 200° C./hour to a temperature of about 1000° C., a one-hour hold at about 1000° C., and then cooling at a rate of about 200° C./hour to room temperature. This intermediate firing procedure is preferably performed prior to firing at a final sintering temperature for the purpose of eliminating the organic additives within the slurry 32, and can be performed according to conventional techniques, for example, in a gas or electric furnace. Full sintering of the facecoat 36 occurs at around 1600° C., which can occur during the mold preheating step of the casting process. As understood in the art, suitable and preferred temperatures, durations, and heating rates during drying and firing will depend on factors such as slurry thickness, composition, particle size, etc. As such, the drying and firing temperatures and durations can vary significantly.
As a result of firing, the facecoat 36 is in the form of a monolithic low-porosity protective barrier on the cavity surface 34 that protects the mold 12 and prevents reactions between the mold 12 and the molten alloy, thereby reducing the likelihood of pitting and other potential surface defects in the casting 38 that can be caused by such reactions. The preferred composition for the facecoat 36 has been observed to react with and adhere to the casting surface 40, even as the casting 38 contracts and the casting surface 40 moves away from the mold 12 during cooling and solidification of the molten alloy. During contraction of the casting 38, the entire facecoat 36 may remain tightly adhered to the casting surface 40 through the oxide layer 42. In practice, the facecoat 36 has been observed to effectively delaminate, in which case a continuous portion of the facecoat 36 remains tightly adhered to the casting surface 40 through the oxide layer 42 while the remainder of the facecoat 36 tends to remain adhered to the surface 34 of the mold 12. The combination of the reacted metal oxide layer 42 and the facecoat 36 (or at least the remnant of the facecoat 36 remaining attached to the surface 40) provides a continuous reaction barrier on the casting surface 40 that serves to physically and chemically separate the entire casting surface 40 from any liquid coolant 24 that may have infiltrated the mold 12. The high-solid loading of the preferred facecoat slurry 32 promotes the formation of a dense facecoat 36, so that the oxide layer 42 and at least the remnant of the facecoat 36 remain tightly adhered to the casting surface 40 and prevent any reaction with the coolant 24 as the casting 38 shrinks away from the mold surface 34.
The composition of the oxide layer 42 will depend on the particular compositions of the casting alloy and facecoat 36. If the casting alloy contains aluminum, as typical with many nickel-based superalloys, the oxide layer 42 is believed to be primarily alumina (Al2O3). However, the oxide layer 42 may alternatively or further comprise other metal oxides, such as chromia (Cr2O3) and/or other oxides of metal elements present in the facecoat 36 and the casting alloy.
Investigations leading to the present invention have shown that the high-solids yttria facecoat 36 having compositions as described above can be successfully employed to cast nickel-based superalloys. For example, in one investigation a nickel-based superalloy was cast by a unidirectional solidification process using a casting apparatus generally as represented in
The casting was then sectioned for metallographic examination and its surface was found to be covered with the facecoat remnant as well as the layer of infiltrated tin.
As evident from the images above, the remnant facecoat layer was continuous on the surface of the casting, and the oxide layer was nearly continuous on the casting surface. These images evidence that the original facecoat and the oxide layer grown in situ on the casting surface had successfully protected the casting surface during the casting operation, and thereafter the remnant facecoat and oxide layer had successfully protected the casting surface during the approximately two-hour immersion in tin during the casting operation. As such, the facecoat was shown to protect the casting surface from surface reactions with molten tin, which is advantageous for protecting a casting from reactions with a molten coolant in the event the mold cracks or is otherwise infiltrated by molten coolant during solidification of the casting.
While the invention has been described in terms of certain embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.
