Patent Application
20090293994
This invention relates generally to superalloy casting methods, and more specifically to casting methods for producing directionally solidified articles exhibiting fine dendrite arm spacing.
The mechanical properties of cast superalloy articles improve by applying directional casting techniques to produce columnar-grained or single crystal articles.
Directional casting techniques used to manufacture such articles start with a mold shaped to produce the desired cast article. One such process of manufacturing directionally solidified cast articles employs a Bridgman-type furnace and comprises the pouring of molten metal into a mold within a heated zone. A chill plate cools the base of the mold. Subsequent solidification of the molten metal occurs by controlled withdrawal of the mold from the heated zone. The mold is initially cooled through the chill plate by conduction and then radiation as the metal solidifies upward along the length of the mold.
Another process for making directionally solidified cast articles is discussed in U.S. Pat. No. 3,763,926 by Tschinkel et al. The process includes pouring molten metal into a superheated mold positioned in a heated zone and withdrawing the mold from the furnace into a liquid metal coolant bath. The liquid metal bath is used to obtain high thermal gradients during the directional solidification process.
The quality and structure of the directionally solidified cast article still needs refinement. Certain mechanical properties are controlled by the microstructure of the cast materials. Due to the dendritic nature of the solidification, certain elements segregate to the dendrite and others to the interdendritic region. The last metal to solidify is in the interdendritic regions and thus porosity and eutectic pools are located herein. As a result, the properties of the cast alloy are decreased by such inhomogeneities. The size of the porosity, carbides, and eutectic pools is significantly reduced by a reduction in primary dendrite arm spacing in the cast article. The primary dendrite arm spacing is the average spacing between adjacent dendrite cores. This spacing is measured normal to the crystal growth direction by the average number of dendrite cores per area. Secondary dendrite arm spacing is the average spacing between adjacent secondary dendrite arms as observed on a section perpendicular to the growth direction. Thus, there is a need to produce unidirectional cast articles with minimal primary and secondary dendrite arm spacing to achieve superior mechanical and physical properties.
Dendrite arm spacing is also directly related to the solidification conditions during casting. Dendrite arm spacing varies inversely with cooling rate (solidification rate times thermal gradient). High thermal gradients are required to prevent nucleation of new grains during directional solidification. In known processes, the casting cavities for single crystal and columnar-grained processes are spaced a relatively large distance away from one another in a mold to avoid re-radiation of heat from mold/casting to mold/casting. The spacing is used to promote uniform thermal gradients to thereby avoid coarse dendritic microstructure and solidification defects. Furthermore, the rate of heat extraction from the mold/casting limits the rate at which the mold can be withdrawn from the hot zone without forming solidification defects.
Accordingly, it would be desirable to provide a method to form cast parts having the desired microstructure utilizing optimized casting packing and withdrawal rate.
The above-mentioned need or needs may be met by exemplary embodiments which provide a method of making a plurality of directionally solidified articles. An exemplary method includes adding a molten superalloy metal to a plurality of cavities in a mold in a heated zone, each cavity being defined at least in part by an associated mold wall spaced at a predetermined minimum spacing from an adjacent cavity. Each mold cavity is shaped to form at least one cast article. In the exemplary embodiment, the mold is withdrawn from the heated zone into a liquid metal cooling tank at a predetermined withdrawal rate, wherein the withdrawal rate and the spacing cooperate to provide a thermal gradient sufficient to solidify the molten metal to form a plurality of directionally solidified cast articles each having primary dendrite arm spacing of between about 6 to about 12 mils (about 150-300 microns).
An exemplary embodiment provides a method for reducing solidification defects in a directionally solidified article. The article may comprise a high-refractory nickel base superalloy. The exemplary method includes adding a molten high-refractory nickel base superalloy metal to at least one cavity in a mold in a heated zone, wherein the cavity is shaped to form at least one cast article and withdrawing the mold with the molten superalloy metal from the heated zone into a liquid metal cooling tank at a predetermined withdrawal rate. The withdrawal rate is sufficient to provide a thermal gradient sufficient to solidify the molten metal to form the directionally solidified cast article having primary dendrite arm spacing of between about 6 to about 12 mils (about 150 to about 300 microns). The primary dendrite arm spacing provides a reduction in solidification defects in at least one cast article relative to an amount of solidification defects in a cast article comprising a comparable high-refractory nickel base superalloy formed using a Bridgman technique.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,
Referring to
An exemplary embodiment provides an optimum casting process for producing directionally solidified articles, such as nozzles, airfoils, and shrouds in a tightly packed arrangement. As used herein, the term “directionally solidified” refers to either columnar-grained or single crystal microstructure, as will be understood by those having skill in the art.
The mold configuration, such as spacing between adjacent outer mold surfaces, and withdrawal rate are interrelated. An optimized combination of spacing and withdrawal rate enables solidification of castings with properties similar to articles formed by less-densely packed processes and/or by the Bridgman method. In an exemplary embodiment, directionally solidified cast articles may be formed in a mold arrangement having minimal spacing D between adjacent outer mold surfaces. In an exemplary embodiment, the spacing D may be as low as about ⅛″ (about 3 mm).
In an exemplary embodiment, the mold (e.g., mold 10, mold 50) is used in conjunction with a liquid metal coolant bath. In order to form castings having the desired microstructure in a densely packed mold, the liquid metal coolant must be able to provide the necessary high temperature gradient. An exemplary coolant is liquid tin (Sn).
The optimized spacing between the mold walls (e.g., mold walls 16) of adjacent mold cavities allows circulation of the liquid metal coolant between the molding cavities in order to extract heat. The circulating flow of the liquid tin bath provides sufficiently high thermal gradients during cooling to produce primary dendrite arm spacing as fine as 6 mils (about 200 microns) when withdrawal rates equal to or greater than 12 in/hr are used. In an exemplary embodiment, the withdrawal rate with such tight packing of the mold cavities may be approximately 12 to 20 in/hr (about 30-50 cm/hr).
Additionally, withdrawal rates of approximately 20 to 30 in/hr (about 50 to 76 cm/hr)may be utilized with less densely packed molds, i.e., wider spaced outer mold surfaces. In other exemplary embodiments, the withdrawal rate may be as high as 50 in/hr (about 127 cm/hr).
For some applications, such as high pressure gas turbine engine blades, the optimal dendrite arm spacing may be approximately 6 mils (about 150 microns). Practically speaking, the primary dendrite arm spacing may be slightly larger than the optimum value, or about 8 mils (203 micron) in thicker sections such as a blade root.
In an exemplary embodiment, a liquid metal cooling process is utilized to provide directionally solidified articles such as nozzles, shrouds, and airfoils, which are formed in a densely packed alumina/silica mold. The articles cast by the exemplary methods exhibit desired microstructures to provide mechanical properties that are similar, or superior, to mechanical properties exhibited by articles cast using prior methods. It is believed that the fine primary dendrite arm spacing promotes the reduction or elimination of solidification defects. The exemplary methods disclosed herein may be further useful for casting higher refractory nickel-base superalloys. For example, superalloy compositions including increased amounts of rhenium or tungsten, which may provide undesirable solidification defects when cast in prior processes, may be utilized in the exemplary embodiments disclosed herein.
Two mold configurations, parallel plate molds and bar molds, were used to determine the relationship necessary for process optimization of tightly packed directional solidified articles by the liquid metal cooling process.
Three molds each with three parallel plates of dimensions 2 inches by 5 inches by 0.5 inches were cast in a DS withdrawal furnace equipped with a liquid metal tin bath for high gradient solidification. The three molds each had three plates spaced 1.5 inches, 1.0 inches, and 0.8 inches, respectively. The outer mold surfaces were spaced ⅞ inches, ⅜ inches, and ⅛ inch apart between adjacent outer mold surfaces.
The nickel base superalloy utilized in the exemplary molding process has a nominal composition, in weight percent, of: Al 6.2, Ta 6.5, Cr 7, W 5, Mo 1.5, Re 3, Co 7.5, C 0.05, B 0.004, Hf 0.15, balance nickel and incidental impurities. This particular composition is known as Rene N5 and is suitable for use in directional solidification processes.
The parallel plate molds/castings were withdrawn from the hot zone at 16 in/hr in a withdrawal furnace using the liquid metal cooling capability. The cast plates were single crystals, free from solidification defects and had a fine dendritic microstructure with primary dendrite arm spacings (PDAS) as low as 8 mils.
The dendritic microstructure of the plates disposed at the center of the mold were compared to the microstructure of the plates disposed toward the outer regions of the mold. The dendritic microstructure of the central cast plates were substantially similar to the microstructure of the outer cast plates. See Table 1.
The six bar mold configuration in which the castings were spaced at greater than 1 inch apart produced castings with PDAS of approximately 10 mils at a slightly faster withdrawal rate of 20 in/hr. The data suggests uniform cooling of all the plates and the ability of the liquid tin to remove the heat uniformly from tightly packed castings. The PDAS from the castings in the 20 in/hr run was similar to the tightly spaced cast plates indicating the ability of the liquid tin to uniformly cool tightly packed castings.
A second mold configuration was a six bar single crystal mold in which the bars were spaced slightly more than one inch apart (between adjacent cast surfaces). The mold was withdrawn from the hot zone at different rates up to 50 in/hr. In this particular example, it was found that, for cast articles of similar size to an airfoil, rates over 35 in/hr were not optimal in terms of both dendritic microstructure and casting defects. The 30 in/hr withdrawal rate resulted in defect-free single crystal castings with microstructures between 5 and 6 mils. For withdrawal rate and microstructure comparisons, the experimental bars are believed to be sufficiently similar to the desired airfoil structures.
Low temperature low cycle fatigue data has shown improvements in cycle life of over an order of magnitude. The improvement in the low temperature low cycle fatigue life is believed to be related to the primary dendrite arm spacing. For example improvements are found in PDAS of 10 mils as compared to 14 mils, even greater improvements with PDAS of 8 mils, and more with PDAS of 6 mils. It is believed the prior Bridgman process is not capable of producing these fine dendrite arm spacings in solidification defect-free castings.
Thus, cast parts having the desired microstructure can be formed in closely packed molding cavities by withdrawal in a liquid coolant at faster withdrawal rates than previously known. Further, molds containing more tightly packed directionally solidified casting than previously realized can be used for the solidification of single crystal or columnar-grained articles with either a similar microstructure or a finer dendritic microstructure than previous realized with typical Bridgman castings. Such “close packing” may occur in a variety of mold cavity arrays including circular, rectangular, and the like. Further, the molding cavities are not confined to a regular or symmetric array. However, the spacing between cavities should enable a sufficient thermal gradient at a selected withdrawal rate to form cast articles with the desired microstructure. Further, it is desired that each cast article exhibit a desired microstructure regardless of cavity location. Thus, it is desired that cast articles formed in a first cavity group, such as an outer region of a mold be substantially similar to articles formed in a second cavity group, such as in an inner region of the mold. Further, this tighter packing of castings could be realized in three dimensions, i.e., castings can be vertically stacked as well as tightly packed around the mold. For example, castings could be packed in a circular manner in several layers around the center of the mold and then additionally packed a plurality of layers vertically.
Based on the results of the Examples provided above, it is envisioned that other nickel base superalloys may be utilized in the exemplary processes disclosed herein to achieve the fine primary dendrite arm spacing and thereby reduce solidification defects in directionally solidified cast articles. The reduction in solidification defects may be particularly useful for high refractory nickel base compositions.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
