Patent Application
20060090871
The invention relates to investment casting. More particularly, the invention relates to the forming of core-containing patterns for investment forming investment casting molds.
Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components.
Gas turbine engines are widely used in aircraft propulsion, electric power generation, ship propulsion, and pumps. In gas turbine engine applications, efficiency is a prime objective. Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures in the turbine section exceed the melting points of the superalloy materials used in turbine components. Consequently, it is a general practice to provide air cooling. Cooling is typically provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. Such cooling comes with an associated cost in engine efficiency. Consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. This may be obtained by the use of fine, precisely located, cooling passageway sections.
A well developed field exists regarding the investment casting of internally-cooled turbine engine parts such as blades, vanes, seals, combustors, and other components. In an exemplary process, a mold is prepared having one or more mold cavities, each having a shape generally corresponding to the part to be cast. An exemplary process for preparing the mold involves the use of one or more wax patterns of the part. The patterns are formed by molding wax over ceramic cores generally corresponding to positives of the cooling passages within the parts. In a shelling process, a ceramic shell is formed around one or more such patterns in a well known fashion. The wax may be removed such as by melting, e.g., in an autoclave. The shell may be fired to harden the shell. This leaves a mold comprising the shell having one or more part-defining compartments which, in turn, contain the ceramic core(s) defining the cooling passages. Molten alloy may then be introduced to the mold to cast the part(s). Upon cooling and solidifying of the alloy, the shell and core may be mechanically and/or chemically removed from the molded part(s). The part(s) can then be machined and/or treated in one or more stages.
The ceramic cores themselves may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into hardened metal dies. After removal from the dies, the green cores may then be thermally post-processed to remove the binder and fired to sinter the ceramic powder together. The trend toward finer cooling features has taxed ceramic core manufacturing techniques. The cores defining fine features may be difficult to manufacture and/or, once manufactured, may prove fragile.
A variety of post-casting techniques were traditionally used to form the fine features. A most basic technique is conventional drilling. Laser drilling is another. Electrical discharge machining or electro-discharge machining (EDM) has also been applied. For example, in machining a row of cooling holes, it is known to use an EDM electrode of a comb-like shape with teeth having complementary shape to the holes to be formed. Various EDM techniques, electrodes, and hole shapes are shown in U.S. Pat. No. 3,604,884 of Olsson, U.S. Pat. No. 4,197,443 of Sidenstick, U.S. Pat. No. 4,819,325 of Cross et al., U.S. Pat. No. 4,922,076 of Cross et al., U.S. Pat. No. 5,382,133 of Moore et al., U.S. Pat. No. 5,605,639 of Banks et al., and U.S. Pat. No. 5,637,239 of Adamski et al. The hole shapes produced by such EDM techniques are limited by electrode insertion constraints.
Commonly-assigned co-pending U.S. Pat. No. 6,637,500 of Shah et al. discloses exemplary use of a ceramic and refractory metal core combination. With such combinations, generally, the ceramic core(s) provide the large internal features such as trunk passageways while the refractory metal core(s) provide finer features such as outlet passageways. As is the case with the use of multiple ceramic cores, assembling the ceramic and refractory metal cores and maintaining their spatial relationship during wax overmolding presents numerous difficulties. A failure to maintain such relationship can produce potentially unsatisfactory part internal features. It may be difficult to assemble fine refractory metal cores to ceramic cores. Once assembled, it may be difficult to maintain alignment. The refractory metal cores may become damaged during handling or during assembly of the overmolding die. Assuring proper die assembly and release of the injected pattern may require die complexity (e.g., a large number of separate die parts and separate pull directions to accommodate the various RMCs).
Separately from the development of RMCs, various techniques for positioning the ceramic cores in the pattern molds and resulting shells have been developed. U.S. Pat. No. 5,296,308 of Caccavale et al. discloses use of small projections unitarily formed with the feed portions of the ceramic core to position a ceramic core in the die for overmolding the pattern wax. Such projections may then tend to maintain alignment of the core within the shell after shelling and dewaxing.
Nevertheless, there remains room for further improvement in core assembly techniques.
One aspect of the invention involves a method for forming an investment casting pattern. A metallic first core element is formed including at least one recess. The first core element is engaged to at least a mating one of an element of a die and a second core element (if present). The recess serves to retain the first core element relative to the mating one. The die is assembled. Sacrificial material (e.g., wax) is introduced to the die to at least partially embed the first core element.
Various implementations involve forming the first core element from sheet stock having opposite first and second faces. The at least one recess may include a first recess in the first face and a second aligned recess in the second face. The first and second recesses may be elongate channels. The engaging may involve translating a first portion of the first core into a slot in the mating one so that a projecting portion of the mating one within the slot is received into the at least one recess so as to provide a mechanical back-locking effect. The forming may involve forming a regular pattern of recesses including the at least one recess. The engaging may leave exposed a number of the recesses of the regular pattern. The regular pattern may be pre-formed in flat sheet stock. The metallic first core element may be cut and/or shaped from such sheet stock.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
According to one aspect of the invention, the sheet 20 may be pre-formed with surface features or other enhancements to serve one or more useful functions during the investment casting process. The exemplary sheet of
The enhancements may be formed in an initial unenhanced sheet by a variety of means including one or more of embossing, engraving, etching, and drilling/milling (e.g., photo-etching, laser etching, chemical milling, and the like). Once so formed, individual RMCs might be cut from the larger sheet and optionally further shaped (e.g., via stamping, bending, or other forming/shaping technique).
The enhancements may serve one or more of several purposes. The enhancements may provide for registration and/or engagement/retention of the RMC with one or more of a pattern-forming mold, another core (e.g., a molded ceramic core), and an investment casting shell formed over a pattern. The enhancements may provide features of the ultimate casting. For example, through-apertures may provide posts for enhanced heat transfer and/or structural integrity. Blind recesses may provide enhanced heat transfer due to increased surface area, increased turbulence, and the like.
Especially for smaller-scale manufacturing applications, use of the pre-enhanced RMC sheet material 20 may have substantial cost benefits in providing the aforementioned utility.
The dovetail RMC-to-die attachment function identified above may be reproduced in other situations. For example, rather than having a regular array of the recess pairs 34 and 36, the sheet 20 might be provided with only a single recess pair adjacent the edge 26 or even a single recess on one side 22 or 24 in the absence of an aligned recess on the other side. The enhancements across the remainder of the sheet (if any) may be otherwise formed (e.g., arrays of the apertures and/or dimples). Individual RMCs may be cut relative to the edge 26 so that the single recess or recess pair may be used to provide the dovetail interaction with the die. In yet another example, such recesses may be post-formed.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, details of the particular part to be cast may influence details of any particular implementation. Furthermore, the principles may be implemented in modifying an a variety of existing or yet-developed manufacturing processes for a variety of parts. The details of such processes and parts may influence the details of any implementation. Accordingly, other embodiments are within the scope of the following claims.
