Core for casting high-temperature, thin wall structures

Abstract: Disclosed is a single-cast, thin wall structure capable of withstanding impinging gases at temperatures of 2371°C and higher, and method of making the same.

Description: This invention relates to a core for casting high-temperature, thin wall structures. Thick walled one-piece alloy structures are disclosed in US patent Nos.3,806,276 and 3,192,578. Laminated structures having thin walls capable of withstanding high temperature impinging gases have heretofore been known. By way of example, such structures are disclosed in US patent Nos.4,245,769; 4,042,162; 4,004,056; 3,963,368; 3,950,114; 3,933,442; 3,810,711; 3,732,031; 3,726,604; 3,698,834; 3,700,418; 3,644,059; 3,644,060; 3,619,082; 3,616,125; 3,610,769; 3,606,572; 3,606,572; 3,606,573; 3,584,972; 3,527,544; 3,529,905; and 3,497,277. The thin walls of such structures are laminated to another thin wall or to a substantially thicker structure by brazing, welding or otherwise bonding. The laminating process involves high temperature brazing, welding or bonding materials that directly affect the alloy or otherwise limit the overall high temperature performance of the structure. Further, these thin wall layers often have holes formed therein by mechanical means or etching which is time consuming, labour intensive and expensive. Although these laminated thin wall structures are capable of withstanding impinging gases at temperatures higher than the melting point of the alloys which the structures are made from, the process of making the structures is time consuming, labour intensive and extremely expensive. Many prior art methods of casting hollow structures utilize ceramic cores. It has been generally accepted that these ceramic cores must have a density sufficiently low enough such that the core is compressive so that it gives as molten alloy solidifies around the core. It has generally been accepted that if the ceramic core has a density above 60 to 70 percent, the core will be crushed and broken by molten alloy which solidifies around it. It has also generally been accepted that ceramic cores having a thickness less than 0.76 mm (0.03 inches) with such low density less would be crushed and broken during casting. The density of prior art ceramic cores ranges from about 50 to about 60 percent. Although 100 percent quartz rods having 100 percent density have been used, such use has been limited to making bent and straight holes or central passageways. Heretofore, a high density ceramic core (above 70 to 99 percent plus density) has not been used to make a radial passageway. Generally, in turbine engine components such as turbine blades, such radial passageways parallel the outer thin wall of the component or turbine blade. It is generally accepted that the use of a high density material for a large core will break the metal. As molten alloy solidifies around a large high density core, the metal shrinks faster than the core and will break due to the high density core. Thus, those skilled in the art use low density cores to compensate for the fast rate of shrink of the molten metal and to prevent the metal from breaking. Another problem recognized by those skilled in the art is the problem of shape distortion during casting. Heretofore, it has been generally accepted that this shape distortion of the casted part is caused by what is known as "mould buckle". This "mould buckle" occurs in the process of building up the shell around the core and pattern. If one of the successive shell layers does not sufficiently dry, the layer moves away from the pattern causing the mould to "buckle" and causes a distorted casting shape. Heretofore, it was not recognized that casting shape distortion could be caused by shell creep. In prior art methods of making laminated thin wall structures such as gas turbine blades, the thin walls are provided by metal which has been cold-rolled to a very thin thickness. The cold-rolled metal is then etched or machined to provide small holes in the surface thereof. The small holes provide a cooling air film over the thin wall as the gas turbine blade is impinged with hot gases. This cold-rolled metal must be formed and bonded (or welded sufficiently to provide heat transfer from the thin wall to the main body of the blade) in a curved shape to produce the outer wall of the turbine blade. The forming process may result in the distortion of the holes in the wall. If the holes are not properly positioned, or the metal not sufficiently bonded, it is possible to develop hot spots at certain sections on the blade which would be undesirable and would limit production yields. Further, the cold-rolled material must be later heat treated which also could possibly result in varying heat transfer properties across the surface of the blade which also would be undesirable. Other casting problems are caused by ceramic cores which are extremely brittle and fragile. These problems increase with decreasing thickness and density. Heretofore, there has been a need for single-cast, high-temperature, thin wall hollow structures and means for making the same which is quick, relatively inexpensive and not labour intensive. A means for satisfying this need has heretofore escaped those skilled in the art. According to the present invention there is provided a core for casting alloy structures comprising a quartz material having a thickness ranging from about 0.127 mm to about 0.508 mm and a density ranging from about 70 to about 100 percent. The material may have a density greater than 99 percent. The core is curved. The present invention is based on the discovery of a variety of phenomena and agencies which gave rise to an idea of a means for repeatably and reliably producing or casting thin wall hollow structures with dimensional accuracy and having the thickness less than about 0.76 mm (0.03 inches). The following statements, by way of example, highlight the discoveries which are part of the present invention as a whole. The present inventors have discovered that the problem of creep of a ceramic shell can be solved by controlling the injection pressure of the molten alloy into a cavity. The injection pressure may be controlled as a function of time to cast walls having a thickness less than about 0.76 mm (0.03 inches) without shell creep. The injection pressure of an alloy into a cavity may be varied as a function of time by using a control orifice to bleed the head pressure off after the thin cavity has been filled. The present invention includes the discovery that very thin passageways of about 0.127 mm to 0.381 mm may be formed using a thin core having a density greater than about 70 percent, and preferably about 99 percent or greater. The present invention includes the dicovery that a thin core having a thickness of about 0.127 mm to 0.381 mm and a density greater than about 70 percent may be used without crushing and breaking when surrounded by solidifying alloy. The present invention includes the discovery that such thin cores may be used to form narrow radial passageways, having a width of about 0.127 mm to about 0.381 mm, in casting and such passageways may be formed substantially parallel to a thin outer wall having a thickness of about 0.127 mm to about 0.762 mm. The present invention includes the discovery that cores having a thickness of 0.127 mm to 0.381 mm may be used in core making, pattern making and casting process without reducing yields. Structures may be cast using high density ceramic cores, preferably quartz, having a minimum dimension of about 0.127 mm to about 13.97 mm to form very small holes in a thin wall structure and to hold the above-described thin core in place during casting. These cores can be of any shape. A pocket may be drilled into a ceramic core so as to receive and hold a narrow diameter rod. A single-piece, hollow multi-wall structure having a very thin outer wall, an inner wall and a very thin passageway therebetween may be cast using a ceramic core, narrow rod and thin ceramic core construction. A very thin curved core in accordance with the present invention may be held in position in a casting mould by forming a first ceramic core: coating the first ceramic core with wax or plastic pattern where metal is desired; placing the very thin curved ceramic core on the pattern; drilling a hole through the very thin ceramic core and pattern, and into the first ceramic core to form a pocket; inserting a rod through the hole so that the rod is received in the pocket in the first ceramic core; covering the very thin ceramic core with a thin layer of wax or plastic pattern where a thin wall of metal is desired; forming a hole through the thin layer of wax and into the thin ceramic core so as to form an angled pocket in the first core at a predetermined position where force is needed to keep the thin ceramic core in its curved shape; inserting a portion of an outer rod through the hole in the thin layer of wax so as to be received in a pocket from in the main ceramic core or in the thin core so that upon casting the structure a passage is provided through the thin wall and into the cavity formed by the thin core or extending through to the cavity formed by the main core; and covering the thin layer of wax and the other portion of the outer rod with a ceramic shell. The present invention includes the discovery of a means for producing single-cast, thin wall structures having smooth outer surfaces and having wall thicknesses as narrow as about 0.127 mm. The present invention enables the manufacture of a single-cast, thin wall hollow structure capable of withstanding impinging gases at temperatures as high as 2371 °C (4300°F) or higher. Another advantage of the present invention is that finer details may be made in the thin ceramic core using a laser due to the high density of the core. The present invention can be utilized to make structures having multiple thin walls each having a thickness less than about 0.762 mm as illustrated in Figure 8. The present invention will now be described, by way of example, with reference to the following detailed description, and the accompanying drawings, in which:- Figure 1 is a sectional view of a prior art casting mould; Figure 2 is a sectional view of a prior art casting mould after the mould has creeped; Figure 3 is a graphic illustration of the process of varying charge pressure with time ; Figure 4 is an illustration of a casting mould ; Figure 5 is an illustration of a casting mould, pattern, core and rod combination ; Figure 6 is an illustration of a thin ceramic core of the present invention; Figure 7 is an illustration of a method of forming single crystal alloy structures ; Figure 8 is an illustration of a multi-wall structure casting using cores of the present invention; Figure 9 is an illustration of a jet engine having nozzles; Figure 10 is an illustration of an atmospheric air/space craft having a leading edge; Figure 11 is a cross-section of a prior art film cool turbine-blade; Figure 12 is a cross-section of a single piece, single cast turbine-blade having a thin outer wall; and Figure 13 is a graphic illustration of various turbine-blade designs developed over time verse practical operating turbine inlet temperatures (°C) for gas turbine engines using the various turbine-blade designs. The techniques described may be used to produce thin walled hollow structures using equiaxed, directionally solidified and single crystal alloy processes. A variety of techniques are known for producing equiaxed, directionally solidified and single crystal alloy structures. The described techniques are particularly suitable for producing very thin walled structures using single crystal casting techniques. One such single crystal casting technique is illustrated in Figure 7, which shows a casting mould 100 carried on a water cooled chill plate 102 and received in a mould heater 104. A casting furnace 106 includes a vacuum system 108 and an induction melting crucible 110 for pouring molten alloy in the mould 100. Once the molten alloy is poured in the mould, the mould is slowly removed from the furnace by an elevator 112. Generally, for single crystal processing, the moulding temperature is preferably heated to at least 1538°C (2800°F), and the single crystal superalloy is heated to 1538°C (2800°F) and the single crystal is grown slowly at about 254 mm/hr (10 inches/hr). Generally, for equiaxed processes, the mould temperature is heated to about 982°C (1800°F) and the equiaxed alloy temperature is heated to 1538°C (2800°F) and the alloy solidified at 1 mm/min. Various types of superalloy compositions and manufacturers of such compositions are known to those skilled in the art. Most alloys of interest are complicated mixtures of nickel, chromium, aluminium and other elements. Prior attempts to cast multi-wall structures having at least one wall less than 0.76 mm (0.03 inches) thick have been foiled by what has been known as "shell creep". In order to get molten alloy to completely fill narrow passages less than 0.76 mm (0.03 inches) thick, a substantial amount of head pressure is necessary. However, the ceramic shell used in such casting processes is not sufficiently strong enough to withstand the head pressure needed to fill these narrow passages. As a result, the head pressure causes the walls of the shell to creep outwardly thus distorting the mould and rendering the part unacceptable. This problem is particularly troublesome when casting at high temperature with slow solidification such as single crystal investment casting. Figure 1 illustrates a cross-sectional view of a standard mould before injecting molten alloy under pressure. The portion of Figure 1 from points A-B represent the alloy charge reservoir, points B-C a charge runner, and points C-D the actual shape of the part to be cast. Figure 2 illustrates a cross-sectional view of the distortion of the mould due to sustained pouring or injecting head pressure of the molten alloy. As can be appreciated from Figure 2, maintaining the head pressure during injection of the molten alloy, causes the wall to become substantially thicker than desired. Points D-E illustrate the distortion of the casting shell due to "shell creep" under substantial head pressure. The "shell creep" phenomenon precludes the casting of structure with dimensional accuracy using prior processes. The present inventors have discovered that the problem of "creep" of a ceramic shell can be solved by varying the injection pressure of a molten alloy into a cavity, having a thickness less than about 0.76 mm (0.03 inches), as a function of time. This concept is graphically illustrated in Figure 3. One means of accomplishing this is illustrated in Figure 4. Figure 4 illustrates a ceramic shell 10 having a part mould 11 having a cavity 12 for producing a structure with a wall having a thickness less than about 0.76 mm (0.03 inches). Molten alloy is injected into the cavity 12 and allowed to solidify to form a desired structure. An alloy charge line 14 is located above the area of the mould cavity 12 designated for the desired structure. Molten alloy is delivered from a reservoir 16 defined by container 15 through the molten alloy charge line 14 to the structure cavity 12 of the mould 11. A charge pressure control means may be a control orifice 18 that is located above the structure cavity 12 of the mould 11 and connected to the alloy charge line 14. The charge pressure control means may be a bleed line 20 to remove excess alloy once the cavity 12 has been filled and to reduce the head pressure of the molten alloy thus eliminating the undesirable "creep" of the ceramic shell. The bleed line 20 communicates with an excess alloy reservoir 22. Preferably the excess alloy reservoir 22 is located immediately below the alloy reservoir 16 and is defined by a hollow column 24 which supports container 15. A plurality of part moulds 11 may be positioned radially around the column 24 with associated charge lines 14 and bleed lines 20 as described above. The typical range of head pressure for casting a wall having a thickness of less than 0.76 mm (0.03 inches) ranges from about 254 mm (10 inches) to about 356 mm (14 inches) of a nickel-based superalloy having a density of about 8.30 Mg/m 3 (0.3 lbs./in. 3 ). The technique may be used to fill cavities less than 0.76 mm (0.03 inches) thick, and up to 1.52 m (5 feet) high and 0.61 m (2 feet) wide. The present invention includes the discovery that very thin passageways of about 0.127 to about 0.381 mm (0.005 to about 0.015 inches) wide. can be formed using a thin core having a density greater than about 70 percent, and preferably about or greater than 99 percent. The present invention is a thin core having a thickness less than about 0.127 to about 0.508 mm (0.005 to about 0.020 inches) and having a density greater than about 99 percent. It has been discovered that the high density of such thin cores gives the thin core sufficient strength to withstand core, wax pattern, mould and casting processes. The material for use in such thin cores is a silica (SiO 2 ) material, commonly called quartz. The surface of the core may be patterned to provide pedestals or indentations by machining or preferably by laser patterning. The very thin ceramic core having a thickness less than about 0.76 mm (0.03 inches) may be shaped before or after making holes in it by, for example, a process of "creep" forming over a contoured die. The very thin ceramic core is made from a material capable of withstanding the alloy casting temperatures and which can be easily removed with a strong acid or base or can be burned out ash free. The material for the very thin ceramic core is quartz. Figure 6 illustrates a suitable thin ceramic core having a pattern formed by a laser on the surface thereof. The pattern forms channels and pedestals in the cast part to increase surface area for heat exchange. The method described includes the discovery that a thin wall structure can be cast using rods having a diameter of about 2.29 to about 14.0 mm (0.09 to about 0.55 inches) to form very small holes in a thin wall structure and to hold the above-described thin core in place during casting. The holes may be formed by a variety of shaped structures in addition to rods depending on the desired objectives. In the present invention, holes in the thin wall may be formed with precision using rods which are placed in a predetermined position so that when the wall structure is cast, the holes would be perfectly positioned on the surface of the thin wall. The use of such rods are particularly advantageous when a curved surface having holes therein is desired. Machining holes on a curved surface of a thin wall is very difficult, time consuming and expensive. Rods are used to cast the structure with very narrow holes thus eliminating machining. However, in order to do so the rods must be received in a pocket formed in the core. The pocket may be formed in the core during the core moulding process or may be subsequently formed in the moulded core by a variety of means such as drilling or directing a laser beam on the core. A pocket 26 (Figure 5) can be drilled into a ceramic core 28 so as to receive and hold the above-described narrow diameter rod 30 in the ceramic core if the core to be drilled is free of the normal protective coating used in the art. This description includes a process which avoids using a protective coating on the ceramic core. As illustrated in Figure 5, the ceramic core 28 free of protective coating is drilled to provide the pocket 26 for receiving and holding the narrow diameter rod 30 in the ceramic core to provide passageways from the void formed by the main core 28 to the thin void formed by the thin ceramic core 32 and to hold the thin ceramic core 32 in fixed relationship between a casting shell 44 and the main core 28. The pocket 26 is made sufficiently large enough to receive the rod 30 yet small enough to firmly hold it in place and withstand any thermal expansion of the rod or ceramic core during the casting process. Generally a clearance ranging from about 0.0127 to about 0.0254 mm (0.0005 to about 0.001 inches) greater than the diameter of the rod 30 is sufficient. The rod 30 may be made out of quartz, Al 2 O 3 or other similar material. A method is provided for holding the very thin, curved core 32 in position in the casting process. According to the method, a first ceramic core 28 is prepared and coated with a first pattern 34 of wax or plastics where metal is desired. Then a curved, very thin core 32 is placed on the first pattern 34 and initially located with locating pins (not shown). The locating pins are positioned near the corners of the thin core 32 and extend into the pattern 34 so as to temporarily hold the thin core in position while the subsequently described hole making process takes place. A hole is drilled through the very thin ceramic core 32, pattern 34 and into the first ceramic core 28 to form a pocket 26 in the first core 28 for receiving and holding in place a small diameter rod 30. A small diameter rod 30 is inserted through the hole so that the rod is received in the pocket 26 formed in the first ceramic core 28. The very thin ceramic core 32 is then covered with a thin pattern 36 of wax or plastics (or other suitable material) where a thin wall of metal is desired. Then holes are formed at an angle through the thin pattern 36 and into the thin ceramic core 32 to form a pocket 38 at a predetermined position where force is needed to keep the thin ceramic core in its curved shape. Although the thin ceramic core 32 may be already curved, the core has a resiliency or elasticity that cause it to want to move out of a curved shape such as that needed to make a gas-turbine blade. A portion of an outer rod 40 is inserted through the hole in the thin pattern 36 so as to be received in the pocket 38 of the thin ceramic core 32. Finally, the thin pattern 36 and the other portion of the outer rod 40 are covered with a ceramic shell 44. The thin core 32 may also be held in position by a geometric relationship of the core and holding rods. A rod may extend through a thin curved core so that the longitudinal axis of the rod is at an angle of ninety degrees or greater to a line tangential to the curved core surface at a point near the longitudinal axis of the rod. This arrangement prevents the core from moving. A substantially straight air passageway may be formed by placing a rod 46 which extend through the thin pattern 36, thin core 32, first pattern 34 and is carried on one end by the casting shell 44 and by the main core 28 on the other end. A labyrinth for air flow may be formed by a first rod 48 held on one end by the casting shell 44 and extend through the thin pattern 36 and the thin core 32. A second rod 50 is positioned a distance laterally from the first rod 48, is held on one end by the main core 28 and extends through the first pattern 34 and the thin core 32. So that when the part is cast and the rods and cores are removed, air may flow from the outer surface of the part perpendicularly through the hole formed by rod 48 in the thin wall associated with the thin pattern 36, parallel to the thin wall through the passageway formed by the thin core 32 and perpendicular through the hole formed by rod 50 in the wall associated with the first pattern 34 out to the void left by the main core 28. This type of labyrinth provides enhanced air contact with the thin wall associated with the thin pattern 36 and provide enhanced cooling of the thin wall such that the wall, in combination with other features described herein, can withstand impinging gases at temperatures of 2371°C (4300°F) and greater. Thereafter the patterns are removed. In the case of wax patterns, the entire mould is preheated to cause the wax to flow out of the mould leaving a cavity for molten alloy and so that the cores are held firmly in place. The alloy is then cast in the mould as described above and the pins and cores are removed, for example, with a caustic solution. Once the above-described thin layer of wax has been coated over the thin ceramic core, the thin layer of wax and other portions of the outer rod are covered with a ceramic shell. The ceramic shell covering is made by first dipping the thin layer of wax in a slurry having ceramic particles in a colloidal silica vehicle. A suitable ceramic powder may have a mesh size of about 325. The mould is then dipped into dry ceramic refractory powder to give strength to the shell. The process of dipping the mould into a ceramic slurry followed by dry ceramic refractory powder is repeated until a sufficient thickness of the shell is achieved, for example, a thickness of about 12.7 mm (1/2 inch). The ceramic slurry and dry powder are dried in an oven at a temperature of about 22-26°C (72-78°F), at 10 percent to 30 percent relative humidity during each dipping step. Using the above-described techniques, it is possible to produce a single-cast, thin wall structure having smooth outer surfaces and a wall thickness as narrow as about 0.127 mm (0.005 inches). Figure 8, illustrates a single piece, thin walled, gas turbine blade with portions removed. Figure 9 is a sectional view of a single piece, multi-wall structure. This single-cast, thin wall structure is capable of withstanding impinging gases at temperatures as high as 2371°C (4300°F). The techniques described can be utilized to produce a variety of products which will be readily apparent from those skilled in the art including gas-turbine blades such as jet engine nozzles 400 (illustrated in Figure 9), leading edge 404 of wings and similar structures for above atmosphere air/space craft 402 (illustrated in Figure 10).

Claims: A core for casting alloy structures comprising a quartz material, the core having a thickness ranging from about 0.127 mm to about 0.508 mm and a density ranging from about 70 to about 100 percent, and wherein said core is curved A core as claimed in claim 1, wherein said material has a density greater than 99 percent.

Inventors: FRASIER DONALD JAMES, MUELLER-LARGENT HEIDI, O'CONNOR KURT FRANCIS, FRASIER, DONALD JAMES, MUELLER-LARGENT, HEIDI, O'CONNOR, KURT FRANCIS
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: B22C9
IPC codes: B22C9, F01D5, B22D19, B22C19