CROSS-REFERENCE TO RELATED APPLICATIONS
- STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
- BACKGROUND OF THE INVENTION
The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to the inventors and/or the assignee of any royalties thereon.
1. Field of the Invention
The present invention relates to methods and materials for manufacture of casting cores utilized to form complex metal components. More specifically, the present invention relates to methods of manufacture of casting cores composed of soluble materials mixed with ceramic materials for casting complex metal components.
2. Description of the Related Art
Conventional investment casting methods typically utilize casting cores composed of machined graphite or steel for manufacture of relatively non-complex shapes. Use of molds lacking the need for expensive and time-consuming machining of metal compounds is preferred to provide cost-effective production of a wide range of complex metal matrix composite (MMC) components. A multitude of technological advances in the automobile and aerospace industries, and in the field of generating exotic composite materials have led to the need for casting molds which are capable of forming complex shaped and structurally rigid MMC components by known pressure molding processes without the need for machining of the casting mold.
Benefits have been identified in the use of a hardened salt compound as a non-machined mold core utilized for pressure molding of complex components, as explained in U.S. Pat. No. 4,904,423, issued to Foreman et al. Methods for pressure infiltration casting have been disclosed for generation of complex metal matrix composite components, as explained in U.S. Pat. No. 6,360,809, issued to J. A. Cornie, et al. As disclosed in U.S. Pat. No. 6,776,219, issued to J. A. Cornie, et al. (hereinafter, the '219 patent), use of graphite or steel molds are excessively expensive when the molds must be machined into complex or intricate shapes. The '219 patent discloses a method and materials for preparing investment molds useful in pressure infiltration casting of near net-shape metal or MMC components. The investment mold materials disclosed in the '219 patent include commercially available castable refractory cement materials modified by the addition of magnesium oxide in order to maintain the stability of the investment mold during pressure infiltration casting with molten metal. The '219 patent discloses use of other refractory materials including alumina, silica, magnesia, graphite, feldspar, and other refractory materials which are closely packed to form a dense, low porosity investment mold. The '219 patent does not disclose use of a molten salt such as sodium carbonate mixed with ceramic particulates, which, when solidified into a casting core, is impermeable when subjected to molten metal and is readily dissolved after casting when exposed to non-corrosive solvents such as water for separating the casting core from a cast MMC component.
- BRIEF SUMMARY OF THE INVENTION
Accordingly, there is a need for a slurry mixture composed of non-reactive soluble materials mixed with ceramic particulates, with the slurry mixture being readily solidified into a complex shaped mold which is resistant to thermal expansion during exposure to molten metal in pressurized casting. Further, a need exists for a process of producing a casting core composed of a mixture of a metal salt and ceramic particulates which, when solidified, provides casting surfaces impervious to molten metals during pressurized casting. In addition, a need exists for a method of manufacturing a casting core composed of selected materials which are impermeable to molten metals during pressure casting to form a complex composite component, while the casting core is rapidly separated from the complex composite component at completion of the casting process by exposure to a non-reactive solvent.
A process of manufacturing a soluble casting core mold is disclosed for the production of complex shaped MMC components which may include hollow portions therein. The soluble casting core is generated by a sequence of steps which provide a molten slurry mixture composed of an alkali metal salt having ceramic particulate materials dispersed therein. After sufficient mixing, the slurry mixture is solidified in a mold pattern to form a casting core mold configured to generate a complex shaped component during a casting process. The casting core mold is utilized for casting molten metal alloys to form a complex shaped metal matrix composite (MMC) component which is readily separated from the casting core mold by dissolving the core mold by immersion in heated water or exposure to steam.
The process includes providing molten sodium carbonate to form the basic component of the slurry mixture in which a ceramic particulate material is dispersed evenly therein. One embodiment includes magnesium oxide particulates mixed throughout the molten sodium carbonate salt in a mixing ratio including about 1 part magnesium oxide mixed with about 3 parts molten sodium carbonate. Alternative ratios of magnesium oxide mixed with molten sodium carbonate are utilized to adjust the density of the casting core slurry and to adjust the CAE of the solidified casting core mold to approximate the CTE of the molten metal composite material selected to form the MMC component. The magnesium oxide particulates serve as heterogeneous nucleation agents within the molten sodium carbonate, and further serve to reduce the grain size of the sodium carbonate salt during solidification of the molten slurry mixture thereby resulting in a dense core mold when the molten sodium carbonate and magnesium oxide when solidified after being poured in a preheated mold pattern formed of metal such as stainless steel or graphite. The dense core mold formed is resistant to surface etching and mold failure upon contact with molten metal during pressure infiltration casting.
The process further includes the casting core slurry mixture being solidified in a preheated mold pattern thereby attaining a casting core mold having smooth surfaces impervious to molten metal during high temperature and pressure casting. Upon removal of the preheated mold pattern from the solidified casting core mold, the non-metal casting core mold is easily machined as needed in order to form exactly configured surfaces utilized as a complex shape mold during pressure infiltration casting with molten metals.
BRIEF DESCRIPTION OF THE DRAWINGS
The process of manufacturing water soluble casting cores further includes applying surface coating materials lacking water contamination onto the contact surfaces of the casting core mold. A step of casting proceeds with the casting core mold filled with liquid aluminum, magnesium, or other alloy matrix composites, preferably utilizing a pressure infiltration casting process. After cooling, the solidified complex shaped MMC component is separated from the casting core mold by exposure of the core mold to a non-reactive solvent such as steam and/or heated water, resulting in dissolution of the casting core mold and separation from the MMC component. Additional machining of the MMC component is not typically required due to the ability of the casting core mold to accurately duplicate each surface contour for generation of the complex shaped MMC components. The complex shaped MMC components generated by the process of manufacture are utilized in various industries including production of automobiles, aircraft and spacecraft.
The present invention is illustrated in the drawings in which like element numbers represent like parts in each figure, including:
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a process flow diagram for manufacturing soluble casting core molds of the present invention, for use in production of complex shaped MMC components.
Referring now to FIG. 1, a process 10 of manufacturing a soluble casting core mold 40 is disclosed, with the soluble casting core mold 40 being composed of a salt material which is water soluble and mixed with metal oxide particulates forming a slurry mixture when molten which is readily poured for casting, and is impervious to molten metal when the slurry mixture is solidified into the core mold which is readily dissolved when exposed to heated water in order to free a complex shaped cast MMC component 50 formed within the soluble casting core mold 40. The soluble casting core mold 40 is utilizable in pressure casting processes such as a pressure infiltration casting process. Benefits of utilizing the soluble casting core mold 40 include the core mold is impermeable when exposed to molten metal under typical pressure and temperatures used for pressure infiltration casting. Further, the core mold is readily dissolved in non-corrosive solutions such as heated water for rapid removal of the casting core mold 40 from the complex shaped cast MMC component 50 generated by the process 10 of manufacturing the soluble casting core mold 40.
The selected materials and steps utilized in the process 10 of manufacturing the soluble casting core mold 40 include providing and heating 12 a soluble salt such as sodium carbonate 32 to form a molten sodium carbonate base solution referred hereinafter as a molten slurry 36. Sodium carbonate is selected due to its low cost, higher melting point, ease of flow when molten, and its dissolution when exposed to heated water. A step of mixing 14 includes selecting a ceramic particulate such as magnesium oxide particulates 34, and mixing the magnesium oxide particulates with the molten slurry 36. One embodiment for the process 10 includes mixing magnesium oxide particulates having a powder size of between 5 microns to about 150 microns throughout the molten sodium carbonate. One proportion for mixing 14 includes adding between about 5 parts of magnesium oxide particulates to about 95 parts of molten sodium carbonate. Alternate proportions for mixing include a range of between 5 parts of magnesium oxide particulates up to about 30 parts of magnesium oxide particulates, mixed with between about 95 parts of molten sodium carbonate, or reduced to about 70 parts of molten sodium carbonate. An alternate proportion range for the molten slurry mixture being composed of molten sodium carbonate by weight percentage in a range of between about 50% to about 97%, and mixing in magnesium oxide particulates by weight percentage in a range of between about 50% to about 3%. The molten sodium carbonate mixed with magnesium oxide particulates forms a casting slurry mixture 36 readily pourable 18 after a step of providing 16 a mold pattern 38 composed of metal such as stainless steel and/or graphite, or a similar high temperature resistant material such as ceramic compounds utilized by those skilled in the art for molds used in pressure infiltration casting processes.
During the manufacturing process 10, the step of providing 16 further includes a step of preheating the mold pattern 38 to a temperature between about 400° C. to about 650° C. before or simultaneously with the molten slurry mixture 36 being poured therein. An alternative step of preheating the mold pattern 38 includes superheating the mold pattern 38 to a temperature between about 850° C. to about 950° C. before the molten slurry mixture 36 is poured therein. The mold pattern 38 is configured to include at least one inlet for rapid receipt of the poured molten slurry mixture 36 in sufficient volume to fill the mold pattern 38 before solidification of the slurry mixture 36 occurs during a step of cooling 20 for the slurry mixture. Further, the mold pattern 38 is configured of sufficient volume and in the preferred design core shape configuration of which the molten slurry mixture adopts upon cooling 20 of the molten slurry mixture 36 to form a solidified casting core mold 40 which can be readily dissolved in heated water. Upon solidification, the casting contact surfaces of the casting core mold 40 are preferably impermeable during an upcoming step of infiltrating 24 with aluminum or similar casting metals heated within typical casting temperature ranges utilized for the pressure infiltration casting processes. In a step of removing 22, the casting core mold 40 is removed from contact with the mold pattern 38. The casting core mold 40 preferably retains complex shaped surfaces 42 configured to include inwardly extending channels or voids to provide the preferred mold pattern 38.
A step of infiltrating 24 includes pouring molten metal such as aluminum and/or magnesium alloys in matrix composite solutions into the solidified casting core mold 40. The step of infiltrating 24 further includes the poured molten metal being retained by the casting core mold 40 without casting surface breakthrough, until the molten metal solidifies to form a cast metal matrix component 50. Pressures utilized by pressure infiltration casting into the casting core mold 40 include pressurization of molten metal for infiltration casting at hydrostatic pressures up to about 75 atmospheres. One preferred pressure for the infiltration casting of molten metals in the casting core mold 40 is about 68 atmospheres.
After removing 22 the casting core mold 40 from the mold pattern 38, a step of machining 23 may be utilized for machining and/or surface polishing of the casting surfaces 42 of the casting core mold 40. The step of machining 23 provides for exact duplication of the plurality of casting surfaces of the casting core mold 40 with a preferred design for the MMC component 50 having outer and inner contours and/or interior void spaces 52, when manufactured with the casting core mold 40. Due to the casting surfaces 42 of the casting core mold 40 being composed of solidified sodium carbonate providing a base solution having a plurality of magnesium oxide particulates dispersed therein, the casting surfaces 42 are readily machined at a lesser expense than typical machining of molds composed of stainless steel or graphite materials. An additional step of coating may be included following the step of machining 23, in order to distribute substantially impervious coating materials on the plurality of casting surfaces of the casting core mold 40. The steps of machining 23 and/or surface coating are completed before the step of infiltrating 24 the molten metal into the casting core mold 40, in order to minimize leakage of molten metal through, or infiltration into the coated casting surfaces 42, and to improve the efficiency for separating the casting core mold 40 upon exposing 26 to heated water 46 or steam 48, from the solidified cast metal component 50. The finished complex shaped surface configuration of the solidified cast metal matrix component 50 is intended to be an exact duplicate of the designed mold pattern 38, without a requirement for additional metal machining or surface finishing other than cleaning the metal matrix component 50.
The benefits of producing a soluble casting core mold 40 having sodium carbonate as the main core content include sodium carbonate is widely available and is inexpensive, and sodium carbonate is readily mixed when in a molten state in any selected ratio of sodium carbonate and magnesium oxide particulates. Alternative ratios of magnesium oxide mixed with molten sodium carbonate are utilized to adjust the density and the CTE of the solidified casting core mold to approximate the CTE of the molten metal composite material selected to form the complex shaped MMC component 50. When dispersed within the molten sodium carbonate, the magnesium oxide particulates provide: (a) heterogeneous nucleation agents in the slurry mixture, (b) reduction of the grain size of the sodium carbonate salt during solidification of the molten slurry mixture thereby resulting in a dense solidified core mold, and (c) acceleration for dissolution of the sodium carbonate material during exposure of the solidified casting core mold 40 to heated water, steam or a similarly non-corrosive heated liquid. The dense casting core mold 40 formed is resistant to surface etching and resistant to core mold failure upon contact with molten metal during pressure infiltration casting. Further benefits of the process 10 of manufacturing and utilizing the soluble casting core mold 40 for generation of complex shaped MMC components 50 include the core mold being impermeable when exposed to molten metal under typical pressures and temperatures used for pressure infiltration casting. Upon completion of the casting process for the complex shaped MMC component 50 and solidification within the core mold 40, dissolving 28 the core mold 40 is achieved by immersion in a heated water solution 46 or another liquid which is not corrosive to the MMC component 50. In the alternative, the core mold 40 is readily dissolved 28 by exposure to steam 48 or another superheated liquid which is not corrosive to the MMC component 50. The step of dissolving provides for rapid release 30 of the complex shaped MMC component 50 without damage to the cast metal matrix components of the complex shaped MMC component 50 and with minimal expense.
While numerous embodiments and methods of use for this invention are illustrated and disclosed herein, it will be recognized that various modifications and embodiments of the invention may be employed without departing from the spirit and scope of the invention as set forth in the appended claims. Further, the disclosed invention is intended to cover all modifications and alternate methods falling within the spirit and scope of the invention as set forth in the appended claims.