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Die for producing a casting and method for making the dieDie for producing a casting and method for making the die description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070181285, Die for producing a casting and method for making the die. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001]The invention concerns a die for producing a casting, particularly a component for a motor or other engine, from a reactive non-ferrous molten metal charge using a casting mould, and a method for manufacturing the die and the use of the die to produce a component. BACKGROUND TO THE INVENTION [0002]The use of casting moulds includes their employment for producing castings of the widest range of types. In the process, a molten charge of a casting material is introduced into a cavity of the casting mould. The casting material then solidifies, giving the casting an external shape which corresponds to that of the casting mould. The casting is subsequently removed from the casting mould. In particular, thin-walled casting moulds, known as ceramic shell moulds in the lost wax casting process, are frequently used. [0003]In the lost wax casting process, materials based on intermetallic compounds are used to produce castings, particularly for high-temperature applications in motive engines. .gamma.-titanium aluminides (.gamma.-TiAl), nickel aluminides and nickel-based alloys are considered particularly suitable. Compared to traditional titanium materials, TiAl alloys are distinguished by better creep properties and lower susceptibility to oxidation. [0004]DE 103 46 953 A1 discloses a die for producing castings and a process for manufacturing the die. The known casting mould is particularly suitable for handling TiAl alloys. The casting mould is manufactured using the slurry dipping and sand coating process. A wax blank in the shape of the casting to be made is prepared, repeatedly coated with a slurry, sand coated and then dried and cured. Finally, the wax blank is removed from the casting mould thus formed. A slurry consisting of water, yttrium oxide, magnesium oxide and calcium oxide is used to produce the known casting mould. In this way, a casting mould is obtained in which at least one surface coming into contact with the reactive non-ferrous molten charge consists of yttrium oxide, magnesium oxide and calcium oxide. [0005]In connection with the use of different slurry compositions, it is also known that further additives matched to the binder system can be added to the slurries, as well as to the ceramic fillers, particularly liquefiers, wetting agents, condensers and antifoaming agents. Liquefiers improve the flow properties and affect the viscosity. Wetting agents improve the wettability of the wax blanks or coatings already present on dipping. The slurry reacts to the addition of even minimal volumes of both constituents, but does not display any pronounced sensitivity to slight "overdoses". Viscosity which is too low can be readjusted by the addition of condensers. Even the lowest concentration of condensers is highly effective. They must be introduced in very small volumes during constant stirring, to prevent agglutination of the slurry. The antifoaming agent is primarily intended to prevent bubbles from forming on the surface of the slurry, which could cause defects in the production of the casting mould. Acids and bases can also be added. Complicated, time-consuming tests are usually necessary to create a slurry system suitable for the purposes of specific applications. [0006]Casting moulds/ceramic shell moulds for handling intermetallic compounds, particularly those of reactive TiAl alloys, are known in which the surfaces of the casting mould/ceramic shell mould facing the front are made of AI.sub.2O.sub.3/Y.sub.2O.sub.3 (G. W. Dickhues: Feingusstechnologie fur intermetallische .gamma.-TiAI-Legierungen [Lost wax casting for intermetallic .gamma.-TiAl alloys], Reihe 5 [Series 5]: Grund- und Werkstoffe [Primary matter and materials], no. 369, VDI-Verlag GmbH, 1994). The casting moulds known as ceramic shell moulds based on the Al.sub.2O.sub.3/Y.sub.2O.sub.3 or Al.sub.2O.sub.3 system are also made by dipping and sand coating within the scope of the known dipping process. [0007]Dickhues represents an attempt to produce SiO.sub.2-free ceramic shell moulds, as earlier lost wax casting methods used slurries which contained not only various ceramic filler systems but also primarily binders in the form of aqueous or alcoholic SiO.sub.2 dispersions (silicic acid brine or ethyl silicate). The reason is that SiO.sub.2 can be reduced by reactive molten charges, such as alloys based on TiAl or NiAl in ceramic shell moulds containing SiO.sub.2. The consequence is considerable oxygen absorption by the molten charge, which may entail drastic losses of strength by the material. In addition, the interaction between the molten charge and the ceramic shell mould may entail pronounced erosion, which may cause the ceramic shell mould/casting mould to be destroyed, the molten charge to absorb ceramic particles and/or poor surface quality of the casting. Dickhues uses acrylic acid ester as a binder for the slurry for the front layers of the casting mould. [0008]Dickhues not only conducted trials of the production of suitable casting moulds/ceramic shell moulds, but also of the production of crucibles which could be used for melting the material subsequently to be cast. Ceramic front layers of a crucible were produced by dipping into a Y.sub.2O.sub.3-titanium chelate slurry and coating with Y.sub.2O.sub.3 sand. However, it was not possible to make a reproducible crucible with a front layer on an yttrium oxide basis. Pronounced cracks consistently appeared when the casting moulds were fired, rendering the crucibles unusable. [0009]Such cracking is presumably attributable to poor fusion of the particles of yttrium oxide to form a mechanically stable layer. SUMMARY OF THE INVENTION [0010]The purpose of the invention is to create a die for producing a casting from a reactive molten charge using a casting mould and a method for manufacturing the die which facilitates reliable, reproducible production of the casting mould. [0011]According to the invention this problem is solved by a die for producing a casting in accordance with independent claim 1 and by a method for manufacturing a die in accordance with independent claim 12. Moreover, independent claim 26 refers to the use of a die to cast a casting. [0012]According to the invention, a die is made for producing a casting; particularly a component for a motor or other engine, from a reactive non-ferrous molten metal charge using a casting mould; which die has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a back-up layer area mechanically stabilising the front layer area, the front layer area consisting of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel. [0013]In accordance with a further aspect of the invention, a method is provided for manufacturing a die for the production of a casting, particularly a motor or other engine component, from a reactive non-ferrous molten metal charge, which method includes the following steps: providing of a wax blank with an external shape which reproduces the casting to be made; formation of a casting mould which has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a back-up layer area mechanically stabilising the front layer area, the wax blank being coated at least once with a slurry; drying and curing the casting mould; removal of the wax blank from the casting mould and firing of the casting mould; the front layer area of the casting mould being formed as a layer consisting of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel. [0014]The particular advantage over the state of the art is that the sintering temperature for the front layer area is reduced and thus achieves improved fusion and ultimately improved physical properties. [0015]A purposeful embodiment of the invention anticipates the minimum of one rare earth oxide being yttrium oxide. [0016]In a preferred embodiment, as the at least one additional metal oxide a metal oxide is used which is an oxide from the same metal comprised in the reactive non-ferrous molten metal charge to be used in the casting mould. Such metal oxide may therefore be referred to as a metal oxide of the same species, namely the same metal. Thereby, a possible contamination of the non-ferrous molten metal charge by external or parasitic elements can be minimized or even completely avoided. Finally, such positive effect refers also to the casting produced using the die. The generation of undesired phases in the casting is avoided. In a further preferred embodiment, exactly one metal oxide of the same species is used in the front layer area. In case of casting a reactive non-ferrous molten metal charge based on a titanium alloy or a titanium aluminide, therefore, in an embodiment of the invention a titanium oxide is used in the front layer area of the casting mould. In a similar way, it is preferred to use a nickel oxide in case of a reactive non-ferrous molten metal charge based on a nickel alloy. There is the advantage of minimized oxygen penetration into the non-ferrous molten metal charge during production of the casting. In addition, the behaviour during the sinter process is farther improved. [0017]A purposeful embodiment of the invention anticipates the yttrium oxide having purity in excess of 99.9%, avoiding problems which could occur due to the presence of impurities in the front layer area. [0018]A purposeful further development of the invention may provide for the ratio by weight between the minimum of one rare earth oxide and the minimum of one metal oxide exceeding 10:1. [0019]In one embodiment of the invention, the front layer area and/or the back-up layer area are purposefully produced by the slurry dipping and sand coating process. This method has the advantage over other methods for manufacturing casting moulds/ceramic shell moulds that even contours with the small dimensions of the wax blank casting reproducing the casting to be produced are coated reliably. [0020]To minimise the disruptive effect of SiO.sub.2 in the front layer area, an advantageous further development of the invention provides for the casting mould in which the front layer area which comes into contact with the reactive non-ferrous molten metal charge contains less than 0.1% SiO.sub.2 by weight. This avoids unwanted reactions between the material of the front layer area and the molten charge. [0021]A preferred embodiment of the invention anticipates that the casting mould in the front layer area and the back-up layer area is openly porous, rendering the casting form gas-permeable. A preferred embodiment of the invention provides for the casting mould in the back-up layer area having pores making up a volumetric proportion of between 20% and 40%. [0022]A purposeful further development of the invention may provide for the casting mould having a surface roughness (Ra) better than 3.2 .mu.m. Such high-quality surfaces are particularly advantageous to fluidic applications in motor and other engine components. Continue reading about Die for producing a casting and method for making the die... 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