MXPA99006861A - INTEGRATED CRUCIBLE AND MOLD FOR LOW COST&ggr;-TiAl CASTINGS - Google Patents

Abstract

An integral crucible for casting of reactive metals incorporates a graphite sleeve (10) adapted for attachment of an investment pattern (14) over which a laminate of alternating layers of facecoat slurry (20) and ceramic stucco (22) are applied to form an investment shell. The layers of the laminate extend over a mating portion of the sleeve and firing of the shell vaporizes the investment pattern leaving the sleeve and shell an integral crucible.

Description

INTEGRATED CRUST AND MOLD FOR FOUNDRY OF? -TiAL, LOW COST REFERENCE TO RELATED APPLICATIONS This application claims the benefit of the filing date of provisional application 60 / 036,041 that has a filing date of January 27, 1997 entitled Integrated Crucible and Low-Cost Casting Mold for? -TiAl.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates, in general, to crucibles for casting. More specifically, the present invention provides a one-piece integrated crucible and wax mold for use in the melting of reactive metals, particularly complex forms thereof.

Description of the Related Art The melting and casting of reactive metals such as titanium or titanium aluminides is difficult due to the reactivity of the molten metals with the crucibles containing alumina, zirconium, or silicates that are commonly used in the casting of metals such as iron, nickel or aluminum. Turboblasting rotors are usually melted using nickel superalloys that can be easily melted and cast using a wax mold with a zircon front coating and an alumina-silicate crucible attached to the top of the mold. The molten titanium aluminide will react rapidly with the alumina-silicate crucible making melting impossible. Candidate crucible materials include yttria (Y2O3), thoria (Th02), calcite (CaO) or other rare rare earth oxides. The toria material is radioactive and has a poor resistance to thermal shock. For this reason it has not been exploited in an industrial way. However, itria crucibles are very expensive, from 50 to 100 dollars. The alumina-silicate crucibles used for the castings of nickel superalloys are less than a dollar. Since titanium aluminide and titanium aluminide foundries compete with nickel smelters in a wide range of applications, the high cost of yttria crucibles effectively removes titanium and aluminide titanium castings from the market. This is particularly important for applications where cost is important, for example automotive applications. The material calcia is also a potential refractory material for titanium and titanium aluminides due to its thermal stability. U.S. Patent No. 4,710,481 to Degawa et al, describes the fusion of titanium and titanium alloys in calcia crucibles. However, the calcium material is highly hydrophilic and spontaneously hydrates at atmospheric humidity levels. Hydration is accompanied by changes in volume that cause fracture and laxation. Calcium crucibles can fracture spontaneously after only hours of exposure to atmospheric moisture. For this reason, calcia is an impractical material for a crucible in commercial, industrial media. Therefore, it is desirable to provide lower, industrially practical molds for casting titanium metals and titanium aluminide that overcome the aforementioned drawbacks. The invention will also find application for other reactive metal castings, for example zirconium alloy castings. It is also desirable to provide a low cost, industry-friendly, one-piece crucible and wax mold for melting reactive and non-reactive metals. It is further desirable to provide a graphite crucible or other suspected inductive compound containing graphite that is directly bonded to a wax mold for melting the? -TiAl components and, specifically, to provide a method for producing smelters at The low-cost lost wax of turbo-charged rotors from? -TiAl.

COMPENDIUM OF THE INVENTION In order to solve the limitation of the prior art and obtain the desired ities, the present invention provides a graphite crucible directly attached to the upper part of a wax mold in the shape of the complex molded part., a turbo-charged rotor pays the specific embodiments described herein to form an integrated unit. A method for producing the integrated crucible and mold is characterized by the steps of attaching a wax model to a cylindrical graphite crucible and constructing a wax mold cover around the model and a matching portion of the crucible. The cover of the mold is created by immersing the model and the matching portion of the crucible in a grout of the inert ceramic front coating, allowing the dip coating layer to dry and / or partially cure, apply a layer of ceramic stucco over the layers of the dip coating and the additional laminate layers of dip coating and stucco to achieve the desired thickness of the mold cover. The embodiment of the invention described in detail below provides a one-piece crucible / mold system suitable for casting a metal part under vacuum at temperatures exceeding 1400 ° C.

The method of employing the apparatus of the invention for the melting of complex components takes advantage of the rapid melting and casting of? -TiAl to avoid carbon contamination of the crucible. Carbon is a known contaminant of? -TiAl. At low levels, that is, below 2000 ppm (% by weight), this is added frequently to improve the plastodeformation properties of? -TiAl. However, at higher levels the tensile ductility of? -TiAl is strongly reduced. The graphite crucible / wax mold in one piece prevents the deleterious contamination of? -TiAl by the crucible since the design allows fast melting times and few superheats, which minimizes the potential for carbon contamination.

BRIEF DESCRIPTION OF THE DRAWINGS The details and peculiarities of the present invention will be more clearly understood with respect to the detailed description and the drawings, in which: Figure 1 is a view of an elevation cut of the integrated graphite crucible and the mold waxy; and Figure 2 is a view of an example of a turbo-charged rotor that can be manufactured from ΔTiAl using the present invention.

DETAILED DESCRIPTION OF THE INVENTION One embodiment of the present invention is shown in Figure 1. The integrated crucible and the mold include a cylindrical graphite sleeve 10 having a lower part 12 adapted to accommodate the attachment of a polystyrene wax mold. with a joint slider 15. In the embodiment shown, the joint slider is sealingly attached to the sleeve with wax 16. The sleeve is sized to accommodate a desired fusion billet 17. A wax cover 18 is constituted by laminations of a front facing slurry 20 and ceramic stucco 22 (shown only on a portion of the section for clarity), which extend over and adhere to a lower coupling portion of the graphite sleeve. After firing, which results in the vaporization of the model, the laminate of the hardened cover is fixedly attached to the graphite sleeve forming an integrated crucible and the mold for casting the desired part. An opening 24 in the lower face of the sleeve provides a flow path of molten metal melt to the mold cover while the edge 26 [sic] retains the cast billet in the crucible in a position to introduce heat. A method for creating and employing the apparatus of the present invention includes the steps of attaching the turbo-wheel, polystyrene wheel model to the cylindrical graphite sleeve by submerging the upper part of the polystyrene model in hot wax and inserting it into the recess in the part. bottom of the graphite sleeve. A wax covering is then built around the polystyrene model and the lower portion of the sleeve. As embodied herein, the wax mold process begins with the immersion of the model made of wax or plastic with the shape of the desired cast in a slurry of the front coating prepared from any ceramic inert to? -TiAl, by example yttria or calcia. A suitable slurry for creating a front cladding is described in co-pending application No. 08 / 644,598 entitled "Inert Front Coatings for Lost Wax Casting of Titanium and Titanium Aluminide Alloys" which has a common assignee with the present application, the description of which is incorporated herein in its entirety as a reference. After allowing the dip coating layer to partially dry or cure, alternate layers of ceramic stucco and dip coating are applied in the model and the bottom area of the crucible until a shell of desired thickness is formed. The crucible / wax mold is left to dry perfectly, and then cooked at temperatures close to 1000 ° C for a period of not less than 0.5 hours in an oxidizing atmosphere such as air.

The cooking results in the complete volatilization of the model in a manner familiar to those skilled in the "lost wax" art. The crucible / wax mold is then transferred, while still hot, from the furnace and is placed in a casting chamber and evacuated. A cylindrical billet of the melting material of the cast metal is inserted into the crucible. The metal is then rapidly melted by inductive heating of the graphite crucible. Since the graphite is a strong susceptor, the crucible is heated rapidly to the melting temperature of? -TiAl. The molten metal falls directly into the wax mold bonded with the help of gravity with a minimum of superheat and retention time. The short retention time and the low superheat avoids significant carbon contamination of the crucible. The short fall distance, achieved as a result of the design of a piece of wax crucible / mold allows the complete filling of thin sections. Larger dropping distances, as would be required by two-piece crucible / wax mold systems, result in poor filling due to premature cooling and solidification in the area of the fill tube. After -cooling, the fusion metal, now in the form of the original model is removed from the model. An example.

EXAMPLE I A turbo-rotor model made of polystyrene plastic was fitted to a recess in the bottom of a graphite crucible using wax to attach the model to the crucible. The plastic was immersed in the slurry of the front coating made of a frontal coating of inert Yttria a? -TiAl. After allowing the dip coating layer to dry and / or partially cure, the alternate layers of ceramic stucco and dip coating were applied both in the model and in the lower area of the graphite crucible until a cover was formed of desired thickness. This was subsequently submerged multiple times in support linings of flint powder and alumina silicate to form an integrated wax mold, as schematically shown in Figure 1. The crucible / wax mold containing the plastic turbojet model was left dry perfectly, and then cooked at temperatures close to 1000 ° C for a period of 0.5 hours in air. The whole of the graphite crucible / wax turbosplant mold was then transferred while it was still hot from the furnace, it was loaded with the melting billet of the metal for casting, placed under a quartz glass bell and evacuated. The graphite crucible and metal were then heated by induction resulting in melting of the metal. A drawing of the molten TiAl rotor made from this process is shown in Figure 2. Having now described the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the modalities thus described. . These modifications and substitutions are within the scope and purpose of the present invention as defined in the following claims.

Claims (10)

1. A method for producing an integrated crucible and mold for the melting of reactive metals comprises the steps of: attaching a wax model to a cylindrical graphite sleeve; forming a wax covering around the model and a coupling portion of the sleeve; cook the cover and the sleeve together for complete curing and volatilization of the model. The method as defined in claim 1, wherein the step of attaching the wax pattern to the sleeve consists of: immersing a coupling portion in hot wax; and insert the coupling portion of the model into a recess in the graphite sleeve. The method as defined in claim 1, wherein the forming step consists of: immersing the model in a front facing slurry; allowing the dip coating layer to dry or partially cure; Apply alternate layers of ceramic stucco and immersion coating on the model and the coupling portion of the sleeve until a cover of desired thickness is formed. 4. The method as defined in step 3, wherein the step of applying alternating layers is followed by a step of allowing the cover to dry perfectly. 5. The method as defined in claim 1, wherein the model is polystyrene plastic. 6. The method as defined in claim 1, wherein cooking is performed at temperatures close to 1000 ° C for a period not less than 0.5 hours in an oxidizing atmosphere such as air. The method as defined in claim 3, wherein the step of applying alternating layers is followed by the step of applying at least one lamination layer of flint powders and alumina silicates. The method as defined in claim 3, wherein the grout of the front coating forms an inert ceramic [sic]. 9. The method as defined in claim 8, wherein the inert ceramic consists of yttria. The method as defined in claim 8, wherein the inert ceramic consists of calcium.