INERT CALCIA FACECOATS FOR INVESTMENT CASTING OF TITANIUM AND TITANIUM-ALUMINIDE ALLOYS

Abstract: A calcia mold facecoat is applied to a mold for casting parts composed of reactive metals such as titanium aluminide. The facecoat is composed of a calcium carbonate based slurry comprising a dense grain calcium carbonate powder and an aqueous based binder. It is applied to a wax or plastic pattern used in the lost wax process for fabricating a casting shell. The mold is built using multiple dipping of alumina-silicate slurries, and then fired at high temperatures in an oxygen rich environment. The metal part is cast before the fired mold can cool below about 800 DEG C. Organometallic based slurry binders are avoided and significant cost savings are realized owing to the benign nature of the aqueous based suspensions with respect to the environment.

Description: [ NERT CALCIA FACECOATS FOR INVESTMENT CASTING OF TITANIUM AND TITANIUM-ALUMINIDE ALLOYS BACKGROUND OF THE INVENTION Field Of The Invention: The present invention relates to mold facecoats for use in the casting of reactive metals, particularly complex shapes thereof Description Of The Prior Art: Melting and casting of reactive metals such as titanium or titanium aluminides is difficult due to the molten metal's affinity for elements such as oxygen, nitrogen and carbon. At elevated temperatures necessary for casting, titanium and titanium aluminides react with ceramic facecoats, for example zirconia and zircon, commonly used in the casting for Fe or Ni based alloy components The reaction of molten titanium and titanium aluminides can result in rough surface quality as well as the formation of a brittle alpha phase at the surface ofthe component. The brittle surface layer results in premature crack formation and a deleterious reduction in mechanical properties and component surface life Removal ofthe brittle surface layer can be accomplished by mechanical and/or chemical methods of polishing. However, this adds cost to the component and is frequently impractical due to the complexity of shape or difficulty in maintaining dimensional tolerance. Formation ofthe brittle surface layer in cast titanium and titanium aluminide components can be done using thermodynamically stable facecoats. Few facecoats exist which have the necessary stability compared with molten titanium and titanium aluminides. Candidate facecoat materials include yttria (Y2O3), thoria (ThO 2 ), calcia (CaO) and other exotic rare earth oxides. Thoria is radioactive and suffers from poor thermal shock resistance. For this reason it has not been exploited industrially. Yttria is a strong candidate and has been pursued as a facecoat mateπal by a number of investigators USP 4,703,806 to Lassow et al discloses a yttπa slurry for the casting of reactive metals The slurry performs well but suffers from high raw mateπal cost Present costs of yttπa are approximately S60/lb Zircon, a facecoat widely used for nickel castings, is less than S l/lb Since titanium and titanium alumimde castings compete with nickel castings in a wide vaπety of applications, the high cost of a yttπa facecoat effectively pπces titanium and titamum aluminide castings out ofthe market This is particularly important for cost sensitive applications, for example automotive applications Calcia is also a potential refractory material for titanium and titanium aluminides due to its thermal stability USP 4,710,481 to Degawa et al. discloses melting titanium and titanium alloys in calcia crucibles However, calcia is highly hydrophilic and spontaneously hydrates in ambient levels of atmospheπc moisture The hydration is accompanied by volume changes which cause cracking and spallation Calcia crucibles can spontaneously crack after only hours of exposure to atmospheric moisture. For this reason calcia is an impractical material for commercial, industrial environments SUMMARY OF THE INVENΗON The present invention provides an industrially practical, low cost facecoat for the fabrication of molds used to cast titanium and titanium aluminide metals, which overcomes the above-mentioned drawbacks. The facecoat ofthe invention also finds application for other reactive metal castings, for example zirconium alloy castings. A specific object of this invention is to provide a low cost, industrially praαical facecoat comprising calcia for casting reactive and non-reactive metals The calcia facecoat is formed in-situ by heating a calcium carbonate precursor facecoat. A further object of this invention is to provide a calcia mold facecoat for use in the fabrication of molds for casting reactive metals such as titanium and ^ titanium aluminide which reduces or eliminates reactivity between mold and the reactive metal. The calcia facecoat is, again, derived from a calcium carbonate precursor facecoat. Another object of this invention is to provide a calcium carbonate based slurry mold facecoat which can be applied relatively smoothly and evenly to a wax or plastic pattern used in the lost wax process for fabricating casting shells for casting reactive metals such as titanium and titanium aluminide The calcium carbonate will transform upon heating into a calcia based facecoat A further objective of this invention is to provide a method for producing high precision investment castings of reactive metals, such as titanium and its alloys, and non-reactive metals, such as nickel and its alloys, at a lower cost than previous techniques A still further objective is the reduce or eliminate the amount of surface grinding or chemical milling required to produce precision investment castings of reactive metals such as titanium and titanium aluminide. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice ofthe invention. The objects and advantages of the invention may be realized and obtained by means of instrumentality's and combinations particularly pointed out in the appended claims To achieve the objects in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method of using a calcium carbonate based slurry comprising a dense grain calcium carbonate powder and an aqueous based binder as a mold facecoat in the fabrication of molds for casting reactive metals. To further achieve the objects and in accordance with the purpose ofthe invention, as embodied and broadly described herein, the invention comprises a method of fabricating a casting shell for casting reactive metals comprising the steps of; preparing a pattern; dipping the pattern in a calcium carbonate based slurry comprised of a dense grained calcium carbonate powder and an aqueous based binder, building a mold using multiple dipping of alumina-silicate slurries, fiπng the mold at high temperatures in an oxygen rich environment, and casting the metal part before the fired mold can cool below about 800°C Another objective of the invention is to avoid the use of organometallic based slurry binders such as metal alkoxide or chelates which are environmentally hazardous By using aqueous based suspension, significant cost savings occur resulting from the benign nature ofthe suspensions with respect to the environment This procedure takes advantage ofthe well known chemical transformation of calcium carbonate to calcia at temperatures above about 750°C At these high temperatures the newly formed calcia does not hydrate Since the metal casting is done before the mold cools, the hydration ofthe calcia is prevented, allowing the non-reactive calcia facecoat to be advantageously utilized As a secondary benefit, sequential operation of mold firing then immediate casting results in reduced energy needs and fast manufacturing throughput. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiments ofthe invention and the accompanying drawings in which: Fig. 1 is an SEM micrograph of a "green" calcium carbonate coating Fig. 2 is an SEM micrograph of a coating after it has been transformed to calcium oxide;and Fig. 3 is a photograph of a γ-TiAl rotor produced from the in-situ formed calcium oxide facecoat. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, a calcium carbonate based slurry comprising a calcium carbonate powder and an aqueous binder is used as a mold facecoat in the fabrication of molds for casting reactive metals. As used herein, the term "reactive metals" means metals such as titanium, titanium alloys and titanium aluminides which have a high negative free energy of formation for the oxide, nitride and carbide ofthe metal. As embodied herein the reactive metals include but are not limited to titanium and its alloys, titanium aluminides, and zirconium and its alloys The invention also pertains to non-reactive metals such as nickel and its alloys While existing facecoat materials, for example zircon, have extensive commercial success in nickel casting, the calcium carbonate to calcia conversion process offers a cost advantage due to the low raw material cost of calcium carbonate For the purposes ofthe present invention, the aqueous based binder preferably has both a low temperature green strength and is a high temperature ceramic binder Preferably, the aqueous based binder is a colloidal suspension of inorganics, for example zirconia, titania, hafhia, or silica For the purposes ofthe present invention, the aqueous based binder may also include additional additives or solvents to effect other desirable characteristics such as to control the viscosity ofthe binder or adjust the pH. In accordance with a preferred embodiment ofthe present invention, the calcium carbonate based slurry, comprising a calcium carbonate powder and a tailored aqueous based binder, is used to form a mold facecoat in the fabrication of an investment casting shell by the "lost wax" process. As embodied herein, a pattern made of wax or plastic having the shape ofthe desired casting is prepared and dipped in the calcium carbonate based slurry After allowing the dipcoat layer to partially dry and/or cure, alternate layers of ceramic stucco and dipcoat are applied until a shell of desired thickness is formed. The mold is allowed to dry thoroughly, and then the mold is fired at temperatures near 1000°C for a peπod of no less than 0 5 hour in an oxidizing atmosphere such as air The fiπng results in the complete volatilization ofthe pattern m a manner familiar to those skilled m the lost wax " art The fiπng also has an additional purpose of converting the green calcium carbonate facecoat into a calcia facecoat The mold is then transferred to a casting chamber while hot and the mold is filled with molten metal with the assistance of gravity, pressure or centrifugal force or other techniques familiar to those skilled in the art The metal is allowed to cool After cooling, the cast metal, now in the shape of the oπginal pattern is removed from the pattern By using the mold while still above about 750°C from the fiπng, the deleteπous hydration of calcia is prevented, resulting in a low cost, inert facecoat compπsing of unhydrated calcia The following examples are presented to provide a more complete understanding of the invention The specific techniques, conditions, mateπal s. proportions and reported data set forth to illustrate the principles and practice of the invention are exemplary and should not be construed as limiting the scope of the invention EXAMPLE I This example illustrates the production of a calcium oxide facecoat using a calcium carbonate slurry precursor A calcium carbonate slurry was made by ball milling 1700 grams of calcium carbonate, 233 5 grams of colloidal silica binder (Ludox Remet 30), 550 grams of de-ionized water and 45 grams of Darvan 821 A dispersant. Ball milling was accomplished with the addition of 7354 grams of 0 375 inch zirconia cylinders Milling was done for 2 5 hours Viscosity was then measured using a Brookϋne II viscometer Viscosity was adjusted by the addition of calcium carbonate powder or xanthum gum and the ball milling resumed Milling, viscosity measurement and adjustment were done periodically for a total of 20 hours to attain a target viscosity of between 50 and 100 as measured by the Broojchne II viscometer The pH and particle size were also monitored with a target pH of 9 5 and a target particle size of 0 3 to 3 micrometers The pH was adjusted by adding either sodium hydroxide to raise the pH or TMA (tetramethvl ammonium hydroxide) The purpose of ball milling the slurry is to de-agglomerate the calcium carbonate powder The de-agglomeration results in the suppression of cracks duπng drying It also has a secondary effect in that the "hiding" power of the coating is greatly enhanced The solution is next applied to a substrate and allowed to dry at ambient temperature to form a "green" calcium carbonate coating, shown in Figure 1 That coating is next heated to 1000°C, transforming it to an adherent calcium oxide coating, shown in Figure 2 EXAMPLES 2-10 Examples 2-10 were produced identical to example 1 except that they used a different binder from the colloidal silica or no binder at all The compositions are listed in Table I Table I EXAMPLE 11 A calcium carbonate slurry as described in Example 1 is made and applied to a polystyrene preform of a turbocharger rotor by dipping the rotor The coating is allowed to dry under ambient conditions, forming a facecoat on the turbocharger rotor pattern. It is subsequently redipped multiple times into backup coatings of fϋntgrains and alumina silicate powders to form a standard investment mold Such techniques are widely used and understood in the investment cast industry The completed investment mold is next passed through a furnace at between 900 and 1 100°C to melt the plastic preform and simultaneously transforming the calcium carbonate facecoat into a calcium oxide. The still hot mold is next filled with a suitably sized charge of titanium-aluminide alloy, placed in a vacuum and inductively melted and cast into the turbocharger rotor cavity Since the mold is . . always held above about 800°C prior to the cast, the facecoat remains calcium oxide which is inert to molten titamum aluminide alloy This prevents the hydration of calcium oxide which would occur at room temperature, destroying the non-reactive nature of the facecoat Such a calcium oxide facecoat could not be otherwise formed by room temperature processes since the calcium oxide in an ambient environment would hydrate, rendering it unusable as a facecoat mateπal The in-situ transformation of calcium carbonate- to calcium oxide duπng the removal of the plastic pattern and its immediate use by casting represents a novel aspect ofthe invention A photograph of the castTiAl rotor made from this process is shown in Figuπr3 Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that vaπous changes and modifications may suggest themselves to one skilled in the art, all falling within the scope ofthe present invention as defined by the subjoined claims

Claims: CLAΓMS FOR PCT FΓLΓNG FOR CASE NO 0030-3875 (2900) I A method for casting metal comprising the steps of forming a facecoat slurry comprising a calcium carbonate power and an aqueous binder; applying the slurry to a casting pattern, forming a casting shell over the casting pattern to create a mold, firing the mold; transferring the mold to a casting chamber; filling the mold with molten metal, allowing the metal to cool, and removing the mold from the cast metal part 2 A method as defined in claim 1 wherein the step of firing the mold is conducted at about 1000°C for greater than 0 5 hour. 3 A method as defined in claim 2 wherein the step of firing is further conducted in an oxidizing atmosphere 4 A method as defined in claim 1 wherein the steps of transferring the mold and filling the mold with molten metal are accomplished while the mold temperature remains greater than about 750°C from the step of firing 5 A method as defined in claim 1 wherein the step of firing the mold is conducted at a temperature sufficient to convert the calcium carbonate facecoat into a calcia facecoat 6 A method as defined in claim 1 wherein the aqueous binder is comprised of a colloidal suspension of inorganics 7 A method as defined in claim 6 wherein the inorganics for the colloidal suspension are selected from the group of zirconia, titania, hafnia, and silica. 8. A method as defined in claim 1 wherein the step of forming a casting shell comprises the steps of: allowing the slurry to partially dry; and applying a ceramic stucco laminate to form a desired thickness shell for the mold. 9. A method as defined in claim 8 wherein the step of applying a ceramic stucco laminate comprises applying alternate layers of ceramic stucco and slurry 10. A method as defined in claim 6 wherein the molten metal for casting comprises a reactive metal selected from the group of titanium, titanium-aluminide, zirconium, alloys of titanium and alloys of zirconium.

Inventors: BARRY EOIN JOSEPH, FANELLI ANTHONY JOSEPH, LASALLE JERRY CAPRIOTTI, SNOW BRIAN JEFFREY, BARRY, EOIN, JOSEPH, FANELLI, ANTHONY, JOSEPH, LASALLE, JERRY, CAPRIOTTI, SNOW, BRIAN, JEFFREY
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1997
Main classification: B22C1
IPC codes: B22C1, B22C3, B22C9