Process for Obtaining Y-Tial Pieces by Casting

Abstract

The present invention concerns the development of a process for obtaining γ-TiAl castings, with a maximum thickness of 20 mm, starting from melting charges constituted by commercially pure Ti and Al. In this process the metallic charge is induction melted in a ZrO 2 ceramic crucible inside coated with Y 2 O 3 , the pouring being performed by centrifugation in ZrO 2 moulds, with an Y 2 O 3 contact coating, obtained by the lost wax moulding process. The whole sequence of operations is accomplished under argon atmosphere. The use of this process, besides avoiding the use of previously melted charges needing very specific furnaces, allows to obtain parts with homogeneous chemical composition and very low contamination with residual elements, without surface oxidation, with a hardness quite uniform from the surface to the inside of castings, which reveal a surface finishing identical to that recommended by the international standards for steel castings obtained by the same moulding process.

1 . A process for obtaining a γ-TiAl alloy casting parts, comprising: melting an alloy charge in a multi-layered ceramic crucible; and pouring the melted charge into multi-layered ceramic moulds, whereby all the processing steps are accomplished in a chamber under controlled atmosphere.  
   
   
       2 . The process of  claim 1 , wherein the controlled atmosphere is an argon atmosphere at a pressure of about 10 −2  to about 10 −4  bar.  
   
   
       3 . The process of  claim 1 , wherein before beginning the step of melting, set-up is formed whereby the crucibles, the mould and the melting charge are pre-heated to a temperature between about 200 to about 400° C. for a period of about 1 to about 3 hours.  
   
   
       4 . The process of  claim 1 , wherein before beginning the step of melting, the chamber undergoes a washing step with an argon flux under pressure of about 1 to about 3 bar, at a flow rate of about 50 to about 100 litters/minute, for a period of about 5 to about 10 minutes, followed by evacuation to a minimum pressure of about 5×10 −6  bar, whereby said step of washing is repeated a minimum of 3 times.  
   
   
       5 . The process of  claim 1 , wherein the melting charge inside the melting crucible, is disposed such that fragments of aluminium (Al) are located close to the wall of the crucible, and fragments of titanium (Ti) are located inside the crucible, preventing direct physical contact of the titanium (Ti) fragments with the crucible wall.  
   
   
       6 . The process of  claim 1 , wherein the step of melting is done by maintaining the molten state above a minimum temperature of 1400° C.  
   
   
       7 . The process of  claim 1 , whereby the step of pouring is done at a metal temperature lower than 1700° C., and wherein the rotation speed of the chamber is not less than 350 rpm.  
   
   
       8 . The process of  claim 1 , wherein the crucible, made of totally or partially stabilized Si0 2 , graphite, mulite, SiC or Zr0 2 , is coated with a Y 2 0 3  film, having at least 100 μm of thickness.  
   
   
       9 . The process of  claim 8 , whereby, in order to coat the melting crucibles, the crucible, is filled with an emulsion of Y 2 0 3 , followed by at least 30 seconds of residence inside the crucible, followed by spilling the excess for at least 5 minutes out, and drying the crucible at no less than 200° C. for a period of at least 24 hours, repeating the whole coating procedure at least once, with the final drying step at a temperature of 500° C.  
   
   
       10 . The process of  claim 8 , wherein the final composition of the coating is comprised of at least 99% of Y 2 O 3 .  
   
   
       11 . The process of  claim 1 , whereby obtaining the mould is done by filling a metallic box with sand agglomerated by a sodiumsilicate/CO 2  process, where a multi-layered ceramic shell comprised of an interior layer of Y 2 O 3  and outer layers of partially or totally stabilised Si0 2 , ZrO 2 , Al 2 O 3  or graphite, was previously made.  
   
   
       12 . The process of  claim 11 , whereby the multi-layered ceramic shell, is produced by application of at least a coating to a wax particle with a Y 2 O 3  based slurry, and successive coatings with a slurry of different type alternated with coatings of granulated refractory materials of the same type.  
   
   
       13 . The process of  claim 12 , wherein the Y 2 O 3  based slurry comprises: a binder; a refractory agent; urea; acetic acid; an anti-foaming agent; and a wetting agent.  
   
   
       14 . The process of  claim 12 , wherein the slurry used in the outer coatings comprises: a binder; a refractory agent; acetic acid; an anti-foaming agent; and a wetting agent.  
   
   
       15 . The process of  claim 12 , wherein the refractory agent is: SiO 2 , Zr O 2 , Al 2 O 3  or graphite, with a granulometry between 30 and 325 mesh.  
   
   
       16 . The process of  claim 12 , wherein the step of applying coatings to a wax particle, comprises the following steps: immersing the wax particle in the Y 2 O 3  slurry for at least 5 seconds; Dripping out of the slurry for at least 10 seconds; Drying of the mould during at least 24 hours, at a temperature of about 30 C. and a relative moisture of 40-50%; immersing the wax particle in the slurry  claim 14 , for at least 5 seconds; Dripping out of the slurry for at least 10 seconds; Coating the mold with the refractory agent of  claim 15 , till the saturation of the slurry,; Drying of the mould for at least 24 hours, at a temperature of about 30 C. and a relative moisture of 40-50%; an repeating the steps of second step of immersing to the second step of drying at least 5 times, wherein in each repetition a refractory agent of growing granulometry, from coating to coating is used.  
   
   
       17 . The process of  claim 13 , wherein the Y 2 O 3  based slurry is prepared using a mixer, and comprises the steps of: adding the urea to the binder with the wetting agent and anti-foaming agent already included; adding the refractory agent, at a rate of 25% every 30 minutes; and slowly adding the acetic acid, in bulk, over a 5 minute period.  
   
   
       18 . The process of  claim 14 , whereby the slurries intended for the outer coatings, is prepared using a mixer using the following steps: adding the anti-foaming agent, followed by the wetting agent to the binder; and slowly adding the refractory agent to the binder-additives.  
   
   
       19 . The process of  claim 13 , wherein the values of viscosity are kept between 12 to 16 seconds flow through a Zahn #4 cup for the Y 2 O 3  based slurry, and 11 to 13 seconds for the slurries intended to the outer coatings.  
   
   
       20 . Parts produced according to  claim 1 , wherein the parts are intended for industrial applications, wherein the average roughness values is not higher than those suggested by the international standards, showing absence of superficial roughness increase enough to force finishing operations for removing the surface rough material and presenting surface finishing conditions in agreement with the established by the 359-01 Technical Recommendation standard of BNIF for castings obtained by ceramic moulding processes.  
   
   
       21 . The process of  claim 13 , wherein the binder, refractory agent, anti foaming agent and wetting agent is colloidal, 325 mesh, RSD-10 burst and Victawet respectively.  
   
   
       22 . The process of  claim 14 , wherein the binder is colloidal SiO 2 , Zr0 2 , Al 2 0 3  or graphite, the refractory agent is 325 mesh Si0 2 , Zr0 2 , A120 3  or graphite, the anti-foaming agent is RSD-10 burst and the wetting agent is Victawet.  
   
   
       23 . The process of  claim 18 , wherein the step of slowly adding the refractory agent to the binder, wetting agent and anti-foaming agent mixture, comprises the steps of adding of 50% of the refractory agent in the first 15 minutes of mixing; adding another 25% of the refractory agent over the next 5 hours, at the rate of 5% per hour; and adding the remaining 25% of the refractory agent over 40 hours, at the rate of 5% each 8 hours.