Mold System for the Casting of Reactive Alloys

Abstract

The present invention relates to novel binder and slurry formulations used to form molds for casting metal alloys and, more particularly, reactive metal alloys. The shell molds lead to more uniform castings which exhibit limited alpha case as compared to other currently available shell mold systems.

1 . A slurry composition for forming investment casting molds for casting titanium and titanium alloys comprising:
 a refractory including one or more materials selected from yttria, zirconia, and rare earth metal oxides;   a positively charged colloidal silica;   a soluble organic polymer;   a stabilizing acid;   water; and   optionally, one or more additives selected from the group consisting essentially of cross-linking agents, sintering aid, surfactant, anti-foaming agent and insolubilizer.   
   
   
       2 . The slurry of  claim 1  wherein said refractory is selected from the group consisting essentially of fused yttria, sintered yttria, fused zirconia, fused calcium zirconate, Er 2 O 3 , Sc 2 O 3 , Pr 2 O 3 , SrO, La 2 O 3  and mixtures thereof. 
   
   
       3 . The slurry of  claim 1  wherein said positively charged colloidal silica is present in an amount of between about 0.5 wt. % to about 20.0 wt. % of the slurry. 
   
   
       4 . The slurry of  claim 1  wherein said organic polymer is present in an amount of up to about 2.0 wt. % on a dry weight basis. 
   
   
       5 . The slurry of  claim 4  wherein said organic polymer is polyvinyl alcohol. 
   
   
       6 . The slurry of  claim 5  wherein said polyvinyl alcohol is at least about 85.0% hydrolyzed. 
   
   
       7 . The slurry of  claim 1  wherein said stabilizing acid is present in an amount of up to about 2.0 wt. % of the slurry. 
   
   
       8 . The slurry of  claim 7  wherein said stabilizing acid is an organic acid. 
   
   
       9 . The slurry of  claim 8  wherein said stabilizing acid is a glycolic acid. 
   
   
       10 . The slurry of  claim 1  wherein said sintering aid is in the form of a ground refractory having an average particle size of less than about 2.0 microns. 
   
   
       11 . The slurry of  claim 1  wherein the sintering aid is present in an amount of between about 0.10 wt. % to about 3.0 wt. %. 
   
   
       12 . The slurry of  claim 10  wherein the refractory material and the sintering aid are pre-mixed prior to addition into the slurry. 
   
   
       13 . The slurry of  claim 1  wherein said positively charged colloidal silica, said soluble organic polymer, said stabilizing acid and water are pre-mixed to form a binder sol prior to addition into said slurry. 
   
   
       14 . The slurry of  claim 1  wherein said slurry is used as a prime coat. 
   
   
       15 . A binder composition for use in slurries for forming investment casting molds comprising:
 a positively charged colloidal silica;   a polyvinyl alcohol;   a stabilizing acid; and   water.   
   
   
       16 . The binder of  claim 15  wherein said positively charged colloidal silica is present in an amount of between about 0.5 wt. % to about 20.0 wt. % of the slurry. 
   
   
       17 . The binder of  claim 15  wherein said organic polymer is present in an amount of up to about 2.0 wt. % on a dry weight basis. 
   
   
       18 . The binder of  claim 15  wherein said polyvinyl alcohol is at least about 85.0% hydrolyzed. 
   
   
       19 . The binder of  claim 15  wherein said stabilizing acid is present in an amount of up to about 2.0 wt. % of the slurry. 
   
   
       20 . The binder of  claim 15  wherein said stabilizing acid is glycolic acid.