Mold System for 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.

What is claimed is: 
     
         1 . A slurry composition for forming investment casting molds comprising:
 a refractory selected from yttria, zirconia, and rare earth metal oxides;   a negatively charged or neutral colloidal silica;   an organic polymer;   water; and   a crosslinker for the organic polymer.   
     
     
         2 . The slurry of  claim 1  wherein the refractory comprises fused yttria, sintered yttria, fused zirconia, Er 2 O 3 , Sc 2 O 3 , Pr 2 O 3 , or La 2 O 3 . 
     
     
         3 . The slurry of  claim 1 , wherein the refractory comprises fused calcium zirconate or SrO. 
     
     
         4 . The slurry of  claim 1  wherein said negatively charged or neutral colloidal silica is present in an amount of between about 0.5 wt. % to about 20.0 wt. % of the slurry. 
     
     
         5 . The slurry of  claim 1  wherein the organic polymer is present in an amount of up to about 3.0 wt. % on a dry weight basis. 
     
     
         6 . The slurry of  claim 1  wherein said organic polymer is selected from the group consisting of latex, polyvinyl alcohol, hydroxypropyl cellulose and polymer emulsions based on styrene, butadiene, acrylics, and vinyls. 
     
     
         7 . The slurry of  claim 1  further comprising a sintering aid in the form of a ground refractory having an average particle size of less than about 2.0 microns. 
     
     
         8 . The slurry of  claim 7  wherein the sintering aid is present in an amount of between about 0.10 wt. % to about 3.0 wt. %. 
     
     
         9 . The slurry of  claim 7  wherein the refractory material and the sintering aid are pre-mixed prior to addition into the slurry. 
     
     
         10 . The slurry of  claim 1  wherein said positively charged colloidal silica, said organic polymer, said stabilizing acid and water are pre-mixed to form a binder solution prior to addition into said slurry. 
     
     
         11 . The slurry of  claim 1  wherein said slurry composition has a pH of at least about 8.0. 
     
     
         12 . The slurry of  claim 1  wherein said slurry composition has a pH of at least about 8.5. 
     
     
         13 . The slurry of  claim 1  wherein said slurry composition has a pH of at least about 9. 
     
     
         14 . The slurry of  claim 1  comprising a negatively charged colloidal silica. 
     
     
         15 . The slurry of  claim 1  comprising a neutral colloidal silica. 
     
     
         16 . A method of preparing a casting shell for use in molding a metal object, comprising
 applying at least one coat comprising a stable slurry according to  claim 1  as a prime slurry onto a wax pattern;   draining excess slurry from the pattern; and   stuccoing the resulting part with a refractory grain.