Shell molds and processes for forming shell molds

Abstract

Shell molds and processes for making the shell molds that exhibit high emissivity in the red and infrared regions. In this manner, thermal resistance within a gap formed between solidifying cast metal and the interior mold surface is decreased. In one embodiment, the facecoat region is formed from a slurry composition comprising an aluminum oxide, a green chromium oxide and a silicon dioxide. In another embodiment, the facecoat region is formed from a slurry composition including zirconium silicate and silica with stucco layer of alumina is included.

1 . A shell mold for casting molten material to form an article, comprising:
 a facecoat disposed on an inner surface of the shell mold that contacts the molten material during use thereof, said facecoat having a phase comprising a high-emissivity alumina solid solution, wherein the high emissivity alumina solid solution is formed from a slurry comprising zirconium silicate and colloidal silica with a stucco comprising aluminum oxide.   
     
     
         2 . The shell mold of  claim 1 , wherein the zirconium silicate is in an amount from 70 to 95% by weight, and colloidal silica is 5 to about 30% by weight, wherein the weight percents are based on a total solid content of the slurry composition after drying. 
     
     
         3 . The shell mold of  claim 1 , wherein the aluminum oxide stucco layer has a grain size of 200 mesh to 40 mesh. 
     
     
         4 . The shell mold of  claim 1 , wherein the stucco further comprises titanium dioxide or chromium oxide. 
     
     
         5 . The shell mold of  claim 1 , wherein the slurry further comprises a refractory material selected from a group consisting of, FeO, Fe 2 O 3 , TiO 2 , TaC, TiC, SiC, HfC, ZrC, oxides thereof, and combinations thereof. 
     
     
         6 . A process for forming a shell mold, the process comprising:
 preparing a fugitive pattern;   dipping said pattern in a slurry composition to form a facecoat layer contacts the fugitive pattern, the slurry composition comprising an aluminum oxide, a green chromium oxide, and a silicon dioxide;   depositing a stucco layer onto the facecoat layer;   drying the shell; and   firing the shell at a temperature greater than a melting point of a metal to be cast.   
     
     
         7 . The process for forming a shell mold of  claim 6 , wherein the aluminum oxide, the green chromium oxide, and the silicon dioxide form, upon firing, substantially mullite and corundum 
     
     
         8 . The process for forming a shell mold of  claim 6 , wherein the aluminum oxide is in an amount of 70 to about 95 weight percent; the chromium oxide in an amount greater than 0 to about 9 weight percent, the silicon dioxide in an amount greater than 0 to about 27 weight percent, wherein the weight percents are based on total solids of the slurry. 
     
     
         9 . The process for forming a shell mold of  claim 6 , wherein the stucco layer is formed of aluminum oxide. 
     
     
         10 . The process for forming a shell mold of  claim 6 , further comprising depositing secondary layers of the slurry composition. 
     
     
         11 . The process for forming a shell mold of  claim 6 , wherein the temperature is within a range of 1200° C. to about 1800° C. and for a period of about 5 minutes to about 10 hours.