Aluminum casting method with helium insertion

Abstract

Helium gas at ambient pressure is inserted into an expendable mold at a fill tube or other port exiting the bottom of the mold. The helium displaces atmospheric air in the mold cavity, and the displaced air exits the cavity through the fill tube. When the air in the mold cavity and fill tube has been completely displaced by the helium, the fill tube is submerged in molten aluminum, and the aluminum is drawn into the mold cavity through the fill tube to form a casting having significantly reduced oxide inclusions and significantly improved fatigue strength.

1 . A method of casting molten aluminum in an expendable mold having a cavity, a body surrounding said cavity and a fill passage that extends through said body and into said cavity, the method comprising the steps of: 
 orienting said mold so that said fill passage opens in a downward direction;    dispensing helium gas at ambient pressure in or under said fill passage so that the helium gas naturally rises through said fill passage and into said cavity to displace atmospheric air in said cavity and said fill passage by virtue of the helium's light weight relative to said atmospheric air; and    filling said cavity with molten aluminum to form a casting.    
   
   
       2 . The method of  claim 1 , where said body of said expendable mold is formed of permeable material, and the method includes the step of: 
 sealing at least a top surface of the mold body to prevent said helium gas from escaping upward out of said cavity.    
   
   
       3 . The method of  claim 1 , where the atmospheric air displaced by said helium gas is expelled through said fill passage.  
   
   
       4 . The method of  claim 1 , including the steps of: 
 positioning a helium dispenser in or under said fill passage; and    dispensing helium gas at ambient pressure with said helium dispenser to fill said cavity and fill passage with helium gas.    
   
   
       5 . The method of  claim 1 , wherein said mold includes a purge passage for purging gases in said cavity when said cavity is filled with molten aluminum, and the method includes the step of: 
 providing a downwardly extending course in said purge passage such that said helium gas cannot escape through said purge passage prior to the filling of said cavity with molten aluminum.    
   
   
       6 . The method of  claim 1 , wherein said casting is a turbocharger impeller wheel.  
   
   
       7 . The method of  claim 1 , including the steps of: 
 securing a fill tube to said fill passage;    partially submerging said fill tube in said molten aluminum; and    filling said cavity with molten aluminum through said fill tube and said fill passage.