Composite metal foam and methods of preparation thereof

Abstract

The present invention is directed to composite metal foams comprising hollow metallic spheres and a solid metal matrix. The composite metal foams show high strength, particularly in comparison to previous metal foams, while maintaining a favorable strength to density ratio. The composite metal foams can be prepared by various techniques, such as powder metallurgy and casting.

1 . A method of preparing a composite metallic foam comprising:
 placing a plurality of hollow metallic spheres in a mold;   casting a matrix-forming liquid metal into the mold, thereby filling interstitial spaces around the hollow metallic spheres; and   solidifying the liquid metal, thereby forming the composite metallic foam having a solid metal matrix around the hollow metallic spheres.   
     
     
         2 . The method of  claim 1 , further comprising preheating the mold and the hollow metallic spheres. 
     
     
         3 . The method of  claim 2 , comprising preheating to a temperature that is greater than the casting temperature of the matrix-forming liquid metal but is less than the solidus temperature of the hollow metallic spheres. 
     
     
         4 . The method of  claim 1 , wherein said placing step comprises vibrating the mold. 
     
     
         5 . The method of  claim 1 , wherein the hollow metallic spheres and the matrix-forming liquid metal comprise different metals or metal alloys. 
     
     
         6 . The method of  claim 5 , wherein the conditions of the method are such that the formation of intermetallic compounds is minimized. 
     
     
         7 . The method of  claim 1 , wherein said casting step comprises filling the mold from the bottom up. 
     
     
         8 . The method of  claim 1 , wherein the metal or metal alloy comprising the matrix-forming liquid metal has a melting point that is at least 25° C. less than the solidus temperature of the metal comprising the hollow metallic spheres. 
     
     
         9 . The method of  claim 8 , wherein the metal or metal alloy comprising the matrix-forming liquid metal has a melting point that is at least 50° C. less than the solidus temperature of the metal comprising the hollow metallic spheres. 
     
     
         10 . The method of  claim 1 , further comprising adding one or more reinforcement materials to the matrix-forming liquid metal prior to said casting step. 
     
     
         11 . The method of  claim 1 , wherein the matrix-forming liquid metal is cast into the mold in such a manner as to facilitate complete filling of the interstitial spaces around the hollow metallic spheres. 
     
     
         12 . The method of  claim 1 , further comprising coating the interior of the mold with a release agent prior to introduction of the matrix-forming liquid metal into the mold. 
     
     
         13 . The method of  claim 12 , wherein the release agent comprises boron nitride. 
     
     
         14 . The method of  claim 1 , wherein said casting step comprises screening the matrix-forming liquid metal before or during introducing the liquid metal into the mold. 
     
     
         15 . The method of  claim 14 , comprising screening the liquid metal through a ceramic screen. 
     
     
         16 . The method of  claim 1 , wherein the hollow metallic spheres have an average diameter of about 0.5 mm to about 20 mm. 
     
     
         17 . The method of  claim 1 , wherein the hollow metallic spheres have an average wall thickness that is about 1% to about 15% of the average sphere diameter. 
     
     
         18 . The method of  claim 1 , wherein the hollow metallic spheres have an average wall porosity of less than about 12%. 
     
     
         19 . The method of  claim 1 , wherein the formed composite metallic foam has a strength, evaluated as the plateau stress, of at least 35 MPa. 
     
     
         20 . The method of  claim 1 , wherein the formed composite metallic foam has a density of less than about 4 g/cm 3 . 
     
     
         21 . The method of  claim 1 , wherein the formed composite metallic foam exhibits an energy absorption of at least about 20 MJ/m 3 .