Advanced Processing of Hollow Sphere Foams

Abstract

Abstract : Closed-cell metallic foams were fabricated by bonding millimeter sized hollow spheres at points of contact. The composition of low density metallic foams with hollow sphere architecture achieved to date includes Ti-6Al-4V, 400 series stainless steel, and Inconel 718. Hollow spheres are formed as powder shells from slurries of non-metallic precursor powders at room temperature. Subsequent heat treatment coupled with either direct reduction or hydride-dehydride processing converts precursors into the appropriate metal alloys . To produce titanium alloy spheres, the starting powder is titanium alloy hydride. Thermal treatment in an inert atmosphere decomposes the hydride and sinters the titanium powder in the sphere walls to greater than 96% relative density. Both titanium and Ti-6V-4V spheres and foams have been produced. However, oxygen content remains a concern for the titanium compositions explored and the titanium alloy foams produced so far behave like brittle foam. For stainless steel spheres, the starting powder is a mixture of iron and chromium oxide. Thermal treatment in hydrogen reduces the oxides to Fe-Cr alloys with less than 2% porosity in sphere walls. The nominal composition is close to that of 405 stainless. Carburization in CO/CO2 atmosphere followed by heat treatment produces foams of either 410 or 420 type stainless steels depending on carbon content. Compressive stress-strain behavior was measured on point contact bonded stainless foams both before and after carburization. A sample with 0.5 wt% carbon at a relative density of 15% indicated a yield strength of 16 MPa. Specific strengths of the foams were positioned between open and closed cell models. This was encouraging because bonding in the foams was less than optimum and the hollow sphere walls contained numerous defects. With improvements in processing, strengths should increase while maintaining ductility.