Ingenious design allows alloy to remain lightweight with the strength to withstand gunfire

May 2016 - Studies on the attributes of metal foams continue to evolve. When Dr. Afsaneh Rabiei began her work at North Carolina State University in Raleigh, she honed in on the shortcomings of the material and became interested in developing a composite metal foam, one that provided an ideal strength-to-weight ratio. More recently Rabiei developed an armor structure using composite metal foam that is currently being tested—that is, shot at—by the U.S. Army to test its strength—and it’s holding up.

“We were looking for a light and cheap metal with a low melting point to cast around steel hollow spheres, a metal easy to play with,” says Rabiei, professor at the Mechanical and Aerospace Engineering Department at NC State. She came to Raleigh after a few years research as a post doc at Harvard. “We decided on aluminum.” Rabiei’s first composite metal foam project was funded by the National Science Foundation. 

Rabiei’s approach focused on the reasons for the premature failure of materials in the past. “The porosity of the metal (steel) [was causing the material to fail],” she says. “There needed to be an infrastructure in place where there was an equal distribution of loading.” That is why she distributed porosities in a uniform manner throughout the metal to prevent stress concentration and premature failure while providing a light material with potential for energy absorption.

Hollows and spheres

Hollow spheres worked well as a lightweight structure but, alone, it wasn’t strong enough. “I added a metallic matrix design to the mix,” explains Rabiei. “Imagine you have a jar full of hollow spheres—you need something to fill the gaps in between those objects to reinforce the overall structure.”

Rabiei casts aluminum around steel hollow spheres. “When you cast the aluminum, the steel spheres do not melt down,” she says. Rabiei also mixes the spheres with metal powder. “The beauty of that technique is we can make the entire matrix and spheres out of the same metal such as steel, a cheaper alloy. 

“We mix, sinter and press them, creating the steel composite foam. Now we have two types of material, one includes a low melting point matrix such as aluminum with higher melting point spheres and the other one is comprised of hollow spheres and matrix made from the same material such as steel.” Rabiei says, adding the composite metal foams could be constructed out of any combination of metals and alloys, but steel and aluminum work well due to low cost and wide availability.

Rabiei’s team is working on transitioning the composite metal foam from lab to commercial applications, such as car bumpers, biomedical devices and what the U.S. Army is currently testing: armor. Rabiei’s hope is that her composite metal foam will protect lives by armoring helicopters or aircraft, for example. 

“We’re working with investors and talking with federal agencies in efforts to move forward to real-life applications—including space shuttles and lunar explorations—there is a lot of potential in different fields for this material,” Rabiei says. 

Composite metal foam will be viable sooner rather than later. “The affordability of steel and aluminum opens up lots of possibilities.” MM

Images: Advanced Materials Research Group, Afsaneh Rabiei