Injection technique expands foam-metal applications 3/1/99

Newark, DE--Foam metal, produced by mixing a little powdered-foaming agent into a commercial metal powder, is a porous metal matrix with unique properties. For example, the density of aluminum foams range from 0.5 to 1 gm/cm³, with even lower densities possible. As a result, aluminum foam can float on water.

When foamed, aluminum and its alloys--and other metals such as zinc, or lead--provide excellent energy absorption and higher strength than foamed polymers. Unlike other engineered composites, the material is recyclable.

Given these advantages, one might wonder why foam metals aren't used in more applications? Up until now, production techniques limited part geometry to sheets or simple profiles. But, a new heated injection system developed at the University of Delaware under the guidance of Fraunhofer Resource Center's Chin-Jye Yu, now makes more complex part geometries possible.

"Until recently, furnace 'free-foaming' was the only way to foam metal," explains Design-project Leader Dina Berlingieri. During free-foaming a billet of material is placed in a furnace, and heated to temperatures greater than the melting point of the matrix metal. As the billet melts, a foaming agent releases gas in a controlled way. The gas discharge expands the metal slowly as a semi-solid, foamy mass. The foaming process stops as the furnace cools. Density is controlled by changing the foaming-agent content and varying the heating conditions.

"Now, our novel heating system with temperature control injects metal-matrix materials in a foaming state, into complex molds. Injection heating lets the foaming process occur inside the cavity--something that was previously impossible. This means metal foams can be used in more applications that require parts with non-uniform geometries," says Berlingieri.

The original system concept sprang from the design of a common glue gun that extrudes a solid glue stick at its melting temperature through the gun barrel's tip. Metal-foam injection is similar, only a billet of solid metal gets pushed through the system. As the billet reaches its melting temperature, it is extruded in the desired foamed state.

Metal-foam injection provides a versatile way to produce complex foam-metal parts without compromising the material's density. Materials produced by this method, could replace honeycomb composites, polymers, and metals, in automobile, aerospace, and other structural applications. 

Until now, metal-foam production was limited to sheets or simple profiles cut from sheets.
Measuring 1/2 x 1/2 x 6-inch, a billet, machined from pre-foamed metal sheets, is placed in the pre-heating stage. The billet heats to 425C while suspended in the center of a 9-inch long, high-temperature steel tube. Radiant heat enters the tube through a 6-inch long window cut out on one side. Once the billet reaches pre-heat temperature, a 3/4-inch diameter piston pushes the billet into the foaming section. Induction coils provide localized heating via electromagnetic waves to bring the billet’s condition to its foaming temperature, a gradient of 200C. The foaming section uses a 9-inch long alumina tube with a 1/8-inch wall thickness.

Additional details…Contact Chin-Jye "Mike" Yu, Fraunhofer Resource Center, 501 Wyoming Rd., Newark, DE 19716, (302) 369-6761

Other Applications

• Energy absorption

• Lightweight construction

• Thermal barrier insulation

• Flame arrestors

• Vibration damping