Scientists continue to seek lightweighting benefits from metal foam

January 2015 - One of the definitions of foam by the Merriam-Webster dictionary includes “a substance that is like a thick liquid made of many small bubbles; or a soft material that is used to make many products.” Instead of a light and frothy liquid concoction, metallurgical foam is a dense substance comprising beads or layers of beads of metal that build structure. 

Researchers like Joachim Baumeister, project manager of powder technology at the Fraunhofer Institute, an application-oriented research organization in Munich, Germany, are working to meet the increasing demand for the material as industrial companies and their customers seek to reduce fuel consumption and CO₂ emissions. 

Baumeister likens the process of creating metal foam to baking a loaf of bread or a cake. “Instead of flour we use metal powder, instead of baking soda, we use another foaming agent (e.g. titanium hydride). The mixture of flour and baking soda cannot be ‘foamed,’” explains Baumeister. “Similarly, the mixture of metal powder and foaming agent cannot be foamed. It has to be compacted. For compacting the metal powder mixture, various methods can be used: pressing extrusion, rolling, etc. The resulting foamable metal is virtually non-porous, but can be foamed up by a final heat treatment (or by ‘baking the cake’).”

The type of aluminum alloy or aluminum powder used in the process is not important, says Baumeister, other than it should be dry, not heavily oxidized and should not contain many impurities. “But we’ve even successfully produced metal foams from metal grinding dusts and chips/cuttings,” he adds.

Open porous aluminum foams are already being used for myriad functions and are undergoing evaluation by several manufacturers for applications such as heat exchangers with high inner surface and high thermal conductivity, like oil and water coolers, and in heat pumps. 

“We produce open porous aluminum foam by high-pressure die casting,” Baumeister explains. As a result, the transition “from the present prototype stage to the production stage will not be that big.”

Fabricating foam

Metal foams can be welded but the welding seam that results is made of solid filler material. “This somewhat increases the weight of the foam part, so we usually try to manufacture the foamed parts near net-shape and avoid welding,” Baumeister says. Metal foams can also be cut and machined but the clamping forces “should be selected as not to compress and damage the foam structure.”

In addition to aluminum, metal foams include magnesium, titanium, nickel, gold and steel. Steel foams are cheaper and lighter than solid steel, but not as strong.

Metal foams are increasingly sought after and produced on an industrial scale (nickel foams, for example, are being used for battery electrodes and aluminum for energy absorbers for Audi Q7 cars). To move farther afield into additional commercialization, however, Baumeister believes the initial costs of starting materials, such as metal powder and foaming agent powder, are drawbacks. “The usage of recycled aluminum sawing chips instead of aluminum powder and cheaper foaming agents (CaCO₃) will help reduce costs,” he says, noting that further simplifying the process will also help boost consumption. MM

All photos courtesy Fraunhofer-IFAM, Bremen

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