Blowing bubbles

Metal foams are finally beginning to come of age. Jon Excell reports on a group of materials that stand on the brink of industry-wide acceptance.

Imagine a material that’s as strong as steel yet light enough to float on water. A material that can damp sound or help amplify it, that can dissipate or repel heat, has fantastic electrical properties and can even be used to improve your golf handicap.

All these properties and many more can be found in a set of materials that are easy to manufacture, relatively cheap and have been around for the last 60 years: metallic foams. In the past poor processing techniques have stymied the huge potential of metallic foams, but now, thanks to improvements in these techniques, the material stands on the brink of industry-wide acceptance.

There are two types of metal foam: closed-cell foams and open-cell foams. Open-cell foams consist of cells joined only by struts – it helps to imagine them as containing exploded bubbles. However, closed-cell foams are, as the name suggests, completely closed off. Both types call for different manufacturing processes and are suited to different applications.

Making a closed-cell foam is relatively simple. Typically, air is blown into a piece of molten metal (usually aluminium) and the resulting foam is allowed to cool. The problem lies in controlling the uniformity of the material. For instance, a common problem with closed-cell foams was bubbles draining away while the metal solidified.

Manufacturers have got over this hurdle by making the materials used more viscous, through, for instance, the addition of calcium. However, more general improvements to the control over the fabrication process are also making a big difference to the quality of the material that can be produced.

Dr Paul Mummery, a materials scientist from the University of Manchester’s engineering department, said that thanks to these improvements, we are now seeing a resurgence of interest in metal foams.

‘In the past the material was poor quality, but manufacturers have gotbetter at exploiting the potential.’

This view was echoed by Dr Jerry Lord, who, two years ago, headed up a National Physical Laboratory (NP) study to gauge the commercial potential of the technology. Open-cell metal foam production is only slightly more complicated. The most popular method uses a polymer open-cell foam as a template for a casting. This is covered in casting sand, the polymer is melted away, then the casting is refilled to make a metal foam.

In general the closed-cell structure is favoured for energy absorption applications while open-celled structures are better suited for acoustic or thermal management. This is because an open-cell foam permits flow throughout.

For instance, many manufacturers of open-cell foam metals are pushing their technology for use in heatsinks for cooling computers, and the huge surface area of open-cell nickel-based foams has already led to them being widely used in the nickel hydride batteries that power mobile phones.

Mummery added that the material also has great potential as a hydrogen storage device and the growth of hydrogen fuel cell technology represents a potentially huge market.

NASA has also reportedly been experimenting with the use of open-cell foams for satellite mirror supports. The advantage of foams here is that they are very light, and the thermal expansion coefficients can be engineered to be either low or negative. This means that moving from sunlight to darkness should not have too much of an effect on the supports.

Closed-cell foams are better suited to structural applications, and there is particular excitement about their use as energy-absorbing materials in vehicles. Siemens has already experimented with metal foams as buffers on trams and trains, but so far their use in the automotive industry has been limited to concept cars by Karmann and Audi.

Clearly metallic foams have many desirable properties for different applications, but the idea of exploiting a number of these properties in one application is what really appeals to designers.

For instance, Mummery said that closed-cell foams would be ideal for the panels on ship doors, which must not only pass stringent flame tests, but must also be light and absorb sound.

Metal foams also exhibit interesting acoustic properties. While civil engineers in Japan are experimenting with putting closed-cell foams under motorways to lower traffic noise, BMW engineers are exploiting the fantastic acoustic transparency of open-cell foams as support structures for loudspeakers.

There are currently some 12 producers marketing a range of metal foams, mostly based on aluminium. But other metals – copper, nickel, stainless steel and titanium – can be foamed and are available on order. Cambridge-based materials expert Granta design has even developed a software tool, the Cambridge Engineering Selector, it claims allows users to compare the properties of 130 different foams with those of conventional metals.

However, while most current methods are concerned with manufacturing aluminium-based foams, researchers at the California Institute of technology (Caltech) have made a foam from bulk metallic glass (BMG), a metallic material with a non-crystalline microstructure, making it amorphous, or ‘glassy’ in its solid state.

The advantage of BMG over, for instance aluminium, is its exceptionally high strength. The Caltech team quotes yield strengths of around 2Gpa (gigapascals), compared to around 250Mpa (megapascals) for aluminium. Foams made from BMG are, therefore, expected to have an even greater ability to absorb energy.

According to Chris Veazey, the researcher involved in the development of the new foam, the material has the stiffness of conventional metal but the springiness of a trampoline. Tentatively named Bubbloy (a combination of ‘bubble’ and ‘alloy’) the material is made of palladium, nickel, copper and phosphorus. This alloy was already known as one of the best bulk metallic glasses around, but Caltech’s contribution was figuring out how to get it to foam.

The manufacturing process involves adding hydrated boron oxide powder to a chamber containing molten palladium mixed with nickel, copper and phosphorus. Water vapour released by heat from the boron oxide aerates the alloy, creating small bubbles.

The mix is allowed to cool to room temperature and is then reheated while air is simultaneously sucked from the chamber. This causes the bubbles to expand slowly within the alloy. Preliminary results indicate that the closer the bubbles are to each other the springier the material is.

Crucially, because these bubbles are in a metal without crystals, they can be compressed by forces that would permanently damage other foams and yet still return to their original shape. And as for its strength-to-weight ratio: initial castings of Bubbloy are strong yet light enough to float in water, added Veazey.

Veazey believes the material has great potential for use in the crumple zones of cars. ‘It should make one car safer than another where the structures in the crumple zone are made of conventional metals,’ he said.

This work in still in the early stages. But it has already taken its first step out of the laboratory, and is currently being further developed by Caltech spin-off company, Liquidmetal which has also developed a range of amorphous alloys that are being used in tennis rackets and golf clubs. However, Liquidmetal’s Otis Buchanan was unwilling to say when Bubbloy is likely to be commercialised.

Although metal foams are already being used, there is a sense that they have not yet been fully exploited and are still waiting to be embraced by a big industry as the material of choice. ‘Manufacturers are currently looking for one or two applications, preferably somewhere like the automotive industry where they can get in large volumes and it becomes the accepted material,’ said Mummery.

They must overcome the conservative instincts of engineers, who have to be convinced the material is safe, and then that its benefits would outweigh the effort of redesigning entire structures around it.

Mummery sounded a note of caution for UK manufacturers. While UK engineers have led the world in developing understanding of metal foams, our dwindling manufacturing base means that we’re now getting left behind, he said.

In the meantime, throughout Europe, the US and particularly in Japan – host of the Metfoam 2005 exhibition – interest in metallic foams is getting very serious.