EUREKA September 1997 cover story

ADVANCED MATERIALS

Metal foam delivers

superlight components

Foamed metals are strong, rigid, fire and impact resistant and a fraction of the weight of their conventional counterparts. Tom Shelley reports

By using metals with a porous, bone-like structure, designers can produce solid metal components that will float on water, provide light weight EMC protection and absorb large amounts of impact energies. These 'foamed metals', which contain 50% or more holes, also boast twice the stiffness per unit weight of conventional metals.

Structures with closed cell structures are likely to be used in structural applications, while open celled structures show promise for heat exchangers and heat sinks. They could also be used to reduce the turbulence over aircraft wings. While commercial product is available, several research projects are looking to improve product quality and find ways of reducing production costs.

Human and animal bones, the ultimate in micro-engineered girder construction form the inspiration for materials with many holes. Structural foams based on polymers are available widely, but those based on metals offer advantage in terms of strength and temperature resistance. The best quality metal foams presently available bear the 'Alulight' name, and were developed at the Institute of Materials and Machine Mechanics, an offshoot of the Slovak Academy of Sciences in Bratislava, Slovakia. Alulight panels are available from the Mepura company, castings from the Illichmann company, both are based in Austria.

Vladimir Gergely, from the Institute in Bratislava, is working with Dr Bill Clyne at the University of Cambridge. He says that production is presently limited to tens of kilogram quantities at a time, but could easily be scaled up.

Dr. Clyne explains that the material is made by coating particles of pure aluminium, aluminium silicon or aluminium magnesium silicon casting alloys with small amounts of titanium or zirconium hydride. The powder is made into sheets and other shapes by compacting the particles, and then heating them up in a mould to a temperature at which the alloy becomes partly liquid. The aluminium combines with the titanium or zirconium to form intermetallic compounds. This leaves hydrogen gas which produces the porous structure. The result is a structure with a pores of very even size, comprising up to 80 or 90% of the overall volume. Sheet is usually made from extruded rods of powder.

The base material has already been seen to have many potential uses (see box), but in its present form, is not particularly cheap.

Since porosity in aluminium castings is normally a property which occurs only too easily, it may seem surprising that research is necessary to increase this effect. Indeed, it is not hard to make such materials porous: the problem comes in controlling the porosity.

The most obvious alternative approach is to bubble air or gas through the molten metal as it solidifies. Such an approach has been investigated by Alcan Aluminium who are able to produce foamed aluminium in large quantities. The only problem is that pores so made tend to be tens of millimetres across, whereas an even and fine structure, more like bone, is more desirable.

Research at Cambridge in concentrating on improving the melt foaming process, retaining its essentially low costs while aiming to produce the sorts of structures seen in powder produced material, with pores only 1mm across. One approach being studied is to use a gas generating powder with a special coating to delay gas release until the metal alloy is partially solidified.

University of Cambridge Department of Materials Science and Metallurgy

Foamed aluminium has many uses

The vendors of Alulight say that it has a density of from 300 to 1,000 kg/m3, that is, 30 to 100% that of water. Components made of the material can therefore be expected to float. Alulight nonetheless shows an electric shielding effectiveness comparable to that of silicon steel up to 10MHz, and is superior at higher frequencies. It can thus be used for light weight EMC shielding. The material also has excellent sound and vibration energy absorption properties, yet unlike polymer foams, remains self supporting up to the onset of melting.

Left hand graph: sound absorption of open celled Alulight plate as a function of frequency and distance from solid wall compared with bulk aluminium (bottom line). Right hand graph: deflection of Alulight panels and sandwiches under load compared with aluminium sheet of the same weight (right hand bars)

The vendors say that it can be used to make impact energy absorbing components for cars, and lifting and conveying systems. It may also be used for self supporting, stiff, light weight panels for road and rail vehicles and architecture. It may be used for its sound deadening properties in buildings and in other situations subject to high temperatures, moisture, mildly oxidising atmospheres, or where there is concern about a fire risk. It may be used as fillings in hollow sections in order to stiffen them against buckling.

Top graph: compressive load-deformation curves of Alulight of different densities compared with that for bulk aluminium (blue area).

Bottom graph: Magnetic field shielding effectiveness as a function of frequency for samples of same weight. Bottom line represents bulk aluminium

In open cell form, it may be used to make heat exchangers, filters and catalysts. In heat exchanger mode, one fluid would be circulated through the open structure, while the other fluid would circulate against the outside. Alternatively, fluid could be circulated through the material, one surface of which was in contact with some power electronic device which required cooling. The very high internal surface area of aluminium foam should render it much more effective, size for size than conventional heat sinks. As normally produced, aluminium foams tend to be sealed or nearly so on all surfaces which interface with the surface of the mould. Dr Clyne says it is 'Quite easy to complete this sealing operation'

A possible exotic use might be to use open celled foam sections in aircraft wings. There is evidence that allowing air to emerge from wing surfaces through arrays of fine drilled holes can reduce air turbulence.

Mepura (Panels)

Illichmann (Castings)

Properties of Alulight panels, without aluminium cover sheet

Density

kg/m3

500

750

1000

Elasticity modulus

GPa

5

9

14

Minimum plastic collapse stress

MPa

3

13

25

Minimum bending strength

MPa

7

15

20

Bending stiffness with respect to Aluminium sheet of same weight

%

210

170

150

Thermal conductivity at 20 deg C

W/m.K

6-15

15-35

25-50

Electric conductivity

106.S/m

3

5

9

Loss factor

 

0.003

0.004

0.004

Energy absorption (Up to 20MPa compressive stress)

MJ/m3

4.5

4.3

0.5

The material can easily be machined, screwed or bolted like wood

The material is decorative, easy to recycle and environmentally friendly.

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