Weight watchers’ sandwich

Advances in foamed aluminium alloy provide strength and stiffness without adding bulk, says Sue Stuckey

At the Detroit Auto Show last month, one car caused a stir, not for a hint of the elegant lines of its legendary predecessor – the VW-based Karmann Ghia sports car – but for the use of a little-known structural material called foamed aluminium alloy.

Foamed aluminium is interesting to builders like Karmann because it seems an ideal solution to a well-known problem. Eighty per cent of a saloon car’s torsional stiffness is normally in the roof. A car without a roof is like a box without a lid – a weak structure that can flop around. Stiffness improves a car’s ride and handling. Sports cars need extra reinforcement without adding weight to make up for the lack of a lid.

Dr Timothy Olind, head of engineering at Karmann in the US, is clear about the benefits. ‘What aluminium foam allows us to do is to take a large body panel that is inherently not stiff and make it 10 times stiffer than the solid material. We gain back stiffness very quickly and simply. That is the logic.’

Key benefits are low density, high compressive and shear strength, fire retardance and the ability to absorb impact energy and high frequency sound.

In the spaceframe design of the Karmann concept car, the floor pan is made from aluminium foam sandwiched between an outer skin of the same material. The firewall – the partition between engine and passenger compartments – is also made from foam sandwich. But it will be several years before the material is used in production models.

Karmann makes its own foam structures using a process developed with the Fraunhofer Institute for Applied Materials Research, which is now the subject of a Brite-Euram project involving a consortium of European transport companies including Adtranz UK. Unlike another foaming process which starts with the molten alloy, or melt, this starts with the powder form. The foam in both processes is made by a gassing agent, typically a metal hydride powder which releases hydrogen.

Starting from a metal powder produces finer, more homogenous voids making the material stiffer than foams produced from melt. It is possible to make complex 3D shapes by extruding the sandwich material as a thin ply up to 2mm thick, then pressing it into the required shape. Parts are then baked in an oven where an endothermic reaction causes the aluminium powder to melt, trapping the expanding gases to produce a porous foam sandwich perhaps 10mm thick.

Melts are suitable for bulk processing of low-cost foams. This summer, Cymat Aluminium will begin production of aluminium foam at its new continuous casting line at Kingston, Ontario under licence from Alcan International. Startup production is 2,000lb/hr and prices are competitive at around $4-5/lb compared with a Japanese foam made from powder at $25-30/lb.

The Alcan formulation includes up to 15% ceramic particles to stabilise the foam and prevent it collapsing – an inherent problem with open cast foams. Even so, according to Jeff Woods, Cymat’s director of research and development, it is possible to control foam density to well within 0.5% of target.

Cymat’s Alcan foam has a density of 2-20% of solid, produced in rectangular section up to 1.5m wide and 150mm thick. Applications include shock absorbing linings for car doors and crash barriers. Unlike honeycomb aluminium material, foamed aluminium is isotropic and withstands impact from any angle.

Cold War contracts have kept ERG Materials & Aerospace of Oakland, California out of the limelight for 20 years as a producer of specialised metal and ceramic foams and parts made from a secret powder process. Unlike closed-cell foams, the voids in ERG foam are exploded in a controlled way to form a network of open cells. This makes the material ideal for reconnaissance satellites where it is used in cryogenic heat exchangers to keep camera optics cold while filming infra red images.

Foamed metals are not exactly new: Dunlop Equipment of Coventry hit the news in 1967 when its ‘reticulated’ foam appeared on BBC TV’s Tomorrow’s World. This high-temperature material capable of operating at 800 degrees C is made from a polymer foam substrate which is first nickel then chrome plated before the polymer is ‘burned’ out, leaving a metal foam made from an intricate pattern of fine, hollow strands. Light and immensely strong, its main use to date is in air-oil separators in aero gas turbine lubrication systems.

But more research is needed into the mechanical properties of foamed metals before they become commonplace.

Researchers under Professor Alan Evans at Harvard University highlight the principal defects affecting stiffness in cast foams which prevent them from being a substitute for more costly powder foams. And Dr Bill Clyne at Cambridge University’s department of materials science is working on ways to improve the casting process for volume production of stiffer foams with smaller, homogenous voids. In cast foam, voids can be 8mm across.

Brian Leyda, ERG’s engineering manager, warns companies against becoming enthralled by a concept which is imperfectly understood and to take professional advice before using metallic foams.