Featherweight steel foam bonded to steel

Tom Shelley reports on a development that has the potential to save weight – a primary consideration in a massive array of products

A new process allows the direct bonding of stainless foam to stainless steel without the use of adhesives, and in such a way that it allows full functionality at high temperatures. Primary applications are seen in automotive and aerospace construction, high temperature filters, fuel cells, heat exchangers and medical implants. Ever since the announcement of practical ways of making components out of metal foams, (Eureka September 1997 cover story) there has been immense interest from the automotive and aerospace industries, both for weight saving and for improving impact energy absorption in crash situations. However, attention is now turning to improving manufacturing and production processes, and one of the big development areas is finding better ways of joining metal foam to solid metal. Dr Amir Shirzadi, of the Department of Materials Science at the University of Cambridge, working in conjunction with the GKSS-Research Centre, in Geesthacht, Germany, has developed a process based on transient liquid phase diffusion bonding, a field in which he has particular expertise (Eureka June 1998 and June 2001). The technique does not damage the foam because it requires very little pressure, due to the formation of a liquid phase at the joint interface. The materials chosen by the head of research group in Germany, as being of particular interest, were a stainless steel-based metal foam with 73% volume porosity and a density of 2.15 g/cm cubed and ordinary 316L stainless steel. Conventional fusion welding leads, inevitably, to considerable local collapse damage of the foam structure. Shirzadi told Eureka that he tried various candidate alloys for the interlayer, before choosing a titanium-copper alloy. Bonds produced using this method have much greater mechanical strengths than the parent metal foam: with tensile, three-point bend and cantilever beam tests all resulting in failure of the parent foam metal away from the bond line. Potential applications include the manufacture of impact and energy absorbing structural components for cars. In ships, cars and aircraft, they could be a serious competitor to existing honeycomb structures. And, because they can endure service temperatures of several hundred degrees C, they also have great potential as air and fluid filters and heat exchangers at elevated temperatures. Other applications under investigation include magnetic shields and sound barriers as well as the fuel cells and medical implants.