Light, Lighter, Materials Research

The equation is simple: If one can make a vehicle lighter by 200 kilograms, it consumes one litre less petrol over 100 kilometres and thus emits also less carbon dioxide. In the battle against surplus kilos, the designers of cars and aeroplanes therefore focus on lighter materials, for example, magnesium.

That way, the weight of individual parts can be reduced by 30 to 70 percent. According to Professor Dr. Karl Ulrich Kainer, 100 kilogram could be substituted by magnesium in a car. Furthermore, alloys from this metal are considerably stiffer than competing materials with the same mass. “Yet the deformation of magnesium still causes major problems”, says Kainer, who heads the Magnesium Innovations Centre (MagIC) at the Helmholtz-Zentrum Geesthacht. This is where Helmholtz researchers develop new, more flexible magnesium alloys. They also optimise production processes towards requiring less energy and improving the material properties at the same time. „The CO2 balance of production processes likewise is included in a vehicle’s total balance“, emphasises Kainer.

So far, magnesium parts are being produced predominantly by casting processes. This requires high temperatures and the variety of shapes is limited. HZG researchers work on alternatives using so-called wrought processes such as extrusion, forging and rolling. As of autumn 2010, they will have a new twin roll caster of industrial scale at their disposal. “It is suitable for basic research but allows us also to translate the production process into industrial scales at a faster rate”, says Kainer.

In the twin roll casting process, the liquid material is poured directly between the rolls, where it is rolled out thinly immediately and solidifies. Compared to rolling a block of magnesium, only a few rolling passes are necessary to obtain a sheet of metal for further processing. This saves on energy and costs, for the material does not have to be warmed up again each time. “Furthermore, the quick cooling has a positive effect on the material’s inner structure”, explains Kainer.

The large facility is augmented by another one in miniature format, which even can be transported. This offers the unique chance to examine all steps of the twin roll casting process in detail at the GKSS outstation at DESY in Hamburg. “At the Storage Ring Petra III, we can use highly brilliant x-rays to observe the microstructure from solidification to deformation”, says Professor Dr. Andreas Schreyer, who heads the division Materials Physics at the HZG. This supplies the researchers not only with valuable information for optimising the process, but helps them also to optimally tailor the material to the new production process.

Primarily, the material characteristics are to be the same in all dimensions. Only then, parts with an even wall thickness can be achieved later. The materials scientists achieve this by adding certain alloy elements. This can be achieved, for example, with magnesium alloys containing zinc and manganese, and which are in the process of being patented.

Another material that has a certain significance in light-weight building is titan-aluminium. In contrast to magnesium, it is suitable for application in very high temperatures from 700 to 800 degrees Celsius and at the same time much lighter than comparable steel or nickel-based alloys. This allows applications with extreme stress such as turbine blades in aeroplanes. “20 years ago, HZG researchers pioneered in this field, by now the material flies in a first aeroplane engine”, reports Schreyer. Now, he and his colleagues want to advance also the latest developments in material to market maturity. The most important goals here are to further increase the temperature range and solve the problems the new alloys still have. “In order to do so, we first have to understand the basics very well, for we don’t want to improve the materials and processes by trial and error, but based on knowledge”, emphasises Schreyer.

The titan-aluminium researchers likewise are able to conduct atomic scale materials analyses with the Helmholtz colleagues at the DESY. For translation into industrial manufacture they closely collaborate with industry partners, as do the magnesium researchers. For instance, they recently licensed a patented HZG alloy to Rolls-Royce and now optimise the material for application in aeroplane engines.