Researchers of the Federal Cluster of Excellence “MERGE: Technologies for Multifunctional Lightweight Structures” at Chemnitz University of Technology and their partners from the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) have created a successful lightweight design with immense importance for tomorrow’s automotive construction:
They were able to reduce the weight of a car wheel by more than 50% through the use of novel materials and structures. Compared to a conventional steel wheel of about 6.8 kg, the novel lightweight wheel weighs only 3.02 kg. The three-part sandwich wheel rim consists of a core of aluminium foam and top layers made of thermoplastic fibre-plastic compounds.
This sandwich design takes advantage of the materials’ respective special properties. Such multi-material-designs are more and more frequently used in the automotive sector. Thus, not only the weight but also the emission of pollutants by motor vehicles can be reduced through combining novel processing technologies with smart material systems.
The combination of thermoplastic fibre-plastic-compounds with metal foams has great lightweight potential, especially for producing on a large scale, as the research fellow at the Cluster of Excellence MERGE Alexander Hackert points out: “Highly porous metal foams such as aluminium foam have excellent mechanical properties at a low density while at the same time they can potentially absorb a lot of energy. That is a contribution to driving comfort, especially when going round a turn in the road. Additionally, they have a distinct damage tolerance.”
The special compound at the core renders the wheel rim extremely stiff and light at the same time. The outside surface of the aluminium foam core is very thin and closed, in order to provide the optimal interface to the carbon fibre-reinforced layers. In addition, there is a buffer layer with glass fibre reinforcing for the harmonisation of the difference in stiffness between the aluminium foam and the carbon fibre-reinforced layers.
“Through an immense increase in pressure when producing the part in a thermic pressing process the thermally induced residual stresses are basically locked in the component. This helps improve its performance, since the high-load areas of the wheel are already under preload”, Hackert explains.
By reducing the unsprung masses the scientists were also able to improve the driving characteristics. The vehicle becomes more agile and changes its behaviour: when the driver hits the brakes, it will stop much faster. “There are special norms and regulations for the application in real traffic, because the safety of all passengers as well as other road users must be guaranteed under all circumstances”, says Hackert.
“Even an unintended crossing onto the curb or going through a pothole must be manageable for such a wheel.” Extensive testing of the sandwich compound as well as complex simulations of the component have proven the construction principle to be adaptable to many other applications. “If we come up with a novel structure for a component we do not just want to make it different per se, but make it better”, as the engineer puts it.
Alexander Hackert’s team has already registered the design of the lightweight wheel as a utility model, and as a patent specification at the German Patent and Trademark Office (DE 20 2014 005 111 U1, DE 10 2014 009 180 A1). The wheel rim’s prototype combines all results from preliminary tests and component simulations and can now be used for the ongoing development up to the point of an actual mass-produced component.
“Our tests of the core compounds have shown the enormous potential of the wheel rim for an automotive application”, Hackert points out. He is more than optimistic that automobile manufacturers who apply alternative concepts such as electric or hydrogen drives will also will make use of the lightweight wheel made in Chemnitz for the development of novel mobility solutions.