Fiber Meets Metal

Lightweight design with steel and aluminum is already reaching its limits. In the MultiMak2 project in the framework of the Open Hybrid LabFactory (OHLF), IAV is therefore working with partners on new solutions based on a multi-material mix where composites play an important role. The new components are to be not only light in weight but also suitable for volume production so that they can be manufactured at low cost.

In the MultiMak2 project, IAV and other OHLF partners such as Braunschweig University of Technology and Volkswagen AG are developing a vehicle tunnel and a suspension strut dome. Both parts are exposed to high forces: the vehicle tunnel is the key part of the body. It provides installation space, for example for a cardan shaft or the exhaust system, but also makes a significant contribution in terms of body stiffness. The suspension strut dome is also subject to heavy loads: for example, it introduces forces from the front axle into the body and has to transfer loads in the event of a front-end crash.

”In the past, the OEMs have frequently used increasingly high-strength steels“, reports Dr. Andreas Löffler, IAV Senior Vice President for Vehicle Exterior and Leightweight Design Technology. ”This resulted in thinner panels with less material usage and a lower weight.“ But this approach is meanwhile hindered by various constraints: modern steel is far more complex in terms of the processes involved, both for material production and in subsequent manufacturing, which is pushing the costs up. And so the focus is increasingly shifting to lightweight metals such as aluminum. But there are constraints too. It is frequently not possible to fully exploit the weight advantage of light metals: reduced strength and stiffness properties mean that in some cases parts have to be designed with much higher wall thicknesses than parts made of steel.

Here it makes sense to supplement the classic materials with fiber composites such as GFRP or CFRP. They guarantee high strength and stiffness properties. ”The MultiMak2 project therefore intends to reinforce metal structures with CFRP or GFRP at critical points so that forces are absorbed by the composite material and the components weigh less”, explains Löffler. ”Although the composite materials cost more, the aim is to compensate for this disadvantage, at least in part, through economies of scale and changed manufacturing processes for volume production.“

Examples here include replacing thermosetting plastics with thermoplastic composites. Thermoplastics have a low melting point and are suitable for volume production by injection molding. Plastic is molded around the fiber matrix in the dye in one shot, resulting in relatively short cycle times. ”As a result, we can make three to four times more parts in the same time compared to a thermosetting matrix“, says Löffler.

Apart from the aspect of costs, the combination of different materials is also a major technical challenge. On the one hand, the metals and fibers have to be firmly connected, with bonding techniques, for example. On the other hand, they have completely different temperature characteristics (delta/alpha problem), which can cause problems when exposed to temperature fluctuations, as during cathodic dip coating (CDC). “When aluminum and steel heat up, they expand more than CFRP“, says Löffler. ”Cracks can be caused at the contact surfaces. Here we need to find the optimum material combinations and joining processes.” This is exactly the focus of research work by the developers in the technical section of the Open Hybrid LabFactory, working in close cooperation with the manufacturing specialists.

The MultiMak2 concept phase has meanwhile been completed where the lightweight design experts examined six different approaches under the aspects of weight, stiffness, strength, costs and life cycle assessment, among others. ”Full life cycle assessment is an important part of MultiMak2“, says Löffler. ”We look in detail at the entire carbon footprint from raw material extraction through to final recycling. After all, it is conceivable and desirable that the CO2savings accumulated throughout the service life of lightweight vehicles will more than compensate for the CO2 generated by material extraction and disposal.”