Lightweight construction has long been a top priority at German carmaker Audi and future innovations will ensure that the brand can continue to reverse the weight spiral. Each new Audi model will be lighter than the one it replaces.
The Audi Lightweight Design Centre (ALDC), first set up as the Aluminium Centre for Development, Production Planning and Quality Assurance at the Neckarsulm site in 1994, is the German carmaker's spearhead in the field of lightweight construction. Around 180 specialists in Neckarsulm investigate all the key topics of the future – materials and their alloys, machining and shaping technologies, methods and processes. The ALDC lays down the foundation for the bodies of the future and also for the methods required for their production.
In mid-2010, Audi established a second idea foundry in Neckarsulm – the FRP Technical Center, which is part of the ALDC. The facility has around 30 permanent and around 20 temporary employees, the latter coming from industry and from universities. The team is focused on the area of fibre-reinforced plastics (FRP), i.e., with CFRP materials, but also with glass fibres and other materials with intriguing potential. The objective is to get the optimum out of all materials with respect to weight, function and manufacturability.
The FRP Technical Center recreates the entire production process chain under one roof. Its employees can produce FRP components in a variety of processes, building subassemblies and bodies, also in combination with steel and aluminium. The finished components are subjected to crash tests, endurance tests and other tests, the ideal conclusion of which is the approval for series production. The installation of the required systems technology will be completed in the course of the year.
Multimaterial Space Frame
The aluminium Audi Space Frame (ASF) was a groundbreaking innovation in car body construction when it debuted 17 years ago. Audi is now taking the technology a decisive step further to the Multimaterial Space Frame (MSF), which combines components of aluminium, steel and fibre-reinforced plastics with one another. The future has already begun with the bodies of the A7 Sportback and the new A6.
Throughout most of the automobile industry, steel bodies are still used for traditional monocoque construction. In contrast to Audi’s space frame principle, the tasks of the individual components cannot be separated here. Many panels have to serve a variety of functions simultaneously. The potential of the monocoque construction for reducing weight is largely exhausted. Audi will gradually phase it out in favour of the new MSF and apply its know-how to steel in new ways. The new A6 and the A7 Sportback show the way. Although their bodyshells are predominantly made of steels of different strength classes, their structure already follows the space frame principle in some areas. The rear longitudinal member of the A6 is a steel section with welded-in bulkheads. The A8 with its classic ASF uses an internally ribbed aluminium casting in this area. The properties and appearance of the two components are similar. The steel side sills of the A6 are manufactured in a rolling process and integrated into the structure in a manner similar to that used for the aluminium extruded sections in the A8.
In the A6, a node assembled from multiple high-strength steel parts serves as the connection between the A-pillar, the side sill and the front cross member. The A8 uses two aluminium die-cast parts here that perform essentially the same functions. And similar to the luxury saloon, the strut mounts in the front of the A6 are complex, particularly high-precision aluminium castings. The links of the suspension are bolted directly to them; the bearing pedestals commonly used in the past have been eliminated. Audi will systematically drive this development forward in its upcoming new models. Many key solutions in the new MSF are already as good as ready for series production.
New joining technologies
MSF construction requires new joining technologies to bring steel, aluminium and fibre-reinforced plastics together in a rigid unit. Audi is developing these technologies in close collaboration with its suppliers. The self-tapping screws already used in models such as the TT are also suitable for joining aluminium and CFRP parts, such as in the area of the longitudinal member. An adhesive layer eliminates the risk of corrosion posed by the carbon-reinforced materials while simultaneously sealing the joint.
Another high-end method, friction stud welding, is used to join steel and aluminium. A steel element, a kind of rivet, penetrates an aluminium panel while rotating at high speed and under great pressure, creating a friction-welded joint with the steel sheet below. There are also many promising developments in rivet technology and in aluminium resistance spot welding.
The mix of material fractions in the Multimaterial Space Frame offers Audi's development engineers virtually unlimited freedom. They can design each body structure optimally for the requirements, whether the desired distribution of the axle loads, vehicle production volumes or body construction flexibility, which is necessary with a large model family. The objective is always to achieve optimum performance while using the least amount of material.
All future Audi bodies will have three strengths in common: easy repair properties, an extremely safe occupant cell in the style of a protective cage and groundbreaking low weight. Even in this first stage, the MSF principle offers the potential to save at least 10 percent compared to the state of the art. Although the superstructure of the new A6 is wider, it is also around 30 kilograms lighter. In the near future, carbon fibre-reinforced plastics and additional new technologies will reduce the weight even more sharply.
Continuous innovation – the materials
Aluminium, ultra-high-strength steels, fibre-reinforced plastics. From its development work, Audi is intimately familiar with the complete range of the materials used in the construction of automobile bodies, with all of their potential and disadvantages. The Audi body of the future will use all of these high-end materials, with the mix varying depending on the model.
Carbon fibre-reinforced plastics: Fibre-reinforced plastics are a particularly intriguing group of materials, particularly carbon fibre-reinforced plastics. Audi already offers a number of CFRP components for the interior and the body, primarily in the R8 model family. These range all the way up to load-bearing structural elements such as the side walls and the cover for the top compartment in the R8 Spyder.
In future models, Audi plans to use CFRP components manufactured using the much more efficient RTM (resin transfer molding) process in the vehicle structure. With RTM, the dried fibre fabric is first draped (formed) before being placed in a closed, heated die and injected with synthetic resin at high pressure. This ensures that the fabric is thoroughly saturated and cured under pressure and temperature. The experts at Production Technology Development have developed specialised expertise in both draping and injection, enabling them to optimise the production process in advance. The new CFRP parts will reinforce steel or aluminium components or will serve as independent components in the structure.
Carbon fibre-reinforced plastic is not only around 60 percent lighter than steel, it is a designer material that allows the mechanical designers to freely define many component properties. CFRP components perform best when they are designed to absorb forces coming from a single direction. The individual layers of fiber fabric placed over top of one another in the synthetic resin matrix can be aligned identically to achieve maximum strength.
Another possible application for CFRP is in the longitudinal members in front of or behind the passenger cell. The members developed by Audi exhibit excellent crash properties. The members in the rear survive a defined-energy frontal collision with no fractures or deformations. Up front, however, the countless individual fibres of the elements fan out in all directions like the petals of a blossom (crushing). This deformation and separation from the matrix consume a very large amount of energy. So-called trigger edges at the front end of the component ensure that the impact is optimally absorbed.
Audi experts are working concurrently on a new type of CFRP structural elements called OLAS (OLAS: oscillating laminated absorbing structures), which visually resemble wave-shaped roofing tiles. This design allows them to absorb extremely large amounts of energy over short distances.
CFRP is not the only fibre-reinforced plastic that Audi is working with. In the current A8, the lower cross-member of the front end is made of so-called organic sheets – a matrix of glass-fibre-reinforced plastic (GFRP) reinforced with embedded aluminium sheets. Weighing only 5.4kg, it is roughly 100 grams lighter than a comparable aluminium component. Audi also offers bucket seats with a GFRP frame as an option for the R8 GT. These are 31.5kg lighter than the standard seats.
The affordable GFRP becomes particularly attractive when it replaces one or more carbon fibre layers in a CFRP matrix. Alternately, aramid layers could be used here, as Audi does with its high-security vehicles. The toughness of the aramid layers prevents the CFRP components from splintering in a crash. Natural fibres such as hemp are also suitable for use in the matrix and are particularly good at damping noise.
Aluminium: Classic aluminium also still harbours many possibilities. Audi is working together with its suppliers to further increase the strength and deformation properties of all semi-finished components. New casting alloys that permit thinner wall thicknesses thanks to increased strength are nearly ready for series deployment. They achieve a strength of more than 500 megapascals – an increase of 60 percent.
New joining technologies are the third approach. Aluminium sections, for instance, can be joined using friction stir welding. The seam is extremely strong and free of distortion. Like topology-optimised parts, ‘built’ sections are characterised by complex geometries. In the next step, Audi will also use friction stir welding to join aluminium panels and sections of different alloy grades.
Steel and magnesium: Many different materials are competing in auto body construction, including steel. New alloys for hot-shaped steels will soon enable ever greater strengths and thus lower weights. Audi also sees potential in magnesium. The lightweight material, with a density of just 1.8 grams per cubic centimetre (0.63 lb per cu in), is already used today in many areas. Examples include the centre tunnel and the gearbox cross-member in the A8, the engine frame of the R8 and add-on engine parts, such as induction manifolds. Magnesium is also an option for the strut brace in the engine compartment or for parts of the engine itself, such as the cover of the camshaft case.