For the next generation Audi A8, Audi plans to use a mix of four materials in the weight-bearing body structure for the first time. With a mix of aluminium, steel, magnesium and carbon fibre-reinforced polymer (CFRP) the lightweight design experts at Audi are establishing a new stage in multi-material construction in the Audi Space Frame (ASF) for the next generation of the Audi A8.
As well as keeping weight down, the new Audi A8’s torsional rigidity – the critical parameter for precise handling and acoustics – surpasses the values of the previous model by a factor of about one quarter.
In terms of its overall dimensions, an ultra-high-strength, torsionally rigid rear panel made of CFRP is the largest component in the occupant cell of the new Audi A8, and it contributes 33 per cent to the torsional rigidity of the total vehicle. To absorb longitudinal and transverse loads as well as shearing force, between six and 19 fibre layers are placed one on top of the other, ensuring a load-optimised layout. These individual fibre layers consist of tapes 50 millimetres wide and can be placed individually in a finished layered package, with any desired fibre angle and minimal trimming of the fibres. The innovative direct-fibre layering process specially developed for this purpose makes it possible to entirely dispense with the normally needed intermediary step of manufacturing entire sheets. Using another newly developed process, the layered package is wetted with epoxide resin and sets within minutes.
A high-strength combination of hot-formed steel components make up the occupant cell, which comprises the lower section of the front bulkhead, the side sills, the B-pillars and the front section of the roof line. Some of these sheet metal blanks are produced in varying thicknesses using tailoring technologies – meaning they are customised – and others also undergo partial heat treatment. That reduces weight and increases the strength, especially in areas of the vehicle that are particularly critical for safety.
The aluminium components make up 58 percent of the new Audi A8 body, the largest share in the mix of materials. Cast nodes, extruded profiles and sheets are the elements characteristic of the ASF design. And here too the competition of materials has been driving progress. New heat-treated, ultra-high-strength cast alloys attain a tensile strength of over 230 MPa (megapascals).
Rounding out the intelligent mix of materials is the magnesium strut brace. A comparison with the predecessor model shows that it contributes a 28-per cent weight saving.
The highly complex yet energy-efficient production operation uses 14 different joining processes, including roller hemming at the front and rear door cutouts. This mechanical, “cold” technology is used to join the aluminium side wall frame to the hot-formed, ultra-strong steel sheets at the B-pillar, roof line and sills. Improvements of up to 36 millimetres at the door cutouts make getting in and out of the car even more comfortable and widens the driver’s field of vision around the A-pillar.