For the next generation of the Audi A8, an intelligent mix of four materials is being used for the first time in the weight-bearing body structure – more materials than in any of the brand’s previous production models. The mix includes aluminum, steel, magnesium, and carbon fiber-reinforced polymer (CFRP) to decrease weight, increase torsional rigidity, and offer greater performance, efficiency and safety. “The lightweight design experts at Audi long ago abandoned the fixation on using a single material in lightweight design,” explained the company, which describes this as a new stage of multi-material construction in its Audi Space Frame (ASF).
The first generation of the A8 luxury sedan was introduced in 1994, with an aluminum unitary body that made the ASF an established presence in the automotive world. Since then, the company has built more than one million production cars in accordance with this design principle, and it has been continually building upon its know-how in the use of materials and joining techniques.
The New Spaceframe
Aluminum remains a key material in the new ASF (as it does in the multi-material structure of the R8 Spyder). The aluminum components make up 58% of the new A8 body, the largest share in the mix of materials. Cast nodes, extruded profiles, and sheet are characteristic of the ASF design. Audi noted that innovative production technologies are improving the specifications of the aluminum components. New heat-treated, ultra-high-strength cast alloys are able to attain a tensile strength of over 230 MPa. The corresponding yield strength in the tensile test of these parts is over 180 MPa, and for the profile alloys it is higher than 280 (i.e. 320 MPa) – significantly higher values than previously seen.
Magnesium is used in the strut brace, contributing a 28%weight savings over the previous model. A comparison with the predecessor model shows that it contributes. Aluminum bolts secure the connection to the strut tower domes, guaranteeing the body’s high torsional rigidity. In the event of a frontal collision, the forces generated are distributed to three impact buffers in the front end.
A combination of hot-formed high-strength 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 customized) and others also undergo partial heat treatment, which reduces weight and increases the strength, especially in areas of the vehicle that are particularly critical for safety.
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. It contributes 33% to the torsional rigidity of the total vehicle. To optimally absorb longitudinal and transverse loads, as well as shearing force, between six and 19 fiber layers are placed one on top of the other, ensuring a load-optimized layout. The innovative direct-fiber 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.
New Body Shop for the A8
In addition to the complete redevelopment of the ASF for the next generation A8, the production halls at the Neckarsulm, Germany, location were specially built for the upcoming flagship. The new, 41-m-high body shop provides highly complex yet energy-efficient production operation using 14 different joining processes.
Among the joining methods is roller hemming at the front and rear door cutouts — a mechanical, “cold” technology used to join the aluminum side wall frame to the hot-formed, ultra-strong steel sheets at the B-pillar, roof line, and sills. The engineers thus realized improvements of up to 36 millimeters (1.4 in) at the door cutouts compared to the predecessor model. That in turn makes getting in and out of the car even more comfortable and widens the driver’s field of vision around the A-pillar, an area that is key to safe driving.
As for the “warm” joining processes, Audi has developed remote laser welding for use with aluminum. Exact positioning of the laser beam in relation to the welding edge considerably reduces the risk of hot cracking during production. The new process makes it possible to precisely control the penetration depth of the laser by means of the heat input. In this way, process control can immediately determine the gap width between parts being joined, and this can effectively be closed using regulating controls. The laser beam’s high feed rate and low energy use reduce the CO2 emissions of this production step by about one fourth. This new process also results in a 95% savings on recurring costs in series production because it eliminates the need for costly process controls required with conventional laser welding. The remote laser welding technology perfectly symbolizes the entire production of the new Audi A8.