For the next generation of the A8, Audi is employing an intelligent mix of four materials in the body structure for the first time, and the new flagship model’s low weight and high rigidity are the basis for greater performance, efficiency and safety.
The experts at Audi long ago abandoned the obsession with using a single material in lightweight design. With a mix of aluminum, steel, magnesium and carbon fiber-reinforced polymer (CFRP) they are establishing a new level in multi-material construction of the Audi Space Frame (ASF) for the next generation of the A8 – in keeping with the principle “the right material in the right place in the right amount”.
The company consistently applies new material technologies and designs that are directly beneficial to the customer – and not only in terms of weight. The upcoming flagship’s torsional rigidity – the critical parameter for precise handling and pleasing acoustics – surpasses its predecessor model’s rigidity value by up to 24 percent.
Innovative production process: The carbon rear panel in the new Audi A8
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 percent to the torsional rigidity of the total vehicle. To optimally absorb longitudinal and transverse loads as well as shearing forces, between six and 19 fiber layers are placed one on top of the other, ensuring a load-optimized layout. These individual fiber layers consist of tapes
50 millimeters (0.2 in) wide and can be placed individually in a finished layered panel, with any desired fiber angle and minimal trimming of the fibers. 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 of carbon fiber. Using another newly developed process, the layered panel is wetted with epoxy resin and cured 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 manufactured in varying thicknesses by means of tailoring technologies – they’re tailor-made in other words – 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.
Aluminum components in the form of cast nodes, extruded profiles and sheets, elements characteristic of the ASF design, make up the biggest share of the new Audi A8 body, at 58 percent. And here too the competition of materials has been driving progress. New heat-treated cast alloys, for example, attain a tensile strength of over 230 MPa (megapascals). The corresponding yield strength in the tensile test is over 180 MPa, and for the profile alloys it is higher than 280 and 320 MPa – significantly higher values than seen previously.
Rounding out the intelligent mix of materials is the magnesium strut brace. A comparison with the predecessor model shows that it contributes a 28-percent weight saving. Aluminum bolts secure the connection to the strut tower domes, making them a guarantor of 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.
Benefits for customers and the environment – the new body shop for the Audi A8
In addition to the complete redevelopment of the Audi Space Frame for the next generation A8, the production halls at the Neckarsulm location were specially built for the upcoming flagship.
A total of 14,400 metric tons of steel were needed just for construction of the new, 41-meter-high body shop, twice as much steel as was used for the Eiffel Tower in Paris.
In 1994 it was the first generation of this luxury sedan, with its aluminum unitary body, that made the Audi Space Frame 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 result is innovative bodies whose low weight and high rigidity form the basis for greater performance, efficiency and safety.
The body shop – 14 joining processes
Along with expanding the mix of materials, Audi consistently focuses on innovative production technologies. The brand is using 14 different joining processes to assemble the multi-material body for the new A8, including roller hemming, grip punch riveting, and remote laser welding of aluminum which is being done here for the first time anywhere in the world.
Roller hemming is used all the way around the complete front and rear door cutouts on the new Audi A8. Thanks to the design made possible by this technique, getting in and out of the car will be even more comfortable and the driver’s field of vision will be wider around the A-pillar, an area that is critical for save driving. It also enabled the engineers to gain up to 36 millimeters (1.4 in) at the door cutouts compared to the predecessor model.
The grip punch riveting, which fixes the side wall frame in its position, accompanies the roller hemming process, which in turn is supported by structural bonding. It was the development and adaptation of these joining technologies to this specific application that first made it possible to use the material concept in the new A8, and to combine the aluminum side wall frame with the hot-formed, high-strength steel sheets at the B‑pillar, the roof line and the sills with their thin flanges.
With remote laser welding of aluminum, Audi has developed a new approach realized by no other premium automaker. Exact positioning of the laser beam in relation to the welding edge considerably reduces the risk of hot cracking because the heat input can be precisely controlled. The size of the gap between parts being joined can immediately be determined and effectively filled in by means of process control strategies. The laser beam’s high feed rate and low energy use reduce CO2 emissions by about one quarter. This new process also results in a 95-percent saving on recurring costs in series production because it eliminates the need for the costly process controls required with conventional laser welding.
Used in the rear of the new A8 – at the water drain channels – is a further development of the conventional aluminum MIG (metal inert gas) welding process based on the established CMT (cold metal transfer) process. The development approach is essentially a geometric modification of the inert gas nozzle, which makes it possible to achieve process speeds of up to 50 mm/s and a very fine weld seam appearance. Compared to the conventional MIG welding process, this corresponds to a threefold increase in speed for equivalent applications in the body shop. This improvement also results in considerably reduced heat input, and therefore also less risk of component distortion. To ensure the welding wire is positioned at the component edge with the required precision, the process is performed in combination with a system for automatic seam detection and seam tracking.
Resistance spot welding (RSW) of aluminum is a highly versatile joining process. Here too, high-performance plant technology combined with control technology adapted to the requirements of working with aluminum are delivering improvements in process stability and reproducibility of welding results. Use of welding tongs with higher electrode forces makes it possible to reduce undesirable adhesions from the copper electrode onto the aluminum component.
Laser welding – a classic Audi highlight in the body shop – is used to join the sides of the A8 roof to the side walls along a practically invisible zero gap.
The equipment, data and prices specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.