As Audi recently revealed details of the structural technology—including 14 advanced joining processes—of its all-new A8, Chairman Rupert Stadler and Porsche Chairman Oliver Blume were also making statements about joining, but on a strategic scale.
“The best brains of both companies will together set the technical course for the future,” said Stadler, and Blume added: “We will utilize the expertise of both companies and take advantage of synergies. We will co-operate wherever it makes sense.”
This means a shared vehicle strategy for architectures, modules and components to shape mobility up to 2025, that will rest on electrification, digitalization and autonomous driving.
But each company will ensure that its respective brands will each retain its individuality. “A Porsche is always a Porsche,” stressed Blume.
And with the upcoming A8, Stadler can rest easy that this Audi will always be an Audi, with its continued use of aluminum and space-frame technology, albeit complemented by other materials.
The model year 2018 car, to be launched this summer and designed to take alternative powertrains and extensive autonomous capability, will be aluminum intensive but will also use steel, magnesium and, significantly for weight saving and torsional rigidity, carbon-fiber reinforced polymer (CFRP). Claimed torsional rigidity is up 24% on the incumbent A8.
Currently, Audi’s Lightweight Design Center employs some 200 specialists, 25 of whom focus on fiber-reinforced polymers, as the battle continues to offset the increased weight of safety systems and expanding specifications plus provision for PHEV technology. Because of this – and despite all the engineers’ best efforts - the new A8’s BIW scales 282 kg (621 lb) against 231 kg (509 lb) for the previous generation.
CFRP is used for the rear panel and associated parcel shelf, the largest component in the occupant cell of the A8. It provides some 33% of the torsional rigidity for the whole car.
Detailing the material, Audi explains that to optimally absorb longitudinal and transverse loads and associated load-shearing forces, between six and 19 fiber layers are placed one above each other to deliver load optimization. The individual fiber layers comprise 50-mm-wide (2-in) tapes placed in a finished layered panel “with any desired fiber angle and minimal trimming of the fibers.”
The process obviates the need for an intermediary step of manufacturing entire sheets of carbon fiber. A further new process sees the layered panel wetted with epoxy resin and cured “within minutes.” Although high strength hot-formed steel components including tailored blanks are used for the occupant cell, aluminum cast nodes, extruded profiles and sheets, account for 58% of the A8’s body.
Detailing the performance of new heat-treated cast alloys, Audi engineers say these attain a tensile strength of more than 230 MPa (33,359 psi); the corresponding yield strength in the tensile test is over 180 MPa (26,107 psi). For the profile alloys, “significantly higher” values are delivered than those previously achieved.
Magnesium is used for a front brace linking the A8’s suspension turrets and provides a 28% weight saving, the engineers claim. Aluminum bolts are used to secure it to the strut tower domes to boost torsional rigidity. In terms of safety performance, a frontal collision would distribute resultant generated forces to three impact buffers in the front end.
A further significant aspect of technologies introduced in the A8’s build is the use of 14 different joining processes in body assembly. These embrace roller hemming, grip punch riveting, and remote laser welding of aluminum “which is being done in Neckarsulm for the first time anywhere in the world,” the company claims.
Interestingly, roller hemming is used round the front and rear door cutouts, described by Audi as aiding passenger accessibility while also improving driver field of view around the A-pillars. Space gain in the cutouts is put at 36 mm (1.5 in) compared to the 2017 car.
The use of grip punch riveting, that fixes the side wall frame in position, accompanies the roller hemming, which is supported by structural bonding.
Audi is particularly proud of its development and adaptation of these joining technologies, combining the aluminum side wall frame with the hot formed HSS sheets at the B-pillar, the roof line and the sills with their thin flanges.
Also significant is remote laser welding of aluminum. Exact positioning of the laser beam in relation to the welding edge is said to “considerably" reduce the risk of hot cracking. The new process is also said to bring a 95% saving on recurring costs in series production, as it eliminates the necessity of expensive process controls.
The car’s spaceframe is being built in a new facility at its Neckarsulm plant. Its robot population is about 500 machines.