The lightweight design experts at Audi long ago abandoned the fixation on 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 stage in multi-material construction in the Audi Space Frame (ASF) for the next generation of the Audi A8 – in keeping with the principle “the right material in the right place and in the right amount.”
Audi consistently applies new material technologies and designs that directly benefit 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 the excellent values of its predecessor by a factor of about one fourth.
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 force, 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 (2.0 in) wide and can be placed individually in a finished layered package, 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. 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 customized – 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 aluminum 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). The corresponding yield strength in the tensile test is over 180 MPa, and for the profile alloys it is higher than 280, i.e. 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 savings. 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.
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 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 stands alone among the premium automakers by virtue of its development of 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 the production process. 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 percent 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.
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 new Audi A8 body shop at the company’s Neckarsulm plant
Audi is building the Space Frame for the new flagship A8 in a newly erected building at the Neckarsulm plant that is full of very sophisticated technologies. The highly automated production flow in the plant is very complex yet also efficient.
The entire A8 body shop was designed to ensure maximum energy efficiency and conservation of resources. The new spot welding tongs are powered by electric motors, and they weigh 35 kilograms (77.2 lb) less than their predecessors – allowing Audi to deploy smaller robots, which in turn use less electricity. The halls are equipped with LED lighting, and intelligent concepts for ventilation and shutting down equipment further reduce energy requirements.
The plant is equipped with about 500 robots, 90 adhesive systems, 60 machines for self-tapping screws, 270 punch riveting systems and 90 resistance spot welding tongs. Many robots perform several process steps, and in the intervals they autonomously switch to the tools needed, such as gripping arms and adhesive guns.
A total of 14,400 metric tons of steel, twice the amount used for the Eiffel Tower in Paris, and more than 16,000 loads of concrete – the body shop for the next Audi A8 has taken shape as an all-new production site. In the plan view, the two directly adjoining buildings resemble an equilateral triangle.
There are three production levels in the new building, which is 41 meters (134.5 ft) high. Each level encompasses 50,000 square meters (538,195.5 sq ft) of floor space, the equivalent of seven soccer fields. Supporting columns divide the floor space of each level into a grid of 500-meter (1640.4 ft) sections. Beneath one of the halls is the plant’s railway loading station, where girders span a distance of 36 meters (118.1 ft) over the rail tracks. The column that bears the heaviest load must support a weight equivalent to that of 1,800 Audi A8 models. At times, 17 cranes were in operation simultaneously during construction, including two of Europe’s biggest tracked cranes, each capable of lifting up to 600 metric tons.
From the longitudinal member to the roof: The superstructure of the ASF body
The ASF body’s superstructure begins with the lower welded assemblies, which include the longitudinal members. They form the foundation for the front and rear body modules. The latter is produced on a separate level of the building. In the next step, the two subassemblies are merged with the floor panels.
The occupant cell takes shape on this underbody, starting with the A-, B- and C-pillars, then the internal and external side panels, and on to the installation of the roof. The big steps take place in the geometry and framing stations, where the parts are positioned and aligned for the welding process with utmost precision. The body shell moves on a conveyor into the adjacent building, where it is fitted with its doors and lids, which have been produced there in advance. After the body has proceeded through the finishing line on the level below, it is transported to the adjacent paint shop. And following cataphoretic painting, the metal ASF cures in an oven at 200 degrees Celsius, where the aluminum alloys reach their final strength.
Inline laser measuring equipment checks the dimensional accuracy of the ASF body at 20 stations during its creation – the first station examines the rear module substructure, and the final station the finished superstructure. Above and beyond these measures, Audi Quality Assurance conducts spot tests of individual components, subassemblies and even complete bodies. And a new measurement center has been set up next to the line for that purpose.
The tools Quality Assurance uses include two coordinate measuring machines, which work with tactile and optical sensors, an ultra-high resolution optical measuring cell, an ultrasound imaging system and a large computer tomograph (CT). Ultrasound imaging and CT enable the specialists to test many joints in the body without having to take them apart. Traditional destructive testing methods and auditing of surfaces round out the spectrum.
The CFRP rear panel: Installation in the final assembly area
The CFRP rear panel is installed in the car during final assembly – already fitted with all components and subassemblies, including the loudspeakers, the rear louver, the three-point seat belts and the center armrest.
An employee uses a handling device to pull the rear panel through the rear window cutout and into the body. A two-component structural adhesive for preventing contact corrosion is used in conjunction with manually installed rivets to join the rear panel to the metal components.
Better qualified than ever: The employees
About 500 people working in three shifts are employed in the new A8 body shop, which involves a high degree of automation. Most of them work in the automated area together with robots, and others in the manual area on the bolt-on and finishing lines.
To ensure a smooth production start for the new model, with its many upgrades, Audi has further expanded its training concept. Audi is training the employees well in advance for the start of series production, with special courses and advanced instruction emphasizing practical, hands-on learning. Depending on the specific type of qualification and the technology involved, a course on automation takes up to ten days.
A new element and special feature of the training concept, something not found anywhere else in the Volkswagen Group, is the finishing booth. The focus here is on working with the material aluminum, which requires great finesse.