For anyone that still thinks vehicle body design is simply a matter of rolling out some steel sheet, bashing it a few times and spot welding the resultant frame and panel parts into a car, let he or she come to Audi. And marvel at an ‘Aluminium Space Frame’ body destined for the redesigned A8, out later this year, that contains no fewer than 29 different materials. Knocked together with 14 joining processes.
Of course, mixing and matching steel types and grades, and welding/fastening techniques, has long been the stuff of body designers at all automakers but Audi is a particular specialist in lightweight design, going back a million ‘aluminiumised’ body shells or so, to the 1994 V8, predecessor of the A8, and this coming generation will be the first to use an “intelligent mix of four materials” in the body structure, plus a clever carbon fibre-reinforced polymer component (CFRP) for the first time – more than in any of the brand’s previous production models.
According to Marc Hummel of the automaker’s materials technical development department at the company’s Neckarsulm plant, home of the Lightweight Design Centre (ALC) established in 1994 and also a key assembly plant for a number of models, those 29 materials consist of 11 sheet steels, three alloy extrusions, three alloy castings, two forgings and eight sheets plus one magnesium casting and the CFRP part. The new car will be 58% aluminium excluding closing panels. All of that requires 14 different joining techniques such as a MIG welding technique now three times faster and new processes such as remote laser welding and a three stage roller hemming process that neatly seams multiple materials to form clean and relatively thin door window frames and very neat door openings – gone are the days of ragged steel spot welds covered in wavy pinch-weld plastic trim.
“In modern lightweight design the focus is on intelligent and flexible use of a wide variety of materials – in keeping with the principle ‘the right material in the right place in the right amount’,” Audi says.
The materials guys talk about being given the vehicle’s spec sheet and noting weight-adding demands for such things as China’s long-wheelbase version, panoramic glass roof and alternative drive. Then they go to work taking as much weight out of the shell as possible. Having cut the weight of the new body by 12% they’ve also boosted torsional rigidity by up to 24%.
“The decisive inspiration for the Audi Space Frame [the in-house term for an alloy body] is found in nature,” according to ALC chief Bernd Mlekusch. “In a bee colony, for example, only the amount of material required to serve its function is used.”
The aluminium ASF body of the first A8 generation made the car more than 40% lighter than had been possible with a conventional steel design. That shook up steel makers and kicked off a development spiral in the competition of materials. Since then, the strength of new high-strength steels has increased by a factor of five.
Among the roughly 200 specialists at the ALC, about 25 experts concentrate on fibre-reinforced polymers (FRP). The FRP Technical Centre covers the development process in its entirety and has a full range of machinery mirroring what is used, on a larger scale, by suppliers with whom very close liaison exists. The centre also develops new joining technologies, works on quality assurance aspects and the development of service and repair techniques.
Carbon rear panel
The CFRP rear panel won’t go in when the rest of the body is made, primed and painted – it will actually be installed, as a complete sub-assembly, during final assembly, which hasn’t started yet (and no SOP was forthcoming from tight-lipped Audi executives). By the time it will be ready to go in, it will already be fitted with such parts as loudspeakers, the rear louvre, three-point seat belts and the centre armrest. An employee will use 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 will be used in conjunction with manually installed rivets to join the rear panel to the metal components.
In terms of overall dimensions, this ultra-high-strength, torsionally rigid rear CFRP panel is the largest component in the occupant cell and contributes 33% to the torsional rigidity of the total vehicle. To optimally absorb longitudinal and transverse loads as well as shearing forces, 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 50mm (0.2 inch) wide and can be placed individually in a finished layered panel, 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 of carbon fibre. Using another newly developed process, the layered panel is wetted with epoxy resin and cured within minutes. Finish is smooth, precise and neat although just-auto has only seen the polymer base, not the fully-fitted sub-assembly.
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 while others also undergo partial heat treatment. That reduces weight and increases the strength, especially in safety-critical areas.
As always with A8s, aluminium components in the form of cast nodes, extruded profiles and sheets, elements long characteristic of the ASF design, make up the biggest share of the new body, at 58%. New heat-treated cast alloys attain a tensile strength of over 230MPa. The corresponding yield strength in the tensile test is over 180MPa and for the profile alloys it is higher than 280MPa and 320MPa – significantly higher than previously.
Rounding out the intelligent mix of materials is the magnesium strut brace, 28% lighter than before. Aluminium 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.
New body shop
Not content with redesigning the shell from top to bottom, Audi has built a brand new body shop for it – sadly operating only in ‘ghost’ mode for our visit with all the (mostly Kuka) robots going through all their motions but not actually handling any parts, though a lot of pre-production sub-assemblies were on display.
The 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 35kg (77.2 lb) less than their predecessors – allowing the use of smaller robots, which in turn use less electricity. The halls are equipped with LED lighting (switched off by area when not needed as robots don’t require lighting to work), 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.
In plan view, the two directly adjoining buildings resemble an equilateral triangle. There are three production levels in the new building, which is 41m (134.5 ft) high. Each level encompasses 50,000 square metres (538,195.5 sq ft) of floor space. Supporting columns divide the floor space of each level into a grid of 500m (1640.4 ft) sections. Beneath one of the halls is the plant’s railway loading station, where girders span a distance of 36m.
As noted earlier, there are 14 different joining processes to assemble the multi-material body, including roller hemming, grip punch riveting, and remote laser welding of aluminium which is a claimed Audi world first.
Roller hemming is used all the way around the complete front and rear door cutouts which allows larger openings (up to 36mm or 1.4 inches) for easier entry and exit and a wider driver’s field of vision around the A-pillar.
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 and to combine the aluminium 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 aluminium, 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. 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% saving on recurring costs in series production because it eliminates the need for the costly process controls required with conventional laser welding
Used at the rear, at the water drain channels, is a further development of the conventional aluminium 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 50mm/s and a very fine weld seam appearance. Compared to conventional MIG welding, this triples speed and 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 aluminium is a highly versatile joining process. High-performance plant technology combined with control technology adapted to the requirements of working with aluminium 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 aluminium component.
Laser welding is used to join the sides of the A8 roof to the side walls along a practically invisible zero gap.
The ASF body’s superstructure begins with the lower welded assemblies which include the longitudinal members which 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.
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 aluminium alloys reach their final strength.
In-line 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. Quality assurance conducts spot tests of individual components, subassemblies and complete bodies. A new measurement centre has been set up next to the line for that purpose.
The tools used 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 are also used.
About 500 people work on three shifts in the new body shop. Most work in the automated area together with robots and others in manual areas on the bolt-on and finishing lines.
Audi is training the employees well in advance for the start of series production, with special courses and advanced instruction emphasising practical, hands-on learning. Depending on the specific type of qualification and the technology involved, a course on automation takes up to 10 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 aluminium which requires great finesse.
It’s all very impressive. I can’t wait to see the finished car which, I hear, is due in July.