Bundled expertise – lightweight design in series production
Keeping things light often requires heavy thinking – not only during the development of lightweight design technologies by Audi, but also during their implementation in production. Audi has developed wide-ranging expertise in this field over the past 15 years. A large number of patents document that Audi is clearly ahead of the competition.
Production expertise is needed during the early development phase of a new model for design, concept and component development. Fabrication concepts are planned in close collaboration with the development engineers to ensure that the prerequisites for series production are met.
One important topic in all this are the materials – the development of new, higher-performance alloys and materials. The manufacturing and processing of wrought components – castings, extruded sections, aluminum panels and steel plates for lightweight body construction – and individual body parts is to a broad extent every bit the responsibility of Audi Production as is the construction of the equipment, fixtures and tools.
“We have mastered the entire process chain,” says Frank Dreves, the Member of the Board of Management of AUDI AG responsible for Production. The chain is made up of numerous links – production planning, technology development, tool making, the press shop, body manufacturing, corrosion protection and painting. It ends with the assembly of all the components at the assembly plant. Each step requires maximum precision. “The integrated approach and the excellent skills and training of each individual employee are the foundation for lightweight design and ensure the high level of quality,” says Dreves.
The secret to the outstanding performance lies in the comprehensive expertise that Audi has in working with different materials – their interaction is highly complex. “Our great strength – our comprehensive process expertise – is fully brought to bear in all of these areas,” says Dreves.
Many hot-shaped components, for example, that make up the ultra high strength backbone of many car bodies, are made at Audi in Ingolstadt. The knowledge gleaned by the engineers during the production of these components can be used to further improve processes in body construction.
The influence of the hot temperatures that occur during portions of the painting process must be considered when laying out the body architecture, when designing the individual parts and during assembly.
Components made of steels of different strength classes and components reinforced with aluminum, magnesium and CFC form a fascinating puzzle during assembly of the body. Audi has refined many joining technologies and made them suitable for series production, including laser brazing, plasmatron brazing, laser beam welding, self-tapping screws and punch rivets. In the TT Coupé and the TT Roadster, aluminum and steel parts enter into a long-term bond; this innovative hybrid construction is the way of the future.
Expertise in lightweight design across the entire production process chain – body construction in steel
Audi's best-seller, the A4, is a lightweight mid-size car, with the 1.8 TFSI model tipping the scales at only 1,410 kilograms. Its steel body is nearly twelve centimeters longer and more than five centimeters wider than the previous model, yet it weighs roughly ten percent less.
The foundation for this good result is the provided by the state-of-the-art material mix used in the modular longitudinal set of the A4 and A5 model families and in the Audi Q5. Conventional deep drawing steel accounts for only 38 percent of the weight of the A4 sedan. High-strength grades account for 32 percent and ultra-high-strength steels for 18 percent. The remaining 12 percent goes to hot-shaped steels, which are both lightweight and ultra high strength.
Stronger and lighter: 125 meters of bonded seams per car
Audi also uses innovative technologies to join the panels. Compared to the previous model, the number of weld points decreased from roughly 6,500 to around 5,500, and the length of the bonded seams increased from 26 to 125 meters. That saves weight. The high-quality adhesive enhances the strength of the connection and thus the entire structure. Audi is also working on another advancement here: 30 percent of the material and roughly 0.2 kilograms of vehicle weight can be saved if the bonded seams are not continuous, but are instead applied in short runs between the weld points.
The seam around the water outlet at the trunk and the so-called invisible seam between the side panel and the roof – a particularly problematic area of any car – are made using the innovative plasmatron brazing process. With their high precision – the permissible tolerance is less than 0.1 millimeters – the nearly invisible seams epitomize the quality philosophy at Audi. The rain gutter covers found on the roofs of many of the competition’s cars are no longer needed.
Audi also applies a high standard when it comes to the sealing of the weld seams. Because the fine seams made by plasmatron brazing at the cutout for the trunk lid are exposed to water, they are protected by narrow strips of PVC applied by robots. The coarse seams – primarily in the underbody area and in the engine compartment –, the wheel wells and the sills are sealed with PVC. The plastic protects against stone chipping and prevents water from entering.
Audi has done away with the bitumen mats previously used for the acoustic insulation of the passenger compartment. Robots instead spray on insulating layers of an acrylic dispersion resembling the material used to seal joints in homes. The spray-on mats have better acoustical performance, yet are 25 percent lighter and save us much as
12 kilograms of weight per car. The body is also painted with the utmost of precision. The coating system, comprising three or four coats, is only one-tenth of a millimeter thick and adds not a single superfluous gram of weight.
Remote laser beam welding is used at the sills and the doors in the A4 family. It is a fast and highly efficient process in which the laser beam is deflected via a mirrors to focus it on the work area. In conventional laser welding, the head of the laser must be moved at a very close distance of just a few millimeters above the spot to be welded. The frequent repositioning of the head drastically impairs the speed of work. Distances of over a meter are possible with remote laser beam welding. This allows the head of the laser to be moved continuously over the component. It also makes possible very narrow welding flanges less than six millimeters wide on the insides of the door frames. These reduce the weight of each door by 60 grams, for a total reduction in weight of 0.25 grams with the A4. The narrow flanges also expand the driver's field of view in the area of the window.
Remote laser beam welding is performed in two steps inside of a closed cell. In the first step, a 7 kW laser shoots very rapid pulses at the surface of the inside of the door. This creates nubs only 0.15 millimeters high that are essential when welding galvanized sheet because they create the joint gap in which the zinc can degas in a controlled manner.
In the second step, a robot uses a focusing lens to draw the weld seams. More than 40 seams provide a strong connection between the inside of the door and its frame. The two focusing mirrors that deflect the laser beam adjust in a preprogrammed sequence while the robot moves. It takes approximately 40 milliseconds to complete a weld, and the next one follows just a few milliseconds later.
Highly decorated: the Audi Q5
All of Audi's mid-size models benefit from this innovative technology, including the Q5. Last year, Audi was awarded the Euro Car Body Award, Europe’s most important prize for innovation in body construction, for the Q5. The Audi SUV, which is built together with the models of the A4 and A5 families according to sophisticated production concept, has the lightest body-in-white in its segment at a class-leading 355 kilograms. The Q5 also heads the field in lightweight design quality, which is the relationship between weight, torsional rigidity and size.
The reason for this is the concerted use of lightweight components. The hot-shaped steels used in the Q5 have a total weight of only 44 kilograms, saving 15 kilograms or a good 20 percent compared to conventional components. Tailored rolled blanks in the rear of the floor section reduce the weight by 1.9 kilograms. Audi also uses aluminum on a grand scale. The crash buffer in front of the fore longitudinal members and the cross-members connecting them are made of aluminum. Together they weigh less than five kilograms. The hood and the tailgate are likewise made of aluminum.
Maximum strength – hot-shaped steels
Currently representing the state-of-the-art in steel body construction, hot-shaped steels combine low weight with maximum strength. Hot-shaped steels form the backbone of all Audi mid-size models - the A4 family, the A5 family and the Q5. They are used to reinforce the center tunnel and in the longitudinal members, the sills, the front cross member in the engine compartment and in the B-pillars.
Shaping and curing in house: furnaces in the press shop
Audi produces a portion of the hot-shaped components itself. Two natural gas-fired furnaces, each measuring 23 meters long, are installed at Press Shop South in Ingolstadt. A robot gripper places the steel plate on an article carrier at the entrance to the furnaces. Ceramic rollers carry the plate through the furnace in under four minutes, during which time the plate is heated to over 920 degrees Celsius. At the furnace exit, a gripper arm rapidly places the red-hot plate into a hydraulic press that works with a closing force of more than 600 tons.
Cooling tubes through which cold water flows are cast into the die, and the panel is cooled to approximately 180 degrees Celsius after hot shaping. The martensitic structure thus formed has an extremely high tensile strength of 1,500 megapascals – the same as the cables of a suspension bridge. A single wire with a cross-section of one square millimeter can suspend a weight of 150 kilograms.
34 kilograms of hot-shaped steel are used in the body-in-white of the Audi A4 for a total weight saving of nine kilograms – with significantly greater strength. The CO2 balance of the production process is also positive: The energy required for hot-shaping produces 21 percent less carbon dioxide than conventional fabrication.
Some of the hot-shaped panels are tailored blanks. These custom components are composed of different types of steel(boron alloy and micro alloy), thus they have different characteristic strength values depending on the area. Audi developed this principle and fabrication process and is the first to use it. The B-pillars, for example, should undergo defined deformation in the event of a side-impact collision, with greater deformation at the bottom than at the top. This not only serves passenger safety, it also protects the roof from expensive damage in the event of a minor accident.
Other plates, called tailored rolled blanks, are rolled to different thicknesses. Audi is a pioneer in this field as well. The engineers want to combine both technologies in the future, which offer additional potential for reducing weight together with greater strength. The hot-shaping plant also harbors new opportunities. Clever tool and process design enables the panels to be heated and/or cooled to different levels in different zones, and thus in part hardened to different degrees.
Aluminum and steel – hybrid construction from Audi
Aluminum and steel are two materials that don’t necessarily make the best of neighbors. They expand at different rates when heated, therefore they cannot be joined directly by welding. Added to this is the risk of contact corrosion at the joints. But Audi has developed solutions to these problems: Aluminum and steel get together in the Audi TT with its innovative hybrid construction ASF.
Production of the components uses appropriate geometries to account for the different expansion behavior of the material when heated, such as when priming the body-in-white using the hot dip method. They are joined using punch rivets, bonded joints and self-tapping screws. The body of the TT Coupé is held together by 229 flowdrill screws; there are 250 of these in the TT Roadster. 98 of them are arranged adjacently and roughly 100 millimeters apart in the vehicle floor.
Driving screws with 1,500 newtons and at 5,000 revolutions per minute
The flowdrill screws are installed by a robot. Due to the high speed of the electric motor – up to 1,500 rpm – and the high axial force – up to 1,500 newtons – the 18 millimeter long screws cause the component to begin to melt. Controlled by a sensor, the robotic system changes the speed and force numerous times during the three second long process. The frictional heat causes the material to begin to melt, enabling the screws to tap their own metric thread into the workpiece without damaging it.
The self-tapping screws offer numerous advantages. They can be used even where components are difficult to reach or can only be reached from one side. The screwing process is flexible in terms of the sequence and reliable. The threads are so durable that they can be reused in the event of a repair. The process causes no thermal distortion, making it an ideal match to the strict quality standards that Audi applies to body construction.
Structural adhesive is used to further reinforce 80 percent of the joints. The adhesive also forms a separating layer, thus solving the problem of contact corrosion. Cameras monitor its proper application.
The hybrid construction of aluminum and steel sheet means that the engineers can use the best material at each location. This principle harbors tremendous additional potential for the years ahead.
Audi has developed many new manufacturing methods that are suitable for this innovative form of ASF construction and also for use in large-volume production. The harmonious overall concept for joining technology, which considers all aspects of body construction – from cycle times to tool changes – gives Audi the decisive Vorsprung durch Technik in this field as well.
Permanent innovation – body construction in aluminum
The R8 high-performance sports car is an Audi icon – including in the field of body construction. Its Aluminum Space Frame (ASF) body weighs only 210 kilograms, easily the best in its class when it comes to the relationship between weight, rigidity and size. It comprises 70 percent extruded sections, 22 percent metal panels and 8 percent diecast aluminum parts. The engine frame, which stiffens the rear end, is made of even lighter weight diecast magnesium.
Just as discriminating as the selection and processing of the wrought components, i.e. the initial workpieces, is the assembly of the body at the Neckarsulm plant. The front end, the central floor and the rear end are assembled separately from one another. After the three assemblies are joined to form the substructure, the pillars and the large metal panels – primarily the roof, the side walls, the doors and the cover panels – are added.
The floor tunnel of the R8 is reinforced by custom, particularly lightweight tailored blanks that save one kilogram of weight. Prior to pressing, panels of different thicknesses are joined by means of friction stir welding. This state-of-the-art process produces a particularly pore-free, dense joint with excellent dimensional accuracy and causes only minimal deformation. Audi is the first carmaker worldwide to use tailored blanks. Friction stir welding harbors great potential for the future, including the joining of different materials.
38 welding machines, five riveting tongs and five robots are used to build the body of the Audi R8. The metal inert gas (MIG) weld seams have a total length of 99 meters. In addition, 782 punching rivets, 308 self-tapping screws that cut their own threads and a row of roller-type hemmings on the add-on parts join the components together.
Just like in large-volume production, maximum precision is the key to the manufacturing of the body. A fully-automatic measuring system checks the dimensional accuracy of each body down to a tenth of a millimeter. The scanner works without contact and uses its 95 laser sensors to check 220 points within five seconds.
The computer tomograph (CT) in which Audi performs random checks measures even more precisely. Its x-rays can detect deviations on the thousandths of a meter scale. The CT checks to ensure that the connecting points on the ASF body are perfectly aligned. The unit is large enough to accommodate a complete R8 body, but it can also check tiny electronic components just a few millimeters in size.
Three kilowatts of power: the laser for the invisible joint on the roof
Body construction for the TT compact sports car is also a high-tech process. Audi uses a good dozen mechanical and thermal joining processes here, including laser beam welding. The three kilowatt laser is mounted on a six-armed robot and uses a lens to focus its beam onto the workpiece.
Laser welding allows large sheet panels to be joined perfectly to the body structure because its linear connecting seams are stronger and more rigid than individual weld points. In the TT, laser welding is used for high-precision invisible aluminum joints between the roof and the side panel as well as for the major welds in the sill area. Together they are 5.30 meters long. The Audi A8 sedan has more than 20 meters of such welds.
A new technology at Audi is the use of a laser to partially clean the aluminum panels prior to assembly. The purpose here is to remove residues from the press, such as greases, oils or release agents, from those edges and surfaces where the welds will be performed. The pulsed laser beam completely vaporizes the cover layer to be removed. This method is not only more thorough, it is also more economical than conventional washing.
Even greater strength – innovative aluminum alloys
When Audi engineers develop new wrought components – casting joints, extruded sections and aluminum panels – for the ASF bodies, strength is a very crucial criterion. Increased strength allows the material thickness and thus the weight to be further reduced.
For the next A8, Audi has written a 25 percent increase in the strength of higher-strength wrought components into the requirement specification. This was accomplished in two ways. First, by further improving a monolithic alloy, i.e. an alloy with consistently uniform properties. Second, by means of an innovative composite material for the aluminum panels called a fusion alloy.
The core layer of the new material is made of an alloy having a tensile strength of more than 250 newtons per square millimeter – a ten ton weight could be suspended from a strip of this material only 13 millimeters thick and 30 millimeters wide. It contains a copper component, which increases strength but slightly reduces its resistance to corrosion.
New solution: two cover layers
A protective cover layer is therefore applied to both sides. Each of the cover layers represents approximately ten percent of the total thickness – with a conventional aluminum panel this is roughly 0.2 millimeters for both together. The cover layer ensures that the panels can be shaped in the press to have tight radii and sharp edges despite the high-strength core, making it possible to take the distinctively drawn designs and implement them in a production vehicle with the high precision and level of quality typical of Audi.
The new aluminum panels with the fusion alloy are used for the load-bearing parts of the future A8 structure – the center tunnel, the cross bracings in the floor, the dashboard mounts, and parts of the rear longitudinal members. In perfect reflection of the Audi philosophy, these are both stronger and lighter than the panels used in the previous model.
They exemplify how Audi consistently raises the bar with each new generation of vehicles by improving material properties as part of its technology development strategy.
Audi will be expanding its use of aluminum in the bodies of its cars in the near future. Another new alloy still under development shows significant promise in this regard. It exhibits virtually the same good characteristics as the known materials, but is significantly less expensive to produce.
Maximum precision – expertise in individual components
Exquisitely precise body construction is a hallmark of Audi. Audi's strict standards apply not only to load-bearing components, but also to individual add-on parts such as the tailgate of the Q5. Made of aluminum and weighing 8.1 kilograms less than the corresponding part made of steel, it is a small work of engineering art.
With its tight radii and sharp contours, the tailgate is a visual symbol for the quality of Audi’s aluminum surfaces. The wrap-around shape with which it runs around the D-pillars is very demanding with respect to rigidity and to the tolerances and gaps between it and the adjacent components such as the roof, side panels, lights and bumpers.
Aluminum generally has lower forming properties than deep drawing steel. It is more sensitive in the press, i.e. the surface is more easily damaged. It has a higher rebound, making it more difficult to maintain dimensional accuracy. Simply put, aluminum requires greater expertise in the production of the press tools, the production of parts in the press shop and subsequent fabrication steps. Audi therefore uses advanced computer programs to simulate the behavior of the components in the press and during construction of the body. The specialists from Production have been engaged in a close dialog with their colleagues from Development ever since the design phase of the tailgate.
Audi uses newly developed aluminum alloys for the tailgate of the Q5 to ensure the perfect surfaces and the required bendability and foldability. Pressing forces of up to 2,000 tons and specially equipped pressing lines are required for the production of the components. The press shop employees have developed a comprehensive understanding of the complex fabrication process over the years so that each individual component can be manufactured to the ultra high quality standard typical for Audi over the entire production period of the car.
A lot of know-how also goes into the individual parts and how they are connected. The outside of the tailgate comprises two parts: a 76 centimeter long laser weld seam joins the upper element to the lower element. The weld is made by a new type of diode laser that requires less space and energy than the lasers used previously.
The fittings for the wide, inward tapering, wedge-shaped taillights, on the other hand, are fabricated from a single piece of metal. This marks Audi's first use of the cold metal transfer (CMT) welding process. The state-of-the-art process produces relatively little heat energy and thus hardly any deformation, making it suitable for complex areas and for joining panels of different thicknesses. CMT welding can also be used to join aluminum to steel, making it particularly attractive to Audi.