COVER STORY
Lightweight Design
Development Tendencies for Lightweight Design of the Future Audi started its development in automotive car bodies made out of 100 % aluminium back in the early 1980’s. The expected result of a car body structure made out of aluminium was an extreme saving in weight. The secondary effects are an increase in linear and non-linear vehicle dynamics and lower fuel consumption and emissions while presenting similar and even better results in body-stiffness, -rigidity and crash-performance.
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1 Introduction This development led into a complete new car body concept, the Audi Space Frame (ASF) whose serial production was launched in 1994 with the A8, which was made out of 100 % aluminium. All goals have been achieved or even exceeded, for example with a weight saving of 43 % to a comparable steel body and being one of the safest cars in its class; the highest ranking of five stars in the frontal impact rating of the American road safety authority was achieved. So far, weight saving in classical steel body design has been achieved by intelligent structural concepts, optimised simulation tools and small increases in the material’s strength. This extreme weight reduction due to using aluminium in a car body structure led into several lightweight steel body development programmes that were followed by an increased use of highest-strength and hightensile steels in today’s modern steel car bodies. The achieved weight reduction has nowadays reached up to 10 % in comparison to classical steel bodies. The social attention on environmental care evolved over the last decades strongly and is nowadays an important factor for the customer’s buying decision. Besides, various legal obligations and voluntary agreements demand the continuous depreciation of emission-limiting values. This changing attention to cars offers next to engine and drivetrain solutions a wide field for developments in lightweight design. Since the introduction of the A8 in 1994, eight passenger vehicles with an ASF body have been developed to serial production. During this time a lot of experience has been gained all over the process chain from the basic material over the serial production up to the end-of-lifetime of a passenger car’s ASF body. To be prepared for the future, various fields of development are investigated to reduce weight and do also increase the performance of the car to fulfil future demands made by law and, most important, from the customer.
ture of the Space Frame, new semi-finished parts found their way into the design of passenger vehicle bodies. The idea of this combination is to optimise the structure in each part to its certain demand. These demands can be on the one hand load cases that have to be met through the geometrical design and the material characteristics, on the other hand there are a number of functional requests, which are often realised by a number of additional parts, such as fasteners and their belonging reinforcements. The use of die-cast parts allows a high degree of integration. Not only fastening elements can be integrated directly into the part and therefore save process steps, like handling and mounting, but also structural reinforcements. While reinforcements in sheet metal-design have to be designed in order to fit into the surroundings and to have an optimal connection to the structure – especially regarding the accessibility of joining tools and the material combination – integrated reinforcements in die-cast parts do save weight due to their perfect geometrical integration through varying wall-thicknesses and local ribs. While sheet metal design needs several supplementary parts to implement for example the connection of a door hinge to a lower a-pillar, an aluminium die-cast part integrates the fastening concept with screw in domes and local adjustments in wall thickness and load optimised rib structures, Figure 1. Secondary effects like the reduction of tools for the production of each supplementary part, logistics and handling
The Authors
Dipl.-Ing. Heinrich Timm is Director Aluminium and Lightweight Design Centre at Audi AG in Neckarsulm (Germany).
Dipl.-Ing. MSc Benjamin König is Assistant to the Management, Aluminium and Lightweight Design Centre at Audi AG in Neckarsulm (Germany).
2 Structural Concepts With the change from classical sheet metal design to the space frame struc-
Figure 1: Topology of the Audi TT’s A-pillar ATZautotechnology 06I2008 Volume 8
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COVER STORY
Lightweight Design
gine compartment strut of the R8. This magnesium die-cast part is a powder coated assembly that is mounted into the structure within the assembly line after the “marriage” of the car body and the drivetrain. It stiffens the rear end of the vehicle and allows significant weight savings in comparison to a design made out of different extrusions, while the economic result is equal.
3 Materials and Material Hybrids
Figure 2: Concept comparison of the Lamborghini Gallardo Spyder engine bonnet, with eleven parts at the aluminium concept (left) and only two parts at the CFK-concept (right)
of the part and most important for higher volumes, the number of steps in production and therefore the production time are reduced. This multi-functionality is one of the main progressive characteristic of die-cast parts and the continuously increasing size of these parts, from vehicle generation to generation, offers more and more potentials to integrate functions directly into the die-cast part. These stated potentials offer as well new possibilities in using materials and semi-finished parts which seems to be more expensive at first view but in the overall result regarding the whole process chain, the more economic decision, especially under the perspective of weight saving and proximate secondary effects. A comparable degree of freedom in design to aluminium die-cast parts can be found in carbon-fibre reinforced plastics (CFRP). With respect to the requested production volume, the concept feasibility study for the Lamborghini Gallardo Spyder’s engine bonnet showed that a CFRP bonnet has an even better economic outcome as an aluminium sheet metal assembly and it offers a weight saving by additional 20 %. The reasons can be found in the overall process assessment, where the aluminium bonnet is made out of eleven pieces to realise the styling and functional demands. Due to the reduction to only two pieces in CFRP design and the relatively low tool costs, the overall result for the 20
ATZautotechnology 06I2008 Volume 8
vehicle project became obvious and was realised with this highly sophisticated technology, Figure 2. When it comes to combinations of different materials or semi-finished parts, several aspects have to be considered: The integration into the whole body concept has to be ensured. Next to the integration into the manufacturing and the resulting demand on suitable joining technologies and surface coating processes, the durability, especially in respect to corrosion, has to be considered. To exhaust the maximal potential in weight saving, assemblies can be integrated into the structure with a cold and removable joining technology after the coating process. One example is the en-
As shown in the case of the R8 engine compartment strut, it is important to know the demands on function and the possibilities in production and their impact on the whole process chain to choose the adequate material to fulfil the demands requested. The development of structural concepts lives from the technical advances in material development. Next to the production characteristics of a material in order to ensure the formability of a part, the functional characteristics have to be considered as well. Big steps in tensile strength have been made in the development from classical steel (H340) to highest strength and warmformed steels. This results into an increase in tensile strength from 340 MPa up to 1650 MPa. An intensive use of warm-formed and Iron Manganese (FeMn)-steels will reach savings of up to 14 % considering a successful feasibility proof over the whole process chain. In parallel, the development of aluminium alloys is focused as well on an
Figure 3: The mixture of semi-finished parts in the TT ASF hybrid structure
enhancement of tensile strength. The R8 offers for the first time and in many parts of the body structure increased tensile strength of 240 MPa. This is a growth of 20 % in comparison to classically used aluminium alloys for extrusions. While potentials in growing tensile strength are more or less exhausted, aluminium alloys still offer the perspective of higher strength. The widespread band of single material characteristics shows that a perfect lightweight design should combine the advantages of certain materials to achieve a successful overall concept. But above all, speaking about the car body concept is not everything; the full vehicle concept has to fit together. With the Audi TT, the vehicle layout was already taken into consideration in order to realise a perfect overall lightweight design for a sporty car. The integration of a steel rear-end into a Space Frame body structure led to a perfectly weight-balanced overall vehicle concept, where the weight distribution of the body perfectly harmonises with the overall vehicle concept; engine mounted transversally in the front with front wheel drive or Quattro, Figure 3. This concept of material hybrid body design offers a customer benefit of perfect driving behaviour due to the high body stiffness, low centre of gravity and well balanced weight distribution at a competitive price and styling.
4 Process Developments While the properties of the materials are ever enhancing, the possibilities to forecast certain behaviours already at the virtual stage of design are enhancing as well. The best way to save weight is to avoid using material, where it is not functionally necessary. One of the most promising approaches to increase the lightweight potential of a part or assembly in its surrounding is the topology optimisation. Defining the available space for the structure and the applied loads onto the part, lightweight structures are created. A special example very close to bionic design is the cross-section of the TT Roadster’s sill as an example for extrusions. The inner contour was optimised to the different load characteristics of an open Roadster,
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COVER STORY
Lightweight Design
Figure 4: Adaption of the extrusion‘s inner structure allows derivatisation with little adjustments in production
ther weight savings: The potential of a reduced number of production steps is in most cases not only an economical benefit. Every part that is saved, avoids unnecessary weight: The development of intelligent joining technologies can cause weight savings in secondary effects. The introduction of the zero-gap laser-welding-seam between the roof and the side frame reduced the number of parts – the roof trim strip became obsolete – and also the material usage. Overlapping flanges to allow the joint of the two parts were no longer needed, Figure 5.
5 Conclusions without changing the outer contour, Figure 4. To achieve maximum performance in lightweight design under the aspect of a modern and economic production, an intensive use of virtual processes from the first step in development up to the serial production is one of the main motivations of efficient lightweight design. From the single part to the whole assembly of the car body are nowadays various virtual tools to accompany the development. Simulations of body-inwhite behaviour in cases of stiffness and crash-performance are meanwhile wellknown and widespread in the automotive industry. But already the production of the semi-finished part offers a potential to reduce weight. During the
Figure 5: Zero-gap laser-welding seams are an established joining technologie in steel car body design, firstly introduced in aluminium car bodies with the TT 22
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development of single components, adjustments on the geometry of the part have to be made to ensure the producibility of the part. These adjustments can lead to an increased weight, due to the fact that the functional demand has to be covered and also the productive demands have to be added. This can be avoided by an early monitoring at the virtual stage of the part. Furthermore, the specific material characteristics can be forecasted in simulating the filling and the heat-processing of the casting die, as well as the heat-treatment. This knowledge allows improvements in the part geometry to increase its functionality and reduce weight. Although the possibilities to predict the behaviour of a part during its production via virtual tools are getting more and more accurate, it is important to develop the production process itself. A major goal, especially for the increase in production volumes, is the reduction of process time in producing semi-finished parts. Regarding aluminium diecasts, a time consuming process step is the heat-treatment to adjust material characteristics to the requirements. A similar situation can be found at the creation of CFRP parts. Although the raw material is quite expensive, the main cost driver of the manufacturing process is the shaping of the single layers and their combination before this assembly is resinated. Not only has the production of each single part offered potentials to save weight. Also the manufacturing process of the whole vehicle body can show fur-
When it comes to lightweight design, it is quite obvious, that the material is not the only origin of a successful design. Especially when adapting lightweight body concepts to high production volumes, the challenges next to the right choice and combination of materials are the development of processes and intelligent structural concepts to increase the economy of the whole process chain: starting with the production and handling of the semi-finished part, leading to the joining technologies to allow the most efficient combination of materials. As a result of this development, lightweight design is no longer a special field of activity for high price and low volume cars; it is gaining importance in every model range. The future challenge for automotive body design is the perfect interaction of demands in functions and weight, production scale and number of derivates with the best offer to the customer regarding safety, environment-friendliness, driving pleasure, economy and emotion. O
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