BMW 7 SerIeS

EfficientDynamics

Innovative Lightweight Material Design in the Body-in-white The body forms the vehicle‘s backbone for achieving a multitude of different and, to a certain extent, contradictory requirements. In the case of many vehicles, vehicle dynamics, passive safety, comfort, quality and styling are vital arguments for the relevant customer and therefore contribute towards market success. The above mentioned vehicle characteristics are extensively influenced by the correct design and conception of the body-in-white.

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1 Introduction Comfort and passive safety generally lead to an increase in the weight of the vehicle, which contradicts the desired, efficient vehicle dynamics – this, in the final analysis, demands a stiff body with low weight. To achieve the initially mentioned characteristics as a whole, it is therefore necessary to design a body which offers high passive safety, good static and dynamic stiffness values, a low center of gravity, balanced axle load distribution and low weight. In comparison with the predecessor, increased crash requirements, the longer wheelbase, the engine‘s relocation backwards, the increase in output, reduced installation spaces and design boundary conditions led to a theoretical increase in weight. However, this was essentially compensated by means of lightweight design in the body-in-white, as is described in the following, Figure 1. Figure 2 provides an overview of the materials used in the body frame and in the body-in-white as a whole, specified according to the minimum stress at permanent set limit in the case of the steel materials and the various aluminum alloys. An overview of the percentages contained in the body-in-white with doors, lids and attaching parts is shown in Figure 3. Never before have so many hot-formed or press-hardened components been used in a BMW. The latter are manufactured using a newly developed procedure [1]. The intensive and

intelligent use of high-strength and ultrahigh strength grades of steel additionally leads to a mean minimum stress at permanent set limit of 392 MPa, a value which has never before been attained in a BMW and which surpasses that of the predecessor by 60 %, Figure 4. The material concept which is described represents the optimum in the sense of function, weight and cost target achievement. All of the lightweight material design measures – aluminum roof, die-cast spring support, highstrength and ultra-high strength coldformed steels plus the new, galvanized hotforming steels – enabled a weight saving of 37 kg in the body frame alone. In addition to steel materials, the use of aluminum for the front spring support and the roof led to further weight savings. To reduce weight even further, aluminum was also used in the hood, the front side walls and the doors, lowering the weight by an additional 33 kg. The individual grades of steel and their importance to passive safety are essentially dealt with in the passive safety chapter. The aluminum components of the body frame, the bonded aluminum roof and the die-cast aluminum spring support will be discussed in the following.

The Authors Dr.-Ing.­ Markus Pfestorf is responsible at body development for mate­ rials strategy and body­ shell materials con­ cepts within the BMW Group in Munich (Ger­ many).

Dipl.-Ing. Ralf Grünn is side frame and roof design and construc­ tion group manager for body development within the BMW Group in Munich (Ger­ many).

Dipl.-Ing. Michael Marks is team manager for body development, front end and bulk­ head, within the BMW Group in Munich (Ger­ many).

2 Bonded Aluminum Roof For the first time in a mass-produced car, an aluminum roof has been joined to a steel body via bonding without addition-

Dipl.-Wirt.-Ing. Marc Müller is a doctoral candi­ date in the joining and ­handling technol­ ogy group within the BMW Group in Mu­ nich (Germany).

Figure 1: Weight reduction in the body-in-white with doors, lids and attaching parts thanks to innovative lightweight material design ATZextra I November 2008

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B M W 7 S e ri es

al joining processes such as e.g. punch riveting. Thanks to the reduction in weight achieved in this manner at the highest point of the vehicle, 5.9 kg in the standard roof variant and 8.3 kg in the sliding/tilting sunroof variant, success was achieved in lowering the vehicle‘s center of gravity, thereby making a further contribution towards top-class vehicle dynamics, Table. In the sliding/tilting sunroof variant, the upper section of the frame is also manufactured from aluminum. To enable all of the desired radii and the cutout for the sliding/tilting sunroof to be implemented, including the surrounding flange for reasons of stiffness, the AlSi-alloy was used, Figure 5. To ensure optimal roof denting resistance, this alloy has a particularly high bake hardening effect thanks to a special process in the rolling mill. The adhesive and the double step in the roof compensate the different expansion rates of steel and aluminum when the body becomes heated, e.g. due to solar radiation, and simultaneously ensure a firm bond in the case of the different stresses which arise during a crash. Development of the joining technology will be dealt with in greater detail in the following. In the past, the experience acquired to date in the attempts made to join an aluminum roof to a steel body with the aid of thermal and mechanical joining processes or with adhesives from the body shell shop, which are usually designed for crash resistance and durability, has encountered process technology and component-specific limits. Nor did the use of adhesives from the assembly department lead to the exclusive achievement of the desired goals in terms of vehicle characteristics. For example, the extremely flexible 1-component polyurethanes previously used for bonding vehicle glazing would not have met the overall vehicle stiffness requirements. The expansion capacity of a (semi-)structural 2-component polyurethane, as is already used to bond carbon fiber-reinforced composite plastic roofs, would also have been insufficient to withstand the stresses to which an aluminum-steel joint is subject under alternating temperature strain. The requirements pertaining to desired long-term durability and the mechanical 58

BMW 7 Series

EfficientDynamics

Figure 2: Overview of materials

characteristics of the adhesive therefore had to be drawn up anew and redefined for this project. The objective was to attain optimum stiffness, expansion, strength, crash safety and long-term durability for the complex process of bonding the aluminum roof onto the steel body. In this specific case, the two materials‘ differences in expansion behavior under alternating temperature strain required particular attention in bonding a large aluminum component onto a steel body. The development work undertaken in the joining and handling technology department not only enabled the determination of process technology influences in processing the newly de-

veloped, cold-cured 2-component adhesive, but also enabled conclusions to be drawn with regard to the deformations and displacements which actually occur on the vehicle as a result of alternating temperature strain. Figure 6 shows a vehicle from the early development phase in an alternating temperature chamber. Together with the results of basic tests, success was achieved in specifically determining the requirements made on the mechanical characteristics of the adhesive and the bonded joint and in using these to derive a corresponding requirement profile. Some of the information which was obtained was

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Figure 3: Materials contained in the body-in-white

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Figure 4: Mean minimum stress at permanent set limit

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curing purposes, in turn enabling the cycle time to be reduced. This technology, which is familiar from the body shell shop, had to be redesigned to meet the special boundary conditions in the field of assembly bonding. The sensitivity of painted surfaces and the cold-curing adhesive systems usually

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integrated directly into development by the adhesive manufacturer, enabling the formulation and implementation of an iteratively optimized adhesive system for this new application case. To prevent the bonded component from moving in relation to the body or even from becoming detached during the further production process, the component has to be secured, once the aluminum roof has been joined to the steel body, until a defined handling strength has been attained. In this case, no further securing elements in the form of clamps, adhesive tapes, etc. are required; instead, the adhesive itself is used for se-

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Table: Weight saving thanks to the aluminum roof

technik bewegt.

ATZextra I November 2008

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B M W 7 S e ri es

EfficientDynamics

employed in the assembly department are process-limiting factors which had to be taken into consideration in qualifying this technology. Thanks to extensive development work in this field, success had already been achieved in securing a carbon fiber-reinforced composite plastic roof bonded to a steel body using this technology and integrating it into the series production process during the market launch of the current BMW M3. Figure 7 shows a joining gripper on a robotic arm, as is also used in the Dingolfing plant in production of the new BMW 7 Series. Figure 5: Surrounding flange

DOI: 10.1365/s40111-008-0101-y

3 Die Cast Aluminum Spring Support The die cast aluminum spring support, consisting of curing AlSi10MgMnFe alloy, is a significant element for increasing stiffness. Under the aspects of function, weight and costs, the die-cast aluminum spring support is the consistent further development of comparable components from the current BMW 5 Series and the BMW X5. Whilst aluminum has a lower modulus of elasticity than steel, the optimized design of the spring support with various wall thicknesses, which are adapted to the relevant strain in the component, and a multitude of function-specific ribs enable the achievement of considerably higher stiffness in the component than is possible with a conventional sheet metal shell design. Integration of the upper radius arm link and specific unit mountings or fastening points additionally enabled a considerable reduction in installation space. A comparable sheet metal shell design would have consisted of up to eleven individual components. Therefore, use of the cast spring support also allowed the required joining operations to be reduced. Components are mounted onto the spring support itself using bolts, clip holes or damping elements in the assembly department. Besides various units and wheelhouse liners, these also include stiffnessrelevant struts manufactured from extruded aluminum profiles, the springs and dampers plus the front axle‘s upper radius arms as the interface to the body, Figure 8. Interfaces for a total of 30 different components are integrated into the spring support. 60

BMW 7 Series

Figure 6: Vehicle from the early development phase in the alternating temperature chamber

Figure 7: Robotic gripper for automated gripping, joining and securing of roofs in automotive production

Figure 8: Assembly of the most important components on the spring support

Due to the different materials, the spring support cannot be joined to the surrounding components by means of spot welding. Joining is carried out using 68 punch rivets per spring support, whereby the contact surfaces between steel and aluminum are additionally bonded with a 2430 mm adhesive seam per spring support. This enables stiffness to be increased even further. Contact corrosion between the steel and the aluminum is additionally avoided. To prevent contact corrosion with absolute certainty throughout the vehicle‘s entire lifetime, the spring supports are additionally CDP-coated prior to joining, and all joints with steel components are sealed using PVC.

Reference

Automotive

[1] F aderl, J.; Radlmayr, K.M.: ultraform und ultraform_ PHS – Innovation made by voestalpine. Conference volume on the 1st Erlangen hot sheet metal form­ ing workshop 2006, Distributed DFG Researcher Group Reporting and Industrial Colloquium FOR 552 Publ.: Geiger, M.; Merklein, M., Meisenbach Bamberg 2006, P. 130-149

Your competent partner for each challenge. Sika AutomotiveSolutions for Bonding, Sealing, Damping and Reinforcing. www.sika.com ATZextra I November 2008

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