Weight savings and superior stiffness: The new Mercedes Benz SL's aluminum body with structural parts and extruded profiles from Martinrea Honsel

With a car body nearly completely made of aluminium, Mercedes Benz launches the new SL. In comparison to the steel body of the predecessor the aluminium structure offers substantial weight savings and superior stiffness. Central elements of this lightweight design body are the cast longitudinal members and beams as well as floor panel structures manufactured by Martinrea Honsel. At the International Aluminum Extrusion Design Competition of the Extrusion Technology for Aluminum Profiles Foundation in Wauconda, Illinois, USA the friction stir welded floor group from Martinrea Honsel was awarded the Grand Prize. Its innovative design convinced the jury as well as the qualified audience.

With a dimension of approximately 1120 mm x 585 mm x 360 mm each the longitudinal member is a central element of the rear end of the new SL model. In spite of the large dimensions every component weighs just 14 kilograms. In order to meet the high crash standards, the cast component material has very high ductility.

These components are produced in low pressure gravity die-casting, a process characterized by a low turbulence fill mode, low porosity and a fine grained microstructure. The cast components can be welded and heat treated. The hollow structure is formed by insertion of a massive sand core into the mould. During the heat treatment the distortion is minimised by aerial quenching and the required mechanical properties are reached. Thus tensile strengths of min. 210 MPa, yield stress of min. 150 MPa with an elongation of min. 12% can be guaranteed.

After machining the surface of the longitudinal member is treated by alodinization and the components are finished by assembly of threaded rivets.

At the Daimler plant in Bremen the longitudinal members are assembled to the other chassis components. Among these are the extrusion profiles of the Martinrea Honsel extrusion plant in Soest that stand out for their high ductility. The three profiles for the right and left floor structure have thin walls in order to meet the required weight targets. The production process is particularly designed for tight tolerances to meet the requirements for friction steer welding. This is also the first application of friction steer welding in a configuration with multi-chamber profiles for body structure.

The longitudinal members connect the rear and the front part of the vehicle and are situated on the outside of the body structure. Because of the high safety standards for front and lateral collision the components are designed as a complex profile with several chambers. At the same time they are very ductile in order to absorb the kinetic energy in case of a lateral collision. All components are machined and delivered to the customer with the required surface treatment.