About I-CAR - Current Events/News: e-newsletters: Advantage Online: August 6, 2007

Figure 1 - The BMW 5 Series front structure is made from aluminum, while the area rearward of the cowl is made of steel.

Figure 2 - The Audi TT Space Frame is nearly 70% aluminum. (Photo courtesy of Audi of America)

Figure 3 - The adhesive provides protection against galvanic corrosion where steel and aluminum are joined.

Figure 4 - The torque boxes on the Audi TT are made from cast aluminum.

ALUMINUM AND STEEL HYBRID-STRUCTURED VEHICLES; THE DESIGN OF THE FUTURE?

In 2004, BMW introduced the 5 Series with a front structure made from aluminum and a rear structure made from steel. An I-CAR Advantage Online article on November 10, 2003, Aluminum/Steel Construction of the BMW 5-Series, discussed some of the reasoning behind using differing construction materials and some of the repair considerations (see Figure 1). The article summarized the potential for galvanic corrosion when two dissimilar metals are used and how the BMW repair recommendations address the concern. Now, the all new Audi TT Coupe and Roadster are built using both aluminum and steel. Is the use of multiple construction materials the design of the future? Capitalizing on the unique properties of each material allows vehicle makers to get the best of both worlds.

The all-new 2007 Audi TT Coupe and Audi TT Roadster are both built using the Audi Space Frame (ASF)^® concept. Different from traditional ASF vehicles that are made from aluminum, the ASF is made from both aluminum and steel for structural parts. The ASF^® on the Audi TT Coupe (see Figure 2) is made of nearly 70% aluminum, with the remaining 30% being steel unitized construction. The use of aluminum over traditional construction materials creates a 48% weight savings and 50% higher torsional rigidity, meaning there is much less flex in the vehicle structure. The total weight of the space frame with the exterior body panels attached is approximately 277 kg (610 lb). The biggest advantage is that steel is used in areas where additional weight is desired. The steel unitized structure provides additional weight to the rear axle, creating more down force to the rear wheels and a more efficient balance between the front wheels and rear wheels. Steel is also used for the door shells and rear hatch.

The aluminum portion of the Audi TT space frame is made up of castings, extrusions, and stampings. Castings make up 22% of the Audi TT Coupe structure, and 18% of the structure of the Audi TT Roadster. Aluminum extrusions make up 16% of the structure on the Coupe and 22% on the Roadster. Steel and aluminum sheet stampings make up the remaining construction materials of the Audi TT space frame.

Galvanic corrosion occurs when two dissimilar metals are in contact with the presence of an electrolyte. An electrolyte is a non-metallic conductor of electricity. Water and salt water are examples of electrolyte that vehicles are typically exposed to. When galvanic corrosion occurs, the more active of two metals transfers electrons to the less active metal. During galvanic corrosion between aluminum and steel, aluminum becomes the anode. The anodic metal is more active and corrodes faster than it would by itself. The anodic metal is sacrificial and corrodes to protect the other metal. The steel becomes the cathode. The cathodic metal is less active and corrodes slower than it would by itself. Steel is cathodic to aluminum. Due to both metals being used on the space frame, there are specific concerns with galvanic corrosion. During the vehicle manufacturing process, all of the connections between aluminum and steel are insulated with a nonconductive structural adhesive (see Figure 3). In addition to the adhesive, there may be another fastening method, including self-piercing rivets, clinches, or rolled hem joints. The cured adhesive also seals the flange, preventing any electrolyte from being introduced to the joint.

During collision repairs to the Audi TT, there are specific recommendations for the type of adhesive that must be used on connections between the steel and aluminum. All of the fasteners that are used have a coating on them to reduce the likelihood of galvanic corrosion.

Another concern is that steel and aluminum have different tensile strengths. When straightening a hybrid structure, the aluminum will straighten at a different rate than the steel structure. Although the Audi repair information stops short of saying do not straighten the structure, as the BMW repair information for the 5 Series does, straightening the structure may cause the adhesively sealed joints to separate. This could potentially lead to galvanic corrosion. Due to the space frame design of the Audi TT, the vehicle torque boxes are constructed of cast aluminum (see Figure 4). There is a concern of adequately anchoring the structure on the aluminum castings and straightening the structure, without causing damage to these parts. In most cases, the damaged parts will typically require removal and replacement.

Vehicle makers have continually strived to come up with methods to reduce the weight of a vehicle, yet maintain crashworthiness and handling characteristics. Although aluminum and steel hybrid-structured vehicles are not real common, vehicle makers have proven that these materials can be used together, provided that the proper corrosion-preventive measures are taken.

It is too soon to tell if the aluminum and steel hybrid structure will reach the mainstream of vehicle construction. When assembled together properly, both Audi and BMW seem confident that the characteristics of each material can be combined to build a solid and durable vehicle.

For comments or suggestions on the Advantage Online, please contact I-CAR Senior Instructional Designer Bob Jansen at bob.jansen@i-car.com.

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