Welding Journal - January 2012

For more than 30 years, body-in-white development has been dominated by the need to achieve weight reduction while improving crash worthiness (Refs. 1, 2). Weight reduction is considered a primary key to improvements in fuel economy. One area of considerable development has been through materials substitution. Thirty years ago, automotive structural elements were made almost exclusively from relatively low-strength steels. Today, however, a variety of materials can be found within the vehicle structure. This includes a range of steels (interstitial free grades up to martensitic grades), magnesium alloys, plastic composites, and of course, aluminum (Refs. 3, 4). Aluminum has been of particular interest for these applications for a number of reasons. First and foremost, the aluminum alloys under consideration today offer strength-to-weight ratio improvements over mild steel on the order of 3:1. This suggests that for an equivalent design, body-in-white weight reductions on the order of 70% could be achieved simply by this direct substitution. Even given strength and stiffnesses between aluminum and steel, weight reductions of 40 to 60% can still be realized. Additionally, aluminum sheets typically offer considerable corrosion benefits over even galvanized steels. This is of considerable advantage when addressing increased reliability requirements on newer generations of vehicles. A recent survey assessing trends in the automotive industry clearly showed an increase in aluminum usage (Ref. 5). This has also been reflected in the numbers of aluminum-intensive vehicles that have been either developed or are under evaluation. These include the Mercedes-Benz CL Coupe (Ref. 6), the Audi A-2 (Ref. 7), Audi A-8 (Ref. 8), and the General Motors EV-1 (Ref. 9), just to name a few. It is well understood that vehicle manufacturing is dominated by three design/assembly strategies. These include the unitized vehicle, body on frame, spaceframe, and check (Refs. 1–4). The unitized vehicle approach is most commonly used for higher volume production vehicles. Unitized vehicle manufacture typically incorporates stamped components as structural elements. Stamped components are then assembled into the unibody assembly, generally incorporating subframes for suspension attachments. Structural load paths are then through the vehicle (unitized) body itself. For steel designs, unitized bodies are assembled almost exclusively by resistance spot welding. Resistance spot welding offers a number of advantages, including low cost, minimal fixturing, application flexibility, and high process robustness. Body-on-frame as well as spaceframe approaches incorporate a structure (frame) that acts as a loadpath in design. Body-on-frame approaches are commonly used for truck and SUV applications. Spareframe applications are less common, and more generally associated with low production run vehicles. In both cases, body panels are designed to be attached to this frame, and are not considered significant to the structural performance of the vehicle. Manufacture Joining Aluminum Sheet in the Automotive Industry – A 30 Year History