By Jim Kerr; photos by Paul Williams
Aluminum has been used in cars for decades. Bugatti used aluminum extensively. In 1924, the Bugatti Model 35 sported aluminum wheels and their famed turned aluminum dashes and engine covers were a highlight of those cars. Bugatti even produced three cars with all aluminum frames and bodies.
Today, many automobile manufacturers use aluminum engine components and body panels to reduce vehicle weight, but few have the experience with aluminum bodies that Audi has. In 1985, Audi exhibited their first “Aluminum Design Study” of the full-size Audi A8 sedan. In 1993, the “Space Frame Concept Car” was shown at the auto show in Frankfurt, and it was followed by an aluminum-bodied Audi A8 production car in 1994. Audi partnered with Alcoa (Aluminum Company of America) in the development of its all-aluminum bodies.
The first volume production aluminum car, the Audi A2 was introduced in 1999, with more than 176,000 cars built in the following years. By 2003, Audi had built a quarter of a million cars with aluminum bodies. Now they are going one step further: Audi is producing cars with an aluminum composite space frame. The Audi R8 mid engine sports car and the 2008 TT Coupe and Roadster models have benefited from this construction technique, first used on the A8 in 2006.
Space frame construction is not new. Aircraft have used it for years. Some may see it simply as a unibody construction similar to many production cars now on the road, but the space frame is much more than unibody construction. It is a combination of aluminum castings, forgings, extrusions and sheet, combined with steel panels to form a light, stiff body structure. In the Audi TT, an all-steel body would be 48% heavier than the aluminum space frame design. The bare TT coupe body weights only 206 kilograms!
Aluminum-bodied 2006 Audi A8L. Click image to enlarge
The combination of materials, forgings and castings have increased the strength of the Audi TT body, with 50% higher torsional rigidity than the original TT design. This provides better handling and ride comfort as well as increased safety.
As for safety, aluminum is used instead of steel for many structural parts. The aluminum has to be 1.8 mm thick to absorb the same impact energy as steel that is only 1.2 mm thick, but even with the thicker section, the aluminum part weighs 50% less. By making the forward sections of the Audi TT out of aluminum, more balanced weight distribution is possible to enhance handling. The centre of gravity is also lowered by one centimetre to reduce body roll. All this increases driving safety.
One of the difficulties in producing an aluminum/steel composite body is preventing electrolysis. Two different metals in contact with each other and exposed to a wet salty environment will soon generate an electrical current, just like a battery, and corrode the metals. To prevent this, Audi uses a variety of joining techniques. One of the ways aluminum castings and sheets are joined is with punch-riveting. Aluminum or coated stainless steel rivets are punched through both material layers, joining them together. Non-conductive adhesive bonding is used for joining aluminum/galvanized steel panels in the body shell where the two dissimilar metals would be susceptible to corrosion.
Another method of joining the two panels is with Flow Drill Screwing. This technique uses a special coated screw that goes through a punched hole in the outer panel and is forced through the inner panel by rotational speed and pressure. The force and speed soften the material so the screw can form threads in the inner panel and clamp the panels together. More traditional thermal joining techniques such as MIG or TIG welding have very limited use on the Audi space frame.
The Audi space frame can cut 100 kg from the vehicle weight. That may not seem like much but it can save 0.3 litres of fuel every 100 km. Audi expects to sell 4000 of the new Audi TTs in Canada over the model life. With an average of 225,000 km of driving on each car, that equals 900,000,000 kilometres. Saving 0.3 litres per 100 km will save 2,700,000 litres of gasoline on this model alone!
Lower emissions, reduced fuel usage and improved performance are all benefits of less vehicle weight. Audi may not produce the sales volume of companies like GM or Ford, but as they like to say, they are “making a difference – one car at a time”.