Technology: Aluminum Is Striving for Mass Appeal, but With a Lot Less Mass October 21, 2006
Just as automakers turn from steel to aluminum as a way to lower fuel consumption and improve performance, Big Steel is fighting back.
ANY expectations that technology would only march forward were dinged, if not quite dashed, when Audi revealed details earlier this year of the 2008 TT, the second generation of its stylish sports model, which goes on sale next spring. A proponent of all-aluminum cars, Audi seemed to be stepping backward by building the framework in a blend that was nearly one-third steel.
The move was not a retreat – after all, the outgoing TT had a steel chassis – but an acknowledgment that an all-aluminum structure, while economically feasible for a luxury-class car like the company’s flagship A8 sedan, may not be the best choice for a car that costs half as much. As is the case with other advanced technologies, costly features arrive first at the top of the line.
Chevrolet, Ferrari and Jaguar, among others, make extensive use of aluminum frames to save weight; Audi has built more than 150,000 cars with aluminum space frames. It is now 16 years since Honda introduced the NSX sports car (sold in the United States as an Acura), the first production car with an all-aluminum unibody.
Many other makers are using aluminum in body panels like hoods and doors, a practice embraced by Land Rover in the 1940′s, to achieve specific weight goals. Aluminum chassis and bodies are the next big step after the long process of adopting aluminum in engines, transmissions and brake calipers – displacing cast iron in each case.
Autos have been made of steel for a long time – and for good reason. Steel has been available in quantity since 1860, so there are many grades and the material is well understood by engineers. It is inexpensive, can be stamped into complex shapes and can be readily joined by welding.
Even so, automakers have turned to aluminum as a way to reduce vehicle weight. A major goal is lower fuel consumption, and another goal is improving performance; other factors being equal, lighter cars are quicker in acceleration and stop in shorter distances.
Substituting aluminum for steel to make a vehicle lighter is not as intuitive as it may seem. Though a given volume of aluminum weighs about one-third as much as steel, it also has only about one-third the stiffness, which seems to imply that an aluminum car would have to weigh just as much as a steel one. How does the lighter metal save weight-
Automobile chassis and bodies are mostly made of steel sheet, formed into shapes that make it stiff, just as rolling cardboard into a tube stiffens it. To reduce weight, sheet metal thickness can be reduced, but there are limits to how much.
This limit is set not by the strength of the material, but by its ability to retain its shape by resisting buckling. If you bend a cardboard tube, it does not break – it buckles. Auto-body steel may be as thin as about .040 inch, but below that, buckling becomes more likely and stamping it without wrinkling can be difficult. Similar rules apply for chassis structures.
At equal weights, a square foot of aluminum sheet would be three times as thick as a steel sheet, and this greater thickness gives the aluminum greater resistance to buckling. Therefore it is possible to reduce the thickness of the aluminum to something like 1.5 times that of the original steel (and half the weight), yet still have equal resistance to buckling. This allows aluminum structures to be lighter than steel yet still adequately stiff and strong. The space frame of an Audi A8 – a design that looks like a cage of aluminum beams – weighs 548 pounds, some 46 percent lighter than it would in steel. The aluminum chassis of the Chevrolet Corvette Z06 weighs 285 pounds, a weight saving of more than 30 percent compared with an equivalent steel structure.
The attachment of suspension parts or engine and transmission mounts to a steel chassis requires the alignment and welding of many small stampings. This is simplified in aluminum structures by substituting a single casting – a process called parts consolidation. Recent developments in aluminum casting yield parts nearly as strong as if they were forged.
Switching to an aluminum structure is more complicated than just opening the computer files for the frame designs, erasing the word steel and typing in aluminum. Aluminum forms quite differently from steel – if too sharply bent, it can crack, and it also springs back more than steel when stamped in a die. The cost per pound is several times that of steel. Partly offsetting this are new methods of aluminum fabrication. Aluminum structures can be designed for computer-controlled laser welding along the seams, replacing many thousands of spot welds that normally hold a steel chassis together.
Other methods of aluminum fastening include structural adhesives and rivets that need no predrilling to install. When one operation replaces many, assembly costs fall. Five years ago the cost penalty of an aluminum frame was estimated at about $500 but experience has steadily reduced that disadvantage. Projects like the one that developed the aluminum structure of the Acura NSX offer valuable experience; similar construction techniques were later used in the Honda Insight hybrid.
Steel is fighting back. Auto engineering magazines carry advertisements for specialized steels. The 1980′s heartbreak of body rust – all but dropping the windshield in your lap at 80,000 miles – has been largely eliminated by antirust treatments.
- Posted in : Uncategorized
- Author : arnold
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