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Aluminum’s Pluses and Minuses

A light metal 45 percent as dense as conventional steel, aluminum has been used as a major structural material in the aerospace industry for many years. Although it is expensive-aluminum sheet sells for about $1.50 per pound, compared with about 30 cents per pound for steel sheet-researchers in the automobile industry have begun to investigate the possibility of substituting aluminum for steel in vehicle bodies.

One of the main advantages of switching to aluminum, compared with other lightweight materials, is that it can be formed using many of the techniques already applied in making automobiles out of steel. Thus the industry could continue to use much of its existing equipment. And designing for aluminum is not drastically different from designing for steel-an important advantage in an industry where engineers are reluctant to experiment with relatively untried materials.

Of course, the fact that automobile bodies are not largely aluminum today suggests that the material also has disadvantages. Besides being more expensive than steel, aluminum is only about one-third as stiff-a crucial limitation in automobile body design. Stiffness can be increased somewhat by changing the geometry of the design (curved shapes are stiffer than flat ones), but this is problematic in an industry where shape and style are important sales concepts. An easier solution is to make flat aluminum body panels-fenders, hoods, and doors-thicker than steel panels to ensure that they perform equally well. This imposes higher material costs, however, and offsets the weight advantage to some extent.

Another problem is the high electrical conductivity of aluminum, which makes spot welding difficult. Spot welding is the standard method for assembling steel automobile bodies. The two parts being joined are clamped between two electrodes and electrical current is applied, thereby heating the two parts at the point of contact, leading to diffusion bonding. (The metal does not actually melt, since this would reduce the material’s performance and lead to corrosion and part failure.)

Because aluminum conducts heat better than steel, it takes a lot more electricity and larger electrodes to make the metal hot enough to bond. And because the electrodes stay in contact with the aluminum longer while the current is being applied, aluminum atoms are more likely to diffuse into the electrode, shortening its useful life. Aluminum vehicles will probably therefore rely on alternative assembly techniques, including seam welding (in which a strip of molten metal is applied more or less like glue), adhesives, and mechanical fasteners.

Unibody versus Space Frame

The challenge facing the automobile industry is how to design an aluminum automobile so as to capture the advantages of the material and minimize the disadvantages. There are two competing possibilities: a unibody, short for “unitized body,” the design used for steel automobiles; or a space-frame design, essentially a large truss structure covered with a thin skin.

In a unibody, the vehicle’s body panels are joined together to form a shell structure. This makes efficient use of the high stiffness of the body panels. Although aluminum is not as stiff as steel, if the panels are made thick enough and appropriate joining techniques are used, the unibody design will work well with this material.

However, the unibody design poses two related problems. First, it is relatively difficult (and therefore expensive) to make complex surfaces, such as cutouts or elaborate curves, from relatively stiff metal body panels. If designers attempt to circumvent this problem by using materials that are easier to form, the second problem arises: because the unibody derives most of its structural performance from the way its parts are attached, those parts must be composed of materials that can easily be joined. Without an inexpensive way to fasten two dissimilar materials to one another, the unibody design essentially requires the automaker to manufacture cars using a single class of materials.

In response to these objections, designers are exploring the space frame. In this design, the vehicle structure is composed, in effect, of a lattice of metal rails, similar to a bridge truss. The vehicle does not rely on body panels for structural performance and in fact can be driven without any panels attached. This design does not work well for steel, in part because complex steel rails are not that much easier to make than complex steel body panels. Today the consensus among automakers is that the unibody is the most efficient way to make a mass-market vehicle out of steel.

However, the space frame is gaining renewed attention from designers working with alternative materials, especially aluminum. It is easier to make complex rails out of aluminum than steel because, unlike steel, aluminum can be extruded-formed into complex tubular shapes-in a process similar to pasta-making. These extruded, hollow rails can be far stiffer than solid bars of equivalent weight. Extrusion is easily adapted to mass production; it is already used on a large scale to manufacture construction shapes such as window frames and pipes. Several designs for aluminum space-frame vehicles have been developed, each using differing combinations of extrusions, castings, and sheet metal. While the jury is still out, with the right combination of materials the space frame may someday challenge the unibody in mainstream automobile production.

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