Materials Selection Criteria

Five key factors affect the auto designer’s selection of materials: manufacturability and cost, performance, weight, safety, and recyclability.

Manufacturability and Cost

A typical mid-size family car costs about $5 per pound on the dealer’s lot, and about $2.25 per pound to manufacture. Of the manufacturing cost, about $1.35 goes to labor and overhead, and $0.90 for materials, including scrap.4 The reason cars are so affordable is that steel sheet and cast iron, the dominant materials, cost only $0.35 to 0.55 per pound. Advocates of alternative materials such as aluminum and composites are quick to point out, however, that the per-pound cost of materials is not the proper basis for comparison, but rather the per-part cost for finished parts. Although they may have a higher initial cost, alternative materials may offer opportunities to reduce manufacturing and finishing costs through reduced tooling, net shape forming, and parts consolidation. In addition, a pound of steel will be replaced by less than a pound of lightweight material. Nevertheless, the cost breakdown given above suggests that, if finished parts made with alternative materials cost much more than $1.00 per pound, overall vehicle manufacturing costs will rise significantly.5 This severe constraint will be discussed later.

For comparison, the per-pound and per-part costs of alternative materials considered in this study are given in table 3-1, along with the expected weight savings achieved by making the substitution. On a per-pound basis, glass fiber-reinforced polymers (FRP), aluminum, and graphite FRP cost roughly 3 times, 4 times, and 20 times as much as carbon steel, respectively.6 Because these materials are less dense than steel, however, fewer pounds are required to make an equivalent part, so that, on a part-for-part basis, the difference in raw materials cost relative to steel is 1.5 times, 2 times, and 5 times, respectively.High-strength steel costs 10 percent more per pound than ordinary carbon steel, but 10 percent less is required to make a part, so, on a part basis, the two have roughly equivalent cost.

Manufacturing costs

As with any mass production industry, cost containment/reduction (while maintaining equivalent performance) is a dominant feature of the materials selection process for automotive components. Customarily, this objective has focused the automobile designer upon a search for one-to-one substitutes for a particular part, where a material alternative can provide the same performance for lower cost. More recently, the focus has broadened to include subassembly costs, rather than component costs, which has enabled consideration of materials that are initially more expensive, but may yield cost savings during joining and assembly. Manufacturers can also reduce costs by shilling production of complex subassemblies (such as dashboards, bumpers, or door mechanisms) to suppliers who can use less expensive labor (i.e., non-United Auto Worker labor) to fabricate components that are then shipped to assembly plants.

Thus, the manufacturer’s calculation of the cost of making a materials change also depends on such factors as tooling costs, manufacturing rates, production volumes, potential for consolidation of parts, scrap rates, and so forth. For example, the competition between steel and plastics is discussed not only in terms of the number of units processed, but also the time period over which these parts will be made. Because the tooling and equipment costs for plastic parts are less than those for steel parts, low vehicle production volumes (50,000 per year or less) and short product lifetimes lead to part costs that favor plastics, while large production runs and long product lifetimes favor steel. As automakers seek to increase product diversity, rapid product development cycles and frequent styling changes have become associated with plastic materials, although the steel industry has fought this generalization. Nevertheless, styling elements like fascias and spoilers are predominantly plastic, and these elements are among the first ones redesigned during product facelifts and updates.

Life Cycle Costs

The total cost of a material over its entire life cycle (i.e., manufacturing costs, costs incurred by customers after the vehicle leaves the assembly plant, and recycling costs) may also be a factor in materials choices. For example, a material that has a higher first cost may be acceptable, if it results in savings over the life of the vehicle through increased fuel economy, lower repair expense, and so forth. However, this opportunity is rather limited. For instance, at gasoline prices of $1.20 per gallon, fuel cost savings owing to extensive substitution of a lightweight material such as aluminum might be $580 over 100,000 miles of driving--about $1 per pound of weight saved. These savings are insufficient to justify the added first cost of the aluminum-intensive vehicle (perhaps as much as $1,500, see below). Moreover, manufacturers are generally skeptical about the extent to which customers take life cycle costs into consideration in making purchasing decisions.

Materials choices also influence the cost of recycling or disposing of the vehicle, though these costs are not currently borne by either the manufacturer or consumer. This situation could change in the near future, however, with increasing policy emphasis on auto recycling around the world (see recycling section below).

Manufacturability

Steel vehicles are constructed by welding together body parts that have been stamped from inexpensive steel sheet materials. Over the years, this process has been extensively refined and optimized for high speed and low cost. Steel tooling is expensive: an individual die can cost over $100,000 dollars, and with scores of dies for each model, total tooling costs maybe several tens of millions of dollars per vehicle. A stamped part can be produced every 17 seconds, however, and with production volumes of 100,000 units or more, per-part costs are kept low.

Aluminium-intensive vehicles have been produced by two methods: by stamping and welding of aluminum sheet to forma unibody structure (a process parallel to existing steel processes); and by constructing a “space frame” in which extruded aluminum tubes are inserted like tinker toys into cast aluminum nodes, upon which a sheet aluminum outer skin can be placed.

An advantage of the stamped aluminum unibody approach is that existing steel presses can be used with modified tooling, which keeps new capital investment costs low for automakers and permits large production volumes. Ford used this method to produce a test fleet of 40 aluminumbodied Sables; as did Chrysler in the production of a small test fleet of aluminum Neons.8 The Honda NSX production vehicle was also fabricated by this method.

The aluminum space frame approach was pioneered by Audi in the A8, the result of a 10-year development program with Alcoa. Tooling costs are reportedly much less than sheet-stamping tools, but production volumes are inherently limited; for example, the A8 is produced in volumes of about 15,000 units per year. Thus, per-part tooling costs for space frames may not be much different from stamped unibodies.

Manufacturability is a critical issue for using composites in vehicle bodies, particularly in loadbearing structures.9 Although composite manufacturing methods exist that are appropriate for aircraft or aerospace applications produced in volumes of hundreds or even thousands of units per year, no manufacturing method for load-bearing structures has been developed that is suitable to the automotive production environment of tens or hundreds of thousands of units per year.

The most promising techniques available thus far appear to be liquid molding processes, in which a fiber reinforcement “preform” is placed in a closed, part-shaped mold and liquid resin is injected. l0 The resin must remain fluid long enough to flow throughout the mold, thoroughly wetting the fibers and filling in voids between the fibers. It must then “cure” rapidly into a solid structure that can be removed from the mold so that the process can be repeated. A vehicle constructed from polymer composites might be built with a continuous glass FRP or carbon FRP structure made by liquid molding techniques, with chopped fiber composite skin and closure panels made by stamping methods.

Liquid molding can be used to make entire body structures in large, integrated sections: as few as five moldings could be used to construct the body compared with the conventional steel construction involving several hundred pieces. However, a number of manufacturing issues must be resolved, especially demonstrating that liquid molding can be accomplished with fast cycle times (ideally 1 per minute) and showing that highly reliable integrated parts can be produced that meet performance specifications. Suitable processes have yet to be invented, which is the principal reason that the composite vehicle is used in the 2015 “optimistic” scenario. At present, manufacturing rates for liquid molding processes are much slower than steel stamping rates (roughly 15 minutes per part for liquid molding, 17 seconds for steel), so that order of magnitude improvements in the speed of liquid molding will be necessary for it to be competitive.

While advocates of automotive composites point to the General Motors (GM) Ultralite as an example of what can be achieved with composites, in some ways this example is misleading. First, the Ultralite was manufactured using the painstaking composite lay-up methods borrowed from the aerospace industry, which are far too slow to be acceptable in the automotive industry. Second, the Ultralite body cost $30 per pound in direct materials alone (excluding manufacturing costs). This is at least an order of magnitude too high for an automotive structural material.