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9 Materials Technologies for Future Vehicles Kathleen C. Taylor and Anil SachUev, General Motors Corporation The motivation for new materials in the automotive sector falls into three categories—government regulations for emissions and fuel economy, technologi- cal advances, and market trends. Government regulations for emissions and fuel economy have had a large impact on the introduction of new lightweight materials and new catalyst systems. Technological advances can and do occur in reaction to government regulation; however, this is not always the case. For example, in California the requirement for electric vehicles did not lead to long-range, low- cost battery systems. Instead, hybrid power train systems are being developed that address energy efficiency with acceptable vehicle range. The market, par- ticularly through consumer demand, provides tremendous challenges and oppor- tunities for the automotive industry. In response to these demands, improved energy efficiency is going to come from three areas--vehicle mass reduction, changes in basic vehicle architecture, and the power train. While the focus of this presentation is materials, improved efficiency of the power train plays a large role in energy savings. The development and use of new materials are crucial to a response to the need for improved energy efficiency. In the year 2000, 17 M tons of iron and steel, 2 M tons of aluminum, and 2 M tons of plastics were used in the automobile industry. However, over the past 20 years, the allocation of these materials in a typical family car and the addition of new materials, have considerably altered the materials composition of automobiles (Table 9.1~. Iron and steel, which made up 75 percent of a vehicle in 1977, have been reduced to 66 percent, while high- strength steel has risen from 3 percent to roughly 10 percent. In addition, the amounts of polymer composites and aluminum used have risen substantially. 56
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MATERIALS TECHNOLOGIES FOR FUTURE VEHICLES TABLE 9.1 Materials Used in a Typical Family Car 57 1977 1999 Total iron and steel 75 percent 66 percent High strength steel 3.4 percent 10 percent Polymer composites 4.6 percent 7.5 percent Aluminum 2.6 percent 7.2 percent Magnesium O percent 0.2 percent Magnesium, which was virtually unused in automobiles in the 1970s, now makes up 0.2 percent of the typical family car. Figure 9.1 shows fuel consumption versus curb weight for a large number of vehicles. The slope indicates that fuel consumption generally is greater as curb weight increases. However, achieving a reduction in vehicle weight by reducing vehicle size reduces functionality and contradicts market trends. To both provide the volume of space that consumers want and meet fuel economy standards requires the use of new materials and new power plants for vehicles. For the automotive industry, tremendous challenges and opportunities are associated with introducing new materials, in terms of both fundamental and applied research. Issues related to performance/function, total accounted cost, design rules, and manufacturing feasibility at production volumes all play key roles. Other issues include joining (welding, bonding, and fastening), lead time for qualifying new materials, durability and reliability, and crash energy management. The performance of a material in its intended application is the first materials requirement. For example, to convert from steel to aluminum for the vehicle body, differences in materials properties must be well understood and new methods must be developed for materials processing and manufacturing. Design rules are different as a material is changed. When changing to a new material, new design guidelines for manufacturing are required in order to achieve high-rate and high-quality production. As new polymer composites are considered, the issues of environmental impact and recycling become even more important. Metals have an advantage over polymer composites because an infrastructure exists for recycling them. However, work is progressing on developing more recyclable polymer composite systems. Aluminum is the leading new material for lightweight vehicle structures. In order to be used more widely in passenger vehicles, a number of issues must be addressed. Forming and joining are of particular importance. Aluminum should deform better than steel based on its face-centered structure, but this is not the case. As a result, joining is difficult with aluminum. Galvanic corrosion presents
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58 ENERGY AND TRANSPORTATION
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60 ENERGY AND TRANSPORTATION another challenge because it restricts the manufacturer to medium-strength alloys. Very high strength alloys pose problems of stress corrosion and cracking. Limits on aluminum alloy composition are mainly driven by recycling con- cerns. Aluminum cannot be readily refined to remove secondary elements with- out added cost. As a result, when developing composites, auto manufacturers try to limit applications to one or two alloy systems.) Painting can be an issue when using aluminum because of formation of the aluminum oxide layer. Because the modus of aluminum is one-third that of steel, aluminum parts must be a little thicker than steel in order to resist denting. General Motor' s development of the Oldsmobile Aurora represents a vehicle that uses aluminum extensively. Aluminum is used in the cylinder head and cylinder block, the hood deck lid, the fenders, and the bumper beams. The use of aluminum in these applications has reduced the weight of this particular vehicle by 285 pounds. An aluminum engine cradle was used for the first time in the 2000 Impala. The engine cradle is made from 19 extrusions and one stamping, which are all put together in a large welded fixture for a weight savings of 20 pounds. GM plans to produce 1.5 million units of this engine cradle. Magnesium is another material that has increased in use. One large applica- tion for magnesium in automobiles has been the instrument panel for GM's full- sized car and full-sized van that consists of a one-piece die casting that provides the entire structure of the front dashboard. The use of polymer composites in automobiles poses many advantages as well as significant barriers. Polymer composites offer low mass and are damage and corrosion resistant, and parts can be consolidated because of molding capa- bility. Balanced against these benefits is the significant barrier in changing current assembly methods. For the manufacture of auto parts from composites, entirely new assembly methods must be developed. Issues that must be addressed include the durability of composites, design issues, and the cycle time for manufacturing. GM recently put into production a new nanocomposite made from thermo- plastic olefin with a clay filler. Only about 2 percent clay is used, compared with 20 to 30 percent talc in the composite material it replaces. Two advantages of this new nanocomposite are that it is both light-weight and recyclable. Presently, this nanocomposite is used in the step assist for the full-sized van, but a future goal is to use it in body applications such as vertical and horizontal panels. Polymer composites are also being used for the pickup box in trucks. Using a composite in this application saves 50 pounds with the truck bed, and another 15 pounds with the lift gate and provides superior impact and corrosion resistance. In the past decade, interindustry-government partnerships such as the Partner- ship for a New Generation of Vehicles (PNGV) have become a significant means tin addition to recycling costs, the costs of aluminum stampings have also placed limitations on additional use of aluminum alloy composites in lightweight vehicles.
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MATERIALS TECHNOLOGIES FOR FUTURE VEHICLES 61 for conducting precompetitive research. Reduced vehicle mass was recognized as a key factor for improving fuel economy in the PNGV program, and a number of companies have each used an aluminum body structure and a light power train to achieve this goal in their year 2000 PNGV concept vehicles. The steel industry is also looking into new ways to utilize high-strength steel in the construction of motor vehicles in order to reduce vehicle mass. Other devel- opments are steel-plastic sandwich structures and innovative manufacturing processes. The American Iron and Steel Institute plans an advanced concept vehicle project targeted to meet the 2004 PNGV vehicle and crash requirement. Future needs are divided into three areas lightweight materials, low-cost materials and processes, and environmental enablers. Light-weight materials pro- vide fuel economy while keeping the vehicle volume constant and maintaining the power of the vehicle for activities like towing. The introduction of new light- weight materials must be done without increasing the base cost of the vehicle, and high-volume manufacturing methods must be used that produce high-quality materials.
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