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Lighter materials are one answer to improved fuel economy. As the U.S. auto industry sought to raise the fuel efficiency of gasoline cars by 50% between 1975 and 1995 to meet government fuel economy standards, one response was to replace bulky cast iron engines with lightweight aluminum engines.
"If cars could be made 40% lighter by using advanced materials such as magnesium and carbon-fiber composites, Americans could increase fuel economy by 25%," says Ray Boeman, director of Oak Ridge National Laboratory's Transportation Program and National Transportation Research Center, which includes University of Tennessee researchers. ORNL researchers have developed procedures for evaluating the durability of glass-fiber composites and carbon-fiber composites that have been adopted by industry. As an example of "lightweighting" progress, 11 pounds of carbon fiber replaced 200 pounds of steel in the 2004 Dodge Viper. Lightweight material tests While working five years with the Automotive Composites Consortium in Detroit, Boeman initiated at ORNL composite manufacturing and evaluations of the ability of advanced lightweight materials to protect car occupants in collisions. His evaluations used the newly developed Test Machine for Automotive Crashworthiness, or TMAC. Computer models and TMAC tests of auto parts made of carbon-fiber composites reveal that composites can absorb as much or more energy from an impact as steel, enhancing protection of occupants. "The two materials absorb energy differently," says Phil Sklad, ORNL researcher and technical manager of DOE's Automotive Lightweighting Materials Program. "When steel is struck in a collision, it collapses like an accordion. When a carbon-fiber composite is crushed, the energy is absorbed through multiple fracture processes." ORNL inherited carbon composite experts from the project in Oak Ridge that developed gas centrifuges for uranium enrichment. Currently, the Laboratory hosts DOE's largest effort dedicated to devising ways of making carbon-fiber composites affordable to the auto industry. Today these composites, which are used in aircraft parts, tennis racquets and wind turbine blades, cost $8 to $15 a pound. DOE funds efforts at ORNL aimed at lowering the cost of composite production to below $5 a pound, a price the auto industry would find attractive. ORNL researchers are evaluating other lightweight materials for vehicles, such as magnesium and high-strength steel. If regular steel in a car were replaced with only one lighter material, Sklad says that the weight of the car's body and chassis could be reduced 60% using only carbon-fiber composites, 50% using magnesium, 35% to 40% using aluminum, and 25% to 30% using thingauge, high-strength steel. "While carbon-fiber composites have the greatest potential, lowering their cost enough will require the most work and investment," Sklad says. "The cost of making magnesium sheet is very expensive because the hexagonal crystal structure makes deformation processes much more difficult. We can easily make cost-effective components using a number of casting techniques, but the processes for producing magnesium sheet for forming specially shaped components are much more costly." Advanced high-strength steels are so strong that thinner material provides the same strength as regular steel in a vehicle, reducing vehicle weight by up to 30%. The American auto industry has more than a century's experience with steel. "Since Henry Ford's day, the steel and automotive industries have learned how to produce, design, weld and recycle inexpensive steel," Sklad says. This century's challenge is to develop a cost-effective process to produce specially shaped, lightweight steel components. DOE's lightweighting program also focuses on finding less costly ways to manufacture new "designer" steels, called transformation-induced plasticity steels. TRIP steels are designed to change their properties and microstructure in desirable ways as a result of stresses imposed by forming. Multi-material vehicle The DOE does not envision a future car made from a single material. Sklad says DOE national labs are working with the auto industry to design a "multi-material vehicle" that is part aluminum, part polymer, part magnesium, part carbon-fiber composite and part high-strength steel. "The government's role is to help develop all these materials, remove the technical barriers to their use by the auto industry and, in turn, provide design engineers with the ability to mix and match the materials according to individual needs," Sklad says. "Our task is to choose the right material for the right application. For example, magnesium might be a suitable material for an engine cradle or radiator support but not for a frame rail." Technical barriers also include joining dissimilar materials, corrosion, disassembly and recycling. Steel is easy to recycle, but carbon-fiber composite parts joined by adhesives are much more challenging. Joining will be a major issue because dissimilar materials have different properties. "Spot welding would melt plastic," Sklad says. "Fusion welding of magnesium to steel is impossible because each melts and solidifies at different temperatures. In addition, when magnesium and steel come into contact, battery-acid-like galvanic corrosion can result." ORNL is pursuing funding to experiment with friction stir welding and to develop other methods to address major joining issues involving dissimilar materials. Although a number of issues remain, all agree that a multidisciplinary effort involving materials scientists, joining researchers, engineers and computer scientists will be needed to remove the technical barriers on the road to building an affordable, lightweight, crashworthy, multi-material car.—Carolyn Krause
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