The Move to Lighter, Stronger, Cheaper Materials for Transportation
Another area of Li's research is the characterization of composites used in the automotive industry, supported by the Ford Motor Company. Using X-ray computed tomography (CT), which combines multiple X-ray images into a three-dimensional image, she studies and characterizes the microstructure of advanced composite materials.
From article, (Advanced manufacturing can require the bonding of incompatible materials. Li is an expert in new techniques that can bond materials when traditional bonding doesn’t work.
“My research is in lightweighting – how to make a car lighter by using advanced materials such as aluminum, magnesium or carbon fiber composites” she explains. “The benefits of such advanced materials lie in their high strength-to-weight ratio, which makes it possible to reduce the vehicle’s weight and thus reduce greenhouse gas emissions.”
But traditional bonding won’t work with dissimilar materials that have different melting temperatures. If the materials can’t fuse, or if they form an intermetallic compound that is brittle or prone to fracture, other bonding methods are required. One such method is called friction stir welding.
“There are two types of welding,” says Li, “fusion, which is what most people are familiar with in welding shops and garages across America, and solid state welding, which doesn’t melt the metal. The latter doesn’t apply any external heat source; friction alone heats the metal to a plastic state. For instance, aluminum is a softer metal than steel and the friction stir technique works well to bind aluminum and steel together.”
Invented in Great Britain in the early years of this century, friction stir uses robots to apply a spinning metal pin at the juncture of two materials, which then soften and bond along a weld.
Li uses a somewhat similar technique, patented by General Motors, that is a combination of friction stir and mechanical riveting. Called friction stir riveting, the technique drives a rivet into friction-softened materials. Once the rivet is locked, the rivet binds the materials.
“In the future, researchers are also looking to friction stir to bond higher strength steels,” she says. “We’ve been talking with Zi-Kui Liu (Penn State professor of materials science and engineering) about simulating friction stir to get a better idea of what is happening in the bond formation.”
Lightweighting is important in the automobile and aerospace industry, both of which are driven to reduce fuel consumption without losing the strength of traditional heavier materials. Boeing’s 787 Dreamliner, for example, uses advanced composites, which account for 50 percent of the airplane’s total weight. Along with allowing for a lighter structure, the composite materials are better at resisting impacts, easier to repair, and do not fatigue or corrode.
In the automotive realm, a 10 percent reduction in vehicle weight can reduce fuel usage by 6-8 percent. The DOE’s Vehicle Technologies Office says that replacing cast iron and traditional steel components with high-strength steel, magnesium alloys, aluminum alloys, carbon fiber, and polymer composites can reduce the weight of a vehicle’s body and chassis by up to 50 percent. Lightweight materials are particularly important for hybrid and electric vehicles, as they could lead to smaller and less expensive batteries and greater driving range.)
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