Metals benefit from addition of toxic waste

Researchers at the Polytechnic Institute of New York University (NYU-Poly) have demonstrated the potential to keep millions of tons of toxic waste out of landfills while improving the performance and lowering the cost of some of manufacturing industries’ most expensive raw materials such as aluminum and magnesium.

Nikhil Gupta, associate professor of mechanical and aerospace engineering at NYU-Poly’s Composites Materials and Mechanics Laboratory, has reported the results of experiments designed to utilize fly ash – a toxic by-product of coal combustion – as an additive to create lightweight composite metal foams that can replace solid aluminum and magnesium in some automotive and consumer product applications.

Gupta and collaborators from the University of Wisconsin-Milwaukee published their findings in a recent issue of Journal of Metals (JOM), a publication of The Minerals, Metals & Materials Society.

More than 70 million tons of fly ash are produced by coal power plants in the United States every year, and more than half that amount is dumped in landfills. Fly ash contains hollow particles that, when added to a molten metal such as aluminum, create a porous metal foam that is lighter than solid metal yet absorbs a higher amount of energy under compression. The team tested aluminum and magnesium alloys filled with fly ash at high compression rates – similar to those experienced in high-speed auto accidents – and found that the lightweight foams absorb more energy than the solid metals.

“Composite metal foams made with fly ash could be seamlessly incorporated into vehicle manufacture with no compromise in performance,” said Gupta. “As a starting point, these materials are ideal replacements in automotive parts that aren’t load-bearing – for example, engine and wheel covers and intake manifolds, where the weight and strength of solid metal doesn’t provide any benefit – in fact, it just costs more and weighs more.”

Diverting fly ash for use in metal foams has significant environmental and cost-saving benefits. First, it keeps toxic ash out of landfills and preserves the $1 billion spent annually disposing of this waste. Second, manufacturers can reduce their costs by purchasing smaller quantities of expensive metals that take a high environmental toll in mining and production. Lastly, because additions of fly ash make automotive parts lighter in weight, the finished vehicle requires less fuel to operate, leading to further energy and cost-savings for consumers.

Studies have shown that reduction in vehicle weight by 10 percent can lead to an improvement of about 5 percent in fuel economy. With 137 billion gallons of gasoline consumed each year in the United States, this can translate into more than $22 billion in savings at the current gas prices. The JOM reports estimate that replacing 10 percent of solid aluminum with fly ash in a manufacturing application would result in an approximate 8 percent overall weight-savings.

While fly ash itself is available at no cost, companies would need to bear the cost of transporting the material and preparing it for use.

Composite foams made from fly ash could be widely useful outside the automotive industry. Everyday items such as highway and runway signs, park benches, lamp posts, sliding tracks for windows and home accessories like doorknobs could all be made lighter and less expensive through the incorporation of metal foams, according to Gupta. “Look around you – anywhere you see aluminum or steel, there’s an opportunity for these materials,” he said.

Research funding was provided by the United States Office of Naval Research and the National Science Foundation. The papers, “The Synthesis, Compressive Properties, and Applications of Metal Matrix Syntactic Foams” and “High Strain Rate Compressive Characterization of Aluminum Alloy/Fly Ash Cenosphere Composites,” were co-authored by Pradeep K. Rohatgi of the University of Wisconsin-Milwaukee. Dung D. Luong, a doctoral candidate at NYU-Poly, was involved in the research.