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Two birds, one stone: Alloy promises better engines, more rare earth production

ORNL researchers and partners at Lawrence Livermore National Laboratory and Wisconsin-based Eck Industries have developed aluminum alloys that are both easier to work with and more heat tolerant than existing products.

What may be more important, however, is that the alloys—which contain cerium—have the potential to jump-start the United States’ struggling production of rare earth elements. 

The team will discuss the technical and economic possibilities for aluminum–cerium alloys in the June issue of JOM, a publication of the Minerals, Metals & Materials Society. 

They’re working as part of the Critical Materials Institute, a DOE Energy Innovation Hub. Based at Ames National Laboratory in Iowa, the institute works to increase the availability of rare earth metals and other materials critical for U.S. energy security.

Uses for rare earths

Rare earth elements are used in electronics, alternative energy and other modern technologies. Modern windmills and hybrid vehicles, for example, rely on strong permanent magnets made with the rare earth elements neodymium and dysprosium.  More than 90 percent of rare earth production, however, comes from China, with no production occurring in North America.

One problem is that cerium accounts for up to half of the rare earth content of many rare earth ores, including those in the United States, and it has been difficult to find a market for all of the cerium mined. The United States’ most common rare earth ore, in fact, contains three times more cerium than neodymium and 500 times more cerium than dysprosium.

Aluminum–cerium alloys promise to boost domestic rare earth mining by increasing the demand and, eventually, the value of cerium. If, for example, the new alloys find a place in internal combustion engines, the new market could quickly transform cerium from an inconvenient byproduct of rare earth mining to a valuable product in itself.

“The aluminum industry is huge,” explained ORNL materials scientist Orlando Rios. “A lot of aluminum is used in the auto industry, so even a very small implementation into that market would use an enormous amount of cerium.” A 1 percent penetration into the market for aluminum alloys would translate to 3,000 tons of cerium, he added.

Rios said components made with aluminum–cerium alloys offer several advantages over those made from existing aluminum alloys, including low cost, high castability, reduced heat-treatment requirements and exceptional high-temperature stability. 

“Most alloys with exceptional properties are more difficult to cast,” said David Weiss, vice president for engineering and research and development at Eck Industries, “but the aluminum-cerium system has equivalent casting characteristics to aluminum–silicon alloys.” 

The key to heat tolerance

The key to the alloys’ high-temperature performance is a specific aluminum–cerium compound, or intermetallic, which forms inside the alloys as they are melted and cast. This intermetallic melts only at temperatures above 2,000 degrees Fahrenheit.

That heat tolerance makes aluminum–cerium alloys attractive for use in internal combustion engines, Rios noted. Tests have shown the new alloys to be stable at 300 degrees Celsius (572 degrees Fahrenheit), a temperature that would cause traditional alloys to begin disintegrating. 

Not only would aluminum–cerium alloys allow engines to increase fuel efficiency directly by running hotter, but they may also increase fuel efficiency indirectly by paving the way for lighter engines that use small aluminum-based components or that use aluminum alloys to replace cast iron components. 

The team has already cast prototype aircraft cylinder heads in conventional sand molds, as well as a fully functional cylinder head for a fossil fuel-powered electric generator in 3-D-printed sand molds. This first-of-a-kind demonstration led to a successful engine test in which the engine was shown to handle exhaust temperatures of over 600 degrees Celsius.

“Three-dimensional printed molds are typically very hard to fill,” said ORNL physicist Zachary Sims, “but aluminum–cerium alloys can completely fill the mold thanks to their exceptional castability.”