Novel aluminum alloy could boost rare earth production

Researchers at the US Department of Energy's Oak Ridge National Laboratory have helped to develop 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 US production of rare earth elements.

ORNL scientists Zach Sims, Michael McGuire and Orlando Rios, along with colleagues from Eck Industries, the Lawrence Livermore National Laboratory and Ames Laboratory, discuss the technical and economic possibilities for aluminum-cerium alloys in an article in JOM.

The team worked under the aegis of the Critical Materials Institute, an Energy Innovation Hub created by the US Department of Energy (DOE) and managed out of DOE's Advanced Manufacturing Office. Based at Ames, the institute works to increase the availability of rare earth metals and other materials critical for US energy security.

Rare earths are a group of elements critical to electronics, alternative energy and other modern technologies. Modern windmills and hybrid autos, for example, rely on strong permanent magnets made with the rare earth elements neodymium and dysprosium. Yet these elements are not currently mined in North America.

One reason for this is because cerium accounts for up to half of the rare earth content of many rare earth ores, including those in the US, and it has been difficult for rare earth producers to find a market for all this cerium. The most common rare earth ore in the US contains three times more cerium than neodymium and 500 times more cerium than dysprosium.

"We have these rare earths that we need for energy technologies," said Rios, "but when you go to extract rare earths, the majority is cerium and lanthanum, which have limited large-volume uses."

Aluminum-cerium alloys offer one potential solution to this problem, by increasing the demand and, eventually, the value of cerium. If, for example, these alloys find a place in internal combustion engines, they could quickly transform cerium from an inconvenient by-product of rare earth mining to a valuable product in itself.

"The aluminum industry is huge," Rios explained. "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% penetration into the market for aluminum alloys would translate into 3000 tons of cerium, he added.

According to Rios, 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 the aluminum-silicon alloys."

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 and only melts at temperatures above 2000°F.

This heat tolerance makes aluminum-cerium alloys very attractive for use in internal combustion engines, Rios noted. Tests have shown that the new alloys are stable at 300°C (572°F), a temperature that would cause traditional alloys to begin disintegrating. In addition, the stability of this intermetallic sometimes eliminates the need for the heat treatments that are typically required for aluminum alloys.

Not only would aluminum-cerium alloys allow engines to increase fuel efficiency directly by running hotter, they may also increase fuel efficiency indirectly. For they could pave the way for lighter engines that use small aluminum-based components or use aluminum alloys to replace cast iron components such as cylinder blocks, transmission cases and cylinder heads.

The team has already cast prototype aircraft cylinder heads in conventional sand molds. The team also cast a fully functional cylinder head for a fossil fuel-powered electric generator in 3D-printed sand molds. This first-of-a-kind demonstration led to a successful engine test performed at ORNL's National Transportation Research Center, which showed that an engine with these cylinder heads could handle exhaust temperatures of over 600°C.

"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."

This story is adapted from material from Oak Ridge National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.