Researchers at the Department of Energy’s Oak Ridge National Laboratory and partners Lawrence Livermore National Laboratory and Wisconsin-based Eck Industries have developed aluminium 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 that contain cerium, have the potential to jump-start the United States’ production of rare earth elements.
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 there is no production occurring in North America at this time.
Aluminium cerium magnesium engine head. Image credit: Carlos Jones, ORNL
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 for rare earth producers 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.
Aluminium 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, they could quickly transform cerium from an inconvenient byproduct of rare earth mining to a valuable product in itself.
Components made with aluminium-cerium alloys offer several advantages over those made from existing aluminium alloys including low cost, high castability, reduced heat-treatment requirements and exceptional high-temperature stability.
Alloyed metals being poured from a furnace into a ladle to fill moulds
Most alloys with exceptional properties are more difficult to cast, but the aluminium-cerium system has equivalent casting characteristics to the aluminium-silicon alloys.
The key to the alloys’ high-temperature performance is a specific aluminium-cerium compound, or intermetallic, which forms inside the alloys as they are melted and cast. This intermetallic melts only at temperatures above 1 093 degrees Celsius.
That heat tolerance makes aluminium cerium alloys very attractive for use in internal combustion engines. Tests have shown the new alloys to be stable at 300 degrees Celsius, a temperature that would cause traditional alloys to begin disintegrating. In addition, the stability of this intermetallic sometimes eliminates the need for heat treatments typically needed for aluminium alloys.
Not only would aluminium-cerium alloys allow engines to increase fuel efficiency directly by running hotter, they may also increase fuel efficiency indirectly, by paving the way for lighter engines that use small aluminium-based components or use aluminium alloys to replace cast iron components such as cylinder blocks, transmission cases and cylinder heads.
Green sand mould of an aerospace engine head
The team has already cast prototype aircraft cylinder heads in conventional sand moulds. The team also cast a fully functional cylinder head for a fossil fuel-powered electric generator in 3D-printed sand moulds. This first-of-a-kind demonstration led to a successful engine test performed at ORNL’s National Transportation Research Center. The engine was shown to handle exhaust temperatures of over 600 degrees Celsius.
Three-dimensional printed moulds are typically very hard to fill, but aluminium–cerium alloys can completely fill the mould thanks to their exceptional castability.
The alloys were jointly invented by researchers at ORNL and Eck Industries. Colleagues at Eck Industries contributed expertise in aluminium casting, and LLNL researchers analysed the aluminium-cerium castings using synchrotron source X-ray computed tomography.
For more information visit www.ornl.gov