The team is working as part 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 there is no production occurring in North America at this time.

One barrier 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 contains three times more cerium than neodymium and 500 times more cerium than dysprosium.

Aluminum–cerium alloys could boost domestic rare earth mining by increasing the demand and, eventually, the value of cerium.

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

—Orlando Rios

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.

In their paper, the researchers observed that two million metric tons of aluminum are consumed annually in transportation. For every 1% of penetration into the aluminum transportation market—assuming a 12 wt.% Ce alloy—2400 t of cerium would be required, with global production of Ce of about 24,000 t annually.

The commercialization path in part will depend on the cost of cerium. At an alloy cost of less than $10.00/kg, there could be significant interest and adoption by the automotive industry. The high strength could result in lighter weight components and significant adoption in applications requiring good high-temperature strength. At alloy costs in the $10 to $20 range, commercialization activities would concentrate on military application, where the alloy strength could be used advantageously for lighter weight structures. At alloy costs greater than $20/kg, commercialization activity would concentrate on the commercial, general aviation, and space markets where strength and high-temperature performance would be justified by the high penalty of weight.

Where high-temperature performance would normally be addressed by the conversion of aluminum to titanium, much higher prices may be justified. The evidence suggests that as much as 25% to 30% of the existing titanium market (4000 t annually) is driven by temperature performance in the 150 °C to 315 °C range, which is within the expected operating range of this alloy system.

—Sims et al.

Air-cooled cylinder head cast from Al-12Ce-0.4Mg. Image credit: Carlos Jones, ORNL. Click to enlarge.

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, but the aluminum-cerium system has equivalent casting characteristics to the aluminum-silicon alloys.

—David Weiss, vice president for engineering and research and development at Eck Industries

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 1093 ˚C (2,000 ˚F).

Al-Ce alloys have the possibility of replacing heavier steel and cast-irons for use in high-temperature applications. Al-Ce alloys castable along a broad range of cerium content, which are compatible with modern casting practices, require no changes to the present foundry infrastructure. Mechanical properties are as high as 252 MPa for tensile and 128 MPa for yield strength. Although high-temperature mechanical properties are not represented here, the combination of thermodynamic properties and stability after heat-treatment suggest that Al-Ce-X alloys hold great promise for high-temperature mechanical properties.

—Sims et al.

That heat tolerance makes aluminum–cerium alloys very attractive for use in internal combustion engines, Rios noted. Tests have shown the new alloys to be 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 heat treatments typically needed 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, by paving 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. The engine was shown to handle exhaust temperatures of more than 600 ˚C.

Three-dimensional printed molds are typically very hard to fill, but aluminum–cerium alloys can completely fill the mold thanks to their exceptional castability.

—Zachary Sims

The alloys were jointly invented by researchers at ORNL and Eck Industries. Colleagues at Eck Industries contributed expertise in aluminum casting, and LLNL researchers analyzed the aluminum-cerium castings using synchrotron source X-ray computed tomography.

Resources

• Zachary C. Sims, D. Weiss, S. K. McCall, M. A. McGuire, R. T. Ott, Tom Geer, Orlando Rios , P. A. E. Turchi (2016) “Cerium-Based, Intermetallic-Strengthened Aluminum Casting Alloy: High-Volume Co-product Development” JOM doi: 10.1007/s11837-016-1943-9