Soaring Study Into Solidification of Lightweight TiAl Alloys

Researchers aiming to model influence of changing gravity levels

Research into production of lightweight titanium aluminide alloys continues at the European Space Agency’s Technical Center in Noordwijk, The Netherlands, where an oversized centrifugal machine has been used to study the effects of gravity (and hypergravity) on molten metal solidification. That research is about to get a boost.

ESA’s Large Diameter centrifuge allows research teams to establish a state of hypergravity on the Earth’s surface. It can spin at up to 67 revolutions per minute, producing gravity levels of up to 20 times “Earth normal” in the gondolas fixed to the ends of four arms.

In the solidification of molten metal, gravity drives convection flows that influence the process. By changing the level of gravity, the microscopic grain size of the alloy also should change.

In ESA;s research of titanium aluminides, one of the centrifuge’s gondolas was fitted with a special furnace filled with a molten combination of titanium and aluminum. After spinning for about an hour, the alloy was allowed to cool and solidify over 15 minutes.

Afterwards, the titanium aluminide material was removed for analysis into the newly formed metal’s microstructure, to learn how it is affected by the elevated gravity level — a rate that ESA described as “eight times stronger than Jupiter’s (gravity).”

“While lightweight, titanium aluminide is strong and corrosion-resistant,” according to Laszlo Sturz of Access, a research company launched by Germany’s Technical University of Aachen. “In particular, its strength increases with temperature, making it particularly promising for building aerospace and automotive engine elements as well as other moving parts.”

He continued: “Right now, titanium aluminide parts are cast in various ways, including centrifugal, where a ceramic mold is spun as the alloy cools. But, such manufacturing follows a trial-and-error approach. Our project aims at creating a detailed mathematical model of how solidification is influenced by changing gravity levels, to help in optimizing future casting technology.”

Gravity-driven convection in the molten metal influences solidification: any change the level of gravity and the microscopic alloy grains may be expected to change their size, too.

While differing levels of hypergravity can been achieved with ESA’s centrifuge, microgravity casting will be tested next year during the 10–15 minutes of weightlessness available on the flight of a suborbital rocket. Four casting furnaces will be flown on the Maxus rocket in that experiment.

“This centrifuge campaign is also serving to qualify them for flight,” according to ESA’s Antonio Verga.

In preparation for that part of the research, the challenge was to design a self-contained furnace that can heat up to the 1,700°C required on the inside, while its exterior surface temperature remains no higher than than 70°C.

The furnace chamber in which electrical heaters melt the alloy is surrounded by ceramic heat shields and buffeted by inert argon gas, with water-cooling pipes wrapping the cylinder’s exterior and  telemetry systems that relay real-time data to researchers throughout the process.

The GRADECET project (Gravity Dependence of Columnar to equiaxed transition in peritectic TiAl alloys) is led by the ESA, but also involves researchers from Germany, Ireland, Slovakia, France and Hungary. Its results — including  the detailed mathematical model of how TiAl solidification is influenced by changing gravity levels should help to optimize casting technology on a wider basis.