Researchers at the Massachusetts Institute of Technology (MIT) have developed a heat treatment that can ‘transform’ the microscopic structure of 3D printed metals, making the materials stronger and more resilient in extreme thermal environments.
The technique has the potential to make it possible to 3D print high-performance blades and vanes for power-generating gas turbines and jet engines, which would enable new designs with improved fuel consumption and energy efficiency.
Most gas turbine blades today are manufactured through conventional casting processes in which molten metal is poured into complex moulds and directionally solidified. The materials used for the components are some of the most heat-resistant metal alloys on Earth, as they are designed to rotate at high speeds in hot gas, extracting work to generate electricity in power plants and thrust in jet engines.
MIT say that there is a growing interest in using additive manufacturing to develop turbine blades, which could allow manufacturers to quickly produce more intricate, energy-efficient blade geometries. The university says that efforts to 3D print turbine blades have yet to clear a big hurdle: creep.
In metallurgy, creep refers to a metal’s tendency to permanently deform in the face of persistent mechanical stress and high temperatures. MIT says that during researcher’s exploration of 3D printing turbine blades, they have found that the process produces fine grains on the order of tens to hundreds of microns in size, a microstructure that is especially vulnerable to creep.
“In practice, this would mean a gas turbine would have a shorter life or less fuel efficiency,” said Zachary Cordero, Boeing Career Development Professor in Aeronautics and Astronautics at MIT. “These are costly, undesirable outcomes.”
Through their research, Cordero and his colleagues discovered a method of improving the structure of 3D printed alloys by adding an additional heat-treating step during post-processing, which changes the as-printed material’s creep potential, since the ‘columns’ are aligned with the axis of greatest stress. The researchers say that the method ‘clears the way’ for industrial 3D printing of gas turbine blades.
“In the near future, we envision gas turbine manufacturers will print their blades and vanes at large-scale additive manufacturing plants, then post-process them using our heat treatment,” said Cordero. “3D printing will enable new cooling architectures that can improve the thermal efficiency of a turbine, so that it produces the same amount of power while burning less fuel and ultimately less carbon dioxide.”
Cordero’s co-authors on the study are lead author Dominic Peachey, Christopher Carter, and Andres Garcia-Jimenez at MIT, Anugrahaprada Mukundan and Marie-Agathe Charpagne of the University of Illinois at Urbana-Champaign, and Donovan Leonard of Oak Ridge National Laboratory.
The method developed by the team is a form of directional recrystallisation, a heat treatment that passes a material through a hot zone at a precisely controlled speed to meld a material’s many microscopic grains into larger, sturdier, and more uniform crystals.
Invented over 80 years ago, recrystallisation has traditionally been applied to wrought materials. In the new study, the MIT team adapted the method for 3D printed superalloys.
The team initially tested the method on 3D printed superalloys that were nickel-based, metals that are typically cast and used in gas turbines. During a series of experiments, the researchers placed 3D printed samples of rod-shaped superalloys in a room temperature water bath placed just below an induction coil. Each rod was slowly drawn out of the water and through the coil at various speeds, dramatically heating the rods to temperatures varying between 1,200 and 1,245 degrees Celsius.
“The material starts as small grains with defects called dislocations, that are like a mangled spaghetti,” Cordero said. “When you heat this material up, those defects can annihilate and reconfigure, and the grains are able to grow. We’re continuously elongating the grains by consuming the defective material and smaller grains, a process termed recrystallisation.”
After the heat-treated rods had been cooled, the microstructure was examined using optical and electron microscopy. The researchers found that the material’s printed microscopic grains had been replaced with ‘columnar’ grains, or long crystal-like regions that the team said were significantly larger than the original grains.
“We’ve transformed the structure,” said the study’s lead author Dominic Peachey. “We show we can increase the grain size by orders of magnitude, to massive columnar grains, which theoretically should lead to dramatic improvements in creep properties.”
Cordero said that this level of control over microstructures can enable manufacturers to 3D print turbine blades with site-specific operating conditions.
Future plans for the research involve testing the heat treatment on 3D printed geometries that more closely resemble turbine blades. The team is also exploring ways to speed up the draw rate, as well as test a heat-treated structures resistance to creep.
“New blade and van geometries will enable more energy-efficient land-based gas turbines, as well as eventually aeroengines,” added Cordero. “This could from a baseline perspective lead to lower carbon dioxide emissions, just through improved efficiency of these devices.”
The research was in part supported by the U.S. Office of Naval Research. The full study can be found here.