The MIT heat treatment transforms 3D-printed metals’ microscopic structures, making them stronger and more resistant to extreme temperatures. This technique could be used to 3D-print high-performance blades, vanes and turbines for power-generating jet engines and gas turbines. It would also allow for new designs that are more fuel efficient.
Today’s gas turbine blades are manufactured through conventional casting processes in which molten metal is poured into complex molds and directionally solidified. These components are made out of some the most heat-resistant metal alloys on Earth. Because they can rotate at high speeds with extremely hot gases, extracting work to create electricity in power plants, and thrust in jet engine engines, they are very durable.
3D-printing is becoming more popular for the manufacturing of turbine blades. In addition to its environmental and economic benefits, it could also allow manufacturers to rapidly produce intricate, efficient blade geometries. However, 3D printing turbine blades has yet to overcome a major hurdle: creep.
In metallurgy, creep refers to a metal’s tendency to permanently deform in the face of persistent mechanical stress and high temperatures. While researchers have explored printing turbine blades, they have found that the printing 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,” says Zachary Cordero, the Boeing Career Development Professor in Aeronautics and Astronautics at MIT. “These are costly, undesirable outcomes.”
Cordero and his colleagues found a way to improve the structure of 3D-printed alloys by adding an additional heat-treating step, which transforms the as-printed material’s fine grains into much larger “columnar” grains — a sturdier microstructure that should minimize the material’s creep potential, since the “columns” are aligned with the axis of greatest stress. According to the researchers, today’s method was described in Additive Manufacturing This opens the door to 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,” Cordero says. “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 emits 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 trigger of a transformation
The team’s new method is a form of directional recrystallization — 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.
Directional recrystallization, which was developed more than 80-years ago, has been used to make wrought materials. The MIT team has adapted directional recrystallization to 3D-printed superalloys in their latest study.
The team tested the method on 3D-printed nickel-based superalloys — metals that are typically cast and used in gas turbines. Researchers placed 3D-printed rod-shaped samples of superalloys in an induction coil-less water bath in a series of experiments. Each rod was slowly pulled out of the water and pushed through the coil at different speeds. This heated the rods to temperatures between 1,200 and 1,245 Celsius.
They found that drawing the rods at a particular speed (2.5 millimeters per hour) and through a specific temperature (1,235 degrees Celsius) created a steep thermal gradient that triggered a transformation in the material’s printed, fine-grained microstructure.
“The material starts as small grains with defects called dislocations, that are like a mangled spaghetti,” Cordero explains. “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 recrystallization.”
You can always creep away
After cooling the heat-treated rods, the researchers examined their microstructure using optical and electron microscopy, and found that the material’s printed microscopic grains were replaced with “columnar” grains, or long crystal-like regions that were significantly larger than the original grains.
“We’ve completely transformed the structure,” says 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.”
The team also showed they could manipulate the draw speed and temperature of the rod samples to tailor the material’s growing grains, creating regions of specific grain size and orientation. Cordero explains that this level of control can allow manufacturers to print turbine blades with microstructures specific to each site, which are more resilient to certain operating conditions.
Cordero intends to test heat treatment of 3D-printed geometries with a more similar appearance to turbine blades. The team is also exploring ways to speed up the draw rate, as well as test a heat-treated structure’s resistance to creep. They also envision 3D printing to create industrial-grade turbine blades with more complicated shapes and patterns using heat treatment.
“New blade and vane geometries will enable more energy-efficient land-based gas turbines, as well as, eventually, aeroengines,” Cordero notes. “This could from a baseline perspective lead to lower carbon dioxide emissions, just through improved efficiency of these devices.”
This research was partly supported by the U.S. Office of Naval Research.