An MIT-developed heat treatment aims to transform the microscopic structure of 3D-printed metals, making the materials stronger and more resilient in extreme thermal environments. The technique also aims to make it possible to 3D print high-performance blades and vanes for power-generating gas turbines and jet engines, which would enable improved fuel consumption and energy efficiency.
There is growing interest in manufacturing turbine blades through 3D printing, but such efforts have a big hurdle: creep — a metal’s tendency to permanently deform in the face of persistent mechanical stress and high temperatures. In addition, researchers 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,” said Zachary Cordero, the Boeing Career Development Professor in Aeronautics and Astronautics at MIT. “These are costly, undesirable outcomes.”
Cordero and his team improved the structure of 3D-printed alloys by adding an additional heat-treating step, which transforms the 3D-printed material’s fine grains into much larger “columnar” grains — a sturdier microstructure capable of minimizing the material’s creep potential. The researchers noted that the method clears the way for the 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 emits less carbon dioxide.”
The new method is a form of directional recrystallization — a heat treatment that passes a material through a hot zone at a controlled speed to meld a material’s microscopic grains into larger, sturdier, and more uniform crystals.
The team tested the method on 3D-printed nickel-based superalloys, metals typically cast and used in gas turbines. The engineers placed 3D-printed samples of rod-shaped superalloys in room-temperature water placed just below an induction coil. They then slowly drew each rod out of the water and through the coil at various speeds, dramatically heating the rods to temperatures varying between 1,200-1,245 °C.
The results showed that drawing the rods at a particular speed (2.5 millimeters per hour) and through a specific temperature (1,235 °C) created a steep thermal gradient that triggered a transformation in the material’s printed, finegrained microstructure.
After cooling the rods, the researchers examined the microstructure and found that the material’s printed microscopic grains were replaced with “columnar” grains.
The team also exhibited an ability to 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. This level of control can enable manufacturers to print turbine blades with site-specific microstructures that are resilient to specific operating conditions.
Cordero plans to test the heat treatment on 3D-printed geometries that more closely resemble turbine blades, while the team is exploring ways to speed up the draw rate as well as test a heat-treated structure’s resistance to creep.
“New blade and vane geometries will enable more energy-efficient land-based gas turbines, as well as, eventually, aeroengines,” said Cordero. “This could from a baseline perspective lead to lower carbon dioxide emissions, just through improved efficiency of these devices.”