Researchers have found a way to make 3D-printed metal parts more heat-resistant, a technique that could pave the way for using additive manufacturing for high-quality parts for gas turbines and jet engines that facilitate more efficient designs, they said. A team of scientists at MIT, the University of Illinois at Urbana-Champaign, and the Oak Ridge National Laboratory developed a heat treatment that transforms the microscopic structure of 3D-printed metals in such a way that the materials become stronger and more resilient even in extreme, heat-intensive environments, they said.
While there has been a lot of interest in manufacturing turbine blades through 3D printing given the numerous benefits it provides—including lower costs and less waste and other environmental strains—it is not yet a possibility for production parts because of some challenges associated with it. 3D-printing these blades also could allow manufacturers to quickly produce more intricate, energy-efficient blade geometries.
The technique that the MIT team developed overcomes one of the biggest issues with 3D-printing these parts—a phenomenon called “creep”—that engineers have faced when trying to 3D-print large blades for gas turbines and other parts that must withstand high temperatures, researchers said.
“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 Zachary Cordero, a professor in Aeronautics and Astronautics at MIT.
Overcoming 3D Printing ‘Creep’
Today’s gas turbine blades—fabricated from some of the most heat-resistant metal alloys that are currently available—are manufactured through conventional casting processes that pour molten metal into complex molds where they solidify. By design, these turbines must be able to rotate at high speeds in extremely hot gas so they can serve to generate electricity in power plants and thrust in jet engines.
In metallurgy, the term “creep” refers to a metal’s tendency to deform permanently in the face of persistent mechanical stress and high temperatures. Unfortunately, the 3D printing process for turbine blades produces a finely grained microstructure about tens to hundreds of microns in size that is especially susceptible to creep.
Thus, if 3D-printed turbine blades fabricated via current technology were put into production systems, they would represent shorter lifespans or less fuel efficiency for the gas turbine—which are “costly, undesirable outcomes,” Cordero said.
Rather than create an entirely new process, researchers focused on making the fine grains of the metal more resistant to creep, they said. They found a way to improve the structure of 3D-printed alloys by transforming the grains into much larger “columnar” grains, which is a sturdier microstructure that minimizes the potential for creep because the columns are aligned with the axis of greatest stress, researchers said.
How It Works
The method itself is a form of what’s called directional recrystallization, a heat treatment invented more than 80 years ago and applied to “wrought” materials, such as wrought iron. It passes a material through a hot zone at a precisely controlled speed, melding a material’s many microscopic grains into larger, sturdier, and more uniform crystals.
In their research, the team tested the method on 3D-printed nickel-based superalloys, which are typically used to manufacture gas turbines. They placed 3D-printed samples of rod-shaped superalloys in a room-temperature water bath placed just below an induction coil, then slowly drew each rod out of the water and through the coil at various speeds. This dramatically heated the rods to temperatures varying between 1200 and 1245 degrees Celsius.
What they found through these experiments was that by drawing the rods at a particular speed (2.5 millimeters per hour) and through a specific temperature (1235 degrees Celsius), they could create a steep thermal gradient that triggered a transformation in the material’s printed, fine-grained microstructure, Cordero said.
“The material starts as small grains with defects called dislocations, that are like a mangled spaghetti,” he explained in a press statement. “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.”
Finding the Solution
Researchers peered under the hood of the heat-treated rods after they cooled 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. This should lead to significant improvements in making the material resistant to creep, they said.
Moreover, researchers also realized that they could use the process 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 over the metal can enable manufacturers to print turbine blades with site-specific microstructures that are resilient to specific operating conditions, Cordero said.
“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,” he said in a press statement.
Researchers published a paper on their work in the journal Additive Manufacturing.
The team plans to test the heat treatment further on 3D-printed geometries that are closer in structure and size to turbine blades. They also are exploring ways to speed up the draw rate as well as test how resistant a structure that’s already been treated is to creep, they said.
After further study and optimization, the heat treatment could be applied to enable the production of industrial-grade turbine blades with more complex shapes and patterns using additive manufacturing, Cordero said.