Thanks to a new heat treatment, 3D printed metals can withstand extreme conditions

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November 14, 2022

(News from Nanowerk) A new heat treatment developed by MIT transforms the microscopic structure of 3D printed metals, making materials stronger and more resilient in extreme thermal environments. The technique could allow high-performance blades and vanes to be 3D printed for gas turbines and power generation jet engines, enabling new designs with improved fuel consumption and energy efficiency.

Gas turbine blades today are made by conventional casting processes in which molten metal is poured into complex molds and directionally solidified. These components are made from some of the most heat-resistant metal alloys on Earth, as they are designed to spin at high speed in extremely hot gases, extracting work to generate electricity in power plants and propelling jet engines.

There is growing interest in manufacturing turbine blades by 3D printing, which, in addition to its environmental and economic benefits, could allow manufacturers to rapidly produce more complex and energy-efficient blade geometries. But efforts to 3D print turbine blades have yet to overcome a major hurdle: creep.

In metallurgy, creep refers to the tendency of a metal to deform permanently in the face of persistent mechanical stresses and high temperatures. As the researchers explored printing turbine blades, they discovered that the printing process produces fine grains on the order of tens to hundreds of microns – a microstructure that is particularly vulnerable to creep.

“In practice, this would mean that a gas turbine would have a shorter lifespan or less fuel efficiency,” says Zachary Cordero, Boeing Career Development Professor of Aeronautics and Astronautics at MIT. “These are costly and undesirable outcomes.”

Cordero and his colleagues have found a way to improve the structure of 3D-printed alloys by adding an additional heat treatment step, which transforms the fine grains of the as-printed material into much larger “columnar” grains – a more microstructure. robust which should minimize the potential for creep, since the “columns” are aligned with the axis of greatest stress. The researchers say the method, described in Additive manufacturing (“Directional recrystallization of an additively manufactured nickel-based superalloy”), paves the way for industrial 3D printing of gas turbine blades.

A thin rod of 3D-printed superalloy is drawn from a water bath and through an induction coil, where it is heated to temperatures that transform its microstructure, making the material stronger. MIT’s new heat treatment could be used to strengthen 3D-printed gas turbine blades. (Photo: Dominic David Peachey)

“In the near future, we envision gas turbine manufacturers printing their blades and blades in large-scale additive manufacturing plants and then post-processing 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 emitting less carbon dioxide. .”

Cordero’s co-authors on the study are lead author Dominic Peachey, Christopher Carter, and Andres Garcia-Jimenez of MIT, Anugrahaprada Mukundan and Marie-Agathe Charpagne of the University of Illinois at Urbana-Champaign, and Donovan Leonard from the Oak Ridge National Laboratory.

Trigger 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 rate to fuse the many microscopic grains of a material into larger, sturdier crystals. and more uniform.

Directional recrystallization was invented more than 80 years ago and has been applied to wrought materials. In their new study, the MIT team adapted directional recrystallization for 3D-printed superalloys.

The team tested the method on 3D-printed nickel-based superalloys, metals typically cast and used in gas turbines. In 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. They slowly pulled each rod out of the water and through the coil at varying speeds, dramatically heating the rods to temperatures ranging between 1,200 and 1,245 degrees Celsius.

They found that pulling the rods at a particular speed (2.5 millimeters per hour) and at a specific temperature (1,235 degrees Celsius) created a steep thermal gradient that triggered a transformation of the material’s fine-grained imprinted microstructure.

“The material starts out as small grains with flaws called dislocations, which look like mangled spaghetti,” Cordero explains. “When you heat this material, these defects can annihilate and reconfigure, and grains can develop. We continually lengthen the grains by consuming the defective material and the smaller grains – a process called recrystallization.

move away

After cooling the heat-treated rods, the researchers examined their microstructure using light and electron microscopy and found that the imprinted microscopic grains of the material were replaced by “columnar” grains, or long, distinctly larger crystalline regions. larger than the original grains.

“We completely transformed the structure,” says lead writer Dominic Peachey. “We show that we can increase the grain size by orders of magnitude, up to massive columnar grains, which should theoretically lead to dramatic improvements in creep properties.”

The team also showed that they could manipulate the draw rate and temperature of rod samples to tailor the growth grains of the material, creating regions of specific grain size and orientation. This level of control, Cordero says, can allow manufacturers to print turbine blades with site-specific microstructures that withstand specific operating conditions.

Cordero plans to test the heat treatment on 3D printed geometries that look more like turbine blades. The team is also exploring ways to speed up the draw rate, as well as testing the creep resistance of a heat-treated structure. Next, they envision that heat treatment could enable the practical application of 3D printing to produce industrial-grade turbine blades, with more complex shapes and patterns.

“New blade and vane geometries will enable more energy-efficient land-based gas turbines, as well as, eventually, aircraft engines,” Cordero notes. “That could, from a basic perspective, lead to a reduction in carbon dioxide emissions, just from improving the efficiency of these devices.”

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