New heat treatment makes printed alloys less creepy

A new heat treatment developed by researchers at Massachusetts Institute of Technology (MIT) and elsewhere can transform the microscopic structure of 3D-printed metals, making them stronger and more resilient in extreme thermal environments. The technique could make it possible to 3D print high-performance blades and vanes for power-generating gas turbines and jet engines, allowing new designs with improved fuel consumption and energy efficiency.

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 from some of the most heat-resistant metal alloys on Earth, as they are designed to rotate at high speeds in extremely hot gas, extracting work to generate electricity in power plants and thrust in jet engines.

There is growing interest in manufacturing turbine blades through 3D-printing, which, in addition to its environmental and cost benefits, could allow manufacturers to quickly produce more intricate, energy-efficient blade geometries. But 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. 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, a professor in aeronautics and astronautics at MIT. “These are costly, undesirable outcomes.”

Cordero and his colleagues have now found a way to improve the structure of 3D-printed alloys by adding an additional heat-treating step. This step 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. The researchers say the method, reported in a paper in Additive Manufacturing, 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,” 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.”

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

They tested the method on 3D-printed nickel-based superalloys — metals that are typically cast and used in gas turbines. In a series of experiments, they placed 3D-printed samples of rod-shaped superalloys in a room-temperature water bath located just below an induction coil. They slowly drew each rod out of the water and through the coil at various speeds, dramatically heating the rods to temperatures varying between 1200°C and 1245°C.

They found that drawing the rods at a particular speed (2.5mm per hour) and through a specific temperature (1235°C) 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.”

After cooling the heat-treated rods, the researchers examined their microstructure with 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 at MIT. “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 researchers also showed that, by manipulating the draw speed and temperature of the rod samples, they could tailor the material’s growing grains, creating regions with specific grain size and orientation. This level of control, Cordero says, could allow manufacturers to print turbine blades with site-specific microstructures that are resilient to specific operating conditions.

Cordero now plans to test this heat treatment on 3D-printed geometries that more closely resemble turbine blades. He and his colleagues are also exploring ways to speed up the draw rate, as well as test a heat-treated structure’s resistance to creep. They envision that this heat treatment could allow 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, aeroengines,” Cordero notes. “This could from a baseline perspective lead to lower carbon dioxide emissions, just through improved efficiency of these devices.”

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.