3D-printed materials are now more durable thanks to MIT researchers' new heat treatment
Created by the Massachusetts Institute of Technology (MIT), a new heat treatment alters the microscopic structure of 3D-printed metals, making the materials more durable and resistant to thermal shock.
As MIT suggests, high-performance blades and vanes for jet engines and gas turbines might be 3D printed using the process, opening the door to novel designs with reduced fuel consumption and increased energy efficiency.
The research was published in Additive Manufacturing on November 11.
Modern gas turbine blades are produced using traditional casting techniques, including pouring molten metal into intricate molds and solidifying in a particular direction. As they are made to revolve at high speeds in extremely hot gas, extracting work to generate electricity in power plants and thrust in jet engines, these components comprise some of the most heat-resistant metal alloys on Earth.
Produced to block creeping
In metallurgy, the term "creep" describes a metal's propensity to irreversibly change shape when subjected to high temperatures and ongoing mechanical stress, says MIT.
While investigating the printing of turbine blades, researchers discovered that the printing method results in refined grains ranging in size from tens to hundreds of microns, a microstructure particularly prone to creep.
“In practice, this would mean a gas turbine would have a shorter life or less fuel efficiency. These are costly, undesirable outcomes,” says Zachary Cordero, the Boeing Career Development Professor in Aeronautics and Astronautics at MIT.
The fine grains of the as-printed material is changed into much larger "columnar" grains, creating a more durable microstructure that should reduce the material's potential for creep since the "columns" are aligned with the axis of greatest stress. Cordero and his colleagues discovered a way to improve the structure of 3D-printed alloys by adding an additional heat-treating step.
“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 novel technique developed by the researchers is a type of directed recrystallization. This heat treatment moves a substance through a heated zone at a precisely regulated pace to combine the substance's numerous small grains into larger, more stable crystals.
The technology was tested on metals that are generally cast and utilized in gas turbines—nickel-based superalloys that were 3D printed. In a series of tests, the scientists positioned 3D-printed samples of rod-shaped superalloys beneath an induction coil in a room-temperature water bath. They drastically heated the rods to temperatures ranging from 1,200 to 1,245 degrees Celsius by carefully drawing each rod out of the water and through the coil at various rates.
“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.”
Metal additive manufacturing processes can create intricate components that are difficult to form with conventional processing methods; however, the as-printed materials often have fine grain structures that result in poor high-temperature creep properties, especially compared to directionally solidified materials. Here, we address this limitation in an exemplary additively manufactured Ni-base superalloy, AM IN738LC, by converting the fine as-printed grain structure to a coarse columnar one via directional recrystallization. The directional recrystallization behaviors of AM IN738LC were characterized through a parameter study in which the peak temperature and draw rate were each independently varied. Recrystallization began when the peak temperature was higher than the γ′ solvus of 1183 °C. Varying the draw rate from 1 to 100 mm/hr while maintaining a fixed peak temperature of 1235 °C and a thermal gradient of order 105 °C/m ahead of the hot zone showed that a draw rate of 2.5 mm/hr maximized the grain size, giving a mean longitudinal grain size of 650 µm. Specimens processed under these optimal conditions also inherited the 〈100〉 fiber texture of the as-printed material. Close inspection of a quenched specimen revealed Zener pinning of the longitudinal grain boundaries by MC carbides and a discrete primary recrystallization front whose position followed the γ′ solvus isotherm. The present results demonstrate for the first time how directional recrystallization of additively manufactured Ni-base superalloys can achieve large columnar grains, manipulate crystallographic texture to minimize thermal stresses expected in service, and functionally grade the grain structure to selectively enhance fatigue or creep performance.
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