On November 20, 2023, researchers at MIT developed an innovative heat treatment method that can significantly improve the strength of 3D printed metal products, making them more resistant to extreme high temperature environments. This technological breakthrough opens up new possibilities for manufacturing high-performance blades and vanes for use in power generation gas turbines and even jet engines. This is a major breakthrough for the metal products industry, as it is now possible to 3D print with stunning precision without sacrificing the quality and reliability of metal parts.
A thin rod of 3D printed superalloy is pulled from a water bath and passed through an induction coil, where it is heated to a temperature that changes its microstructure, making the material more elastic.
Gas turbine blades are usually manufactured using traditional casting processes. Manufacturers pour molten metal into complex molds, allowing it to directionally solidify, then use a variety of machining tools to finish the final metal parts. The blades must be able to spin at high speeds in extremely hot gases to generate electricity in power plants and provide thrust for jet engines.
However, there is growing interest in manufacturing turbine blades through 3D printing, a method that is environmentally and cost-effective and allows manufacturers to produce more complex and energy-efficient blade geometries. Unfortunately, there's a big hurdle to overcome: creep.
Creep is the tendency of metals to permanently deform under sustained mechanical stress and elevated temperatures. Previous research has found that the 3D printing process produces fine particles ranging from tens to hundreds of microns in size. Although barely visible to the naked eye, this microstructure is particularly susceptible to creep.
"In practical terms, this means the gas turbines will have a shorter service life or be less fuel efficient," explains Zachary Cordero, Boeing Career Development Professor of Aeronautics and Astronautics at MIT.
To solve this problem, Cordero and colleagues found a way to improve the structure of 3D printed alloys by adding a new heat treatment step. This method transforms the fine grains of the printed material into larger "columnar" grains, a stronger microstructure that minimizes creep in the material. The grain "pillars" are aligned with the axis of maximum stress.
The authors of the new study claim that a new heat treatment method could revolutionize industrial 3D printing of gas turbine blades.
Cordero said: "In the near future, we expect gas turbine manufacturers to print their blades and vanes in large additive manufacturing plants and then post-process them using our heat treatment. 3D printing will enable new cooling architectures that will improve Thermal efficiency of the turbine, thereby producing the same amount of electricity while burning less fuel and ultimately emitting less carbon dioxide."
Directional recrystallization setup. Remove the sample from the coolant through the hot zone. The steep thermal gradient in front of the hot zone maintains a high dislocation density leading to the recrystallization front.
Directional recrystallization of high temperature alloys
The MIT team's new method is a form of directional recrystallization, a heat treatment that moves a material through a hot zone at precisely controlled speeds, fusing the material's many microscopic grains into larger, stronger, more durable Uniform crystals.
The researchers used directional recrystallization of a 3D printed superalloy, which is commonly cast and used in gas turbines. They tested the method on rod-shaped 3D printed nickel-based superalloys, which were immersed in a room-temperature water bath directly beneath the induction coil. They slowly pulled each rod out of the water and controlled the coils at different speeds, sharply heating the rods to temperatures between 1,200 and 1,245 degrees Celsius.
They found that pulling the rod at a specific speed (2.5 mm/hour) and a specific temperature (1,235 degrees Celsius) created a steep thermal gradient that triggered a transformation in the fine-grained microstructure of the material printed.
"The material starts out as small particles with defects called dislocations, like broken spaghetti," Cordero explains. "When you heat the material, these defects disappear and reconfigure, and the grains grow." We continue to elongate the grains by consuming defective material and smaller grains, a process called recrystallization.
Finally, examination of the heat-treated rods using optical and electron microscopy confirmed that microscopic grains on the surface of the 3D printed metal parts were placed in "columnar" grains, resulting in significantly improved creep properties. By controlling the drawing speed and temperature of the rod sample, the printed particles can achieve specific sizes and orientations. This level of control may be welcomed by turbine manufacturers. This advancement not only marks an important milestone in materials science but also opens up new avenues for innovation across industries.