Recrystallization Heat Treatment Technology

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.

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