Gas turbine or jet engine blades can be 3D printed using an energy-efficient method that changes the microscopic structure of metals.
A new heat treatment developed by MIT changes the microscopic structure of 3D-printed metals, making the materials more durable and resistant to thermal shock. High-performance blades and blades for jet engines and gas turbines can be 3D printed using this technique, opening the door to new designs that reduce fuel consumption and increase energy efficiency.
Modern gas turbine blades are produced using traditional casting techniques that involve pouring molten metal into complex molds and allowing it to solidify in a particular direction. These components are made up of some of the most heat-resistant metal alloys in the world, as they do the work by rotating at high speeds in the extremely hot gas to generate electricity in power plants and to provide thrust in jet engines.
Besides the financial and environmental benefits, there is a growing interest in manufacturing turbine blades using 3D printing that could allow manufacturers to create more complex, energy-efficient blade shapes faster. But a major hurdle called creep still stands in the way of 3D printing turbine blades.
In metallurgy, the term "creep" describes the tendency of a metal to deform irreversibly when exposed to high temperatures and sustained mechanical stress. In examining the thrust of turbine blades, the researchers discovered that the printing method results in fine particles ranging in size from tens to hundreds of microns, making it a particularly creep-prone microstructure.
According to Zachary Cordero, Boeing Career Development Professor of Aeronautics and Astronautics at MIT, “in practice, this means that a gas turbine will have a shorter lifespan or less fuel efficiency.” These are expensive and undesirable consequences.
The fines of the printed material transform into much larger “columnar” grains, creating a more durable microstructure and reducing the creep potential of the material as the “columns” align with the greatest stress axis. Cordero and colleagues discovered a way to improve the structure of 3D-printed alloys by adding an additional heat treatment step. The approach, which the researchers describe in today's issue of Additive Manufacturing, they claim paves the way for commercial 3D printing of gas turbine blades.
“We anticipate that gas turbine manufacturers will soon print blades and blades in sizable additive manufacturing plants and then process them using our heat treatment,” Cordero says.
The development of new cooling architectures enabled by 3D printing will allow turbines to generate the same amount of power, use less fuel and ultimately emit less carbon dioxide.
Lead author Dominic Peachey, co-authors Christopher Carter and Andres Garcia-Jimenez from MIT, Anugrahaprada Mukundan and Marie-Agathe Charpagne of the University of Illinois at Urbana-Champaign and Donovan Leonard of Oak Ridge National Laboratory are co-authors of the study with Cordero. .
The new technique developed by the researchers is a type of directed recrystallization, a heat treatment that moves a substance through a heated region at a precisely regulated rate to combine large numbers of tiny grains of matter into larger, more stable crystals.
Directional recrystallization was developed more than 80 years ago and has been used on processed materials ever since. In their recent work, the MIT team has modified directional recrystallization for 3D-printed superalloys.
The technology has been tested on metals that are often cast and used in gas turbines – 3D-printed nickel-based superalloys. In a series of tests, scientists placed 3D-printed samples of rod-shaped superalloys under an induction coil in a room-temperature water bath. By slowly pulling each rod out of the water and through the coil at various speeds, they significantly heated the rods to temperatures ranging from 1.200 to 1.245 degrees Celsius.
They discovered that moving the rods at a precise speed of 1.235 millimeters per hour over a temperature range of 2,5 degrees Celsius causes a sharp thermal gradient that changes the printed, fine-grained microstructure of the material.
According to Cordero, the material initially consists of microscopic grains with dislocations reminiscent of shredded spaghetti. “When this material is heated, these defects can disappear and rearrange, allowing the grains to expand. Recrystallization is the process by which we continually elongate grains by ingesting faulty material and smaller grains.
After cooling the heat-treated rods, the researchers discovered that the printed microscopic grains of the material were replaced with "columnar" grains, or elongated crystal-like regions, that were significantly larger than the original grains.
According to main author Dominic Peachey, the building has undergone a complete transformation. We have shown that theoretically it is possible to greatly increase the grain size to form columnar grains, which should result in a significant improvement in creep properties.
The scientists also demonstrated how they could control the temperature and shrinkage rate of the bar samples to adjust the growing grains of the material and produce sections with specific grain size and orientation. According to Cordero, it is possible to print turbine blades with region-specific microstructures that are resistant to certain operating conditions, thanks to this level of control.
Cordero aims to test the heat treatment using 3D-printed shapes that more closely mimic turbine blades. The team is also testing the creep resistance of heat-treated structures and looking for ways to speed up the shrinkage rate. Next, they think heat treatment will make it possible to use 3D printing in a practical way to create industrial-grade turbine blades with more complex shapes and patterns.
New blade and blade shapes will enable the development of land-based gas turbines and ultimately more energy efficient aircraft engines. “Even by simply increasing the efficiency of these devices,” he writes, “this could lead, from a fundamental point of view, to reductions in carbon dioxide emissions.”
Source: news.mit.edu – Jennifer Chu | MIT News Office