‘Secret Sauce’ Enables New Way to Fabricate Compositionally Graded Alloys

Technology shows promise in additive manufacturing for extreme environments

Published: Tuesday, August 8, 2023 - 11:01

(Oak Ridge National Laboratory: Oak Ridge, TN) -- Research into a unique technology to fabricate composite metal parts for a wide range of applications operating in extreme environments across the aviation, space, and energy industries is showing promise for additive manufacturing.

Developed by scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory, this technique enables the design of compositionally graded composite parts. These components transition from high-strength superalloys to refractory alloys that can withstand extremely high temperatures—so no welding is needed. Though superalloys and refractory alloys typically can’t be welded or joined with each other, many applications require materials with site-specific, high-temperature, and high-strength properties.

“We can enable compositions that transition from one alloy to another seamlessly,” says Soumya Nag, an ORNL materials scientist who is leading the studies. “We can tune a composite part that we can grade from one end to another and have high-strength and high-temperature capability on each side.”

Shown are additively manufactured, thin-walled, functionally graded builds from IN718 to C103 alloys via a thick transition layer having high specific strength. Credit: Brian Jordan, Soumya Nag, ORNL/U.S. Department of Energy

The secret is in the “sauce.”

“It’s like cooking,” says Nag. “You basically have different ingredients. So, if you have more pasta on one side and more risotto on the other side, how do you continuously change from a pasta to a risotto? You change the ingredients as you move along from one end to another, and that’s exactly what we do.”

The “sauce” in this case is a powder composed of a third transition alloy that possesses lightweight or high-temperature characteristics. Nag and team members used an additive manufacturing modality called directed energy deposition to deposit different powder compositions in an inert argon environment, changing the deposition rate as they went along.

For most structural applications, Nag says, a single alloy composition is often used to fabricate components for corrosive, high-temperature, or radiative environments. But this process is expensive and compromises performance. For components requiring widely varied properties, welded parts made from dissimilar materials are often fabricated, leading to abrupt interfaces that can adversely affect performance.

In the latest studies, the scientists used powders of Inconel 718, a nickel-based alloy, and C103, a niobium-based alloy. These alloys—one high-strength and the other high-temperature resistant—do not want to join and tend to create cracks when they do. But by using a blown-powder, directed-energy deposition beam machine and changing the rate at which the powders flow, the scientists can change the composition of the joined metals so that the composition has the beneficial properties of both.

The scientists designed the actual nonlinear gradient pathway by coupling state-of-the-art computational thermodynamics with experimental data gathered via multiscale, high-throughput characterization tools. By doing so, they successfully circumvented welding issues and joined normally nonweldable superalloys with refractory alloys. The team analyzed the stress states of these integrated builds with neutron diffraction-based studies at ORNL, validating the computational alloy design. Currently, melt pool, thermal, and strain models are being generated based on the experimental data.

“Few additive manufacturing modalities have this capability of mixing powders on the go during the build,” Nag says. “This is a unique attribute that can blow powders at different rates. Now you can do temperature transition from a relatively low-temperature nickel alloy to an extremely high-temperature niobium alloy without any problem.”

An electron microscopy image shows the entire gradient transition from In718 to C103 and high magnification highlighting defect-free compositional transition. Credit: Tracie Lowe, Andres Marquez Rossy/ORNL, U.S. Department of Energy

First published July 21, 2023, in ORNL News.

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Oak Ridge National Laboratory

Oak Ridge National Laboratory is a multiprogram science and technology laboratory managed for the U.S. Department of Energy by the University of Tennessee-Battelle LLC. Scientists and engineers at ORNL conduct basic and applied research and development to create scientific knowledge and technological solutions that strengthen the nation's leadership in key areas of science; increase the availability of clean, abundant energy; restore and protect the environment; and contribute to national security.