Super-Hard Materials for Friction Stir Welding

Three researchers from Missouri University of Science and Technology are part of a nationwide collaboration to synthesize inexpensive materials hard enough to literally stir two pieces of steel together and create some of the hardest materials ever produced.

With funding from a five-year, $7.5 million grant through the U.S. Department of Defense’s Multidisciplinary University Research Initiative (MURI) competition, the researchers will also develop a suite of artificial intelligence-materials tools capable of designing similar materials on demand with properties tailored to a wide range of applications.

Researchers at Duke University’s Center for Autonomous Materials Design are leading the project. In addition to the Missouri S&T team, the project includes researchers from Pennsylvania State University, North Carolina State University and State University of New York at Buffalo.

The class of so-called “high-entropy” materials derives enhanced stability from a chaotic mixture of atoms rather than relying solely on the orderly atomic structure of conventional materials. After first demonstrating this approach with carbides in 2018, the researchers plan to add borides into the irregular self-organized structures to produce extremely hard materials.

That’s where Missouri S&T comes in.

“We will be synthesizing and characterizing bulk ceramic materials that are designed in the computational studies,” says Dr. William Fahrenholtz, Curators’ Distinguished Professor of ceramic engineering and one of three S&T ceramic engineering professors on the project. S&T’s expertise in developing materials that can withstand extreme environments – such as the ultra-high temperatures experienced by hypersonic vehicles during high-speed flight – will be applied to developing materials for this project.

“We’ve already developed the computational machinery needed to tell us when this phenomenon will produce these stable, super-hard materials,” says Dr. Stefano Curtarolo, professor of mechanical engineering and materials science at Duke and leader of the new MURI award. “Our goal now is to develop the necessary ‘cooking’ procedures as well as AI-materials tools that can automate the discovery of new recipes to fit different needs.”

While high-entropy materials could be useful for many applications, one of the priorities at the top of the Department of Defense’s list is called friction stir welding. Invented in the early 1990s, the technique uses a rotating drill-like tool to join two pieces of metal without melting them.

As the tool rotates, it heats and softens the metal, allowing the surrounding material to swirl and mix as the machine moves along a line. Because neither metal is heated to its melting point, friction stir welding produces an extremely strong and durable joint with few defects. The technique can be used on a wide range of materials and even multiple types of metals at once.

But it struggles with steel.

For a friction stir welding tool to make a successful joint between two pieces of steel, it must be incredibly hard to avoid wearing too fast, thermally stable to withstand the high temperatures, chemically inert to avoid polluting the weld and inexpensive enough to mass produce. Diamond is hard enough for the job, but it sheds carbon atoms during the process, which makes the weld brittle. Polycrystalline cubic boron nitride – the current material of choice – wears down too quickly to justify the high cost of manufacturing it.

“If the right material could make friction stir welding a viable choice for large projects involving steel, it could revolutionize the construction of ships and other defense equipment,” says Curtarolo. “And we’ve got the idea for the perfect material for the job.”

The material that Curtarolo and his colleagues have in mind is a combination of carbon, boron, nitrogen and five other inexpensive metallic elements, all stabilized by chaotic entropy.

Curtarolo’s team already knows how to model and predict the formation of high-entropy carbides, and they are getting close to figuring out high-entropy borides. For help on that front, Curtarolo is turning to the Missouri S&T team members for their expertise on borides and other high-temperature materials. Joining Fahrenholtz on the project is Dr. Greg Hilmas, Curators’ Distinguished Professor of ceramic engineering and chair of materials science and engineering, and Dr. David Lipke, assistant professor of ceramic engineering.

“Once the computational experts predict a composition that they want to test, we will be tasked with making the materials,” says Fahrenholtz, who is also director of Missouri S&T’s Materials Research Center. “In addition, we will be measuring the properties of the materials that we make in our lab. We will be testing the hardness as well as other properties such as strength, fracture toughness, thermal conductivity and elastic constants for comparison to existing materials.”

Testing these materials will require specialized equipment that can take elemental powders, heat them at several thousand degrees under high pressures (without melting them) for days, and then slowly let them cool. Once the project is complete, the researchers hope to not only have produced a super-hard high-entropy carbide-boride capable of friction stir welding steel, but to have devised a system for designing similar materials to fit other needs as well.