Researchers have demonstrated a new technique for manufacturing strong magnetic materials that improves the quality of the magnets, produces the magnets quickly, uses less energy and is less expensive.
Strong permanent magnets are used in a wide variety of applications, and demand for these materials is increasing due to their use in technologies such as electric vehicles, wind turbines and robotic devices.
However, manufacturing these magnets is challenging.
"Currently, to manufacture a magnet, industry relies on sintering metal alloy powders into a bulk solid at high temperature and under high pressure," says Bharat Gwalani, corresponding author of a paper on the work and an assistant professor of materials science and engineering at North Carolina State University. "This is a complex, time-consuming process that requires a lot of energy and often results in the creation of flawed magnets.
"For example, the conventional manufacturing process often results in uneven distribution of porosity throughout the magnetic material, with high porosity at the center," Gwalani says. "This means the magnetic properties of the material are also unevenly distributed.
"One last challenge we wanted to address is related to the fact that the rare earth metals used in producing magnetic materials are very reactive to oxygen," Gwalani says. "Oxidation adversely affects the magnetic properties of these materials, and high heat increases the rate of oxidation, so we wanted to see if a different process could improve magnet quality."
To that end, the researchers experimented with a new magnet manufacturing process that makes use of a technique called friction stir consolidation (FSC). Essentially, the alloy powder is placed in a chamber, placed under pressure and stirred with a rotating tool.
"The energy from the rotational motion and the forge force - the pressure - sinters the powder into a solid without ever melting the alloy," Gwalani says. "Because the metal is not exposed to high heat and doesn't actually melt, this process results in less oxidation and unwanted phase transitions in the material. Also, while we are applying pressure, it's less than one megapascal (MPa), whereas conventional magnet manufacturing techniques apply more than 100 MPa."
The researchers also found that the new approach eliminated porosity in the magnetic material.
"This is because conventional techniques apply pressure in only one direction, which means the pressure is applied largely to the top and bottom of the material, forcing porosity into the center," Gwalani says. "Because our technique involves rotating the material as pressure is applied, that pressure is distributed throughout the material.
"In addition, our approach relies on the frictional heating caused by the alloy powder particles rubbing together," says Gwalani. "This means the heat is being generated at the exact site where it is needed to fuse the powders into a bulk solid - there isn't an external source of heat being applied everywhere, as there is in conventional techniques."
Ultimately, the combination of pressure, rotation and frictional heating means there are no pockets or bubbles in the magnetic material.
"The result is a faster manufacturing method that reduces energy consumption and produces strong permanent magnets with less oxidation and uniform magnetic properties throughout the material," says Gwalani.
"Now that we have figured out a way to consolidate magnetic materials without porosity, we are experimenting with the development of next-generation magnets that incorporate non-magnetic binder agents. Our goal is to develop strong permanent magnets that have more desirable physical properties. We want to make low-density, tougher magnets that are less reliant on difficult-to-obtain rare-earth materials."
The paper, "In-Situ Thermo-Mechano-Chemical Transformation and Consolidation of Sm-Co Powders via a Single-Step Route for Bulk Magnet Fabrication" appears in the journal Nature Communications. The paper was co-authored by Aniruddha Malakar, a former postdoctoral researcher at NC State who is now at the Pacific Northwest National Laboratory; Andrew Martin, a postdoctoral researcher at NC State; Farhan Ishrak, Caleb Schenck and Michael Lastovich, Ph.D. students at NC State; Joseph Tracy and Martin Thuo, professors of materials science and engineering at NC State; Anqi Yu, Mayur Pole, Jens Darsell, Tianhao Wang, Libor Kovarik, Glenn Grant and Mert Efe of the Pacific Northwest National Laboratory; Joseph Helsing of the Stevens Institute of Technology; and John Thornton of Bruker Nano.
This research was done with support from the Office of Naval Research under grant N00014-23-1-2758, the National Science Foundation under grant 2243104, and the Pacific Northwest National Laboratory, which is part of the Department of Energy's Office of Science.
