Electron backscatter diffraction yields microstructure insights

November 14, 2017
Comparison of the influence of different cutting methods on the deformation of the edge of electrical steel sheets: a) rough cutting using a blunt object, b) industrial punching tool and c) laser cutting. Credit: the authors

High-efficiency electric motors with tailored speed-torques, determined by their magnetic components, are essential for sustainable, successful electric automobile drive concepts. Soft magnetic core engineering plays a key role in these motors, where the main soft magnetic materials used today are electrical steels. But for higher-frequency applications, soft magnetic composites (SMCs) are also promising candidates.

Each of the different stages of motor construction, such as rolling, punching, laser cutting, or annealing, affects the material's microstructure and can lead to magnetic losses. Understanding the details of the microstructure is paramount to reaching higher efficiency for electrical motors. Due to the high rotational speed of traction drives, more than 5 percent of the electric energy produced is lost as heat.

In a paper published this week in the journal AIP Advances, researchers at the University of Aalen in Germany created an advanced characterization method to closely examine microscale structural characteristics and changes during manufacturing processes using electron backscatter diffraction (EBSD).

"When you have deformations from machining, it is a great help to make the deformations visible," author and professor of physics of magnetic at Aalen University Dagmar Goll said. "In order to get deeper insight into the material's structure, is really useful. For example, grain size and shape, texture and degree of elastic strains and plastic deformations can be determined."

The authors compared the effects of various machining types on the microstructure of electrical steel. During machining, the cutting edge of the material is damaged, changing the crystallographic structure. "We evaluated the misorientations of grains in the material. So in the case of the cutting edge, we evaluated inhomogeneous areas with plastic deformations," Goll said. While these characteristics are microscopic, the cumulative effect in the material's structure adds up to a loss of efficiency in the final product.

"In the case of the powder metallurgical soft magnetic composites, which allow a higher degree of freedom in the design and construction of electric motors, we evaluated the recrystallization process during manufacturing as a function of compaction pressure, annealing parameters and powder particle size," author David Schuller said.

"We are improving the ratio between particle size and grain size distribution of the material," he said. "Depending on the annealing temperature, we can control grain growth and recrystallization in order to tailor the magnetic properties and minimize the magnetic losses."

The methodology developed by Schuller and colleagues provides a new tool to see exactly how, where and to what extent the crystalline structure is disrupted in machining processes and can be recovered during annealing. Their results show that EBSD is a powerful and versatile characterization technique for investigating and tailoring soft magnetic materials.

Explore further: Pumping efficiency into electrical motors

More information: "Analysis of soft magnetic materials by electron backscatter diffraction as a powerful tool," AIP Advances (2017). DOI: 10.1063/1.4994200

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