Abstract
The current work demonstrates that electron backscatter diffraction (EBSD) is a powerful and versatile characterization technique for investigating soft magnetic materials. The properties of soft magnets, e.g., magnetic losses strongly depend on the materials chemical composition and microstructure, including grain size and shape, texture, degree of plastic deformation and elastic strain. In electrical sheet stacks for e-motor applications, the quality of the machined edges/surfaces of each individual sheet is of special interest. Using EBSD, the influence of the punching process on the microstructure at the cutting edge is quantitatively assessed by evaluating the crystallographic misorientation distribution of the deformed grains. Using an industrial punching process, the maximum affected deformation depth is determined to be 200 - 300 μm. In the case of laser cutting, the affected deformation depth is determined to be approximately zero. Reliability and detection limits of the developed EBSD approach are evaluated on non-affected sample regions and model samples containing different indentation test bodies. A second application case is the investigation of the recrystallization process during the annealing step of soft magnetic composites (SMC) toroids produced by powder metallurgy as a function of compaction pressure, annealing parameters and powder particle size. With increasing pressure and temperature, the recrystallized area fraction (e.g., grains with crystallographic misorientations < 3°) increases from 71 % (200 MPa, 800°C) to 90% (800 MPa, 800°C). Recrystallization of the compacted powder material starts at the particle boundaries or areas with existing plastic deformation. The progress of recrystallization is visualized as a function of time and of different particle to grain size distributions. Here, large particles with coarse internal grain structures show a favorable recrystallization behavior which results in large bulk permeability of up to 600 – 700 and lower amount of residual misorientations (>3°).
Highlights
High-efficiency-electric-motors with tailored speed-torque characteristic are essential for the sustainable success of electric vehicles
High-frequency applications can be realized by soft magnetic composites (SMC) which allow a high degree of freedom in the design and construction of aDavid.Schuller@hs-aalen.de 2158-3226/2018/8(4)/047612/8
The present paper focuses on the development of a reliable electron backscatter diffraction (EBSD)-based approach to detect and quantify local grain misorientations introduced by mechanical load during a punching process of electrical steels
Summary
EBSD-analysis can be a powerful tool to visualize and quantify the content of deformation and recrystallized structure.[10,11,12,13] This enables the establishment of structure-property relations and the possibility for specific material design. A qualitative assessment of the influence of the punching tool condition (wear conditions) on resulting microstructure and magnetic performance is presented in Ref. 14. The present paper focuses on the development of a reliable EBSD-based approach to detect and quantify local grain misorientations introduced by mechanical load during a punching process of electrical steels. The developed approach is deployed for the analysis of SMC materials to quantify the degree of recrystallization during annealing which in turn is compared with magnetic performance
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