Shear-based deformation processing and characterization of electrical steel sheet

Andrew Benjamin Kustas, Purdue University

Abstract

Processing of electrical steel (i.e., iron-silicon) alloys containing up to 6.5wt%Si into sheet is demonstrated by application of highly confined shear deformation in the form of metal cutting. Iron-silicon (Fe-Si) alloys, of major interest to electromagnetic applications, are characterized by poor workability. Through the interactive combination of simple shear, high strain rates, near-adiabatic heating and large hydrostatic pressure in the deformation zone, sheet and foil forms of bulk Fe-Si workpieces are created in a single deformation step. A wide range of microstructures could be achieved in sheet through selection of different deformation parameters. Under ambient conditions (i.e., room temperature) and slower cutting velocities (1-2 m/s), a homogeneous, cold-worked structure resulted. With increased workpiece temperature and cutting velocity (≥4 m/s), a diffuse transition from homogeneous to localized material flow, in the form of periodic adiabatic shear bands, was observed. Suppression of these inhomogeneous flows is demonstrated by confining the deformation through a secondary cutting tool in a hybrid cutting-extrusion configuration called large strain extrusion machining (LSEM). Crystallographic textures of the sheets are characterized by strong simple shear textures, defined by partial {110} and <111> fibers, inclined relative to the sheet length. The orientation (inclination) of these fibers is shown to be predictably controllable over a wide range through selection of the deformation path. A one-to-one correspondence is observed between the deformation path (determined by upper bound models) and measured texture orientations (pole figures). The simple shear textures developed during deformation are found to be retained following recrystallization and grain growth treatments and models are proposed to explain the observed texture evolution. Bulk magnetic properties of the shear-textured sheets are assessed using a closed-loop hysteresisgraph (i.e., permeameter). Full quasi-static hysteresis loops are obtained and properties including: maximum relative permeability, coercivity and hysteresis loss are tabulated. These properties are analyzed as a function of sheet structure (i.e., texture) and alloy composition and compared directly to commercially rolled Fe-Si sheet (courtesy of AK Steel). A basic model is presented to describe the processing-texture-property relationships of the shear-textured sheets with implications.

Degree

Ph.D.

Advisors

Chandrasekar, Purdue University.

Subject Area

Mechanical engineering|Materials science

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