PNNL

Abstract

Raising Si beyond 3.5?wt% reduces core loss but embrittles electrical steels, constraining processing and application. Here we assess friction stir processing (FSP) as a solid-state route to enable Fe–4.0?wt% Si with improved mechanical performance. Temperature-controlled FSP with a PcBN tool produced a defect-free processed zone. SEM/EBSD revealed dynamic recrystallization and grain refinement from an average 110 µm in the base material to ~20 µm in the processed region, with ~ 80% of grains 40 µm and a reduction in high-angle boundary fraction to ~65% (vs >85% in the base). Comparative mechanical property analysis shows a moderate increase in yield strength in FSP samples relative to the base material, with a significant improvement in ductility (~35–50%). Post-fracture EBSD identified 60° twin boundaries in the base while sub-grain formation with elevated KAM in the FSP sample. Fractography revealed a shift from quasi-cleavage in the Base-faceted planes with river patterns, steps, and occasional secondary cracks—to ductile micro void coalescence after FSP. Following FSP, the material retains soft-magnetic behavior, with modest shifts in saturation magnetization consistent with grain refinement and stored strain, accompanied by a clear gain in formability. These results indicate that FSP provides a viable route to deploy higher-Si electrical steels with concurrent gains in strength and ductility.