Effect of heat treatment on the microstructure and magnetic properties of laser powder bed fusion processed equiatomic Co-Fe

Equiatomic Cobalt-Iron (Co-Fe 50%at.) has great potential as a soft magnetic alloy, but its wider use has been limited by its poor workability and strength. Recent advancements in Laser Powder Bed Fusion (LPBF), an Additive Manufacturing (AM) technique, provided a new pathway for constructing fully dense, structurally sound, complex-shaped components from bulk equiatomic Co-Fe in a single process step. To obtain the desirable soft magnetic performance in the alloy, thermal post-processing with a controlled slow cooling needs to be applied. In order to identify the optimum heating conditions, several single and multiple step heat treatment profiles were performed and compared. The effects of the thermal post-processing on the microstructure, structural ordering, and functional properties of the alloy after each heat treatment were investigated using electron microscopy, neutron diffraction (ND), electron backscatter diffraction (EBSD), and quasi-static magnetic characterisation in a closed loop magnetic circuit. Results have shown that a normalisation heat treatment at 1300 K for 2-hours followed by a 4-hour primary heat treatment at 1123 K and slow cooling to room temperature produced a recrystallised microstructure characterised by predominantly equiaxed grains with an average size of up to 61 mu m, and a fully ordered B2 structure. The quasi-static soft magnetic properties obtained were favourable compared to those of commercial Co-Fe grades, with maximum relative permeability higher than 8000, coercivity as low as 112 A/m and magnetic saturation polarization of 2.39 Tesla. These findings provide the basis to enable the manufacturing of three-dimensional complex-shaped electromagnetic cores by LPBF.