Microstructure of sintered permanent magnets of Co–25 % Sm KS 25 grade

A metallographic study of the microstructure of the surface of sintered rare-earth magnets Co-25 % Sm grade KS-25 after Electrical Discharge Machining (EDM) was carried out. Chemical composition of the studied magnets was 25 wt. % Sm; 18 wt. % Fe; 9 wt. % Cu; 3 wt. % Zr; and the rest – Co. One of the sample surfaces was subjected to the EDM in various ways; the parameters of the EDM (the processing speed in a straight line and the offset) were varied. Scanning electron microscopy and micro-X-ray spectral analysis (EDS-analysis) revealed the presence of dendrites in the microstructure of magnets containing four coexisting phases, i.e. SmCo5, Sm2Co17, Zr5Co3FeSm and Sm2O3. The grain size was 10–50 µm. Crystals of Zr5Co3FeSm intermetallic compound had size of 1–5 µm; inclusions of Sm2O3 samarium oxide of globular shape had size of 2–10 µm. The EDM influenced the thickness and the chemical composition of the white layer. In general, the chemical composition varied slightly with distance into the white layer: the content of Sm, Cu, O and Zr decreased; the content of Fe and Co increased. The grain size, when moved from the white layer into the sample by 500 µm, increased by 40–50 %, while the porosity decreased. The size of Sm2O3 oxide particles increased slightly with distance from the white layer.