Suppression of Solidification Defects in Partial Non-Magnetization Improvement for Silicon Steel

Leakage flux in rotor core bridges is a problem specific to interior permanent-magnet (IPM) motors. It is widely known that the partial non-magnetization of bridges reduces the magnetic flux leakage. In a previous study, a process was proposed whereby a part of the silicon steel sheet that bridges after pressing was non-magnetized by melting and mixing Ni–Cr alloy powder with a silicon steel sheet using a laser, and the rotor core was produced by laminating them. However, because the final solidification part had solidification defects, such as cracks and shrinkage cavity, the process was proposed to leave a homogenous part free of solidification defects. Therefore, the area of the improved portion increased. We focused on developing a new alloy for non-magnetic improvement to suppress solidification defects. The improved portion was melted and mixed using a laser with various B contents to obtain a composition of Fe–(15–20) mass%Ni–(15–20) mass%Cr–(2–3) mass%Si–(0–1.6) mass%B. Large cracks and large shrinkage cavity were observed in the boron-free alloy. The cracks and shrinkage cavity decreased with an increase in the B content. The minimization of the area of non-magnetic improvement is possible by suppressing solidification defects. Consequently, the laser processing speed per piece and the amount of expensive nickel were reduced. These new alloys show promise for practical applications in the partial non-magnetization process.