Suppression of Shrinkage Cavity Formation in Silicon Steel by Partial Non-Magnetization

抄録

Internal permanent magnet motors suffer from leakage flux in their rotor core bridges, which can be removed through the partial non-magnetization of the bridges. In a previous study, a process was proposed in which the bridged portion of the silicon steel sheet was non-magnetized after pressing by melting, an Ni–Cr alloy powder was added to the silicon steel sheet using a laser, and the rotor core was produced via lamination. However, because the final solidification step produced solidification defects, such as cracks and shrinkage cavities, a homogenous alternative step that does not produce solidification defects needs to be developed. To reduce the area of improvement, we previously investigated a chemical composition that suppressed solidification defect formation. The addition of B to the improved portion suppressed the formation of cracks and shrinkage cavities, and a crack-suppression mechanism was proposed. In this study, we focus on the mechanism of shrinkage cavity suppression. We determine that the solidification shrinkage rate decreases owing to the precipitation of Cr₂B, which has a lower density than the austenite phase, and results in the suppression of the shrinkage cavities. Minimization of the area of non-magnetic improvement is possible by suppressing solidification defects. Consequently, the laser processing speed per piece and amount of expensive nickel are reduced. These new alloys show promise for practical partial non-magnetizing applications.

Adding B to the improved portion suppressed the formation of cracking and shrinkage voids. The mechanism for suppressing shrinkage voids was also revealed. The solidification shrinkage ratio decreased with the addition of B. Only a few eutectic structures were observed in the sample without B, whereas the sample containing a B content of 0.9% showed several eutectic structures. The left-most photograph shows the precipitates in the eutectic structure using STEM-HAADF imaging. The precipitates mainly consisted of the Cr₂B phase. Subsequently, the solidification shrinkage rate decreased owing to the precipitation of Cr₂B, which has a lower density than the austenite phase, and the formation of shrinkage cavities was suppressed.