Microstructure control of Nd-Fe-B sintered magnets by two-step sintering

Abstract

Nd-Fe-B sintered magnets have been widely applied to a high performance permanent magnet. Nd-Fe-B sintered magnets are normally composed of Nd 2Fe14B hard magnet phase as a matrix and Nd-rich phase on grain boundary [1,2]. Many researchers have shown that the coercivity of Nd-Fe-B magnets was sensitive to microstructure [3,4]. Therefore, modifying a sintering process has been an important method for better homogeneous microstructure. In this study, Nd-Fe-B powder was fabricated by strip-casting and jet milling process. The powder was compacted under magnetic field of 20kOe. After that, the green compact was sintered at temperature between 950°C and 1050°C. Two-step process was employed for the sintering of Nd-Fe-B powder to obtain homogeneous grain size and restrain grain growth in bulk part. The microstructure was investigated by scanning electron microscopy and electron probe micro analyzer. Magnetic properties were measured by a B-H loop tracer. Densification over 97% was obtained by the two-step sintering of Nd-Fe-B powder under 6 μm in Nd2Fe14B phase and the Nd-rich phase was homogeneously distributed around Nd2Fe14B phases. Moreover, the two-step sintering process led to uniform grain size distribution which had the improved magnetic properties.