THE IMPACT OF THE DIRECTIONAL SOLIDIFICATION ON THE MAGNETIC-PROPERTIES OF NDFEB MAGNETS

Microstructures of cast Nd15Fe78B7 ingots with thicknesses of 8.7 and 4.4 mm have been investigated. Usually, a flat cast ingot consists of three different types of grains: fine, coarse, and columnar grains. Fine grains appear near the cast interface, columnar grains usually show in regions which undergo directional solidification and coarse grains exist in regions of a lower solidification rate. Low solidification rate results in a segregation of dendritic Fe, which is undesirable in magnet processing. Magnets made from a 8.7-mm-thick ingot yield a B(r) of 11.7 kG and a BH(max) of 34.0 MGOe while magnets made from a 4.4 mm ingot give rise to a B(r) of 12.3 kG and a BH(max) of 37.5 MGOe. The magnetic properties of final magnets have been correlated to the microstructures in flat cast ingots. Columnar grains have been shown to be desirable for achieving higher B(r) and BH(max) in final magnets.