The role of wheel surface quality on structural and hard magnetic properties of nd-fe-b permanent magnet powders
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Rapidly solidified Nd15Fe77B8 magnetic alloy powders have been produced by using melt-spinning method. Different wheel surface structures such as smooth and textured were tested to investigate the effect of wheel surface morphology on powder shape, microstructure, Curie temperature, and magnetic properties. Produced powders were subjected to surfactant-active ball-milling process for powder size reduction and to enhance magnetic and thermal properties. Generally, flaky-shaped powders were obtained with smooth surface wheels and spherical-shaped powders were produced with textured surface wheel. Microstructure of flaky-shaped powders consisted of polygonal fine equiaxed grains with an average grain size of 0.45 A mu m. Spherical-shaped powders had dendritic microstructure regardless of their sizes. Both flaky and spherical powders involved hard magnetic Nd2Fe14B phase located in the grain interior and a fine network of Nd-rich phase at the grain boundaries. Surfactant-active ball-milling process significantly contributed to magnetic and thermal properties of powders. The coercivity values of flaky-shaped powders milled for 90, 150, 210, 270, 330, and 390 min milling times were substantially higher than that of spherical-shaped powders milled for the same period of times. The Curie temperatures of Nd15Fe77B8 ingot alloy and spherical- and flaky-shaped powders were found as 279, 305, and 321 C-ay, respectively. On the other hand, the Curie temperatures of flaky- and spherical-shaped powders milled for 390 min were measured as 331 and 311 C-ay, respectively.