Abstract
In this study, effect of wheel cooling on magnetic properties of Nd<sub>15</sub>Fe<sub>77</sub>B<sub>8</sub> alloy powders produced by melt spinning method has been investigated. The present method includes the cooling of the copper wheel by externally contacting a coolant block which is cooled by internally circulating freon gas. Within this framework, the effect of wheel temperature on the microstructure and magnetic properties of Nd<sub>15</sub>Fe<sub>77</sub>B<sub>8</sub> powders have been investigated. The temperatures of cooling block and melt spinning wheel were measured as -15°C and -5°C, before experimental run, separately. Produced powders exhibited different morphologies depending on the powder sizes. The smallest size of powders was formed as spherical, ligamental and fiber-like morphologies. As powders get larger, the amount of spherical, ligamental and fiber-like shaped powders decreased and the length of the fibers declined. The microstructural cell sizes for 5 µm and 48 µm size powders were measured as 0.22 µm and 1.23 µm, respectively. The cooling rates of 4 µm, 28 µm and 52 µm sized powders were measured as 5.95 x 10<sup>6</sup> K/s, 0.85 x 10<sup>6</sup> K/s and 0.45 x 10<sup>6</sup> K/s, respectively. The Curie temperature of produced powders was 321.5°C. The coercivity value of melt-spun powders was obtained as 2.842 kOe.
Highlights
Hard magnetic materials have attracted considerable attention and intensive research because of their excellent hard magnetic properties
The Curie temperature of Nd15Fe77B8 ingot alloy, melt-spun powders produced with cooled wheel and surfactant active ball-milled powders were obtained as 279°C, 341°C and 346°C, respectively
The mean cell sizes of produced flaky shaped powders by melt spinning method were decreased with increasing cooling rate
Summary
Hard magnetic materials have attracted considerable attention and intensive research because of their excellent hard magnetic properties. Rapid solidification is a widespread technique and has been broadly used to produce highest performance of NdFeB based permanent magnets either by rapid quenching or by undercooling [13] In this method, thermal energy of material which include both superheat and latent heat is rapidly extracted during solidification process from liquid state at high temperatures to solid state at room temperature. The most popular technique is the melt spinning process and today’s, this method is standard industrial practice for producing starting NdFeB powders with high cooling rate This method uses a rapidly rotating copper wheel as substrate, which contacts with the molten alloys, quenches the melt, and ribbon-shaped particles are formed [16, 17]. On the other hand, produced melt-spun powders were subjected to surfactant-assisted high energy ball milling method with a newly designed vacuum ball milling apparatus to further enhance their magnetic properties and Curie temperatures
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