Abstract
Nd-Fe-B ring magnet (RM) prepared with backward extrusion method shows deep and wide cracks at the top part but crack-free inner surface at the bottom part. To reveal the mechanism for the elimination of cracks at the bottom part, trisected hot-pressed sections was stacked and backward extruded into medium magnets with different height of ring-shaped part. The evolution in shape and height of the trisected sections revealed that the friction from the bottom surface of cylindrical punch resulted in accumulation of worse-flowability Nd-Fe-B under the punch. The consequent bow-shaped worse-flowability Nd-Fe-B lump could cover the bottom surface of punch and formed smooth transition with the cylindrical punch for outward flow of Nd-Fe-B. It is such smooth transition that avoided the formation of cracks on the inner side surface at the bottom part of RM. Similarly, the punch with semispherical-end shape could eliminate the cracks at the top part of RM due to the smooth transition between the bottom surface and side surface of punch. Moreover, the semispherical-end punch can also lead to improved remanence at the top part of RM than traditional cylindrical punch.
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