Design of Punch Shape for Crack-Free Radially Oriented Nd–Fe–B Ring Magnet Prepared by Backward Extrusion Method

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 were 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 the 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 form 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.