Effects of Length-to-Width Ratio on Magnetic and Microstructural Properties of Die-Upset Nd-Fe-B Magnets

In this study, the effects of the length-to-width ratio on the magnetic and microstructural properties of die-upset Nd-Fe-B magnets were examined. A die-upset magnet with a uniform shape and no significant cracking was successfully developed. During the die-upset process, the applied pressure was not uniform across the magnet and varied depending on its shape and position. In the case of the magnet with a 4:1 ratio, which had the largest length-to-width ratio, there was a higher concentration of stress at the edges compared to the center, resulting in easier grain deformation and growth at the edges, which led to the lowest remanence and (BH)_max. As the length-to-width ratio approached 1:1, the grain size increased slightly, reducing the coercivity; however, the magnets maintained a uniform grain-size distribution. The change from a rectangular to a square pressing surface resulted in a more uniform stress distribution, particularly at the center and corners, which improved the consistency of the magnetic properties across different regions of the magnet. Consequently, an 11.9% enhancement in (BH)_max, reaching 285.6 kJ/m³, was achieved in the 1:1 ratio die-upset magnet, which is attributed to the uniform grain size and improved grain alignment.