Application of hot pressing in fabricating radial polarized Sm-Co magnets

Abstract

The properties of hot pressed Sm-Co magnets are compared to those obtained in sintered magnets. Hot pressing was performed several hundred degrees below the normal sintering temperature using powder compacts of three types: (1)isotropic discs, (2)axially aligned discs and (3) radial aligned rings. In all cases densities in excess of 97%, of the theoretical value were obtained which is somewhat higher compared with sintered material. Intrinsic coercivity values, H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ci</inf> , were between 15 and 20kOe, which is comparable to annealed, sintered magnets. Despite the higher density of the hot pressed samples, values of B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> and (BH) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> were lower than in sintered magnets. Inverse pole figures obtained by x-ray diffraction indicated that the alignment of magnetic poles after hot pressing does not change from that of the original powder compact. This is in contrast to the improvement in alignment as a result of normal sintering, which presumably occurs as a result of well aligned grains growing at the expense of poorly aligned grains. By heat treating aligned, hot pressed magnets at the sintering temperature, a similar improvement in alignment was obtained. Values of (BH) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> as hot pressed and after this heat treatment were 12MGOe and 16MGOe, respectively. The results of this study demonstrate the importance of sintering and grain growth processes as they relate to metallurgical structure and final second quadrant characteristics. In the case of radial ring fabrication, the results are particularly encouraging since hot pressing in a confining die eliminates the distortion and cracking which occurs during sintering.