The production of a Nd16Fe76B8 sintered magnet by the hydrogen decrepitation/hydrogen vibration milling route

The present work has been undertaken to investigate the potential of hydrog en vibration milling (HVM) as a possible replacement for roller milling (RM ). The aim was to develop a more rapid laboratory method for producing a fi ne clean powder, suitable for the production of Nd-Fe-B sintered magnets. H VM has been applied to a Nd16Fe76B8-type alloy, using the highest possible amplitude (A) and frequency (nu) within the capacity of the present hydroge n vibration mill (HV mill). Under these conditions, the correlations betwee n the amount of the material being milled at each stage, milling time, aver age particle size, and the magnetic properties of the sintered magnets made from these powders, have been investigated. The average powder particle si zes achieved in the HVM technique, for a batch of 50 g milled for 4 h, or f or a batch of 300 g milled for 10 h, was well below 5 mu m This was lower t han those of the powders produced by the RM or jet milling (JM) processes. Thus, intrinsic coercivities (H-ci) of up to 654 kA m(-1) (8.2 kOe) with co rresponding energy products [(BH)(max)] of up to 331 kJ m(-3) (41.5 MGOe) w ere obtained for the sintered magnets made from the HV milled powder. These values were significantly higher than those attained for the sintered magn ets made by the hydrogen decrepitation (HD)/RM and HD/JM processes. As with conventional processing, applying an annealing treatment at 600 degrees C for 1 h followed by air cooling in vacuum increased further the H-ci and (B H)(max) values. The improvements in the magnetic properties of the HD/HVM m agnets have been related to (a) the finer powder particles, (b) in-situ KD of the alloy, (c) the dry milling condition, and (d) the shielding nature o f the hydrogen. Factors (b)-(d) should result in less oxygen pickup during milling. (C) 1999 Elsevier Science S.A. All rights reserved.