Sintering behaviour of NdFeB magnets

The dimensional changes that occur during the sintering of NdFeB magnets were investigated using a high-temperature vacuum dilatometer. These measurements were made on samples produced by the conventional powder metallurgical method and on compacts produced using hydrogen decrepitation (HD) as a premilling technique. The amount of shrinkage at a given temperature was found to be greater for the HD compact, with pronounced contractions being observed at the temperatures where hydrogen is desorbed. Shrinkage measurements, carried out on sintered compacts produced with a range of alignment fields, revealed anisotropic shrinkage of the compact, with the increased growth being along the a-axis direction. In sintering experiments NdFeB magnets were cooled at different rates to room temperature and subsequently given a postsintering heat treatment. The results of the experiments showed that the intrinsic coercivity decreases linearly with increasing cooling rate. However, when the samples were annealed for 1 h at 650°C all coercivity values came up to the same level, more or less nullifying the influence of the previous cooling rate