The processing and characterisation of recycled NdFeB-type sintered magnets

A study of the processing and characterisation of sintered NdFeB magnets made from recycled feed stock was undertaken. Initially the hydrogen decrepitated (HD) powder was investigated using two different milling techniques. The powders were analysed with optical microscopy, with the aid of a magnetic field. It was found that with light milling the HD powder breaks up to a similar particle size to that of the grain size of the starting material. A data logging system was built to investigate the desorption behaviour of green compacts during sintering. Desorption traces showed desorption from the matrix phase and the intergranular Nd-rich phase. The start of desorption was seen to shift to lower temperatures as the mean particle size of the green compact was reduced. For the processing route used in this work intergranular additions of neodymium hydride were required to increase the density and magnetic properties. To investigate the oxidation behaviour of lightly milled HD powder, powder was exposed to air for varying times. The exposed powder was aligned pressed and sintered. The Nd-rich desorption peak reduced with exposure time, the density and magnetic properties also reduced. Post exposure additions of Intergranular neodymium hydride to the powder recovered density and magnetic properties