Densification of Nd-Fe-B Powders by Hydrostatic Extrusion

Hydrostatic extrusion is a modern method of densifying materials. This method is rapid and permits extruding materials of various properties. In the present experiments, Nd-Fe-B powder, provided by Magnequench, was subjected to densification by hot pressing and subsequent hydrostatic extrusion. The powder was initially pressed mechanically and subsequently placed in a copper capsule. The densification was conducted at room temperature and at temperatures above and below the melting point of the Nd-rich phase (T_Nd), respectively. When densified at a temperature below T_Nd, the sample was strongly porous and the powder particles were not well bonded, whereas at the temperature above T_Nd the interparticle bonds were good. In scanning electron microscopy images we can see solid regions which are fragments of the starting powder particles and, between them, porous regions which also contain small fragments of the powder particles. It seems that, during the deformation, the surface layer of the polycrystalline powder particles is segmented into smaller particles. After the extrusion new regions appear, containing the Nd-rich phase which has been forced out from the intergranular spaces just as it is the case of die-upset forging. The extrusion, even at room temperature, affects the magnetic properties of the material, whereas when conducted at higher temperatures, it resulted in a slight decrease of the coercivity. This can be due to the grains growth when the powder is heated prior to extrusion. No anisotropy of the magnetic properties was observed in the extruded materials.