The influence of pressing pressure on particle alignment in Nd-Fe-B green compacts

Abstract

Different processing routes have been presented in the literature as appropriate for Nd-Fe -B permanent magnet production, such as melt spinning of molten alloy [1, 2], hot pressing of as-cast alloy [3-5] and the powder metallurgy technique followed by thermal treatments [6-8]. Because the powder metallurgy route is the most widely employed, research efforts have concentrated on sintering parameters in relation to the magnetic properties and microstructural parameters. However, less data have been published on powder production and especially on powder compaction of magnetically aligned Nd-Fe -B powder. It is well known that the ideal pressing pressure must be sufficient to give enough cohesion and allow handling of the green compact but it should not cause severe particle misorientation. The powder is usually compacted by uniaxial pressing or by cold isostatic pressing, the latter giving the better characteristics with respect to density, homogeneity and particle alignment [9-11]. In both cases the pressing pressure usually applied is close to 200 MPa. Of course, perfect magnetic alignment is never found, independently of the magnitude of the pressure or the method used, which may be attributed to several factors, such as the particle size, its shape and the possible pressence of second phases on individual particles. This study analyses the relative degree of magnetic alignment in different regions in the green compact when the particles are submitted to different pressing pressures by isostatic and unixial pressing of previously aligned Nd-Fe -B powders. An ingot of nominal composition NdlsFe77B 8 was prepared by arc-melting under pure argon atmosphere from starting elements commercially available. Heat treatment for 1 h at 1000 °C was carried out to achieve microstructure homogenization. After crushing, the material was ball-milled for 2 h under toluene protective medium to produce a powder with mean particle size of approximately 3 #m. For simple uniaxial pressing (SUP), the powder was aligned in a non-magnetic die by a pulsed magnetic field of 3.7 T and discretely pressed at 100, 200 and 300 MPa with magnetic punches parallel to alignment direction. No die lubrificant was used. For cold isostatic pressing (CIP), the powder was fed into a rubber mould and aligned along the longitudinal direction. The pressure used was 160 MPa. The samples for metallography observation were vacuum infiltrated by epoxy resin before sectioning