B. Michalski

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 (TNd), respectively. When densified at a temperature below TNd, the sample was strongly porous and the powder particles were not well bonded, whereas at the temperature above TNd 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.

Effect of Composition and Structure on the Properties of Ti Containing Nd-Fe-B Rapidly Quenched Alloys

The effect of neodymium and boron content was studied for the Nd_7+xFe_79-x+y-zB_14-yTi_z (x=0, 1, 2; y=0, 1, 2; z=0, 4) system. The alloys were prepared by melt-spinning and suction casting. The highest magnetic properties (_JH_c=648 kA/m, J_r=972 mT, (BH)_max=143 kJ/m³ ) were achieved for the melt-spun Nd₈Fe₇₄B₁₄Ti₄ alloy. This was caused by Ti addition and was related to the convenient combination of the phase constitution and fine crystallite size in the range of 15-30 nm.

Recycling of Nd-Fe-B Magnets from Scrap Hard Disc Drives

A hydrogen-based treatment, including Hydrogen Decrepitation (HD) and Hydrogen Disproportionation-Desorption-Recombination (HDDR), was used as part of a recycling procedure for scrap neodymium-iron-boron magnets. Chemical methods of removing nickel coating out of magnets were tested, however ineffectively. Powders were obtained from magnets after the HD and were further processed by the HDDR. Finally, material with maximum energy product (BH)max of 74 kJ/m3 was produced. Chemical composition of magnets (MS, EDS), magnetic properties (VSM) and microstructure observations (SEM) were carried out.

Application of the HDDR method for recycling of Nd-Fe-B magnets

In the light of tremendous technological development, rapidly grows the demand for raw materials. The Rare Earth Elements have been recognized by the US and Europe as critical ones [1]. The waste of electrical and electronic equipment becomes an urban ore - an important source of precious metals. Nd-Fe-B magnets, widely used in hard disc drives, electric motors and other devices, are considered to be very valuable for recovery because their chemical composition is based mostly on the critical elements (Nd, Dy, Pr). The HD (Hydrogen Decrepitation) and HDDR (Hydrogenation, Disproportionation, Desorption and Recombination) methods are widely tested as a prospective procedure for processing of scrap sintered Nd-Fe-B magnets into new, resin-bonded magnets [2].

Hydrogen disproportionation phase diagram and magnetic properties for Nd15Fe79B6 alloy

Transformation-temperature-hydrogen pressure phase diagram was constructed for a Nd15Fe79B6 alloy in order to estimate appropriate conditions for hydrogenation, disproportionation, desorption and recombination reaction (the HDDR). Optimised recombination time (the highest coercivity) was found to be 10 min. for 5 g samples processed at 740 °C. Several HDDR processes were carried out at 30 kPa of hydrogen pressure at various temperatures. No correlation between magnetic propertiec and a direction of measurement was observed for the samples processed at 740 °C. Remanence anisotropy was induced along an alignment direction when the temperature of the HDDR process was increased up to 800 °C and 850 °C for <100 μm and 100–160 μm particles, respectively. Simultaneously, a small drop in coercivity was observed in the direction of alignment for <100 μm particles, but no for 100–160 μm particles. Furthermore, probably an ordered phase was found by TEM microstructure analysis in the bulk sample disproportionated at 850 °C under 150 kPa of hydrogen. Grains with antiphase domains were observed and corresponding electron diffraction patterns were resolved, likely indicating superlattice structures.

Correlation Between The Size Of Nd60Fe30Al10 Sample, Cast By Various Techniques And Its Coercivity

The present study is concerned with the correlation between the magnetic properties of the Nd60Fe30Al10 sample and its size, in particular its dimension measured in the direction of heat removal. We compared samples produced using the three methods: melt‐spinning, die casting under pressure, and suction casting. The samples were of various shapes such as ribbons, plates, rods and pipes. We found that despite the differences in the shape of the samples and the technique of their casting, their magnetic properties did not differ significantly. Hence we may conclude that it is the sample dimension measured along the direction perpendicular to the heat‐removing surface which is the parameter that mostly decides about the cooling rate.

Effect of Gallium Addition on the Magnetic Properties of Nd

In this study, the effect of gallium (Ga) for Al substitution in the Nd60Fe30Al(10-x)Gax (in at.%) system was analyzed. Cylindrical samples (rods) with a diameter of 1 mm were prepared by casting the material into a copper die. Substitution of gallium for aluminum leads to substantial growth of the coercivity up to 407 kA/m for x = 8. The remanence is constant up to x = 8. Magnetic properties for higher Ga concentrations of hard magnetic properties abruptly decrease. On the basis of the differential scanning calorimeter and X-ray diffraction data, it may be concluded that the Ga addition reduces the ability for amorphous structure formation; for x = 10 crystalline phases predominate.