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Abstract

Nd-Fe-B permanent magnets have become indispensable components of many different devices in the past few decades. Their excellent overall magnetic performance originates in the intrinsic properties of the hard magnetic Nd2Fe14B phase. Bulk magnets are polycrystalline and their magnetic properties are governed by the material’s microstructure. To compensate for the unfavorable microstructural characteristics such as the large grain size and imperfect magnetic isolation of the Nd2Fe14B grains, heavy rare earth (HRE = Dy or Tb) elements are often added to the Nd-Fe-B alloy to increase the magnetocrystalline anisotropy of the hard magnetic phase. This improves the material’s resistance to demagnetization, measured by the intrinsic coercivity value. A major drawback of this approach, in addition to the high prices and low natural abundancy of the HRE elements, is the reduced saturation magnetization due to the antiferromagnetic coupling of the magnetic moments of the HRE atoms with the moments of iron, which results in lower magnetic flux density.
Magnetic properties deteriorate with increasing temperature. Some applications like traction motors for electric vehicles and wind turbines require magnets with high concentrations of the HRE elements, since the operating temperatures are high and certain parts of the magnets are exposed to large demagnetizing fields. The local reversal of magnetization, if not prevented, can lead to an overall reduction of the magnetic flux density, which has a pronounced negative impact on the magnet’s performance. A novel approach to achieve the resource efficiency and to minimize the loss of the flux density without degrading the magnet’s resistance to demagnetization is presented in this thesis. As an alternative to the homogeneously distributed Dy or Tb throughout the magnet, multicomponent magnets containing regions with different magnetic properties were manufactured from Dy-containing and HRE-free Nd-Fe-B magnetic powders.
Manufacture of magnets starts with the production of magnetic powders. Preparing a dense magnet from the powder is a crucial step that determines the material’s final properties. Consolidation of the polycrystalline nanostructured powders is especially challenging due to the undesired grain growth that can significantly degrade their magnetic properties when the consolidation temperatures are too high. By using a special kind of hot-pressing technique called Pulsed Electric Current Sintering (PECS), the material can be densified at lower temperatures and with shorter processing times compared to the conventional high-temperature sintering and hot pressing techniques. This is achieved by simultaneously applying pressure and heat, generated internally by means of a pulsed DC current that passes through the conductive powder compact. Consequently, the grain growth is suppressed and the magnetic properties of the initial powder are retained. Three different types of Nd-Fe-B magnetic powders (melt-spun, gas-atomized and hydrogenation-disproportionation-desorption-recombination-processed) were considered as possible candidates for the preparation of the multicomponent magnets. Processing parameters have been optimized for each of the materials investigated with the aim of finding a combination of powders that can be consolidated together without deteriorating the materials’ respective magnetic properties. Among the materials chosen for the study, only the neodymium-rich melt-spun powders are commonly used for hot pressing, while the hydrogenation-disproportionation-desorption-recombination-processed materials are intended for bonded magnets. The gas-atomized powders are seldom used for magnetic applications and a novel type of Nd-Fe-B magnet was prepared from a neodymium-rich gas-atomized powder with the PECS technique. On the other hand, the Nd-lean gas-atomized powder proved to be unsuitable for the PECS processing: the material did not condense at temperatures low enough to preserve the powder’s magnetic properties. This was correlated to the absence of the Nd-rich liquid phase, which hinders the densification kinetics.
The hydrogenation-disproportionation-desorption-recombination-processed powder needed the highest temperature to reach the maximal density (≈ 800 °C) and the dense sample required an additional post-consolidation heat treatment to retain the powder’s initial magnetic properties. A strong dependence of the samples’ intrinsic coercivity values on the PECS processing parameters was observed. Based on the microscopic investigation, the trend was attributed to the redistribution of the Nd-rich phase during the consolidation and subsequent heat treatment. In general, the Nd-rich phase has an important role in the development of coercivity in the Nd-Fe-B-type materials.
As a rapid solidification technique, the gas atomization process can be adopted for the manufacture of the Nd-Fe-B powders, but the cooling rates are not sufficiently high to prevent the formation of α-iron and large Nd2Fe14B grains in alloys with commonly used compositions. Another reason that this technique is rarely used is that the Nd-rich powders experience a significant loss of hard magnetic properties at temperatures close to or higher than 150 °C. In the PECS experiments, the maximal density was reached at 700 °C with the Nd-rich gas-atomized powder and the sample’s intrinsic coercivity was much higher than the value of the initial powder, even after the latter was heat-treated in order to improve its magnetic properties. Another advantage of the bulk magnet prepared with the PECS technique, compared to the raw powder, is improved high-temperature stability of magnetic properties. This was achieved with the reduction of the surface area, which hinders the surface nucleation reversal mechanism, believed to be responsible for the deterioration of the magnetic properties of the raw Nd-rich gas-atomized powders.
The melt-spun powders performed exceptionally well in the PECS experiments and dense single component magnets were prepared from the Dy-free and Dy-containing powders at similar consolidation temperatures (650 and 670 °C, respectively) without deteriorating their magnetic properties, which were significantly better than the properties of other types of powders chosen for the study. For these reasons, the melt-spun powders were selected for the preparation of multicomponent magnets.
Two different approaches were considered for the manufacture of magnets with locally different magnetic properties. The first sample was prepared from the raw powders by placing one powder on top of the other, after which the powder compact was densified in a single PECS consolidation step. The second sample was manufactured from the dense single component Dy-free and Dy-containing magnets, previously prepared from the respective powders. The two parts were then condensed into a single magnet body in a second PECS consolidation step. SEM and EDX analysis of the as-prepared multicomponent magnets revealed that the compositions of the respective materials remained unchanged. This corresponded with the results of the magnetic characterization, which showed that the magnetic properties of the respective materials were preserved in the multicomponent magnet.