Sintered NdFeB Magnets Research Articles

Preparation of Ni–Co alloy coating on sintered NdFeB magnet by potentiodynamic electrodeposition

A protective Ni–Co alloy coating was successfully prepared on sintered neodymium–iron–boron (NdFeB) magnet via potentiodynamic electrodeposition in this paper. The microstructure and surface morphology analysis of Ni–Co alloy coating showed that the surface morphologies were quite compact and more uniform than that prepared by direct current (DC) electrodeposition; significantly, the chemical composition analysis indicated that the content of Co in the alloy coatings amounted to 33–34 wt-%. Potentiodynamic polarisation and long term immersion test results revealed that the Ni–Co alloy coating could supply excellent corrosion protection for NdFeB substrate in 3.5 wt-% NaCl solution. Overall, the Ni–Co alloy coating with higher Co content prepared by potentiodynamic electrodeposition exhibited better corrosion protection for sintered NdFeB magnet compared with that prepared by DC electrodeposition.

Influence of Dysprosium Distribution on Properties of Sintered and Aged Dy-doped NdFeB Permanent Magnets

Sintered and aged Dy-doped NdFeB magnets were investigated. The magnetic properties, the microstructures and the compositions were characterized by hysteresis loop instrument, thermal field emission scanning electron microscopy (TFESEM) and energy disperse spectroscopy (EDS), respectively. The results indicate that Dy element is mainly distributed in the Nd-rich phase, Nd-Dy oxides and Dy-rich particles located at the grain boundaries of the sintered magnets, in addition to the main crystal phase Nd 2 Fe 14 B. The optimized aging process is beneficial to promote a reasonable diffusion and distribution of Dy element. The Dy contents of Nd-riched phases, Nd-Dy oxide, and Dy-riched particle decrease successively in the sintered, the high temperature aged and the optimally two-stage aged Dy-doped NdFeB permanent magnets. The measurements demonstrate that the enhancement of the coercivity of the aged Dy-doped NdFeB Magnet is caused mainly by the Dy element reasonable distribution.

Corrosion Behavior of Sintered NdFeB Magnets Coated with Ni Coatings Deposited by Ion Beam Sputtering

To improve the corrosion resistance of a sintered NdFeB magnet, pure Ni coatings were deposited on the magnet by ion beam sputtering, and the effect of the deposition process parameters upon the microstructure and the corrosion resistance of the Ni coating was investigated by scanning electron microscopy and potentiodynamic polarization curves, respectively. A dense Ni coating was obtained via room temperature deposition for 3 h, which exhibited high adhesive strength with the NdFeB substrate and excellent corrosion resistance. The Ni coatings deposited at high temperature, however, possessed a lower corrosion resistance that can be attributed to the atom diffusion and crystallization of the coatings grown at high temperature. A magnetic field is found to affect the corrosion behavior of NdFeB magnets with or without Ni coatings, reducing its corrosion resistance. The corrosion resistance of a Ni-coated NdFeB sample in the presence of a magnetic field is better than that of an uncoated sample.

Coercivity and Thermal Stability Enhancement for Spark-Plasma-Sintered Nanocrystalline NdFeB Magnets With Dy<sub>2</sub>O<sub>3</sub> and Zn Additions

The main problems with the sintered NdFeB magnets are their bad formability, low oxidation resistance, and poor temperature stability. Relatively large grain size, typically more than 3 mm, is also not beneficial to the high coercivity. Current efforts are directed to improve the magnetic properties and reduce the cost of NdFeB magnets by compositional modification, microstructure optimization, and new process employment. Spark plasma sintering (SPS) is one of the novel sintering techniques for preparing NdFeB magnets in a laboratory scale. Low sintering temperature and short holding time make it possible to sinter nanocrystalline NdFeB powders into fully dense bulk magnet. However, it is still not possible to completely avoid the grain coarsening, which leads to inevitable decrease of coercivity. To improve coercivity, the magnetic powders can be mixed with some metal powders such as Cu and Zn, which are beneficial for densification during SPS. Another effective way to enhance the coercivity is to add the powdered Dy compounds. In this paper, we report the effects of separated and combined Dy 2 O 3 and Zn additions on the magnetic properties, microstructure and thermal stability of SPSed nanocrystalline NdFeB magnets.

Effect of Refractory Metal Substitution on Magnetic Property and Corrosion Behavior of Sintered NdFeB Magnets

Abstract Nd24Fe65.5-xRMxB0.97Tb7.0Dy1.1Co1.0Al0.2Cu0.23 (RM=Nb, Zr, Ti;x=0.2,0.4,0.6) magnets with high property were prepared by powder metallurgy process. The measurements of high temperature magnetic properties indicate that the value of α (temperature coefficient of remanence of NbFeB magnet) is obviously increased when the temperature is below 200 °C, and β (temperature coefficient of coercivity of NbFeB magnet) is well improved after iron is substituted by the refractory metal, e.g. in Nb and Ti-substituted magnets. The result of PCT test shows that the mass loss of Nb-substituted magnets is only about 0.07 mg/cm2, which indicates that the corrosion resistance is enhanced greatly in Nb-substituted magnets, but the opposite phenomenon is observed in Ti-substituted magnets. EDX results reveal that Dy could be prevented from oxidation when more than 0.2 wt% Zr or Ti is doped in the magnets.

Magnetic microstructure and magnetic properties of spark plasma sintered NdFeB magnets

Nanocrystalline NdFeB magnets were prepared by spark plasma sintering (SPS) technique using melt-spun ribbons as starting materials. A distinct two-zone structure with coarse grain zone and fine grain zone was formed in the SPSed magnets. Multi-domain particle in coarse grain zone and exchange interaction domain for fine grain zone were observed. Intergranular non-magnetic phase was favorable to improve the coercivity due to the enhancement of domain wall pinning effects and increased exchange-decouple. The remanent polarization of 0.83T, coercivity of 1516kA/m, and maximum energy product of 118kJ/m3 are obtained for an isotropic magnet.

Processing of Grinding Slurries Arising from NdFeB Magnet Production

AbstractNeodymium‐iron‐boron (NdFeB) grinding slurries are the residual output of grinding as shaping process step in the production process of sintered NdFeB magnets. The treatment of the grinding slurries with the aim of recovering rare earth elements may lead to a reduced demand in primary raw materials like the rare earth elements neodymium, praseodymium and dysprosium. In this study, the possibility of recycling the contained rare earth elements, which account for up to 30 wt % in the alloy, through a pyrometallurgical process was investigated. The necessity of a thermal conditioning prior to the pyrometallurgical process is described.

Properties improvement and structural optimization of sintered NdFeB magnets by non-rare earth compound grain boundary diffusion

Abstract Grain boundary diffusion using rare earth (RE)-containing compounds has recently become an effective approach for improving the coercivity and reducing the heavy RE content in sintered NdFeB magnets. Here we report the enhancement of magnetic properties and corrosion resistance of NdFeB magnets by a non-RE compound diffusion process. The Dy-free sintered NdFeB magnets were coated with an MgO layer by magnetron sputtering, followed by solid diffusion heat treatment. With the successful diffusion of MgO into the magnet, the coercivity increasing from 1094 to 1170 kA/m and the maximum energy product increasing from 240 to 261 kJ/m 3 , together with the enhanced temperature stability and corrosion resistance, have been demonstrated. The underlying mechanisms for these enhancements have been analyzed. Microstructural investigations show that MgO entered mainly into the intergranular regions and modified the composition and structure of the grain boundary phase. The intergranular Nd–O–Fe–Mg phases observed in the MgO diffused magnet contribute to the improved performance. The current non-RE compound grain boundary diffusion process has significance in further minimizing the use of rare earth (RE).

Anticorrosion performance of sintered NdFeB magnets by EDM in different dielectric fluids

Purpose – This paper aims to present an experimental investigation of improving the surface corrosion resistance of sintered neodymium-iron-boron (NdFeB) magnets by electrical discharge machining (EDM) in different dielectric fluids. Design/methodology/approach – Scanning electron microscope and X-ray diffraction were used to analyze the surface morphology and chemical structure of recast layers formed by EDM using kerosene and distilled water as the dielectric fluids. Polarization scans and electrochemical impedance spectroscopy were applied to investigate the post-machining corrosion resistance. Findings – The test results indicated that the recast layer produced during EDM had amorphous characteristics, and the newly formed amorphous structure could improve the corrosion resistance of the NdFeB material. The corrosion resistance of the recast layer formed in kerosene was better than that formed in distilled water. Originality/value – Surface corrosion modification of sintered NdFeB magnets by means of electrical discharge with an ordinary copper electrode is proposed in this paper. The layer formed by EDM exhibits different behavior to that of the interior of the bulk material and improves the anti-corrosion performance of NdFeB magnets.

Corrosion Behavior of Sintered NdFeB Magnets in Different Acidic Solutions

Abstract Corrosion behaviors of sintered NdFeB magnets in H 2 SO 4 solution, HCl solution and HNO 3 solution of different concentrations were investigated by pickling, immersion test and electrochemical test. The results show that the corrosion mechanism of sintered NdFeB magnets in HNO 3 solution is uniform corrosion, while the one in H 2 SO 4 solution and HCl solution is selectively intergranular corrosion. The corrosion rate of NdFeB magnets increases with increasing of the concentrations of the three acid solutions, and the corrosion rate of NdFeB magnets in the HNO 3 solution is the smallest among the three acid solutions both in immersion test and electrochemical test. Therefore, the HNO 3 solution is more suitable to be used as the pickling solution for the surface pretreatment of sintered NdFeB magnets.

Electroplating mechanism of nanocrystalline NdFeB film

Nanocrystalline NdFeB film was fabricated onto the copper substrate through direct current electroplating method, and characterized by scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscope (EDS), vibrating sample magnetometer and potentiodynamic polarization techniques. The initial electroplating behavior was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Results revealed that the corrosion resistance of the NdFeB film was better than that of the traditional sintered NdFeB magnet. The depositing process of the NdFeB film followed the three-dimensional nucleation and subsequent grain growth mechanism, and was controlled by charger transfer. With the increase of the negative potential bias, the deposition mechanism of NdFeB film changed from heterogeneous to homogeneous nucleation/growth, which consequently resulted in the decrease of charge-transfer-resistance.

Corrosion Resistance Analysis of Sintered NdFeB Magnets Using Ultrasonic-Aided EDM Method

Sintered neodymium-iron-boron (NdFeB) permanent magnets are widely used in many fields because of their excellent magnetic property. However, their poor corrosion resistance has been cited as a potential problem that limits their extensive application. This paper presents an experimental investigation into the improvement of surface corrosion resistance with the ultrasonic-aided electrical discharge machining (U-EDM) method. A scanning electron microscope was used to analyze the surface morphology of recast layers formed through the EDM and U-EDM processes. The chemical structure and elements of these recast layers were characterized using x-ray diffraction and energy dispersive spectroscopy. Corrosion resistance was also studied by means of potentiodynamic polarization, electrochemical impedance spectroscopy, and immersion tests in 0.5 mol/L H2SO4 solution. Experimental results show that an amorphous structure was formed in the recast layer during the EDM and U-EDM processes and that this structure could improve the corrosion resistance of sintered NdFeB magnets. Moreover, the corrosion resistance of U-EDM-treated surface was better than that of the EDM-treated surface.

Corrosion Behavior of Detonation Gun Sprayed Al Coating on Sintered NFeB

Pure Al coating was prepared by a detonation gun (D-gun) spraying process to protect sintered NdFeB magnets. The detonation gun sprayed coating is very uniform and has a low porosity of 0.77%. The thickness of the Al coating is approximately 16 μm. The corrosion current density for the coated sample was 1.30 × 10−5 A/cm2 immediately after immersion in 3.5% NaCl solution, compared to 6.54 × 10−5 A/cm2 for the uncoated sample. X-ray photoelectron spectrometry results indicate that the formation of Al2O3 film contributes to the increased corrosion resistance of Al coating. Meanwhile, electrochemical impedance spectroscopy with equivalent electrical circuit was used to ascertain the corrosion process of the Al coatings. Results show the corrosion procedure consists of two stages which agree with the potentiodynamic polarization test. It can be concluded that the Al coating deposited by the D-gun spray process can improve the corrosion resistance of sintered NdFeB.

Effects of Al coating on corrosion resistance of sintered NdFeB magnet

Pure Al coating was deposited on sintered NdFeB magnet by direct current (DC) magnetron sputtering to improve the corrosion resistance of magnet. The influences of coating thickness and sputtering power on microstructure and corrosion resistance of Al coating were investigated. The surface morphology of Al coating was characterized by scanning electron microscopy (SEM). The corrosion properties were investigated by potentiodynamic polarization curves and neutral salt spray (NSS) test. The formation of the uniform and compact Al coating is a necessary condition to achieve excellent corrosion resistance. And the optimal corrosion resistance can be obtained in the sample with 6.69 μm thick Al coating deposited at 51-82 W.

Dysprosium-free melt-spun permanent magnets

Melt-spun NdFeB powders can be formed into a number of different types of permanent magnet for a variety of applications in electronics, automotive and clean technology industries. The melt-spinning process produces flake powder with a fine uniform array of nanoscale Nd2Fe14B grains. These powders can be net-shape formed into isotropic polymer-bonded magnets or hot formed into fully dense magnets. This paper discusses the influence of heavy rare earth elements and microstructure on the magnetic performance, thermal stability and material cost of NdFeB magnets. Evidence indicates that melt-spun nanocrystalline NdFeB magnets are less dependent on heavy rare earth elements for high-temperature performance than the alternative coarser-grained sintered NdFeB magnets. In particular, hot-pressed melt-spun magnets are an attractive low-cost solution for applications that require thermal stability up to 175–200 °C.

Corrosion Resistance of Sintered NdFeB Magnets with Various Surface Coatings

Pure Ni coating and multi-layered Ni-P coatings were deposited on sintered NdFeB magnets by ion beam sputtering and electroless plating to improve the corrosion resistance of magnets, respectively. The structure and preferred growth orientation, surface morphology, the adhesive strength between coatings and magnets, and corrosion properties of samples were investigated by XRD, SEM, high-low temperature shock test and potentiodynamic polarization curves, respectively. It was found that electroless Ni-P coatings are amorphous with nodules distributed, Ni coating deposited by ion beam sputtering is crystalline and spherical Ni grains are densely packed. Ni coating deposited by ion beam sputtering can provide better protection for sintered NdFeB than that of electroless Ni-P coating.

Magnetic Behavior of Sintered NdFeB Magnets on a Long-Term Timescale

Stable polarization of permanent magnets over the lifetime of the application is an important aspect in electrical machine design. Specification of the long-term stability of magnet material is difficult, since knowledge of the phenomenon is incomplete. To be able to optimize magnet material selection, the long-term magnetic behavior of the material must also be understood. This study shows that material with a very squareJHcurve is stable until a certain critical operating temperature is reached. Major losses are detected as the critical temperature is exceeded. Material with a rounderJHcurve does not show a well-defined critical temperature, but increasing losses over a large temperature range. The critical temperature of a material is also dependent on the field conditions. Results differ whether the tests are performed in an open or closed magnetic circuit. In open-circuit tests, the opposing field is not homogeneously distributed throughout the volume of the magnet and thus the long-term behavior is different than that in closed-circuit conditions. Open-circuit tests seem to give bigger losses than closed-circuit tests in cases where the permeance coefficient of the open-circuit sample is considered to be the average permeance coefficient, calculated according to the dimensions of the magnet.

Metal Injection Molding (MIM) of NdFeB Magnets

Due to the increased and unstable prices for Rare Earth elements there are activities to develop alternative hard magnetic materials. Reducing the amount of material necessary to produce complex sintered NdFeB magnets can also help to reduce some of the supply problem. Metal Injection Molding (MIM) is able to produce near net shape parts and can reduce the amount of finishing to achieve final geometry. Although MIM of NdFeB has been patented and published fairly soon after the development of the NdFeB magnets there has never been an industrial production. This could be due to the fact that MIM was very young at that time and hardly developed. Thus, the feasibility of the process needs to be revaluated. This paper presents results of our work on determining the process parameters influencing the magnetic properties of the sintered magnets as well as the shrinkage during processing. The role of binder and powder loading on the alignment of the particles as well as on the carbon and oxygen contamination was examined.

Study on The Corrosion Behavior of NdFeB Permanent Magnets in NaCl Solutions with PVD Techniques

The corrosion resistance of NdFeB magnets is improved by direct NiCrAlV and NiCrAlVN thin film treatment of a sintered NdFeB magnet surface. X-ray diffraction and transmission electron microscopy are used to examine the microstructure of the thin films. The hardness, wettability, and surface roughness of the thin films are investigated using nanoindentation, contact angle measurements, and atomic force microscopy, respectively. The NiCrAlV and NiCrAlVN thin films exhibit a typical amorphous microstructure and a smooth surface with an average roughness of about 2 nm. The results of alternating-current (ac) impedance spectroscopy in 5% NaCl solution show that the ac impedance of the thin films increased with time. The |Z |10 mHz values for NiCrAlVN ( 15\thinspace000 ohms/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), NiCrAlV ( 4000 ohms/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and uncoated NdFeB magnets ( 550 ohms/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) show an eight-fold increase in the low-frequency impedance due to the amorphous state.

Cold-Sprayed Al Coating for Corrosion Protection of Sintered NdFeB

A protective Al coating was achieved on the sintered NdFeB magnet by cold spray. The sprayed Al particles generate plastic deformation and hang together. The thickness of the coating is about 170 μm. The corrosion currents of Al coating and NdFeB without immersion tested by potentiodynamic polarization in 3.5 wt.% NaCl solutions are 1.350 × 10−6 and 4.361 × 10−6 A/cm2, respectively. X-ray photoelectron spectrometry results confirm that the oxide film is Al2O3 and the corrosion process can be derived into two different stages. The Al coating can provide long-term protection for NdFeB effectively.