Hydrogenation Disproportionation Desorption Recombination Research Articles

Recycling of additively printed anisotropic Nd-Fe-B bonded magnets

A high potential cost-effective and environmentally friendly method has been applied for recycling anisotropic Nd-Fe-B bonded magnets. Waste additively printed Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) Nd-Fe-B anisotropic bonded magnet was pulverized into composite powder containing Nd-Fe-B particles and nylon binder through cryomilling at a liquid nitrogen temperature (∼77 K) under Ar inert atmosphere. Then, the cryomilled composite powder was warm compacted into a bonded magnet. The magnetic particles were aligned during post-compaction annealing under a magnetic field of 30 kOe. The recycled bonded magnets have a higher density (3 % enhancement), but slightly inferior magnetic properties compared to the original magnets, i.e., the magnetic remanence, coercivity and maximum energy product are reduced by 2 %, 3 % and 8 %, respectively. The scanning electron microscopy revealed that some HDDR Nd-Fe-B powder crumbled into fine particles during cryomilling. Powder X-ray diffraction showed a small amount of Nd-oxide impurity in the cryomilled powder. The slightly deteriorated magnetic properties are ascribed to the oxidation of Nd-Fe-B particles due to formation of fresh fracture surface during cryomilling. The approach enables the direct reuse of end-of-life bonded magnets in an economical and environmentally friendly way.

Effects of Misalignment of c-axis on the Properties of Hydrogenation-Disproportionation-Desorption-Recombination Particles.

Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) Nd2Fe14B particles have excellent magnetic properties, but the magnetic properties of powder are not uniform across different particle sizes. The remanence and maximum magnetic energy products of samples with a particle size of 120 μm are 14.0 kGs and 41.35 MGOe, while the products of samples with a particle size of 60 μm are only 13.3 kGs and 36.31 MGOe. The macroscopic morphology of HDDR Nd2Fe14B particles and the gradient distribution of microstructures in different micro-regions were observed. By modifying the macroscopic morphology of the particles, the poorly oriented clusters on the surface of the particles were precisely eliminated, and the remanence and maximum magnetic energy products of the particles increased to 14.5 kGs and 45 MGOe, respectively. Compared with the original particles, the samples after mechanical grinding had better grain arrangement. The effects of the nanocrystalline c-axis and field misalignment angle θ on the magnetic properties of HDDR Nd2Fe14B particles were investigated through micromagnetic simulation. The targeted removal of macroscopic defects on the particle surface contributed to a 3.6% increase in remanence and an 8.8% increase in the maximum magnetic energy product, offering a promising approach to enhance the microstructure of high-performance HDDR Nd2Fe14B particles.

Influence of preparation processes with or without long-time homogenization heat treatment on performance of Nd–Fe–B HDDR products

We investigated the influence of hydrogenation-disproportionation-desorption-recombination (HDDR) preparation processes, with or without long-time homogenization heat treatment, on the microstructure and hard magnetic properties of Nd12.7B6.5Co2.1Cr0.1Al0.5Nb0.3Ga0.5Fe77.3 powders/bonded magnets. Hard magnetic properties such as remanence (Br = 1.01 T), coercivity (Hcj = 949.39 kA/m), and maximum magnetic energy product (BH)max = 181.54 kJ/m3 were achieved for HDDR bonded magnets with homogenization heat treatment. In contrast, magnets prepared without homogenization heat treatment exhibited Br = 0.65 T, Hcj = 854.69 kA/m, and (BH)max = 66.98 kJ/m3. The notable decrease in remanence for magnets prepared without homogenization treatment is attributed to the presence of α–Fe in the parent alloy, which inhibits the hydrogenation-disproportionation process and consequently affects the orientation of the crystallographic texture. Furthermore, we fabricated bonded quadrupole magnetic rings using HDDR powder prepared with homogenization heat treatment. The study also explored the relationship between disproportionation temperature and surface magnetic field intensity for these quadrupole magnetic rings.

Pre-orientation sintering: an alternative process to prepare high-performance anisotropic Nd–Fe–B HDDR magnets

Hydrogen–disproportionation–desorption–recombination (HDDR) Nd–Fe–B magnetic powders are promising to prepare bulk anisotropic magnets, but high magnetic performance has not been achieved due to the absence of an Nd-rich phase in the powder and the low degree of orientation of the bulk magnets. In this study, an alternative process of pre-orientation sintering via magnetic alignment followed by spark plasma sintering was proposed to prepare the precursor of hot-deformation (HD) magnets, and a high maximum energy product of 294 kJ m−3 was achieved in the HD magnet with a relatively low height reduction of 35%, then an improved coercivity of 1107 kA m−1 could be obtained followed by a grain boundary diffusion of Pr40Tb30Al20Cu10. Microstructure analysis indicates that pre-orientation of HDDR powders facilitates grain rotation and alignment during the HD process, thereby reducing the minimum deformation ratio. It helps to obtain the deformed grains with lower shape anisotropy and smaller grain size, enabling a good compatibility of magnetic and mechanical behaviors. In-situ Lorentzian transmission electron microscopy results show that the magnetic domains have been strongly fixed by the thick intergranular RE-rich phase and the fully Tb-diffused grains, which contributes to the improved coercivity after grain boundary diffusion. This study provides a guiding significance for the preparation of the anisotropic Nd–Fe–B HDDR magnets with optimized performance.

Design of High-Remanence Nd-Fe-B Hot-Pressed Magnets by Manipulating Coercivity of Hydrogenation-Disproportionation-Desorption-Recombination Treated Anisotropic Precursors.

We propose a method of manipulating the coercivity of anisotropic hydrogenation-disproportionation-desorption-recombination (HDDR) powders to fabricate high-remanence and fine-grained Nd-Fe-B magnets using only hot-pressing without a subsequent hot-deformation process. By reducing the Nd content of anisotropic HDDR precursors such that their coercivity (Hcj) is lowered, the c-axis of each HDDR particle is well-aligned parallel to the direction of the applied magnetic field during the magnetic alignment step. This is because the magnetic repulsive force between adjacent particles, determined by their remanent magnetization, decreases as a result of the low coercivity of each particle. Therefore, after hot-pressing the low-Hcj HDDR powders, a significantly higher remanence (11.2 kG) is achieved in the bulk than that achieved by hot-pressing the high-Hcj HDDR powders (8.2 kG). It is clearly confirmed by the large-scale electron backscatter diffraction (EBSD) analysis that the alignment of the c-axis of each anisotropic HDDR particle in the bulk is improved when low-Hcj HDDR powders are used to fabricate hot-pressed magnets. This coercivity manipulation of HDDR powders can be a helpful method to expand the use of HDDR powders in fabricating anisotropic Nd-Fe-B bulk magnets.

Effect of Phosphate Treatment on the Corrosion Resistance of Nd–Fe–B Anisotropic Magnetic Powder

Anisotropic Nd–Fe–B hydrogenation disproportionation desorption recombination powder with high corrosion resistance was prepared by phosphoric acid treatment. The treated samples baked at temperatures higher than 250 °C exhibited high corrosion resistance. In addition, the change ratio of coercivity before and after the anticorrosion test was within 5%. Microstructural observations indicated the formation a uniform phosphate layer with a small amount of Nd oxide at high baking temperature.

Anisotropy optimization of HDDR magnetic powders by precursor alloy annealing

The anisotropy of hydrogenation–disproportionation–desorption–recombination (HDDR) method to form Nd2Fe14B powders was effectively improved by annealing the strip casting (SC) alloy precursors at 1080 °C. According to the analysis of the evolution of microstructure, the excellent anisotropy was attributed to the nearly consistent orientation of the nanoscale Nd2Fe14B grains in the HDDR magnetic powders, which perfectly inherited that of the coarse grains in the treated SC alloy flakes. In contrast, for the untreated specimen, more nanoscale clusters of different orientations were generated after smashing and HDDR treatment of fine columnar Nd2Fe14B grains with different orientations in the original SC alloy, severely affecting the anisotropy of the magnetic powders. This study provides new insights into the development of high-anisotropy and high-performance Nd-Fe-B magnets.

Effect of temperature on particle shape, size, and polycrystallinity of Nd-Fe-B powders obtained by hydrogen decrepitation

This work presents a detailed study of hydrogen decrepitation (HD) to obtain monocrystalline Nd-Fe-B powder. The effect of decrepitation temperature has been investigated to optimize both particle size and shape. Differential scanning calorimetry was applied to analyze the hydrogenation kinetics of Nd2Fe14B and Nd-rich phases in the range of 25 to 300 °C. Thermogravimetry and X-ray diffraction allowed determining the hydrogen absorption of the whole alloy and the matrix phase, respectively. While scanning electron microscopy (SEM) was used to visualize particle shape and size, dynamic image analysis was applied to evaluate quantitatively these properties. The high monocrystallinity of the powder was confirmed by electron backscattering diffraction. The partial pressure of hydrogen required to initiate the hydrogenation reactions decreases when the temperature increases. The hydrogen absorbed by the whole alloy and, in particular, by the Nd2Fe14B phase decreases with temperature. Below 150 °C, the hydrogen absorbed by the Nd2Fe14B phase produces a significant transgranular cracking that is undesirable for particle shape. At 300 °C, the fast and limited absorption of hydrogen by the Nd-rich phase causes insufficient intergranular fracture and, hence, polycrystallinity. Between 150 and 300 °C, the controlled fragmentation resulted in monocrystalline particles with a more equiaxial shape, which is a suitable precursor to develop anisotropic ultrafine powders by the hydrogenation, disproportionation, desorption, recombination (HDDR) process.

Coercivity enhancement of hydrogenation disproportionation desorption recombination Nd-Fe-B powders by grain boundary diffusion process using Tb70Cu15Al15 alloy

The hydrogenated disproportionation desorption recombination (HDDR) Nd-Fe-B powders mixed thoroughly with Tb70Cu15Al15. The samples were subjected to a grain boundary diffusion process using various heat treatment conditions. The results show that the coercivity of the magnetic powders is significantly influenced by the diffusion conditions. Under the optimum treatment process, the coercivity of treated powders increased from 1074.3 kA/m to 1424.4 kA/m. Furthermore, the irreversible flux loss of the treated powders was also dramatically decreased by 50% at 150 °C for 100 hrs, demonstrating a significant improvement in temperature stability. The nanoscale core–shell structure leading to a notable improvement in magnetic properties was observed using transmission electron microscopy (TEM). Micromagnetic simulations were used to explain how diffusion processes affected the properties of magnetic powders.

Reduction of the mechanical anisotropy in hot-deformed Nd-Fe-B magnets by the construction of heterogeneous structure

Conventional hot-deformed Nd-Fe-B magnets exhibit significant mechanical anisotropy in the two directions perpendicular and parallel to c-axis, resulting in a complicated machining process. In this work, hydrogenation-disproportionation-desorption-recombination (HDDR) powders were added to reduce the mechanical anisotropy of hot-deformed magnets. The effects of the addition of HDDR powders on the compressive strength of hot-deformed magnets and their corresponding microstructures have been investigated. The mechanical anisotropy was maximally decreased by 68 % at 20 wt. % addition. Microstructure observation indicated that a heterogeneous structure containing two kinds of grains with different morphology was formed after the addition of HDDR powders. Further analyses revealed that the reduction of mechanical anisotropy was mainly attributed to the change of the shape anisotropy of grains and the disruption of the regular stacking structure of the melt-spun ribbons. This work provides a new insight for the preparation of high performance hot-deformed Nd-Fe-B magnet with outstanding comprehensive service capability.

On the feasibility of using Sm2Fe17Nx powders obtained via HDDR process for laser powder bed fusion of bonded permanent magnets

Powders to be used as the main feedstock component for additive manufacturing processes, such as Laser Powder Bed Fusion (LPBF) technique, must present suitable technological properties, mainly high values of apparent density (AD) and flow rate (FR). These features are directly related to particle size, morphology, and size distribution. Although these issues have already been addressed to feedstocks based on Nd-Fe-B powders, the potential use of Sm-Fe-N powder as a raw material for feedstock processes has been scarcely explored thus far. If on one hand it is well established in the literature that Sm-Fe-N particles should be small (≈ 2 μm) to present sufficiently high values of intrinsic coercivity (Hcj, > 700 kA/m), on the other hand, however, smaller particles exhibit AD and FR values that are not appropriate for use in LPBF. An alternative to circumvent this apparently conflicting situation is to refine the microstructure of the particles in order to increase coercivity, instead of reducing particle size itself, which could be achieved via Hydrogenation-Disproportionation-Desorption-Recombination (HDDR). Considering this context, the main objective of this work is to study the feasibility of using Sm2Fe17Nx-based powders produced via the HDDR process followed by nitrogenation to obtain as-printed bonded magnets by the LPBF process. AD and FR of sieved powders with two distinct granulometry (< 45 μm and 45 < x < 63 μm) were evaluated. The obtained HDDR Sm-Fe-N powders presented values of Hcj = 683–777 kA/m, comparable to commercial nanocrystalline powders obtained by melt spinning. Powders were mixed with polyamide-12 (PA12) powder and used as feedstock to obtain bonded magnets by the LBPF technique. Microstructural properties of the as-printed bonded magnets were analyzed by Scanning Electron Microscopy (SEM). The effect of the LPBF process on the structural and magnetic properties of HDDR Sm-Fe-N powders was evaluated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). Both coarser and finer powders presented no structural modifications or reduction in magnetic property values after the laser processing. However, denser as-printed bonded magnets were obtained using the coarser one, emerging as a potential candidate to be used as feedstock in LPBF process.

Magnetic and microstructural modification of the Dy-free HDDR Nd-Fe-B powders with micron BN addition by the grain boundary diffusion engineering

The micron BN powders were studied as diffusion sources for the grain boundary diffusion process to HDDR (hydrogenation-disproportionation-desorption-recombination) Nd-Fe-B powders. The coercivity of HDDR powders was enhanced by about 52.1 %. The only downside is that the remanence was slightly reduced because BN powders were a non-magnetic additive. Microstructural investigations showed that the nonferromagnetic Nd-rich grain boundary phases envelop the Nd2Fe14B grains because the wettability of the interface between the matrix and the grain boundary phases was improved by diffusing BN powders. The preferred residence of BN in the intergranular phases and their reaction with the grain boundary phases might both contribute to the enhancement of the coercivity. As a result, the research and development of the grain boundary diffusion process with non-rare earth materials have significance in further minimizing the use of rare earth (RE).

Grain boundary infiltration in HDDR processed Nd2Fe14B magnets

We investigate the grain boundary infiltration process of various low melting eutectic alloys for the coercivity enhancement of hydrogenation disproportionation desorption recombination (HDDR) processed Nd-Fe-B powders. Nd-based as well as heavy rare earth (Tb) and light rare earth (La, Ce) containing alloys were systematically studied: Nd70Cu30, Nd90Al10, Nd80Ga15Cu5, Nd62Fe14Ga20Cu4, Nd60Tb10Cu30, La71Cu29 and Ce72Cu28. Moreover, the Fe content in the quaternary Nd-Fe-Ga-Cu system was varied to investigate the effect of grain boundary phase magnetism on the resultant coercivity.The largest coercivity enhancement, from 0.42 T in the as-HDDR powder to 1.88 T after infiltration was observed in the case of ternary Nd80Ga15Cu5 composition. Furthermore, it also shows the best temperature stability with the infiltrated sample still exhibiting a coercivity of 0.58 T at 200 °C. Infiltration of light rare earth (La, Ce) based alloy did not increase coercivities due to poor wetting at the grain boundaries. Adding Fe to the grain boundary alloys was shown to enhance magnetization up to a certain extent without significant loss in coercivity. These findings demonstrate the effectiveness of grain boundary infiltration in HDDR-processed magnets without using heavy rare earths. The infiltration with non-magnetic material strongly decreases the intergranular interaction, reflected in a different magnetic domain evolution during the magnetization reversal process.

Preparation of anisotropic (Ce, Nd, Pr)-Fe-B powder with HDDR method from wasted sintered magnets

With the wide application of Ce-contained rare earth (RE) permanent magnets, recycling of wasted Ce-contained magnets has drawn more and more attention. Hydrogenation-disproportionation-desorption-recombination (HDDR) method was used to prepare anisotropic magnetic powder from wasted (Ce, Pr, Nd)-Fe-B sintered magnet (N45 magnet). The influence of hydrogen pressure in disproportionation (Pdis) and desorption (Pdes) and the disproportionation time (tdis) on the magnetic properties and microstructures of HDDR powders was studied. The Pdis, Pdes and tdis were optimized to be 50 kPa, 5 kPa and 5 h, respectively. The optimized anisotropic HDDR (Ce, Pr, Nd)-Fe-B powder possessed the coercivity of 10.15 kOe, remanence of 11.14kG and the degree of alignment of 0.77. This coercivity has reached 83 % of that of the recycled N45 magnet. Microstructure examination with scanning electron microscope revealed that the optimized HDDR powder is consisted of flake-shaped particles with hierarchical structure. The distribution of rE-rich phase has been significantly influenced by Pdis, Pdes and tdis. The optimized HDDR particles exhibited the best homogeneous distribution of rE-rich phase. The result of transparent electron microscopy (TEM) and energy dispersive spectrometry demonstrated the nature of (Ce, Pr, Nd)-rich phase and (Ce, Pr, Nd)2Fe14B for the rE-rich phase and RE2Fe14B grain phase, respectively. Examination with high-resolution TEM revealed that the optimized tdis resulted in the largest thickness of (Ce, Pr, Nd)-rich phase at grain boundaries and the consequent maximum coercivity of HDDR particles. The orientation of the optimized HDDR particles was analyzed with X-ray diffractometer and electronback-scatter diffractometer. The result revealed well textured HDDR particles and the subordinate nanograins with preferred (001) orientation, suggesting the inherited texture of these HDDR particles from the initial (Ce, Pr, Nd)2Fe14B grains in N45 sintered magnet. This HDDR method was proved to be a prospective strategy in recycling of Ce-contained sintered magnets.

High Coercive RE-Fe-B Powders Recycled via HD and HDDR Process from Waste Magnets

In order to fabricate high-coercivity hard magnetic RE-Fe-B powders from the waste bulk magnets, the hydrogen decrepitation (HD) and hydrogenation-disproportionation-desorption-recombination (HDDR) processes were employed to the waste RE-Fe-B magnets. The optimum HD and HDDR conditions for the waste RE-Fe-B magnets were established through the systematic investigation on the effect of HD and HDDR treatment temperature on the magnetic and microstructural properties of final recycled powders. We found that the waste magnets were crushed into the flakes with a uniform size distribution when the HD treatment was carried at 200 ℃. By applying the HDDR process to the HD treated flakes at 920 ℃, high coercivity of 11.2 kOe could be achieved in the recycled RE-Fe-B HDDR powders. Based on the results from the microstructural characterization, the underlying reasons for the magnetic and microstructural changes of the recycled powders as a function of the HD and HDDR treatment temperature is discussed in detail.

Magnetic properties and microstructures of hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B powders by grain boundary diffusion of Nd-Cu-Al

The relatively low coercivity of the anisotropic bonded Nd-Fe-B powders prepared by hydrogenation-disproportionation-desorption-recombination (HDDR) method hinders their application in high-tech fields, such as new energy vehicles and robots. The low melting point Nd80Cu10Al10 (wt.%) ternary alloy is employed as the source for grain boundary diffusion to enhance the coercivity of the HDDR powders. The effect of the content and diffusion temperature on the magnetic properties is systematically investigated. It is found that the coercivity is enhanced after grain boundary diffusion. The coercivity increases monoclinically from 1026.0 to 1301.0 kA m−1 with the addition of 2% ∼ 6% (wt.%) diffusion source at 820 °C. Through the initial magnetization curve, the S-shape indicates that the coercivity mechanism is more inclined to pinning type. Meanwhile, the derivation curve shifts towards higher value gradually, indicating that the pinning field increases due to the grain boundary diffusion. Correspondingly, TEM micrographs show that the content of the grain boundary phase is increased, and the grain boundary is widened. In the range of 770-870℃, the coercivity increases at first and then decrease as the temperature exceeds 820 °C. The maximum of the coercivity of the HDDR powders of Hcj = 1301.0 kA m−1 is obtained with the addition of 6 wt% Nd-Cu-Al, diffusion treated at 820 °C.

An engineering route to synthesize stable bulk nanocrystalline magnesium with an average grain size of 20 nm

A simple and effective route to preparing bulk nanocrystalline (NC) pure Mg with an average grain size of 20 nm is first proposed by hydrogenation-disproportionation-desorption-recombination (HDDR) followed by spark plasma sintering (SPS) and hot extrusion (HE) techniques. NC Mg is thermodynamically super-stable against annealing at 550 °C and straining under compression and tension. Moreover, the as-extruded NC Mg samples show high tensile yield stress (TYS) of 259 MPa and compressive yield strength (CYS) of 157 MPa, which are about 2 times and about 7–10 times higher than those of their coarse-grain counterparts and the as-cast samples, respectively.

Study on Magnetization Reversal Processes of Anisotropic HDDR Pr2Fe14B-Type Magnetic Materials

The magnetization reversal processes of hydrogenation–disproportionation–desorption–recombination (HDDR) anisotropic Pr–Fe–B magnetic powders were systematically investigated. It is found that with the increase of external magnetic field, the magnetization first increases rapidly, and then increases slowly, and finally increases rapidly again. The increasing trend of coercivity with the increase of external magnetic field is just opposite to that mentioned above. Further micromagnetic simulation studies suggest that the coercivity is mainly controlled by the nucleation of reversed domain, and this is also indicated by the subsequent investigation for the evolution of the magnetic domain under the magnetic field.

Coercivity Enhancement of Hydrogenation Disproportionation Desorption Recombination Nd-Fe-B Powders by Grain Boundary Diffusion Process Using Tb70cu15al15 Alloy

Coercivity Enhancement of Hydrogenation Disproportionation Desorption Recombination Nd-Fe-B Powders by Grain Boundary Diffusion Process Using Tb70cu15al15 Alloy

Coercivity improvement in Nd–Cu infiltrated Nd-Fe-B hot-deformed magnets by controlling microstructure of initial HDDR powders

We have studied influence of the microstructure of initial hydrogenation-disproportionation-desorption-recombination (HDDR) treated isotropic powders on the magnetic and microstructural properties of Nd–Cu infiltrated Nd-Fe-B hot-deformed magnets. To control the microstructure of the initial HDDR powders, the starting strip-cast alloys were heat-treated prior to the HDDR process (HDDR powder A). Compared to the HDDR powders prepared from the un-treated strip-casts (HDDR powder B), non-uniform and discontinuous Nd-rich intergranular phases were formed in the HDDR powder A. Therefore, most of the Nd-rich phases in the HDDR powder A were agglomerated at the triple junctions after the hot-deformation (Hot-deformed magnet A). In contrast, homogeneous Nd-rich intergranular phases enveloping the laterally elongated grains are observed in the hot-deformed HDDR powder B (Hot-deformed magnet B). As a result, the coercivity of hot-deformed magnet A is lower than that of the hot-deformed magnet B. Interestingly, after the Nd–Cu infiltration, the coercivity of hot-deformed magnet A increases more rapidly than that of the hot-deformed magnet B because the faster diffusion of liquid Nd–Cu via the c-plane intergranular phases occurs in the hot-deformed magnet A, while the agglomeration of Nd–Cu into the triple junctions occurs in the hot-deformed magnet B. High resolution transmission electron microscopy showed that the Nd–Cu infiltration on the hot-deformed magnet A leads to the formation of thicker and non-ferromagnetic intergranular phases on the (001) planes of Nd 2 Fe 14 B grains, resulting in the magnetic isolation of individual grains and increase of pinning force against the domain wall motion.