Preparation and Investigation of Anisotropic HDDR NdFeB Magnetic Powders
The NdFeB permanent magnetic materials with the greatest properties and developmental future can be divided into the sintered and bonded NdFeB magnets, according to their different preparation processes. Compared with the sintered magnets, the preparation technology of bonded magnets is simple and has low cost, high toughness and performance of resisting the damage and crack. Thus, the bonded magnets can be prepared into products with different shapes. The anisotropic magnetic powders used for producing the bonded magnets are usually prepared by the HDDR(Hydrogenation, Decomposition, Desorption, Recombination)process. By using this process, the original large Nd2Fe14B grains in the cast ingots are transformed into fine grains with diameter of around 0.3¦̀m, which is close to single domain size of Nd2Fe14B phase. The special grain microstructure of HDDR magnets is different from that of both sintered magnets and nanocomposite magnets.
Due to the coercivity of magnets closely related to the grain microstructure, so, the coercivity mechanism of HDDR bonded magnets is different from not only that of sintered magnets, but also that of nanocomposite magnets. So far, the coercivity mechanism of HDDR bonded magnets has not been completely clear yet.The HDDR magnetic powders are commonly prepared from the segregated master ingots, and their magnetic properties are low due to the existence of soft magnetic phase¦Á-Fe in the master ingots. The alloy flakes prepared by the SC (strip casting) process have a good columnar crystalline structure, the main phase Nd2Fe14B is uniformly separated by the symmetrical Nd-rich phase fine lamella and a-Fe is nonexistent, which is very suitable for preparing the HDDR magnetic powders. Morimoto et al. reported that the HDDR magnetic powders prepared directly from the SC alloy flakes without any heat treatment are isotropic. Subsequently, they prepared the anisotropic magnetic powers by homogenizing SC alloy flakes before the HDDR process. Up to date, the preparation of anisotropic magnetic powers directly from the SC alloy flakes without any heat treatment has still not been reported.Early experiment results indicated that the addition of elements such as Co, Zr, Nb and Ga is prerequisite for the inducement of anisotropy in the NdFeB-type alloys treated by the HDDR process. The subsequent experiments showed that purely ternary NdFeB anisotropic magnetic powders can also be obtained by using a modified HDDR process treatment.
Not only the addition elements, but also the HDDR process plays an important role in the inducement of anisotropy. Because the composition and HDDR process used for preparing anisotropic magnetic powders have not ripened completely, thus, the inducement mechanism of anisotropy of magnetic powders during HDDR process has not been clear completely.Our research work includes the theoretical and experimental investigation. Theoretically, considering the special microstructure of HDDR magnetic powder grains, we established a theoretical model of anisotropy at the grain boundaries, and investigated the coercivity mechanism of HDDR NdFeB magnets and the effect of intergranular phase on the coercivity of magnets. Experimentally, we prepared the NdFeB magnetic powders by using the SC+HDDR process, probed the possibility of preparing anisotropic magnetic powders directly from hexahydric alloy Nd12.8Fe72Co7.8B7Zr0.1Ga0.3 flakes without any heat treatment, and further clarified the origin of anisotropy of HDDR magnetic powders taking Nd13.5Fe79.5B7 as example.
As long as one reasonably adjusts the alloy¡¯s composition and technical process, to ensure the HDDR magnets containing small amount of Nd-rich phase, and excluding a-Fe, the HDDR NdFeB magnets with high performance could be obtained. When d is 1nm and the structure defect thickness ro is close to the thickness of domain wall (-4 nm), the calculated coercivity based on the two different anisotropic expressions are both consistent well with the interrelated experiment results.2. Experimental research1) The experiment equipments were updated and transformed according to the experimental requirement. The wheel speeds and quartz nozzle pressure of SC process were steadied and controlled, respectively, the diffusion furnace was transformed, the air inlets were increased, and the aero-pressure control system was deployed.2) The NdFeB alloy flakes were prepared by using the SC process, and the effects of wheel speeds of SC process on the microstructure of alloy flakes were investigated. The China magnets results indicated that the alloy flakes prepared with wheel speed of 3m/s display the optimal microstructure. The flakes contain the main phase Nd2Fe14B phase lamellae with a width ranging from 0.5 to 2¦̀m separated by about 0.05 to 0.1¦̀m wide Nd-rich phases. The main phase lamella crystals show parallel orientation. The Nd-rich phase is distributed uniformly and a-Fe isn¡¯t present. Such SC alloy flake subjected to the HDDR process can obtain the fine powder grains with a uniform dimension distribution. If the wheel speeds are slower than 3m/s, the free surface of SC alloy flake displays a-Fe dendrites.
However, if the wheel speeds are higher than 3m/s, there is a pool region of Nd-rich phases near the cooling surface, which leads to the segregation of the Nd-rich phase. Both higher and lower cooling speeds will all lead to the reduction of magnetic performance of powders. Thus, the wheel speed of 3m/s is considered the optimized cooling speed.3) Using the hexahydric alloy flakes with the composition of Nd12.8Fe72Co7.8gB7Zr0.1Ga0.3, we investigated the effect of hydrogen pressure of slow recombination treatment (HPSR) on the magnetic performances of powders, and probed the possibility of preparing anisotropic magnetic powders directly from the SC alloy flakes without any heat treatment. The results indicated that the remanence Br, coercivity Hcj, magnetic energy product (BH)max and degree of orientation of anisotropy (DOA) all increase firstly, and then decrease with increasing HPSR. While the HPSR is 30kPa, the magnetic powders are obviously anisotropic, and the magnetic performances achieve the maximum values of Br=1.3T, Hcj=954.3kA/m, (BH)max=259 kJ/m3 and DOA=0.87, respectively. Morimoto et al. reported that the HDDR magnetic powders prepared directly from the SC alloy flakes without any heat treatment are isotropic. The difference between two processes is that in the recombination stage during the HDDR process, the disproportionated products are firstly carried out a slow desorption-recombination reaction in low hydrogen pressure before they are carried out a fast desorption-recombination reaction in high vacuum in our paper, however, the disproportionated products are directly subjected to a fast desorption-recombination reaction in high vacuum in Morimoto¡¯s paper.
When the disproportionation time further increases, the lamella crystal completely disappears, and transforms into the columnar crystal. This illustrates that the obvious anisotropy of the magnetic powders corresponding to a short disproportionation time originates from the lamella disproportionated mixture. This may be attributed to that the lamella disproportionated mixture (intermediate phase) remains or inherits the alignment of hard magnetic phase of original SC alloys, and may also be related to the alignment of the newly formed Nd2Fe14B grain, which results in the magnetic powders displaying the evidently anisotropic alignment. The generating mechanism of anisotropy is in accordance with "anisotropy-mediating phase" model.Theoretically, the model of anisotropy at the grain boundaries of HDDR NdFeB magnets was established based on the special microstructure of HDDR grains, and the demagnetization process and coercivity mechanism of magnets were understood, which will provide an importantly theoretical reference for preparing the magnets with high coercivity.Our experimental investigation indicated that whether or not adds the element, the anisotropic NdFeB magnetic powders can be obtained by using the SC alloy flake without any heat treatment subjected to an modified HDDR process treatment. Whether the element added and SC flakes subjected to the homogenization heat treatment or not are not necessary for obtaining the anisotropic magnetic powders. The key to the China magnets adjustment of HDDR process, i.e., appropriately speed disproportionation reaction course, slow desorption-recombination reaction course, and control the appropriate hydrogen pressure during the slow recombination stage. Our research avoids the homogenizing heat treatment which expends long time and huge amounts of energy, so, it will provide an important reference for reducing cost and preparing highly anisotropic magnetic powders and bonded magnets.
Tags: neodymium magnets Anisotropic HDDR, NdFeB Magnetic Powders Smco magnets