Study on local demagnetization behavior of sintered NdFeB rare earth permanent magnets

The excellent magnetic properties of sintered NdFeB promote the development of miniaturization and high energy efficiency of permanent magnet synchronous motor. However, the reverse magnetic field, abnormal temperature rise and mechanical vibration in the working environment may lead to irreversible demagnetization, seriously affect the output power and torque of the motor, and even cause serious motor failure. At present, the risk assessment of irreversible demagnetization is mostly focused on the material flux In this paper, the local irreversible demagnetization behavior which may be caused by the reverse magnetic field in the actual working process of the motor is simulated under the off load condition, and the local demagnetization behavior and microstructure of the material are studied, which has important reference value for the application risk assessment and risk control of the material under the actual working condition of the permanent magnet motor.

As the third generation of rare earth permanent magnet materials, sintered NdFeB has excellent room temperature comprehensive magnetic properties and absolute cost advantages compared with the previous two generations of samarium cobalt materials [1]. Since it was discovered in 1983, it has attracted continuous research investment all over the world and has been widely used [2]. However, the lack of stability of sintered NdFeB materials is also very obvious. The reverse magnetic field and local magnetic field in the working process of permanent magnet motor are also obvious Both temperature rise and mechanical vibration may affect the output magnetic field of sintered NdFeB materials. In extreme cases, the complete demagnetization of permanent magnet will lead to the complete failure of the motor. In order to ensure that the material can meet the actual requirements of the motor, the overall flux change of the material at different temperatures is usually tested before installation to evaluate the influence of irreversible demagnetization of the magnet on the motor.
Motor simulation results have shown that according to different shapes, the demagnetization process of the permanent magnet in the motor often starts from some areas, so it is of great practical significance to study the local demagnetization behavior of the permanent magnet for material development and motor design [3]. So far, the research on the local demagnetization process of the permanent magnet is still in the simulation level, because the demagnetization cost of the whole machine is high and the cost is low Due to the lack of effective characterization methods, the experimental verification of local demagnetization of permanent magnets in the industry is still blank. This paper designs and optimizes the research methods of local demagnetization characteristics of permanent magnets in the off installation state, selects representative magnets for testing, analysis and verification, and preliminarily discusses the relationship between local demagnetization behavior and microstructure of materials.

Test method and process

N45h and n50h sintered NdFeB permanent magnet materials were selected to fabricate 40 × 30 × 5mm permanent magnet. Local demagnetization of the permanent magnet was realized by adding local reverse demagnetization field at two symmetrical positions of the magnet center. The surface magnetic field distribution of the two materials was characterized by three-dimensional waveform tester after local reverse demagnetization field was applied. The surface magnetic field distribution was analyzed by scanning electron microscope (SEM) The intrinsic magnetic properties of the two materials were tested by high-precision pulsed magnetic field testing equipment.

Result Analysis

The reverse magnetic field required to reduce the magnetization or magnetic induction to zero after saturation of permanent magnet is called coercivity. Coercivity is the core index parameter of permanent magnet materials, which reflects the difficulty of the process of demagnetization.
As shown in Table 1, the coercivity of n45h and N50h sintered NdFeB permanent magnet materials under indoor conditions is 19.28koe and 19.84koe respectively. Therefore, by applying 2T reverse magnetic field locally to the two materials, irreversible demagnetization occurs inside the permanent magnet. The surface magnetic fields of the two materials are characterized after applying the reverse magnetic field locally. The results are shown in Fig. 1 and Fig. 2.
Table.1 room temperature intrinsic magnetic properties of two kinds of sintered NdFeB permanent magnets

Grade	Br（kGs)	Hcb（kOe)	Hcj（kOe）	BHmax（MGOe)
N45H	13.3	12.64	19.28	42.53
N50H	14.05	13.67	19.84	48.13

Fig.1 local demagnetization test results of N45h sintered NdFeB

Fig.2 local demagnetization test results of N50h sintered NdFeB
Under the reverse magnetic field which is higher than the intrinsic coercivity of the material, the surface magnetic field distribution test results show that although the overall magnetization direction of the material has not changed, the obvious local irreversible demagnetization has occurred, which proves that the risk of local demagnetization in the process of motor simulation is objective.
The more strange phenomenon is that comparing the results of local irreversible demagnetization of N45h and N50h permanent magnets, it can be found that there are obvious differences in the irreversible demagnetization behavior between them: the demagnetization of N45h permanent magnet at the position where the local reverse magnetic field is applied is more diffuse, which reflects that there is a strong magnetic coupling interaction between the main phase grains in the material The demagnetization is also driven in the region adjacent to the applied local demagnetization field.
The coercivity mechanism of sintered NdFeB permanent magnet material shows that although the intrinsic magnetic properties of the rare earth transition group compound are excellent, the defects of the hard main phase boundary and the magnetostatic coupling between the main phase grains lead to the low coercivity, which affects the anti demagnetization ability of the material at high temperature. According to the traditional understanding in the industry, the sintered NdFeB material is mainly composed of the main phase and the grain boundary Earth phase and a small amount of boron rich phase are composed of 2:14:1. In order to improve the intrinsic coercivity of the material, the magnetostatic coupling between the main phase grains is usually reduced by adding heavy rare earth elements with stronger magnetic anisotropy or optimizing the microstructure of the material [4]. It is generally believed that the main phase grains in the material are separated by non-magnetic rare earth rich phases
The wider the soil phase spacing, the weaker the magnetic coupling between the main phase grains. For the N45h and N50h materials studied here, the rare earth content in the n45h material is about 32wt%, while the rare earth content in the n50h material is only 30wt%. The rare earth rich phase spacing between the main phase grains of n45h material is much larger than that of N50h material, but from the analysis of the local demagnetization behavior of the two materials, N45h has a stronger magnetic coupling Coupling effect.
The above abnormal magnetic coupling can not be explained by the conventional phase structure and microstructure. In order to analyze the cause of abnormal demagnetization process of N45h sintered NdFeB material, the microstructure is characterized, as shown in Figure 3.

Fig.3 SEM results of N45h sintered NdFeB
It can be seen from Fig. 3 that N45h has a very ideal microstructure. The main phase grain morphology is round and separated by a large number of rare earth rich phases. However, in the analysis results of the grain boundary phase by SEM, in addition to the conventional white rare earth rich phase, there are also a large number of gray grain boundary phases. Further EDS micro composition analysis shows that N45h is a 6:13:1 phase, which is the most common in the world Based on the local demagnetization behavior of N45h, it can be inferred that a certain amount of ferromagnetic grain boundary phase is distributed in the grain boundary of the magnet. Therefore, it is very likely that the 6:13:1 phase is weak ferromagnetism, which leads to the enhancement of magnetic coupling between the main phase grains.

Conclusion

Through the design and optimization of the research method of local demagnetization characteristics of permanent magnet under off load condition, and through the verification of N45h and N50h materials, it is proved that there is a potential risk of local demagnetization of permanent magnet in motor operation. In the analysis of local demagnetization behavior of N45h, it is indirectly found that the grain boundary phase with weak magnetism causes the abnormal local demagnetization behavior. For the application fields with high stability requirements, it is necessary to In order to improve the anti demagnetization ability of permanent magnet, the local magnetic hardening treatment should be carried out according to the motor simulation results.
Author: Shi Dawei
Source: China Permanent Magnet Manufacturer – www.rizinia.com
Reference:

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