Analysis on machining property and micro-structure of the sintered NdFeB by the electrical discharge machining milling using the deionized water

The sintered NdFeB is widely used in many fields due to its excellent magnetic properties. However, the sintered NdFeB is characterized by its high hardness and brittleness. Therefore, its application has been limited. In order to improve its machining properties by electrical discharge machining (EDM), the deionized water is used as the dielectric medium, and then the sintered NdFeB is processed by the electrical discharge machining milling (EDM-M). The machining properties are studied by the comparative experiments. Changing the set value of the current, get the material removal rate (MRR), the surface roughness (SR) and the tool wear rate (TWR) of the die-sinking EDM in the kerosene-based medium (KEDM), the EDM-M in the kerosene-based medium (KEDM-M), the die-sinking EDM in the deionized water (DEDM), and the EDM-M in the deionized water (DEDM-M), respectively. By the scanning electron microscope (SEM), the influences of current on the material removal mechanism and surface formation mechanism of the DEDM-M are analyzed. Meanwhile, the characteristic of its surface micro-morphology influenced by current is analyzed. Combined SEM with energy-dispersive spectroscopy (EDS), the cross-section of the sintered NdFeB by the DEDM-M is analyzed. It is revealed by the comparative experiments that, for the sintered NdFeB by the DEDM-M, the interval of the machining parameter is wide; the value of SR is lowest obviously under low current set value; the variations of its MRR, SR, and TWR are regular; the controllability is good. The regression equations for the machining properties of the DEDM-M are obtained and its analysis of variance (ANOVA) is good. The analysis on the SEM of its machined surface indicates that the material removal and surface formation of the DEDM-M are effected synthetically by fusion and stress. The change of current leads to the difference in surface micro-morphology by fusion removal, globule, microvoid, microcrack, and surface micro-morphology by stress removal, and then leads to differences in MRR and SR, respectively. The results of analysis on the cross-section reveal that there are four categories from the surface to the matrix. They are recast layer after fusion with plenty of rare earth elements and minor oxygen which is amorphous and sheet, recast layer after fusion with plenty of iron element and minor rare earth elements which is strip and granular, micro-morphology by stress, and heat affected layer that is composed of large grains with thin grain boundaries.

Automated summary

Sintered NdFeB is widely used due to its excellent magnetic properties, but its high hardness and brittleness limit its application. In this study, the sintered NdFeB was processed by electrical discharge machining milling (EDM-M) using deionized water as the dielectric medium to improve its machining properties. Comparative experiments were conducted to study the machining properties by changing the current set value. The influences of current on the material removal mechanism and surface formation mechanism were analyzed using SEM, and the characteristic of surface micro-morphology influenced by current was also analyzed. Cross-section analysis of the sintered NdFeB by DEDM-M was conducted using SEM combined with EDS. The results showed that the machining parameters of DEDM-M had a wide range, and the surface roughness was lowest under low current set value. The MRR, SR, and TWR variations were regular, indicating good controllability. Regression equations for the machining properties were obtained, and the analysis of variance was good. SEM analysis of the machined surface revealed that material removal and surface formation in DEDM-M were influenced by fusion and stress. The change of current led to differences in surface micro-morphology and affected MRR and SR. Cross-section analysis showed four categories of materials from the surface to the matrix.