Microstructural Analysis, Property Evaluation And Process Optimization For Sintered NdFeB Magnets

As a key material for energy-saving devices, sintered NdFeB magnet has a very broad applications in the field of electric cars and hybrid vehicles, inverter air conditioner, consumer electronics and wind power generation. In this dissertation, we summarized the current status on the production and applications of sintered NdFe B magnets industry at home and abroad. Based on market research for sintered NdFe B companies, Hitachi Metals and Zhong Ke San Huan were selected to analyze the range, performance, and applications of their products. Especially, based on the author’s engineering practice experience in domestic sintered NdFeB production enterprise XXX, the production process and waste utilization of NdFeB sintered magnets were recorded. The current development and application of sintered NdFeB magnets have been analyzed. The main problems and the future trends of domestic sintered NdFeB industry are also mentioned.For the experimental work, four grades of domestic production N35, N38, N40 and N45 were firstly selected to analyze the density, average grain size, phases and phase content, magnetic properties, microstructure, corrosion resistance and mechanical properties. The orthogonal experimental method were then used to optimize the heat treatment process of N35 magnet, and the effects of two stage tempering on microstructure and properties were compared. Finally, Nd₇₀Cu₃₀ eutectic alloy with a low melting point were made as a diffusion medium for N38 and N40 magnets in grain boundary diffusion process. The effects of diffusion process on the magnetic properties and microstructure has been analyzed, the mechanism of using pressure diffusion processes to increase coercivity has been discussed. The main contents and conclusions of this work are as follows:（1） The average grain size of various magnets were between 5⁶ μm, and the uniformity of grains and process stability still needed to be improved. In all four grades of magnets, Nd₂Fe₁₄ B phase content was about 81% to 88%, Nd-rich grain boundary phase content was about 11% to 17% and porosity was about 0.8% to 1.8%. All grades magnets showed uneven distribution of thin grain boundary phase and relatively low magnetic properties, corrosion resistance and mechanical properties indicating there is still enough room for improvement.（2） Orthogonal experiments showed that the process time of the second stage tempering is the major factor affecting the coercivity. Nd-rich phase accumulation in the grain boundary led to coercivity decrease after the first stage tempering. After the second stage tempering, coercivity improved significantly because of the disappearance of Nd-rich phase accumulation and the reduction of main phase sharp corners.（3） After NdCu diffusion, the coercivity of N38 and N40 magnets had different degrees of improvement, and discontinuous and continuous thin Nd-rich grain boundary phase appeared, especially in pressure diffusion samples. Moreover, Cu showed uneven distribution inside the magnet. In particular, Cu tends to concentrate in the thin grain boundaries, while the triple junction shows much less Cu. During the diffusion process under pressure, the microcracks plays a role as diffusion channels for Nd-rich liquid phase, so the NdCu alloy distributed among the magnet more evenly.