NdFeB Magnets Fabricated By Spark Plasma Sinteirng And Hot Deformation

Nd-Fe-B-based permanent magnets exhibit the higher remanence, coercivity andmaximum energy product, and they have been used in a wild variety of applications such ashard disk drives （HDD）, the traction motor for hybrid, magnetic resonance imaging （MRI）and speakers of mobile phone. As the application of hybrid requires NdFeB magnets withgood magnetic properties, exceptional corrosion, thermal stability and fracture toughness, it isimportant to improve the comprehensive performance of NdFeB magnets. The performanceof the NdFeB permanent magnets has a close relationship with the chemical composition andthe microstructure, including the grain size, the grain orientation and the distribution of grainboundaries phase. However, the processing method controlling the grain size and orientationis one of the key points for improving the over-all properties of NdFeB permanent magnets.In this thesis, HDDR processed NdFeB magnetic powders were used as the starting materials.Effects of the spark plasma sintering （SPS） conditions and subsequent hot-deformation （HD）conditions on magnetic properties and microstructure of NdFeB magnets were investigated.Besides these, efforts were paid in investigating the relationship between the hot press （HP）sintering comditons and magnetic properties of NdFeB magnets.The effects of sintering temperature (T_sps) on the magnetic properties and density ofNdFeB magnets fabricated by SPS from HDDR powders were studied. With increasing thesintering temperature in the range of650°C⁹00°C, the remanence （B_r）, intrinsic coecivity(H_cj) and maximum energy product (（BH）_max) all increased first and then decreased. Theoptimum magnetic properties of B_r=0.78T, H_cj=577kA/m,（BH）_max=78kJ/m³wereobtained in the samples SPSed at800°C and the density reached99%of the theoretical. Themicrostructure, XRD patterns and magnetic properties indicated that the sample (T_sps=800°C) is anisotropic to a certain degree. But the Nd₂Fe₁₄B grains grew obviously when theT_spsreached900°C. B_r, H_cjand （BH）_maxall increased gradually when the holding time (t_sps)increased from0min to20min. Compared to the T_spsand t_sps, sintering pressure （Psps） hadlittle effects on magnetic properties and density of the sintered NdFeB magnets.The SPSed magnet with optimal magnetic properties (T_sps=800°C, t_sps=20min, Psps=50MPa) was subjected to HD subsequently. The HD conditions were as follows:（I） hotdeformation temperature (T_HD)750°C⁹00°C;（II） compression ratio (d_HD)41%⁶9%.The hot deformation made the Nd₂Fe₁₄B grain oriented in such a way that the c-axis of theNd₂Fe₁₄B grain is parallel to the pressing direction and the c-axis crystallographic alignmentin the HDed magnets became stronger with the increase in T_HD(d_HD=60%). However, the alignment of grains was disrupted by a small amount of abnormal large grains. The H_cjand（BH）_maxof the HDed magnets raised first, peaked at800°C and then fell with increasing T_HD,whereas, the B_rincreased gradually when T_HDincreased and the maximum value appeared at900°C. The c-axis crystallographic alignment in the HDed magnets became stronger with theincrease in d_HD. The abnormal growth of Nd₂Fe₁₄B grains could be observed when d_HDcameup to60%. The H_cjand （BH）_maxof the HDed magnets increased with the increase in d_HD,passing through a maximum at50%and decreasing thereafter. However, the B_rincreasedwith the increased T_HDand the maximum value appeared at69%. The optimal magneticproperties parallel to the c-axis of the HDed magnet were obtained when T_HD=800°C andd_HD=50%and they were as follows: B_r=1.16T, H_cj=449kA/m,（BH）_max=178kJ/m³.Meanwhile, the effects of sintering temperature of HP sintering process on themicrostructure and magnetic properties of the sintered NdFeB materials were researched. Theresults showed that when the sintering temperature （THP） ranged from800C to950C, the B_r,H_cjand （BH）_maxall increased first and decreased thereafter. The optimum magnetic propertieswere obtained in the samples HPed at900°C and the density reached99%of the theoreticaland the Nd₂Fe₁₄B grain size was uniform. The Nd₂Fe₁₄B grains grew obviously and themagnetic properties fell sharply when THPcame up to950°C.