Structural Regulation And Application Of Sintered NdFeB Magnets With Low Dysprosium

Based on powder metallurgy process, this thesis proposes two technologies to prepare sintered NdFeB magnets with low dysprosium and high coercivity, and one to develop sintered NdFeB magnets with low dysprosium and low cost. Sintered magnets prepared by the proposed dual main phase technology have higher coercivity than conventional magnets. A simplified model was used to explain the enhancement of coercivity of dual main phase magnets. The efficient use of heavy rare earth resources will be achieved when magnets is prepared with this technology. Impact demagnetization of sintered NdFeB magnets has been investigated. These principal conclusions of the thesis are presented as follows:(1)A kind of sintered NdFeB magnet 48SH ((BH) m=382 kJ/m3, Hcj=1548 kA/m) with low dysprosium and high coercivity, whose nominal composition is Nd28.2Dy2Fe68.8B was developed successfully. An appropriate thickness of Nd-rich phases could make use of to get high quality heavy rare earths.(2) Analyze the reason for the abnormal grain growth, based on which, this thesis found out a set of technology to manufacture sintered NdFeB magnet with low dysprosium and low cost. With pulverizing management making most of the oxygen existing in grain boundaries, this technology reduces the waste of materials, production time and significantly reduces the cost of magnet. And through regulating the content of oxygen in the grain boundaries, this technology will improve the sintering process of high coercivity magnet.(3) Found out a technology to lower dysprosium in sintered magnet. The magnets are named as dual main phase magnets, whose coercivity is greatly increased comparing to the magnet made by the conventional single alloy method. This thesis characterizes for the first time the different anisotropy field grain structure existing of the dual main phase magnet and provided the simplified model. Deducing thesemi-classical formula, Hcj=HN=2K1*/μ0Ms*αexαφαK1*-NeffMs*,K1*=(1-c)K1g+cK1d,the author provides a qualitative explanation to the coercivity increase mechanim of dual main phase magnet.(4) Dual main phase magnets with different grain sizes were prepared. The best magnetic properties of magnets is, (BH)m=389.93 kJ/m3,Hcj=1282.79 kA/m. The author also studies the associated effect of grain size and grain boundary phase:with the decrease of average grain size, the main gain size distribution becomes concentrated and its morphology becomes regular and smooth. When the size of Nd-rich phase is smaller at the grain conjoint boundary phase, the strip-type gain boundary phase will increase and the conjoint boundary phase will decrease, which will improve coercivity of magnet.(5) Compressions of the unmagnetized Nd2Fe14B permanent magnets are executed by using shock wave with different pressure in one stage light gas gun system. The shock wave pressures in the NdFeB magnets decreased almost linearly. The critical pressure of demagnetization of NdFeB magnets should be less than 4.99 GPa. When the pressures are in the range of 6.26≤P≤7.21 GPa, the presence of intergranular fracture is easily detected in the surface of magnet. The crystallographic alignment decreases when shock wave pressure increases. It can be concluded that maybe the demagnetization are derived from three aspects:first of all, these matrix phase grains connect together through thin grain boundaries phase which are crushed to move elsewhere during the impact process; On the other hand, it is considered that a lot of microcracks and pores existed in these grain boundaries phase of shock wave compressed magnet. The nucleation field may be weakened by the microcracks and pores at the grain boundaries. In addition, the slip or fracture of grain boundaries affectes the orientation relationship between matrix phase and grain boundaries phase.The pulsed power output could be improved by raising the maximum energy product and reducing impact resistance of the magnet.