| A brief review is given in this paper of microstructure characteristics, magnetic properties, mechanical properties, corrosion resistance and temperature stability of sintered Nd-Fe-B magnets, and advanced preparation techniques for high performance sintered Nd-Fe-B magnets. The effects of alloy compositions and melting process on microstructure of Nd-Fe-B strip cast alloys and magnetic properties of sintered Nd-Fe-B magnets were studied. The effects of sintering and annealing on microstructure and magnetic properties of Nd-Fe-B magnets were analyzed. By optimizing the alloy compositions and improving the melting process, powder-preparing process magnetically-aligning process, sintering process and annealing process,44SH and40UH high-performance sintered Nd-Fe-B magnets have been successfully mass-produced.The strip cast alloys with an average thickness of0.3～0.4mm show a good microstructure that lamellar Nd2Fe14B phase grows in the direction perpendicular to the wheel surface, and Nd-rich phase with an interlamellar spacing of3～5μ.m evenly exists within the main phase grains and in the grain boundaries of the main phase. There are small amounts of a-Fe near the free surface of strip cast alloys with an average thickness of0.8～1mm. However, the interlamellar spacing of Nd-rich phase of only about2μm is too small in the strip cast alloys with an average thickness of0.2～0.3mm. A higher heavy rare earth element content of6%～8%in the alloy composition increases the supercooling degree when the alloy melt crystallizes, which causes the presence of many over-small equiaxial grains less than3μm in the microstructure of Nd-Fe-B strip cast alloys. The strip cast alloys with ideal microstructure could be prepared by using through appropriate pouring process. When alloy melt is poured, too low melt temperature leads to the appearance of many over-small equiaxial grains in the microstructure of strip cast alloys, and lower melt temperature and slower melt-pouring rate inhibit the lamellar grains growth. By improving the microstructure of (Pr-Nd)26.4Dy4.0Ho2.0FebaiCo1.5Nb0.55Cu0.2Al0.45B1.02strip cast alloys, avoiding the existence of over-small equiaxial grains, a good alloy powder particle size distribution was achieved, and the sintered magnet orientation degree was increased. The magnets with dense, fine-grained and homogeneous microstructure were prepared, and their magnetic properties were Br=1.213T, Hcb=954.40kA·m-1, Hcj=2009.90kA·m-1,(BH)max=288.79kJ·m-3, Hk/Hcj=87.0%。The use of reasonable sintering process is a necessary condition for preparing magnets with dense, fine-grained and homogeneous microstructure and hence improved magnetic properties and thermal stability. Under the experimental conditions described in this paper, magnets sintered at1338K for2h has a density of7.58g·cm-3, and an average grain size of7.4μm and a uniform grain size distribution; their Br, Hcj and (BH)max were1.22T,1753.7kA·m-1and290.1kJ·m-3respectively; when L/D=0.1, the irreversible flux loss was only-0.65%after exposure at393K for2h. The sintered Nd-Fe-B magnets also could have dense microstructure, and the average grain size was small, by the two-step sintering methods of sintering at1363K for5min, then sintering at1298K for1h. But the grain size distribution uniformity was bad, and there were abnormal grains with sizes of17-19μm exist which made the magnetic properties and thermal stability poor. The Hcj of as-sintered magnets could be increased greatly by the use of first-step annealing+second-step annealing heat treatment. When first-step annealing temperature and time are1163~1193K and2.5h, and second-step annealing temperature and time are758-798K and30min respectively, the Hcj of magnets increases by332kA·m-1. The Hcj of sintered magnets also could increases by the way of only second-step annealing heat treatment, but the increase is lower than the former. When second-step annealing temperature and time are758-798K and30-60min respectively, the Hcj of magnets increases by278kA·m-1. With or without the first-step annealing heat treatment, the Hcj increases gradually and reaches a maximum value around30-60min with the second-step annealing temperature of758-798K, but the Br decreases to a certain extent.By optimizing the alloy compositions and improving the smelting process, powder-preparing process, magnetically-aligning process, sintering process and annealing process,44SH and40UH high-performance sintered Nd-Fe-B magnets have been successfully mass-produced from common commercially-available raw materials and equipments. And the sintered Nd-Fe-B magnets with a lower Dy content can save the cost and improve the market competitiveness of products. Typical magnetic properties for44SH sintered Nd-Fe-B magnets are Br=1.340T (13.40kGs), Hcj=1685.13kA·m1-(21.17kOe),(BH)max=348.89kJ·m-3(43.83MGOe). Typical magnetic properties for40UH sintered Nd-Fe-B magnets are Br=1.274T (12.74kGs), Hcj=2017.86kA·m-1(25.35kOe),(BH)max=316.49kJ·m3(39.76MGOe). The44SH sintered Nd-Fe-B magnets were obtained with a temperature coefficient of induction(a) of-0.107%/℃and a temperature coefficient of intrinsic coercivity(β) of-0.543%/℃at22-150℃. The40UH sintered Nd-Fe-B magnets were obtained with a temperature coefficient of induction(a) of-0.085%/℃and a temperature coefficient of intrinsic coercivity(β) of-0.468%/℃at22-150℃. The sintered Nd-Fe-B magnets exhibit an outstanding thermal stability. |
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