Research On Preparation Process And Properties Of Bonded Nd-Fe-B Permanent Magnets

Bonded Nd-Fe-B magnets were prepared by the method of compression molding. The thermodynamic properties of Nd-Fe-B alloy system were discussed. The influences of silane coupling treatment, binder content, high magnetic field, compaction process and thermosetting technology on the microstructure and properties of bonded Nd-Fe-B magnets were studied by the analysis of DSC, SEM, bending strength, compressive strength and MATS-2010 Permanent Magnet Test. The preparation process of bonded Nd-Fe-B magnets was optimized.The analysis of thermodynamic properties of Nd-Fe-B alloy system showed that the Gibbs free energy change (AG) in the phase transformation of Nd₂Fe₁₄B phase was less than zero (AGO) during the melting and cooling of Nd-Fe-B alloy. The formation of Nd₂Fe₁₄B phase was feasible in the Nd-Fe-B alloy system, which was the premise condition of preparation of Nd-Fe-B permanent magnet. The elements of Nd, Fe and B were oxidated easily and the formation of their oxides was easier than that of Nd₂Fe₁₄B phase. So reducing the oxygen content of Nd-Fe-B alloy system was significant during the preparation of Nd-Fe-B permanent magnet.The antioxidation, relative density, mechanical properties and magnetic properties of Nd-Fe-B magnets were improved after the silane coupling treatment of Nd-Fe-B magnetic powders. The coupling effect was the best when the content of silane coupling agent was 1wt%. The effects of binder content on the properties of magnets indicated that the mechanical properties of bonded magnets increased with the increase of binder content, but the magnetic properties decreased.The preparation process of epoxy resin bonded Nd-Fe-B magnets had been studied. The technology C was an uniaxial compaction molding, without dwell time, with simple preparation process and high production efficiency, which was suitable for mass production. The mechanical properties of bonded magnets with technology C obtained the best values when the preparation process was in the following:â‘ the proportion between epoxy resin and curing agent was 7:3;â‘¡the thermosetting temperature was about 160℃;â‘¢the compaction pressure was 620MPa. The B_r and (BH)_max of bonded magnets increased with the increase of compaction pressure, but the H_ci decreased. The epoxy resin bonded Nd-Fe-B magnets were magnetized in high magnetic field. The B_r and (BH)_max increased almost linearly when the magnetic field strength rose from 1.5T to 5T. But the B_r and (BH)max increased slowly when the magnetic field strength exceeded 5T. Moreover, the B_r and (BH)_max increased no longer with increasing magnetic field up to 10T. However, the H_ci kept almost invariably with the magnetic field increasing.The phenolic resin bonded Nd-Fe-B magnets were prepared by the method of warm compaction molding. The magnetic properties of warm compaction magnets were better than that of room compaction magnets. When the phenolic resin content was 3wt% and compaction pressure was 620MPa, the properties of magnets at room temperature were in the following: relative density =89.4%, B_r =0.6176T, H_ci =704.2 kA/m, (BH)_max=72.1kJ/m³; but the properties of magnets at warm compaction were in the following: relative density =97.3%, B_r=0.6786T, H_ci =714.2 kA/m, (BH)_max=79.7kJ/m³. The phenolic resin bonded Nd-Fe-B magnets might have no need for second thermosetting treatment and the best warm compaction temperature was 180℃. The magnetic properties of magnets with second thermosetting treatment decreased because the hot loss, thermal stress and oxidation degree of magnets increased. So the second thermosetting treatment had adverse effects on the magnetic properties of magnets.The nylon (nylon 12) bonded Nd-Fe-B magnets were prepared by the method of warm compaction molding. The mechanical properties of 160℃warm compaction magnets were better than that of room compaction magnets. The mechanical properties of magnets increased a little when the warm compaction temperature rose from 160℃to 190℃. The mechanical properties kept almost invariably when the warm compaction temperature rose from 190℃to 200℃. The mechanical properties increased and had no turning point with the increase of compaction pressure. The B_r and (BH)_max of bonded magnets increased with the increase of compaction pressure, but the H_ci decreased. The magnetic properties decreased with the increase of thermosetting temperature. Scanning Electron Microscopy (SEM) was used to examine the fracture surfaces of epoxy resin and nylon bonded Nd-Fe-B magnets. The bonded Nd-Fe-B magnet was a kind of brittle materials. The binder content and bonding strength were the decisive factors on the mechanical properties of bonded Nd-Fe-B magnets. The strength of the magnetic powder also helped to increase the mechanical properties of magnets a little.The composite bonded (epoxy resin and nylon66) Nd-Fe-B magnets had been studied, whose properties compared with the epoxy resin and nylon 12 bonded Nd-Fe-B magnets. The mechanical properties of nylon 12 bonded Nd-Fe-B magnet were relatively low because of their low densities, many interspaces and low bonding strength. The densities of composite bonded Nd-Fe-B magnets (EP:PA=1.0:3.5) were equivalent to that of epoxy resin bonded Nd-Fe-B magnets. Because the nylon66 content was more 2.5wt% than the epoxy resin content and the mechanical properties of nylon66 were higher than that of epoxy resin, the mechanical properties of composite bonded Nd-Fe-B magnets were higher than that of two other magnets. Because the densities of nylon bonded Nd-Fe-B magnets were lower than that of two other magnets and the magnetic powders in unit volume were less, the B_r and (BH)_max of nylon bonded Nd-Fe-B magnets were lower than that of two other magnets, but the H_ci was higher than that of two other magnets.