The Study On Deforming Process And Properties Of High Performance Nd-Fe-B Magnetic Ring By Hot Extrusion

In this thesis,the backward-extruded Nd-Fe-B ring and the composite-extruded Nd-Fe-B ring were prepared by backward extrusion deformation and composite extrusion deformation.The magnetic properties,microstructure and phase structure of the magnetic rings were analyzed.The effects of hot-pressing holding time,hot-extrusion deformation temperature,deformation pressure,deformation speed and other process conditions on the magnetic properties,texture strength and microstructure of the magnetic rings were investigated.The stress,strain and velocity field of the composite extrusion Nd-Fe-B ring were analyzed by Deform 3D software,and the influences of friction coefficient between blank and die on the stress and strain of reverse extrusion Nd-Fe-B ring and composite extrusion Nd-Fe-B ring were explored,which can provide a reference for the preparation process optimization of magnetic rings.The main research contents and conclusions are as follows:（1）The backward-extruded Nd-Fe-B magnetic ring with a height of 29.7 mm was prepared,and the height of the backward-extruded Nd-Fe-B magnetic ring with a magnetic energy product,（BH）_max=31.8 MGOe～32.8 MGOe was about 5 mm,accounting for only 16%of the total height of the magnetic ring.It is found that the grain coarsening is related to the long deformation time and the long hot-pressing holding time,which leads to the properties decrease of the magnetic ring.However,the appropriate extension of the hot-pressing holding time is more conducive to the properties improvement of magnetic ring by strengthening grain orientation.（2）The composite-extruded Nd-Fe-B ring with a height of 36 mm was prepared.The height of the ring with a magnetic energy product of 31.8 MGOe～38.5 MGOe was about 12 mm,accounting for 33.3%of the total height of the ring,which was higher than that of the backward-extruded ring.It is found that the composite-extruded ring has the alternating arrangement micromorphology of coarse and fine crystal regions,but there is a difference between the backward-extruded part（BE）and forward-extruded part（FE）.By adjusting the preparation process,it is found that reducing the deformation speed and prolonging the hot-pressing holding time are more conducive to the deformation.However,reducing the deformation temperature has different effects on the BE and FE parts.The deformation of the FE part needs a higher temperature than that of the BE part to obtain higher properties.（3）The distribution of stress,strain and velocity field of the composite-extruded Nd-Fe-B magnetic ring was analyzed by using Deform 3D.The results show that the effective stress and effective strain of the composite-extruded Nd-Fe-B ring are not uniform,and there is a stress concentration in the transition regions between the BE part and the FE part,and the effective strain of the FE part is smaller than that of the BE part.In addition,the velocity field analysis results show that the FE part has a faster deformation speed because it suffers less resistance during deformation.（4）The effects of friction coefficient on the stress and strain of the two kinds of magnetic rings were investigated by using Deform 3D.It is found that with the increase of friction coefficient,the stress is more concentrated near the bottom of the back-extruded ring and in the transition regions of the BE part and FE part of the composite extrusion ring.However,the friction coefficient has little effect on the equivalent strain of the magnetic ring.In addition,a moderate increase of the friction coefficient contributes to a more uniform distribution of the equivalent strain in the FE part of the composite-extruded Nd-Fe-B ring.