Study On The Preparation And Properties Of Nd-Fe-B Magnetic Rings Using Different Hot Extrusion Forming Methods

At present,hot extruded Nd-Fe-B permanent magnet ring is mainly prepared by backward hot extrusion.This deformation method has the advantages of high forming quality,but it is limited in wall thickness and height,easy to produce micro cracks in the ring and uneven performance.In this thesis,the mold design and the performance analysis of the magnetic ring are carried out for the three forming methods of backward hot extrusion,compound hot extrusion and forward hot extrusion.Through simulation and experiment,the effect of extrusion speed on the properties of magnetic rings prepared by three forming methods was studied,analysis of the conventional and boss type backward hot extrusion magnetic ring,the stress,strain,velocity field distribution and Influence on magnetic ring performance.The main research contents and conclusions are as follows:(1)Nd-Fe-B magnetic rings with outer diameter of 39.5 mm and wall thickness of4.5 mm were prepared by backward hot extrusion(BE)technology.The effects of extrusion speed on the magnetic properties,microstructure and structure of Nd-Fe-B magnetic rings were studied.The results showed that with the increase of extrusion speed,the magnetic properties of Nd-Fe-B magnetic rings first increased and then decreased.The magnetic properties of the magnetic ring with extrusion speed of 0.05 mm/s are the best,and the remanence,coercivity and maximum magnetic energy product are 12.385 k Gs,10.705 k Oe and 32.808 MGOe,respectively.At the same time,a backward hot extrusion(BE-TU)mold with a boss is designed and made.The Nd-Fe-B permanent magnet ring prepared by the BE-TU mold is compared with the magnetic ring prepared by the BE mold.The maximum magnetic energy product of BE and BE-TU ring samples above 25 MGOe was 15% and 31%,respectively.The results show that the strain distribution inside the BE-TU magnetic ring is more uniform and the flow rate is faster,and the magnetic properties of the ring are better.(2)The composite hot extrusion die is designed and made.The composite hot extrusion Nd-Fe-B magnetic ring is prepared by the composite hot extrusion process.The height of the backward extrusion part(BE)and the forward extrusion part(FE)of the composite hot extrusion magnetic ring is 26.4 mm,and the total length-diameter ratio is1.31.Maximum magnetic energy product greater than 25 MGOe reaches 22 mm,accounting for 43% of the height of the magnetic ring.Compared with the backward hot extrusion,the forming height has been significantly improved.The effects of extrusion speed on the properties,microstructure and structure of magnetic ring were studied.With the increase of extrusion speed,the magnetic properties of magnetic ring first increased and then decreased.When the extrusion speed was 0.15 mm/s,the magnetic properties of magnetic ring were the best,the partial remanence,coercivity and maximum magnetic energy product were 12.94 k Gs,12.156 k Oe and 36.933 MGOe,respectively.The partial remanence,coercivity and maximum magnetic energy product of FE are 11.383 k Gs,15.111 k Oe and 29.131 MGOe,respectively.(3)The forward extrusion mold of magnetic ring was designed and manufactured,and the Nd-Fe-B permanent magnet ring with height of 32 mm and aspect ratio of 2.06 was prepared by the forward hot extrusion molding method.The inner wall of the forward extrusion magnetic ring is smooth and has no obvious cracks.Compared with the reverse hot extrusion and compound hot extrusion forming methods,the length-diameter ratio is significantly improved,and the inner wall cracks are also significantly improved.The magnetic properties in the transition area of the magnetic ring forming are the lowest,and the magnetic properties in the middle of the magnetic ring are the best.Remanence,coercivity and maximum magnetic energy product are 9.92 k Gs,11.79 k Oe and 19.01 MGOe,respectively.DEFORM-3D software was used to simulate and analyze the transition form,extrusion speed and die height of the forward extrusion die,and the influence on the magnetic ring was simulated and analyzed.The results showed that the conical transition extrusion die was more conducive to the billet flow and the improvement of the magnetic ring crack,and the lower extrusion speed was conducive to the improvement of grain orientation.Reducing the mold height can not only improve the magnetic properties of the magnetic ring,but also save the raw materials.