Hot deformation in nanocrystalline Nd-Fe-B backward extruded rings

Radially oriented Nd-Fe-B rings are prepared by backward extrusion of fine grained melt-spun powder. Melt- spun powder with the nominal composition of Nd30.5Febal.Co6.0Ga0.6A10.2B0.9 （wt%） is used as starting material. The effects of process variables, such as deformation temperature （Td）, strain rate （ε） and height reduction （△h%）, on the magnetic properties of the rings are investigated. A scanning electron microscope （SEM） equipped with an energy spectrum device is used to study the metallograph and microfracture of the extruded rings. The Br and （BH）max reach the optimum values at Td =800℃,ε= 0.01 mm/s, and △h% = 70%. It is found by SEM observations that the particle boundaries, which seemingly correspond to the interfaces of the starting melt-spun powders, emerge after the corrosion of metallography specimens. This is helpful for studying the effects of powder-powder interface on the local deformation and deformation homogeneity in the rings. For different spatial positions of the extruded rings, there are characteristic metallographies and microfractures. The upper end of the rings has the least deformation and worst texture, and therefore the worst magnetic properties. The magnetic properties in the radial direction increase slightly along the axis from the bottom to the middle, then steeply decrease at the upper end of the ring. The deformation and the formation-of-texturing processes are discussed. The deformation and the texturing formation of melt-spun Nd-Fe-B alloys probably involve grain boundary sliding and grain rotation, the solution-precipitation process and preferential growth of Nd2Fel4B nanograins along the easy growth a-axis.

Rare earth permanent magnets prepared by hot deformation process

Hot deformation is one of the primary methods for fabricating anisotropic rare earth permanent magnets.Firstly,rapidly quenched powder flakes with a nanocrystal structure are condensed into fully dense isotropic precursors using the hot-pressing process.The prepared isotropic precursors are then hot-deformed to produce high-anisotropy uniaxial bulk rare earth permanent magnets and a highly textured structure is produced via this process.The resulting magnets possess many advantages such as near-net-shape,outstanding corrosion resistance,and ultrafine-grain structure.The influence of the preparation parameters utilized in the hot-pressing and deformation processes on the magnetic properties and microstructure of the permanent magnets are systemically summarized in this report.As a near-net-shape technique,the hot deformation process has notable advantages with regard to the production of irregular shapes,especially for radially oriented ringshaped magnets with high length-diameter ratios or thin walls.The difficulties associated with the fabrication of crack-free,homogeneous,and non-decentered ring-shaped magnets are substantially resolved through an emphasis on mold design,adjustment of deformation parameters,and application of theoretical simulation.Considering the characteristics of hotdeformed magnets which include grain shape and size,anisotropic distribution of intergranular phases,etc.,investigation and improvement of the mechanical and electric properties,in addition to thermal stability,with the objective of improving the application of hot-deformed magnets or ring-shaped magnets,is of practical significance.