| In order to meet the requirement of sintered Nd-Fe-B permanent magnet materials in the fields of wind power generation,motors and hybrid vehicles.It is necessary to improve the magnetic properties of magnets and promote the efficient use of heavy rare earth resources.Grain boundary diffusion process（GBDP）has been used to enhance the coercivity of the magnet and to reduce the amount of heavy rare earth and production costs.However,there are problems such as limited diffusion depth and low diffusion efficiency.Furthermore,a large amount of diffusion sourcere remins on the surface after GBDP which causes the wastge of resources and increase in production costs.To address the above issue,the DyMg heavy rare earth film layer is co-deposited by magnetron sputtering on the magnet.The low melting element Mg was used to optimized the grain boundary,increased the diffusion depth and efficiency of Dy element diffusion,and improved the distribution uniformity of（Nd,Dy）2Fe14B core-shell structure.The main work was done as below:（1）The Dy and Mg elements was separately deposited and diffused on the magnet,and the effect of different element on the coercivity of sintered Nd-Fe-B magnet was analyzed.（2）The DyMg was co-deposited and diffused on the magnet,and the synergistic diffusion effect between Mg and Dy was revealed.（3）The magnetic properties and microstructural changes of DyMg co-diffusion and Dy diffusion magnets were compared and analyzed,the mechanism of DyMg co-deposition diffusion and the coercivity enhancement were studied.Under the optimal diffusion process（900℃×10 h）,the Dy element forms a hard magnetic phase epitaxial layer at the surface area of the magnet from 010μm.The coercivity is increased from the initial 13.26 kOe to 17.34 kOe,while the remanence was mrginlly reduced.The Mg element（800℃×5 h）diffused into the magnet to form a low-melting liquid phase with good wettability.The grain boundary phase distribution was uniform,and the coercivity was increased to 17.12 kOe.In the DyMg co-deposited diffusion magnet,when the deposition amount of Dy element was reduced by 39%,the coercivity of the magnet was increased to18.21 kOe,which showed an increase of 37.3%.These results indicted better diffusion efficiency and lower preparation cost of grain boundary diffusion using co-deposition method.The temperature coefficient of remanence of the DyMg co-diffused magnet increases from-0.142%/K to-0.128%/K,the temperature coefficient of coercivity increased from-0.836%/K to-0.678%/K,which indicated the enhanced thermal stability.The corrosion current density of DyMg co-diffused magnet was reduced from 1.1151×10-5 A/cm2 to 1.985×10-66 A/cm2,indicting the enhanced corrosion resistance.In the process of DyMg co-deposition and diffusion,the Mg element and the Dy-Mg alloy preferentially diffused to form a low-melting diffusion channel,and the diffusion rate and depth of the Dy diffusion is significantly inproved.the core-shell layer was located at the depth of110μm.The coercivity was enhanced significantly because of the optimized microstructure.The diffusion of Mg caused the capillary contraction of the grain boundary phase to form a continuous and thin layer of GB phase,which weakend the interaction of adjacent grains in the short-range exchange coupling and suppressed the cascade propagation of reverse magnetic domains in the grains.The introduction of Mg element induced the formation of the Nd-O-Fe-Mg steady-state phase during the heat treatment,which was an important factor to improve the corrosion resistance of the magnet.The coercivity of the magnet was improved significantly by DyMg co-deposition diffusion,and the theory system of low melting point alloy diffusion has been completed.A preliminary solution to the problems of low diffusion rate and limited diffusion depth of heavy rare earth elements in sintered Nd-Fe-B magnets was proposed.The efficient use of heavy rare earth resources was realized for the reduction of heavy rare earth applications. |
