Two-step diffusion method for preparing high-performance dual-main-phase sintered mischmetal-iron-boron magnet

A two-step diffusion method for preparing high-performance dual-main-phase sintered mischmetal-iron-boron magnet, wherein the high-performance dual-main-phase sintered mischmetal-iron-boron magnet comprises a Pr/Nd2Fe14B main phase A and a (E, Nd)2Fe14B main phase B, hydrogen decrepitation coarse powders of the main phase B is subjected to two-step rotating diffusion treatment, then mixed with hydrogen decrepitation coarse powders of the main phase A, a mass ratio of the main phase A to the main phase B is 1:9-5:5 with the sum being 10;

• wherein nominal composition of the main phase A is Pr/NdxFe100-x-y-zMyBz (wt. %), and nominal composition of the main phase B is [EaNd1-a]xFe100-x-y-zMyBz (wt. %), where E is mischmetal, and mass percent of each component is Ce:
• 48-58%, La:
• 20-30%, Pr:
• 4-6%, and Nd:
• 15-17%;
• M is one or more of Nb, Ti, V, Co, Cr, Mn, Ni, Zr, Ga, Ag, Ta, Al, Au, Pb, Cu, Si, a x, y, z satisfies the following relationships:
• 0≤a≤1, 25≤x≤35, 0.5≤y≤3, 0.3≤z≤1.5;
• the two-step diffusion method comprising the following steps:
• (1) according to the main phase A with the nominal composition of Pr/NdxFe100-x-y-zMy Bz, and the main phase B with the nominal composition of [EaNd1-a]x Fe100-x-y-zMyBz, praseodymium, mischmetal (E), other metals (M), neodymium, iron, and iron boron alloy are selected and put into a crucible;
• after drying under vacuum and filling argon, mixed metals are smelted and then poured on a rotating water-cooled copper roller with a rotation speed of 1-4 m/s;
• A and B strip-casting alloys with a thickness of 180-400 μm are obtained, respectively;
• (2) a PrHoFe alloy and a ZrCu alloy are prepared into PrHoFe strip-casting alloys and ZrCu strip-casting alloys using a vacuum induction rapid-quench furnace, respectively;
• then the PrHoFe strip-casting alloys and the ZrCu strip-casting alloys are roughly broken into square pieces with a size of (0.5-1.5) cm*(0.5-1.5) cm;
• (3) the A and B strip-casting alloys of step (1) are broken by hydrogen decrepitation, respectively, and coarsely crushed powders are obtained after dehydrogenation;
• (4) the hydrogen decrepitation coarse powders of the B strip-casting alloys of step (3) and the PrHoFe strip-casting alloys of step (2) are respectively placed in inner and outer cavities of a coaxial double-layer circular barrel for a first step diffusion treatment;
• a mass ratio of the B strip-casting alloys and the PrHoFe strip-casting alloys is 2:1 to 1:2;
• a molybdenum mesh separates the inner cavity and the outer cavity;
• first-step diffusion coarse powders are obtained by diffusion heat treatment at a speed of 1-10 r/min and 500-700° C. for 3-6 h in a rotary heat treatment furnace;
• an external shell of the coaxial double-layer circular barrel is made of solid material plates;
• a coaxial inner layer is a molybdenum mesh cylinder;
• an annular cavity structure between the molybdenum mesh cylinder and the external shell of the coaxial double-layer circular barrel is the outer cavity;
• a cavity in the molybdenum mesh cylinder is the inner cavity;
• a mesh diameter of the molybdenum mesh cylinder is less than 5 μm;
• (5) the first-step diffusion coarse powders of step (4) and the broken ZrCu strip-casting alloys of step (2) are respectively placed in the inner and outer cavities of the coaxial double-layer circular barrel for a second-step diffusion treatment to obtain second-step diffusion coarse powders;
• a mass ratio of the first-step diffusion coarse powders and the broken ZrCu strip-casting alloys is 2:1 to 1:2;
• a diffusion heat treatment is carried out in a rotary heat treatment furnace at a speed of 1-10 r/min and 800-950° C. for 2-5 h;
• the rotary heat treatment furnace is connected with a glove box filled with inert gas to protect raw materials during moving in and out of the rotary heat treatment furnace in the glove box;
• (6) the hydrogen decrepitation coarse powders of the A strip-casting alloys of step (3) are mixed with the second-step diffusion coarse powders after two-step diffusion treatment of step (5) to make a mass ratio of main phases A and B between 1:9 and 5:5;
• fine powders with a diameter of 1-5 μm are obtained by jet milling after adding 0.01-5 wt. % lubricant and 0.01-5 wt. % antioxidant;
• the weight percentage of the lubricant and the antioxidant is based on a total weight of the hydrogen decrepitation coarse powders of the A strip-casting alloys of step (3) and the second-step diffusion coarse powders after two-step diffusion treatment of step (5);
• (7) the fine powders of step (6) adding 0.01-5 wt. % lubricant and 0.01-5 wt. % antioxidant again are mixed well, then aligned and compacted under a magnetic field of 1.5-2.0 T in an inert gas to obtain compacts;
• the compacts are vacuum-encapsulated and subjected to cold isostatic pressing;
• the weight percentage of the lubricant and the antioxidant is based on the weight of the fine powders of step (6);
• (8) the compacts of step (7) are put into a vacuum sintering furnace for sintering at 980-1080° C. for 1-4 h and then cooled by argon;
• to restrain inter-diffusion between the two phases, the high-performance dual-main-phase sintered mischmetal-iron-boron magnet is only annealed at low temperature at 400-600° C. for 2-5 h.