Method for preparing a permanent magnet material
The disclosure discloses a method for preparing a permanent magnet material. In this method, an ionic liquid electroplating process is used to electroplate a heavy rare earth metal onto a surface of a sintered magnet to form a magnet with a coating, wherein the sintered magnet has a thickness of 10 mm or less in at least one direction; in the ionic liquid electroplating process, an electroplating solution comprises an ionic liquid, a heavy rare earth salt, a group VIII metal salt, an alkali metal salt and an additive, an anode is a heavy rare earth metal or a heavy rare earth alloy, a cathode is the sintered magnet, an electroplating temperature is 20-50° C., an electroplating time is 15-80 min. The preparation method of the disclosure can improve an intrinsic coercive force of the magnet with low cost and high production efficiency. A utilization rate of heavy rare earth is high.
1. A method for preparing a permanent magnet material, comprising steps as follows:
S1) magnet preparation step:
S1-1) smelting step: formulating a raw magnet material consisting of the following components with weight percent: 27.4% of Nd, 4.5% of Dy, 0.97% of B, 2% of Co, 0.2% of Cu, 0.08% of Zr, 0.2% of Ga, 0.3% of Al, and the balance of Fe; putting the raw magnet material in a vacuum melting casting furnace to smelt and form an alloy sheet having an average thickness of 0.3 mm;
S1-2) powdering step: subjecting the alloy sheet to a hydrogen absorption and dehydrogenation treatment in a hydrogen decrepitation furnace so as to allow the alloy sheet to form a coarse magnetic powder with a D50 of 300 μm, and then crushing the coarse magnetic powder in an air jet mill using nitrogen as a medium to obtain a fine magnetic powder having a D50 of 3.8 μm;
S1-3) shaping step: applying an aligning magnetic field of 1.8 T to the fine magnetic powder under a protection of nitrogen in a forming presser, and pressing the fine magnetic powder to make a green body, wherein a density of the green body is 4.3 g/cm 3 ;
S1-4) sintering and cutting step: placing the green body in a vacuum furnace with an absolute vacuum degree above 0.1 Pa, and sintering the green body under a temperature of 1,055° C. for 5 hours to obtain a magnet with a density of 7.63 g/cm 3 and a dimension of 50 mm×40 mm×30 mm; cutting this magnet into a sintered magnet having a dimension of 38 mm×23.5 mm×2.2 mm;
S2) ionic liquid electroplating step:
subjecting the sintered magnet to degreasing, rust cleaning, acid cleaning activation and drying treatment to obtain the sintered magnet to be electroplated;
uniformly stirring anhydrous terbium chloride, anhydrous ferric chloride and 1-butyl-3-methylimidazole trifluoromethanesulfonate as ionic liquid with a molar ratio of 1:1:1 at a temperature of less than 80° C. in a glove box protected by nitrogen, adding lithium chloride with a concentration of 40 g/L based on an electroplating solution, adding toluene with a volume ratio of 30 vol % of 1-butyl-3-methylimidazole trifluoromethanesulfonate, and then stirring the mixture uniformly to form the electroplating solution;
sealing an entire electroplating tank with the glove box, charging the entire electroplating tank with nitrogen, electroplating the sintered magnet to be electroplated at a temperature of 35° C. for 30 min to obtain a electroplated magnet, and immediately rinsing the electroplated magnet with absolute ethanol, and then drying; wherein an alloy block of Tb and Fe having 75% of Tb by mass is used as the anode, the sintered magnet to be electroplated is used as a cathode, an average value of the pulse voltage is 7 V, a pulse frequency is 3.0 kHz, and a duty cycle is 40%;
S3) diffusion step: subjecting the magnet with a Tb coating obtained from the ionic liquid electroplating step S2) to a heat treatment at a temperature of 925° C. and an absolute vacuum degree above 0.01 Pa for 5 hours;
S4) aging treatment step: subjecting the magnet obtained from the diffusion step S3) to a heat treatment at a temperature of 510 Pa for 3 hours.