Neodymium-Iron-Boron Magnet having Gradient Coercive Force and its Preparation Method

A preparation method of a NdFeB magnet having gradient coercive force, comprising following steps of:

• (1) preparing at least two alloys having components of R—Fe—B-M, wherein the alloys comprise at least alloy A and alloy B;
• R contains at least one rare earth element selected from Pr, Nd, Dy and Tb;
• M contains one or two more elements selected from Co, Cu, Ga, Nb, Al, Mn, Zr and Ti;
• and the weight percentage of M is below 5%, and the content of Dy and Tb in the alloy A is higher than the content of Dy and Tb in the alloy B, and the content of Dy and Tb in the alloy B is higher than the content of Dy and Tb in other alloys;
• (2) pulverizing the alloy obtained from the step (1) into powder via a pulverizing device by at least one of following preparation methods of:
• (a) putting alloy flakes respectively into a hydrogen treatment furnace for a hydrogen pulverization in a protective environment of inert gas or N2 and then into a jet mill for a fine pulverization;
• and
• (b) grinding and pulverizing alloy flakes respectively and then processing the alloy flakes with a fine pulverization via a jet mill;
• (3) molding the powder obtained from the step (2) in a magnetic orienting and molding device, wherein at least one separating board is pre-provided therein;
• the powder is respectively filled into different separated cavities;
• and the separating board is removed until the powder filling is finished, wherein the powder of the alloy A is filled into at least an external cavity;
• and
• (4) sending the compact into a sintering furnace to be sintered at 1000° C. to 1120° C. for 1 hour to 6 hours and subsequently processing with aging treatments at 850° C. to 950° C. for 1 hour to 6 hours and at 450° C. to 600° C. for 1 hour to 6 hours, so as to obtain a NdFeB magnet having gradient coercive force.