Solidification Behavior of Laser Melting Layer of Nd_(15)Fe_(77)B_(8) Sintered Magnets and Its Microstructures Evolution

摘要

To research the solidification behavior and microstructures of a laser remelting/solidification layer on aniso-tropic Nd_(15)Fe_(77)B_8sintered magnets with their magnetization direction parallel to X, Y, Z-axis respectively, their surfaces (parallel to XOY plane) were scanned by 5 kW Roffin-Sinar 850 type of CO_2 laser along Y axis. The rapid solidification of the molten alloy in the layer results in three distinct zones. The transition zone close to the unmolten portion of a magnet (substrate) , consists of the columnar Nd_2Fe_(14)B phase (matrix), the 10.0%～15.1% dendrite primary iron phase dispersing in the matrix, and the Nd-rich phase along Nd_2Fe_(14)B grain boundaries. The columnar crystal zone in the middle of the layer consists of the long columnar Nd_2Fe_(14)B grains and their grain boundary Nd-rich phase. And the dendrite crystal zone near the free surface of the layer consists of dendrite Nd_2Fe_(14)B grains and their grain boundary Nd-rich phase. When the laser scanning velocity is lower, the growing direction of the microstructures in the layer tends to the laser scanning direction step by step. When the velocity is not lower than 25 mm·s~(-1), the laser remelting/solidification layer thins and the columnar crystal zone comprises almost the whole layer. Under this condition, on the substrate with its magnetization direction along X or Y-axis respectively, the columnar Nd_2Fe_(14)B grains in the layer grow in the direction of Z-axis (that is their long-axis along Z-axis), their alignment of the easy magnetization axis [001] is parallel to the magnetization direction of the substrate correspondingly; but on the substrate with its magnetization direction along Z-axis, the columnar Nd_2Fe_(14)B grains in the transition zone grow at an angle of 30°～50° between Z-axis and their long-axis. And the columnar Nd_2Fe_(14)B grains in the columnar crystal zone gradually tend to the Z-axis, and their easy magnetization axis [001] arrange in the range of 0°～360° of the plane perpendicular to their long-axis.