Sintering and heat treatment

The blanks pressed by magnetic field orientation are sintered in a high vacuum or pure inert atmosphere to reach a high density close to 95% of the theoretical density. The magnet holes are in a closed structure, which ensures the uniformity of the magnetic flux density and the chemical stability of the metal; The characteristics of permanent magnets are closely related to their metallographic microstructure. The heat treatment process after sintering is very important for the adjustment of magnetic properties. The sintering process is carefully controlled.

In order to avoid the decrease of coercive force caused by the grain growth of the main phase as much as possible, Nd-Fe-B magnets need to be sintered at a temperature lower than 1100°C. The usual sintering temperature is 1050~1080°C, and close to zero porosity can be obtained. The solid density, the grain size is in the range of 5~15μm; in order to obtain high coercive force, two-stage heat treatment at around 900°C and 500°C is usually required, and rapid cooling is required after sintering and heat treatment to fix the corresponding microstructure. structure. The optimal combination of heat treatment temperature and time is closely related to the added elements and their components in Nd-Fe-B magnets, but a large number of experiments have shown that the primary heat treatment temperature (900 ° C) has a wide range of universality, because it is rich in The Nd phase is in a liquid state. As a grain boundary phase, the surface of the main phase grains can be repaired. As long as the time is not too long, it will not cause excessive growth of the main phase grains or enrichment of the Nd-rich phase. This effect is not related to the composition. Large; the second stage heat treatment is very important to the adjustment of the phase composition and microstructure of the magnet. In this temperature range, the eutectic reaction will occur, and the total amount, composition and distribution of the liquid phase will change, so it will be sensitive It affects the intrinsic coercive force of the magnet, the squareness of the demagnetization curve and the high temperature irreversible loss of the magnet.


   Due to the characteristics and technical limitations of the magnetic field orientation forming process, it is difficult for sintered magnets to directly achieve the shape and size accuracy of practical applications at one time, so machining of sintered blanks is inevitable. The main reasons are:

1. Many finished magnets are small in size and complex in shape, and can only be processed by rough magnets of a certain shape;

2. Even for the near-finally formed blank magnet, due to the low bulk density and poor fluidity of the powder, the filling uniformity of the female mold is not good, and it is difficult to avoid the fluctuation of the shape or size of the sintered magnet blank;

3. Due to the obvious difference in the sintering shrinkage of the Nd-Fe-B blank magnet in parallel and perpendicular to the orientation direction, and the difference in the sintering shrinkage of the boundary and center of the blank magnet, it is finally difficult to meet the dimensional accuracy requirements of the finished magnet.

Due to the consideration of raw materials and labor costs, Japanese, European and American companies mostly choose near-finish forming technology, supplemented by subsequent mechanical processing; Chinese companies produce a variety of rare earth permanent magnet products, mainly adopting the comprehensive production process of rough magnets combined with post-processing, and fully refer to ceramics And the technological advantages of crystal processing have brought the mechanical processing level of rare earth permanent magnets to the extreme. With the increase of raw material cost and labor cost pressure, near net forming and automatic forming technology are developing rapidly in our country.

The rare earth permanent magnet prepared by powder metallurgy is a typical cermet product, which is hard and brittle. For hard and brittle materials, only cutting, drilling, grinding and drilling are required for general machining. Grind and tumble too. Subdivision can be carried out according to the basic characteristics of the processing surface: Blade cutting usually uses diamond or cubic boron nitride powder electroplated blades. Different blade thicknesses and blade edge positions are selected according to the depth of incision and shape and position tolerance requirements. The edge of the inner circle cutter is supported by the blade and the outer hoop. Good flatness can be guaranteed during the cutting process, so the thickness of the blade can be 0.1mm, but the depth of the incision and the size of the magnet to be cut are limited by the difference between the inner diameter of the blade and the difference between the inner and outer diameters. The cutting edge of the outer circle cutter floats on the outer edge, and the supporting capacity of the knife edge is inferior to that of the inner circle cutter. Therefore, to ensure the same tolerance level, a slightly thicker blade is required, generally in the range of 0.2~0.5mm, and the resulting material loss is also bigger. For products with large batches and single size specifications, the use of wire saws is very efficient.

EDM cutting and laser cutting belong to direct thermal processing, which can be used for cutting complex shapes, but relatively speaking, the cutting efficiency is low and the processing cost is high, and some studies have found that the processed surface of sintered NdFeB magnets has a thickness of about the 15μm Nd-rich layer reduces the chemical stability of the material. Magnet drilling depends on diamond or laser. In order to improve the utilization rate of materials, the technology of hollow drilling and hollowing has been developed. The solid cylinder dug out from the center of products with larger inner diameters can also be used to make other small-sized products. The hole pattern can alleviate brittle damage, which is more beneficial to the processing of high-brittle high-performance or high-thermal-stability magnets.

There are two types of grinding wheels: metal base or resin base. Profile grinding is to make the base of the grinding wheel according to the contour of the grinding surface, then coat it with diamond or BN powder, and modify the shape to meet the final product requirements. Machining will produce defects on the surface of the magnet, which seriously affects the performance and corrosion resistance of the magnet, especially for small and thin products, so it needs to be repaired by removing or repairing the surface defect layer.


Leave a Reply