Basic Composition and Microstructure of Sintered NdFeB Magnet (neodymium-iron-boron)

Sintered NdFeB magnets have excellent magnetic properties and are widely used in electronics, medical equipment, toys, aerospace, and other fields. You can find the sintered NdFeB magnets in permanent magnet motors, speakers, magnetic separators, computer disk drives, magnetic resonance imaging equipment and meters, etc. Then how are sintered NdFeB magnets made? In this article, we will try to answer this question.

Sintered NdFeB magnet is produced by the powder metallurgy method, and there are 16 process steps in total. The production of sintered NdFeB is a systematic project, and each process must lay a good foundation for the next. The following are the main steps of producing sintered NdFeB magnets.

The hydrogen absorption and desorption characteristics of the hydrogenation disproportionation desorption and recombination (HDDR) process on scrap sintered neodymium-iron-boron (NdFeB) type magnets have been investigated. At each stage of the process, the microstructural changes have been studied using high resolution scanning electron microscopy. It was found that the disproportionation reaction initiates at grain boundaries and triple points and then propagates towards the centre of the matrix grains. This process was accelerated at particle surfaces and at free surfaces produced by any cracks in the powder particles. However, the recombination reaction appeared to initiate randomly throughout the particles with no apparent preference for particle surfaces or internal cracks. During the hydrogenation of the grain boundaries and triple junctions, the disproportionation reaction was, however, affected by the much higher oxygen content of the sintered NdFeB compared with that of the as-cast NdFeB alloys. Throughout the entire HDDR reaction the oxidised triple junctions (from the sintered structure) remained unreacted and hence, remained in their original form in the fine recombined microstructure. This resulted in a very significant reduction in the proportion of cavitation in the final microstructure and this could lend to improved consolidation in the recycled magnets.Basic Composition and Microstructure of Sintered NdFeB Magnet (neodymium-iron-boron)

Basic Composition and Microstructure of Sintered NdFeB Magnet (neodymium-iron-boron)

Basic Composition and Microstructure of Sintered NdFeB Magnet (neodymium-iron-boron)

1. Raw Materials
Good raw materials are the basis for the production of high-quality magnets.

2. Composition Design
The composition design of sintered NdFeB is very important, it relates to whether the product quality and magnetic performance indicators can meet customer requirements. Because many intrinsic magnetic properties of materials, such as magnetic polarization, Curie temperature, etc., are determined by the composition of the material. The basic principle of composition design is to ensure a sufficiently high intrinsic performance, while comprehensively considering the material cost.

3. Smelting
Smelting is the first process for sintered NdFeB magnets to enter the production process. The smelting furnace produces alloy spun strips. The process requires furnace temperature to reach about 1300 degrees and lasts for four hours to complete.

4. Powder Milling
The purpose of powder milling is to crush large alloy ingots into powders of a certain size. In order to obtain a well-oriented magnet, the powder particles are required to have a small size (3-4μm) and concentrated size distribution, and the powder particles are spherical or nearly spherical.

5. Orientation
Powder magnetic field orientation is one of the key technologies for producing high-performance sintered NdFeB. After the crushed magnetic powder is put into the mold, an external magnetic field is applied for orientation, and the powder is compacted after the orientation. There are three types of molding methods commonly used at present: membrane pressure method, mold pressing and cooling isostatic pressing, and rubber mold isostatic pressing. In the case of the same neodymium content, the isostatic pressing of the rubber mold can obtain a larger magnetic energy product.

6. Sintering and Tempering
The relative density of the sintered NdFeB powder compact is relatively high, the contact between the particles is mechanical contact, and the bonding strength is low. In order to further increase the density, improve the contact properties between the powder particles, increase the strength, and make the magnet have the microstructure characteristics of high permanent magnetic performance, the compact needs to be heated to a temperature below the melting point of the basic phase of the powder and heat-treated for a period of time. This process is called sintering.

After the sintered magnet is quenched at high temperature, the grain boundary phase distribution is uneven and the grain boundary is not clear. Therefore, it is necessary to perform tempering treatment at a certain temperature to optimize the structure and obtain the best magnetic properties. Tempering refers to cooling the sintered magnetic powder to a certain temperature and then heating it up again. The tempering temperature shall be determined by test or thermal differential analysis.

7. Machining and Surface Treatment
In practical applications, the shapes of sintered NdFeB magnets are diverse, such as discs, cylinders, rings, squares, tiles, sectors, and various irregular shapes. Due to the different shapes and sizes of the permanent magnet elements, in the production process, except for the large-sized regular permanent magnet elements, it is difficult to form other magnets at one time.

Therefore, in the powder metallurgy process, a large billet is first produced, after sintering and tempering treatment, and then through mechanical processing, grinding processing, and surface coating treatment to produce magnetic materials of the shape and size that meet the needs of customers.

There are 3 types of machining, including:

1. Cutting Processing: cut cylindrical and square columnar magnets into disc-shaped and square-shaped elements.

2. Shape Processing: process round and square magnets into fan-shaped, tile-shaped, or grooved, or other complex-shaped magnets.

3. Punching Processing: Process round bar and square bar magnets into cylindrical or square cylindrical elements.

The machining methods include grinding and slicing, electric discharge cutting, and laser processing.


Thank you for reading our article and we hope it can help you to know how are sintered NdFeB magnets made better. If you want to learn more about NdFeB magnets, we would like to advise you to visit HS Magnets for more information.
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