Sintered Nd - Fe - B magnets are produced by the powder metallurgy method, involving sixteen process steps, and several monitoring and analysis are also included at different stages. Strictly speaking, none of these sixteen steps can be omitted. The production of sintered Nd - Fe - B is a systematic project where each step is closely linked, and every step needs to lay a good foundation for the next one. Once a process step fails to meet the requirements, the permanent magnets produced may not meet the target performance requirements and become rejects, or the yield may be low.
Raw Materials
There is a saying that "Good raw materials make good products", which is highly applicable to the production of sintered Nd - Fe - B magnets. High-quality raw materials are the foundation for producing excellent magnetic materials. Generally, raw materials are purchased according to the corresponding national standards based on the requirements of magnetic material manufacturers for producing high-grade, medium-grade, or low-grade sintered Nd - Fe - B magnets. Before smelting, the raw materials should undergo cutting and surface treatment.
Composition Design
The composition design of sintered Nd - Fe - B is extremely important as it relates to whether the product quality and magnetic performance indicators can meet the requirements of customers. Many intrinsic magnetic properties of materials, such as magnetic polarization intensity and Curie temperature, are determined by the composition of the materials. The basic principle of composition design is to ensure sufficiently high intrinsic properties while comprehensively considering the material cost. (The cost of raw materials accounts for about 65% - 90% of the total cost of sintered Nd - Fe - B materials. Under the premise of meeting the magnetic performance requirements of users, inexpensive component elements should be used as much as possible, and rare earth metals and other precious metal materials should be used sparingly.)
Smelting and Ingot/Casting Sheet Formation
Smelting is the first process in the production process of sintered Nd - Fe - B strong magnets. The smelting furnace produces alloy strip-casting sheets. In this process, the furnace temperature needs to reach about 1300 degrees Celsius and last for four hours to complete. Through this process, the raw materials are processed into alloy sheets through hot melting and cooling, and then proceed to the next process.
Powder Preparation
The purpose of powder preparation is to break the large alloy ingots into powder bodies of a certain size. The latest powder preparation process is to prepare powder from Nd - Fe - B strip-casting sheets (SC sheets) through hydrogen decrepitation and jet milling. In order to obtain magnets with good orientation, it is required that the powder particle size is small (3 - 4μm) and the size distribution is concentrated, and the powder particles are spherical or approximately spherical.
Orientation and Pressing
Powder magnetic field orientation is one of the key process technologies for producing high-performance sintered Nd - Fe - B. After the crushed magnetic powder is loaded into the mold, an external magnetic field is applied for orientation, and then the powder is pressed after orientation. Currently, there are three commonly used pressing methods: diaphragm pressing, die pressing plus cold isostatic pressing, and rubber mold isostatic pressing. Under the same neodymium content, rubber mold isostatic pressing can obtain a larger magnetic energy product.
Sintering and Tempering
The relative density of the sintered Nd - Fe - B powder compact is relatively large, and the contact between particles is mechanical contact with low bonding strength. In order to further increase the density, improve the contact properties between powder particles, increase the strength, and make the magnets have the microstructural characteristics of high permanent magnetic performance, it is necessary to heat the compact to the temperature close to the melting point of the basic powder phase and conduct heat treatment for a period of time. This process is called sintering. After the sintered magnets are quenched at high temperature, the grain boundary phase is unevenly distributed and the grain boundaries are not clear. Therefore, tempering treatment needs to be carried out at a certain temperature to optimize the organizational structure and obtain the best magnetic performance. Tempering refers to reheating the sintered magnetic powder compact after it has been cooled to a certain temperature. The tempering temperature needs to be determined through experiments or differential thermal analysis.
Machining and Surface Treatment
The shapes of sintered Nd - Fe - B magnets used in practical applications are diverse, such as discs, cylinders, rings, cubes, tiles, sectors, and various irregular shapes. Due to the different shapes and sizes of permanent magnetic components, during the production process, except for large-sized regular permanent magnetic components, it is difficult for other magnets to be formed in one step. Therefore, generally in the powder metallurgy process, large blanks are first produced, and after sintering and tempering treatment, machining (including cutting, drilling, etc.), grinding, and surface plating treatment are carried out to produce magnetic materials with shapes and sizes that meet the needs of customers. There are three types of machining, including cutting processing, which cuts cylindrical and square columnar magnets into disc-shaped and square-shaped elements; contour processing, which processes circular and square magnets into sector-shaped, tile-shaped, or magnets with grooves or other complex shapes; and drilling processing, which processes round bar and square bar magnets into cylindrical or square tubular elements. Machining methods include grinding and slicing processing, electrical discharge cutting processing, and laser processing.
Quality Inspection
The quality monitoring during the production process of sintered Nd - Fe - B permanent magnets and the quality inspection of the final products should include the items listed in the following table. However, not every item needs to be inspected, and it is determined by the requirements of the product purchase contract.
