Sintered NdFeB magnets
Sintered NdFeB magnets are high-quality magnets used in multiple processes in the engineering and manufacturing industries. The core tetragonal lattice of sintered NdFeB magnets, combined with the strength and magnetism of its constituent elements – neodymium, iron, and boron – shapes these magnets as some of the best permanent magnets known.
Their distinct strength and ability to function under high temperatures make sintered NdFeB magnets suitable in electronics and aerospace, as well as in the production of medical tools. This article further explores sintered NdFeB magnets.
What are sintered NdFeB magnets?
The Japanese scientist Masato Sagawa produced the earliest batches of sintered NdFeB magnets almost four decades ago. The magnets resulted from a mixture of neodymium, iron, and boron and were described as the strongest magnets known right from their creation. They became commercially available immediately and have been trusted for many metallic projects since then.
Sintered NdFeB magnets are manufactured through a powder metallurgical process that allows the substitution of their constituents elements with other magnetic elements to enhance specific properties of the magnets. For instance, a portion of the neodymium can be exchanged with dysprosium or any other rare earth metal to increase the strength of the end product slightly. Cobalt can also replace some iron portions to alter the magnet’s Curie temperature limit, stability under heat and temperature, and corrosion resistance.
The desired output determines the modification done during its production process. To control the microstructure of the sintered NdFeB magnet, you can include some elements that impart doping effects, such as aluminum and copper. These controlled magnets yield different grades of magnets when the manufacturing is complete.
The strength of sintered NdFeB magnets can be deduced from several magnetic properties. The maximum energy product, BH max, a product of a magnet’s magnetism and intrinsic coercivity, is often used to grade the magnetic performance of sintered NdFeB magnets. Presently, there are seven classes of these magnets, with the N28AH magnet having the least maximum energy product (BH) max. The mega-gaussian-oersted (MGOe) is used to represent a magnet’s (BH) max, with 1 MGOe equalling 7.958 kJ/m3 of the magnet.
As a group, the energy products of sintered NdFeB magnets range from 30 MGOe to 52 MGOe, with possible improvements in the coming years. Sintered NdFeB magnets will also operate between – 20C to 300C, depending on the grade of the magnet.
How are sintered NdFeB magnets made?
Depending on the manufacturer, the steps involved in producing sintered NdFeB magnets range between seven and ten. These magnets are made using the powdered metallurgy method, which involves mixing the alloy powders before heating and sintering the resulting mold in an atmosphere-controlled furnace to bond the articles.
The process is a series of closely-linked steps divided into front-end processing and post-processing. In the latter, the magnetic molds are processed into finished products. Magnet manufacturers may major in both production phases or choose the post-processing phase. The production of sintered NdFeB magnets begins with:
- Sourcing of the raw material. Good raw materials are the basis of any premium magnetic material. The choice of raw material depends on the desired grade of finished product demanded by a client or in need in the market. Once the rare earth metal ore is mined, the ore is refined and surface treated to prepare it for smelting. The alloys’ composition affects the magnet’s intrinsic properties, including the limits of its Curie temperature. Thus, manufacturers prepare the raw material to conform to the expected standards or the standards set by the industry.
- The composite raw materials are left in a furnace with temperatures as high as 1300 celsius. The intense heat breaks down the molecules in the elements and facilitates the fusion of the raw materials. Smelting also marks the first step in preparing sintered NdFeB magnets, lasting up to four hours.
- This is a combination of two processes – hydrogen breaking and jet milling. They both involve external pressure to contain the metallic alloys to powdered materials of about 4 micrometers in radius. The size distribution of the grains must be well distributed, and the grains must attain a spherical or near-spherical shape. This will ease the effect of the external magnetic field applied on the alloys during orientation.
- Here, the alloys are magnetically charged by exerting an external magnetic field. This is done through one of three methods: membrane pressure, mold pressing, and isostatic cooling. The isostatic cooling method boosts the maximum energy product of NdFeB magnets. After this, the powdery form is then compacted into molds.
- Expectedly, sintered magnets undergo sintering to improve and enhance specific magnetic features. The metallic mold, though magnetic, has a high relative density and a low bonding strength. When the mold is heated to a temperature beyond its melting point, the bonding strength increases, the mold takes on a microstructure shape, and the frictional contact between the mold components is reduced. Sintering is thus complete.
- Tempering is a loop process that involves the cooling down of the heated mold before reheating it to its previous temperature. This evens out the grain boundary phase of the magnetic mold and marks the grain boundary within the fold. Overall, the magnetic properties of the mold are optimized.
- A magnetic mold does not immediately fit into the desired shape for industrial applications. Machining helps manufacturers achieve pre-determined shapes and sizes for different magnets. Different companies adopt other machining methods, considering cost requirements and labor demands. The machining process could be cutting (converting the mold to square and round shapes), shaping (converting the molds into unique shapes), or punching (for making cylindrical magnets).
- Grain diffusion. This optional part increases the magnetic strength of sintered NdFeB magnets by adding the rare earth metals dysprosium and terbium. The elements increase the overall production cost. Thus the process is bypassed by manufacturers.
- Surface treatment. Finished products may contain voids and pores that facilitate the oxidation and corrosion of the metals. To prevent this, the magnets are taken under surface inspection and treated by electroplating, plating, or electrophoresis.
- Sintered NdFeB magnets are strong magnetic materials, and the action of a magnetizer can enhance their magnetism. This also allows the magnets to reach their magnetic saturation limit, increasing their remanence and coercive force.
Before shipping, the magnets are packaged in durable containers to prepare them for clients. Due to their magnetic properties, sintered NdFeB magnets are mostly shipped on land, though they can also be shipped on a flight.
Why should you use sintered NdFeB magnets?
Sintered NdFeB magnets are highly magnetic and considerably lighter when compared with other magnets with similar strengths. Thus, they are recommended for applications that require premium magnets. Choosing to use NdFeB magnets will depend on the working environment; sintered NdFeB magnets have a temperature range within which they are at optimal performance.
Below 200C, these magnets may become prone to corrosion and oxidation, especially if used in extensive procedures. Using the magnets at elevated temperatures may also reduce the remanence and maximum energy products; this can be improved by combining the magnets with alloys that have a high intrinsic coercivity.
Sintered NdFeB magnets are relevant in the following processes:
- Production of large-capacity internal and external hard drives and light headphones.
- Designing anti-lock braking systems in modern vehicles.
- In making effective motors for vehicles, especially for electric cars.
- Industrial windmill generator design and operation.
Are sintered NdFeB magnets inferior to bonded NdFeB magnets?
The procedural manufacturing of sintered NdFeB magnets differs from bonded NdFeB magnets. Bonded magnets are produced through a short-phase series without post-processing to furnish the products. They are much more flexible in shape and design and are cost-saving for industrial magnet manufacturers.
Sintered NdFeB magnets undergo a more complex manufacturing process; their maximum energy product, which reaches up to 50M, surpasses that of bonded NdFeB magnets. They incur large costs during the production, but their incomparable magnetic strength makes them fit for highly-complex industrial applications.
Bonded NdFeB magnets are optimized for manufacturing products such as automation machines, CD-ROM, mobile phones, and other industrial needs that do not require magnets with a strong magnetic field. In such cases, sintered NdFeB magnets are preferred.
Sintered NdFeB magnets serve several purposes in manufacturing and production; despite the remarkable volume loss recorded while they’re being processed from alloys to molds, the relevance of these magnets ensures that they’re in constant demand within the industry.
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