How are Neodymium Magnets Made?
Neodymium Magnets are strong permanent magnets made from alloys of rare-earth metals.
Neodymium magnets are primarily made with the alloy of neodymium, iron, and boron (NdFeB). They also have small amounts of elements like praseodymium (Pr), dysprosium (Dy), aluminum (Al), and niobium (Nb). These may be added to enhance properties such as strength, temperature tolerance, and resistance to demagnetization and corrosion.
Preparation of the Neodymium alloy begins by melting the metals to a vacuum induction furnace. The melted alloy is cooled by strip casting, a rapid cooling technique, resulting in thin flakes of the material.
These flakes are broken down and placed in a jet mill where they are pulverized into a fine powder.
Sintered neodymium magnets are made by vacuum heating the rare earth metal particles used as raw materials in a furnace. The elements– chiefly neodymium, iron, and boron – are selected to result in a designated grade of magnet. The chemical composition of the of the magnet is adjusted to determine magnetic polarization, Curie point, flux density, and coercivity.
Once melted, the NdFeB (neodymium, iron, boron) mixture is cast into a mold and cooled to form ingots. The ingots are ground into tiny grains and milled, typically in a jet mill. This fine powder is pressed into a shaped mold. Magnetic energy from a wire coil is applied while the powder is heated and melted.>
This forms the neodymium into form dense blocks. The magnetism from the coil is generated when electrical current passes through it.
After the crushed magnetic powder is put into the mold, an external magnetic field is applied for orientation. The directional orientation of the magnetism is fixed as the mixture is being pressed. The powder is fully compacted after the orientation.
The resulting magnet is said to be anisotropic, i.e., the direction of the magnetism aligns with the particle structure. By maximizing the magnetic orientation in the direction of the magnet’s poles, the strength is enhanced.
There are three distinct methods used to press sintered NdFeB magnets, each yielding a slightly different end product. The common methods are axial, transverse, and isostatic pressing. Each represents a particular relationship between the pressing axis and the magnetic alignment axis.
With axial pressing, the pressing and alignment axes are the same. Transverse pressing indicates that the pressing axis is perpendicular to the alignment axis. Finally, applying pressure equally from all directions is known as isostatic pressing. When pressing magnets isostatically, the magnetism is aligned before the magnets are pressed.
After the magnetic direction is locked, magnetized material is demagnetized. Because the material is too brittle for practical use, it must now be sintered. Sintering heats it in an oxygen free environment to near its melting point so that the magnetic particles fuse together.
After sintering, the magnet is quenched. The heated material is rapidly cooled, imbuing the material with greater strength and hardness. After the sintered magnet is quenched, a tempering treatment is performed to cool the magnetic powder.
Once it reaches the designated temperature it is reheated. The rapid cooling enhances the performance of the magnet by reducing the areas of poor magnetism.
The magnets can now be machined into their appropriate, useful shapes. Diamond plated cutting tools are used, due to the magnets’ hardness. The machining methods include grinding and slicing, laser processing, and electrical discharge machining (EDM).
Bonded NdFeB magnets are rare earth magnets made from NdFeB magnetic powder and a binder. The powder is prepared by grinding the NdFeB alloy into a powder and combining it with a polymer. Bonded magnets are not only extremely useful as finished magnets, they are also used as components in many other products. Compared to other types, these magnets often contain less neodymium and more iron.
Bonded magnets may be made by injection molding, extrusion, calendering, or compression bonding.
With injection molding, a melted thermoplastic compound is injected into a mold. There, it cools and solidifies into the right shape. For neodymium magnets, NdFeB is used as the magnetic powder in this mixture. Magnets can be shaped and formed by this process, which works well with assembles and over molding manufacturing techniques.
The extrusion process pushes the mixture through a heated barrel with a large screw. The mixture is pressed through a heated die, and that material is cut to the right length.
Calendering is a way to make continuous magnet sheets. This is often used for flexible magnets. A powdered compound of iron powder and elastomer is pushed through a set of hot rollers. These rollers stretch and smooth the strip, creating a uniform sheet.
In compression bonding, NdFeB is processed through a powder refinement process, blended with a plastic material and compression molded. Compression bonded neodymium magnets can be magnetized in any direction and with multiple poles. They are typically used in small motors, mobile phones, electronics, automobiles, etc. Other applications include brushless motors, speakers, buzzers and toys etc.
Because neodymium magnets are brittle, they are prone to chipping and breaking. The NdFeB substrate can also oxidize quickly without a protective layer. To prevent this, they are coated, cleaned, and plated to protect the magnet against corrosion.>
Before the material is re-magnetized, a protective coating is applied to extend the lifespan of the magnet. This is usually an electroplated coating of three layers consisting of nickel, copper, and nickel.
Any coating or plating must be applied to a sintered magnet before the it is saturated (charged). High heat can demagnetize the magnet, and the magnetic field can disrupt the electroplating process. The most common plating is a nickel-copper-nickel mixture, but other metals or PTFE polymers can also be applied.
Bonded Neodymium magnets are also typically coated prior to use, usually with an Electrophoresis Coating (“E-coating”) or Spray Coating process. Alternative coatings and methods can be used for magnets used in extreme temperature applications or corrosive environments. E-coating is widely used because it is suitable for different applications and has a uniform thickness. Spray coating is more suitable for smaller magnets and not recommended for corrosive environments.