The Production Process of Neodymium Magnets
From raw materials to finished products, neodymium magnets have experienced countless difficulties to become the “King of Magnets” we know today. This article will explain the complete production process of neodymium magnets.
Step 1: Mixing Raw Material
The core raw material of neodymium magnets is rare earth. The main elements in NdFeB magnets are Fe(65%), Nd (32%) and B(2%) and it also contains some additive elements, such as Dy, Tb, Co, Al, Cu, Ga, etc..
Based on the different requirements of magnetic performance, the factory will customize unique formulas of NdFeB magnets. If customers require stronger magnetic force (i.e., higher Br), the factory will increase the content of neodymium ; if customers need magnets with high-temperature resistance (i.e., higher Hcj), the factory will increase the content of heavy rare earth.
Step2: Melting
Put the raw material into a Vacuum Melting Furnace for high-temperature melting. The purpose of it is to melt metal blocks into a liquid form at high temperature. During the process, continuous stirring is required to ensure that raw materials are thoroughly mixed and become the uniform alloy melt. Then by spinning process, the alloy melt will be transformed into the thin sheets in the picture below.
Step 3: Milling
Fine powders will improve the performance of neodymium magnets. Therefore, the large alloy castings produced by smelting need to be crushed into powders with a particle size of 3-5 μm. Since neodymium particles are easily oxidized, during the entire milling process, it is necessary to add H2 and N2 to protect the raw material from oxidation. The milling process involves two steps: Hydrogen Decrepitation and Jet Milling.
a. Hydrogen Decrepitation（HD）
Hydrogen decrepitation can achieve the initial crushing of the alloy and improve the crushing efficiency of subsequent jet milling. This is because rare earth have strong hydrogen absorption ability. And hydrogen is slowly absorbed into the solid alloy, making the alloy break and become fine powders. Additionally, hydrogen decrepitation allows hydrogen atoms to adsorb onto the particle surface, further protecting the powder from oxidation. The powder produced by the HD process is coarse powder.
b. Jet Milling
Put the coarse powder into a jet milling, where is the high-pressure airflow environment, causing the powder to collide with each other and become fine powder. The powder smaller than 3-5μm will be delivered into a cyclone separator for separation, while the powder that does not meet the requirements will be returned to the jet milling for further grinding until it reaches the 3-5μm requirement and is then separated.
Step 4: Pressing
Neodymium magnet is an “anisotropic” magnet. Each particle itself is a “small magnet” with a north and south pole. Without a magnetic field, they point randomly in all directions. This means that the entire magnetic strength will be strongest only when the magnetic directions of all the particles are aligned. This process includes two steps: Orientation and pressing.
a. Orientation
Place 3-5μm neodymium alloy powder into a mold. At the same time, apply a very strong magnetic field. When this external strong magnetic field is suddenly applied to these powder particles, they begin to rotate rapidly, as if they had received the command “At ease! Turn right!”, aligning the direction in which they are easily magnetized. The purpose of this is to ensure that the magnetic force generated by all the particles combine in the same direction, rather than canceling each other out. This is a necessary step to produce the high-performance neodymium magnets.
b. Pressing
While maintaining the applied magnetic field, using Isostatic Pressing Machine to apply a huge pressure to the already aligned powder, compressing it into a dense magnet blank. The pressure tightly squeezes the loose powder particles together, forming a certain strength and shape magnet blank such as a disc, square, or ring. The purpose of pressing is to shaping and solidifying the orientation result. Without pressing under a magnetic field, once the magnetic field is removed, the powder particles will become disordered again.
Step 5: Sintering
Place the pressed products into the Vacuum Sintering Furnace for high-temperature sintering (usually at 1000-1100°C). Under high temperature, solid-phase diffusion and liquid-phase sintering (if a liquid-phase is present) occur between the powder particles. It will increase the density of magnets and form a dense sintered body.
Step 6: Initial inspection
The initial inspection of raw magnet blanks mainly includes magnetic performance parameters, such as Remanence (Br), Intrinsic Coercivity (Hcj), Maximum Energy Product ((BH)max) etc.. The purpose is to identify and reject defective products, ensuring the effectiveness of subsequent processing.
Step 7: Machining
Sintered magnets are large in size and have rough surfaces. Therefore, the raw magnets need to be cut, ground, and drilled to become the specific shape required by customer. Due to the high hardness and brittleness of neodymium magnets, it is necessary to use precision machining equipment such as Slicing Machine, Grinding Machine and Multi-wire Cutting Machine.
Step 8: Surface Treatment
Neodymium magnets are prone to oxidation and corrosion, so surface treatment of magnets is essential. Nickel plating, Zinc plating, Epoxy are the common surface treatment of magnets. The coating not only isolates the magnet from humid air, extending its lifetime, but it also meets the functional requirements of specific applications. There are many factors that influence the choice of coating, such as usage environment, budget, appearance etc.. If you don’t know how to choose the suitable coating, please refer to this article: Which Coating Should I Use to Protect Neodymium Magnets?
Step 9: Final Inspection
The final inspection of the magnets including magnetic performance (such as surface magnetic field strength, magnetic flux, etc.), dimensional accuracy, appearance quality, surface coating quality and corrosion resistance to ensure all indicators will meet customer requirements.
Step 10: Magnetization
Magnetization is a crucial step to activate the performance of magnets. The magnetizer generates a strong magnetic field that enables the neodymium magnets to reach a state of saturated magnetization and allowing them to show strong magnetic performance. Depending on the customers’ requirements, our factory can achieve various types of magnetization, including monopolar magnetization, multipolar magnetization radial magnetization, etc..
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