Ultimate Guide: How Are Sintered Neodymium Magnets Made?

Table of Contents

Sintered neodymium magnets, also known as NdFeB magnets, are made from a powder composed of neodymium, iron, and boron. The powder is placed in a mold that is the desired shape of the finished magnet. The mold is then placed in a furnace and heated to a high temperature. The heat causes the particles in the powder to “sinter.” Once the sintering process is complete, the mold is cooled, and the magnet is removed. The magnet is then coated with a layer of nickel to prevent corrosion. Finally, the magnet is magnetized before packaging.

To make Neodymium magnets, dozens of process steps must be followed precisely. Below, we will summarise some of these key processing steps.

Let’s briefly go over the manufacturing process of Neodymium magnets, which are as follows:

  1. Raw Material Composition
  2. Melting and Strip Casting
  3. Pulverization (HD Crushing& Jet Milling)
  4. Isostatic Pressing
  5. Sintering and Annealing
  6. Machining
  7. Coating
  8. Magnetizing

Step #1: Material Composition for Sintered NdFeB Magnet 

The primary method of controlling the suitable component ratio for custom-sintered NdFeB magnets is to add heavy rare-earth elements and micro-quantity additives. This allows you to obtain the required magnet grade and magnetic properties.

The ratio example: (PrNd) 30-32%, (B) 1%, ((Dy): Grade H 3%, Grade M 2%, Grade N 1%), (Co) 0.4-1.5%, (Cu) 0.1-0.15%, (Al) 0.2%, (Tb) 0-1%.

Three essential elements form sintered NdFeB magnets: rare earth neodymium (Nd), iron (Fe), and boron (B). The atoms of Neodymium couple with ferromagnetic iron atoms, helping the magnet to achieve a high remanence and maximum energy product. This makes it more extraordinary compared to other permanent magnets. Although Boron only has around 1 percent weight in the makeup of the magnet, it is necessary for phase stability, resulting in stable magnetic properties for this type of magnet.

Usually, commercial sintered magnets contain Nd with some Praseodymium (Pr) substitution. This combination’s physical and chemical properties make it more economically viable to produce an alloy of PrNd instead of pure Nd. This way, a stronger magnet can be created from the raw material.

The coercivity (Hcj or Hci) of a neodymium-iron-boron magnet can be increased significantly by substituting Dy and/or Tb elements for Nd due to their higher magnetocrystalline anisotropy field HA. Neodymium-iron-boron magnets have high coercivity at room temperature but lose much of it as the temperature rises above 100 °C (212 °F) until reaching the Curie temperature (around 320 °C or 608 °F). The fall in coercivity limits the efficiency of magnets under high temperatures. Dysprosium (Dy) or terbium (Tb) is added to decrease performance loss from temperature changes. However, this makes the magnet more expensive. The total content of Dy and Tb elements in magnets is typically less than 10 wt% because of cost and remanence (Br) loss.

The iron (Fe) element can be substituted for a portion of the cobalt (Co) element to increase the magnets’ Curie temperature Tc, thermal stability, and corrosion resistance. The Curie temperature of cobalt (1403K) is higher than that of iron (1043K). As such, each at% substitution of Co for Fe will increase the final magnet’sCurie temperature by 11oC.

Furthermore, Magnet microstructure homogeneity can be significantly improved by adding just a tiny amount of some aspects like aluminum (Al), copper (Cu), niobium(Nb), and gallium (Ga). This not only increases the maximum magnetic field strength obtainable ((BH)max) but also improves the coercivity((Hcj).

Step #2: Melting & Strip Casting Process for Sintered NdFeB Magnets

The alloy, composed of neodymium rare-earth alloys, iron, boron, and other additive elements at a designated composition rate, is melted in a vacuum induction melting furnace from industrial-grade raw materials that are over 99.5% pure. Which then proceeds to the strip casting (SC) technique into SC alloys.

Vacuum induction melting

The vacuum induction melting furnace generates an eddy current in the metal charge through electromagnetic induction so that it can be heated to a high enough temperature, allowing various metals to melt and fuse into an alloy.

Advantages of vacuum induction melting

The vacuum induction melting furnace is easy to control, so the alloy’s purity, composition, quality, and stability are guaranteed. This technology is the essential means of Alloying and cannot be replaced by others.

Processing notes for vacuum induction melting

The main issue with vacuum melting is that due to the low-pressure surface of the molten pool and the electromagnetic stirring effect, some non-metallic inclusions may float on the surface of the molten pool and form an oxide film. If this film mixes into the alloy, it will lower product quality. The inclusions are mainly removed from the alloy by decomposition, volatilization of low-valent oxides, and combination with carbon and oxygen. However, at melting temperature, the partial pressure of O2 and N2 is too high for decomposition. As a result, other processes must be done to remove them altogether.

Strip casting (SC) and its advantages

The strip casting (SC) process is a quick technique for creating alloys sintered NdFeB magnets. With this method, molten alloys are poured onto a spinning copper wheel which rapidly cools the alloy and forms an optimally advantaged microstructure including less alpha iron than conventionally cast processes and a finer grain structure.

Advantages of strip casting (SC)

As one of the most useful new technologies to come into production in China, this new method of preventing base material corrosion increase the material’s intrinsic resistance and enable improvements in reducing costs, size, and wasted energy.

Processing notes for strip casting:

The control of the strip casting process must be exact, which is problematic. 

To create optimally sized grains, the molten metal’s temperature, the metal’s flow rate, the stream’s shape, the chill medium’s velocity, and temperature must be carefully monitored.

Controlling the thickness of NdFeB SC alloys is essential.

The varying thickness of SC alloys resulted in a difference in microstructure and phase composition. With a 0.2 to 0.3 mm alloy, complete columnar grains could grow well from the rolling surface to the free surface. Additionally, there was good dispersion of Nd-Pr-rich phases along the main phase grains absent α-Fe. When SC alloys are 0.3-0.4 mm thick, the volume fraction of total columnar grains decreases; however, when they are 0.5 mm or more viscous, α-Fe phases begin to appear on the surface, and delicate grain areas develop near the sides of the rolling surface. As thickness gradually increases, more α-Fe phases precipitate, with lumpy Nd-Pr rich phase appearing in this area. Therefore, controlling appropriate thickness is key to producing high-quality SC alloys in mass.

Step #3: Pulverization (HD Crushing& Jet Milling)

After the strips are cast, they undergo hydrogen decrepitation crushing, breaking them down into coarse, fragile particles. A jet milling procedure mills these coarse particles until their particle size is about 3-4 μm.

Principle of hydrogen explosion treatment

The principle of hydrogen explosion treatment (HD) is a physicochemical method that uses explosive force to break down the material. It is beneficial for alloys or intermetallic compounds that contain rare earth elements, as they have an affinity for absorbing hydrogen.

For example, when NdFeB is hydrogenated to form a hydride, the resulting lattice expansion produces enormous stress in the NdFeB crystal. This results in many micro-cracks and makes the material lose or revert to its original coarse powder form. If you heat it to dehydrogenate it, part of the primary phase hydride will return to a coarse powder while some of the nd-rich hydrides remain in the material.

Important notes for the HD crushing process

Hydrides: Remember that the HD-produced coarse powder isn’t only the original casting strips’ coarse powder. The latter still has some hydrides whose chemical makeup and magnetic properties have changed. Therefore, this must be considered during the later steps of NdFeB magnet production.

Particle Size: the HD crushing method is only suitable for metal or alloy hydrogenation that can be broken down into pieces no larger than 0.1-100 mm. The powder size will typically be 10-1000 μm and is ideal for hydrogen storage alloys.

Reaction Tank Placement: The reaction tank will be placed vertically after completing the HD treatment. All the alloy material in the tank will be injected into the aggregate tank via funnel and valve door–this method prevents leakage. Keep in mind that not only does this apparatus make it easy to achieve requirements, but it is also more secure and effective.

Jet Milling of Hydrogenated NdFeB Magnet

The HD process shrinks the powder, but it still needs to be reduced even more in size through jet milling (JM). The JM procedure uses almost supersonic jets of gas. These confined jets create a vortex, and HD powder is introduced into this tornado. When the particles made from HD crash into each other in this way at high speed, their average particle size decreases significantly.

NdFeB magnets are best prepared with a powder size of 0.7-7 μm and an average particle size of 3-4 μm. Varying the distribution of particle sizes can alter the microstructure and magnetic properties of sintered NdFeB magnets.

Sintered magnets usually have better remanence and maximum energy product when fewer fine particles are present, while coercivity increases by eliminating coarse particles. However, if the powder is classified as too fine or too rough, the magnetic properties will suffer due to excessive grain growth or poor densification. Magnets made from powder with mostly fine or coarse particles had lower density values.

Step #4: Pressing in Magnetic Field Process

After milling, the HD powder is usually aligned in an external magnetic field and pressed to form green compacts in the shape of a cylinder or block.

The pressing process has three primary goals: compressing magnetic powders into green compacts of a specific density, molding the powders into the required size and shape, and preserving the magnetic orientation achieved in the presence of a magnetic field.

To achieve strong magnetic properties, all powder particles’ easy axes must be aligned in the same direction as the final magnetization. This is done by placing the powder in a magnetic field and pressing it, so its orientation stays compact.

Alignment and mold pressing co-occur, after which isostatic pressing takes place.

The two main techniques for alignment and mold pressing are axial pressing (where the compact experiences a magnetic field parallel to the axis it’s being pressed on) and transverse pressings (with a magnetic field perpendicular to the axis).

After mold pressing, these pressed compacts have a density of 2.0g/cm³. To achieve properties, this needs to be isostatically pressed to increase its density and magnetic capabilities further.

To avoid oil immersion and oxidation, the green compacts will be vacuum sealed before being placed into an isostatic pressing machine. Fluid pressure is applied outside the sealed packing, furthering its compactness on all sides equally.

Isostatically pressed compacts have better alignment than axially or transversely pressed compacts, resulting in a denser compact with a higher energy product. These types of compacts are also referred to as green compacts and generally have a density of 4.0g/cm³.

After sealing, we extracted the bag from the machine and removed any oil on its surface before transferring it to storage.

Step #5: Sintering and Post-sintering Heat Treatment

The green compacts are then densified by sintering in a vacuum at a temperature over 1000°C. Post-sintering heat treatment in a vacuum at about 600°C is also necessary to enhance the coercivity of the magnet. 

Sintering will solidify the magnets. Careful control of the sintering temperature, sintering time, and oxygen content is crucial to the development of the properties of the final magnets.

The influence of sintering temperature on the densification of NdFeB magnets

The sintering temperature has to be high enough to allow the grain boundary phase to liquefy but not too high since the grain growth is quickly activated at high temperatures. The optimal sintering temperature varies slightly with the composition of the magnet.

After the sintering process, heat treatments (known as annealing) can improve coercivity by creating a thin Nd-rich grain boundary phase with a Cu-enriched layer. This is expected to decouple the exchange interaction between the Nd2Fe14B hard magnetic grains and increase coercivity.

The influence of sintering time on the densification of NdFeB magnets

The sintering dwell time affects grain growth. To achieve good magnetic properties, tiny grains of Nd2Fe14B surrounded by the Nd-rich phase are required. Hence, the dwell time needs to be sufficient to achieve density; any other holding will result in grain growth.

The influence of oxygen content on the densification of NdFeB magnets

If the oxygen content is increased, it can form Nd2O3, which results in a lower density. However, if the oxygen level becomes too low, it can cause grain growth abnormalities. Density and grain size both affect magnetic properties.

Step #6: Machining Process

Because of magnets’ characteristics and technical limitations during the magnetic field orientation forming process, sintered magnets are often inaccurate in shape and dimension. To get a variety of sizes and shapes, machining is required on the rough blanks after the sintering process.

Before machining, rough blanks usually come in one of two shapes–cylinders or blocks. The chosen shape depends on what final form the required magnets will take. If the necessary magnets are block-, oval-, arc-, square stair-step-, or square countersunk-shaped, then they are based on block-shaped rough magnets. When the magnets required are in the shape of a disc, ring, rod, round stair-step, or countersunk, they’re based on a cylinder rough magnet.

Due to brittleness and poor mechanical properties, Neodymium magnets are generally machined by slicing, grinding, drilling, and chamfering to obtain the required small sizes, precise tolerance, and complex shapes. 

Slicing procedure of neodymium magnet

Slicing neodymium magnets can be done in a few ways. The most common way is to use a diamond wire saw. The saw has a diamond wire used to cut the neodymium magnet. The advantage of using the diamond wire saw is that it is very accurate and leaves a clean surface on the magnet. Another way to slice neodymium magnets is with a water jet. A water jet uses high-pressure water to cut through the magnet. The advantage of using a water jet is that it can cut through very thick magnets. The last way to slice neodymium magnets is with a laser. A laser can be used to cut through fragile magnets.

Grinding procedure of neodymium magnet

Grinding neodymium magnets is a typical process to achieve the desired shape and size. There are many different grinding machines, each with advantages and disadvantages. Neodymium magnets are ground using diamond grinding wheels because of their hardness and sharp edges. The grinding process can be done dry or with coolant. When grinding dry, the chips produced by the grinding wheel can cause problems such as clogging the cooling system and affecting the accuracy of the machined part. Grinding with coolant helps to remove the chips and prolongs the life of the grinding wheel.

Drilling procedure of neodymium magnet

The drilling procedure of neodymium magnets is essential to prevent damage to the magnet. Drilling deep holes with a standard twist drill can cause the magnet to shatter because of the high brittle strength of neodymium magnets. Using a diamond-coated bit and a low drilling speed can minimize this possibility.

Ultrasonic drilling is a process that uses ultrasonic vibrations to create holes in materials. The process is slower than traditional drilling methods but does not cause any damage to the material being drilled. In addition, since ultrasonic vibrations produce little heat, there is no need to use a cooling liquid when ultrasonically drilling neodymium magnets.

The chamfering procedure of neodymium magnet

Chamfering neodymium magnets is a process that is used to remove the sharp edges of the magnet. This is done by putting the magnet in a chamfering machine and running it against a wheel. The chamfering wheel is made of a softer material than the magnet and wears down the sharp edges of the magnet.

Step #7: Surface Treatment And Coating

The surface coating process is used to protect neodymium magnets from corrosion and to improve their mechanical properties. The most common type of coating is nickel-copper-nickel (Ni-Cu-Ni) plating, which gives the magnets a silver appearance. Other coatings that can be used are zinc, gold, and epoxy.

Surface plating procedure

The Ni-Cu-Ni plating process coats neodymium magnets with nickel, copper, and nickel. Here are the steps:

  1. Clean the neodymium magnet with a solvent to remove oils or dirt.
  2. Place the magnet in a bath of nickel-plating solution.
  3. Place the magnet in a bath of copper plating solution.
  4. Placed into another Nickel Plating Solution for final layer coverage
  5. Dried, then Inspected for Quality Standards

The production procedure of another plating of neodymium magnets is the same as Ni-Cu-Ni plating. The only difference is the type of plating solution used. The magnets are placed in a bath of zinc plating solution for zinc plating. The magnets are placed in a tub of gold plating solution for gold plating. And for epoxy coating, the magnets are coated with a layer of epoxy resin.

Step #8: Magnetizing

Magnetization is aligning the magnetic dipoles in a material to create a magnetic field. This can be achieved by applying a magnetic field or exposing the material to an existing magnetic field. The resulting magnetization creates a magnetic force that can attract or repel other magnetic materials.

The strength of the magnetization and the resulting magnetic field depends on the type of material and the strength of the applied magnetic field.

Conclusion

Magnets are a fascinating product that has many uses. In this blog post, we’ve looked at the manufacturing process of sintered Neodymium magnets. You may better understand how they are made of, the importance of each step in the manufacturing process, and how it contributes to the final product.

Contact us today if you are interested in finding out more about these magnets or need help finding a reliable factory to produce them. We would be happy to help!

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