The Guide to Quality Inspection In NdFeB Magnet Manufacturing

Table of Contents

Sintered neodymium-iron-boron magnets are the strongest type of permanent magnet made. They are used in various applications, from electric motors to wind turbines and MRI machines. The manufacturing process for neodymium magnets is complex and requires a high degree of quality control to ensure that the finished product meets specifications. This article will discuss the various aspects of quality inspection in sintered neodymium magnet manufacturing.

The Quality Inspection in The Manufacturing Process of Sintered Neodymium Magnet

Chemical composition analysis of NdFeb material

Chemical composition analysis is an essential part of the manufacturing process for neodymium magnets. By carefully monitoring the chemical composition of the neodymium-iron-boron alloy, manufacturers can ensure that the finished product meets all required specifications. This paragraph will discuss the various methods used for chemical composition analysis in neodymium magnet manufacturing.

ICP Spectrometric: analyzing the neodymium-iron-boron magnet component.

ICP spectrometric analysis measures the concentration of neodymium, iron, and boron in the neodymium-iron-boron alloy. By accurately measuring the concentrations of these elements, manufacturers can ensure that the finished product meets all required specifications. This paragraph will discuss the various methods used for ICP spectrometric analysis in neodymium magnet manufacturing.

Flame AAS:

A standard method for ICP spectrometric analysis. It uses a flame to heat the neodymium-iron-boron alloy sample, causing it to emit light. This light is then captured by a spectrometer, which measures the intensity of the light at specific wavelengths. This information calculates the concentration of neodymium, iron, and boron in the alloy sample.

Inductively Coupled Plasma (ICP):

An alternative method for ICP spectrometric analysis. It uses a high-powered radio frequency generator to generate an inductive plasma field. This field causes the neodymium-iron-boron alloy sample to ionize, and a mass spectrometer then detects the ions. This information calculates the concentration of neodymium, iron, and boron in the alloy sample.

Carbon Sulfur Analyzer: analyzing the levels of carbon and sulfur content.

Carbon and sulfur are two elements that can significantly impact the strength and durability of neodymium magnets. Carbon sulfide (CS2) can form during manufacturing, leading to reduced magnet strength and shortened lifespan. Manufacturers can detect and identify any traces of CS2 in the neodymium-iron-boron alloy using a carbon sulfur analyzer. This allows them to make necessary adjustments to the manufacturing process to ensure that the finished product meets all required specifications.

LECO: analyzing the content of oxygen, nitrogen, and hydrogen in neodymium magnets

A critical aspect of quality control is analyzing the content of oxygen, nitrogen, and hydrogen in neodymium magnets. By accurately measuring the concentrations of these elements, manufacturers can ensure that the finished product meets all required specifications.

Methods Of Measurement Of The Magnetic Properties

Three methods commonly used for testing magnetic material characterization are Helmholtz coil measurement, demagnetization curve measurement, and magnetic field scanning.

Helmholtz coil measurement

  • General Introduction: You can determine a permanent magnet’s magnetic properties by measuring its external magnetic field in surrounding space. The easiest way to figure out a magnet’s polarization or flux density is to measure the total induction with a Helmholtz coil, then divide that number by the sample’s volume. Doing so only shows you the state of Magnet at measurement time, though.
  • Measuring Theory: Helmholtz coil consists of two identical coils separated from each other by a distance that equals the radius of the coils. They form an area of the homogeneous field between them. This can be reversed to use the coils as a flux-sensing device instead of a flux-generating one. A magnetic flux generated by the magnet can be determined by measuring the current induced by the coils when a magnet is brought to the center of the system. The current is usually detected by a fluxmeter, which integrates the signals from the coils.
  • Measuring Ways: You can measure the sample in one of two ways. First, zero out the integrator, then bring the sample to the center of the coils from a long distance away. Or, second, place the magnet at the center of the coils and zero out the integrator again; this time, move the magnet far enough from the coils that it won’t interfere with readings. To get an accurate reading, the second practice is to turn the sample around inside the coils. If you zero out the integrator before inserting the magnet, you’ll get an average of two readings without significant changes. However, if you zero out the integrator after inserting the magnet, you’ll only get half of the reading.

The demagnetization curve measurement

  • General Introduction: To understand how a material will respond to an external magnetic field, we measure its hysteresis loop. This is difficult for sintered NdFeB magnets, so instead, we focus on the demagnetization curve, which provides enough information for our purposes. Typically, demagnetization curves are measured using hysteresis graphs. The resulting curve shows how the material typically responds to changes in external fields.
  • Measurement: The demagnetization process is complete when the external magnetic Review field reaches a certain intensity, at which point the magnet’s flux density no longer changes. We attach samples to an iron yoke that provides a closed magnetic circuit to measure this change. We then calculate the changing flux density in the magnet by using pick-up coils wrapped around the sample. Finally, we integrate these signals using a fluxmeter.

Magnetic field scanning

  • General Introduction: The demagnetization curve measurements do not give information about the magnet’s local flux density variations. To detect homogeneity of flux density, scan the sample’s surface with a Hall probe.
  • Measurement: The magnetization inside the magnet can be studied by scanning the surface of the magnet with a Hall probe. The Hall sensor detects the axial component of magnetic flux density near the surface. The Hall probe is fixed to measure flux density perpendicular to the sample surface at a short distance from the surface. The resulting flux density profile depends on the sample shape and homogeneity of magnetization. Results of shaped samples can be compared if scans are performed along the same line section.
  • Gaussmeter: A gaussmeter is an essential tool for measuring the magnetic field strength of neodymium magnets. Using this meter, you can get readings of a neodymium magnet’s magnetic flux density (in Teslas). This information is essential for determining how strong the magnet is and whether it will suit your application

Corrosion Resistance Testing Methods For Sintered NdFeb Coatings

Sintered neodymium-iron-boron magnets are routinely coated with nickel, zinc, or gold to extend durability by protecting them from corrosion. The level of protection is not only determined by the substrate’s corrosion resistance but also by coating type and thickness, as well as the process used.

The table below lists the average test time for sintered NdFeb samples under three typical environmental conditions. The coating should not have any visible defects within this time frame, such as foaming, peeling, rusting, or powdering. Some discoloration, blackening, and darkening of the coat are allowed, as well as slight corrosion of the zinc layer.

Various methods can be used to evaluate the corrosion resistance of these coatings.

PCT Test: High Temperature And Pressure Acceleration Testing

High pressure accelerated aging test (PCT) can test the corrosion resistance of the magnet’s substrate and the finished NdFeB magnets that have been processed and surface-coated. To put it another way, PCT is used to gauge a sample magnet’s tolerance to harsh temperatures, saturated humidity, and pressure over an extended period (e.g., 12 hours, 24 hours, 72 hours).

To perform the PCT test, set it in high-pressure equipment that uses distilled or deionized water with a resistivity exceeding 1.0MΩ·cm. The less severe unsaturated mode has these conditions: temperature 120℃±2℃, air pressure 0.2MPa, and relative humidity between 100_05 %, while the more intense saturation mode has these parameters: temperature 120℃±2℃, air pressure 0.2MPa, and always 100% relative humidity.

Salt Spray Test

The salt spray test is a way to measure the corrosion resistance of neodymium magnets by artificially recreating the conditions found in salt water.

The test was conducted under these specific conditions: the temperature at 35℃±2℃, with a 5%±1% concentration of NaCl solution (by mass), and pH readings of the collected salt spray varied from 6.5 to 7.2. Additionally, the angle at which the sample was placed in the tank affected the results, requiring an inclined angle placement of 45°±5°.

How long does one hour of salt spray test correspond to field use? Based on the data:

• Neutral salt spray test 24h  ⇌ natural environment for 1 year

• Acetate mist test 24h ⇌ natural environment for 3 years

• Copper salt accelerated acetate spray test 24h  ⇌ natural environment for 8 years

Conditional Tester:

Using this test method, we can accelerate the rate at which temperature and humidity affect samples.

The sample is continually under high pressure from unsaturated, damp steam for a long time. The following were the test conditions: temperature 85℃±2℃, relative humidity 85%±5%, and the humidification water was either distilled or deionized. The most severe level, Level 1, requires 168 hours of testing.

Drop Test

The drop test is a way to measure the adhesion of neodymium magnets’ surface electroplating. To do this, the magnet is dropped from a certain height and onto a hard surface. The height of the drop can be varied to get different results.

Thickness Meter

The thickness of the coating will significantly determine how resistant it is to corrosion. A thickness meter is a device that helps you measure the electroplating on neodymium magnets. This way, manufacturers can check how thick it is and then give customers a report to show if the thickness meets their requirements.

Dimensions, Tolerances, Appearance Inspection

To maintain the quality of neodymium magnets, their dimensions and appearance must be checked for each batch. Check out some most common testing methods below:

Projector Inspection

A projector can inspect the specification, dimension, and tolerance of neodymium magnets. This will help ensure that each batch of magnets meets the required specifications.

Automatic visual inspection machine:

This device automatically inspects neodymium magnets for appearance, size, aperture, and any internal cracks or other defects. By using such a machine, manufacturers can ensure that each batch of magnets meets the required specifications.

Furthermore, neodymium magnets must also pass environmental protection tests to ensure safety.

XRF Tester:

The XRF tester is used to detect the presence of ROHS and halogen in neodymium magnets. This helps ensure that the magnets meet all safety requirements and comply with environmental regulations.

Conclusion

Maintaining the quality of neodymium magnets is essential to ensure they meet customers’ requirements. This can be done through various quality control measures, as listed previously. Additionally, manufacturers must ensure that neodymium magnets pass environmental protection tests on complying with safety regulations. By using these methods, it can be guaranteed that neodymium magnets are of the highest quality possible.

If you have questions about quality control or want to know more about a specific test, feel free to contact us. We would be happy to help!

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