Demagnetization, also known as magnetic cleaning, demagnetization, etc., refers to the process of magnets returning to a magnetic neutral state, and can also be called magnetic neutralization.

In industrial processing, there are three methods of demagnetization:

1. Static demagnetization

Add a magnetic field opposite to the original magnetization direction of the magnet. The strength of this diamagnetic field should ensure that when it is removed, the magnetic induction intensity of the magnetic body will become zero. The resulting magnetic neutral state is called the static magnetic neutral state.

In the hysteresis loop, the red line segment in the second quadrant represents the demagnetization curve, that is, when a magnet is applied with a magnetic field opposite to the magnetizing direction, its magnetic induction intensity decreases with the increase of the reverse magnetization field. When the magnetization field intensity reaches -Hc, the magnetic induction intensity of the magnet drops to 0, and then the magnet no longer has magnetism.

The hysteresis loop is measured at room temperature, and the demagnetization curve of the magnet is not the same at different working temperatures, as shown in the figure below. Therefore, the strength of the reverse magnetic field applied for demagnetization is different under different temperature conditions.

2. Dynamic demagnetization

Apply a sufficiently strong alternating magnetic field to the magnetic body, and then gradually reduce the amplitude of the alternating magnetic field to zero. The resulting magnetic neutral state is called dynamic magnetic neutral state.

The principle of this method is to place the workpiece in an alternating magnetic field and demagnetize it with the decrease of the hysteresis loop. As the amplitude of the alternating magnetic field gradually decays, the trajectory of the hysteresis loop becomes smaller and smaller. When the magnetic field gradually decays to zero, the residual magnetism in the workpiece will be close to zero. The principle of demagnetization is shown in the figure below. It can be seen that the direction and size of the current and the magnetic field must be changed at the same time during demagnetization.

(1) Alternating current demagnetization

The workpiece that has been electromagnetized by alternating current is demagnetized by alternating current, which can be passed or attenuated.

A Passing method

For batch demagnetization of small and medium-sized workpieces, it is best to put the workpieces on a demagnetizer equipped with rails and carriages for demagnetization. When demagnetizing, place the workpieces on the carriages 30cm in front of the coil. When the coils are energized, move the workpieces along The track slowly passes through the coil and is at least 1m away from the coil. For heavy or large workpieces that cannot be demagnetized on the demagnetizer, you can also put the coil on the workpiece, slowly pass the coil and stay away from the workpiece when power is applied, and cut off the power at least 1m away.

B attenuation method

Because the direction of the alternating current is constantly changing, it can be demagnetized by using an automatic attenuation demagnetizer or a voltage regulator to gradually reduce the current to zero. Place the workpiece in the coil, clamp it between the two magnetized chucks of the flaw detector, or use a support rod After the contact contacts the workpiece, the current is reduced to zero for demagnetization.

The price of the demagnetizer is mainly related to the capacity of the energy storage capacitor and the charging voltage (the energy of the demagnetizer). When purchasing the demagnetizer, you should mainly consider the demagnetization product brand or intrinsic coercivity and the size of the demagnetization sample.

(2) DC demagnetization

By continuously changing the direction of the direct current, the current through the workpiece is reduced to zero for demagnetization. The waveform of the DC demagnetization current is shown in the figure below. In the figure, T1 is the current on time interval, and T2 is the current off time interval. It is necessary to ensure that the current reverses when the power is off. The decay times of the current should be as many as possible (generally more than 30 times are required), and the current amplitude of each decay should be as small as possible. If the decay amplitude is too large, the purpose of demagnetization cannot be achieved.

3. Thermally induced demagnetization

It is a method of heating the magnetic body above the Curie temperature and then cooling and demagnetizing it without the action of an external magnetic field. Sintered NdFeB can be baked at a high temperature of 350℃ for 30 minutes to 1 hour by thermal demagnetization.

In the working temperature, the magnetic force of the magnet will decrease when the temperature rises, but most of the magnetic force will recover after cooling. If the temperature reaches the Curie temperature, the molecules inside the magnet move violently and demagnetize, which is irreversible.

No matter which of the above three methods is used to demagnetize the magnet, the internal structure of the magnet will be permanently changed. After demagnetization and then magnetize the magnet, the magnetic performance cannot be restored to the previous level.