What is AlNiCo magnet？ Classification, characteristics and application

Table of Contents

• What is AlNiCo magnet?

• Production process of AlNiCo magnets

• Characteristics of AlNiCo magnets

• AlNiCo vs SmCo

• Application of AlNiCo

What is AlNiCo magnet?

AlNiCo permanent magnet: an alloy composed of aluminum, nickel, cobalt, iron and other trace elements. Alnico permanent magnetic material was successfully developed in the 1930s. It is the first permanent magnetic material developed in history and is an alloy composed of aluminum, nickel, cobalt, iron and other trace metal elements. Before the discovery of rare earth permanent magnetic materials in the 1970s, AlNiCo alloy was the strongest permanent magnetic material.

Production process of AlNiCo magnets

There are two different production processes for Al-Ni-Co permanent magnets: casting and sintering:

• Cast AlNiCo is produced by traditional casting method using resin bonded sand mold. The casting process can be processed into different sizes and shapes; Compared with the casting process, sintered products are limited to small size. The dimensional tolerance of the blank produced by sintered products is better than that of the casting products. The magnetic property is slightly lower than that of the casting products, but the machinability is better.

• Sintered aluminium-nickel-cobalt magnets are formed by powder metal manufacturing method. Sintered Al-Ni-Co is suitable for complex geometric shapes.

Most of the AlNiCo produced are anisotropic, which means that the magnetic direction of the grains is in one direction. This orientation is achieved by heat treatment after casting or sintering. This process includes heating the casting or sintered part to above Curie temperature, and then cooling at a controlled rate in the presence of a strong directional magnetic field. The final forming of aluminium-nickel-cobalt materials is achieved by abrasive grinding and cutting, which requires close tolerance.

Characteristics of AlNiCo magnets

• High temperature resistance, magnetic properties can be maintained at 450 ℃, and the maximum working temperature can reach about 520 ℃.

• Aluminum, nickel and cobalt have low iron content, so they have strong corrosion resistance, and the surface does not need electroplating treatment.

• Cast Al-Ni-Co magnets have various sizes and shapes, while sintered Al-Ni-Co magnets are usually small in size and high in accuracy, but their magnetic properties are slightly lower than those of cast Al-Ni-Co magnets. By casting or sintering, they can be made into complex shapes that other magnetic materials cannot achieve.

• Low coercivity, easy to magnetize, but also easy to demagnetize. In the design and manufacture of device magnetic circuit, special attention should be paid to its particularity, and the magnet must be stabilized in advance.

AlNiCo vs SmCo

Both belong to high temperature resistant permanent magnetic materials. The maximum working temperature of AlNiCo magnets can reach 550 ℃, the maximum working temperature of SmCo 1:5 type is 250 ℃, and the maximum working temperature of SmCo 2:17 type is 350 ℃.

No surface treatment is required for both (except for use in special environment).

Al-Ni-Co is a cubic crystal with three cubic axes in the direction of easy magnetization, so it has many magnetic poles.

However, whether it is 1:5 type (hexagonal crystal) or 2:17 type (rhombic crystal), there is only one easy magnetization direction and a single magnetic pole.

The energy required to magnetize to saturation in the direction of easy magnetization is small, while the energy required to magnetize to saturation in the direction of difficult magnetization is large. The difference between the two energies is called magnetocrystalline anisotropy, and the magnetocrystalline anisotropy energy per unit volume is called magnetocrystalline anisotropy constant, which is expressed by the letter K. The K value of a single magnetic pole is large, and the intrinsic coercivity of permanent magnetic materials with large K value is mainly determined by the magnetocrystalline anisotropy.

In addition to magnetocrystalline anisotropy, there is also shape anisotropy. The K value of aluminium-nickel drill is small, so shape anisotropy is needed to improve its intrinsic coercivity. In general, magnetization in the long axis direction is easier, but it is difficult in the short axis direction. Therefore, increasing the length-diameter ratio of AlNiCo magnets will help to improve their intrinsic coercivity.

The Al-Ni-Co magnet is cheap, with the best chemical stability and temperature stability, but its magnetism is weak, its coercivity is low, and it is not resistant to vibration and shock (vibration and shock will reduce its magnetic flux by about 2%). It is easy to demagnetize under strong magnetic field, so many people suggest that it should be assembled first and then magnetized; Because samarium cobalt contains a lot of rare earth elements, it is estimated that the price is relatively expensive, generally 4~5 times of that of aluminum nickel cobalt of the same specification (some say up to 10 times), but its magnetism is strong, second only to neodymium iron boron, with high coercivity, and is not easy to demagnetize; The banknote cobalt can withstand 10g vibration and 100g impact without demagnetization, and contact or friction with iron will not demagnetize it, but it cannot withstand the corrosion of acid, alkali and salt mist; The contact and friction between aluminum nickel drill and iron will demagnetize it.

Relatively speaking, samarium cobalt material is fragile, and impact can not demagnetize it, but can break it, so special attention should be paid during use; The performance of Al-Ni-Co magnet is relatively good.

It should be noted that although the vibration and impact can not demagnetize the cobalt magnet, the resulting internal microcracks may cause its magnetism to weaken;

Application of AlNiCo

With ferrite and Al-Ni-Co magnets being gradually replaced by other magnets, they still have better performance than other magnets in some specific fields.

Cast Al-Ni-Co products are mainly used in automobile parts, instruments and meters, electroacoustic, electrical machinery, teaching, aerospace, military and other fields.

Sintered Al-Ni-Co is widely used in electrical machinery (now gradually replaced) instruments, communication, magnetoelectric switches and various sensors.