Alnico is an term referring to a series of permanent magnet alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co).
Introduction
AlNiCo is an term referring to a series of permanent magnet alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co). The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminum had a much higher coercivity than the best steel magnets of the time. The composition of alnico alloys is typically 8–12% Al, 15–26% Ni, 5–24% Co, up to 6% Cu, up to 1% Ti, and the balance is Fe.
Various Alnico grades with differing magnetic properties are achieved by modifying the chemistry and manufacturing process. For example, specialized casting techniques are used to achieve the unique crystalline grain orientation found in the Alnico 5-7 grade.
Before the development of rare earth magnets in the 1970s, Alnico was the strongest type of permanent magnet. In many applications Alnico has been replaced by other type of magnets. However, the temperature stability of Alnico magnets, and the fact that they can be used in very high temperatures, up to 500C, make them indispensable in certain applications.
Manufacturing process
Alnico magnets are manufactured either by casting or sintering. Alnico is produced by conventional foundry methods using resin bonded sand molds or powder metal manufacturing methods. Sintering Alnico is suitable for complex geometries.
Most Alnico produced is anisotropic meaning that the magnetic axle of the grains is oriented in one direction. The orientation is achieved by a heat treatment process after casting or sintering. The process involves heating the casting or sintered part above its Curie Temperature and then cooling it at a controlled rate in the presence of a directionalized magnetic field. Final shaping of the Alnico materials is achieved by abrasive grinding and cutting where close tolerances are required.
Cast AlNiCo Manufacturing Process
Properties typical for Alnico are high remanence, low coercitivity and very low temperature dependence of the both. Properties given in the table 1 and 2 are typical for each grade at the room temperature. More information, including demagnetisation curves in various temperatures, for each grade can be found in the datasheets (possibility to download). The same information is available in numerical form for magnetic modelling (request for info – e-mail).
Table 3 shows some general properties of Alnico magnet materials.
Figure 2 below shows demagnetisation curves of Alnico 5 grade at various temperatures, from -180 C to +300 C. The figure illustrates the temperature stability of Alnico magnets. It can been seen that when the working point is near BHmax, the output of the magnet is nearly constant over a large temperature span.
Table 1: Typical magnetic properties of cast Alnico magnet grades at 20℃
Table 2: Typical magnetic properties of sintered Alnico magnet grades at 20℃
In Tabel 3 typical physical properties of Alnico materials are given. These values must not be understood as quaranteed, as the properties are not controlled in manufacture.
Table 3:Physical Properties of Alnico magnets
Surface coating
Alnico magnets are not prone to corrosion and can normally be used without any surface protection. In some application, however, smooth surface is preferred.
Note:
The corrosion resistance of these coatings in different environment is influenced by the shape of magnets, e.g. chamfer, inner rings.
Design considerations
As the coercivity of Alnico magnets is low, it’s important to design the magnet system in such a way that the permeance coefficient is high enough to quarantee the stability of the magnet in all temperatures (the working point is above the knee of the J-H-curve). For example, the length of an Alnico 5 rod should be > 5 times the diameter.
As Alnico magnets are easily demagnetized by careless handling, it is recommended to make the magnetizing after assembly.
Alnico magnets offer outstanding temperature stability. The output from Alnico magnets varies the least with changes in temperature, making it an ideal choice for temperature sensitive applications. The reversible temperature coefficient of induction is -0.02%/deg C from ambient and the reversible temperature coefficient of Intrinsic Coercivity is +0.01%/deg C from ambient. This makes Alnico a perfect choice in performance critical applications such as medical and military. Alnico magnets can actually be used up to 530 C, without irreversible losses in magnetizing.
