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 Alnico Magnets

Allstar Magnetics sells many grades of alnico magnets. Alnico is an acronym[1] referring to a family of iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co), hence al-ni-co. They also include copper, and sometimes titanium. Alnico alloys are ferromagnetic, with a high coercivity (resistance to loss of magnetism) and are used to make permanent magnets. Before the development of rare earth magnets in the 1970s, they were the strongest type of magnet. Other trade names for alloys in this family are: Alni, Alcomax, Hycomax, Columax, and Ticonal.

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. The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminum had a coercivity of 400 oersted (Oe; 32 kA/m), double that of the best magnet steels of the time.

 

Alnico Magnets

Properties

Alnico alloys make strong permanent magnets, and can be magnetised to produce strong magnetic fields. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth's magnetic field. Some brands of alnico are isotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, are anisotropic, with each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico's remanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersted (80 kA/m), its energy product ((BH)max) can be up to 5.5 MG·Oe (44 T·A/m). This means alnico can produce a strong magnetic flux in closed magnetic circuit, but has relatively small resistance against demagnetization.

Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximum working temperature is normally limited to around 538 °C (1,000 °F). They are the only magnets that have useful magnetism even when heated red-hot. This property, as well as its brittleness and high melting point, is the result of the strong tendency toward order due to intermetallic bonding between aluminium and its other constituents. They are also one of the most stable magnets if they are handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets.

Magnetic MaterialsDensity( g/cm )Maximum Energy Product BH (max)Residual Induction BrCoercive Force HcIntrinsic Coercive Force HcNormal Maximum Operating Temp F°
Alnico 5 (cast) 7.3 5.5 12800 640 640 975
Alnico 8 (cast) 7.3 5.3 8200 1650 1860 1020
Alnico 5 (sintered) 6.9 3.9 10900 620 630 975
Alnico 8 (sintered) 7.0 4.0 7400 1500 1690 1020

Manufacturing

Alnico magnets are produced by casting or sintering processes. Anisotropic alnico magnets are oriented by heating above a critical temperature, and cooling in the presence of a magnetic field. Both isotropic and anisotropic alnico require proper heat treatment to develop optimum magnetic properties — without it alnico's coercivity is about 10 Oe, comparable to technical iron, which is a soft magnetic material. After the heat treatment alnico becomes a composite material, named "precipitation material"—it consists of iron and cobalt rich precipitates in rich-NiAl matrix.

Alnico's anisotropy is oriented along the desired magnetic axis by applying an external magnetic field to it during the precipitate particle nucleation, which occurs when cooling from 900 °C (1,650 °F) to 800 °C (1,470 °F), near the Curie point. Without an external field there are local anisotropies of different orientations, due to spontaneous magnetization. The precipitate structure is a "barrier" against magnetization changes, as it prefers few magnetization states requiring much energy to get the material into any intermediate state. Also, a weak magnetic field shifts the magnetization of the matrix phase only, and is reversible..

Temperature

Alnico magnets are widely used in industrial and consumer applications where strong permanent magnets are needed; examples are electric motors, electric guitar pickups, microphones, sensors, loudspeakers, magnetron tubes, and cow magnets. In many applications they are being superseded by rare earth magnets, whose stronger fields (Br) and larger energy products (BHmax) allow smaller size magnets to be used for a given application .

Production

Alnico magnets offer the best temperature characteristics of any standard production magnet material available. Alnico magnets can be used for continuous duty applications where temperature extremes up to 930°F (500°C) can be expected.

Design Considerations

Because Alnico magnets are coarse-grained, hard, and brittle, they cannot be drilled or conventionally machined. Given these mechanical properties, Alnico parts are not normally used as structural members. Where a choice exists, select simple shapes. Slots are preferred over holes. Cross sections of less than .125" (3.18 mm) should be avoided. Finished surfaces, when required, may be produced by grinding; however, for many applications, the "as cast" or "as sintered" surface yields a satisfactory result at a significantly lower cost. To facilitate mounting and give adequate protection to the magnet, special assemblies such as rotors and magnetron magnets may be supplied with an aluminum jacket. Additionally, l ow carbon steel pole pieces may be attached to magnet pole faces by adhesive bonding or with studs or bolts. Plastisol or paint can be applied for improved appearance.

 

 Grade  Br (KGs)  Hci (Oe)  BHmax (MGOe)  Tc (C)  Tw (C)
 AL2 7.2 600 1.55 810 450
 AL3 6.0 500 1.20 810 450
 AL5 12.5 600  5.0 860 525
 AL5DG 13.0 700  6.5 860 525
 AL5-7 13.5 740  7.5 860 525
 AL6 10.0 720 3.5 860 525
 AL8HC 7.0 1750 4.5 860 550
 AL8 8.0  1380 5.0 860 550
 Al9 10.5  1400 9.0 860 550