Alnico magnets are manufactured by a cast or sintered process. Production level cast Alnico magnets are produced by conventional foundry methods using resin bonded sand molds. The magnetic characteristics for some exotic grades of Alnico magnets are achieved during the casting operation and are due to the unique crystalline grain orientation developed during the process.
Sintered Alnico is a powdered metal and it is manufactured by compacting finely milled Alnico powder in a metallic mold. The resulting geometry is not fully dense and it must be sintered in a furnace to achieve a solid state.
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 are made from aluminum, nickel, cobalt, iron and a few other elements, such as copper, titanium, and sulfur. Alnico magnets are relatively easily demagnetized, but resisting oxidation well.
|Grade||Br||Hc||(BH)max||Curie Temp.||Temp.Coe||US standard|
|LNG52||1250/12500||55/690||52.0/6.50||860||0.020||Alnico 5 DG|
|Total||8 cast Aluminium (Al) Nickel (Ni) Cobalt (Co) magnets|
|Grade||Br||Hc||(BH)max||Curie Temp||Temp.Coe||US standard||IEC standard|
|FLNG34||1100/11000||48/600||34.0/4.25||890||0.016||S.Alnico 5||Alnico 37/5|
|Total||8 grade of high temperature Aluminium (Al) Nickel (Ni) Cobalt (Co) Alnico magnet|
A chipped edge is considered acceptable if no more than 10% of the surface is missing. Cracks are acceptable as long as they do not extend across more than 50% of pole surface.
Cast alnico is melted and poured into a mold. Once solidified, the material is rough ground and then heat-treated and cooled, sometimes in a magnetic field. When treated in the presence of a magnetic field, the magnet is called anisotropic.
This orients the material to take on maximum magnetization. A cast magnet that is not heat-treated in a magnetic field is called isotropic. After heat treatment and cooling, the material can be ground to specific tolerances and then magnetized.
Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss, 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 may exceed 12,00, its coercivity can be up to 1000 oersted (80 kA/m), its energy product can be up to 5.5 MG·Oe. This means alnico can produce a strong magnetic flux in closed magnetic circuit, but has relatively small resistance against demagnetization.