AlNiCo, Alnico Magnets, Casting & Sintered AlNiCo Permanent Magnets
Alnico magnets are manufactured through either casting or sintering processes.Cast magnets are manufactured by pouring a molten metal alloy into a mold and then further processing it through various heat-treat cycles. The resulting magnet has a dark gray exterior appearance, and may have a rough surface. Machined surfaces have a shiny appearance similar to steel.
Sintered magnets are manufactured by compacting fine Alnico powder in a press, and then sintering the compacted powder into a solid magnet.
The corrosion resistance of Alnico is considered excellent , and no surface treatments are required. However, Alnico magnets are easily plated for cosmetic reasons if required.
A high working temperature limit (550 °C / 1020 °F) makes Alnico especially well suited for sensitive automotive and aircraft sensor applications. Alnico is produced in many grades to fit the requirements of these applications, from Alnico 1 to Alnico 12, but the most popular grades are 2, 5 and 8. By comparison to newer materials, the coercivity of Alnico is low.
Where Alnico is appropriate, magnet size can be minimized if it can be magnetized after assembly into the magnetic circuit. If used independent of other circuit components, as in security applications, the effective length to diameter ratio (related to the permeance coefficient) must be great enough to cause the magnet to work above the knee in its second quadrant demagnetization curve. For critical applications, Alnico magnets may be calibrated to an established reference flux density value. A by-product of low coercivity is sensitivity to demagnetizing effects due to external magnetic fields, shock, and application temperatures. For critical applications, Alnico magnets can be temperature stabilized to minimize these effects.
Alnico magnet material is made by alloying aluminum, nickel and cobalt with iron. Some grades also contain copper and/or titanium. The alloying process is casting or sintering. The process and the heat treatment needed to optimize magnetic properties produces hard (Rc45) and brittle parts that are best shaped or finished by abrasive grinding. Cast parts are generally under 70 pounds and may be used as-is, but polar surfaces are usually ground flat and parallel. Sintering is confined to high volume parts in sizes under one cubic inch and an effective press length to diameter ratio under four.
AlNiCo is most useful in the following applications: