Alnico Magnets Info

Cast Alnico 5 is the most commonly used of all the cast Alnicos. This material is used extensively in rotating machinery, meters, instruments, sensing devices, and holding applications, to name a few. Excellent temperature stability, high residual induction, and relatively high energies characterize Alnico materials, composed primarily of alloys of Aluminum, Nickel, and Cobalt. They are manufactured through either a casting or sintering process. Cast magnets may be manufactured in complex shapes, such as horseshoes, not possible with other magnet materials. Sintered Alnicos offer slightly lower magnetic properties but better mechanical characteristics than cast Alnicos.

Alnico is hard and brittle. Machining or drilling cannot therefore be accomplished by ordinary methods. Holes are usually cored in at the foundry, and magnets are cast close to final size and then finish machined to closer tolerances.

Alnico has a low coercive force, and is easily demagnetized if not handled with care. For optimum performance of Alnico 5, the magnetic length should be approximately 5 times the pole diameter or equivalent diameter. For example, a 0.250" diameter magnet should be about 1.250" long.

Because of its higher coercivity, Alnico 8 may be used in shorter lengths and in disc shapes.

Manufacturing Methods

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. Assemblies Assemblies can be fabricated by adhering magnets with adhesives to suit a range of environments, by mechanically fastening magnets, or by a combination of these methods. Due to the relatively brittle nature of these magnet materials, press fits are not recommended.

Surface Treatments  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.

Machining Alnico is hard and brittle, and prone to chipping and cracking. Special machining techniques must be used to machine this material. Holes must be made by EDM methods. We are fully equipped to machine these materials to your blueprint specifications.

Magnetizing and Handling  Alnico magnets require magnetizing fields of about 3 kOe. Because of their relatively low coercivities, special care should be taken to assure that these magnets are not subjected to adverse repelling fields, since these could partially demagnetize the magnets. Magnetized magnets should be stored with keepers to reduce the possibility of partial demagnetization. If Alnicos are partially demagnetized, they may be easily remagnetized.

Temperature Effects  Up to about 1,000 F, changes in magnetization are largely reversible and re-magnetizable, while changes above this are largely structural and not fully reversible or re-magnetizable. 

Approximately 90% of room temperature magnetization is retained at temperatures of up to 1,000 F. 

Material Characteristics  Cast Alnico materials commonly contain casting voids and hairline cracks within the material. These can be exposed by finish machining. The following is an extract from MMPA standards regarding such imperfections:
"These are materials used primarily for their magnetic capabilities as permanent magnets without regard to mechanical properties. These materials, prepared by good metallurgical practice, by their very nature will unavoidably contain a degree of physical imperfection. It is not generally recommended that these materials be used for structural or decorative purposes unless the physical requirements of the magnet are previously suitably specified".

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