What are Alnico Magnets?
Alnico magnets derive their magnetic properties and name from these elements – ALuminum, NIckel, and CObalt. They have the widest range of temperature stability of any standard magnetic material – up to 1,000°F, at which approximately 85% of room temperature magnetization is retained. Temperature changes above this are largely structural and not fully reversible or re-magnetizable. Alnico magnets are manufactured using a powder metallurgy process (sintered alnico) or a foundry process (cast alnico). Other characteristics include high residual induction as well as relatively high energies. The corrosion resistance of alnico is considered excellent and no surface treatments are required, but alnico magnets can be easily plated.
Cast Alnico Magnets
Cast alnico magnets are manufactured by pouring a molten metal alloy into a mold and then processing it through various heat-treat cycles. The resulting magnet has a dark gray appearance and may have a rough surface. Machined surfaces have a shiny appearance similar to steel. Cast magnets may be manufactured in complex shapes such as horseshoes, which is not possible with other magnet materials.
Types of Cast Alnico
- Cast Alnico 5 -- This is the most commonly used. This alnico material is used extensively in rotating machinery, meters, instruments, sensing devices, and holding applications, to name a few. For best results with alnico 5 magnets, the length should be no less than 5 times the cross-section diameter; or 5 times the diameter of a circle equal in area to the cross section.
- Cast Alnico 8 HE -- This magnet has the highest temperature stability of any commercially available magnetic material. Improved crystal structure and alloying techniques achieve a 6.0 energy product and high resistance to demagnetization. Typical uses include computer keyboards, drives, printers, microphones, meters, motors, generators, relays, reed-switch relays, and transducers.
Since samarium cobalt offers excellent corrosion resistance, these magnets typically do not require a surface treatment. They also have good resistance to external demagnetizing fields because of their High Intrinsic Coercive Force (Hci). This resistance makes samarium cobalt rare earth magnets an excellent choice for electromechanical applications.
Sintered Alnico Magnets
Sintered alnico magnets are manufactured by compacting fine alnico powder in a press, and then sintering the compacted powder into a solid magnet. Sintered alnico has marginally lower magnetic properties, but better mechanical properties, than cast alnico. Both are hard and brittle materials and require skillful machining on specialized equipment. It is generally not recommended that these materials be used for structural or decorative purposes. Alnico magnets can be pressed directly into nonmagnetic materials. For steel pressings, they should be enclosed in a nonferrous bushing. Sintered alnico 8H has a high temperature stability, coercivity, and demagnetization resistance similar to Cast alnico 8, but it can be manufactured to closer tolerances. Its fine grain structure results in highly uniform flux distribution and mechanical strength, so it is ideally suited to applications requiring short magnetic length or involving high-speed motion. Some applications include core meters, traveling wave tube stacks, polarized relays, reed switches, torque transmitting devices, and sandwich-type holding assemblies.
Things to Know When Using Alnico
Alnico is hard and brittle, and prone to chipping and cracking. Special machining techniques must be used to machine this material. Alnico magnets require magnetizing fields of about 3 kOe (kilo Oersted). Because of their relatively low coercivities, special care should be taken to assure that alnico 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 alnico magnets are partially demagnetized, they may be easily re-magnetized.