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Alnico
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.
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.
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
We are able to manufacture metal
and other components of finished sub assemblies using
our CNC machining facilities.
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 re-magnetized.
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".
Holding Force or
Pull Data
Any pull values shown, are approximate and offered only
for comparison. They have been measured when pole surfaces
are in contact with a 1/2" thick, ground, mild steel
plate. Due to the nature of magnetism, it is very difficult
to establish a definite holding force to fit all applications.
We suggest that each customer make his own pull tables
on an actual model.
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