Alnico Magnets

General Information

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 process.

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 1000 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 1000 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".


PROPERTIES OF CAST ALNICO MAGNET


Material
Grade
Equivalent
MMPA Class
Br Hc (BH)max Density Temperature Coefficient Curie
Temperature Tc
Maximum
Working
Temperature Tw
Remark
mT Gs KA/m Oe KJ/m3 MGOe g/cm3 Near Br %/K Near Hcj %/K Isotropic
LN10 ALNICO3 600 6000 40 500 10 1.20 6.9 -0.03 -0.02 810 450
LNG13 ALNICO2 700 7000 48 600 12.8 1.60 7.2 -0.03 -0.02 810 450
LNGT18 ALNICO8 580 5800 100 1250 18 2.20 7.3 -0.025 +0.02 860 550
LNG37 ALNICO5 1200 12000 48 600 37 4.65 7.3 -0.02 +0.02 850 525 Anisotropic
LNG40 1250 12500 48 600 40 5.00 7.3
LNG44 1250 12500 52 650 44 5.50 7.3
LNG52 ALNIC05DG 1300 13000 56 700 52 6.50 7.3
LNG60 ALNICO5-7 1350 13500 59 740 60 7.50 7.3
LNGT28 ALNICO6 1000 10000 57.6 720 28 3.50 7.3 -0.02 +0.03 850 525
LNGT36J ALNICO8HC 700 7000 140 1750 36 4.50 7.3 -0.025 +0.02 860 550
LNGT38 ALNICO8 800 8000 110 1380 38 4.75 7.3 -0.025 +0.02 860 550
LNGT40 820 8200 110 1380 40 5.00 7.3 -0.025 +0.02 860 550
LNGT60 ALNICO9 900 9000 110 1380 60

7.50

7.3 -0.025 +0.02 860 550
LNGT72 1050 10500 112 1400 72 9.00 7.3 -0.025 +0.02 860 550


PROPERTIES OF SINTERED ALNICO MAGNET


Material
Grade
Equivalent
MMPA Class
Br Hcj Hcb (BH)max Density Temperature
Coefficient
Curie
Temperature Tc
Maximum
Working
Temperature Tw
Remark
mT Gs KA/m Oe KA/m Oe KJ/m3 MGOe g/cm3 Near Br %/K Isotropic
SLN8 ALNICO3 520 5200 43 540 40 500 8-10 1.0-1.25 6.7 -0.02 760 450
SLNG12 ALNICO2 700 7000 43 540 40 500 12-14 1.5-1.75 7 -0.014 810 450
SLNGT18 ALNICO8 600 6000 107 1350 95 1200 18-22 2.25-2.75 7.2 -0.02 850 550
SLNGT28 ALNICO6 1000 10000 57 710 56 700 28-30 3.5-3.8 7.2 -0.02 7.3 525 Anisotropic
SLNG34 ALNICO5 1100 11000 51 640 50 630 34-38 3.5-4.15 7.2 -0.016 890 525
SLNGT31 ALNICO8 780 7800 106 1130 104 1300 33-36 3.9-4.5 7.2 -0.02 850 550
SLNGT38 800 8000 126 1580 123 1550 38-42 4.75-5.3 7.2 -0.02 850 550
SLNGT42 880 8800 122 1530 120 1500 42-48 5.3-6.0 7.25 -0.02 850 550
SLNGT38J ALNICO8HC 730 7300 163 2050 151 1900 38-40 4.75-5.0 7.2 -0.02 850 550
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