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owner of the online magnet store, www.Custom-Magnets.com. Custom-Magnets has been supplying
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Alnico magnets are composed from an alloy of ALuminium, NIckle and CObalt. They may include minor amounts of some other elements like Titanium and Copper. Magnets made from this alloy permit the creation of various designs with high indications, high energies and relatively high coercivities. Excellent temperature stability and good resistance to demagnetization from vibration and shock characterize alnico magnets.
Alnico however, is a very hard and brittle alloy. Ordinary machining and drilling cannot be performed with Alnico. There are two key processes that are used in the manufacturing of Alnico magnets.
Cast Alnico Magnets
Alnico magnets manufactured using the casting process can be made into many shapes and sizes. Cast magnets are manufactured by pouring the molten metal alloy of Alnico into a mold and then further processing it through various heat-treat cycles. The resulting magnet has a dark gray appearance on the exterior and may be roughly surfaced. Magnets that have been polished using machines, may have a shiny gray surface similar to that of steel.
To manufacture cast alnico magnets, cast alnico is melted. The molten alnico is then poured into a mold. Once it solidifies the material is rough ground and then heat treated and cooled alternatively. Sometimes the material is treated in the presence of a magnetic field. This results in the material taking on maximum magnetization and allows a higher gauss level. Such magnets are called anisotropic magnets. A cast magnet that is not treated in a magnetized field is called isotropic magnet. After heating and cooling treatments the material can then be ground to specific tolerances and magnetized.
Specialized casting techniques are used to make the Alnico grades 5 and 8. In both the grades, there is a unique crystalline grain orientation to be found and both are anisotropic. They are designed to produce high magnetic output in a specific desired direction. Orientation is achieved during the heat treatment. The casting is cooled from 2000F at a controlled rate within a magnetic field, which conforms to the preferred direction of magnetization.
Cast 5 Alnico is the most commonly used of all the Alnico’s. It combines high indications with a high-energy product. Cast 5 is very popular in the communications industry, for meters and instruments, in rotation machinery, sensing devices and holding applications. Alnico 8, on the other hand, has a very high resistance to demagnetization. The cobalt content is 35% and allows this grade to function well for short lengths or for length diameter ratios of less than 2 to 1.
Sintered Alnico Magnets
Sintered magnets are formed by compacting fine Alnico powder in a press and then sintering the compacted powder into a solid magnet. The powdered mixture of contents is pressed into a die under tons of pressure. It is then sintered in a hydrogen atmosphere at 2300F and then cooled – either within a magnetic field or without a magnetic field. When cooled within the magnetic field it is known as an anisotropic magnet and when cooled outside of a magnetic field it is called an isotropic magnet.
Sintered alnico magnets have lower magnetic properties than cast magnets but better mechanical properties. They are most suitable for small sizes that are less than 1 oz. The sintering process is well suited to large volume production. It also results in parts that are structurally stronger than the cast alnico magnets. With the sintering process, one can achieve relatively close tolerances without any grinding.
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