The term alnico is an acronym of aluminium, nickel and cobalt; the three primary elements which are used to create alnico magnetic material. Despite being the first permanent magnets to be developed in the 1920s, rare earth magnets aside, alnico magnets are the strongest permanent magnets available. The introduction of alnico magnets meant that expensive electromagnets could be replaced with permanent magnets in devices such as motors, generators and loudspeakers.
Alnico magnets have been manufactured for nearly 100 years and during this time the process has not drastically changed. The primary elements used to create alnico magnets are aluminium, nickel and cobalt, however, like other types of permanent magnets, traces of other elements are added to produce specific characteristics. Alnico magnets are manufactured using one of two methods:
Quantities of the individual elements used to manufacture alnico magnets are put into an induction furnace and melted at over 1750? The molten material is then poured into a shell mould or larger green sand moulds. As the molten metal cools, the shell moulds start to burn and by the time the magnets are cold, the shell is almost disintegrated. Patterns are a similar size to the required magnet size but with an additional allowance for shrinkage and machining to size. The magnet cavities inside are linked by runners which allow the molten metal to reach each magnet cavity and the runners are then scrapped and re-melted once the magnets are cast.
The newly cast magnets are then removed from the moulds before being fettled on grinding wheels to remove the runner gates. The magnets are then heated to incredibly high temperatures and then placed into magnetisers so that they can start to cool down whilst in the presence of a very strong magnetic field.
The magnets are then placed in large tempering ovens for a few days to temper and stabilise the magnets. The heat treatment stage of the process in conjunction with the alloy mixture gives the magnetic material its temperature handling characteristics and its final magnetic properties. Finally, the magnets are inserted into a coil or solenoid magnetisers and in one-fifth of a second, the electrical pulse generates the necessary field needed to fully magnetise the magnets.
Before sintered alnico magnets can be produced the raw elements must first be finely ground by milling into tiny particles. The powdered magnetic material is then pressed under tonnes of pressure in a die, which closely resembles the final desired shape.
Once the powder has been pressed, the material is sintered in a furnace in a hy drogen atmosphere at over 1200?. This process fuses all the pressed particles together to form one magnet.
The red hot material is then cooled in the presence of an external magnetic field to manufacture an anisotropic magnet or without an external magnetic field to produce an isotropic magnet. Generally, sintered alnico magnets do not require further shaping or machining as the die they are pressed in is made to be very close to the desired shape of the magnet. Finally, the raw magnetic material is magnetised by placing it inside a coil or solenoid magnetiser to magnetise the magnets.
Alnico magnets are used in applications that require exceptional magnetic stability when operating at high temperatures as they lose less of their overall magnetic strength per increase in temperature than other permanent magnets. They have the highest maximum operating temperatures of any permanent magnetic material, even up to 550? for some grades. However, despite their resistance to high temperatures they are susceptible to demagnetisation, so much so that forcing two alnico magnets together in repulsion can permanently demagnetise both of them.