Alnico magnets have now been manufactured for nearly 100 years, in which time the process has not drastically changed.
Alnico magnets are permanent magnets that are primarily made up of a combination of aluminium, nickel and cobalt but can also include copper, iron and titanium.
The exact chemical composition of an Alnico magnet depends on the grade being produced. Alnico magnets can be made in two ways, by sintering or more commonly, using a casting process. Alnico magnets produced by casting are greater in density and mass, which gives them better magnetic characteristics and performance. However, sintered Alnico magnets are more economical to produce. Both anisotropic and isotropic variations of Alnico magnets can be produced by sintering or casting.
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When a large Alnico magnet is required weighing tens of kilograms then it will nearly always be produced by casting. Really small Alnico magnets, weighing grams rather than kilograms are usually created using the sintering process. Sintering is generally the process used when a high volume of magnets are required.
Each grade of Alnico has a special recipe in terms of the percentage of aluminium, nickel, cobalt and iron used. Quantities of the individual elements are put into an induction furnace and melted at over 1750oC. Extra aluminium than what is actually required to form the final magnet is added because some is wasted as it melts at a much lower temperature, generally around 680oC.
During the melt and before casting, a sample is taken, cooled and analysed using an x-ray spectrometer to confirm that the exact ratios of materials are correct and to allow any last minute adjustments to be made.
The molten material is 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. A shell mould is made using a pattern plate and a moulding machine.
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 harvested from the moulds before they are then fettled on grinding wheels to remove the runner gates.
The magnets are then heated to a very high temperature (they actually glow red hot) and are placed into magnetisers so that they can start to cool down whilst in the presence of a very strong magnetic field. This process is known as hardening the magnets.
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 foundry mix of alloys, gives the magnetic material its temperature handling characteristics and its final magnetic properties.
The magnets often need finish grinding to tight tolerances. As the magnets are very hard, this is done by grinding.
Alnico magnets can be tested with an hysteresis graph testing machine, gauss meter or flux meter.
If a specific coating is required, this is done right at the end of the process. The poles of the magnets may be fettled to leave them bright and shiny.
Magnets are inserted into a coil or solenoid magnetiser 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 magnets desired shape.
Once the powder has been pressed, the material is sintered in a furnace in a hydrogen atmosphere at over 1200oC. This process fuses all the pressed particles together to form one magnet.
The red hot material is then cooled. At this point if the material is cooled in the presence of an external magnetic field then it will have anisotropic properties and a preferred magnetic direction, making it stronger. If an external magnetic field is not applied then the magnet produced will be isotropic without a preferential direction of magnetism.
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. If a specific coating is required, this is done right at the end of the process, just before the magnets are finally magnetised. The poles of the magnets may be fettled to leave them bright and shiny.
Finally the raw magnetic material is magnetised by placing it inside a coil or solenoid magnetiser and in one fifth of a second, the electrical pulse generates the necessary field needed to fully magnetise the magnets.
At first4magnets.com we stock a small range of red epoxy painted high quality Alnico magnets in a range of different shapes and sizes. Every magnet sold by first4magnets.com is quality assured before it is dispatched to the customer to be used in hundreds of different applications.