How Alnico Magnets are made
The Original Online Magnet Company
Alnico is an alloy made mainly from a combination of Aluminium, Nickel, Cobalt and Iron plus varying levels of Copper, Titanium and Niobium
The exact chemical composition within Alnico grade depends on the grade of Alnico. An example of the composition is given below.
Typical composition of Alnico alloy
|Main Elements within Alnico||Percentage by weight|
|Aluminium(Al)||6% - 13%|
|Nickel (Ni)||13% – 26%|
|Cobalt (Co)||0% - 42%|
|Copper (Cu)||2% - 6%|
|Titanium (Ti)||0% -9%|
|Niobium (Nb)||0% -3%|
|Iron (Fe)||Balance (e.g. 30% -40%)|
An example for sintered Alnico 8 is – 31.5% Iron, 36% Cobalt, 13.5% Nickel, 7.2% Aluminium, 3.5% Copper, 7.5% Titanium and 0.8%Niobium.
Alnico magnets can be made from either sintering or by the more common casting processes. The Alnico structure is one of a strongly magnetic Fe-Co rod microstructure in a weakly magnetic Ni-Al matrix – the shape anisotropy of the rod shape gives the Alnico its coercivity.
Each Alnico grade has its own composition of elements in varying proportions. The elements for the Alnico are put into a furnace (an induction furnace is commonly used) and are melted at 1750-1780 deg C. A higher amount of aluminium (Al) than required is added initially because some Al is lost over time due to it melting at 680 deg C. As a result, the time taken during the casting process is very important as it affects the Ni:Al ratio.
The molten Alnico alloy is poured into sand molds. The sand molds that have been pre-prepared for the required shape of magnet. The sand mold contains volumes that are the shapes for the magnet which are linked by horizontal tubes (runners) and vertical tubes (risers). When the molten magnet alloy is poured into the mold it can easily displace all the air to fill up all the spaces and voids (the method to successfully create sand molds is a technical process). Because casting uses runners and risers, the yield is not 100%. The sand mold and casting is cooled quickly to avoid gamma second phase forming. Sometimes there are special plates at the bottom of the molds called “chills” which aid cooling. And sometimes the sand used is an exothermic sand. The exothermic sand gives a different cooling rate and the “chill” plate provides means for a columnar or semi-columnar crystal growth to produce grades of Alnico with the highest Br values. The cooling rate and also the surface finish of the magnet are affected by amount of sand used in the cast, the sand grain size and the sand properties.
Once the magnet has cooled down, the sand mold is broken down and the magnets retrieved (the sand is recycled; the runners and risers and removed and recycled). A heat treatment cycle in the presence of a magnetic field removes any unwanted second phase and gives some of the final magnetic properties. A final temper stage gives the temperature coefficients and final magnetic properties (improving the magnet’s coercivity Hc and Hci). The heat treatment and temper stages can be modified to give a reduction in the brittleness of these magnets but this is very expensive.
If required, the magnets are ground to finished size. The as cast look of Alnico is a dark grey/black colour with a roughened texture (the texture is from the sand mold). The machined finish is a smooth bright metallic colour. There is no detriment to the magnet’s performance is the non pole faces are left as cast (this is cheaper) although some customers may prefer all the faces to be machined for aesthetic reasons. If needed, the magnets are also magnetised before packing.
Casting is the only method for producing large Alnico magnets. Magnets up to 30kg, perhaps even heavier, can be produced by casting. There may be very small air pockets within cast Alnico magnets – this is perfectly normal and is not detrimental to the magnet.
Sintered Alnico is made by a pressed powder process. The desired mix of metal powders is pressed in a die that is very close to the desired final shape. The resulting “green” magnet is then sintered at around 1260 deg C in a hydrogen atmosphere to fuse the powder together. The magnets are actually structurally stronger than cast Alnico (it is still recommended to avoid putting magnets into applications where they are load bearing as they will break). Although sintering may cause a minor degree of shape variation from the “green” shape, relatively close tolerances are still achieved without the need for further machining/grinding. Generally, sintered Alnico magnets are small, typically weighing less than 30 grams. The process of producing sintered Alnico is suitable for volume production.
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