Until about 1975, Alnico Magnets were the most widely used magnets in the world. The last 35 years have seen a sharp decline in the demand for them mainly because of the soaring prices of Cobalt and the availability of high coercivity materials like Ferrites and Rare Earths, the former being also the cheapest. And yet, Alnicos still play an important role in many applications because of their vastly superior ability to withstand high temperatures (up to 550°C) and a very low temperature co-efficient (less than 0.02% per degree C) as compared to Ferrite and Rare Earth magnets.
Worldwide, Alnicos are produced in several grades, some isotropic (unoriented) and the others anisotropic (oriented). Of these, we produce Alnico-5 and Alnico-6 which are the most popular anisotropic grades and Alnico-3 which is isotropic and primarily used for speedometer magnets. We also produce Alnico-8 (isotropic), which is primarily used in powder form for making bonded magnets.
The word Al-Ni-Co obviously is an acronym formed by combining the chemical symbols for Aluminum, Nickel and Cobalt. Besides these three metals, Copper and the base element Iron are also required, the latter constituting between 40% to 50% of the alloy. Small quantities of some other metals like Silicon, Titanium and Niobium may also be added depending upon specific requirements of certain grades. Out of these we have to import Cobalt, Nickel and Iron. All these metals are required in their purest form, the purity exceeding 99.8%, and with the Carbon content kept below 0.03%.
These metals, in requisite proportions appropriate to the alloy grades, are melted at a temperature of about 1,600°C in a medium frequency induction furnace which is the heart of the plant and which is essential for maintaining a critical control of temperature. The thoroughly homogenized alloy is cast at a critical temperature into previously prepared and stacked sand shell-moulds. The cast ingots are shot –blasted, cleaned and are then subjected to a two-stage heat-treatment.
In the first stage the ingots are heated up to 1000° to 1200°C (depending upon the alloy grade) and in the case of anisotropic alloys (Alnico-5 and Alnico-6) they are then cooled to about 600°C at a controlled rate under the influence of a very strong magnetic field. The direction and strength of this field are similar to those of the field, which the magnets will be required to produce when finished. This technique produces the property of ‘Anisotropicity’, that is, the magnets are so oriented as to have a very high magnetic strength in the predetermined direction (also known as the direction of orientation or the preferred axis), at the cost of reduction in strength in other directions.
The second stage of the heat-treatment consists of an aging process in which the magnets are held at this lower temperature (about 600°C) for a long period in order to increase the coercivity and to ensure metallurgical stability.
Finally, the magnets are ground, if necessary, but particularly on pole faces, to bring them within the desired dimensional tolerances. Inspection and quality control testing is done at every stage of production and the final testing is done after grinding.
Lastly, they are washed and cleaned to make them dust-free before packing. Normally, our magnets are packed and delivered in the unmagnetised condition only, unless the customers specifically ask them to be supplied in magnetised condition.
Our Alnico magnets conform to ISI specification No. IS:6077 (Part 1) – 1971