Alnico magnets
ORIGIN
ALNICO magnets were discovered around 1930 and Euromag started manufacturing them in 1942 and this activity still continues today.
The ALNICOs were the first magnets to truly deserve the title of “permanent”. Previously, magnets were made from special steels with poor magnetic properties.
PROCESS & COMPOSITION
ALNICO magnets are made from metal alloys mainly composed of cobalt (5 to 40%), nickel (10 to 20%), aluminium (7 to 9%), and iron (40% to 70%). Depending on the grades, copper and titanium are also added.
At Euromag, ALNICOs are manufactured by a Croning sand-mould casting process.
After the molds have been manufactured, the previously molten alloy is casted into the cavities of the molds.
After cooling and stripping, the parts are subjected to a more or less complex succession of heat treatments consisting in controlled heating and hardening phases, under magnetic field for anisotropic grades. An annealing process releases some of the stresses on the alloy.
The parts are then machined to give them the required geometric accuracy (up to ±0.01mm). Due to the high hardness of the alloy, this is a grinding wheel grinding.
The design and composition of the mold, have a significant impact on the solidification structure of the alloy. Thus, some molds are able to generate grain growth in a preferred direction. This is known as directed grain grades where the solidification profile shows parallel columns. The magnetic properties will then be amplified in the direction parallel to the columns.
The foundry process is particularly suitable for medium sized parts (a few grams) up to large sizes (several tens of Kg) and complex shapes.
ALNICO magnets can also be produced by powder metallurgy, this process is rather intended for high-volume production of small parts (≤ to a few grams) and simple shapes (parallelepiped, disc…). Euromag can offer parts in this technology.
Finally, ALNICO powders can also be mixed with curable resins to produce isotropic magnets by molding. The main application in the 1970s in the automotive industry was small magnets of magnetic couplings to connect the speedometer cable to the electric meter. As the magnetic properties are considerably reduced, these magnets are rarely used today.
MAIN FEATURES
ALNICO magnets have unique characteristics that have enabled them to survive despite the emergence of new families of magnets with much higher performance.
Indeed, they remain among the materials with the highest persistent induction. Their moderate coercive induction field is without context their weak point and requires the design of magnets with relatively large dimensions parallel to the magnetization direction. However, this weakness becomes an advantage when it comes to magnetizing, demagnetizing or reversing the polarity of the magnet as desired, operations that require little energy compared to other families of magnets.
ALNICOS have a high recoil magnetic permeability compared to all other magnets. This feature gives ALNICOs the ability to amplify the variations in the magnetic field usually found in soft ferromagnetic materials. It is particularly useful in applications where the magnet is used as a sensor in variable gap magnetic circuits.
The high quality of ALNICO magnets lies in their exceptional temperature behaviour. Indeed, the ALNICO Curie temperature is the highest of all magnetic materials for industrial magnets. ALNICO magnets can be used up to 500 – 550°C without significant loss, which is a record among all magnet families.
In addition, these magnets show excellent stability of their magnetic properties against temperature variations. With a standard magnetization variation of 0.02%/°C that can be improved to only a few PPM/°C, the stability of ALNICOs is at least equal to or better than that of the best Samarium Cobalt magnets for much wider temperature variation ranges.
The composition of the ALNICO alloy, close to a stainless steel, as well as the particular metallurgical texture of the material, give the ALNICO magnet excellent chemical stability. Thus if red oxidation can develop on the surface of the magnet and in the presence of moisture, it will remain superficial, giving the surface a form of passivation. However, due to the high aluminium content, the alloy can be attacked by solutions with PH > 7.
The intrinsic stability of the ALNICO material combined with magnetic and thermal stabilization techniques gives the magnets produced a longevity and recognized stability over very long periods (several decades).
Finally, it should be noted that, of all the families of magnets, ALNICOs have the best shock resistance.
APPLICATIONS
Historically, ALNICO magnets have been used in most permanent magnet applications. However, with the arrival of other magnet families, their use has gradually focused on applications that require high precision in terms of effect accuracy, in harsh environments (high temperature, aggressive environments) and where stability and longevity are major criteria. Because ALNICO magnets are easy to magnetize and demagnetize, they are nowadays indispensable in systems with magnetic induction flux switching, i.e., requiring the establishment, cutting or reversal of a magnetic field. They are therefore widely used in magnetic trays for the machine tool industry, in quick tool change devices for plastics processing, and in many magnetic handling devices.
For the same reasons, they are also found in magnetic hysteresis couplings or magnetic torque limiters, which allow motion to be transmitted at constant force or torque.
They are used in many sensors and measuring devices where the aim is to avoid the effects of temperature and obtain an accurate and calibrated response. This is particularly the case with tachogenerators where, in addition to its natural insensitivity to temperature, ALNICO magnet offers the possibility of adjusting the output voltage by magnetic calibration. It is also used in position, level, angle and acceleration sensors, scales and various counters.
ALNICO is also the only magnet that can be used in extreme environments in terms of temperature, aggressiveness and risk. It is therefore still widely used in high-frequency wave guidance and focusing amplification elements and in safety relays for rail, aeronautics and space applications.
IMPLEMENTATION
Design of magnets:
In order to prevent the magnet from demagnetizing itself, the ALNICO magnet must be designed in such a way that its dimension parallel to the magnetization direction is large in relation to the pole surface. This dimension depends on the magnetic circuit into which the magnet is inserted and its magnetic properties. Do not hesitate to contact us for advice. We can also offer calculation and optimization services for your magnetic circuit.
For a cylindrical magnet, surrounded by air, depending on the coercive field of the grade used, it is customary to apply a ratio of 3 to 5 between this dimension and the diameter of the poles.
The low anisotropy of the product makes it necessary to pay particular attention to the contact between the poles of the magnet and the rest of the magnetic circuit. Its surfaces must be machined with excellent flatness (≤ 0.05mm) to limit magnetic losses.
Machining & tolerances:
Like most permanent magnet materials, ALNICO is a very hard material. This will limit machining to the minimum necessary, knowing that it consists of grinding with a grinding wheel.
The standard machining operations applicable are internal and external cylindrical grinding and flat grinding. Wire machining is possible but it always presents a significant risk of cracking or chipping. Tribofinishing and sandblasting are mainly applicable for grades up to ALNICO 800. Beyond that the alloy becomes very hard and flakes too easily.
The tolerances that can be achieved on ALNICO magnets are as follows:
Handling and assembly:
If ALNICO magnets are to be handled and assembled with magnets, it is important to observe the following precautions or risk of significant loss:
Do not approach magnets near a magnetic field source;
Do not approach the magnets between them and in particular do not put their poles in opposition with those of another magnet, even at a distance;
Do not rub the magnets against each other or against a ferromagnetic component;
Magnetization and calibration:
In the vast majority of cases, ALNICO magnets are supplied without magnets. It is preferable to magnetize them at the last moment before use and preferably in the final magnetic circuit. For small quantities we can offer magnetization services for your magnetic assemblies, do not hesitate to contact us.
On the other hand, ALNICO magnets are easily adjustable by progressive and controlled demagnetization, including in the application.
For magnetization, a commonly accepted rule is that the saturation field must be at least 3 times greater than the coercive field of the magnet. In practice, a field of 6000 Oe (480 kA/m) is a sure way to saturate most shades. Due to their low electrical resistivity (40 to 70 Wcm) and the generally large dimensions of the magnets, it is preferable to use a field source with slow variation over time (electromagnet).
EffeCT OF TEMPERATURE:
The magnetic induction field generated by the magnet decreases by 0.02%/°C when the temperature increases. It increases in the same proportion as the temperature decreases. For reasonable temperature variations (about ±100°C), the variations are generally totally reversible if the magnet is well designed.
The magnetic attraction force generated by the magnet varies in the double ratio of the magnetic induction field (0.04%/°C).
MAGNETIC & PHYSICAL CHARACTERISTICS
Magnetic properties of the shades:
The values of remanence, coercive field and maximum specific energy depend a lot on the size and shape of the part.
The minimum values indicated have been established for simple shaped parts (parallelepiped, solid cylinder) with saturation magnets, whose volume is between 1 and 200 cm3 and whose dimensions exceed 8mm. For anisotropic grades, these data are only valid in the direction parallel to the magnetic orientation of the material.
The values of the temperature coefficients of the remanence are given only as an indication. The behaviour of the magnetic induction field created by the magnet also depends on the working point B/µoH in the magnet.
The curie temperature has only a bibliographic value. In reality, the maximum operating temperature is limited by other elements from the manufacturing process and is about 550°C. The maximum operating temperature also depends on the working point in the magnet.
Other physical properties:
The table below is a collection of data from the specialized literature. The values are given for information only.
Characteristic curves :
You can download the B(H) characteristic curves for each of the grades by selecting the links below: