Aluminum Nickel Cobalt
Alnico (Aluminum Nickel Cobalt) magnet alloy exhibits excellent temperature stability and a high Residual Induction (Br). Alnico magnets are largely comprised of Iron, Cobalt, Aluminum, and Nickel with trace amounts of other elements used to tailor the alloy’s magnetic and mechanical properties. Alnico magnet alloy is a mature technology and is manufactured by casting or sintering. The Alnico magnet manufacturing process and the method used to develop anisotropy allow for geometries that are more complex compared to other magnet alloys.
Alnico magnets have a low Coercive Force (Hc) which results in this alloy being easily demagnetized. Commercially viable magnet designs with Alnico require a long magnetic length relative the magnet’s pole cross-section. To counteract this necessity, Alnico magnets are usually used in conjunction with high permeability materials which act as a yoke or return path. There are grades of Alnico magnets which have higher Coercive Forces, and therefore, exhibit higher degrees of resistance to demagnetization, but the cost increases and the mechanical characteristics degrade with these grades. Alnico magnets also have a low Energy Product (BHmax) when compared to the Rare Earth magnet alloys, even though their Residual Inductions (Br) are comparable. In many applications Alnico is now being replaced with much smaller Rare Earth magnets.
Alnico Magnet Manufacturing Process
Alnico magnets are manufactured by a cast or sintered process. Production level cast Alnico magnets are produced by conventional foundry methods using resin bonded sand molds. The magnetic characteristics for some exotic grades of Alnico magnets are achieved during the casting operation and are due to the unique crystalline grain orientation developed during the process. Sintered Alnico is a powdered metal and it is manufactured by compacting finely milled Alnico powder in a metallic mold. The resulting geometry is not fully dense and it must be sintered in a furnace to achieve a solid state.
Properties of all anisotropic or oriented cast and sintered Alnico magnet alloys are optimized during a heat treatment process. The direction of orientation is determined in this phase of the manufacturing process which involves heating the alloy above its curie temperature, then cooling at a controlled rate in the presence of a directional magnetic field.
Some cast and sintered Alnico magnets are isotropic or un-oriented and they skip the orientation process. These grades of Alnico magnets usually have lower energy products compared to the anisotropic grades, but they lend themselves to specialized magnetizing.
The surfaces of cast Alnico magnets are usually dark grey, have wide tolerances, and have a rough finish. Critical dimensions are abrasively cut or ground in order to maintain close tolerances and improve the fit and finish. Sintered Alnico magnets usually require minimal grinding because they can be produced with tighter dimensional tolerances. It is usually desirable to have features developed in the casting or sintering operations because conventional machining methods are difficult to employ when fabricating Alnico magnets.
Alnico Magnets Temperature Characteristics
Alnico magnets offer the best temperature characteristics of any standard production magnet material available. Alnico magnets can be used for continuous duty applications where temperature extremes up to 930°F (500°C) can be expected. Temperatures above 1000°F will result in permanent metallurgical changes which can only be recovered by reheat treating. Excursions to lower temperatures pose less of a problem for most alnico applications; however, each individual circuit should be examined carefully to determine the effects which may occur as a result of operating at those extremes. A Dura team member can provide specific guidelines concerning the temperature characteristics for a given Alnico grade magnet and make recommendations for their proper use.
Alnico Magnets Corrosion Characteristics
Alnico magnets resist corrosion very well and they are usually employed in applications with no coatings or plating. Alnico magnets can be plated or coated with a variety of materials if an application so requires. Some applications which may require Alnico surface treatment are: cosmetic necessity, bonding, internal use, or to increase the surface hardness.
Alnico magnet material is very hard and brittle. Alnico magnet material is very hard and brittle. On average the material’s hardness is 45 Rc and conventional machine tools and cutters are inappropriate. Abrasive grinding and electrostatic discharge machines (EDM) are the typical means of fabrication for this magnet alloy. Most magnet materials are machined in the un-magnetized state. Once the fabrication and cleaning operations are finished the magnet is then magnetized to saturation
Dura Magnetics is capable of fabricating simple or complex shapes from Alnico magnet alloy. We stock a variety of standard and exotic Alnico magnet grades for production or prototype fabrication.
A Dura Magnetics team member can help determine if custom machining is necessary or “cast to size” geometry is possible. The determining selection factors are usually the lead-time, cost, and the alloy required.
The lower coercive force of Alnico makes magnetizing a simple matter in most cases. In order to optimize the performance of Alnico magnets, it is advisable to magnetize the magnet after assembly with other circuit components. This helps control particle contamination, simplifies assembly operations, and helps reduce demagnetization of the Alnico magnet from external influences. These influences can be external demagnetizing fields from other permanent magnets or electromagnets, vibration, and impacts to the Alnico magnet. This material is often supplied with keepers to help ensure the integrity of the magnet or assembly.
Calibration or conditioning of Alnico magnets can also be accomplished after the magnetizing process. During this operation, the Alnico magnets are exposed to a small demagnetizing field or elevated temperatures which partially demagnetizes the magnet. The domains in the Alnico magnet which are influenced are considered “weak” and they would have demagnetized at some point in the near future. The Alnico magnet conditioning operation essentially demagnetizes the magnet to a stable level which will resist normal magnet aging effects. The usable fields of the resulting magnets are more consistent between magnets and age at a similar rate.
These techniques can be used in large quantities to “stabilize” a production run, or individually to meet an operational threshold. The individual method is usually employed when a high and low threshold exists for an application.