Alnico magnets are permanent magnets derived from the combination of aluminum, nickel and cobalt. These magnets come in several forms, namely, isotropic, anisotropic, non-directional, and monodirectional. They can have 5-17 times more magnetic force than that of the naturally occurring magnets magnetites and lodestone.
These specialized magnets have a low temperature coefficient and are commonly calibrated for high residual induction necessary for high temperature applications reaching up to 930°F or 500°C. These are also helpful for applications in the industry where corrosion resistance is a must. These are commonly seen on sophisticated sensors.
How are Alnico Magnets Manufactured
Alnico magnets are manufactured by a series of in depth processes involving sintering and casting a mixture of aluminum, nickel, and cobalt. It is not limited to those materials, in special cases where magnetic force needs to be enhanced further, iron, copper or titanium are opted to be included in the mixture. The casting process sticks to the traditional method of pouring a liquified (molten) mixture of the needed materials into a mold made of a bonded sand.
Choosing the right process in producing alnico magnets will usually depend on the size, weight and the application of the desired product. In general, huge magnets weighing several pounds are cast, while smaller magnets that need to be mass produced are usually made by sintering. Below is the explanation of the important processes in manufacturing these types of magnets.
The grade of an alnico magnet will vary depending on the specific combination of its basic elements. The correct amount of these elements will then be placed in a furnace to be melted at 1750 degree Celsius. In some cases, extra aluminum is added to compensate for the aluminum that melts away. Extra aluminum is added to replace the aluminum that melts away. For quality assurance, a sample of the mixture will be cooled for inspection.
The molten mixture will be poured into a mold of green sand. Molds are designed to be made larger to allow for shrinkage that might happen in the cooling process. The mold shell will disintegrate and leave a small residue during cooling.
Custom manufacturing complex shapes for cast alnico magnets is also possible and this is one of the advantages of this type of magnet from others that are not capable of being formed as such. Alnico magnets, however, are hard and brittle, therefore it is prone to cracking and chipping off. Because of this properties of alnico magnets, machining requires special methods. Cutting or grinding these should be done prior to the magnetization process.
The cooled magnets are removed from the mold then they are fettled on a grinding wheel to clean out the part and remove the mold’s runners.
These magnets will be then heated and placed in a magnetizer. These magnets will go through a process called hardening, where as they cool, they are exposed to a strong magnetic field.They are then placed in an oven to temper and stabilize the magnetic field. This will give its final magnetic properties.
Magnets will be ground for the last time to achieve the magnet’s dimensional and surface finish tolerance and requirements.
Hysteresis graph testing is used to inspect and check for quality of the magnet’s design, which is the B (Flux Density) and H (Magnetizing force) curve, or hysteresis loop. This characterizes a magnet’s material.
Alnico magnets do not need a protective coating, but they are oftentimes painted or coated to cover their dark grey appearance. Painting or coating is the final process in manufacturing these magnets. The poles may be left unpainted and shiny.
The finalized coated magnet is placed in a coil or solenoid. The final magnetizing will be done through an electrical pulse. See picture below for a clearer view of this process.
Pressing & Sintering
Sintering involves the grinding of the right mixture of the elements into tiny particles to form a powder, which will be placed under tons of pressure in a die.
After pressing the ground materials, these will be placed in a sintering furnace with a hydrogen atmosphere reaching a temperature of up to 1200 degrees Celsius, a process that fuses the small particles.
During the cooling process, the magnetic field then again is formed. The magnet will be anisotropic if it is cooled near a magnetic field, while on the other hand, the magnet will be isotropic if no magnetic field is present.
Coating and Magnetizing
Sintered alnico magnets do not require any shaping or grinding because they have achieved their desired form when they were pressed. Just the same as in the casting process, coating is completed prior to magnetizing.
The magnet will be then magnetized by placing it in a coil or solenoid.
Grades of Alnico Magnets
There are 29 grades of alnico magnets, 17of these are cast, 10 of these are sintered, and 2 of these are bonded. The grading classification is based on the magnet’s chemical composition, residual induction, coercive force, maximum energy, the percent of cobalt, and operating temperature. As a rule of thumb, the grade of the magnet goes higher as the operating temperature also rises.
The most powerful and most common alnico grades usually fall within the anisotropic cluster.
Alnico Magnets are one of the most powerful magnets available in the market. It has a magnetic force that is 5 to 17 times stronger than that of magnetite and lodestone. Due to its reputable strength and material integrity, these types of magnets are used widely in the industry and are also trusted for use in applications operating at high temperature