Alnico Magnets

Chapter One – What are Alnico Magnets?

An Alnico magnet is a permanent magnet made by combiming aluminum, nickel, iron, cobalt, and other elements. They come in isotropic, non-directional, or anisotropic, mono-directional, form. Once magnetized, they have 5 to 17 times the magnetic force of magnetite or lodestone, which are naturally occurring magnet materials that attract iron.

Alnico magnets have a low temperature coefficient and can be calibrated for high residual induction for use in high temperature applications as high as 930°F or 500°C. They are used where corrosion resistance is necessary and for various types of sensors.

Developed in the 1930s, alnico magnets are widely used in permanent magnet motors due to their magnetic properties and temperature coefficient. They maintain their magnetic field, are hard to demagnetize, and retain their magnetism at very high temperatures. Alnico magnets cannot be cold worked and are produced using sintering or casting.

The first magnets were naturally occurring lodestones that explorers used to find the magnetic north pole. During the first industrial revolution, developers created man made magnets, which led to the development of the alnico magnets.

Chapter Two – How Alnico Magnets are Made

Alnico magnets are produced by sintering or casting using a mixture of aluminum, nickel, and cobalt. Iron, copper, or titanium are added to enhance the magnetic force. The casting process follows the traditional method of pouring a molten mixture into a mold, which is usually made of a resin bonded sand. Alnico magnets made by casting are larger and have better magnetic properties and performance compared to ones made by sintering.

Cast alnico magnets can be custom designed and manufactured into complex shapes, configurations, and forms, which is not possible with sintering. Whether sintering or casting is used, alnico magnets are too brittle and hard to be cold worked and must be formed using heated raw materials.

The chemical composition of its mixture determines the grade of an alnico magnet. The chart below shows a sampling of some of the different grades of alnico magnets.

The choice of the process used to produce an alnico magnet depends on the size, weight, and final use. Very large complex magnets, weighing several pounds, are cast, while smaller magnets, to be manufactured in large quantities, are cast made by sintering.

Casting Alnico Magnets

Heating Magnet Materials

Each grade of alnico magnet is produced using different combinations of the raw materials of cobalt, aluminum, and nickel. An alnico 6 magnet contains 8% aluminum, 16% nickel, 24% cobalt, 3% copper, 1% titanium, and less than 1% of ferrous metals. The precision measured quantities are placed in an induction furnace to be melted. For the mixture to be sufficiently combined and melted, it is heated to 1750° C or 3182° F.

Casting Process

The casting process involves pouring the molten mixture into green sand molds, which have been made large enough to account for any shrinkage that may take place during cooling. For some sizes of alnico magnets, several molds are placed together and connected by gates. When the molten magnet material is poured into the mold, it travels through the gates to all of the individual molds such that several magnets are produced in a single casting.

After the molten elements are placed in the mold, they are allowed to cool. The cooling rate and surface finish of the magnets are affected by the amount of sand used, the grain size of the sand, and the properties of the sand. During cooling, the mold shell burns away and disintegrates, leaving a small accumulation of sand on the alnico magnets.

Shell molds are made of a combination of sand and resin. After the molten alnico magnet material is poured into the mold, the sand and resin start to melt until they completely disappear, leaving the alnico magnet shapes.

Molds for casting alnico magnets can be complex and intricate due to the flexibility of the molds and the materials. Shrinkage in the molding process is a continual concern that is always factored into the process.

Fettling Method

Fettling is a method for removing excess material from a casting. In the case of alnico magnets, removing the gates allows the molten material to enter the casting for the individual magnets. The process of fettling is completed using various methods of grinding. Since alnico magnets are brittle and chip easily, fettling is a delicate process that is completed without damaging the magnets.

Heat Treatment

Alnico magnets gain their magnetic properties during heat treatment. Temperatures for the heat treatment process reach 1250° C or 2282° F. During heat treatment, the magnets are placed in a magnetizer. As they cool, they are exposed to a strong magnetic field, a process that is known as hardening. The magnets are placed in a large tempering ovens for several days to temper and stabilize the magnetic field, which gives them their final magnetic properties. In the heat treatment stage, the magnetic materials develop temperature handling characteristics and their final magnetic properties.

Finish Grinding

The use of the grinding process is dependent on the manufacturer. In some cases, alnico magnets are not ground and are shipped after treatment. In other processes, the magnets are ground to tight tolerances. Additionally, grinding helps achieve the correct dimensional requirements for the magnets.

Testing

The purpose of testing final products is to inspect the design, performance, and quality of the finished alnico magnets. Some of the tests that can be performed on the magnets include the hysteresis loop, gauss meter, and flux meter.

• Hysteresis Loop Test - The hysteresis loop offers information about the magnetic properties of a material and shows the relationship between the induced magnetic flux density and the applied magnetic force. The size and shape of the loop shows what kind of material the magnet is made from. For high quality alnico magnets, the loop is as wide as possible. Alnico magnets are one of the four materials that meets the hysteresis loop parameters.

• Gauss Meter - A gauss meter measures the direction and intensity of a magneticfield using the hall effect, which is the relationship between electric current and magnetic fields. When an electric current is passed through a magnetic field, the force of the field will push the electrons to one side.

• Fluxmeter - A fluxmeter is used to measure the flux of a permanent magnet. Magnetic flux is the number of magnetic lines that pass through a surface. The calculation of the flux provides a measurement of the overall magnetic field of a surface.
• Magnetic field lines are closed loops that leave the north pole and move to the south pole, as in the diagram below.

Coating Alnico Magnets

It is not necessary to coat alnico magnets since they are exceptionally resistant to corrosion. The level of their corrosion resistance depends on their grade and the types of chemicals to which they will be exposed. In conditions where there is salt water, alkali solutions, and inorganic acids, alnico magnets may be coated with nickel, zinc, or certain types of paint.

In conditions where a coating is not necessary, they may still be coated for aesthetic and appearance purposes, such as uses for school projects. When painted, the color that is always used is red, which has led to their being named red magnets. Painting or coating is the final step of the process. The poles may be left unpainted and shiny - or to be buffed and polished.

Magnetizing Alnico Magnets

An alnico magnet is not a magnet until it passes through the magnetization process. During the heat treatment, alnico magnets are aligned, which gives them their designated direction or poles. Prior to being magnetized, the magnets are referred to as slugs that need to have their magnetic field activated. The majority of alnico magnets are anisotropic or magnetized in a single direction.

There are two methods for magnetizing magnets, which are static and pulse. Static is used for small magnetic fields while pulse is used for stronger and more powerful magnetic fields. Regardless of the method, the magnetizer applies a magnetic field over the entire magnet in the predetermined direction.

The pulse method does not supply a continuous magnetic field since the magnetic coil would need to be cooled. A power capacitor bank is used to discharge power into the magnetic coil. Each pulse lasts less than a second but has an intensity of tens of thousands of ampere producers. When being magnetized, alnico magnets are placed between the poles of the magnetizer to receive complete saturation.

Sintering Alnico Magnets

Pressing Powder

Prior to the initiation of the sintering process, the raw elements have to be prepared by being ground into a very fine powder. The components of the powder are very carefully proportioned to create the right mixture of the elements. The finely ground powder is placed under tons of pressure to form a tightly packed form.

Sintering is a powder metallurgical process that aligns the magnetization axis during the pulverization of the raw materials prior to it being pressed into a mold. The pressing process can be perpendicular to create a perpendicular magnetic field, creating higher performance magnets. Ring alnico magnets are pressed using the parallel pressing process.

Sintering or Frittage

Sintering, known as frittage, forms a solid by applying heat and pressure without melting the material. It is a heat treatment process designed to combine loose, aggregate materials into a solid mass. During the process, the atoms of the material diffuse and fuse together to form a solid piece. The sintering point of the powdered material is where it has been heated enough to drastically reduce the porous spaces between the pulverized particles and squeeze them close together without being melted.

Sintering is an ideal method for manufacturing small alnico magnets since it can easily combine aluminum, iron, nickel, cobalt, and other added elements. Additionally, it increases the properties of the elements such as their strength and integrity.

The die into which the material has been compressed is heated at 2300° F or 1260° C in a hydrogen furnace, which is used to reach extremely high temperatures. The bodies of hydrogen furnaces are made of stainless or carbon steels with molybdenum as the heat shield.

Cooling the Magnets

After sintering, the alnico magnets are red hot and need to be cooled. During cooling, the magnets are placed in the presence of an external magnetic field to make them stronger. For the majority of alnico magnets, the applied magnetic field makes them anisotropic, the most common magnetic field. If an external magnetic field is not applied, the magnets will be isotropic without a directional magnetic field.

Coating Alnico Magnets

Sintered alnico magnets are similar to cast alnico magnets in that they do not require a coating. Additionally, since they are shaped in a die, they do not require grinding, shaping, or forming. Coatings are added for identification or aesthetic purposes. The process of applying zinc, nickel, or paint coatings is completed prior to the magnets being magnetized.

Magnetizing in Sintering

The final step of the sintering process is to magnetize the magnets. Although the direction of the magnets has been determined during cooling, the magnetic field has to be increased by magnetization since the magnetic field is not sufficiently strong. Sintered magnets are magnetized by a pulse generator that supplies pulses at one fifth of a second to fully magnetize the magnet.

Alnico magnets require a magnetizing field of 3 kOe. Electrical coils or solenoids are used to magnetize sintered magnets.

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Chapter Three – Types of Alnico Magnets

Alnico magnets come in a wide variety of shapes of sizes to fit any type of application. They can be found in small toys and electric guitars or in factories for industrial applications. The different shapes of alnico magnets can be produced by sintering or casting with casting being the most common method.

Shapes and Types of Alnico Magnets

Bar or Block Magnets

Bar or block magnets are used in schools for experiments, such as demonstrating magnetic fields. They are the weakest shape because of the small area of the poles but are the most common shape. Bar magnets can be used in automation, separation, and holding applications.

Other items can be magnetized using a bar magnet. When a bar magnet is rubbed against a metal object, it aligns the electrons in the object and magnetizes it. The magnetism that is created is not permanent but will hold for a while.

Cylinder Magnets

Cylinder magnets have straight parallel sides, circular cross section, and a hollow center parallel to their sides. They are used for seals, sensors, coin collection machines, and guitar pickups. Amplifiers, actuators and motor applications use cylinder alnico magnets for their flux density. In medical applications, they are used for their strength and ability to be adjusted.

Rod Alnico Magnets

Rod alnico magnets are cylinder shaped and are used in tools and parts. They have high coercivity, strong flux output, and remain stable at high temperatures up to 1000o F or 537o C. Alnico rod magnets have higher residual energy induction and low coercive force compared to ceramic and rare earth materials.

Horseshoe Magnets

Horseshoe magnets are bar magnets that have been formed into a U shape, which makes them strong because the poles are pointed in the same direction. They can be used to pick up metal objects of any size depending on their strength.

Pot Magnets

Pot, holding, or mounting magnets are used for holding applications. They are an alternative to normal fixture and fastening methods. Mounting magnets can be easily removed and relocated, unlike permanent approaches.

Ring Magnets

Ring magnets are round with a hole in the middle and are sometimes referred to as donut magnets. The location of their north and south poles depends on the way they were polarized. One half of the magnet is always north, while the other is south. Certain kinds of polarization create magnets that are split into quarters or eighths.

Ring magnets are used in science experiments and for medical applications. People with implanted cardioverter defibrillators, or ICDs, automatically receive shocks to their hearts if the rhythm becomes irregular. When the device malfunctions, it shocks the patient unnecessarily, which leads to irregular rhythms. Placing ring magnets on a patients‘ chest, over the ICD, disables the device.

Alnico Magnet Coatings

Alnico magnets have strong resistance to oxidation and corrosion and do not require a protective coating, plating, or surface protection. Their corrosion resistance is so strong that they are not affected when exposed to oil, solvents, gasoline, or alcohol. Their one vulnerability is their iron content. If they are exposed to water for an extended period of time, they get surface corrosion. This same weakness can occur if they are exposed to alkali solutions and inorganic acids.

Though a coating is not needed, manufacturers will paint alnico magnets to improve their appearance and cover their natural dark gray color. In the majority of cases, red, as seen in the example below, is the color of choice. They can also be coated with chrome or nickel.

Alnico magnets can be coated or plated using a wide array of materials. In many cases, they will have a coating applied if it is necessary for an application. Cosmetic treatments, bonding, internal use, and to increasing surface hardness are some of the reasons for alnico magnets to be coated.

Chapter Four – Uses of Alnico Magnets

Alnico magnets are used for applications with high operating temperatures. They are used where a strong permanent magnet is required. such as electric motors, electric guitar pickups, microphones, sensors, loudspeakers, and cow magnets.

The fact that they have high flux density, and magnetic lines of force, at a reasonable price makes them a primary option for many uses. Alnico magnets have the lowest reversible temperature coefficient of any available magnet at 0.02% per degree of centigrade, which creates temperature stability.

Uses of Alnico Magnets

Motors

Alnico rotor magnets have multiple poles with alternating polarity. The hole in the rotor allows it to be mounted on a shaft. They are excellent for use in synchronous motors, dynamos, and air turbine generators.

Cow Magnets

Cow magnets are tools to save the lives of cows, when they eat grass, dirt, nails, staples, and bits of baling wire. When iron items work their way through the stomachs of cows, they cause "hardware disease", which threatens the cow‘s vital organs and causes irritation and inflammation. The magnet is inserted into the cow's mouth at around one year of age as a pill and lodges in the cow's second stomach to attract the stray metal.

Alnico Magnets in Speakers

Alnico magnets are preferred in speakers because they allow for precision tuning of the magnetic field in the air gap, which creates optimum acoustic performance. The air gap is the space between the poles of a magnet, or the space between the magnet and the surface on which it is placed. In the case of speakers, one pole of the magnet surrounds the other pole. The magnetic field in the air gap is very strong and requires precise control, which is possible with alnico magnets.

Electric Guitar Pickups

One of the most popular uses of alnico magnets is in electric guitar pickups. The purpose of pickups to convert the vibrations of the guitar string's into electrical signals, which are sent on to the amplifier to boost the guitar strings volume. The magnetic properties of alnico 2, 3, 4, and 5 magnets makes them ideal for this application. The pickup makes it possible for an electric guitar to be heard over loud noises.

Grate Magnets

Grate magnets are a protective device installed in hoppers, floor openings, vertical chutes, and ducts. They are designed to catch and trap fine iron contamination. Weakly magnetic or very fine iron contaminants become trapped in the grate and are later wiped off.

Alarm Systems

Extremely sensitive and accurate equipment depend on alnico magnet‘s electrical conductive capacity to activate and deactivate electrical circuits. In alarm systems, alnico magnets are used to complete the electrical circuit on the system when the system is closed and in the attraction position. When there is unauthorized access, the circuit is broken and the alarm is activated. In certain applications, the system will turn off a device or other equipment.

Chapter Five – Grades of Alnico Magnets

Alnico magnets come in several variations with different amounts of aluminum, nickel, and cobalt. The varying percentages of the base metals alters the strength of the magnet, which creates a variation in their individual magnetic fields.

There are 29 grades of alnico magnets, which are seventeen cast, 10 are sintered, and two are bonded. The grading system is based on the magnet‘s chemical composition, residual induction, coercive force, maximum energy, the percent of cobalt, and operating temperature. As the operating temperature rises, so does the grade of the magnet.

The majority of alnico magnets are anisotropic, which are the most powerful of the alnico grades. Below is a discussion of a view of the alnico grades and their use.

The chart below provides an overview of some of the grades of alnico magnets and their properties.

Grades of Alnico Magnets

Alnico 2

Alnico 2 is the second weakest of the alnico magnets, with a low cobalt content. It is isotropic with low magnetic output and widely used with electric guitars. They are famous in audio sound recording and performance for their ability to clarify sound. Though alnico 2 magnets are weaker than alnico 4s and 5s, when they are used on an electric guitar pickup, they produce a cleaner sound.

Alnico 3

Alnico 3 has the lowest strength of the Alnico magnets with zero cobalt content. As with alnico 2, it is isotropic with low magnetic output and is used with electric guitars. They have the same sound capabilities as alnico 2 magnets.

Alnico 4

Alnico 4 is a weak magnet, similar to alnico 2 and 3. It is an isotropic with low magnetic output and is used for sound recording and performance.

Alnico 5

Alnico 5 is the most widely used of the alnico magnets and has four cast versions and one sintered one. It delivers magnetic energy economically to magnetic circuits. It is a premium grade anisotropic magnet with superior magnetic output. The directional force of an alnico 5 magnet is created by its crystal orientation during its molten state. Large magnetic fields can be generated with alnico 5 magnets. For that to happen, they must be magnetically long and have iron or steel elements to reduce demagnetization.

Alnico 8

Alnico 8 has the highest coercive resistance of all alnico grades and has excellent energy per unit volume with a low reversible temperature coefficient. It is used for applications exposed to demagnetizing fields or where a short magnetic length is required.

Alnico 9

Alnico 9 has high magnetic properties and a single crystal structure. They are used in geological detection devices and for defense and aerospace technology. As with most alnico magnets, alnico 9 magnets are resistant to high temperatures.

Conclusion

• Alnico magnets are permanent magnets made of aluminum, nickel, and cobalt.
• Alnico magnets have 5 to 17 times the magnetic force of magnetite or lodestone.
• Alnico magnets come in several variations with different amounts of aluminum, nickel, and cobalt. Copper and titanium may be added to increase magnetic strength.
• There are 29 grades of alnico magnets that are 17 cast, 10 sintered, and 2 bonded.
• Alnico magnets are used for applications that involve high operating temperature applications.

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