Alnico magnets: Cast and Sinter Alnico Explained

When it comes to crafting exceptional sound experiences, the role of magnets in subwoofers cannot be underestimated.

Among the various magnet types available, alnico magnets stand out due to their unique composition and remarkable properties.

Comprising a blend of aluminum, nickel, cobalt, and iron, these magnets offer power, efficiency, and durability.

Alnico magnets have carved a niche for themselves in the world of subwoofers, where delivering impactful bass and outstanding performance is paramount.

Key Takeaways

  • The main elements that make Alnico magnets are Aluminum, Cobalt, and Nickel.
  • Alnico magnets are a type of permanent magnets widely used for different applications with high operating temperatures such as electric motors, high-end sensors, speakers, subwoofers, etc.
  • The term alnico is derived from the initials of its main components i.e. Al-Ni-Co refers to Aluminum-Nickel-Cobalt.

What is an alnico magnet in subwoofers?

Alnico magnets are made from special metals like aluminum, nickel, cobalt, and iron.

These metals come together to create a super powerful magnet that used to be the star of speakers a long time ago.

But even though other magnets took over later on, alnico magnets are still used in fancy speakers today.

The name “alnico” is like a secret code for the metals it’s made of aluminum, nickel, and cobalt.

To make these magnets, they heat up these metals and then cool them down quickly. This makes the magnet really strong and it doesn’t lose its power easily.

How Do Alnico Magnets Work?

These magnets generate powerful magnetic fields, which interact with the subwoofer’s voice coil.

This interaction results in the vibrations responsible for producing the distinctive bass notes that enrich our audio experience.

However, while alnico magnets bring forth an array of advantages, they also come with certain drawbacks.

Their higher cost and weight compared to alternatives like ferrite and neodymium magnets require careful evaluation, especially when optimizing subwoofer design.

Let’s take a quick look at their merits followed by potential disadvantages.

Benefits of Using Alnico Magnets for Subwoofers

Alnico magnets offer a range of advantages that make them a preferred choice for subwoofer enthusiasts.

These advantages are derived from their unique composition and magnetic properties, which contribute to an enhanced audio experience:

Exceptional Sound Performance

Alnico magnets play a pivotal role in delivering superior sound quality.

Their strong magnetic fields ensure precise control over the subwoofer’s diaphragm, allowing for the accurate reproduction of low-frequency notes.

This results in a sound profile that is rich, deep, and resonant, perfectly complementing the overall audio spectrum.

Enhanced Bass Response

Alnico magnets excel in this department, providing the necessary magnetic force to drive the diaphragm with the force needed to generate the desired low-frequency vibrations.

This translates to a bass response that’s both authoritative and immersive.

Durability and Longevity

Alnico magnets are renowned for their durability and longevity.

These high-performance magnets maintain their strength over time, ensuring consistent sound quality even after years of use.

Efficient Energy Conversion

The efficiency of Alnico magnets contributes to their appeal in subwoofer design.

They effectively convert electrical energy into mechanical vibrations, maximizing the conversion process and leading to greater energy efficiency.

This efficiency translates to increased sound output without excessive power consumption.

Also Read: Which Amplifier is right for your Subwoofer? (Answered)

Wide Frequency Range

Alnico magnets contribute to subwoofers that excel in reproducing a wide range of frequencies.

This versatility allows the subwoofer to not only produce deep bass notes but also extend its performance into the higher bass frequencies, resulting in a more comprehensive and balanced audio experience.

Disadvantages of Using Alnico Magnets for Subwoofers

As with any technology, alnico magnets are not without their drawbacks:

Cost

Alnico magnets are generally more expensive to manufacture than some other magnet types.

This cost can influence the overall price of a subwoofer, potentially making it less accessible to budget-conscious consumers.

Weight

The high-performance nature of alnico magnets often translates to increased weight.

This added weight can impact the overall design and portability of the subwoofer, which is a crucial consideration, particularly for mobile or compact setups.

In light of these pros and cons, the selection of alnico magnets for a subwoofer requires careful consideration.

It’s essential to strike a balance between the benefits they bring and the challenges they pose.

What are the main elements of alnico magnets?

The main elements of alnico magnets are aluminum, nickel, and cobalt, however, the composition of alnico alloy may also include copper (Cu), Titanium (Ti), Niobium (Nb), and Iron (Fe).

The accuracy of the chemical composition within these magnets depends on the grades of Alnico.

However, you can refer to the table (image) below to know the typical composition range of its main elements according to their weight.

Alnico magnet composition diagram

How are Alnico Magnets Made?

There are typically two methods or processes used to make alnico magnets such as the Sintered and Casting process.

Let us look into their individual steps below for a general understanding of their making.

Process 1: Pressing/Milling

The sintered process includes pressing, sintering, cooling, coating, and magnetizing.

Sintered alnico magnets are typically made by a pressed powder process and under high temperatures.

First, the raw materials or the elements are finely ground by milling them into tiny particles which turns them into a powder form.

Next, the mixture or the powdered form of elements is pressed in a dye under tonnes of pressure to offer a close or desired shape to the magnet.

The resulting magnet typically has a greenish shade or appears green in color.

Sintering

Sintering is the process where the pressed powder or the resulting magnet is transferred in a furnace to fuse the pressed particles under excess heat.

This process is typically carried out in a hydrogen atmosphere at temperatures of around 1200 to 1300 degrees Celsius.

Treating the resulting green magnet under excess hot temperature is termed as sintering. This process helps fuse the pressed particles together.

Note: Sintered alnico magnets are structurally stronger than cast Alnico, however, it is always advised to use them with their suitable applications. In general, the sintered alnico magnets are lighter and smaller and hence are suitable for volume production.

Cooling

The hot magnet is then cooled for further applications such as coating or painting and magnetizing.

After the magnet cools, an external magnetic field is presented to make the magnet stronger.

It also helps in building anisotropic properties and a preferred magnetic direction.

Without the presence of an external magnetic field, the resulting magnet will be isotropic and won’t have a preferred direction of magnetism.

Hence, it is one of the most crucial steps after the magnet cools.

Coating or Painting

The coating or painting depends on the requirement or the type of application the alnico magnets are going to be used for.

In general, sintered alnico magnets are die-pressed firmly to offer them the desired shape and hence do not require further machining for shaping.

Typically, coating or painting is done at the end of the process and before they are magnetized.

In this specific process, the poles are sometimes fettled to make them appear shiny and brighter.

Magnetising

Magnetizing of alnico magnets is the final step of their manufacturing process.

In this step, the resulting magnets are placed in a solenoid or a coil, and the required amount of magnetic pulse is generated to completely magnetize the alnico magnets.

Finally, the magnets become well-prepared to be used in their respective applications.

Also Read: Subwoofer getting hot? Overheating causes and solutions

The process of making sintered alnico magnets (Diagram)

You can refer to the diagram below to understand the process of making sintered alnico magnets in brief:

sintering process of Alnico magnets

Process 2: Casting

When it comes to making a magnet as a finished product the quantity or the percentage of each element varies according to their grades and applications.

The main compounds used to make alnico magnets are aluminum, nickel, cobalt, and iron.

Let us look into the steps below used to make cast alnico magnets:

Foundry Furnace

The raw materials which are the elements used to make alnico magnets are placed in a large industrial induction furnace in the required quantities.

These elements are melted at very high temperatures mounting around 1700 to 1800 degrees Celsius.

Out of all the elements added Aluminum tends to melt quickly and hence its quantity is usually added in excess to account for its waste dose.

Finally, once the mixture melts to the desired level it is set to cool to proceed with the further steps of casting alnico magnets.

Note: Aluminum tends to melt around 800 degrees Celsius.

Inspection

Inspection of the melted compound is another crucial step to ensure the presence of the required ratio of each element in the mixture.

The samples picked for inspection are cooled, and inspected with an X-ray spectrometer before casting them.

This ensures that the required ratio of each element is present in the solution and if required some adjustments can be made before proceeding with the next step.

Casting

The melted compound is added into shell molds or green molds typically made from sand and these molds are pre-prepared in the required shape of magnets.

These shell molds are like containers used to shape the magnets and are technically made with molding machines and pattern plates.

These molds are linked with horizontal tubes known as runners and vertical tubes known as risers. It helps to fill the molds evenly, prevents voids, and ensures no space for air.

The molten compounds usually cool down quickly in the shells, but, in some cases, bottom plates (known as ‘chills’) are used to boost the cooling process.

Moreover, some specialized molds are made using exothermic sands which offer different cooling rates..

Fettling

After that, the finished magnets are taken out of the shell molds and ground on grinding wheels to remove the runner gates.

It lends a finishing touch to the shapes of the magnets.

Heating

The heating process helps in hardening the magnets to further retain their shape in the long run.

Moreover, it helps the magnetic compounds acquire temperature-handling abilities and gain substantial magnetic properties.

So how does it work?

The resulting magnets are treated in industrial ovens at such high temperatures that they start glowing red hot.

They are then placed in a magnetizer to cool down in the presence of a very strong magnetic field to fully magnetize.

Finally, to temper and stabilize the magnets they are placed in tempering ovens for a few days.

Grinding

Due to their high hardness, magnets typically need to be finished by grinding to tight tolerances. This ensures that they meet the required specifications for their intended use.

Testing

Magnets go through quality checks and a few tests which are typically done using Gauss meters, flux meters, or hysteresis graph machines.

After successful tests and desired results, they are forwarded for the final coating and magnetization processes.

Coating / Painting

The required coating is actually done right at the end of a finished cast alnico magnet where the poles are fettled to leave a shiny and bright appearance.

Magnetizing

Similar to the step used in the making of sintered alnico magnets, the magnets are placed in a coil or a solenoid magnetizer.

Finally, necessary fields are generated by the electric pulse which results in a fully magnetized cast alnico magnet within a fraction of time.

Also Read: Subwoofer parts explained (Anatomy of subwoofer speaker)

What is the difference between cast and sintered alnico magnets?

There is a very marginal difference in cast and inserted alnico magnets.

Process of production

Cast alnico magnets are made by first melting the raw materials or necessary components at extremely high temperatures, and then placing them in dye for shape.

In contrast, sintered alnico magnets are created by first applying great pressure on the raw materials.

It aids in their transformation into powder form, after which the tiny particles are placed into various-shaped dyes and heated.

Note: Cast alnico magnets are the only magnets made in foundries

Magnetic properties

Sintered Alnico magnets have marginally lower magnetic properties, however, they hold better mechanical properties compared to cast Alnico magnets.

Moreover, sinter alnico magnets can offer high and consistent flux distribution and better mechanical strength due to their fine grain structure.

Applications

Sintered alnico magnets can offer better magnetic properties in slightly smaller sizes compared to cast alnico magnets.

Hence, sintered alnico magnets are suitable for applications that require small or short magnets i.e., applications such as micro-motors, communications, instrumentation, magneto-electricity switches, sensors (that require short magnets), etc.

Cast alnico magnets have better mechanical properties and are used for applications such as large sensors, meters, subwoofers, and speakers.

Are alnico magnets corrosion resistant?

Alnico magnets are quite popular due to their anti-corrosive properties and strong resistance especially when they come in contact with motor oil, organic solvents, petrol, etc. alcohol.

To prevent further, there are special coatings offered over alnico magnets such as Ni, Zn, paint, etc.

Hence, you will mostly find alnico magnets coated or painted with red color and sometimes black. Because of this alnico magnets are also popularly known as red magnets.

However, the corrosion resistance also depends on the grade of the magnet and the chemicals they are exposed to.

Lastly, over time, alnico magnets may show signs of corrosion when exposed to water (especially, salt water), strong alkali solutions, and inorganic acids.

Choosing Alnico Magnets for Subwoofers

When it comes to optimizing the performance of your subwoofer, selecting the right alnico magnets is a critical step.

Several factors come into play that can influence the overall sound quality, power output, and durability of your subwoofer setup:

Magnet Size

The size of the magnet directly impacts the power output of your subwoofer.

A larger magnet typically produces more bass, contributing to a richer audio experience.

However, it’s essential to find the right balance, as larger magnets can also increase the weight and cost of your setup.

Power Rating

The power rating of the magnet dictates how much energy it can handle.

A subwoofer equipped with a higher power-rated alnico magnet can produce louder bass without the risk of damage.

Matching the power rating of the magnet to the intended use and output of the subwoofer is crucial for optimal performance.

Frequency Response

The frequency response of the subwoofer is a key consideration.

A wider frequency response means that the subwoofer can accurately reproduce a broader range of frequencies, providing a more versatile and immersive sound experience.

When selecting alnico magnets, consider their role in enhancing the frequency response of your subwoofer.

Magnet Placement

The placement of the alnico magnet within the subwoofer assembly can influence its performance.

Proper alignment and integration with other components, such as the voice coil and diaphragm, play a crucial role in ensuring optimal sound quality and efficiency.

Budget and Preferences

Ultimately, your choice of alnico magnets should align with your budget and preferences.

While alnico magnets offer unparalleled benefits, it’s important to weigh their advantages against their associated costs.

Consider your priorities, whether it’s achieving exceptional sound quality, maximizing efficiency, or finding a balance between the two.

Also Read: Subwoofer Low Pass Filter (LPF ) Explained

Final Thoughts

Due to their remarkable qualities and capacities, alnico magnets have established themselves as an essential component in subwoofer manufacturing.

While they may come with certain disadvantages, their unique blend of power, performance, and durability makes them a compelling choice for those seeking an immersive bass experience.

Alnico magnets were once the go-to magnets for a variety of applications, including subwoofers and speakers.

But these days neodymium and the latest magnets rule the market.

Armed with a Diploma in electrical engineering and a remarkable 12 years of expertise, he excels in rejuvenating music systems, washing machines, dryers, and laundry-related appliances. Manish's profound insights and practical know-how establish him as a credible authority in appliance and music system repairs. Contact: manish.singh (at) subwooferblog.com. He's always ready to lend a helping hand!