You know about the ability of powder metallurgy to lower component cost. One of the other benefits of powder metallurgy (PM) is that you can tailor the material properties to your needs.
When the goal is to improve your solenoid or similar application to get a specific performance or property increase, many in the PM game -- and even those using a competing process -- can and should turn to sintered soft magnetic materials.
Then you can turn around and streamline your design -- cost-efficiency included. Good deal, right?
What Are Soft Magnetic Materials?
Sintered metal and soft magnetics are complex pieces of manufacturing. The most important things you, the engineer or decision-maker, should know are:
- Properties
- Effects of density and porosity and how advanced processes such as high-temperature sintering effect those properties
- Applications
The definition of soft magnetic materials starts with this: A soft magnet is not permanent. You can easily magnetize and demagnetize a soft magnet.
The magnet you attach to your kid’s homework on the fridge is a hard, or permanent magnet. A soft magnet wouldn't work there because it doesn't consistently maintain magnetism.
So why bother using a soft magnetic powder? We primarily use them to enhance or channel the flux that an electric current produces.
Properties of Sintered Soft Magnetic Materials
One of the key steps in powder metallurgy is sintering. Metal is stubborn, and sintering helps make sure the powder behaves appropriately after your part is formed.
As far as the magnetic side of things go, this BH curve provides some key information -- if you know how to interpret it. In the right hands, it can translate to high-performing soft magnetic materials. This seemingly simple chart teaches a lot about magnetic performance, including:
- Permeability
- Coercive force
- Induction max
Permeability
Magnetic permeability is a measure of how easily you can magnetize a material. A high permeability implies that to achieve a certain level of magnetic induction, you would need less applied field or less current. This can actually represent an energy savings with the right soft magnetic material.
(Note: The permeability is actually the instantaneous slope of the BH curve. The units are oersted / gauss; however, most science folks consider it a unitless number.)
Coercive Force
Coercive force, or coercivity, is a measure of how easy it is to demagnetize the material. In other words, it’s the opposite of permeability.
The lower the coercive force, less current is required to get back to zero induction.
(Note: Coercive force is measured in oersted or A/m [ampere per meter].)
Induction Max
This is a measure of how much induction your component achieves at a given level of applied field. Knowing the maximum is important if you need a specific level of induction or force.
The characteristics listed below are what you could call structure-sensitive properties.
The threshold for induction depends on:
- Material type
- Permeability
- How the manufacturer sinters your part
- Sintering conditions
- Any residual carbon and oxygen
- Any cold work done to the part
Other Considerations
One factor we haven’t mentioned is maximum saturation. This is simply a function of density.
(Note: The units are gauss or tesla.)
How Does High-Temperature Sintering Affect Magnetic Performance?
Conventional sintering temperatures get the job done most of the time, but not all the time.
Iron-phosphorus and iron-silicon, two common powder metal magnetic materials, are actually premixes of pure iron and either ferrophosphorus or ferrosilicon. Your manufacturer can sinter iron-phosphorus at 2050°F and get the benefits of the phosphorus in your part. However, ferro-silicon additions need 2300° to homogenize the silicon in your part.
Raising the sintering temperature above the required level improves the magnetic performance of your part thanks to:
- Better alloy homogenization
- Reduction of oxides
- Greater sinter neck formation
- Grain growth of the material
- Increase in sintered density
Applications of Soft Magnetic Materials
All of this material and process wizardry isn’t just for show. Like powder metallurgy as a whole, the properties of soft magnetic materials allow for several niche applications.
Sintered soft magnetic materials’ uses include:
- Solenoids (likely the best example)
- DC rotors and stators
- Rotors for brushless DC motors
In the case of brushless DC motors, sintered soft magnetic powders lend excellent strength coupled with high magnetic performance.
How Are They Different From SMCs?
If you’ve spent more than 2 minutes on our blog our website, you’ve heard us talk about soft magnetic composites (SMCs). These powder metal materials are soft magnetics, but are not sintered, which means they have different qualities and uses than sintered soft magnetic materials.
With sintering, you have the option of alloying, which isn’t possible with current SMC options. So if you want to add in the benefits of phosphorus or silicon to your mix, you’ll need to go with a sintered part.
There are also times from an application standpoint where SMCs make more sense than sintered materials. If you were to use a sintered powder metal part as a stator for a household motor, all you would have is a very good heater and a very poor electrical device. That’s where you would need an SMC or lamination steel to minimize the heat inefficiency and maximize performance.
Future Opportunities to Advance Your Product
One of the many advantages of powder metallurgy is that you can tailor performance to your needs. Today, PM is the technology of choice not only because of its magnetic qualities, but also its shape-making capabilities.
Whether it’s with sintered or non-sintered materials, you can eliminate scrap waste and create complex shapes without adding extra secondary operations. (Though sometimes secondary operations are a good thing in PM.)
Mechanical strength is no longer the issue it once was with sintered powder metals. Another myth: Some PM parts producers are afraid to compact iron-phosphorus materials to high densities because of tooling issues. We’ve developed the technology to control that.
One potential advantage of high density relates back to the maximum induction and its direct relationship to density. Your lower-density PM part may give the required performance, but there maybe some flux leakage because of the strong magnets used. This often requires adding a second part or adding mass to your existing part with the sole purpose of capturing that excess flux. Going to a higher-density powder metal part may allow you to reduce a two-part assembly to one part.
A lot of powder metallurgy suppliers offer traditional sintered magnetics today, but a select few can even get you to 7.3 density consistently. With ongoing advances in powder metallurgy such as silicon powders and ultra-high-temperature sintering, the future is bright for sintered soft magnetics -- and PM as a whole!
