Our recent series of articles has focused heavily on the highs and lows of materials used in powder metallurgy (PM) based on parameters such as durability and weight. Porosity is a little twist, however, in the PM formula.
Porosity is defined as empty space in a material. Low porosity prevents/minimizes corrosive substances from entering your components and is often associated with improved mechanical and magnetic properties.
We can give you many porous metals examples. But is porosity in powder metallurgy really about choosing the right material … or is it more about your design, processing technique, and application?
Let that sink in as we take a deeper dive.
Porosity in Powder Metallurgy: Friend And Foe?
The correct amount of porosity for your part really depends on how your part will be used.
Porosity is very application-based, rather than a property for which a single material fits all needs. When you need parts with high strength, porosity can be your enemy, but there are many applications where powder metallurgy can give you precise control over porosity.
When Would You Want High Porosity?
In design and engineering, we tend to talk about porosity as a bad thing, but the fact that we can control porosity with powder metal is actually a plus.
With high porosity, you create something known as a porous filter, which can control the size of particles that’ll be removed from the fluid you are filtering. Particulate matter that should not be in the component or the media being filtered becomes trapped.
In general, a density of 4-5 g per cc is considered high porosity. As you start getting into 5 to 6.5 grams, porosity becomes a balancing act: How much porosity and strength does your part require? A part with more tiny holes is structurally weaker than one with few pores.
If your application is structural, you may learn If you want high performance, it’s all about density, although ultra-high temp sintering may disprove some of that (more on that in a future article!).
So, Are There Any Ideal Materials for Porosity?
There aren’t really any go-to materials from a porosity standpoint, but there are certain applications where porosity needs might determine what material is selected. Here are a few examples:
Porous Stainless Steel
Stainless steel is a frequent choice when the material being “filtered” can be corrosive to other, less durable metals.
It may seem counterintuitive, but iron is just fine in certain applications. One example is self-lubricating bearings. The pores will allow fluid lubricants to flow inside the part -- no external lubricant necessary!
This material is pretty inert to most debris, which makes it good for certain applications. Copper is easy to work with and can be treated in a way that improves its usability further.
Any Materials to Avoid?
Don’t use iron powder for a component that will filter water. As a general rule, porous iron should not be used in corrosive environments because it will inevitably rust.
Also be aware that the iron-copper-carbon mix which most powder metal parts are made from can have a high degree of porosity that can leave an unwanted hole in the part. Consider that these materials are mostly used in structural applications, making porosity a big no-no.
Although the volume porosity of porous aluminum can be controlled to about 55%, you don't want a very porous aluminum part due to flammability concerns. It has high pyrophoricity -- a fun word that means the ability of a powder to fundamentally burn, and how much energy it gives off upon doing so. Any sparks that come in contact with such a composition are an accident in the making.
NASA used to launch space shuttles using aluminum powder in the solid boost reactors. Once they are lit, they burn themselves to extinction. Unfortunately, this is what happened in the Challenger space shuttle explosion in 1986 -- the scientists couldn’t stop the burndown, the spacecraft ignited.
In short, aluminum can be very porous, but unfortunately it can soak up bad stuff that will react to its chemical composition.
Time to Turn Your Attention Elsewhere
The very nature of PM is that porosity is present. The challenge for the parts maker is designing the process to minimize the variation of porosity with the part itself. Secondly, he must minimize variation of porosity from part to part.
Many projects require smart management of density in your application. With powder metal, we have an ability to control porosity to the point where material choice usually doesn’t matter. Porosity is really exclusive from material choice, because what can be done with one metal or grade of metal, can be done with all of them.
It’s more effective to think about porosity in terms of the application. And with powder metal, you can also turn your attention to other properties you need, such as magnetic performance.