As your English teacher probably told you, a few letters or punctuation marks can change the meaning of a word or sentence completely. The classic example is “Let’s eat Grandma” vs. “Let’s eat, Grandma.”
In manufacturing, “powder metal” vs. “powdered metal” is another, far less dramatic example. Both relate to powder metallurgy but could technically mean different things. Here's an explanation.
Powder Metal Vs. Powdered Metal
This statement sums up the difference: Powder metal is defined as finely divided or powdered material compacted into a hardened shape.
In other words, the materials are powdered. The finished forms are powder metal pieces. So when we’re talking about the material, it’s correct to say “powdered metal” and not-so-correct to say “powder metal.”
Now let's answer your next question: What is powdered metal?
The Net-Shape Compaction Process
If you've ever made sand castles on the beach, you've used a powder compaction process. When you press wet sand or powder together, the grains interlock and take on the shape of the mold.
In powder metallurgy, (the process of making metal shapes from powder,) those grains are metallic. After compaction, the “green” compact goes through a furnace, which bonds them together.
Powdered Metal Production
There are several ways of reducing a metal into a powder. The most common are:
- Solid-state reduction
- Communition (powder grinding)
Atomization involves spraying molten metal through a nozzle. Spraying creates droplets that freeze quickly to form powder.
Solid-state reduction entails a chemical reaction between metal particles and carbon in a furnace. It's followed by sieving to yield the target particle size distribution.
Comminution is another process you might come across. This is basically grinding up solid metal into very small pieces.
Effect of Powder Grain Size
Very fine grains pack closely together but don't flow readily. (They tend to clump together.) Larger grains flow better but don't pack as closely.
For powder metal processes using compaction, the preferred size is generally between 30 and 175 microns. (If you're unfamiliar with that metric unit, for comparison a human hair is typically 80 to 100 microns in diameter.)
Similar Words, Different Meaning
So, what is powder metallurgy in a nutshell? It's the process of turning metal that's been powdered, (hence “powdered metal”) into shapes of powder metal.
Only two letters distinguish those terms, but they can make a difference. The “ed” form refers to a verb. Either way, it’s a fascinating process that’s growing in capability and diversity.
To learn more, check out Powder Metal 101: 5 Things to Know About Powder Metallurgy and Sintering. Or, if you want to see the special material that’s revolutionizing electric motors, click the graphic below.