Making components through powder metallurgy is an exercise in trimming the fat. Its ability to create a net shape part means you can skip some -- or all -- secondary operations, including machining.
However, there are times when secondary operations in powder metallurgy can unlock added benefits. They can help seal pores, prevent corrosion, control component size, and a whole lot more.
Secondary operations can be more than just shaving off a bunch of metal. Additive, or enhancing, processes are also available to improve on the inherent advantages of powder metallurgy (PM).
Could your project use some of these hidden elements? We’ll explore both types of secondary operations below.
Important Secondary Operations in Powder Metallurgy
Secondary operations are broken down into two primary categories: common, traditional processes and PM-enhancing techniques. Both categories of operations help engineers keep tight tolerances and provide a better powder metal product.
- Steam treating
Keep reading to learn more about all of these secondary operations!
Traditional Secondary Operations in PM
Traditional secondary operations, while occasionally subtractive in nature, can add great detail to a component. Common secondary processes in powder metallurgy include, but are not limited to:
A part that goes through the sintering furnace can change in size or otherwise distort. The scope of the change depends on the material of your part.
Sizing is a repressing operation that “requalifies” the part so it can hold tighter tolerances. In some cases this process can improve tolerance limits by as much as 50%.
Machining powdered metal parts helps your supplier add features that can’t be molded into the part. It’s also a solution when the sintering process isn’t capable of holding the tolerances a customer requires, either because the metal will change significantly in the furnace or the customer has super-tight tolerance requirements.
For example, it’s hard to compact holes perpendicular to the pressing direction. Machining the part after the fact is a simple work-around.
- Grinding is one machining process that can be used among many others to refine tolerances, while also improving the surface finish.
- Turning helps you meet exact tolerances and form undercuts and features impossible with compaction tools.
- Milling is popular for forming undercuts and slots
Note that machining powder metal parts requires specific, specially developed coolants.
Burrs are small metal protrusions left on a part after forming.
These little imperfections can turn into big issues for two reasons:
- They can be sharp and cut those that handle the part.
- They can come free and float around inside the product. It sounds innocent, but when a loose burr gets caught in the oil of an engine, serious damage could occur
Powder metal manufacturing does not usually produce parts with many burrs, but it can happen in certain situations. Fortunately, there are a variety of ways to remove burrs.
Powder metal parts manufacturers are usually highly motivated to remove burrs to guarantee the quality of their work.
Coining adds little features to your “sintered” component.
The term actually comes from the process of making coins. If you look at a penny, you’ll see that the embossed features of Abraham Lincoln are raised on one side of the coin -- this was done using coining. Pennies are manufactured from big sheets with blanks cut out of the sheet. Then they go through a die that stamps the features upward for the 3D effect.
Coining can be used to add a feature to a part that would not be able to be molded during compaction. In most cases this is because the compaction tool wouldn’t be able to survive the process very long. Anyone who works with metal manufacturing knows it’s not very cost-effective to replace tooling several times for a single project.
Another example of coining is adding an ID number to a part. You could braze it on after sintering using coining.
Remember that many secondary operations aren't as necessary when using PM! To learn more about PM vs. cometing processes, check out this ebook!
Now we return to our regularly scheduled program...
PM-Enhancing Secondary Processes
Now to the fun stuff that doesn’t get talked about enough.
You can also add powder metallurgy secondary processes to the basic structure to enhance performance. There are even some features you can add with powder metallurgy that are impossible with other manufacturing processes. Again, these processes include but are not limited to:
- Shot peening
- Steam treating
Powder metallurgy components are inherently porous. Impregnation, also called infiltration, involves filling most of the pores with:
- Another material
Putting a porous component under pressure will lead to leakage, but if you impregnate the part, it will become pressure-tight.
The material used to impregnate a part with depends on factors including cost and application. For example, copper can swell during sintering, which can mess with dimension stability. Impregnating with oil can make a part self-lubricating. It all comes down to your design needs.
Plating is a great replacement for stainless steel for aesthetic or functional needs -- making the part more visually appealing, improving corrosion resistance, and so on.
Plating gives you those qualities while allowing you to “sandwich” more inexpensive material inside the original part.
3. Shot Peening
Shot peening is a localized densification process that can improve the surface of a part by removing burrs and imparting a surface compressive stress to the part. This can be beneficial in certain fatigue applications. The blasting also creates little pockets that can catch lubricants on the part’s surface.
Fatigue cracks generally begin due to surface imperfections. Shot peening is highly effective in preventing surface crack formation and possibly delay the overall crack growth.
4. Steam Treating
When applied to an iron-based component, steam treating creates a thin, tenacious oxide layer. The oxide layer is not rust; it’s a specific substance that adheres to the iron.
This layer can improve:
- Corrosion resistance
- Pressure tightness
Rethink Your Component
The beauty of powder metallurgy is that in most cases, it makes secondary operations optional rather than a must. But don’t collect your cash savings and start pumping out parts just yet. Consult with a powder metallurgy supplier to see if powder metallurgy’s secondary processes can improve your part’s performance and look:
Editor's Note: This post was originally published in December 2019, but has recently been updated.