Several assembly techniques that compete with machining are used to make components with metal powders. These are on the rise, partly to fill the automotive industry’s growing need for lightweight, low-cost, high-volume parts.
Other industries like industrial tooling and biomedical devices are taking advantage of these highly automated processes that offer low part-to-part variability, less material waste, and fewer steps. New processes and more materials choices are expanding these techniques, making them increasingly attractive for some of the structural components used in commercial aerospace.
The automotive industry is the largest consumer of powder metal (PM) parts, followed by industrial motors and controls. An automotive transmission might contain as many as 55 parts made with PM, according to Jim Dale, vice president of the Metal Powder Industries Federation.
“We’re seeing the amount of PM used in vehicles increasing each year,” Chris Franks, vice president of sales and marketing for the Americas at GKN Sinter Metals, told us. “We’re modifying materials to develop processes more tailored to a specific application.”
This automotive turbocharger impeller is made with BASF’s Catamold catalytic debind process from the company’s GHS-4 alloy, which contains iron, nickel, chromium, molybdenum, carbon, silicon, manganese, vanadium, and tungsten.
Manufacturing processes for creating near-net-shape structural components with PM form parts with either heat or pressure. Press-and-sinter PM shapes metal powders with custom dies under high pressure, then heats parts by sintering. Another process, hot isostatic pressing (HIP), is used for larger parts. “There’s a limit on PM part size of around 42 pounds,” Dale said. “Most PM parts weigh less than five pounds. Right now, individual parts are getting bigger and heavier as both presses and applications get bigger.”
Powder injection molding (PIM) combines the ability of conventional injection molding machines to make complex geometries with the precision and materials flexibility of PM. PIM can produce medium to high volumes of consistent components with complex shapes, multiple textured surfaces, and intricate details. Components can combine several parts, eliminating process steps and reducing cycle times.
The biggest users of PIM are the medical, consumer electronics, mechanical devices, aerospace, automotive, and general consumer goods industries, said Scott Justus, BASF’s business manager for Catamold products, North America. BASF is getting more inquiries about PIM across all industries, and growth rates are rising, he said. “More companies are looking to do lean manufacturing and continuous improvement, so they’re taking a harder look at PIM since it provides good overall value.”
Interesting story, Ann. Are the powder metals a niche market in automotive and aerospace, or are they becoming a mass market for auto and aerospace parts?
Rob, powder metal manufacturing techniques are growing as a percentage of metal parts manufacturing in automotive, where they're already responsible for a large proportion of those parts, as well as industrial controls. Aerospace is also getting interested, but volumes are still quite small. Other major industries are medical and consumer electronics.
Sounds like this is much more than a niche product in automotive. Once again, the auto industry is leading in new materials and technology. It's quite a different industry than it was when I was growing up in the Detroit area in the 60s and 70s.
It made have started out as a niche set of manufacturing techniques, but I don't think it can be called that anymore, especially in automotive and industrial parts.
Obviously, the powder metal industry would like to compare the cost of PM processes to the cost of machining parts out of mill products. This comparison makes PM look very attractive for all but extremely small-volume production. However, as Ann points out, PM's real competition comes from investment casting and forging. It would be nice to see some cost comparisons here.
Another important factor to consider is that the mechanical properties of PM products usually aren't as good as forged or cast products. As Jim Dale points out, a fully-dense PM part will have mechanical properties comparable to a casting -- but achieving full density in a PM part is no easy task. You won't get it in a traditional pressed and sintered part.
That being said, PM is a good option for certain applications. The article does a good job of pointing out its advantages.
Dave, thanks for the input about PM vs other metal component fabrication techniques. We know you're a fan of metals and especially of machining and welding, so it was interesting to see your input on investment casting and forging. I agree, cost comparisons for a given example product would have been revealing but, as usual, they're very hard to come by for publication.
@Ann: I got my start as a process engineer in an investment casting foundry, so I have a certain bias in favor of casting and against PM. I suspect that most people tend to be biased towards materials and processes they are familiar with. I'm aware that it's a bias, and try to keep an open mind.
Unfortunately, this bias has been confirmed to some extent by bad experiences with PM parts. These bad experiences were mostly due to designs which didn't take the nature of the PM material or the limitations of the PM process into account.
Of course, you could say the same about casting, or any other process. Designers ignore the limitations of manufacturing processes at their own risk.
Dave, I know what you mean about low-quality PM parts. I've been on the receiving end of low-quality cast parts (and probably also low-quality PM; I find those harder to identify visually or tactually). My operating principle as a consumer is either it's the design or the materials or the combination that makes a bad part. You can also accuse QC, but QC may only be able to notice whether the duck walks and quacks like it's supposed to, not whether it breaks because it's actually a badly designed goose. That said, I was impressed at what PM can do when it's done right.
@Ann: You bring up a good point -- the relationship between design and quality.
To me, "low-quality PM parts" are parts that are poorly compacted, poorly sintered, cracked prior to sintering, or made using contaminated powder. The good news is that these are all problems that can (potentially) be fixed. Process the material correctly, and the part will work.
On the other hand, if a part is not properly designed, it won't work, no matter how well it is made. For example, using a PM part in an application which involves significant impact loads is almost always a bad idea.
Sometimes the presence of a quality defect may lead you to believe that you're dealing with the first situation, when you're actually dealing with the second.
Dave, thanks for that observation. I think you're right--we tend to blame materials quality first, design quality second, when in fact the opposite may be true, or even both may be faulty.
I always love to see better methods of making parts! Suzuki was making powder metal transmission gears in the 80's. The methods are well known, so it seems that we are seeing better materials being used? It looks like we are getting much better in materials formulating than ever before, bravo!
For our moving mechanism designs, I really appreciate the porosity of powder metal which allows us to impregnate oils in the material matrix. This gives us a great low-cost, durable bearing with relatively good tolerances.
Greg, thanks for that input from the field. Do you have any comments about the differences between PM and cast metal along the lines of what Dave said below?
Ann, I think Dave was spot on when he stated "if a part is not properly designed, it won't work, no matter how well it is made" For many of our medical and electro-mechanical parts (that do not have significant impact loads) we have great success when using an oil-impregnated sintered bronze as a low-cost bearing. Tooling and piece part costs are low and tolerances are very good (assuming a good supplier with consistent process control). However, not every design is suited for powder metal and we use a combination of design experience and historical application to guide us when to use the powder metal process.
Yes, for relatively moderate structural loads that are well within the strength limits of the PM material. For example, PM oil-impregnated bronze bearings work well supporting the sliding portion of a lamp mechanism on an electroless nickel plated steel rod. Designed properly, PM can successfully be applied to a wide variety of moving part designs.
Thanks, Greg. So it sounds like you've found that, for your needs, PM is good for certain moderate-load, both structural and impact, designs. What I still find interesting is the fact that there are so many automotive parts made with PM with high tensile and yield strength, and that PM use is also increasing in aerospace.
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