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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.
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.
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.
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?
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.
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.
@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, 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: 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.
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