"...Powder metal (PM) rods are fine for stock applications. Most people think that the powder process (and potential porosity) is the problem, but usually it isn't.
Dmax rods are pressed into a "green" compact and then hot forged. This densifies the compact and most of the time, it's 100% dense. If you cut it up, you may find a pore, but you'll spend a lot of time looking. And depending on where it is, it may not affect the part strength at all.
However, where the problem really lies is the requirements of fracture splitting.
For the big picture view, the vast majority of current auto con rods are fracture split. This saves a ton of machining and can reduce the cost of the rod by 30% or more (and is made much more quickly). The powder metal associations work very hard to push the technology further, and their industy consortiums will push until all rods are powder metal. It makes total sense for stock applications. Fracture splitting saves cost and makes a great joint. So what's not to like?
What's not to like is the alloy.
There was a big fight some time back between the powder rod people and the steel rod people. Steel companies want to sell forged metal rods, but they couldn't be competive against PM (because of later having to machine the cap surfaces). So they went and worked out an alloy that would fracture split like a PM rod. It came very close, but to make a long story short, it failed because of cost. Ingot metallurgy went to war with PM and got its butt handed to it because of cost.
So back to the alloy. It takes a specific composition of iron alloy to work well in a PM application for fracture splitting. There are a number of PM alloys out there, but the key point is this: the alloys being used for rods come nowehere near the properties of an high-strength ingot-metallurgy alloy like 4340 or its variants (which could never be fracture split because they're too strong). The main reason is, they don't have to. They're not going into F1 cars, for Pete's sake, so all they need to be is decently strong and be 100% repeatable when compacted, forged, and split.....with little yield loss.
The fact is, if the rods are bending in compression, we're simply exceeding the yield stress of the PM alloy. You can redesign the rod and put a bit more metal in the right place and help (LBZ rods), but again, you're nowhere near the yield stress levels that can be achieved in expensive ingot-metallurgy alloys.
Now, could you make a 4340 PM (or other high-alloy) rod? Maybe. But it probably wouldn't fracture split, so there would be no point in doing it vs. a ingot-metallurgy piece. So if you want 4340-level of strength, you have to suck it up and pay for all the yield losses from ingot to slab to plate, take all the machining time hit, and voila, you have a beautiful rod.
Make sense?..."
"... To visualize why they bend, just get a skinny yardstick, stand it up on end, and apply a load.
The first thing is does is buckle. It's because its length is many times longer than the dimensions of its cross section. Get this, it's called a "slender column" in complicated engineer geekspeak.
Our conrods are doing the same thing when under load. They're trying to buckle to relieve the stress applied to them. For light loads, if they buckle a hair and stay under the yield stress, when unloaded, it goes back to the original shape. When you hit the cylinder pressure that causes the metal in the rod to yield, then you've got permanently bent rods.
As with the yardstick, the fibers on the convex side are in tension, and the fibers on the concave side are in compression. That's why the convex side splinters when you push too far. The conrod is similar, and although the metal doesn't have "fibers" like wood, you can visualize the stresses that way in your head."
