I don't know that friction forging would offer any advantages that the PM process doesn't already give you.
It's virtually the same process, spin a carbide tool at high speed while forcing it into the substrate. Great friction is created turning the metal into a hot pliable state, think alloy taffy. The hardest part to wrap your head around is that it doesn't melt. It's right on the edge of melting but instead of creating a re-solidification cast structure you get a homogenous fine grain.
What did you find out? I'd really like to know.
This was a good article, as usual. But, it does not answer some of my other questions, such as why D2 and not S90V, etc.? How much hardness can be gained and what is the optimum temperature? At what point is the grain the finest and is there such a thing as being too fine?