The company I work at explosively welded the material for the end caps on the MMRGT. It was sourced through a contractor so we did not know the application until a reporter called to verify that our parts were on mars!
What is explosive welding?
It's exactly what it sounds like... using explosives to bond two metals together. I don't know much about it other than in passing though, so I can't speculate what the parent's application was. I think it's most commonly done to clad a thin layer of one metal onto a thicker substrate, so probably something related to that.
This is a great book, by the way -- surprisingly technical and thorough. I preordered it thinking I was getting a coffee-table book full of glossy photos and not much else, and was (pleasantly) surprised to be wrong.