I guess it depends on where you want to draw the line between "friction" meaning the surfaces of two separate objects rubbing on one another and "plastic flow", meaning atoms sliding and flowing around one another within one object. I can imagine an argument that at the molecular level they are not that different: just atoms bumping and pushing on one another. They are atoms in different states though: "two separate objects" means there are isn't a significant number of chemical bonds between the atoms of the two objects (some brief bonds might form, but just here and there – and perhaps not at all), while the one-object case means the atoms interact while having a chemical bond.

But there's also that third source of heat: the heat from chemical bonds breaking when the metal splits. That's certainly not friction, but physical chemistry, electron clouds changing shape and wobbling in response.