3-4 mangled metal being returned for throwing axes would make the recipe out of balance with the rest of weapon recipes. Even the largest weapons return no more than two mangled metals on failure. On success, you get 25 throwing axes for those 9 ingots, so the risk/return ratio is pretty high already there. Full suits of armor and large tower shields return 3-5 mangled metals, depending.
This kind of thing was a balancing decision rather than one of "realism". As such, I wouldn't expect the recipe to return more than 2 mangled metals, but someone else should chime in on that, as balancing isn't really my thing.