hollow-forge

[ hol-oh-fawrj, -fohrj ]
/ ˈhɒl oʊˈfɔrdʒ, -ˈfoʊrdʒ /

verb (used with object), hol·low-forged, hol·low-forg·ing.

to produce (a tube or vessel) by trepanning a hole in a forging and expanding it with further forging on a mandrel.