means To join two pieces of metal (or other materials) by heating them until they fuse into a single solid bond.

from A late offshoot of the old verb 'well,' meaning to boil up or gush — the same sense found in a 'well' of water. By the 16th century 'well' had picked up the idea of metal heated until it bubbles and flows together, and an added 'd' (probably borrowed from its own past tense) gave us 'weld.' So at root, to weld is to bring metal to a rolling boil until it pours into one.