From Old English 'wellan' (to boil)—welding is heating metal until it 'boils' enough to fuse.
To join metal pieces by heating them to the point of melting and pressing or hammering them together.
An alteration of 'well' (to boil, to spring up), from Old English 'wellan' (to boil, to bubble), from Proto-Germanic *walljan (to boil). Originally meant to heat metal until it 'boiled' or became fluid enough to fuse. The -d was added by analogy with past tenses