: Choose target artifact a player controls and target artifact card in that player's graveyard. If both targets are still legal as this ability resolves, that player simultaneously sacrifices the artifact and returns the artifact card to the battlefield.Goblin Welder can exchange an expendable artifact for something much more valuable in the graveyard. Its tap ability targets an artifact a player controls and an artifact card in that same player's graveyard. When both targets remain legal, the battlefield artifact is sacrificed and the graveyard artifact returns simultaneously.
Artifact decks can use a Treasure or a small utility artifact to bring back Wurmcoil Engine. Ichor Wellspring is another useful piece because sacrificing it and returning it can each contribute cards at different points in the engine. Discard and self-mill effects help place desirable artifacts in the graveyard before the Welder becomes active.
The ability can also target an opponent's artifacts, provided both belong to the same player for the exchange. Replacing a threatening artifact with a less useful one can disrupt a board, though you must evaluate what their graveyard actually offers. This is not a general ability to steal artifacts or exchange pieces between different players.
Both targets need to stay legal. If an opponent removes the graveyard card or the battlefield artifact before resolution, the exchange does not happen. Because the ability uses a tap cost, summoning sickness also delays an ordinary newly cast Welder. Haste and protection help it begin working, while untap effects can multiply its use. The strongest Welder decks give it several worthwhile exchanges so one removed target does not shut down the entire plan.
