Welding is a new gameplay element introduced in Metroid Prime 3: Corruption.
While working to destroy the Elysia Seed, Samus Aran vaccinates Aurora Unit 217. Afterwards, she encounters Ghor, who uses his Plasma Beam to damage several circuit boards. This action severs the Aurora Unit's link to SkyTown's network and leaves it unable to concoct a plan to deal with the Seed. 217 asks Samus to kill Ghor for his Plasma Beam. In this game, the Beam has the ability to repair damaged circuitry using a welding attachment, allowing Samus to restore power to doors or activate mechanisms.
The welding must be done quickly. If Samus takes too long to completely weld together a circuit board, the part she has repaired will cool off and she will need to start over. The ability is later stacked with the Nova Beam, which gives it a greenish glow.