A welder is a workman who is specialized in combining materials such as metal or plastic including the use of pressure and heat.
In 2151 a crewman served as welder in a section of Enterprise, moments after the ship was damaged by an unidentified alien vessel. (ENT: "Silent Enemy")
In 2154 a crewman of the Enterprise served as welder in the team which prepared a shuttlepod for a mission to the surface of Vulcan. (ENT: "Awakening")
In 2373, one of the Maquis refugees on Marva IV was welding. (DS9: "For the Uniform")
In 2375 one of the Bajorans who followed Gul Dukat and the pah-wraiths was welding on the second level of Terok Nor's promenade. (DS9: "Covenant")