An automatic welder is used in an internal welder unit which travels along the interior of a pipeline and welds the abutting ends of the pipe segments together. The automatic welder has a carriage assembly that contains a laser source for producing a laser beam, first and second line-up clamp assemblies for aligning the respective abutting ends of the pipe segments together, at least one laser beam conductor, or alternatively a reflecting surface positioned between said first and second line-up clamp assemblies along the axis of said beam, for directing said beam to the abutting ends of said first and second pipe segments, and a drive for rotating the reflecting surface around the axis of said beam to direct the beam to the abutting ends of said pipe segments for welding said pipe segments together. The automatic welder can also be configured as an external welder in which the automatic welder carrying a laser source is carried on a track mounted on the outside of the pipeline. As the external welder orbits around the pipeline, the laser source generates a laser beam which is directed at the junction of the abutting ends of the pipe segments for welding them together.