Expander system for MIG welding of bottoms on tanks of diameters between 1200 mm and 2000 mm, thickness from 2 mm to 5 mm and maximum length of 5mt. The line is composed of a trailer-mounted structure, a fixed expander and a moving beam with an expander at its extremity in order to work different sizes of tanks. The machine is equipped with two expanders complete with a hydraulic system locking the bottom at the tank.