A welding device for weld-assembly of first and second parts, defining therebetween a sealing surface, along a weld joint. The device includes: a supporting mechanism configured to move the device along the weld joint; at least one welding mechanism; and an automatic command and control mechanism. The welding mechanism is mounted on a single supporting element oriented substantially parallel to the sealing surface. A first motorized actuator secured to the supporting mechanism includes a first projection cooperating with an open housing in a first end face of the supporting element, and a second motorized actuator secured to the supporting mechanism includes a second projection cooperating with an open housing in a second end face of the supporting element. The first and second projections are movable in a plane substantially perpendicular to the sealing surface and generate controlled movement of the supporting element of the welding mechanism.