Abstract: The module is formed from two sheets welded to each other by laser welding. Longitudinal weld beads define between them longitudinal ducts for the flow of a first fluid. The sheets are also joined together by a peripheral weld bead. The weld bead defines, beyond the longitudinal ducts, a transition zone which narrows from the longitudinal ducts up to the gap. Weld spots are previously formed in this zone. Once the welds are completed, the module is formed by the injection of a fluid under pressure through a gap in the weld bead. Utilization for rendering mutually compatible the variations in width of the module during hydroforming in the end regions and for forming, with the transitions zone, a flow distributer to the various channels during operation.