A process for friction stir welding at least a first part made of metal alloy A with at least a second part made of a dissimilar metal alloy B, in which the first and second parts are placed side by side, and a joint is made between the first and second parts using a rotating tool moving at a speed referred to as the travel speed. The tool travel speed has at least two alternated modes in continuous service, a first mode wherein a first average travel speed S1 and a second mode wherein a second average travel speed S2 is used, the speeds S1 and S2 being significantly different, typically at least 30% of the highest travel speeds, with the slowest advance speed being zero. The assemblies welded according to the invention are particularly advantageous for the fabrication of panels or structural sections used in the manufacture of transport vehicles.