Joining is critical in shipbuilding impacting significantly on several aspects, i.e., properties, lightness, aesthetics, assembly/disassembly, maintenance employed workforce, emissions of fumes or gases. Consequently, it requires a significant study on impacts and risks. The aim of this work is to apply the Life Cycle Assessment to a friction stir welding process between aluminium and steel. The results confirmed that this welding is among the most sustainable (i.e., low energy, absence of filler, inert gases, and consumables). Moreover, was introduced a functional unit (i.e., length of weld divided by the thickness squared) that allows to compare different geometries and process parameters.