Friction stir welding (FSW) is a solid-state joining process that gives welds with excellent mechanical properties. The drive towards electrification has seen FSW adopted by the automotive sector for the fabrication of lightweight aluminium car bodies and battery assemblies. Element Six utilised their expertise in high performance, abrasion and temperature resistant materials to develop a FSW tool for steel and this was trialled at TWI where welding techniques were developed to allow welds to be made both in air and under water. A rigorous, independent assessment of weld quality was undertaken by the Technical University of Delft (TUD) and publications and dissemination resulting from this work has identified a number of potential other applications across the wider transport sector.