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.

Facing multiaxial fatigue testing challenges with respect to non-proportional loading conditions, a custom-built hexapod has been used to establish the mode-{I, III} resistance characteristics of high-quality welds in steel maritime structures. Assessment of the hexapod test data using the effective notch stress and total stress, respectively the best performing multiaxial intact and cracked geometry parameters, shows a fit in the reference quality literature data scatter band and provides conservative lifetime estimates. In order to improve the lifetime estimate accuracy, strength, geometry, material and mechanism aspects are investigated. Welding induced residual stress, a strength aspect, predominantly affects the mode-I fatigue resistance including a mean (residual) stress contribution. The weld notch radius, a geometry parameter, primarily influences the mode-III fatigue resistance. Similar material microstructure compositions of the high-quality welds and reference quality ones are observed, implying comparable mode specific mechanism parameters for the effective notch stress and total stress, respectively the material characteristic length and elastoplasticity coefficient. The material microstructure properties and classification criteria for high-quality welds support the residual stress estimates and suggest a smaller welding induced defect size. In general, the high quality is mainly reflected in the larger resistance curve intercept and slope, another strength and mechanism parameter, implying a larger initiation contribution to the total lifetime. For a high-quality resistance curve involving the representative strength, geometry, material and mechanism contributions, more accurate lifetime estimates are obtained, even though the parameter confidence is reduced because of the relatively small data size in comparison to the reference quality one.