Fatigue assessment of friction stir welded DH36 steel

A fatigue performance assessment of 6mm thick friction stir welded DH36 steel has been undertaken, filling a significant knowledge gap in the process for steel. A comprehensive set of experimental procedures has been proposed; the consequent study extensively examined the weld microstructure, hardness, geometry and misalignments of the samples in support of the tensile and fatigue testing. The effect of varying weld parameters was also investigated. The typical fatigue performance of friction stir welded DH36 steel plates has been established, exhibiting considerably extended fatigue lives, well above 105 cycles at a stress range of 90% of yield strength, irrespective of minor instances of small surface breaking flaws which have been identified. An understanding of the way in which these flaws impact on the fatigue performance has been developed, concluding that surface breaking defects emanating from the friction stir tool’s shoulder marks on the weld top surface can act as the dominant factor for crack initiation under fatigue loading