Residual stresses are an inescapable consequence of manufacturing and fabrication processes, with magnitudes that are often a high proportion of the yield or proof strength. Despite this, their incorporation into life prediction is primarily handled through sweeping assumptions or conservative application of statistics. This can lead to highly conservative fatigue design methodologies or unforeseen failures under dynamic loading. The push from the desire for higher levels of materials performance, coupled with the pull of more sophisticated techniques for residual stress measurement, favours a reassessment of the accuracy of assumptions made about residual stresses and the modification during fatigue cycling. A viewpoint is also emerging that the fatigue performance of welded joints might be optimised through careful process control, coupled with understanding of the relative positions of, and interaction between, residual stress peaks, weld defects, hardness and microstructure. This paper will present information regarding the residual stress profiles in aluminium and steel welds obtained via synchrotron and neutron diffraction at the ESRF/ILL in Grenoble. Specimens were then subjected to specific cases of fatigue loading and the residual stress field was again measured. Difficulties associated with determining the strain-free lattice spacing will be mentioned, and the import of these data for life prediction modelling will be considered.

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A B S T R A C T Friction stir (FS) welding is a relatively new solid-state welding process that offers high levels of joint performance with minimal preparation and little post-weld dressing. The high levels of plastic work induced in the weld zone produce a very fine grain size in the stirred region of the weld (e.g. the nugget), while the low heat input limits residual stresses to a low fraction of the proof strength of the weld metal. These effects are generally beneficial to weld dynamic performance. The peculiar thermomechanical history in the FS weld region leads, however, to particular defects with some unusual effects on crack path, whose occurrence depends partly on crack speed, or growth rate. This paper presents observations regarding specific influences of the FS welding process on crack paths and dynamic performance for 5083-H321 and 5383-H321 aluminium alloys, and proposes an explanation for the observations in terms of the weld microstructures and thermomechanical history. The insights presented in this paper can be used to inform optimisation of the weld process parameters, through on-line feedback and control of tool geometry, force footprint, torque and temperature.