Synchrotron X-ray Residual Strain Scanning of a Friction Stir Weld

Synchrotron X-ray strain scanning has been used to determine the residual stress distribution over the cross-sectional area of a friction stir weld across an I-section made of extruded aluminium alloy type AA 7108 in the T79 condition. Measurements were made using the 311 reflection and a wavelength of about 0.35 angstrom. The dominant longitudinal residual stress field is tensile within and to just outside the friction band where the field drops steeply. Further out there are regions of balancing compression falling to near zero at the outer edges of the sample. The longitudinal residual stresses vary within the range from -60 to 140 MPa. The transverse residual stresses are generally weaker but show a more marked through-thickness variation below the friction band. The residual stress maps show detailed features which are related to the heat flow from crown to root faces and differences between the advancing and retreating sides. The results demonstrate that synchrotron strain scanning is a practicable technique for determining non-destructively internal and through-surface residual strains in light element components. The synchrotron technique complements and extends the range at present covered by neutron and X-ray diffraction methods and is capable of substantial development.