The d 0 -distribution in Al-2024 Friction Stir Welds

Abstract

Neutron and synchrotron radiation have been employed to create 2D maps of the variation of strains and the unstrained lattice spacing d₀ in longitudinal and transverse direction over a cross-section of a friction stir weld (FSW) of an aluminium alloy AA-2024. Strains have been measured directly in the FSW using neutrons. The variation of the unstrained lattice parameter has been measured with synchrotrons on specially prepared stress-relaxed ‘combs’ of the original weld. The variation of d₀ across the weld varies on the same scale as the strains on a magnitude of ∼ l000 µε thus influencing greatly the magnitude of inferred residual stresses in the sample. Both the d₀ variation and strain map show detailed features reflecting the characteristic microstructure generated during the welding process. In particular, they are asymmetric about the weld centre-line. The dominant longitudinal residual stress field is tensile beneath the edge of the tool and compressive in the weld centre-line. Transverse stresses are smaller by a factor of 3 and tend to be compressive. Stresses measured by diffraction agree well with results from conventional holedrilling performed in the same sample.