The variation of the unstrained lattice parameter in an AA7010 friction stir weld

We present the results of an investigation into the spatial variation of the unstrained lattice parameter a0 across a friction stir weld in an AA7010 aluminium alloy. This has been measured using a novel variant of the traditional sin2 ψ method using low scattering angles in transmission rather than backscattering in the conventional reflection geometry that exploits the high penetration of high energy synchrotron X-rays. Relative changes in the unstrained lattice parameter of the order of 10−3 across the weld zone have been mapped along with significant changes in the diffraction peak full-width at half-maximum height using synchrotron X-ray diffraction. It is found that such maps are a very sensitive measure to microstructural changes providing important additional metallurgical insight complementary to direct electron-optical observation. The origin of the variation is considered to be the dissolution, coarsening and/or reprecipitation of the hardening precipitates, which leads to a change in the chemical solute balance. The change in the precipitate distribution due to the welding cycle has been characterised qualitatively by using scanning electron microscopy and Vickers hardness mapping and quantitatively by using small-angle X-ray scattering.