Precipitates in laser beam welded aluminium alloy AA6056 butt joints studied by small-angle neutron scattering

The volume fraction and the size distribution of strengthening secondary precipitates in laser beam welded sheets of Al-alloy AA6056 were studied non-destructively using small-angle neutron scattering. The volume fraction of precipitates was determined in the base material and the fusion zone for sheets welded in T4 and T6 tempers. Also, the effect of post-weld heat treatments on the precipitate dispersion was analyzed. Additionally, a lateral spatial resolution of 1 mm was used for small-angle neutron scattering measurements as a function of the distance from the weld line in a 6 mm thick sample welded in T6. Moreover, the effect of post-weld heat treatment on the residual stresses in the weld zone was analyzed. The results show that the precipitate distribution in the fusion zone was almost independent of the initial temper before welding. The width of the reduced hardness region adjacent to the fusion zone was found to correspond with the absence of precipitates with a size of a few nanometres that are present in T6 base material. The post-weld heat treatments (T6 or T78) induced nanometre-size precipitates in the fusion zone and the heat affected zone. Thus, the solute, and Mg in particular, was not lost totally in the fusion zone through vaporisation during the welding. Furthermore, the over-aging treatment T78 hardly changed the precipitate dispersion in the base material.