Fine equiaxed zone induced softening and failure behaviour of hybrid laser welded 7050 aluminium alloy

A fine (~3-10 μm) equiaxed zone (FQZ) between the heat affected zone and the weld metal is a unique microstructural feature associated with fusion welded Zr-containing aluminum alloys, posing a significant threat to the structural integrity of welded components. This work examines the softening and failure behavior of a 7050-T7451 hybrid laser welded joint. Nanoindentation test results show the FQZ to have the lowest hardness across the welded joint, which is attributed primarily to a significant reduction in strengthening precipitates within the grains. The high-angle grain boundaries in the FQZ are decorated with an interconnected network of coarse brittle ω (Al7Cu2Fe) and S (Al2CuMg) phases. The damage sequence recorded by time-lapse X-ray computed tomography (CT) and in situ scanning electron microscopy (SEM) shows that these phases give rise to intergranular failure through the FQZ by void nucleation and link-up at low overall levels of plasticity, aided by the plastic incompatibility with these phases and the strain localisation.