Liquation cracking in aluminium alloy welds

Abstract

Failure in 7000 series aluminium alloys due to liquation cracking has been investigated using two approaches, namely microstructural characterisation and residual stress analysis. The microstructural features in the region of the weld-parent metal interface are described in terms of precipitation of particles, size and shapes of grain structure and the nucleation of cracks and failure of the weld. Cracking takes place adjacent to the fusion line in a fine equiaxed region of the parent plate commonly referred to in the 7000 series as the ‘white zone’ (WZ). Moreover, cracking can also take place remotely from the weld toe adjacent or perpendicular to the fusion line, known as liquation cracking. The microstructural results have been correlated with measurements of residual stresses over the region of liquation in the weld. The residual stresses have been measured by two techniques; X-ray diffraction and a modified Sachs technique. The former technique indicated that stresses varied both across and along the weld-metal interface and were generally compressive near the root of the weld and tensile at the weld toe. For the latter technique, a three-dimensional stress distribution is obtained along the white zone of the weld. Based on these results, a model is developed incorporating both microstructural and residual stress aspects of liquation cracking.