A methodology is proposed to evaluate the influence of weld-line surface roughness on the failure of AA5754 tailor-welded blanks (TWBs) loaded in plane-strain tension with major strains oriented along the weld-line. Failure is predicted when a band of elements within the finite element model satisfies a bifurcation criterion signalling the onset of shear band localization. Such an approach provides both a quantitative basis for assessing the role of surface defects on weld-line localization behaviour as well as a tool for predicting the relative formability of the weld and parent sheet materials. Significant reductions in failure strain are predicted for the TWB as a result of the decreased work hardening rate of the weld-metal relative to the base metal. On the other hand, changes in failure strain for different sized surface defects are found to be comparatively small.