The role of external and internal weld defects during the onset of localization in an aluminum alloy tailor-welded blank (TWB) is presented. Measures of weld surface roughness and weld porosity are acquired and incorporated into a representative finite element method (FEM) model to determine their relative influence on TWB ductility. A bifurcation analysis is employed to predict the onset of shear localization and failure, which is found to be sensitive to even small levels of porosity. A probabilistic analysis of the weld defects indicates that, given the high frequency of macro-pores, weld porosity has the most detrimental influence on formability for the TWB system under study.