The application of tailor welded blanks offers significant potential for reducing the weight of future automobiles and improvements are being made in the development and understanding of the welding process. There are several geometric and internal weld features that make the complete numerical description of tailor welded blank forming challenging. The variation of the experimental formability results found in the literature for tailor welded blanks appears to be large, and a combined theoretical tensile instability and statistical analysis of internal weld porosity may explain at least some of the variation reported.