In this study, the mechanical properties, formability and forming limit diagrams (FLDs) of the tailor-welded blanks (TWBs) produced by friction stir welding (FSW) were analyzed experimentally. At first, the suitable FSW parameters were achieved. The formability and FLDs of TWBs were evaluated for sheets with the same or different thicknesses and compared to the base metal sheet. This study was performed on low carbon steel (St14) sheets with a lot of applications in automobile industries. The welded zone properties were evaluated by some experiments. The tensile test, micro hardness testing, and metallographic studies were done. The effect of welding seam directions on formability and FLD were investigated concerning two aspects. The welding process was carried out in rolling and transverse directions with no effect on formability, and then the welding process was performed in the principal stress direction and perpendicular to it; these directions had a significant effect on formability. Also, it was revealed that welding caused formability decrease, and in TWBs with different thicknesses, this negative effect was higher. In addition, the initial orientation of sheets had none effect on FSW process. © 2017 The Society of Manufacturing Engineers