In the present article, the seam lap joining of the Al6061 and galvanized steel sheet is carried out using friction stir welding. A unique metallographic feature named 'hook' is observed near the stir zone boundary in the weld nugget. The influence of hooks and the related stress zones on fracture of the joint is discussed in brief. Three different types of fracture are observed during tensile shear testing of the welded joints. The mechanisms of the various types of fracture are understood comprehensively. Further, a correlation between the sizes of the hooks and the type of fracture is presented. The optimum size of the advancing side hook was found to be about 8–15% of the plate thickness, and the retreating side to advancing side hook ratio should be greater than 1.5 for a strong and ductile joint. Metallographic and mechanical testing such as scanning electron microscopy, energy-dispersive X-ray spectroscopy, stereo micrography, tensile testing, and microhardness testing were performed to validate the analysis.