In this research, the deformation behavior during biaxial loading of the joints between 2024 and 7075 aluminum sheets made by friction stir welding (FSW) was investigated. A FSW tool with simple cylindrical pin and concave shoulder was used to join similar and dissimilar pairs of these aluminum sheets. Cruciform specimens of these joints were prepared by electrical discharge machining and milling machine in order to test under biaxial loading. Digital image correlation (DIC) technique was used to measure the strain during the biaxial loading. The biaxial testing was performed in the welding direction and perpendicular to the welding. The results showed that a strain concentration exists in the heat affected zone (HAZ) of the joints due to the softening effect during FSW. In the similar joints with 2 passes of FSW, the strain distributions in both directions are more similar due to the softening effect in the stir zone. Softening in the stir zone of AA2024-AA2024 with double-pass weld was higher than that one of AA7075-AA7075. The results of the present study help to decide on design of the structures joined by FSW.
