Friction stir welding parameters were developed for a 6 mm thick dissimilar butt joint made of mild steel and Ni based alloy 625. A composite W-Re/polycrystalline cubic born nitride tool was used to study the effect of tool offset (between 0 and 3.13 mm away from the alloy 625 plate) and rotational speed (between 250 and 350 rev min-1) on joint consolidation. When non-optimal parameters were used, macrovoids were observed at the advancing side and faying surface near the joint interface root. Defect free joints were obtained using 300 rev min-1, 100 mm min-1, tool offset of 1.63 mm, and axial force between 25 and 30 kN, under force controlled mode. © 2014 Institute of Materials, Minerals and Mining.194343349Mishra, R.S., Ma, Z.Y., Friction stir welding and processing (2005) Mater. Sci. Eng. R, R50, pp. 1-78Nandan, R., Debroy, T., Bhadeshia, H.K.D.H., Recent advances in friction-stir welding - process, weldment structure and properties (2008) Prog. Mater. 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