An immiscible metal combination of stainless steel and copper was butt-welded using friction stir welding (FSW). A W-Re tool was used for FSW; tool dimensions included flat and featureless tool shoulder diameter of 16.0 mm, conical tool pin diameters are 5.0 mm at the tip and 7.0 mm at the bottom, and pin length of 4.0 mm. Metallographic samples for base as well as welded cross sections were prepared by polishing down to a surface finish till 0.02 lm diamond suspension. Base material was observed by optical microscopy (OM). Two-dimensional (2D) digital image correlation (DIC) was applied for localized deformation measurements during tensile testing. Microhardness was conducted on the cross section of welded joints with load of 200 g and a dwell time of 10 seconds. Processed copper region was weakened by thermal softening and strengthened by finer recrystallized copper grains. The combined effect of the thermal softening and grain refinement-related strengthening was dependent on the welding parameters. Lower rotation rate and higher traverse speed led to lower heat input and resultant higher hardness in stir zone except from the region steel fragments existed. In addition, welded samples fractured in the processed copper region adjacent to steel/copper interface due to the stress concentration during tensile testing.