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The microstructure, stacking fault energy (SFE), tensile properties, fractured surface, and the damping capacity of the Fe–17Mn (wt.%) alloy with a dual phase structure of γ austenite and ε martensite were systematically investigated at different tool rotation speeds (i.e., 120, 280, and 440 rpm) during friction stir welding (FSW). The FSWed specimens have a larger fraction of the ε phase than the base material (BM) despite finer grains and higher SFE because of the increased transformation kinetics on subsequent cooling after FSW caused by the introduced dislocations during FSW. With the higher speeds up to 440 rpm, the ε fraction increases gradually, owing to the lower γ stability by grain coarsening and lower SFE. Both the yield and ultimate tensile strengths (YS and UTS) are enhanced in the FSWed specimens relative to the BM regardless of the rotation speed, while total elongation drops at 120 and 280 rpm …