Microstructure, mechanical properties, and damping capacity in stir zone after friction stir welding of Fe–17Mn damping alloy

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 and rises again at 440 rpm. Both grain coarsening and reduction in the accumulated dislocations at higher speeds result in a decrease in YS. The reason for the improved UTS of FSWed specimens is the increase in the strain hardening rate (SHR) caused by active transformation-induced plasticity from γ phase. With increasing speeds up to 440 rpm, an excellent balance of the UTS and total elongation is obtained from the sustained SHR by the increased mechanical stability, the facilitation of the pyramidal slip and twinning in the ε phase, and reduction of the quasi-cleavage fracture. The damping capacity after FSW decreases owing to the pinning effect of the partial dislocation by the introduced dislocations during FSW.