Stacking-fault energy, mechanical twinning and strain hardening of Fe-18Mn-0.6C-(0, 1.5)Al twinning-induced plasticity steels during friction stir welding

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The effect of friction stir welding (FSW) on the microstructure, stacking-fault energy (SFE) and strain hardening rate (SHR) of Fe-18Mn-0.6C-(0 and 1.5)Al (wt.%) twinning-induced plasticity steels using three welding speeds (50, 100 and 200 mm min−1) was investigated. The yield strength of the FSWed 0Al and 1.5Al steels improved due to both grain boundary strengthening by grain refinement and dislocation hardening by the introduction of dislocations with an increase in the welding speed. Their SHR with three stages and without the yield drop increased due to the active mechanical twinning and the introduction of dislocations during the FSW when the welding speed was increased. Among the 0Al steels, 0Al-200 steel with a fine grain exhibited more active twinning than the coarse-grained specimen (0Al-50), which is contrast to the 1.5Al steel. Regardless of the specimens, the slight increase in the SFE, which was attributed to both the shear strain energy caused by the introduction of dislocations and the excess free energy by the grain refinement during the FSW, leads to an increase in the critical twinning stress (σtw). Despite the fine grain of the 0Al steel, the origin of its active twinning was the highly increased yield strength relative to σtw, and the promoted dislocation interactions, giving rise to an increase in the number of sites at which twin nucleation occurred.