Hot deformation behavior and dynamic softening mechanisms for the friction stir processing of lightweight medium Mn steel

In this study, the hot deformation behavior of lightweight medium manganese steel during friction stir processing (strain rate range is 0.01-10 s-1, deformation temperature range is 600-900 degrees C) was analyzed by combining experiments and simulations. The microstructure evolution under varying strain rates and deformation temperatures was analyzed. The softening mechanisms and the austenite reverse transformation behavior were clarified. Results show that the maximum flow stress of lightweight medium manganese steel during hot deformation decreases with increasing deformation temperature and decreasing strain rate, describable by the strain-compensated Arrhenius constitutive equation. The main softening mechanisms during deformation include dynamic recovery, continuous dynamic recrystallization, and discontinuous dynamic recrystallization. As deformation temperature rises, the dominant softening mechanism shifts from discontinuous dynamic recrystallization to a combination of continuous and discontinuous dynamic recrystallization. As strain rate increases, the primary mechanism transitions from discontinuous dynamic recrystallization to dynamic recovery. Optimal conditions for achieving a high austenite volume fraction occur at low temperatures and low strain rates, where discontinuous dynamic recrystallization significantly refines and stabilizes austenite grains. The optimal friction stir processing window combining austenite volume fraction and processing maps is in the range of strain rate of 0.01-0.07 s-1 and deformation temperature of 643-770 degrees C. This work enhances understanding of the relationship between processing parameters and microstructure evolution in medium manganese steel during friction stir processing, offering guidance for selecting process parameters.