Microstructure evolution and strengthening mechanisms in friction-stir welded TWIP steel

Corrosion Behavior and Microstructure of Stir Zone in Fe-30Mn-3Al-3Si Twinning-Induced Plasticity Steel after Friction Stir Welding

“…This grain boundary serration subsequently transformed into several recrystallization nuclei, resulting from a bulging of parts of the serrated HABs accompanied by the formation of sub-boundaries (=LAB) and/or Σ3 twin boundaries, as represented by the black and red arrows in Figure 1c, respectively. The low mobility of LAB under severe plastic deformation at a high temperature stems from the inactive dislocation mobility caused by the stacking-fault formation in the fcc metals (e.g., Au, Ni, Cu, and austenitic Fe) having low/medium stacking fault energy [9,27,28]. It matches with the present study, given that the stacking-fault energy of C-free TWIP steel is the medium value of 54 mJ m −2 based on the thermodynamic calculation [7].…”

“…The introduced LAB in the stir zone drives the higher average kernel misorientation value (0.75 ± 0.43) relative to that (0.43 ± 0.24) in the base metal (Figure 2c), implying an increase in the geometrically necessary dislocation density by the deformation during FSW. Regarding the Σ3 twin boundaries in the stir zone, it was reported that the twin boundaries during FSW caused the crystallographic rotation of twin boundaries by repetitive strain, gradually transforming the random boundaries (=HAB) [9,27], indicated by the red arrows in Figure 1c. In this study, given the much shorter twin boundaries of the stir zone than the lengthy twin boundaries of the base metal, it was concluded that the newly developed twin boundaries were strained during FSW [9,27].…”

“…When the stacking-fault energy of TWIP steel is between 20 and 50 mJ m −2 , TWIP phenomena occur actively, which is attributed to the optimal critical twinning stress [2][3][4]. For extensive industrial applications of TWIP steels, many researchers have given consideration to the formability [5], weldability [6][7][8][9], and corrosion susceptibility [10,11] as well as the optimization of mechanical properties by tuning alloying elements and heat treatment conditions [1]. For the weldability of Fe-18Mn-0.6C-(0, 1.5)Al (wt.%) TWIP steels, friction stir welding (FSW) can suppress the degradation of mechanical properties in weldment, because of both grain refinement by dynamic recrystallization and active twinning by optimal twinning stress [6].…”

“…The advantages are attributed to a solid-state process at relatively low temperature, unlike the material re-melting occurring during fusion welding at high temperatures [12], leading to a reduction in mechanical properties due to brittle martensitic formation and elemental segregation [13]. The grain refinement of Fe-13Mn-0.5C-1.6Al (wt.%) TWIP steel after FSW results from two types of discontinuous-and continuous-dynamic recrystallization [9]. Additionally, Fe-18Mn-0.6C-(0, 1.5)Al and Fe-30Mn-3Al-3Si (wt.%) TWIP steels after FSW show an increase in the value of stacking-fault energy.…”

Microstructure and Mechanical Properties of Medium Manganese Steel after Different Deformation and Thermal Treatments

“…It was shown in earlier works that medium manganese steels with a high carbon content of 0.6 wt % are subject to friction stir welding. The area of the weld joint is then stronger than the base material, which should improve the reliability of welded parts [8]. Unfortunately, friction stir welding is not widely used in manufacturing automobiles.…”