The Grain Structure and Phase Transformations of TWIP Steel During Friction Stir Processing

The microstructural evolution of severely deformed twinning-induced plasticity steel was studied under various processing conditions. The material was subjected to the friction stir processing in three rotational speeds of 800, 1600, and 2000 rpm and at a constant traveling speed of 50 mm/min. It was found that the initial coarse-grained structure of the alloy (~40 μm) had been significantly refined in the processed zone (~0.8 μm). This substantial grain refinement was attributed to the extensive occurrence of dynamic recrystallization (DRX) in the course of deformation. Moreover, the Ferriteoscope results indicated a considerable drop in the ferrite volume fraction after processing under all of the applied rotational speeds. This was justified considering the reverse ferrite-to-austenite transformation. The present study has also explored the room-temperature mechanical properties of the processed material using microhardness measurement and tensile tests. The hardness profile coincides with the progression of DRX where the grain-refinement effect overcomes the hardness loss that resulted from decreasing the ferrite volume fraction. The lower formability of the friction stir processed material compared with the as-received metal was also explained by considering the suppression of mechanical twinning in the ultrafine-grained structure.