Microstructural development and texture evolution in the stir zone and thermomechanically affected zone of a ferrite-martensite steel friction stir weld

The microstructure development and texture evolution during friction stir welding of a dual-phase DP700 steel were investigated. Based on electron backscattered diffraction analysis, it was found that continuous dynamic recrystallization and discontinuous dynamic recrystallization are the operative recovery mechanisms for the ferrite phase in the stir zone. However, in addition to these mechanisms, geometrically dynamic recrystallization is also involved within the thermomechanically affected zone which is due to the lower strain rates in the thermomechanically affected zone. However, the continuous dynamic recrystallization was the predominant recovery mechanism for ferrite in both the stir zone and thermomechanically affected zone. Micro-texture examination showed that FSW develops typical shear texture components such as D₁, D₂ and F as well as the cube recrystallization texture for the ferrite phase within the stir zone. However, because of different thermomechanical conditions in various regions of the thermomechanically affected zone, additional shear texture components are developed, such as E, J and J¯ together with as D₁, D₂ and F, which were already observed in the stir zone. The absence of the cube recrystallization texture of ferrite in thermomechanically affected zone is attributed to insufficient recrystallization due to lowered strain and temperature in this zone. Study of the retained austenite constituent revealed that both shear textures, B and B¯ as well as recrystallization textures are developed in this phase within both the stir zone and thermomechanically affected zone.