Dynamic recrystallization and solute precipitation during friction stir assisted incremental forming of AA2024 sheet

Abstract

Abstract Friction stir assisted incremental sheet forming (FS-ISF) has been proved to further improve the formability of aluminum alloy sheets. Under such thermal forming condition, it is quite necessary to explore if the recrystallization and solute precipitation would happen, which might have potential in further improve the strength of aluminum alloy conventionally realized by heat treatment aging. In the present work, FS-ISF experiments are conducted for AA2024 sheet with proper parameters to achieve more solute precipitates and fine recrystallized grains with stable boundaries, of which microstructure evolutions are characterized by Transmission Electron Microscopy (TEM) and Electron Backscattered Diffraction (EBSD). It s found that cyclic and incremental plastic deformation during FS-ISF induces dislocation jogs and dislocation loops, and finally makes solute move into vacancies and generates solute precipitates. And friction stir also contributes to more solute precipitates than conventional incremental sheet forming (CISF) at room temperature. Meanwhile, precipitation zone inside grains helps to resist ductility damage and stable boundary with pinning contributes to higher strength. Besides, irregular sub-boundaries are merged and sub-boundaries consume dislocations to form new boundaries by the slip and arrangement of dislocations during dynamic recrystallization so that fine equiaxed grains with high angle boundaries are obtained, which results in higher yield strength (YS), ultimate tensile strength (UTS) and better ductility than those by CISF. Furthermore, investigations also demonstrate that static recrystallization cannot arise during FS-ISF.