Investigation of stagnation point and microstructure evolution in forming Al-Mg-Si alloy sheets with synergistic strength-ductility

Plastic flow machining (PFM), an efficient process of severe plastic deformation, aims at preparing gradient-structured sheets in one step without cumbersome equipment. Nevertheless, material stagnation points and microstructure evolution in sheet forming process are often neglected in previous studies. Based on simulation and experiments, this paper reported in detail the forming mechanism, stagnation point position, and mechanical properties of Al-Mg-Si alloy sheets under different thicknesses prepared by PFM. Furthermore, the EBSD, TEM, and XRD methods were used to deeply analyze the microstructure evolution. The results indicated that there existed a material stagnation zone (MSZ) during the PFM process, and the stagnation point position was closely related to sheet thickness. Additionally, a prediction expression for the stagnation point position was proposed, which generally agreed with the actual results. As sheet thickness increased, the MSZ area enlarged and the stagnation point moved left, leading to more material extruding into the forming channel. Compared to initial sheets, the grains, effective strain and hardness of the prepared sheets exhibited a distinct gradient along the thickness direction, improving the strength without compromising much ductility. Accordingly, the Al-Mg-Si alloy sheets fabricated by PFM exhibited excellent strength-ductility synergy, providing insights into the clean processing of lightweight materials with gradient structure.