Achieving gradient ultrafine-grained heterostructure by friction stir-assisted incremental sheet forming

Metallic sheets with gradient ultrafine-grained (GUFG) or gradient nano-grained heterostructure demonstrate a superior strength-ductility synergy compared with the homogenous structured counterparts, which show great application potential in function specific components. However, it is a challenge to efficiently achieve GUFG heterostructure on the industrial thin-walled components by a cost-effective way. This work reports an interesting GUFG structure across the thickness on the component of mild steel DC05, which is fabricated by a friction stir-assisted incremental sheet forming (FS-ISF) method. Further investigations indicate that geometric dynamic recrystallization and continuous dynamic recrystallization are responsible mechanisms in generating GUFG structure. The spatial kinetics for the formation of GUFG structure are also clarified by evaluating the temperature, stain rate and strain gradients along the thickness direction of the formed part. Meanwhile, the relationship between the Zener-Holloman parameter and the refined grain size across thickness is estimated. Compared with conventional ISF process, an enhanced combination of strength and ductility is acquired by the proposed method, which can be further improved by subsequent annealing process. The present work provides a new insight in understanding the mechanical property of GUFG structure on the formed part by FS-ISF, which could be used to develop a potential route in fabricating structural parts with promising heterostructure.