Unveiling the mechanism behind high formability in ultrafine-grained Cu with low ductility

Abstract

To explore the formability of ultrafine-grained (UFG) materials, hemispherical spinning tests are conducted on the UFG Cu for the first time. The limit half-cone angle of spinning for UFG Cu is experimentally determined to be 15.2°, slightly higher than the 7.84° for coarse-grained (CG) Cu, despite UFG Cu exhibiting significantly lower tensile ductility than the latter. The exceptional formability of UFG Cu is primarily attributed to the mechanism of deformation-induced dynamic recovery during spinning, which promotes dislocation absorption via grain boundary migration and releases the strain/strain concentration at grain boundaries. Finite element method (FEM) simulations confirm that fracture in both UFG and CG materials predominantly occurs in the cone wall region due to its biaxial tensile stress state. In contrast, the spinning deformation zone exhibits a triaxial compressive stress state, which serves as an optimal constraint for accommodating large plastic strains within large ranges of strain rates. By adjusting key processing parameters, such as feed ratio and rotational speed, the extent of dynamic recovery can be effectively activated and enhanced. These findings provide new insights into the plastic forming behavior of UFG Cu and expand its potential for engineering applications.