Microstructural evolution during severe plastic deformation by gradation extrusion

In this contribution, we study the microstructural evolution of an age hardenable AA6082 aluminum alloy during severe plastic deformation by gradation extrusion. A novel die design allowing an interruption of processing and nondestructive billet removal was developed. A systematic study on the microstructure gradient at different points of a single billet could be performed with the help of this die. Distinct gradients were investigated using micro-hardness measurements and electron microscopy. Our results highlight that gradation extrusion is a powerful method to produce graded materials with partially ultrafine-grained microstructures. From the point of view of obtaining an ultrafine-grained surface layer with maximum hardness, only a small number of forming elements is needed. It is also found that large incremental deformation by too many forming elements may result in locally heterogeneous microstructures and failure near the billet surface caused by localization of deformation. Furthermore, considering economical aspects of processing, less forming elements are preferred since several processing parameter-related cost factors are significantly lower.