On radial microstructural variations, local texture and mechanical gradients after cold extrusion of commercially pure aluminum

Cold extrusion is an unconventional approach that can potentially combine the benefits of plastic shaping of metals and simultaneous strengthening by grain refinement and strain hardening. In this study, cast billets of the commercially pure AA1080 aluminum alloy were extruded at room temperature in a conventional extrusion press with various extrusion diameters, corresponding to extrusion ratios in the range of about 10 up to 54. (Micro-)hardness measurements as well as microstructural analyses by means of electron microscopy were carried out at several locations along the extrudates' longitudinal directions in order to characterize axial and radial deformation gradients. Interestingly, the extruded profiles exhibit a radially graded microstructure with four distinct annular sections independent of their respective extrusion ratio, reported here for the first time: A double-fiber textured center followed by a single-fiber textured ring, a double-fiber textured region with grains arranged alternatingly in an iris-like shape, and an (ultra-)fine grained surface layer. The microstructural features of these sections can be directly related to hardness distribution, with pronounced gradients in the center and surface sections. The results highlight that cold extrusion of AA1080 results in a pronounced multi-gradient deformation structure characterized by distinct differences in terms of texture, grain size and strain hardening.