Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik IWU
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, continuous-cast billets of the commercially pure AA1080 aluminum alloy were extruded in the directions along and opposite to the casting direction. Extrusion was carried out at room temperature in a conventional extrusion press with an extrusion ratio of about 20. Microstructural analyses by means of optical and electron microscopy and mechanical testing by (micro-)hardness measurements as well as tensile testing were carried out to characterize the cast material and radial deformation gradients in the extrudates. The as-cast condition exhibits coarse columnar grains that are inclined by an angle of about 20-22° to the center axis. The longitudinal axes of these grains are aligned with a <001> crystallographic orientation, which leads to macroscopically anisotropic properties. Changing the extrusion direction therefore results in strongly graded microstructures with four distinct annular sections each that differ significantly from another. The profile extruded along the casting direction exhibits 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. For the profile extruded in the opposite direction the double-fiber textured area covers two regions in the center (distinguished by differences in grain size and orientation of grain boundaries), followed again by a single-fiber textured ring and a fine grained surface layer. The microstructural features of these sections can be directly related to hardness and tensile strength distributions. The results highlight that the extrusion direction (along or opposite to the casting direction) determines the multi-gradient deformation structure after cold extrusion of AA1080.
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