On the influence of processing temperatures on microstructure and mechanical properties of low-temperature extruded AA6060Thursday (29.09.2016) 12:00 - 12:15 Part of:
Compared to conventional materials, ultrafine-grained materials may exhibit exceptionally high strength and ductility. An effective way to achieve grain sizes below 1 µm is the application of severe plastic deformation below recrystallization temperature. While different severe plastic deformation processes could be established, none of them incorporates a change of shape. Due to their high strength and low thermal stability, the semi-products therefore commonly need to be further processed to near net shape by costly and time-consuming cutting processes. In the present study, a shaping extrusion process operating below recrystallization temperature is investigated. Cast billets of the age hardening aluminum alloy 6060 were solid solution annealed and extruded at room temperature (RT) and – in order to reduce pressing forces – at a slightly elevated temperature (170 °C) close to the alloy’s aging temperature (AT). Electron back-scatter diffraction measurements in the center of the circular rods show a coarse-grained microstructure with a double fiber texture, consisting of a strong <111> and a weak <100> component. Near the outer surface, fine- and ultrafine-grained pancake structures were observed, with fiber textures that were significantly rotated due to the pronounced shear deformation during extrusion. The microstructural differences between center and surface regions are also related to a strong gradient of mechanical properties. For RT extrusion, higher hardness (103 HV1) and strength (ultimate tensile strength: 332 MPa) were found in the surface region. After aging, slightly increased values (105 HV1, 348 MPa) and a much more homogeneous distribution were observed. The AT extruded material exhibits a reversed gradient with higher values in the center (108 HV1, 368 MPa) that can be further improved by additional heat treatments. Compared to the initial material, the novel low-temperature extrusion approach presented here leads to strongly accelerated precipitation kinetics with about 10 % higher hardness at a significant reduction of aging time by 97.5 (RT) to 100 % (AT), which offers a high potential for the integration in industrial process chains.