A comparison of the effects of severe plastic deformation and heat treatment on the tensile properties and impact toughness of aluminum alloy 6061

In this study, the tensile properties and fracture behavior of the nanostructured/ultra-fine grained aluminum alloy 6061 were investigated. Severe plastic deformation (SPD) by the equal-channel angular pressing (ECAP) method was adopted to induce large plastic strain into the billets of Al6061. The results revealed that the yield and the ultimate strengths increased significantly after two passes. However, after two additional passes, the yield and the ultimate strengths decreased slightly. Nevertheless, negligible decrease in strength was followed by a noticeable increase in ductility and toughness, which was very interesting for structural applications. Inspection of the fractured surfaces after tensile tests demonstrated the fracture mechanism in the processed samples. The large and deep dimples, which were present in the fractured surface of annealed specimens, were replaced by small, well-developed and uniformly distributed dimples after two ECAP passes. In addition, further increase in the number of ECAP passes caused the formation of parabolic shaped micro-voids indicating the shear type fracture mode in the uni-axial tensile test. The results of Izod impact test showed that the impact toughness of annealed specimens decreased by increasing the number of ECAP passes. However, the impact toughness of T6 heat-treated billets increased after ECAP. The higher volume fraction of the β-AlFeSi phase was the main reason for this observation. Micro-necks and shear voids were formed along the direction of the impact causing absorption of more energy during the impact test of annealed specimen. Assessment of the fractured surface of the specimen after four passes of ECAP showed that the dimples were very small and shallow confirming the lower values of impact toughness obtained using impact test.