Friction-stir processing of an AA8026-TiB2-Al2O3 hybrid nanocomposite: Microstructural developments and mechanical properties

In this study, micro- and nano-sized TiB2 and Al2O3 particles were incorporated separately and simultaneously through the AA8026 aluminum base alloy during multi-pass friction stir processing (FSP) with 100% overlapping to fabricate metal matrix mono and hybrid nanocomposites. Various FSP conditions including different rotational speeds (w), traverse velocities (v), and processing pass numbers were assessed to attain a homogenous distribution of reinforcing particles through the Al-metal matrix. Moreover, the impacts of size (micro or nano) and type of reinforcement particles (TiB2 and Al2O3) on the process-ability of single and hybrid nanocomposite systems were examined. Microstructures of different zones and distributions of reinforcing ceramic particles through the Al-matrix under various processing conditions were studied and characterized by using optical (OM), scanning (SEM), and transmission electron microscopy (TEM) techniques, respectively. The main mechanical characteristics of the prepared nanocomposites, such as, indentation Vickers hardness, tensile properties, and wear resistance were measured and compared for all of the various processing conditions. By optimization of the FSP parameters, as a rotational speed of 1600 rpm and a traverse velocity of 40 mm/min after 4 passes, a uniform AA8026-TiB2-Al2O3 hybrid nanocomposite was attained with significant improvements (~70-100%) in the different mechanical properties. As a result, the tensile yield strength of ~270 MPa, elongation of ~4.5%, and indentation Vickers hardness of ~141 HV were obtained. Also, the average wear rate was reduced from the 21×10−3 mg/m value for the AA8026 base alloy down to 2.6×10−3 mg/m for the best processed nanocomposite. A direct relationship between the wear rate and the indentation hardness resistance was demonstrated. Finally, effects of FSP processing conditions and reinforcement particles (type and size) on the microstructure and mechanical properties of the FSPed Al-matrix nanocomposites were addressed and discussed.