Through-thickness inhomogeneity in microstructure and tensile properties and tribological performance of friction stir processed AA1050-Al2O3 nanocomposite

Friction stir processing (FSP) is used to fabricate Al-Al2O3 nanocomposite by addition of Al2O3 nanoparticles into a groove in AA1050 aluminum alloy and being processed using FSP. Evolution of grain structure, distribution of the Al2O3 nanoparticles, tensile properties and tribology are investigated in four samples extracted from each 2 mm of the fabricated composite, respectively from top surface. It was found that the finest grain structure formed in the second region from the top surface due to the most efficient stirring in this region. In addition, the presence of nanoparticles with an efficient distribution in this region may inhibit grain growth and lead to an eventually fine grain structure. After one pass of FSP, nanoparticles were found to be efficiently distributed in the top surface and less uniformly distributed in other regions leading to formation of particle depleted regions (PDRs) and coarse agglomerated particles. Fracture in the tensile test samples changed from ductile to brittle in the sample with agglomerated particles. Application of the second cycle of FSP leaded to further grain refinement, more uniform distribution of nanoparticles and less agglomerated coarse particles and consequently leaded to more uniformity in tensile properties and tribology behavior of the samples. This was the most effective on grain refinement for the regions which were not efficiently refined or with more extended PDRs in the first cycle. Coefficient of friction (CoF) of aluminum was significantly reduced with addition of Al2O3 particles which was attributed to increase in strength and reduction in sliding contact area.