The effect of milling time on the morphology, microstructure, physical and mechanical properties of pure Al-5 wt % Al₂O₃ (Al-5Al₂O₃) has been investigated. Al-5Al₂O₃ nanocomposites were fabricated using ball milling in a powder metallurgy route. The increase in the milling time resulted in the homogenous dispersion of 5 wt % Al₂O₃ nanoparticles, the reduction of particle clustering, and the reduction of distances between the composite particles. The significant grain refining during milling was revealed which showed as a reduction of particle size resulting from longer milling time. X-Ray diffraction (XRD) analysis of the nanocomposite powders also showed that designated ball milling contributes to the crystalline refining and accumulation of internal stress due to induced severe plastic deformation of the particles. It can be argued that these morphological and microstructural variations of nanocomposite powders induced by designated ball milling time was found to contribute to an improvement in the density, densification, micro-hardness (HV), nano-hardness (HN), and Young’s modulus (E) of Al-5Al₂O₃ nanocomposites. HV, HN, and E values of nanocomposites were increased by ~48%, 46%, and 40%, after 12 h of milling, respectively.

In the present study, friction stir processing was adopted for surface treatment of Al5083 by incorporation of CNT and Al₂O₃ nanoparticles. Microstructural, mechanical and tribological properties of the surface of Al5083/CNT, Al5083/Al₂O₃ and hybrid Al5083/CNT/Al₂O₃ nanocomposite were investigated and compared. The friction stir processing yielded a nearly uniform dispersion of CNTs and Al₂O₃ nanoparticles, irrespective of nanoparticle reinforcements and their weight fraction. Microstructural observations revealed that Al₂O₃ nanoparticles have dispersed in different zones including intra-grain and grain boundary zones while, CNTs are pinned into grain boundaries during friction stir processing. From mechanical point of view, hybridization of CNT/Al₂O₃ enhances the hardness (126 HV at stirred zone), strength (UTS of ∼487 MPa) and also wear resistance of Al5083/CNT/Al₂O₃ nanocomposites. The enhancement is attributed to the presence and combination of features of both CNT and Al₂O₃ nanoparticles which are different in nature; one spherical oxide and one nanotube.