Effects of mechanical alloying on an Al6061–CNT composite fabricated by semi-solid powder processing

For powder-based metal composite manufacturing, mechanical alloying has been widely used to disperse the clustered carbon nanotubes (CNTs) in the metal matrix. However, the effects of mechanical alloying time and CNT dispersion on the microstructure and mechanical properties of the metal–CNT composite have not been fully investigated. In this study, aluminum alloy 6061 (Al6061)–CNT composites were synthesized by semi-solid powder processing from powders that had been mechanically alloyed for different durations. The powder morphology, microstructure, hardness, flexural strength, fracture surface, and composition of the Al6061–CNT composites were analyzed. The results showed that the multi-walled CNT agglomerates were first crushed into thin layers and then dispersed during the mechanical alloying procedure. The CNTs on the surfaces of Al6061 particles at various mechanical alloying times showed that the CNT length decreased quickly to about 1.2 μm after 1 h of mechanical alloying and below 1 μm after 3 h from the original length of over 5 μm. The grains of the Al6061–CNT composites deformed severely during the mechanical alloying process. The hardness improved as the mechanical alloying time increased. However, the bend test and fracture surface indicated reduction of the composite ductility with increasing mechanical alloying time. Compositional analysis also showed formation of aluminum carbide (Al4C3) due to the CNT addition. In addition, a modified shear lag model was used to predict the strength of Al6061–CNT at different mechanical alloying times.