Selective laser melting of advanced Al-Al2O3 nanocomposites: Simulation, microstructure and mechanical properties

Aluminium-based metal matrix composites are widely used in the aerospace and automotive industries, but their manufacturability and mechanical properties are not well understood when these new materials are employed in additive manufacturing. This is an important consideration because, compared with traditional manufacturing technologies, additive manufacturing technologies such as selective laser melting (SLM) offer the ability to manufacture engineering parts with very complex geometries. This paper systematically studied the SLM of an advanced Al-Al2O3 nanocomposite that was synthesised using high-energy ball-milling. A finite element model was developed in the study to predict the thermal behaviour of the composite in order to narrow down the process parameters to be explored in the experiments; in particular, the SLM hatch spacing and scanning speed were found to be worth further experimental investigation for the composites. The experimental results demonstrated that the optimum laser-energy density and scanning speed in fabricating nearly fully dense composite parts were 317.5 J/mm3 and 300 mm/s, respectively. Furthermore, the as-fabricated composite parts were observed to exhibit a very fine granular-dendrite microstructure due to the rapid cooling, while the thermal gradient at the molten pool region along the building direction was found to facilitate the formation of columnar grains. Compared to pure Al, the addition of 4 vol% Al2O3 nanoparticulates was found to contribute to a 36.3% and 17.5% increase in the yield strength and microhardness of the composite samples, respectively, because the reinforcement particulates improved the dislocation density by offering more grain boundaries. The paper also examines the influence of cold working on microstructural and mechanical properties. In comparison with the as-fabricated composite sample, the cold-worked composite sample was found to offer a 39% increase in microhardness; this is thought to have been caused by plastic deformation, which in turn resulted in grain deformation and elongation.