Development and Characterisation of Aluminium Alloys and Aluminium Matrix Composites Produced via Laser Powder Bed Fusion

Nowadays, despite the growing interest about metal additive manufacturing, only a few commercial alloys used for casting are available for these technologies. Therefore, there is the necessity to develop new materials that can exploit the unique opportunities of additive manufacturing processes. The goal of this thesis was to create and characterize new aluminium based materials for powder bed additive manufacturing technologies. At first, the feasibility of the production of aluminium matrix composites by laser powder bed fusion (LPBF) was investigated. Three different composites were produced in order to study the effect of the reinforcement size and properties on the consolidation. Even if the introduction of a ceramic particles alters the consolidation phenomena of the base alloy, the LPBF process allows the production of dense and crack free aluminium matrix composites. The mechanical characterization of these composites revealed that the mechanical properties, and in particular the yield and ultimate strength, are strongly related to the building parameters used during the LPBF process. Regarding the study of existing or new alloys, it was observed that the laser scanning and the phenomena that arise in the melt pool allow the obtainment of an almost homogenous composition even when realizing the alloy in situ, i.e. starting from mixed powders of different composition. A specific alloy that takes advantage from the fast cooling that arises during the laser scanning, and that therefore has excellent mechanical properties, was selected in this study. For both composites and alloys, the laser-powder interaction and the consolidation phenomena were studied and compared by means of single scan tracks analyses which proved to be a promising solution to foresee the behavior of a specific material during the LPBF process. Finally, the effect of a post processing heat treatment on the microstructure on the mechanical properties of LPBF samples was analysed. In most of the cases, the as-built state can be considered as a supersaturated solid solution and therefore a direct-ageing heat treatment allows the achievement of enhanced mechanical properties. This effect was clearly visible on the A357 alloy and on a modified-7075 alloy, in which 15% and 20% respectively higher hardness values were achieved by the selection of the most suitable ageing heat treatment.