Caratterizzazione meccanica e microstrutturale di campioni estratti da getti in lega di alluminio per componente motore

This thesis aimed to characterize mechanically and microstructurally an engine component realized in a quaternary aluminium casting alloy and subjected to T6 heat treatment. A complete cross-section microstructural characterization was performed by optical (OM) and scanning electron (SEM) microscopes, the last equipped with an energy dispersive spectrometer (EDS). The analyses were focused on: defects morphology, Dendrite Arm Spacing (DAS) measurements, evaluation of the eutectic silicon modification, identification of intermetallic compounds. Hardness tests were done, to correlate the microstructural and the mechanical behaviour. These analyses showed the good quality of the casting, which presented a quite low number of defects, but also an inhomogeneous microstructure along the sections. A non-homogeneous distribution of modifying elements, like Sr, and different solidification conditions led to zones with eutectic silicon is only partially modified. EDS analyses showed the presence of Fe-based and complex (Al-Mg-Cu-Si) intermetallic phases whose dimensions are comparable to Si particles. The hardness tests showed values in agreement with the ones expected for the T6 heat-treated quaternary alloy. The tensile tests were carried out on specimens extracted from different zones of the casting (general and local), some of these were subjected to HIP treatment. The resulting tensile and ultimate strength did not satisfy the requested values for the general areas, while the local mechanical properties reached the target. The measured ef was lower than the one requested for all the areas. This was confirmed by the fractographic analysis, which showed zones with an absent presence of dimples. A comparison among HIP and non-HIP specimens was also conducted, confirming that the specimens subjected to HIP presented better general behaviour, especially concerning ductility, making this treatment recommended.