Analisi termodinamica di leghe di Al multicomponente per impieghi in additive manufacturing

In the context of Additive Manufacturing (AM) for metallic materials, aluminium alloys play a key role in the production of objects with increasingly better properties. As AM technologies are developing more and more rapidly, the need for new ad-hoc metallic alloys is constantly rising. In particular, one of the most used Al-based metallic system, both in traditional casting and in AM, is the ternary Al-Mg-Si. In recent years the scientific community has been investigating multiple possibilities to modify these ternary alloys, for example with the addition of rare earths such as erbium. These additions can act as inoculants and improve the properties of the modified alloys. In this work, we have then focused on the quaternary Al-Mg-Si-Er system, with special attention to the properties of interest in AM processes. This thesis work was focussed on a computational analysis of this quaternary system using the ThermoCalc software. In order to perform such calculations, it was necessary to produce a thermodynamic database that can describe the quaternary system using the so called CALPHAD method. The method is based on mathematical models which describe the Gibbs free energy of each phase of a system. The use of this method and the ThermoCalc software which implements it allowed us to completely describe the thermodynamics of the quaternary system. This was the goal of the thesis; to achieve it, it was also necessary to fully reoptimize the binary Er-Si subsystem according to the CALPHAD methodology. Furthermore, all binary and ternary subsystems were carefully examined. From the optimized binary and ternary subsystems, it was then possible to construct a quaternary database and calculate properties of Al-Mg-Si-Er alloys. Once the quaternary database had been built, it was important to validate it as much as possible by comparing the calculated results with experimental data. Unfortunately, experimental analysis in literature concerning this type of alloys are scarce, but it was still possible to compare several calculated quantities and experimental ones. The simulations of equilibrium and non-equilibrium (Scheil) solidification allowed us to investigate which phases solidify and in what order. This information was compared with literature theoretical data from similar quaternary systems but with different rare earths. DSC experimental results (liquidus/solidus temperatures, melting enthalpies) were also compared with calculated values. A calculation of the driving forces for nucleation of the solid phases from the liquid was also done to get more insight into the solidification process. Finally, an analysis of the variation of the “solidification window” (the difference between liquidus and solidus temperatures) at various quaternary compositions was made. In fact, in AM alloys large solidification windows lead to several detrimental effects during processing, such as segregation and inhomogeneities in the final components. It was then important to explore the quaternary composition space searching for alloys with a low solidification window. In some future work, it would also be very interesting to further study this quaternary system experimentally in order to compare more calculations results produced with ThermoCalc with targeted experimental data.