Formazione di composti primari ricchi di Fe in leghe di alluminio da riciclo

The present work analyzes the formation of primary iron-rich intermetallic compounds, commonly referred to as sludge, in secondary EN AB-46000 (AlSi9Cu3(Fe)) aluminium alloys, which are widely utilized in die casting processes. The study focuses on determining the onset temperature and the mass fraction of these intermetallic compounds as a function of chemical composition, cooling rate, and the cleanliness of the molten metal. The research was conducted using two distinct yet complementary approaches. A preliminary study was performed using the CALPHAD method (via Thermo-Calc and JMatPro) under both equilibrium and non-equilibrium (Scheil) conditions to estimate the formation temperature and quantity of the primary phase. Experimental tests were carried out on five alloys with varying chemical compositions, specifically, iron (0.8% to 1.2% wt.), manganese (0.25% to 0.55% wt.), and chromium (0.06% to 0.10% wt.) were the primary variables, resulting in increasing Sludge Factor values. The aluminium alloys used came from biscuits from previous die casting processes and were therefore characterized by contaminant elements: the alloys were analyzed in both non-cleaned conditions and after fluxing treatment with salts. The experimental techniques employed include: • Density measurements and Reduced Pressure Tests (RPT) to evaluate melt cleanliness. • Differential Scanning Calorimetry (DSC) to determine characteristic temperatures and the primary phase fraction. The results demonstrate a clear correlation between the Sludge Factor and the onset temperature of primary precipitation. An increase in the Sludge Factor leads to a rise in both the formation temperature and the quantity of primary compounds. Furthermore, lower cooling rates favor a higher fraction of the primary phase, while the effect of the cleaning treatment was found to be overall limited. Regarding thermodynamic calculations, JMatPro showed good qualitative consistency with experimental data, although it tended to overestimate both the temperatures and the quantities of the primary phases. Conversely, Thermo- Calc proved to be inadequate for the specific alloys analyzed. In conclusion, the risk of primary phase formation in secondary EN AB- 46000 alloys is primarily governed by the chemical composition—synthesized by the Sludge Factor—and the thermal process conditions. Accurate control of these parameters is essential to mitigate issues related to fluidity, surface quality, and die wear in die casting.