Innovative Metal Alloys for Industrial Products with Higher Sustainability and for More Efficient Recycling

Al–Mg–Si alloys are widely used in the automotive, electrical, and construction sectors. However, their manufacturing processes are limited by a critical solidification defect known as hot tearing. Hot crack formation during solidification arises from the combined effects of chemical composition, solidification sequence, and resulting microstructure. This susceptibility may be further enhanced by the increasing use of recycled aluminium alloys, which introduce higher levels of impurities such as iron and manganese. This study investigates the influence of Mg, Si, Fe, and Mn on the hot tearing susceptibility of innovative 6XXX series secondary aluminium alloys, while also developing a predictive framework capable of correlating thermodynamic modelling with actual casting practice. In addition, the potential of semi-solid metal processing is explored as an alternative route to improve the castability of wrought aluminium alloys. A full factorial design of experiments was employed to generate 256 nominal alloy compositions by varying the contents of Mg, Si, Fe, and Mn over four levels. CALPHAD-based thermodynamic calculations were performed to assess the solidification paths of the alloys and to predict hot tearing susceptibility using the Kou criterion over different solid fraction ranges. Statistical analysis showed that the reliability of the criterion strongly depends on the selected solid fraction interval. Three representative alloy compositions were selected for experimental validation. Their solidification paths were characterised by computer-aided thermal analysis, and the evolution of solid fraction was determined through Fourier analysis. A comparison between calculated and experimental hot tearing indices showed good agreement in the 0.87–0.94 solid fraction range. The discrepancy between absolute values was addressed by introducing a correction coefficient. Microstructural investigations revealed that hot tearing resistance is primarily governed by the precipitation sequence and the morphology of intermetallic phases. Alloys with higher Fe and Mn contents promoted the formation of branched α-Al₁₅(Fe,Mn)₃Si₂ phases, which form a solid skeleton capable of accommodating thermally induced stresses. Conversely, alloys richer in magnesium and silicon were characterised by acicular Fe-rich intermetallics that act as stress concentrators, facilitating crack initiation and propagation. Nevertheless, an increase in magnesium content was also found to promote a higher eutectic fraction, which contributes to reducing hot tearing susceptibility by improving the feeding ability of the alloy during the final stages of solidification. Hot tearing susceptibility was further assessed by constrained rod casting tests, which confirmed the trends predicted by the improved thermodynamic model. CRC tests also showed that the addition of an Al–Ti–B grain refiner does not significantly enhance resistance to hot tearing. This result indicates that, when the as-cast structure is already fine, intermetallic morphology and solidification sequence play a more dominant role than grain size alone. Finally, the application of an innovative semi-solid casting route based on rapid slurry formation resulted in a marked reduction in hot tearing susceptibility and overall shrinkage compared to conventional fully liquid casting. The semi-solid process promotes a globular or rosette-like microstructure, enhances mushy-zone permeability, and improves feeding capability during the critical stages of solidification. These effects, combined with a favourable redistribution of intermetallic phases, reduce strain accumulation and mitigate hot crack formation. Overall, this study demonstrates that careful optimisation of alloy composition and processing route represents an effective strategy to improve the castability of Al–Mg–Si alloys, even when produced from recycled feedstock.