Quantifying the effects of cooling rates on Fe-rich intermetallics in recycled Al-Si-Cu alloys by machine learning

In this study, three casting processes named gravity casting (GC), vacuum casting (VC), and centrifugal casting (CC) were introduced to prepare the recycled Al-Si-Cu alloy in order to cover a wide cooling rates range of 15.77–75.25 ℃/s. Machine learning methods have been used to establish a regression model for predicting the Fe-rich intermetallic size of the recycled Al alloys under different cooling rates and verified by experiments. Further, the effects of cooling rate on the size, morphology, and percentage of Fe-rich intermetallics has been investigated, and the correlation between microstructure and mechanical properties were analyzed. With increasing cooling rate from 15.77 ℃/s to 75.25 ℃/s, the morphology of β-Fe gradually changed from coarse platelet to thinner and shorter shape, and finally to granular α-Fe particles. The cooling rate to fully suppress the formation of β-Fe has been found to be 75.25 ℃/s, at which the percentage of α-Fe increased to 2.01 %, which was increased by 25.65 %, leading to the improvement in YS, UTS, and EL by 12.56 %, 11.01 %, and 8.61 %, respectively. After T6 treatment, the percentage of α-Fe in CC process reached 2.15 %, which was increased by 12.05 %, resulting in an improvement in elongation by 57.36 %. By X-ray computed tomography, the statistical number density of Fe-rich intermetallics and microporosities were 4141.25 mm⁻³ and 241.48 mm⁻³ in VC process, while those were increased by 21.29 % and 56.12 % in CC process, respectively. Moreover, the quantified volume fraction of Fe-rich intermetallics and microporosities with equivalent diameter greater than 30 μm were 0.20 % and 0.59 % in VC alloy, while those were decreased by 95.10 % and 9.91 % in CC alloy, respectively, indicating that high cooling rate promotes the nucleation of Fe-rich intermetallics and microporosity, thus refining its size. This study provides a guide for regulating the morphology of Fe-rich intermetallics in recycled Al alloys by controlling cooling rate.