Abstract
A forced flow was experimentally shown to influence the solidification microstructure of metal alloys by modifying the coarsening/ripening law. In some technical alloys (AlSi7Fe1), this flow effect can also be significantly suppressed due to the formation of intermetallic precipitates (β-Al5FeSi) that can block the flow in the mushy region. The forced flow was induced by a rotating magnetic field (RMF). Herein, a three-phase volume-average-based solidification model is introduced to reproduce the above experiment. The three phases are the melt, the primary solid phase of columnar dendrites, and the second solid phase of intermetallic precipitates. The dynamic precipitation of the intermetallic phase is modelled, and its blocking effect on the flow is considered by a modified permeability. Dendrite coarsening, which influences the permeability, is also considered. The RMF induces a strong azimuthal flow and a relatively weak meridional flow (Ekman effect) at the front of the mushy zone during unidirectional solidification. This forced flow reduces the mushy zone thickness, induces the central segregation channel, affects the distribution of the intermetallic precipitates, and influences dendrite coarsening, which in turn modifies the interdendritic flow. Both interdendritic flow and the microstructure formation are strongly coupled. The modelling results support the explanation of Steinbach and Ratke—the formed intermetallic precipitates (β-Al5FeSi) can block the interdendritic flow, and hence influence the coarsening law. The distribution of β-Al5FeSi is dominantly influenced by the flow-induced macrosegregation. The simulation results of the Si and Fe distribution across the sample section are compared with the experimental results, showing good simulation–experiment agreement.Graphic During alloy solidifications the flow can influence the mushy zone by inducing macrosegregation, modifying the solidification microstructure, and influencing the formation of intermetallic precipitates. The resulting microstructural features can in turn affect the melt flow by changing the flow intensity and flow pattern. A three-phase volume-average-based solidification model is introduced to study the flow-solidification interaction, and hence to improve the knowledge on the formation mechanism of intermetallics and their effect on solidification. (a) Schematic for the flow pattern and formation of different phases; (b) experiment–simulation comparison of macrosegregation (Fe) across the diameter of as-solidified sample.
Highlights
MELT flow is a natural phenomenon that occurs during the alloy solidification of almost all casting processes
The resulting microstructural features in the mushy zone can in turn affect the fluid flow by changing the flow intensity and flow pattern.[5]
The solidification of the AlSi7Fe1 alloy starts with the development of primary aluminium dendrites, followed by the formation of b-Al5FeSi during the binary eutectic reaction, and ends with the ternary eutectic reaction at 850 K
Summary
MELT flow is a natural phenomenon that occurs during the alloy solidification of almost all casting processes. 3008—VOLUME 52A, JULY 2021 magnetic field (RMF).[2,6] Interestingly, the same experiments were repeated with another alloy (AlSi7Fe1) in which intermetallic precipitates formed (b-Al5FeSi) during solidification, and the results showed that the applied RMF seemed to have no influence on the coarsening/ripening law, i.e., b was maintained at 1/3. The main goal is to quantify the effect of the intermetallic phase and dendrite coarsening on the solidification of AlSi7Fe alloy under RMF, and to explain the experimental results of Steinbach et al.,[6] i.e., the experimental observation of b = 1/3 (coarsening/ripening law exponent) in the presence of RMF when sufficient intermetallic precipitates are present. The solidification of AlSi7Fe1 alloy was numerically studied by Budenkova et al.,[16] the blocking effect of the precipitates and the dendrite coarsening were ignored
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