The Influence of Fluid Flow on the Microstructure of Directionally Solidified AlSi-Base Alloys

Abstract

To obtain a quantitative understanding of the effect of fluid flow on the microstructure of cast alloys, a technical Al-7 wt pct Si-0.6 wt pct Mg alloy (A357) has been directionally solidified with a medium temperature gradient under well-defined thermal and fluid-flow conditions. The solidification was studied in an aerogel-based furnace, which established flat isotherms and allowed the direct optical observation of the solidification process. A coil system around the sample induces a homogeneous rotating magnetic field (RMF) and, hence, a well-defined flow field close to the growing solid-liquid interface. The application of RMFs during directional solidification results in pronounced segregation effects: a change to pure eutectic solidification at the axis of the sample at high magnetic field strengths is observed. The investigations show that with increasing magnetic induction and, therefore, fluid flow, the primary dendrite spacing decreases, whereas the secondary dendrite arm spacing increases. An apparent flow effect on the eutectic spacing is observed.

Interplay Between Melt Flow and the 3D Distribution and Morphology of Fe-Rich Phases in AlSi Alloys

“…[30] on 2D microstructure did not document the large intermetallics (e.g., as seen in Figures 5(h), (k), and (l)). The fluid flow effect on the microstructure of the Al-5/7 wt pct Si-0.2/0.5/1.0 wt pct Fe alloys is also clear in the X-ray tomography results, in agreement with earlier research, [25] where Steinbach reveled the appearance of a center enriched in Si and Fe in 2D microsections of samples solidified under the influence of melt flow (RMF). The current study and [26,27] revealed larger b-phase particles in the center, but only 3D X-ray visualization verified an increased number density of intermetallics.…”

“…This was observed in all specimens solidified with melt stirring in the present study, as well as in some earlier studies. [25,30] Samples solidified without RMF showed dendrites and eutectic evenly spread in the cross section (data not shown).…”

“…The two furnaces are controlled independently (LabView Software) and set to ensure a constant solidification velocity, cooling rate, and a temperature gradient. [25][26][27] Three pairs of coils located very close to the sample can yield a magnetic field of up to 6 mT at a frequency of 50 Hz, which induces azimuthal flow as well as secondary flows in the radial and axial directions inside the specimen. [28,29] These flow velocities are around 10 mm/s.…”

“…[35,[51][52][53] The fluid flow, as used here, has been confirmed to reduce the primary stem spacing and increase the SDAS. [25,27] The b needles (marked in blue on Figure 15, which shows area G from Figure 2(a)) start to form a few millimeters behind the dendrite tips. The simulation results [28,29] suggest (as presented on Figure 15) that axial flows might reach 12 mm deep in the bulk melt, and with a temperature gradient of G = 3°C/ mm, this would give a temperature range of 36 K (36°C) from the dendrite tips into the bulk of the mesh.…”

“…[25][26][27] The present paper studies fluid flow effects during directional solidification of Al-Si alloys with Fe intermetallics using three-dimensional visualization methods. X-ray tomography aided the assessment of the stirring effect on the intermetallics and microstructure in AlSi alloys.…”