Comparative study of the influence of natural convection on directional solidification of Al–3.5wt% Ni and Al–7wt% Si alloys

Abstract

We present numerical simulations of thermosolutal convection for directional solidification of Al-3.5 wt% Ni and Al-7 wt% Si. Numerical results predict that fragmentation of dendrite arms resulting from dissolution could be favored in Al-7 wt% Si, but not in Al-3.5 wt% Ni. Corresponding experiments are in qualitative agreement with the numerical predictions. Distinguishing the two fragmentation mechanisms, namely dissolution and remelting, is critical during experiments on earth, when fluid flow is dominant.

Comparative study of directional solidification of Al-7 wt% Si alloys in Space and on Earth: Effects of gravity on dendrite growth and Columnar-to-equiaxed transition

“…Then, natural convection in general changes the thermal and solutal conditions ahead of the dendrite front and also in the mushy zone [3,9]. Based on previous results [15,19,26,50], from both experiments and simulations, the qualitative liquid flow patterns induced by the gravity in the mushy zone and ahead of the dendrite front during the solidification can be estimated in this experiment. In practice, solutal convection dominates in the case of metallic alloys owing to their large Lewis number defined as the ratio of the thermal diffusivity to the solutal one (Le = D th /D), typically of the order of 10 3 to 10 4 .…”

“…Generally speaking, in presence of gravity and depending on the alloy (partition coefficient k and respective densities of alloy elements), the convection flow is different in the two regions illustrated in Fig.8 [15,26,50]: within the mushy layer (generally considered as a porous medium) and in the region ahead of the mush/liquid interface. In the latter case, the characteristic length scale may vary from several times the solutal length D/V (V being the solidification velocity) up to the sample size far from the S/L interface.…”

“…Si. But the convection induced by the radial temperature gradient also existed, which would enhance the liquid flow [3,26]. As mentioned by Jung [53], under the solidification parameter G=0.5 K/mm and v=0.002 mm/s, the maximum convection velocity for Al-7wt%Si (2.3 μm/s) was much higher than that for Al-3.5wt%Ni (0.9 μm/s), even though the density difference between Al and Ni is much larger.…”

“…Such fragments can participate to the CET if they are transported from the columnar front into the undercooled region where they can continue to grow and possibly stop the advancement of the columnar front [26][27][28]. In the case of grain refined alloys, the liquid flow might also lead to the transportation of the grain refiners and therefore can affect the CET [29][30][31].…”

Non Newtonian annular alloy solidification in mould

“…One alternative is to include the different physical levels (micro-, meso-and macroscopic) or the two-level (micro-macro) strategy, but with the inconvenience of requiring large computational resources. A second alternative uses macroscopic models that include the macroscopic influence on the system properties and behavior [1][2][3][4][5][6][7].…”