In-Situ Analysis of Coarsening during Directional Solidification Experiments in High-Solute Aluminum Alloys

Abstract

Coarsening within the mushy zone during continuous directional solidification experiments was studied on an Al-30 wt pct Cu alloy. High brilliance synchrotron X-radiation microscopy allowed images to be taken in-situ during solidification. Transient conditions were present during directional solidification. Under these conditions, solute-rich settling liquid flow affects the dendritic array and thus coarsening. Coarsening was studied by following the secondary dendrite arm spacing (SDAS) of a developing dendrite at different local solidification times according to the mush depth and instant interface velocity. Solute enrichment and liquid flow cause deceleration and acceleration of the solidification front, which in turn influences both the mush depth and local growth and coarsening due to variations in solutal gradients and thus local undercooling. In addition, spacing between neighboring dendrites (i.e., primary dendrite arm spacing), which determines permeability within the mushy zone, affects the development of high-order branches.

On the Coupling Mechanism of Equiaxed Crystal Generation with the Liquid Flow Driven by Natural Convection During Solidification

“…[5] through [7], k 1 and k 2 are closely related with G and V, therefore N d can be related with G and V and the relation is shown in Eq. [9]:…”

“…Remelting of tertiary branches at the root to the parent dendrite remains a better pathway for dendrite detachment. Previous to the dendrite fragmentation by remelting, a minor deceleration [9] of the columnar front velocity is noticeable. Ruvalcaba et al hypothesized that this deceleration would induce enhancement of solute at the columnar front and subsequently the solute-rich liquid will settle into the mush and initiate dendrite detachment.…”

A Numerical Method of Macrosegregation Using a Dendritic Solidification Model, and Its Applications to Directional Solidification via the Use of Magnetic Fields

“…Because the flow velocity vector v i and the EM braking force f i interact with each other under an externally applied magnetic field B i , Eq. [38] has been coupled with Eq. [30] via the body force term X i : Thus, Eq.…”

“…Also, it is well known that columnar dendrites grow toward the upstream side of the liquid flow at solidification front, which has been interpreted by constitutional supercooling theory [36] : Tan and Zabaras [37] have shown this by the numerical simulation of an alloy dendritic solidification (see Figures 2.4 and 2.5 of Reference 37). Incidentally, Ruvalcaba et al [38] have carried out the in situ observations during downward DS (liquid density inversion within the mushy zone) of an Al-Cu alloy by means of X-ray microscopy, and they have shown that where the primary dendrite arm spacing becomes uneven, the growth rates of secondary dendrite arms perpendicular to the growth direction exhibit the difference. Although the detailed mechanism is not known, the liquid flow in the mushy zone is considered responsible for this difference.…”

“…[38] are expressed as follows: Furthermore, the electric conductivity r is large for most molten metals, and then the induced electric field strength E = Àgrad w becomes small. In that case, by neglecting E, Eqs.…”