Investigations on Solidification-Flow Interaction

Modelling of metallurgical processes is a rapidly expanding field and the research activities in the last decades cover a wide range of areas including melt pre-treatment, solidification and subsequent manufacturing routes. Among those activities solidification stands in the central position, because the primary structure of the materials and even many defects such as porosity, macro- or microsegregation and inclusions form during solidification. Those primarily formed structures or defects once existing are difficult to be removed or modified by the subsequent material processing. In many solidification processes inertia or natural buoyancy generate strong flow turbulence. The turbulence is known to change the effective heat transfer of the melt flow, which in turn can modify the rate and the structure of the solidification. In region partly solidified the turbulence will be damped, while falling equiaxed crystals might amplify or dampen the fluctuations. A reasonable model of those processes must include a good representation of the turbulence-solidification interaction in term of momentum and energy exchange. Unfortunately, papers on the interaction between a turbulent flow and solidification can not be found in literature. Within a past FWF-project at the chair of the applicant, a particle image velocimetry (PIV) technique has been applied to a solidification system, in which both liquid and crystal flow velocity could be simultaneously measured. Successfully applied in laminar conditions, the present project aim to apply this technique to configurations where the flow is turbulent and interaction with solidification occur.

Solidification is a very old research field but deep understanding of the complex processes involved still need clarification. Experimental works at small scale, comparing with industrial scale, are very important for the profound understanding and constitute a good data base for numerical simulations. During solidification liquid phase and solid phase coexist and their interaction results in different solid types (columnar, equiaxed or columnar to equiaxed transition (CET)). The main objective of this project is to understand the influence of the melt flow on the solidification.In the present study solidification experiments with transparent alloys from Ammoniochloride- Water were performed and the melt flow was measured and characterised through different stages of solidification. The flow is influenced by density changes induced by temperature and/or concentration differences. When solidification occurs generally lighter solute will be rejected at the solid/liquid interface. This local change in the solute concentration generates so- lute buoyancy which can be very strong and even creates turbulence.It is also known that thermally buoyant flows witness a transition from laminar to turbulence if the height of the cooled walls is sufficiently high. Therefore, we have performed experiments with different cooling conditions using differently large rectangle cartridges. The results show that the solidification type (columnar, equiaxed or CET) is closely linked to the flow intensity and occurrence of turbulence.If diffusion of heat and species happens simultaneously, corresponding processes are termed double-diffusive. In our experiments a meander flow regime establishes by double-diffusive convection. Although double-diffusive convections were studied by many groups, this is the first time that such a meandering flow is described. The facts we have learned from the numerical interpretation of this finding could explain certain stratifications observed in oceanography where salt and temperature change stepwise in the ocean depth.