Mohammed M’Hamdi

Coupled Modelling of Air‐Gap Formation and Surface Exudation during Extrusion Ingot DC‐Casting

During casting of aluminium extrusion ingots the surface against the mould experiences a pull-in force that magnifies the air-gap during solidification close to the mould surface. This is a global phenomenon that results in early and large air-gap formations compared to the shape-casting situation. Due to the semi-solid surface created under such conditions, exudation through the surface may appear. In this study a coupled heat and fluid flow, stresses and deformation modelling tool are applied on the process. Results from the mechanical calculation are back-coupled to the thermal boundary conditions. The metallostatic head is the driving force for exudation through the dendritic network and the resulting fluid flow through this network is used to calculate a dynamic thickness of the exuded layer. Measurements from two different alloys, with rather small changes in composition, but with large variations on surface quality, are compared with the modelling results.

Impact of Inlet Flow on Macrosegregation Formation Accounting for Grain Motion and Morphology Evolution in DC Casting of Aluminium

Several transport mechanisms contribute to macrosegregation formation in Direct Chill (DC) casting of aluminium. The latter include solidification shrinkage induced flow, thermal-solutal convection and grain motion. The relative importance of these transport mechanisms depends on process parameters such as cast velocity, inlet melt flow, cooling rate, cast dimensions etc. Of these, the inlet melt flow due to vertical jet is known to cause significant modification in macrosegregation by resuspending equiaxed grains at the center of the ingot/billet. In this paper, we investigate by means of modeling the macrosegregation formation due to an inlet vertical jet. For this purpose, a three-phase, multiscale solidification model accounting for above mentioned transport phenomena except for shrinkage induced flow is applied on a DC cast billet. The model considers grain motion accounting for both globular and dendritic equiaxed grain growth and we investigate their interaction with the inlet vertical jet. We show that an interplay of morphology evolution of equiaxed grains and inlet vertical jet together contribute to grain resuspension which in turn modifies macrosegregation formation. We also compare these results with a case with open inlet.

Numerical Modeling of Stress Distribution in a Bi‐Grain Small Scale Silicon Ingot Including Crucible Deformation

In a previous work, a small scale Bridgman furnace has been used to study silicon bi-grain crystallization at different cooling rate. This work expands the analysis by studying the mechanical interaction between the crucible and ingot during the solidification and cooling. The thermal model is based on a 2D-axisymetric heat-transfer model. The flux histories are then transferred to the ingot-crucible 3D-model. Anisotropic Elastic and Crystal Plasticity model are used to model the silicon deformation. Four different assumptions are applied to model the mechanical contact crucible-ingot and three grain misorientations are considered. The results show the strong impact of the alumina crucible contraction on the stresses and deformations in the silicon ingot.

Macrosegregation Modelling of DC-Casting Including Grain Motion and Surface Exudation

The as cast chemical composition of a DC-cast billet or slab involves mechanisms as movement of solid grains, that are lean on alloying elements, as well as movement of interdendritic liquid, that are enriched on alloying elements. These phenomena can result in the composition being out of the specification at the surface area for billets or in the centre for large slabs. In this study macrosegregation including thermal and solutal convection, solidification shrinkage, grain motion and surface exudation is simulated on a billet cast at the Alcoa Lista plant. The model includes the air-gap formation against the mould during DCcasting and the flow of liquid through the semi-solid surface that is created under such conditions. Modelling results are compared with measurements of chemical composition through the billet from centre to surface and on the exuded layer itself as well as a characterization of the thickness of the exuded layer.