Multiscale Thermomechanical Simulation of Solid-Liquid Interactions in Solidification
This subproject adresses precision-limited characteristics of partially solidified metallic molten masses, which are defined by the microstructure in a solid/liquid multiphase state. Light plasticity/deformability and the clear and volatile changes of many thermophysical and mechanical characteristics during phase transition should be considered in the process layout with regards to dimensional accuracy and defect formation. This is shown, e.g. in the moulding of components with thin walls, where the water feed is determined by the permeability characteristics of the mostly dendritic solidification morphology. A reduced post feed leads to a number of moulding errors, such as macro-cavities or the creation of hot cracks. For the warpage of geometrically complex building components caused by residual tension, volume changes in the phase transition play a role as the source of mechanical stress for the resulting stress fields as do the mechanical material characteristic values of the solidified state. The quality of the building component surface in relation to roughness or defects like surface pores is determined by the rheological characteristics in the partially crystalized state.
The goal of this subproject is to use a multiscale approach to calculate thermomechanical characteristics, such as thermal expansion, permeability, or flow curves in a partially solidified state and then in a solid state during cooling, in order to consider the local variations of these characteristics based on the mould in simulations of the building components. The acquisition of precision in the prediction of warpage and remaining stress is quantified and compared to experimental results from Subproject B8 and predictions based on conventional materials laws. The local cooling conditions are filtered from the thermal macrosimulation of the moulding process and transferred onto the microscale. On the microscale the corresponding solidification morphology is simulated, spatially resolved with the help of a phase field model [STE06] for multi-component metal alloys. The thermal and metallurgical relationship between the solid's skeleton and the molten mass are particularly taken into consideration in the phase field model.
Building on the calculated framework, effective anisotropic characteristics are derived using the homegnization method [LAS10]. In order to formulate material law for individual thermophysical characteristics (permeability, thermoconductivity,...), which are based on the microstructure characteristics of the partially solidified state, the morphology of the calculated framework is quantified through suitable dimensions (e.g. phase ration, primary and secondary dendrite arm spacing, size and orientation of individual punches). Since a 3D microstructure simulation for each node of a macroscopic FE building component is not possible due to the necessary computational cost, the effective characteristics determined for selected areas and the respective microstructure parameters are transferred to the interpolation algorithm developed in TP B9. This program interpolates the effective characteristics transmitted in each discretization point of the component. These locally determined punches flow into an expanded two-phase model of the coherent solid-liquid area on the macroscale. Using the developed thermomechanical multiscale simulation, the warpage and development of stress of the aluminum component during solidification and cooling should be locally and precisely predicted.
This fundamentally oriented simulation project contributes to a better understanding and to the analysis of complex metallurgical and thermomechanical interactions between the molten mass and skeleton of the solid. The control of the geometric precision in particular requires mastering the variation of materials characteristics in the solid-liquid area. The fundamental knowledge about the solid-liquid interaction in aluminum cold casting gained using the multi-scale model is transferred over the course of the SFB to other melting-related processes such as soldering, MSG welding, and laser welding.
Microstructure of a solidifying aluminium alloy (A356)
Microstructure of a solidifying aluminum alloy (A356), resulting from a phase field simulation. Starting from melt (transparent) first aluminum (light grey) grains solidify during primary solidification, then silicon (dark grey) alongside aluminum during eutectic solidification. The simulation domain has an edge length of 150 mm. The simulated solidification takes 5.5 s (slowed down by a factor of 3 in the video) for a heat extraction rate of 240 W/cm3.
Animated representation of the melt flow in a dendritic structure at a vertical pressure gradient
Animated representation of the melt flow in a dendritic structure at a vertical pressure gradient. The microstructure was obtained from a phase field simulation of the solidification of an aluminium alloy (A356). The flow results from a CFD simulation, which is carried out for different points in time during the solidification process, in order to determine the permeability of the microstructure to the melt. The flow is shown at approx. 50% solid phase content.