Measurement of flow fields during equiaxed solidification

Modeling 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 micro) segregation 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 equiaxed solidification simultaneous liquid and solid flow is outstanding amongst other phenomena because the resulting solid-liquid multiphase flow pattern strongly depends on the microstructure of the equiaxed crystals which, in turn, is governed by grain nucleation, growth mechanisms and mass and momentum transfer. Because the coupled liquid-solid flow causes structural and chemical inhomogeneities in the final solidified products, a fundamental understanding of the multiphase transport phenomena coupled with the grain nucleation and growth mechanisms is required. The aim of the proposed project is the development and combination of three optical methods to quantitatively measure of the number, size and movement of NH 4 Cl crystals in an aqueous solution during solidification depending on the cooling rate and geometry. The applied measurement methods will be: 1) stereoscopic Particle Image Velocimetry (PIV) to measure melt convection flow fields 2) particle tracking (PT) to measure the movement of NH 4 Cl grains within the melt and 3) advanced shadowgraph (SG) measurements to extract grain size distributions from shadowgraph images. Additionally, the experimental results will be compared to numerical simulations using a two phase model. The combination of the experimental results with numerical simulations will allow a better understanding of several details of the microstructure formation during solidification on one side and the applicability of a numerical two phase model on the other side.

Modeling 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 micro) segregation 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 equiaxed solidification simultaneous liquid and solid flow is outstanding amongst other phenomena because the resulting solid-liquid multiphase flow pattern strongly depends on the microstructure of the equiaxed crystals which, in turn, is governed by grain nucleation, growth mechanisms and mass and momentum transfer. Because the coupled liquid-solid flow causes structural and chemical inhomogeneities in the final solidified products, a fundamental understanding of the multiphase transport phenomena coupled with the grain nucleation and growth mechanisms is required. The aim of the proposed project is the development and combination of three optical methods to quantitatively measure of the number, size and movement of NH4Cl crystals in an aqueous solution during solidification depending on the cooling rate and geometry. The applied measurement methods will be: 1. stereoscopic Particle Image Velocimetry (PIV) to measure melt convection flow fields 2. particle tracking (PT) to measure the movement of NH4Cl grains within the melt and 3. advanced shadowgraph (SG) measurements to extract grain size distributions from shadowgraph images. Additionally, the experimental results will be compared to numerical simulations using a two phase model. The combination of the experimental results with numerical simulations will allow a better understanding of several details of the microstructure formation during solidification on one side and the applicability of a numerical two phase model on the other side.