Modeling of Temperature Dependent Surface Tension Force

V. Nilsson. Master Thesis, Chalmers University of Technology, 12 December 2018. Supervisors: Johan Göhl and Andreas Mark


One of the challenges with CFD simulations of metal AM is to properly model the temperature dependent surface tension force driving the melt pool flow. High density ratio multiphase flows, as between the gas and the liquid metal in melt pool flow, are considered difficult to model due to the generation of spurious currents at the interface. At Fraunhofer Chalmers Center (FCC) a state-of-the-art CFD solver, IBOFlow is developed. In this project the existing surface tension framework in IBOFlow is improved and extended. A temperature dependent surface tension model together with a thermo-capillary force is proposed. The new surface tension framework is assessed and validated so that the melt pool dynamics of metal AM is accurately modeled.

Different curvature estimation techniques and a technique for calculating the inter-face normal direction are thoroughly tested and evaluated in order to reduce the influence of spurious currents on the results. The numerically calculated curvature and pressure is evaluated and validated against analytical results for a case involving a static droplet in equilibrium. Further more a temperature dependent surface tension model is also proposed and validated together with a thermo-capillary surface tension force. The benchmark case to evaluate the temperature dependent surface tension and the thermo-capillary surface tension force include a comparison with thermo-capillary cavity flow.

The result of the static droplet case show a substantial improvement when calculating the interface curvature and pressure difference across the interface, with results in line with exact analytical calculations. Furthermore, these improvements also substantially reduces the spurious currents around the interface. The temperature dependent surface tension model and the thermo-capillary surface tension force are validated against an analytical solution and compared against other numerical results of the thermo-capillary cavity flow. The results show perfect agreement with analytical values and outperforms other numerical studies on the subject. The improved and extended surface tension framework is then used to successfully simulate a single line melt of a selective laser melting process on a powder bed.

Photo credits: Nic McPhee