CFD-methods applied to dispersed particle flow within melt pools during laser induced surface modification processes of ceramics

Abstract

Laser surface melting and alloying are important techniques to improve the properties of a material locally without affecting the bulk. In ceramic components these methods can be used to optimise the fracture toughness, friction and wear but also the thermal and electrical conductivity [1]. In order to predict the results of the modification process and to reduce the development time numerical models which simulates the physical processes are gaining importance. The process is characterised by a moving solid-liquid phase boundary during the laser induced melting [2]. Shear forces at the surface of the melt pool lead to convection streams within the liquid phase which determines the mass transport and particle flow of the dispersed phase. Therefore, the modelling of the temperature and the velocity field within the molten region can be used to predict the geometry of the melt pool and the distribution of the second phase particles, respectively. Numerical computations has been performed on the basis of a finite element model (FEM) in order to get more insight in the heat and particle transport during melting and solidification. The model considers the liquid-solid phase change and also the heat and mass transport due to convection within the molten pool. Commercial software packages (FEMLAB , PHOENICS) has been used to solve the Navier-Stokes equations which describes the fluid flow within the melt pool due to the surface tension gradient which is induced by the localised heat source travelling with a constant speed. Temperature dependent material properties like the thermal conductivity and the heat capacity has been included in the model. In order to simulate the phase change at the moving liquid-solid boundary on a fixed grid the enthalpy-porosity model has been implemented in the FE-model, which fixes the velocity components to zero in the solid phase and allows for a reconstruction of the solid-liquid interface. The results of the calculations give quantitative information about the temperature and the velocity field during the laser process in the ceramic substrate. Above the melting temperature of the ceramic a velocity field appears in the melt pool which is induced by the negative surface tension gradient. Further calculations on the basis of two-phase flow models and particle tracking algorithms has been performed to evaluate the particle distribution after introducing additional materials into the melt. Detailed numerical studies have been performed to calculate the extension of the melt pool and the volume fraction of the particles for a wide range of process parameters like the laser power density and scanning velocity. The calculated results have been confirmed by experiments. It could be shown that the extension of the melt pool and the dispersion zone, as well as the particle distribution depend not only on the laser process parameters process parameters but also on the thermodynamic properties of the ceramic.