Kinetics of the molten Al–Si triple line movement during a brazed joint formation

Kinetics of liquid-metal spreading over a substrate is of fundamental importance for applications of a number of high-temperature capillary-driven phenomena in technological processes, e.g., bonding by soldering or brazing. The sharp interface model combined with the Navier–Stokes equations and no-slip condition at the solid–fluid interface, results in unphysical stress singularities. Moreover, molecular dynamics studies indicate that the motion of the triple line proceeds by uncorrelated movement of fluid atoms on the solid surface, i.e., diffusion. Hence, diffuse interface (phase field) models are the natural framework for modeling such physical phenomena, whereby the triple line movement is described in terms of a local surface diffusion of fluid. Moreover, they are the only computational models that can describe topological changes associated with capillary flows (breaking up and coalescence of fluid domain). This paper offers comprehensive experimental evidence involving spreading over substrates, and associated phase-field modeling. A 2-D wedge-tee joint configuration was considered. The phase-field model parameters are related to the physical parameters (density, viscosity interface energies, kinetic barrier for surface diffusion), and the computational parameters. The latter are chosen so that neither kinetics nor equilibrium is affected. Numerical solution of the model indicates excellent agreement with the ultimately reached equilibrium state, and follows fully an empirically established trend of the triple line kinetics. Model is tested by using additional benchmark processes of spreading water and silicon oil over non-reactive substrates before implementing it to a high temperature non-reactive approximation of the liquid-metal (Al–Si over aluminum) wetting.