Diffusion-controlled melting and re-solidification of metal micro layers on a reactive substrate

The paper offers a theoretical approach to a prediction of residue formation inherent to melting and subsequent solidification of micro layers of molten aluminum alloys. The residue formation follows a reactive flow of a portion of the melt that is removed by a surface tension action. The residue portion solidifies in situ. The phenomenon studied is associated with materials' processing during controlled atmosphere brazing of aluminum. The model assumes that diffusion of Silicon, present in an Al+Si clad of a brazing sheet, has a twofold role. First, a solid state Si diffusion prior to melting and across the clad-core interface of a composite brazing sheet takes place and modifies alloys' composition on both sides of the interface. Subsequently, Si diffusion within clad controls the melting process. Both processes are essential for clad residue formation. The approach advocated in this paper leads to a prediction of the residue formation through a modeling of the non-equilibrium diffusion-controlled melting. A heuristic interpretation of physical mechanisms was discussed and a related mathematical model devised. The model was solved numerically in terms of Si concentration distributions for a moving boundary problem and corroborated with empirical data. Empirical data were gathered using an experimental controlled atmosphere brazing facility. The results of the modeling and their corroboration with the experimental data indicate a strong dependence of residue formations on the pre-melting state of the clad, in particular on the grain size within Al-clad matrix. A good agreement between numerically predicted residue mass and experimental findings is documented in detail.