On an unusual behavior of the melt Al – Si in thin capillaries

Abstract

The numerical simulation is devoted to the division of binary metal melt Al – Si filling the thin alundum or quartz capillary. The results of calculations have been gotten for cylindrical channel heated from above. Earlier the behaviour of the similar eutectic melts Pb – Sn, Pb – Bi, Zn – Al, Cd – Bi, Bi – Sb, Cu – Sn, Cu – Pb was considered experimentally and theoretically. The adequate theory of the observed physical phenomena was constructed for these pairs of metals. In experiments the binary melt was prepared to be homogeneous at the initial moment of time. The relatively quick and significant division of a mixture on components was observed for each binary melt. The specific feature of the process was found, namely, the heavy component moved to the lower part of the capillary. The character concentrations difference on the edges of the channel reached to the sufficiently big value 20 %. The explanation of the similar phenomenon has been done by the authors of this article and is based on the assumption of the key role of the interfacial tension on the boundary between melt and alundum wall. It is supposed that the condition of non-wetting is fulfilled on the alundum or quartz walls of the capillary for the metal pairs listed above. Thus, the surface active component appears on the interphase over the phenomenon of adsorption. In experiments under discussion the small temperature gradient oriented vertically upward takes place always in the working space of the smelting furnace. So the thermocapillary force arises on the interphase and generates the large-scale movement in the volume. The equations and laws of interfacial hydrodynamics are valid in this case and can be applied to describe the heat and mass transfer in the liquid binary metals. The transfer of heavy surface-active agent takes place along the direction where the surface tension increases i.e. to the down edge of the capillary. Heavy component continues to accumulate in lower part of the channel so long as all fluxes, caused by external forces, will be equilibrated in the volume and on the surface by the concentration-capillary force and diffusion. On the other hand it has been discovered experimentally that the alloy Al – Si behaves as anomalous melt in relation to all tested pairs of metals. On the contrary to the previous data the heavy component (silicon) accumulates in the top part of the capillary. The explanation of such anomaly is given in this article. It is necessary only to take into account in theoretical model the fact that the light aluminum is surface-active agent in our pair of metals. This component is pressed out of the volume on the surface and under the action of thermocapillary force is transferred downwards along boundary of a channel. Then it returns inside the volume near the bottom by means of the desorption. This process generates the unstable stratification since the light component is located in lower part and at the same time the heavy one is lifted to the top of the capillary. In spite of convective instability there is no large-scale transfer of the light admixture upwards because of heating from above and big hydrodynamic resistance of a thin capillary. The estimations and direct numerical modeling show that the thermocapillary mechanism dominates above the resulting Archimedean force for given geometry, therefore inflow of a light component to the bottom part of a capillary prevails of a loss that causes accumulation of the aluminum in the bottom of channel.