When heating the reactive flux of the KF–AlF$_3$–K$_2$SiF$_6$ system on an aluminium substrate, two processes occur; reduction of silicon from potassium hexafluorosilicate; contact-reactive melting of silicon with aluminium. As a result, an Al–Si alloy layer is formed on the reaction surface, the composition of which is close to hypereutectic, which is confirmed by the results of X-ray microanalysis. The microstructure of the crystallized Al–Si metal layer contains aluminium-based solid solution grains and a hypereutectic component with a silicon concentration (% wt.): 17.18 in intercrystalline areas, as well as individual discrete inclusions of the lamellar phase, which is close in stoichiometric composition to the compound FeSiAl$_5$. Comparative experiments on a graphite substrate showed that the silicon content in the flux residues after heating corresponds to its content in the initial flux composition.