Chemical Changes at the Interface Between Low Carbon Steel and an Al-Si Alloy During Solution Heat Treatment

The aim of this work was to characterize the chemical changes during solid state solution heat treatment of a metallurgically bonded steel/Al-Si interface. For this purpose, low carbon steel plates covered with the A-S7G03 aluminium alloy (7 wt.% Si, 0.3 wt.% Mg analogous to A356) were prepared by dip coating, water-quenching to room temperature and reheating in the solid state at 480-560 °C for 3-160 h. Upon reheating at 535 °C, a reaction layer was observed to grow at the interface between steel and the iron-saturated Al-Si alloy. As long as an intimate contact could be maintained, the total thickness, x, of the reaction layer increased with time, t, according to a nearly parabolic growth law x2 = K·t − b. At 535 °C, the value of the growth constant was K = 4.045 × 10−14 m2 s−1. This constant was found to be thermally activated [K = K0 exp(−Q/RT)] with K0 = 4.37 × 10−4 m2 s−1 and Q = 153 kJ mol−1. The whole chemical interaction process was controlled by solid state volume diffusion and the reaction layer sequence corresponded to a diffusion path in the Al-Fe-Si phase diagram. A striking feature of the reaction process is the unbalanced diffusion of aluminium atoms through the reaction zone which rapidly results in the formation of Kirkendall voids. As these voids coalesce, solid state diffusion becomes more and more difficult and the steel/alloy bond gets weakened. Oxidation appears to be an aggravating factor, where applicable.