Microstructural evolution during unsteady-state horizontal solidification of Al-Si-Mg (356) alloy

The increasing demand for reducing vehicle weight in the automotive and aerospace industries has raised the need to develop improved structural aluminum-based alloys. Thus, horizontal solidification experiment with the Al-7% Si-0.3% Mg (mass fraction) alloy was carried out. A water-cooled horizontal directional solidification device was developed and used. Microstructural characterization was carried out using traditional techniques of metallography, optical microscopy and SEM microscopy. The Thermo-Calc software was used to generate the solidification path of the investigated alloy with addition of 0.17% Fe (mass fraction). The effects of the thermal parameters such as the growth rate (V-L), cooling rate (T-C) and solidification local time (t(SL)) on the formation of the macrostructure and on the dendritic microstructure evolution were evaluated. A columnar to equiaxed transition (CET) was found for VL and TC values from 0.82 to 0.98 mm/s and from 1.71 to 2.55 degrees C/s, respectively. The microstructure was characterized by the measurement of the primary and secondary dendrite arm spacings (lambda(1) and lambda(2), respectively). Experimental laws of lambda(1) = f(V-L, T-C) and lambda(2) = f(t(SL)) were proposed. It is observed that the interdendritic region is composed of the following eutectic mixture: alpha(Al)+Si+pi-Al8Mg3FeSi6+theta-Mg2Si.