Experimental analysis on the effect of initial concentration in the direction solidification of Sn-Pb alloy

In this paper, an experimental model for the directional solidification of Pb-Sn alloy is built and the effects of five different initial concentrations on the solidification microstructure are investigated. In the experiment, the constant temperature gradient and growth rate are given by the designed control mechanism. These can lead to directional solidification, which can be seen in the macro and micro photos of the resulting microstructures. From the microstructure observation, the casting can be divided into three zones, the chill-affected, directional solidification and air-affected zones and the directional columnar structures can be obtained in the latter two zones. The non-lamellar eutectic structure is found in the case with the initial concentration closest to the eutectic one and it is also seen between the columnar structures, whose existence probability increases with the initial concentration. In the cases of low initial concentration, the columnar structures are cells, but in the cases of high concentration, they are dendrites. For either cellular or dendritic structure, the higher initial concentration has the smaller microstructure size. Due to the cooling effect of chill, as the location in the casting is farther from the chill, it has a slower cooling rate, which leads to larger microstructure size.