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.
In this paper, an experiment model for the directional solidification of Lead/Tin alloy is built and the effects of different-shape seeds on the microstructures on the solidification microstructure are investigated. In a casting process, the temperature and concentration fields will affect the microstructures of materials and this influence is the key point of improving their mechanical and physical properties. It is not easy to control the morphology of solidifying microstructures. The scheme of directional solidification can make the microstructures grow along a fixed direction and it is also the base of single-crystal growth. In the experiment, a poly-grain seed with the same initial concentration of the solidifying casting is used to induce the columnar growth at the bottom portion of the casting, which could avoid the equiaxed growth due to the high undercooling or cooling rate there. In the experimental analysis, we studied the influences of different geometry seeds on the constrained growth, the preferential growth direction of dendrite, the grain size, the temperature gradient, the growth rate, the primary arm spacing and the secondary arm spacing. From the microstructure observation, the adding seed casting reduced the chill-affected and extended the directional solidification zone. This is expected to have the better or more complete structure of directional solidification. Keywords: Directional Solidification, Seed, Heat Transfer and Microstructure
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