Cellular/dendritic transition and microstructure evolution during transient directional solidification of Pb-Sb alloys

Recent studies of lead-antimony alloys, used for the production of positive electrodes of lead-acid batteries, have assessed the influences of both the microstructural morphology and of solute redistribution on the surface corrosion resistance in sulfuric acid solution, and have shown that cellular structures and dendritic structures have different responses on the corrosion rate of such alloys. The present article focuses on the search of adequate solidification conditions (alloy composition, cooling rate, and solidification velocity), which determine the occurrence of a microstructural transition from the cellular to the dendritic regime during the transient unidirectional solidification of hypoeutectic Pb-Sb alloys and on the microstructural evolution after such transition. The experimental data refers to the solidification of four hypoeutectic Pb-Sb alloys (2.2, 2.5, 3, and 6.6 wt pct Sb) and of the eutectic composition. The experimental results include transient metal/mold heat-transfer coefficients, liquidus isotherm velocity, cooling rate, and cellular and dendritic spacings. It was found that the cooling rate dependence on cellular and primary dendritic spacings is characterized by an experimental law of the form lambda(1) = A.T(-0.55), which seems to be independent of composition where A = 60 represents the alloys undergoing a cellular growth and A = 115 can describe the dendritic growth. The sudden change on such multiplier has occurred for the Pb 2.2 wt pct Sb alloy, i.e., for the cellular/dendritic transition.