Solidification parameters during the columnar-to-equiaxed transition in lead-tin alloys

The columnar-to-equiaxed transition (CET) was studied in lead-tin alloys, which were solidified directionally from a chill face. The main parameters analyzed include the temperature gradients, solidification velocities of the liquidus and solidus fronts, and grain size. The transition was observed to occur when the temperature gradient in the melt decreased to values between -0.8 degrees C/cm and 1 degrees C/cm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The comparison with available analytical models and the observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites.