The columnar-to-equiaxed transition in Al 3 Pct Cu

A columnar-to-equiaxed transition is observed in Al 3 pct Cu solidified directionally from a chill face. The transition occurs when the temperature gradient in the melt ahead of the columnar dendrites decreases to 0.6 ‡C/cm at dendrite growth rates of about 5 x 10^-3 cm/s. Increasing the nuclei density by adding 171 ppm of TiB₂ to the melt produces a fine-grained structure without columnar growth. Adding 100 ppm TiB₂ has no effect on the cast structure or columnar-to-equiaxed transition. The results are considered in relation to the model for the columnar-to-equiaxed transition proposed by Hunt.^2]