Dendrite spacing in unidirectionally solidified Al-Cu alloy

Abstract

Directional solidification experiments have been carried out to study the variation in primary and secondary arm spacings with solidification parameters in the Al-Cu system. It is found that the primary arm spacing Z 1 obeys the common correlation Z 1 = KG -aV -b in the high velocity regime or low temperature gradient regime, where a and b are constants, the value of K included the composition dependence, G is the temperature gradient in the liquid and V the growth rate; however, it does not obey this correlation for the low velocity regime or high temperature gradient regime but goes through a maximum or a catastrophe as a function of V or G at V = V cs k or G = kG cs, where k is the equilibrium distribution coefficient, and V cs and G cs are the critical velocity and temperature gradient at the limit of constitutional undercooling respectively. The initial secondary arm spacings Z 20 are nearly independent of G and mainly depended on V, Z 20 = 0.016 V -0.54 (mm). The secondary arm spacing Z 2 tends to c oarsen with time and thus is a function of coarsening time t f, Z 2 = 0.016t 0.34 f (mm). Theoretical analyses of the primary arm spac ing and the initial secondary arm spacing have been proposed, and the derived relationships agree reasonably well with the above experimental results.