Undercooling and Cracking During Solidification

How the solid fraction f_S increases with decreasing temperature T during solidification, i.e., the f_S(T) of an alloy, can play a critical role in its susceptibility to cracking during solidification as demonstrated by various models of solidification cracking. In the present study the classic analytical modeling of microsegregation during rapid solidification was used to calculate f_S(T) using Al–Cu alloys as an example. The present study showed significant undercooling can occur during fast cooling and affect f_S(T) significantly. For the purpose of illustration, |dT/d(f_S)^1/2| near (f_S)^1/2 = 1 was used in the present study as a simple index for the susceptibility to solidification cracking. The f_S(T) calculated by any solidification model (e.g., the present analytical model or a phase-field model with undercooling, or the Scheil–Gulliver model without undercooling) can be used to calculate the curve of T vs (f_S)^1/2 and hence the index. The present study showed undercooling can increase the index and hence the cracking susceptibility significantly. It can also change the composition most susceptible to solidification cracking.