Effect of Zr on recrystallization in a directionally solidified AA7050

A high purity Al-Zn-Cu-Mg alloy based on AA7050 was cast with varying Zr concentrations by directional solidification (DS) and static casting (SC). Specimens were homogenized, hot rolled and solutionized to study the recrystallization behavior. In the DS ingot a gradient of Zr concentration existed along the growth direction, but across the transverse direction the distribution of Zr was uniform, while in SC ingots a dendritically cored Zr concentration gradient was found. The variations in solidification method and Zr concentration resulted in difference in size, number and spatial distribution of Al3Zr-type dispersoids, and thus different degrees of recrystallization after solutionization. Recrystallization was delayed both in the SC specimen with 0.11 wt% Zr and in the specimen from the top of the DS ingot with 0.03 wt% Zr, whereas full recrystallization and grain growth were found in the SC specimen free of Zr and the DS bottom specimen with 0.11 wt% Zr. The inconsistency between the recrystallization behavior of the DS bottom specimen and its relatively high Zr concentration is likely related to the precipitation and coarsening of the dispersoids during slow DS cooling.