Hot-tearing susceptibility of Al–Zn alloys

Abstract

The modern concepts of the causes of hot tearing are considered and the influence of the solid fraction growth rate of an alloy is studied. The hot-tearing susceptibility (HTS) of binary Al–(5–73) wt % Zn alloys is investigated using backbone tests. The HTS is found to be maximal at ~25 wt % Zn. This maximum cannot be explained by a change in the effective solidification range, since this range of the alloys decreases monotonically with increasing zinc content. The calculations of nonequilibrium solidification by the Scheil–Gulliver model and the solid fraction growth rate of the alloys under study demonstrate that the increase in the HTS induced by an increase in the zinc content from 5 to 25 wt % is related to the decrease in the solid fraction growth rate at the final stages of solidification. The decrease in the HTS at >25 wt % Zn is associated with an increase in the fraction of eutectic in the alloys (the solid fraction growth rate during the eutectic reaction tends toward infinity) and with a change in its morphology.