Inhibited coarsening of solid-liquid microstructures in spray casting at high volume fractions of solid

Abstract

Experimental studies on coarsening in fine grained solid-liquid microstructures at high volume fractions of solid ( f s ) have been carried out to determine if inhibited coarsening under these circumstances could account for the anomalous fine cell sizes observed in spray castings. The materials investigated included a chill-cast dendritic binary alloy of Al-Cu, two spray cast alloys—AA2014 and Cu-Ti, whose grain size was the segregate spacing, and a d.c.-cast alloy Al-4.5 wt% Cu-1.5 wt% Mg in both coarse-grain and grain-refined conditions. The observed segregate spacings after coarsening were smaller than that predicted by empirical correlations of dendrite arm spacing and freezing time. In all cases, the coarsening was found to become slower as the temperature was reduced and f s increased. Conventional coarsening theories and experiments predict the opposite, i.e. faster coarsening at higher volume fractions of solid. Two additional coarsening models were developed for the grain growth at high volume fractions of solid by processes whose rates are limited by migration of liquid at grain boundaries as liquid films on 2-grain surfaces or liquid rods on 3-grain triple points. In both models, the conventional diffusion-limited t 13 coarsening law was reproduced, but the rate constant K contained the term 11−f s and so also predicted accelerated coarsening as f s → 1 . Three possible explanations for the observed lower K values at increasing f s are proposed. The first is the effect of the increasing difference between the solute contents of solid and liquid as the temperature is reduced. This produces a1/X 1 dependence of the coarsening rate constant K. The second inhibiting effect, specific to dendritic structures, is in-grain coalescence of dendrite arms at high f s which produces isolated liquid particles within the grains. The final possibility is particle-inhibition of grain boundary migration by minority (impurity) particles at the grain boundaries. Such particles were seen, however, for only two of the alloys, viz. the grain defined d.c. cast Al-4.5 wt% Cu-1.5 wt% Mg and the spray case AA2014, but they or gas-filled pores are proposed as strong possibilities to account for the fine grain sizes observed in all spray cast microstructures.