Endogenous-exogenous freezing characteristics of pure and impure as-cast Zn-Al alloys

A study was initially made of the cast structures of pure binary Zn-Al alloys having compositions within the eutectic horizontal. For the fixed casting conditions employed it was observed, in hypoeutectic alloys, that there is a coarsening of the macrostructure with increasing aluminium content accompanied by a change in the mode of freezing from endogenous (equiaxed grain structure predominating) to exogenous (columnar grain structure predominating). On the other hand, hypereutectic alloys were observed to freeze as completely equiaxed structures for all compositions examined. When Pb was added as an impurity to alloys in the range 3–10 wt.-%AI, the general effect was a coarsening of the as-cast macrostructure. However, two major differences were noted between the solidification of hypo- and hypereutectic alloys. In hypoeutectic alloys of neareutectic composition the mode of freezing changed from exogenous to endogenous, whereas the opposite effect occurred in the hypereutectic alloys. Furthermore, the Pb had a marked influence on the number, appearance, and distribution of primary zinc solid solution dendrites but this was not so in the case of primary aluminium solid solution dendrites. The results are explained in terms of the intrinsic solidification characteristics of Zn-AI alloys and the manner in which these are influenced by the addition of Pb.