Globular structure of ZA27 alloy by thermomechanical and semi-solid treatment

The dendritic structure of a cast ZA27 alloy can be transformed into a globular structure, if it is cold worked prior to being held in a semi-solid condition. The globular or the equiaxed particles are surrounded by the lower melting matrix. The dendritic structure was swaged and recrystallised prior to holding in a semi-solid state in this investigation. The semi-solid holding of the alloy at temperatures above and below that of peritectic reaction was carried out for various lengths of time. It was shown that the liquid fraction is decreased to below the peritectic temperature which leads to a reduction of lubrication between the solid globules and an increasing deformation force. Holding the alloy directly above the peritectic temperature or keeping below the peritectic temperature first, and then treating it above this temperature provides a different structure to the globules and entrapped pools. In particular it is shown that the surfaces of the globules are perturbated as a result of peritectic decomposition and solute pile up adjacent to the solid/liquid interface at 455 °C. In addition, it is indicated that the semi-solid deformation is influenced by globules with rough surfaces formed in some heating trials. The desired microstructure is achieved as a result of rapid cooling of the melt in the casting step and by performing the single stage treatment at 455 °C.