The effect of Zr as a grain refiner on the solidification behavior, micro-and macrostructure of a new Al-Zn-Mg-Cu aluminum super high strength alloy containing high Zn content was studied. The addition of 2 wt. % Zr reduced the grain size from 1500μm to 190μm. Moreover, the dendritic structure of the alloy altered from a coarse, elongated and non-uniform morphology to a rosette-like shape and more uniform one. The parameters of liquidus region of cooling curve obtained from thermal analysis were in a good correlation with grain size results. The maximum of first derivative in the liquidus region was introduced beside recalescence undercooling which could predict the grain refinement level even after disappearing of recalescence in the cooling curve. Furthermore, the addition of 1 wt. % Zr enhanced fraction of solid in dendrite coherency point (DCP) from 21% to 31% and lessened the amounts of porosity from 2.3% to 1.4%
