The critical problem in the preparation process of large scale products, such as thick plates and heavy for- gings, is the inhomogeneity of properties from surface to center [3–
5]. The key factor therein is the quench sensitivity, which means the properties of the aged alloy are influenced by the quenching rate. For its practical importance, many investigations [5–9] have been done on the quench sensitivity of Al–Zn–Mg–Cu alloys.